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Lipoprotein (a) in pregnancy: A critical review of the literature
European Journal of Obstetrics & Gynecology and Reproductive Biology 122 (2005) 13–21 www.elsevier.com/locate/ejogrb
Review
Lipoprotein (a) in pregnancy: A critical review of the literature Gwendolyn T.R. Manten a,*, Hieronymus A.M. Voorbij b, Ton M. Hameeteman c, Gerard H.A. Visser a, Arie Franx a a
Department of Perinatology and Gynecology, University Medical Center, P.O. Box 85090, KE.04.156.1, 3508 AB Utrecht, The Netherlands b Department of Clinical Chemistry, University Medical Center, P.O. Box 85090, 3508 AB Utrecht, The Netherlands c Department of Obstetrics and Gynecology, St. Antonius Hospital, P.O. Box 2500, 3430 EM Nieuwegein, The Netherlands Received 15 March 2004; received in revised form 24 November 2004; accepted 22 March 2005
Abstract In this article the literature on lipoprotein (a) during normal pregnancy and pregnancy complicated by preeclampsia or intrauterine growth restriction is reviewed. MEDLINE, from January 1966 to May 2003, was searched to locate relevant articles in English. Additional publications were identified by reviewing references in selected articles. Studies were reviewed by predefined and strict criteria. It appeared that methodology and results of studies on lipoprotein (a) during normal and complicated pregnancy were very diverse. Lipoprotein (a) increased with advancing gestation or remained unaltered during normal pregnancy. Women with preeclampsia had higher, unaltered or lower lipoprotein (a) concentrations as compared to normal pregnant controls. Only few studies were in agreement with most of the review criteria. In conclusion, published studies on lipoprotein (a) in pregnancy differ substantially in the used methods to measure lipoprotein (a), sample size, study design and ethnicity of the study population. Therefore, these studies yielded conflicting results and no unequivocal view on the role of lipoprotein (a) in normal and complicated pregnancy. Recommendations for future studies are amongst others: the use of an apo(a) independent method for measuring Lp(a), inclusion of sufficient numbers of patients, the use of a longitudinal study design when the objective is to study the changes of Lp(a) during pregnancy and selection of a study population that is ethnically representative for the general population. # 2005 Elsevier Ireland Ltd. All rights reserved. Keywords: Lipoprotein (a); Normal pregnancy; Preeclampsia; Intrauterine growth restriction; Review
Contents 1.
2.
3.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1. Lipoprotein (a) and pregnancy . . . . . . . . . . . . . . . . . 1.2. Lipoprotein (a) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3. Important factors influencing Lp(a) concentration. . . . 1.3.1. Lp(a) measurement . . . . . . . . . . . . . . . . . . . 1.3.2. Apo(a) isoform typing, sample size and study 1.3.3. Ethnicity . . . . . . . . . . . . . . . . . . . . . . . . . . Study design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Search methods . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Review criteria. . . . . . . . . . . . . . . . . . . . . . . . . . . . Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Search results. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Summary of the results of the included studies . . . . .
..... ..... ..... ..... ..... design ..... ..... ..... ..... ..... ..... .....
* Corresponding author. Tel.: +31 302504000; fax: +31 302505320. E-mail address: [email protected] (Gwendolyn T.R. Manten). 0301-2115/$ – see front matter # 2005 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.ejogrb.2005.03.013
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3.2.1. Lp(a) in normal pregnancy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.2. Lp(a) in pregnancy complicated by preeclampsia . . . . . . . . . . . . . . . . . . . . . . . . 3.2.3. Lp(a) in pregnancy complicated by IUGR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Evaluation of the studies according to the review criteria . . . . . . . . . . . . . . . . . . . . . . . . 3.3.1. The use of an apo(a) independent method to measure Lp(a) concentrations. . . . . . 3.3.2. Apo(a) isoform determination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.3. Sufficient sample size for case control studies . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.4. Longitudinal study design when changes of Lp(a) during pregnancy are evaluated . 3.3.5. Information about and equal distribution of ethnicity in study groups . . . . . . . . . . 3.3.6. Statistical analysis with non-parametric tests or with log-transformed Lp(a) data . . Conclusions and recommendations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1. Introduction 1.1. Lipoprotein (a) and pregnancy Pregnancy is a state of hyperlipidemia [1] and hypofibrinolysis [2]. Lipoprotein (a) (Lp(a)), a carrier of cholesterol, plays an important role in lipid metabolism and is known to hamper fibrinolysis [3,4]. Preeclampsia, a disorder characterized by hypertension and proteinuria after the 20th week of gestation, is associated with fibrin deposition, thrombosis and infarction in the placental vascular bed very similar to the lesions observed in atherosclerosis [5]. Lp(a) has been identified as an inherited risk factor for a variety of vascular diseases, including atherosclerosis [4]. Two case reports, one on high Lp(a) concentrations in a single family with two cases of severe preeclampsia [6], and one on a women with high Lp(a) concentrations giving birth to three children with very low birth weights [7], suggested that Lp(a) may interfere with placental circulation and may be involved in the pathogenesis of preeclampsia. Not surprisingly therefore, Lp(a) has been extensively studied in normal pregnancy and pregnancy complicated by preeclampsia during the last 15 years. The results of these studies are, however, diverse and conflicting. 1.2. Lipoprotein (a) Plasma concentrations of Lp(a) in humans range from less than 10 mg/L to greater than 1000 mg/L, and increased risk for cardiovascular disease is associated with concentrations above 300 mg/L. Lp(a) concentration is almost entirely genetically determined, and is relatively resistant to most dietary and drug interventions [3,4,8]. Lp(a) consists of a low-density lipoprotein (LDL) particle with one additional distinguishing protein known as apolipoprotein (a) (apo(a)). Apo(a) is bound to the apolipoprotein B-100 of the LDL particle by disulphide linkage [3,4,8]. The apo(a) gene is located on the long arm of chromosome 6 and cDNA sequencing of apo(a) has revealed a high degree of homology with plasminogen, a precursor protein of the fibrinolytic enzyme plasmine [9]. Competitive binding of Lp(a) to endothelial plasminogen receptors reduces the physiological fibrinolytic activity of plasmino-
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gen [10,11]. This may be explanatory of the association of elevated concentrations of Lp(a) and atherosclerotic cardiovascular disease [4]. Apo(a) contains a kringle 4 domain which consist of 10 different types of proteases of which type 2 is present in multiple copies. The number of type 2 copies varies and is responsible for the size heterogeneity of the apo(a) phenotype, or so-called isoform, and consequently of Lp(a). The plasma concentration of Lp(a) is inversely related to the size of its apo(a) isoforms [12]. The Lp(a) concentration varies in different ethnic groups. The distribution in Caucasians is highly skewed to the right with many low values and a median Lp(a) concentration of 140 mg/L. Therefore, statistical analysis should be performed with non-parametric tests or with log-transformed data. A significantly different, more bell-shaped Lp(a) distribution with mean concentrations twice as high as those of whites has been reported in populations of American black men and women [13]. 1.3. Important factors influencing Lp(a) concentration 1.3.1. Lp(a) measurement Lp(a) measurement is difficult, because Lp(a) assays are not standardized. Results differ substantially between laboratories and can hardly be compared. The reason for this is the use of different assay methodologies (Laurell technique, enzyme-linked immunosorbent assay (ELISA), immunoturbidimetry, quantitative electroimmunoassay, radioimmunoassay or nephelometry) and the use of different antibodies and standards. Some of these antibodies may recognize the different apo(a) isoforms not independent of their size. This results in over- or underestimation of the Lp(a) concentration, since the size of the apo(a) isoform is inversely related to the Lp(a) concentration. It is therefore, recommendable to use an apo(a) independent method to measure Lp(a) concentrations [12–14]. 1.3.2. Apo(a) isoform typing, sample size and study design The distribution of Lp(a) concentrations in Caucasian populations shifts to low concentrations and is extremely
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wide [3]. Therefore, results of studies measuring Lp(a) concentrations are very susceptible to random deviations. Consequently, case control studies with a low number of cases and/or controls must be considered with caution, in particular when true differences between two groups are small. One way to improve this is to stratify groups for apo(a) isoforms. Especially in small groups it is essential
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that the apo(a) isoforms are determined to identify possible random deviations of small or large isoforms. Another way is to study populations of sufficient size, with group sizes of 50 individuals or more [15]. Apo(a) isoform typing and sample size are less important when Lp(a) is measured in a longitudinal study design. When subjects are studied longitudinally the changes of Lp(a) concentrations, within
Table 1 A review of studies on Lp(a) in normal pregnancy Authors
Year of publication
Study design
Zechner et al.
(1986) [18] Longitudinal
Panteghini et al. (1991) [19] Cross-sectional
Study population
Method used Apo(a) for Lp(a) isoform measurement typing
Ethnicity of study population
Study results
22 women from first trimester, until delivery and 6–8 wks pp
Laurell technique
No
Unknown
193 pregnant women 39 women 1 week pp 178 pregnant women in second and third trimester 58 non-pregnant controls 178 pregnant women in second and third trimester 68 non-smoking and 118 smoking pregnant women 29 non-pregnant, non-smoking controls 68 non-smoking and 118 smoking pregnant women, from 6–40 wks GA 114 Caucasian and 118 African/Afro-Caribbean women in third trimester of pregnancy 36 women at 35–36 wks GA and at 6 wks pp 62 women from first trimester until delivery and 6–12 wks pp 13 women at term, 25 non-pregnant controls 140 pregnant women, 25 women 1 week pp, 25 non-pregnant controls 22 women from 12–39 wks GA
ELISA
No
Unknown
Immunoturbidimetry
No
Unknown
"Lp(a) during pregnancy until 19 wks GA, thereafter #Lp(a) till baseline values at delivery, and = Lp(a) pp "Lp(a) during pregnancy and #pp =Lp(a) in pregnancy vs controls
Immunoturbidimetry Isoform dependent ELISA
No
Unknown
No
Unknown
Isoform dependent ELISA Immunoturbidimetry
No
ELISA
No
Isoform dependent ELISA ELISA
No
#Lp(a) during smoking pregnancy, "Lp(a) during non-smoking pregnancy Caucasian "Lp(a) in African/AfroAfrican/Afro- Caribbean pregnant women Caribbean vs Caucasian pregnant women Hispanic =Lp(a) in third trimester vs pp Unknown =Lp(a) during pregnancy and pp
No
Unknown
ELISA
No
Unknown
ELISA
Yes
Unknown
Latex a gglutination ELISA Immunoturbidimetry
No
Unknown
No
Caucasian
=Lp(a) in pregnancy vs pp = Lp(a) in pregnancy vs controls
Immunoturbidimetry Immunoturbidimetry
No
Unknown
"Lp(a) during pregnancy
No
Unknown
=Lp(a) during pregnancy
Mazurkiewicz et al.
(1994) [20] Case control
Mazurkiewicz et al. Wersch et al.
(1994) [20] Cross-sectional (1994) [21] Case control
Wersch et al.
(1994) [21] Cross-sectional
Koukkou et al.
(1994) [16] Case control
Silliman et al.
(1994) [22] Longitudinal
Chiang et al.
(1995) [23] Longitudinal
Neary et al.
(1995) [24] Case control
Baroni et al.
(1996) [25] Cross-sectional
Brizzi et al.
(1999) [26] Longitudinal
Sattar et al.
(2000) [41] Longitudinal
Rymer et al.
(2002) [27] Case control 29 women at term of and longitudinal which 22 also12 wks pp, 27 non-pregnant controls (2002) [42] Longitudinal 23 women from first until third trimester (2002) [42] Cross-sectional 64 women first trimester, 48 women second trimester, 67 women third trimester
Belo et al. Belo et al.
10 women from 10–35 wks GA
No
=Lp(a) during second half of pregnancy "Lp(a) in non-smoking vs smoking pregnant and non-pregnant women
Unknown
wks: weeks; pp: post partum; GA: gestational age; ELISA: enzyme-linked immunosorbent assay; vs: versus.
"Lp(a) in pregnancy vs controls "Lp(a) during pregnancy until 16 wks GA, thereafter = Lp(a) until 1 week pp "Lp(a) during pregnancy in women with large apo(a) isoforms, = Lp(a) during pregnancy in women with small apo(a) isoforms "Lp(a) during pregnancy
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subjects, will not be influenced by the large interindividual variation in Lp(a) concentration. Thus, a longitudinal study design is preferred, to study changes of Lp(a) in the course of pregnancy. 1.3.3. Ethnicity Racial differences for Lp(a) concentrations have been documented and in pregnancy these differences seem to persist [16]. These differences are not solely caused by differences in apo(a) isoform distribution [17]. When Lp(a) concentrations between groups are compared, ethnicity should be equally distributed in both groups. When changes of Lp(a) concentrations during pregnancy are studied, ethnicity is less important, since it will only influence the absolute height but not within-subject changes of the Lp(a) concentration. This paper is a critical review of the literature on Lp(a) in normal pregnancy and pregnancy complicated by preeclamspia and intrauterine growth restriction (IUGR). Our aim was to elucidate why conflicting results have emerged from the literature, and to give recommendations for future studies on Lp(a) in normal and complicated pregnancy.
in pregnancy, toxemia or toxaemia in pregnancy, intrauterine growth retardation, intrauterine growth restriction and small for gestational age. Additional publications were identified by reviewing references in selected articles. No formal attempt was made to identify unpublished studies. 2.2. Review criteria We submitted the selected articles to the following criteria: 1. the use of an apo(a) independent method to measure Lp(a) concentrations; 2. apo(a) isoform determination, especially in case control studies with small sample size; 3. sufficient sample size of at least 50 individuals in the study groups for case control studies; 4. longitudinal study design when changes of Lp(a) during pregnancy are evaluated; 5. information about and equal distribution of ethnicity in study groups; 6. statistical analysis with non-parametric tests or with logtransformed Lp(a) data.
2. Study design 3. Results 2.1. Search methods 3.1. Search results MEDLINE, from January 1966 to May 2003, was searched to locate relevant articles in the English language. The medical subject headings used were: lipoprotein (a), pregnancy, complicated pregnancy, preeclampsia, eclampsia, pregnancy-induced hypertension, hypertensive disorders
The search yielded 32 articles on Lp(a) in normal or complicated pregnancy. Eleven studies reported on Lp(a) in normal pregnancy [16,18–27] and thirteen studies discussed the results of studies on Lp(a) in pregnancy complicated by
Table 2 A review of studies on Lp(a) in pregnancies complicated by preeclampsia or IUGR Authors
Year of Study design publication
Study population
Method used for Lp (a) measurement
Apo (a) isoform typing
Ethnicity Study results of study population
Meekins et al. (1994) [28] Case control
18 placentas of normal pregnancies, 24 placentas of PE pregnancies
Immunohistochemical technique
NA
Unknown
Uslu et al.
(1996) [29] Case control
Isoform dependent ELISA
No
Unknown
Djurovic et al.
(1997) [30] Case control
Quantitative electroimmunoassay and radioimmunoassay
No
Unknown
#Lp(a) in severe PE, #Lp(a) in mild PE, #Lp(a) in PE + IUGR vs controls
Leerink et al.
(1997) [31] Case control
Isoform independent ELISA
Yes
Mixed
Wang et al.
(1998) [32] Case control
23 women with PIH, 20 normal pregnant controls 31 women with severe PE, 105 women with mild PE, 18 women with PE + IUGR, 76 normal pregnant controls 39 women with history of PE, 47 controls, with history of uncomplicated pregnancy 8 women with severe PE, 18 women with mild PE, 24 normal pregnant controls
Isoform dependent ELISA
No
Unknown
=Lp(a) in history of PE vs controls =Apo(a) isoform distribution in history of PE vs controls ""Lp(a) in severe PE, "Lp(a) in mild PE vs controls
"Lp(a) and "atherosclerosis in spiral arteries of PE pregnancies vs normal pregnancies "Lp(a) in PIH vs controls
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Table 2 (Continued ) Authors
Year of Study design publication
Study population
Method used for Lp (a) measurement
Apo (a) isoform typing
Ethnicity Study results of study population
van Pampus et al.
(1999) [33] Case control
Isoform dependent ELISA
No
Mixed
"Lp(a) in severe PE vs PE + HELLP and controls
Nagy et al.
(1999) [34] Case control
ELISA
Yes
Unknown
=Lp(a) in PE vs controls, = Apo(a) isoform distribution in PE vs controls
Nagy et al.
(1999) [34] Longitudinal
40 women with history of severe PE, 35 women with history of PE + HELLP, 67 controls with history of uncomplicated pregnancy 59 women with PE, 51 normal pregnant controls, 59 non-pregnant controls 10 women with PE
ELISA
Yes
Unknown
Paidas et al.
(1999) [35] Case control 14 asymptomatic women and longitudinal who developed PE, 19 normal pregnant controls (2000) [41] Case control 10 women with PE, 10 normal pregnant controls (2002) [36] Case control 16 women with PE, 35 normal pregnant controls 18 non-pregnant controls (2002) [36] Nested case 82 women at risk for PE control at 14–24 wks GA of whom 9 developed PE, 19 developed other complications, 54 controls, without complications (2002) [37] Case control 75 pregnant women with obstetrical complications, 66 normal pregnant controls
Sensitive immunoassay
No
Mixed (matched)
Latex agglutination No ELISA Immuno-turbidimetry No
Unknown
"Lp(a) during preeclamptic pregnancy =Lp(a) in PE vs controls prior to onset of disease or at any gestational age during PE =Lp(a) in PE vs controls
Aksoy et al.
(2002) [38] Case control
Belo et al.
(2002) [42] Case control
Mori et al.
(2003) [39] Case control
Var et al.
(2003) [40] Case control
Sattar et al. Bar et al.
Bar et al.
Ogunyemi et al.
13 women with severe PE, 15 women with mild PE, 15 normal pregnant and 20 non-pregnant controls 51 women with PE, 67 normal pregnant controls 68 women with placental insufficienty, of whom 24 developed PE, 75 normal pregnant controls 20 women with PE, 20 normal pregnant controls
Mixed (matched)
"Lp(a) in PE vs controls
Immuno-turbidimetry No
Mixed (matched)
ELISA
No
Mixed
Nephelometry
No
Unknown
=Lp(a) in developing PE vs controls, "Lp(a) in developing other complications (IUGR, preterm delivery, fetal loss) vs controls "Lp(a) in complicated pregnancies (oligohydramnios, IUGR, PE, recurrent abortion, abruption, fetal demise) vs controls "Lp(a) in PE vs controls
Immuno-turbidimetry No
Unknown
=Lp(a) in PE vs controls
ELISA
No
Unknown
""Lp(a) in severe PE, "Lp(a) in mild PE vs controls
Immuno-turbidimetry No
Unknown
=Lp(a) in PE vs controls
NA: not applicable; PE: preeclampsia; vs: versus; PIH: pregnancy-induced hypertension; ELISA: enzyme-linked immunosorbent assay; IUGR: intrauterine growth restriction; wks: weeks; GA: gestational age.
preeclampsia [28–40]. Two studies reported on both subjects [41,42]. In Table 1 the studies on Lp(a) in normal pregnancy are summarized and Table 2 gives a review of the studies on Lp(a) in pregnancy complicated by preeclampsia. The results of one of the studies [33] summarized in Table 2 had been published earlier as a letter to the editor [43]. Studies addressing both normal and complicated pregnancy or containing two different types of study design are mentioned twice [20,21,34,36,41] or three times [42] in Tables 1 and 2. In addition there were two case reports. One reported on two sisters with severe preeclampsia/eclampsia in whom very
high concentrations of Lp(a) were found [6], and the other reported on a woman with very high Lp(a) concentrations who gave birth to three children with very low birth weight [7]. Three of the 32 articles were excluded from this review, since the patients suffered from medical disorders associated with abnormal concentrations of Lp(a) and lipid metabolism. One of these was a case report on a woman with high Lp(a) concentrations and a myocardial infarction during pregnancy [44], and another one a case report on a woman with homozygous familial hypercholesterolemia in whom Lp(a) concentrations were measured during preg-
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nancy [45]. We also excluded a longitudinal study on Lp(a) concentrations during pregnancy in 15 women with diabetes [46]. Our review, therefore, comprises 29 articles about Lp(a) in normal pregnancy and pregnancy complicated by preeclampsia or IUGR. 3.2. Summary of the results of the included studies 3.2.1. Lp(a) in normal pregnancy In most studies Lp(a) was elevated during normal pregnancy [18,19,21,24–26,41,42]. In five studies Lp(a) increased with advancing gestational age until the maximum was reached in the third trimester [19,21,26,41,42] and after delivery the Lp(a) concentration decreased till baseline concentrations were reached [19]. The authors suggested that the increase in Lp(a), in parallel with other lipoproteins, was influenced by the progressive hormonal changes of pregnancy [19,21,26,41]. One study described that Lp(a) increased during pregnancy until 19 weeks of gestational age and thereafter decreased to baseline values, that remained at the same level after delivery [18]. Another study showed that Lp(a) concentrations increased until 16 weeks of gestational age, where after Lp(a) concentrations remained stable until at least 1 week after delivery [25]. It was postulated by these authors that there is an independent metabolic control of Lp(a) that is not influenced by pregnancy related hormonal changes [18,25]. The increase in Lp(a) concentration during pregnancy might also be explained by a pregnancy related but aspecific stimulation of the liver biosynthetic activity [25]. Lp(a) is involved in extrahepatic cholesterol transport and Lp(a) may play a role in supplying the substrate for increased cell turnover, steroid hormone synthesis and an increased metabolic need for cholesterol in pregnancy [18,21,25]. It is also known that Lp(a) influences fibrinolysis in pregnancy and therefore, may play a role in normal placental development and delivery [41]. Five other studies showed no change in Lp(a) with advancing gestation [20,22,23,27,42]. The authors of these studies stated that there is a separate metabolic control for Lp(a) as compared to other lipids during pregnancy [20,22,23,27]. The wide range of values for Lp(a) could explain these results too [23,42]. One study described a decrease of Lp(a) concentrations during pregnancy among smoking women and the authors suggested that increasing Lp(a) concentrations during pregnancy might be a physiological necessity, and decreasing Lp(a) concentrations might be unfavourable for the normal development of the rapidly growing fetus [21]. 3.2.2. Lp(a) in pregnancy complicated by preeclampsia In six studies there were higher Lp(a) concentrations in women with preeclampsia or pregnancy-induced hypertension as compared to normal pregnant controls [29,32,36–39]. In two studies Lp(a) concentrations were even associated with the severity of the disease [32,39]. In one study placentas from preeclamptic pregnancies showed more atherosclerotic
lesions in the spiral arteries, containing more Lp(a) as compared to placentas from uncomplicated pregnancies [28]. According to the authors of the forementioned studies there is an abnormal placentation in preeclampsia with, as in atherosclerosis, fibrin and fat deposition in the walls of the spiral arteries, causing placental infarction and hypoperfusion. According to the currently most accepted hypothesis, the impaired placental circulation is responsible for the release of blood-born materials in the maternal circulation, causing endothelial dysfunction, which is the pathophysiological key feature in preeclampsia [5]. Lp(a) is able to bind to plasminogen activator receptors in the endothelium, thus inhibiting plasminogen activation and fibrinolysis, and promoting fibrin deposition in the placenta vessels of preeclamptic patients [29,32,39]. Furthermore, Lp(a), a carrier of cholesterol, is known to deliver large amounts of cholesterol to cells to promote regeneration after endothelial damage. Therefore, high Lp(a) concentrations seem to be associated with endothelial dysfunction [32,36,38]. The results of these studies suggest that Lp(a) is associated with preeclampsia and may even play a role in the pathogenesis of the disorder [28,29,32,36–39]. One study found lower Lp(a) concentrations in women with preeclampsia as compared to normal pregnant controls [30]. The authors suggested that the decrease in Lp(a) concentrations in preeclampsia could be due to the ability of the molecule to accumulate in the arterial wall, and move from the blood into the arterial wall of the placenta resulting in a lower Lp(a) plasma concentration. Five studies found unaltered Lp(a) concentrations in women with preeclampsia as compared to normal pregnant women and the authors argued against a significant role for Lp(a) in preeclampsia [34,35,40–42]. This is supported by the finding in two recent studies that women destined to develop preeclampsia did not have elevated Lp(a) concentrations before the appearance of the disorder [35,36]. Two studies report on the Lp(a) concentrations in women with a history of preeclampsia as compared to women with a history of uncomplicated pregnancy. van Pampus et al. [33] found higher Lp(a) concentrations in women with a history of severe preeclampsia while Leerink et al. [31] found no difference at all in Lp(a) concentration between women with a history of preeclampsia or uncomplicated pregnancy. Differences in selection or ethnicity of the study groups may account for this. 3.2.3. Lp(a) in pregnancy complicated by IUGR Only two studies report on Lp(a) in IUGR [36,37]. Both studies found significantly higher Lp(a) concentrations in pregnant women with obstetric complications (as IUGR, preterm delivery, fetal or neonatal loss, a history of recurrent abortion, placental abruption and unexplained oligohydramnios) as compared to pregnant women without obstetric complications. In both studies about 30% of the women with obstetrical complications had IUGR. Unfortunately no subanalyses were done to compare Lp(a) in pregnancies
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complicated by IUGR and uncomplicated pregnancies. On basis of these results and a case report of a woman with high Lp(a) concentrations who gave birth to three children with very low birth weights, one may speculate that Lp(a) interferes with placental circulation and thus high concentrations might cause fetal growth restriction [7]. 3.3. Evaluation of the studies according to the review criteria
19
3.3.5. Information about and equal distribution of ethnicity in study groups For most of the studies the ethnicity of the study population was unknown [18–21,23–26,28–30,32,34,38– 42]. In two of the studies matching was applied for race [35,36]. The influence of ethnicity on study results is demonstrated by the study of Koukkou et al. [16] who showed significantly higher Lp(a) concentrations in African/ Afro-Caribbean pregnant women as compared to Caucasian pregnant women.
None of the studies fulfilled all of the six criteria. 3.3.1. The use of an apo(a) independent method to measure Lp(a) concentrations Most of the studies used an apo(a) isoform dependent method to measure Lp(a) concentrations [16,20,21,23,27, 29,30,32,33,36,38,40,42]. In other studies it was not mentioned whether the assay used was apo(a) independent or not [19,22,24–26,34,35,37,39,41]. The Laurell technique used by Zechner et al. [18] is a rather insensitive assay, limiting measurement of Lp(a) concentrations to the high range, not representative of those in the general population. For only one study the use of an apo(a) independent method to measure Lp(a) was reported [31]. 3.3.2. Apo(a) isoform determination Three studies performed an apo(a) isoform determination [26,31,34]. The influence of apo(a) isoform size on study results is demonstrated by the study of Brizzi et al. [26] who showed that only in women with large apo(a) isoforms there was a significant increase in Lp(a) concentration during pregnancy. In women with small apo(a) isoforms there was no change in Lp(a) concentration with advancing gestation. 3.3.3. Sufficient sample size for case control studies Of the 14 case control studies on Lp(a) in women with preeclampsia or women with a history of preeclampsia [29– 42], only five studies [30,34,37,39,42] included sufficient individuals in both the cases and the control groups. 3.3.4. Longitudinal study design when changes of Lp(a) during pregnancy are evaluated Six of the 12 studies on Lp(a) in the course of normal pregnancy had a longitudinal study design to describe changes of Lp(a) concentrations during pregnancy and after delivery [18,22,23,26,27,41]. In two of these there were only two sample time points; in the third trimester of pregnancy and after delivery [22,27]. Five studies had a cross-sectional study design [19–21,24,25]. One study used both designs and showed the influence of the choice of cross-sectional or longitudinal design on study results [42]. This study demonstrated that there was a significant increase in Lp(a) concentration during pregnancy in a longitudinal analysis, but no significant change in Lp(a) concentration during pregnancy in a cross-sectional design.
3.3.6. Statistical analysis with non-parametric tests or with log-transformed Lp(a) data Of all 26 studies, eight did not use the appropriate statistical method to analyse the data [18,22,23,26,33– 35,40]. Parametric tests were used without log-transformation of the Lp(a) data in these eight studies. From the studies measuring Lp(a) during normal pregnancy, four studies met three of the criteria [22,26,27,41], although for three of these studies [22,26,41] it was unclear whether the assay used to measure Lp(a) was apo(a) independent or not. For the other nine studies on Lp(a) in normal pregnancy fewer than three criteria were fulfilled. Two studies had only two sample time points; in the third trimester and 6–12 weeks post partum [22,27]. The other two studies described Lp(a) changes during pregnancy, but no data were available on Lp(a) post partum [26,41]. The latter two studies on Lp(a) during pregnancy suggested that Lp(a) increases during normal pregnancy [26,41], whereas the two studies measuring Lp(a) in the third trimester and post partum did not show a difference between the pregnant and the non-pregnant state [22,27]. Thus, the course of Lp(a) during and after pregnancy remains unclear. Therefore, a prospective longitudinal study on Lp(a) concentrations during pregnancy is needed. Such a study should include healthy women with a normal pregnancy course and outcome from distinct ethnicity. Lp(a) should be determined preconceptionally or, if this is not feasible, from early pregnancy onwards until 3–6 months after delivery. An apo(a) independent method should be used to measure Lp(a) concentrations and apo(a) isoform determination should be performed to estimate the influence of apo(a) isoform size on the study results. From the studies comparing Lp(a) concentrations between women with (a history of) preeclampsia and controls, only two studies were in agreement with four of the criteria [31,37], but from one study it was unclear whether the assay used to measure Lp(a) was apo(a) independent or not [37]. For the rest of the studies fewer than four criteria were fulfilled. The study by Leerink et al. [31] found no difference in Lp(a) concentrations between women with a history of preeclampsia and women with a history of uneventful pregnancy. The apo(a) isoform distribution was not different between the two study groups. The study population was predominantly Negroid,
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which questions its validity for Caucasian populations. Furthermore, this study provides no information on Lp(a) concentrations during manifest preeclampsia. The study by Ogunyemi et al. [37] found that a higher percentage of women with obstetrical complications, such as preeclampsia, had Lp(a) concentrations over 300 mg/L, as compared to healthy pregnant women. No information was given on the absolute values of Lp(a) and no separate comparison was made between the subgroup of women with preeclampsia and healthy pregnant controls. To summarize, the relationship between Lp(a) concentrations and preeclampsia remains unclear. Therefore, a prospective case control study is needed. In such a study a group of women with preeclampsia according to strict criteria should be compared with healthy controls with an uneventful pregnancy, with no ethnic differences between the two groups. Both study groups should contain at least 50 individuals, or apo(a) isoform determination should be performed. An apo(a) independent method should be used to measure Lp(a) concentrations. There are only three articles on Lp(a) and IUGR [7,36,37]. Two of these are case control studies [36,37] and the third paper is a case report [7]. The two case control studies addressed the relationship of Lp(a) with a range of pregnancy complications, including IUGR. In both studies the Lp(a) concentration was higher in women with complicated pregnancy as compared to women with uncomplicated pregnancy. No subanalysis for IUGR only was done. Neither of these studies used an apo(a) isoform independent method to measure Lp(a). More prospective data are needed to decide whether there is a difference in Lp(a) concentration between women with pregnancy complicated by IUGR and women with uncomplicated pregnancies. For such studies we propose a similar design as for studies on the relationship of Lp(a) with preeclampsia.
4. Conclusions and recommendations The literature up to now is divided on whether Lp(a) increases during normal pregnancy and on the possible association of Lp(a) with preeclampsia and IUGR. Studies performed on Lp(a) in normal and complicated pregnancy, thus far yielded very diverse results. These differences are likely to be due to differences in methods to measure Lp(a) concentrations, differences in sample size, differences in study design and differences between and heterogeneity of (ethnicity of) study populations. Therefore, comparison of the results of the different studies is hazardous, if justifiable at all. The changes of Lp(a) in normal and complicated pregnancy are not clarified. Recommendations for future studies include: (1) the use of an apo(a) independent method for measuring Lp(a); (2) consideration of apo(a) isoforms to avoid misinterpretation of results, which are caused by random deviation, especially
in case control studies; (3) inclusion of sufficient numbers (at least 50) of patients and controls in case control studies; (4) the use of a longitudinal study design when the objective is to study the changes of Lp(a) during pregnancy; and (5) selection of a study population that is ethnically representative for the general population.
Acknowledgements We thank Dr. M.L. Bots of the Julius Center for Health Sciences and Primary Care, for his critical comments and his help with statistical issues.
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[33] van Pampus MG, Koopman MMW, Wolf H, Bu¨ller HR, Prins MH, van den Ende A. Lipoprotein (a) concentrations in women with a history of severe preeclampsia; a case control study. Thromb Haemost 1999;82: 10–3. [34] Nagy B, Rigo´ Jr J, Fintor L, Romics L, Papp Z, Kara´di I. Distribution of apolipoprotein (a) isoforms in normotensive and severe preeclamptic women. J Matern Fetal Med 1999;8:270–4. [35] Paidas MJ, Kuczynski E, Salafia C, Lockwood CJ. Plasma levels of lipoprotein (a) [Lp(a)] are not elevated in women destined to develop preeclampsia (PE). J Soc Gynecol Invest 1999;6:124. [36] Bar J, Harell D, Bardin R, et al. The elevated plasma lipoprotein (a) concentrations in preeclampsia do not precede the development of the disorder. Thromb Res 2002;105:19–23. [37] Ogunyemi D, Ku W, Arkel Y. The association between inherited thrombophilia, antiphospholipid antibodies and lipoprotein A levels with obstetrical complications in pregnancy. J Thromb Thrombolysis 2002;14:157–62. [38] Aksoy H, Kumtepe Y, Akc¸ay F, Yildirim AK. Correlation of P-selectin and lipoprotein (a), and other lipid parameters in preeclampsia. Clin Exp Med 2002;2:39–43. [39] Mori M, Mori A, Saburi Y, Sida M, Ohta H. Levels of lipoprotein (a) in normal and compromised pregnancy. J Perinat Med 2003;31:23– 8. [40] Var A, Kus¸cu NK, Koyuncu F, et al. Atherogenic profile in preeclampsia. Arch Gynecol Obstet 2003;268:45–7. [41] Sattar N, Clark P, Greer IA, Shepherd J, Packard CJ. Lipoprotein (a) levels in normal pregnancy and in pregnancy complicated with preeclampsia. Atherosclerosis 2000;148:407–11. [42] Belo L, Caslake M, Santos-Silva A, Pereira-Leite L, Quintanilha A, Rebelo I. Lipoprotein (a): a longitudinal versus a cross-sectional study in normal pregnancy and its levels in preeclampsia. Atherosclerosis 2002;165:393–5. [43] van Pampus MG, van den Ende A, Wolf H. Elevated levels of lipoprotein (a) in women with preeclampsia. Am J Obstet Gynecol 1998;179:1103–4. [44] Ulm MR, Obwegeser R, Ploeckinger B, Nowotny C, Pidlich J, Sinzinger H. A case of myocardial infarction complicating pregnancy—a role for prostacyclin synthesis stimulating plasma factor and lipoprotein (a)? Thromb Res 1996;83:237–42. [45] Kroon AA, Swinkels DW, van Dongen PWJ, Stalenhoef AFH. Pregnancy in a patient with homozygous familial hypercholesterolemia treated with long-term low-density lipoprotein apheresis. Metabolism 1994;43:1164–70. [46] Pekelharing HLM, Aalders NL, Visser GHA, et al. Method-dependent increase in lipoprotein (a) in insulin-dependent diabetes mellitus during pregnancy. Metabolism 1995;44:1606–11.
Review
Lipoprotein (a) in pregnancy: A critical review of the literature Gwendolyn T.R. Manten a,*, Hieronymus A.M. Voorbij b, Ton M. Hameeteman c, Gerard H.A. Visser a, Arie Franx a a
Department of Perinatology and Gynecology, University Medical Center, P.O. Box 85090, KE.04.156.1, 3508 AB Utrecht, The Netherlands b Department of Clinical Chemistry, University Medical Center, P.O. Box 85090, 3508 AB Utrecht, The Netherlands c Department of Obstetrics and Gynecology, St. Antonius Hospital, P.O. Box 2500, 3430 EM Nieuwegein, The Netherlands Received 15 March 2004; received in revised form 24 November 2004; accepted 22 March 2005
Abstract In this article the literature on lipoprotein (a) during normal pregnancy and pregnancy complicated by preeclampsia or intrauterine growth restriction is reviewed. MEDLINE, from January 1966 to May 2003, was searched to locate relevant articles in English. Additional publications were identified by reviewing references in selected articles. Studies were reviewed by predefined and strict criteria. It appeared that methodology and results of studies on lipoprotein (a) during normal and complicated pregnancy were very diverse. Lipoprotein (a) increased with advancing gestation or remained unaltered during normal pregnancy. Women with preeclampsia had higher, unaltered or lower lipoprotein (a) concentrations as compared to normal pregnant controls. Only few studies were in agreement with most of the review criteria. In conclusion, published studies on lipoprotein (a) in pregnancy differ substantially in the used methods to measure lipoprotein (a), sample size, study design and ethnicity of the study population. Therefore, these studies yielded conflicting results and no unequivocal view on the role of lipoprotein (a) in normal and complicated pregnancy. Recommendations for future studies are amongst others: the use of an apo(a) independent method for measuring Lp(a), inclusion of sufficient numbers of patients, the use of a longitudinal study design when the objective is to study the changes of Lp(a) during pregnancy and selection of a study population that is ethnically representative for the general population. # 2005 Elsevier Ireland Ltd. All rights reserved. Keywords: Lipoprotein (a); Normal pregnancy; Preeclampsia; Intrauterine growth restriction; Review
Contents 1.
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3.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1. Lipoprotein (a) and pregnancy . . . . . . . . . . . . . . . . . 1.2. Lipoprotein (a) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3. Important factors influencing Lp(a) concentration. . . . 1.3.1. Lp(a) measurement . . . . . . . . . . . . . . . . . . . 1.3.2. Apo(a) isoform typing, sample size and study 1.3.3. Ethnicity . . . . . . . . . . . . . . . . . . . . . . . . . . Study design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Search methods . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Review criteria. . . . . . . . . . . . . . . . . . . . . . . . . . . . Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Search results. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Summary of the results of the included studies . . . . .
..... ..... ..... ..... ..... design ..... ..... ..... ..... ..... ..... .....
* Corresponding author. Tel.: +31 302504000; fax: +31 302505320. E-mail address: [email protected] (Gwendolyn T.R. Manten). 0301-2115/$ – see front matter # 2005 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.ejogrb.2005.03.013
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3.2.1. Lp(a) in normal pregnancy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.2. Lp(a) in pregnancy complicated by preeclampsia . . . . . . . . . . . . . . . . . . . . . . . . 3.2.3. Lp(a) in pregnancy complicated by IUGR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Evaluation of the studies according to the review criteria . . . . . . . . . . . . . . . . . . . . . . . . 3.3.1. The use of an apo(a) independent method to measure Lp(a) concentrations. . . . . . 3.3.2. Apo(a) isoform determination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.3. Sufficient sample size for case control studies . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.4. Longitudinal study design when changes of Lp(a) during pregnancy are evaluated . 3.3.5. Information about and equal distribution of ethnicity in study groups . . . . . . . . . . 3.3.6. Statistical analysis with non-parametric tests or with log-transformed Lp(a) data . . Conclusions and recommendations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1. Introduction 1.1. Lipoprotein (a) and pregnancy Pregnancy is a state of hyperlipidemia [1] and hypofibrinolysis [2]. Lipoprotein (a) (Lp(a)), a carrier of cholesterol, plays an important role in lipid metabolism and is known to hamper fibrinolysis [3,4]. Preeclampsia, a disorder characterized by hypertension and proteinuria after the 20th week of gestation, is associated with fibrin deposition, thrombosis and infarction in the placental vascular bed very similar to the lesions observed in atherosclerosis [5]. Lp(a) has been identified as an inherited risk factor for a variety of vascular diseases, including atherosclerosis [4]. Two case reports, one on high Lp(a) concentrations in a single family with two cases of severe preeclampsia [6], and one on a women with high Lp(a) concentrations giving birth to three children with very low birth weights [7], suggested that Lp(a) may interfere with placental circulation and may be involved in the pathogenesis of preeclampsia. Not surprisingly therefore, Lp(a) has been extensively studied in normal pregnancy and pregnancy complicated by preeclampsia during the last 15 years. The results of these studies are, however, diverse and conflicting. 1.2. Lipoprotein (a) Plasma concentrations of Lp(a) in humans range from less than 10 mg/L to greater than 1000 mg/L, and increased risk for cardiovascular disease is associated with concentrations above 300 mg/L. Lp(a) concentration is almost entirely genetically determined, and is relatively resistant to most dietary and drug interventions [3,4,8]. Lp(a) consists of a low-density lipoprotein (LDL) particle with one additional distinguishing protein known as apolipoprotein (a) (apo(a)). Apo(a) is bound to the apolipoprotein B-100 of the LDL particle by disulphide linkage [3,4,8]. The apo(a) gene is located on the long arm of chromosome 6 and cDNA sequencing of apo(a) has revealed a high degree of homology with plasminogen, a precursor protein of the fibrinolytic enzyme plasmine [9]. Competitive binding of Lp(a) to endothelial plasminogen receptors reduces the physiological fibrinolytic activity of plasmino-
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gen [10,11]. This may be explanatory of the association of elevated concentrations of Lp(a) and atherosclerotic cardiovascular disease [4]. Apo(a) contains a kringle 4 domain which consist of 10 different types of proteases of which type 2 is present in multiple copies. The number of type 2 copies varies and is responsible for the size heterogeneity of the apo(a) phenotype, or so-called isoform, and consequently of Lp(a). The plasma concentration of Lp(a) is inversely related to the size of its apo(a) isoforms [12]. The Lp(a) concentration varies in different ethnic groups. The distribution in Caucasians is highly skewed to the right with many low values and a median Lp(a) concentration of 140 mg/L. Therefore, statistical analysis should be performed with non-parametric tests or with log-transformed data. A significantly different, more bell-shaped Lp(a) distribution with mean concentrations twice as high as those of whites has been reported in populations of American black men and women [13]. 1.3. Important factors influencing Lp(a) concentration 1.3.1. Lp(a) measurement Lp(a) measurement is difficult, because Lp(a) assays are not standardized. Results differ substantially between laboratories and can hardly be compared. The reason for this is the use of different assay methodologies (Laurell technique, enzyme-linked immunosorbent assay (ELISA), immunoturbidimetry, quantitative electroimmunoassay, radioimmunoassay or nephelometry) and the use of different antibodies and standards. Some of these antibodies may recognize the different apo(a) isoforms not independent of their size. This results in over- or underestimation of the Lp(a) concentration, since the size of the apo(a) isoform is inversely related to the Lp(a) concentration. It is therefore, recommendable to use an apo(a) independent method to measure Lp(a) concentrations [12–14]. 1.3.2. Apo(a) isoform typing, sample size and study design The distribution of Lp(a) concentrations in Caucasian populations shifts to low concentrations and is extremely
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wide [3]. Therefore, results of studies measuring Lp(a) concentrations are very susceptible to random deviations. Consequently, case control studies with a low number of cases and/or controls must be considered with caution, in particular when true differences between two groups are small. One way to improve this is to stratify groups for apo(a) isoforms. Especially in small groups it is essential
15
that the apo(a) isoforms are determined to identify possible random deviations of small or large isoforms. Another way is to study populations of sufficient size, with group sizes of 50 individuals or more [15]. Apo(a) isoform typing and sample size are less important when Lp(a) is measured in a longitudinal study design. When subjects are studied longitudinally the changes of Lp(a) concentrations, within
Table 1 A review of studies on Lp(a) in normal pregnancy Authors
Year of publication
Study design
Zechner et al.
(1986) [18] Longitudinal
Panteghini et al. (1991) [19] Cross-sectional
Study population
Method used Apo(a) for Lp(a) isoform measurement typing
Ethnicity of study population
Study results
22 women from first trimester, until delivery and 6–8 wks pp
Laurell technique
No
Unknown
193 pregnant women 39 women 1 week pp 178 pregnant women in second and third trimester 58 non-pregnant controls 178 pregnant women in second and third trimester 68 non-smoking and 118 smoking pregnant women 29 non-pregnant, non-smoking controls 68 non-smoking and 118 smoking pregnant women, from 6–40 wks GA 114 Caucasian and 118 African/Afro-Caribbean women in third trimester of pregnancy 36 women at 35–36 wks GA and at 6 wks pp 62 women from first trimester until delivery and 6–12 wks pp 13 women at term, 25 non-pregnant controls 140 pregnant women, 25 women 1 week pp, 25 non-pregnant controls 22 women from 12–39 wks GA
ELISA
No
Unknown
Immunoturbidimetry
No
Unknown
"Lp(a) during pregnancy until 19 wks GA, thereafter #Lp(a) till baseline values at delivery, and = Lp(a) pp "Lp(a) during pregnancy and #pp =Lp(a) in pregnancy vs controls
Immunoturbidimetry Isoform dependent ELISA
No
Unknown
No
Unknown
Isoform dependent ELISA Immunoturbidimetry
No
ELISA
No
Isoform dependent ELISA ELISA
No
#Lp(a) during smoking pregnancy, "Lp(a) during non-smoking pregnancy Caucasian "Lp(a) in African/AfroAfrican/Afro- Caribbean pregnant women Caribbean vs Caucasian pregnant women Hispanic =Lp(a) in third trimester vs pp Unknown =Lp(a) during pregnancy and pp
No
Unknown
ELISA
No
Unknown
ELISA
Yes
Unknown
Latex a gglutination ELISA Immunoturbidimetry
No
Unknown
No
Caucasian
=Lp(a) in pregnancy vs pp = Lp(a) in pregnancy vs controls
Immunoturbidimetry Immunoturbidimetry
No
Unknown
"Lp(a) during pregnancy
No
Unknown
=Lp(a) during pregnancy
Mazurkiewicz et al.
(1994) [20] Case control
Mazurkiewicz et al. Wersch et al.
(1994) [20] Cross-sectional (1994) [21] Case control
Wersch et al.
(1994) [21] Cross-sectional
Koukkou et al.
(1994) [16] Case control
Silliman et al.
(1994) [22] Longitudinal
Chiang et al.
(1995) [23] Longitudinal
Neary et al.
(1995) [24] Case control
Baroni et al.
(1996) [25] Cross-sectional
Brizzi et al.
(1999) [26] Longitudinal
Sattar et al.
(2000) [41] Longitudinal
Rymer et al.
(2002) [27] Case control 29 women at term of and longitudinal which 22 also12 wks pp, 27 non-pregnant controls (2002) [42] Longitudinal 23 women from first until third trimester (2002) [42] Cross-sectional 64 women first trimester, 48 women second trimester, 67 women third trimester
Belo et al. Belo et al.
10 women from 10–35 wks GA
No
=Lp(a) during second half of pregnancy "Lp(a) in non-smoking vs smoking pregnant and non-pregnant women
Unknown
wks: weeks; pp: post partum; GA: gestational age; ELISA: enzyme-linked immunosorbent assay; vs: versus.
"Lp(a) in pregnancy vs controls "Lp(a) during pregnancy until 16 wks GA, thereafter = Lp(a) until 1 week pp "Lp(a) during pregnancy in women with large apo(a) isoforms, = Lp(a) during pregnancy in women with small apo(a) isoforms "Lp(a) during pregnancy
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subjects, will not be influenced by the large interindividual variation in Lp(a) concentration. Thus, a longitudinal study design is preferred, to study changes of Lp(a) in the course of pregnancy. 1.3.3. Ethnicity Racial differences for Lp(a) concentrations have been documented and in pregnancy these differences seem to persist [16]. These differences are not solely caused by differences in apo(a) isoform distribution [17]. When Lp(a) concentrations between groups are compared, ethnicity should be equally distributed in both groups. When changes of Lp(a) concentrations during pregnancy are studied, ethnicity is less important, since it will only influence the absolute height but not within-subject changes of the Lp(a) concentration. This paper is a critical review of the literature on Lp(a) in normal pregnancy and pregnancy complicated by preeclamspia and intrauterine growth restriction (IUGR). Our aim was to elucidate why conflicting results have emerged from the literature, and to give recommendations for future studies on Lp(a) in normal and complicated pregnancy.
in pregnancy, toxemia or toxaemia in pregnancy, intrauterine growth retardation, intrauterine growth restriction and small for gestational age. Additional publications were identified by reviewing references in selected articles. No formal attempt was made to identify unpublished studies. 2.2. Review criteria We submitted the selected articles to the following criteria: 1. the use of an apo(a) independent method to measure Lp(a) concentrations; 2. apo(a) isoform determination, especially in case control studies with small sample size; 3. sufficient sample size of at least 50 individuals in the study groups for case control studies; 4. longitudinal study design when changes of Lp(a) during pregnancy are evaluated; 5. information about and equal distribution of ethnicity in study groups; 6. statistical analysis with non-parametric tests or with logtransformed Lp(a) data.
2. Study design 3. Results 2.1. Search methods 3.1. Search results MEDLINE, from January 1966 to May 2003, was searched to locate relevant articles in the English language. The medical subject headings used were: lipoprotein (a), pregnancy, complicated pregnancy, preeclampsia, eclampsia, pregnancy-induced hypertension, hypertensive disorders
The search yielded 32 articles on Lp(a) in normal or complicated pregnancy. Eleven studies reported on Lp(a) in normal pregnancy [16,18–27] and thirteen studies discussed the results of studies on Lp(a) in pregnancy complicated by
Table 2 A review of studies on Lp(a) in pregnancies complicated by preeclampsia or IUGR Authors
Year of Study design publication
Study population
Method used for Lp (a) measurement
Apo (a) isoform typing
Ethnicity Study results of study population
Meekins et al. (1994) [28] Case control
18 placentas of normal pregnancies, 24 placentas of PE pregnancies
Immunohistochemical technique
NA
Unknown
Uslu et al.
(1996) [29] Case control
Isoform dependent ELISA
No
Unknown
Djurovic et al.
(1997) [30] Case control
Quantitative electroimmunoassay and radioimmunoassay
No
Unknown
#Lp(a) in severe PE, #Lp(a) in mild PE, #Lp(a) in PE + IUGR vs controls
Leerink et al.
(1997) [31] Case control
Isoform independent ELISA
Yes
Mixed
Wang et al.
(1998) [32] Case control
23 women with PIH, 20 normal pregnant controls 31 women with severe PE, 105 women with mild PE, 18 women with PE + IUGR, 76 normal pregnant controls 39 women with history of PE, 47 controls, with history of uncomplicated pregnancy 8 women with severe PE, 18 women with mild PE, 24 normal pregnant controls
Isoform dependent ELISA
No
Unknown
=Lp(a) in history of PE vs controls =Apo(a) isoform distribution in history of PE vs controls ""Lp(a) in severe PE, "Lp(a) in mild PE vs controls
"Lp(a) and "atherosclerosis in spiral arteries of PE pregnancies vs normal pregnancies "Lp(a) in PIH vs controls
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Table 2 (Continued ) Authors
Year of Study design publication
Study population
Method used for Lp (a) measurement
Apo (a) isoform typing
Ethnicity Study results of study population
van Pampus et al.
(1999) [33] Case control
Isoform dependent ELISA
No
Mixed
"Lp(a) in severe PE vs PE + HELLP and controls
Nagy et al.
(1999) [34] Case control
ELISA
Yes
Unknown
=Lp(a) in PE vs controls, = Apo(a) isoform distribution in PE vs controls
Nagy et al.
(1999) [34] Longitudinal
40 women with history of severe PE, 35 women with history of PE + HELLP, 67 controls with history of uncomplicated pregnancy 59 women with PE, 51 normal pregnant controls, 59 non-pregnant controls 10 women with PE
ELISA
Yes
Unknown
Paidas et al.
(1999) [35] Case control 14 asymptomatic women and longitudinal who developed PE, 19 normal pregnant controls (2000) [41] Case control 10 women with PE, 10 normal pregnant controls (2002) [36] Case control 16 women with PE, 35 normal pregnant controls 18 non-pregnant controls (2002) [36] Nested case 82 women at risk for PE control at 14–24 wks GA of whom 9 developed PE, 19 developed other complications, 54 controls, without complications (2002) [37] Case control 75 pregnant women with obstetrical complications, 66 normal pregnant controls
Sensitive immunoassay
No
Mixed (matched)
Latex agglutination No ELISA Immuno-turbidimetry No
Unknown
"Lp(a) during preeclamptic pregnancy =Lp(a) in PE vs controls prior to onset of disease or at any gestational age during PE =Lp(a) in PE vs controls
Aksoy et al.
(2002) [38] Case control
Belo et al.
(2002) [42] Case control
Mori et al.
(2003) [39] Case control
Var et al.
(2003) [40] Case control
Sattar et al. Bar et al.
Bar et al.
Ogunyemi et al.
13 women with severe PE, 15 women with mild PE, 15 normal pregnant and 20 non-pregnant controls 51 women with PE, 67 normal pregnant controls 68 women with placental insufficienty, of whom 24 developed PE, 75 normal pregnant controls 20 women with PE, 20 normal pregnant controls
Mixed (matched)
"Lp(a) in PE vs controls
Immuno-turbidimetry No
Mixed (matched)
ELISA
No
Mixed
Nephelometry
No
Unknown
=Lp(a) in developing PE vs controls, "Lp(a) in developing other complications (IUGR, preterm delivery, fetal loss) vs controls "Lp(a) in complicated pregnancies (oligohydramnios, IUGR, PE, recurrent abortion, abruption, fetal demise) vs controls "Lp(a) in PE vs controls
Immuno-turbidimetry No
Unknown
=Lp(a) in PE vs controls
ELISA
No
Unknown
""Lp(a) in severe PE, "Lp(a) in mild PE vs controls
Immuno-turbidimetry No
Unknown
=Lp(a) in PE vs controls
NA: not applicable; PE: preeclampsia; vs: versus; PIH: pregnancy-induced hypertension; ELISA: enzyme-linked immunosorbent assay; IUGR: intrauterine growth restriction; wks: weeks; GA: gestational age.
preeclampsia [28–40]. Two studies reported on both subjects [41,42]. In Table 1 the studies on Lp(a) in normal pregnancy are summarized and Table 2 gives a review of the studies on Lp(a) in pregnancy complicated by preeclampsia. The results of one of the studies [33] summarized in Table 2 had been published earlier as a letter to the editor [43]. Studies addressing both normal and complicated pregnancy or containing two different types of study design are mentioned twice [20,21,34,36,41] or three times [42] in Tables 1 and 2. In addition there were two case reports. One reported on two sisters with severe preeclampsia/eclampsia in whom very
high concentrations of Lp(a) were found [6], and the other reported on a woman with very high Lp(a) concentrations who gave birth to three children with very low birth weight [7]. Three of the 32 articles were excluded from this review, since the patients suffered from medical disorders associated with abnormal concentrations of Lp(a) and lipid metabolism. One of these was a case report on a woman with high Lp(a) concentrations and a myocardial infarction during pregnancy [44], and another one a case report on a woman with homozygous familial hypercholesterolemia in whom Lp(a) concentrations were measured during preg-
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nancy [45]. We also excluded a longitudinal study on Lp(a) concentrations during pregnancy in 15 women with diabetes [46]. Our review, therefore, comprises 29 articles about Lp(a) in normal pregnancy and pregnancy complicated by preeclampsia or IUGR. 3.2. Summary of the results of the included studies 3.2.1. Lp(a) in normal pregnancy In most studies Lp(a) was elevated during normal pregnancy [18,19,21,24–26,41,42]. In five studies Lp(a) increased with advancing gestational age until the maximum was reached in the third trimester [19,21,26,41,42] and after delivery the Lp(a) concentration decreased till baseline concentrations were reached [19]. The authors suggested that the increase in Lp(a), in parallel with other lipoproteins, was influenced by the progressive hormonal changes of pregnancy [19,21,26,41]. One study described that Lp(a) increased during pregnancy until 19 weeks of gestational age and thereafter decreased to baseline values, that remained at the same level after delivery [18]. Another study showed that Lp(a) concentrations increased until 16 weeks of gestational age, where after Lp(a) concentrations remained stable until at least 1 week after delivery [25]. It was postulated by these authors that there is an independent metabolic control of Lp(a) that is not influenced by pregnancy related hormonal changes [18,25]. The increase in Lp(a) concentration during pregnancy might also be explained by a pregnancy related but aspecific stimulation of the liver biosynthetic activity [25]. Lp(a) is involved in extrahepatic cholesterol transport and Lp(a) may play a role in supplying the substrate for increased cell turnover, steroid hormone synthesis and an increased metabolic need for cholesterol in pregnancy [18,21,25]. It is also known that Lp(a) influences fibrinolysis in pregnancy and therefore, may play a role in normal placental development and delivery [41]. Five other studies showed no change in Lp(a) with advancing gestation [20,22,23,27,42]. The authors of these studies stated that there is a separate metabolic control for Lp(a) as compared to other lipids during pregnancy [20,22,23,27]. The wide range of values for Lp(a) could explain these results too [23,42]. One study described a decrease of Lp(a) concentrations during pregnancy among smoking women and the authors suggested that increasing Lp(a) concentrations during pregnancy might be a physiological necessity, and decreasing Lp(a) concentrations might be unfavourable for the normal development of the rapidly growing fetus [21]. 3.2.2. Lp(a) in pregnancy complicated by preeclampsia In six studies there were higher Lp(a) concentrations in women with preeclampsia or pregnancy-induced hypertension as compared to normal pregnant controls [29,32,36–39]. In two studies Lp(a) concentrations were even associated with the severity of the disease [32,39]. In one study placentas from preeclamptic pregnancies showed more atherosclerotic
lesions in the spiral arteries, containing more Lp(a) as compared to placentas from uncomplicated pregnancies [28]. According to the authors of the forementioned studies there is an abnormal placentation in preeclampsia with, as in atherosclerosis, fibrin and fat deposition in the walls of the spiral arteries, causing placental infarction and hypoperfusion. According to the currently most accepted hypothesis, the impaired placental circulation is responsible for the release of blood-born materials in the maternal circulation, causing endothelial dysfunction, which is the pathophysiological key feature in preeclampsia [5]. Lp(a) is able to bind to plasminogen activator receptors in the endothelium, thus inhibiting plasminogen activation and fibrinolysis, and promoting fibrin deposition in the placenta vessels of preeclamptic patients [29,32,39]. Furthermore, Lp(a), a carrier of cholesterol, is known to deliver large amounts of cholesterol to cells to promote regeneration after endothelial damage. Therefore, high Lp(a) concentrations seem to be associated with endothelial dysfunction [32,36,38]. The results of these studies suggest that Lp(a) is associated with preeclampsia and may even play a role in the pathogenesis of the disorder [28,29,32,36–39]. One study found lower Lp(a) concentrations in women with preeclampsia as compared to normal pregnant controls [30]. The authors suggested that the decrease in Lp(a) concentrations in preeclampsia could be due to the ability of the molecule to accumulate in the arterial wall, and move from the blood into the arterial wall of the placenta resulting in a lower Lp(a) plasma concentration. Five studies found unaltered Lp(a) concentrations in women with preeclampsia as compared to normal pregnant women and the authors argued against a significant role for Lp(a) in preeclampsia [34,35,40–42]. This is supported by the finding in two recent studies that women destined to develop preeclampsia did not have elevated Lp(a) concentrations before the appearance of the disorder [35,36]. Two studies report on the Lp(a) concentrations in women with a history of preeclampsia as compared to women with a history of uncomplicated pregnancy. van Pampus et al. [33] found higher Lp(a) concentrations in women with a history of severe preeclampsia while Leerink et al. [31] found no difference at all in Lp(a) concentration between women with a history of preeclampsia or uncomplicated pregnancy. Differences in selection or ethnicity of the study groups may account for this. 3.2.3. Lp(a) in pregnancy complicated by IUGR Only two studies report on Lp(a) in IUGR [36,37]. Both studies found significantly higher Lp(a) concentrations in pregnant women with obstetric complications (as IUGR, preterm delivery, fetal or neonatal loss, a history of recurrent abortion, placental abruption and unexplained oligohydramnios) as compared to pregnant women without obstetric complications. In both studies about 30% of the women with obstetrical complications had IUGR. Unfortunately no subanalyses were done to compare Lp(a) in pregnancies
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complicated by IUGR and uncomplicated pregnancies. On basis of these results and a case report of a woman with high Lp(a) concentrations who gave birth to three children with very low birth weights, one may speculate that Lp(a) interferes with placental circulation and thus high concentrations might cause fetal growth restriction [7]. 3.3. Evaluation of the studies according to the review criteria
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3.3.5. Information about and equal distribution of ethnicity in study groups For most of the studies the ethnicity of the study population was unknown [18–21,23–26,28–30,32,34,38– 42]. In two of the studies matching was applied for race [35,36]. The influence of ethnicity on study results is demonstrated by the study of Koukkou et al. [16] who showed significantly higher Lp(a) concentrations in African/ Afro-Caribbean pregnant women as compared to Caucasian pregnant women.
None of the studies fulfilled all of the six criteria. 3.3.1. The use of an apo(a) independent method to measure Lp(a) concentrations Most of the studies used an apo(a) isoform dependent method to measure Lp(a) concentrations [16,20,21,23,27, 29,30,32,33,36,38,40,42]. In other studies it was not mentioned whether the assay used was apo(a) independent or not [19,22,24–26,34,35,37,39,41]. The Laurell technique used by Zechner et al. [18] is a rather insensitive assay, limiting measurement of Lp(a) concentrations to the high range, not representative of those in the general population. For only one study the use of an apo(a) independent method to measure Lp(a) was reported [31]. 3.3.2. Apo(a) isoform determination Three studies performed an apo(a) isoform determination [26,31,34]. The influence of apo(a) isoform size on study results is demonstrated by the study of Brizzi et al. [26] who showed that only in women with large apo(a) isoforms there was a significant increase in Lp(a) concentration during pregnancy. In women with small apo(a) isoforms there was no change in Lp(a) concentration with advancing gestation. 3.3.3. Sufficient sample size for case control studies Of the 14 case control studies on Lp(a) in women with preeclampsia or women with a history of preeclampsia [29– 42], only five studies [30,34,37,39,42] included sufficient individuals in both the cases and the control groups. 3.3.4. Longitudinal study design when changes of Lp(a) during pregnancy are evaluated Six of the 12 studies on Lp(a) in the course of normal pregnancy had a longitudinal study design to describe changes of Lp(a) concentrations during pregnancy and after delivery [18,22,23,26,27,41]. In two of these there were only two sample time points; in the third trimester of pregnancy and after delivery [22,27]. Five studies had a cross-sectional study design [19–21,24,25]. One study used both designs and showed the influence of the choice of cross-sectional or longitudinal design on study results [42]. This study demonstrated that there was a significant increase in Lp(a) concentration during pregnancy in a longitudinal analysis, but no significant change in Lp(a) concentration during pregnancy in a cross-sectional design.
3.3.6. Statistical analysis with non-parametric tests or with log-transformed Lp(a) data Of all 26 studies, eight did not use the appropriate statistical method to analyse the data [18,22,23,26,33– 35,40]. Parametric tests were used without log-transformation of the Lp(a) data in these eight studies. From the studies measuring Lp(a) during normal pregnancy, four studies met three of the criteria [22,26,27,41], although for three of these studies [22,26,41] it was unclear whether the assay used to measure Lp(a) was apo(a) independent or not. For the other nine studies on Lp(a) in normal pregnancy fewer than three criteria were fulfilled. Two studies had only two sample time points; in the third trimester and 6–12 weeks post partum [22,27]. The other two studies described Lp(a) changes during pregnancy, but no data were available on Lp(a) post partum [26,41]. The latter two studies on Lp(a) during pregnancy suggested that Lp(a) increases during normal pregnancy [26,41], whereas the two studies measuring Lp(a) in the third trimester and post partum did not show a difference between the pregnant and the non-pregnant state [22,27]. Thus, the course of Lp(a) during and after pregnancy remains unclear. Therefore, a prospective longitudinal study on Lp(a) concentrations during pregnancy is needed. Such a study should include healthy women with a normal pregnancy course and outcome from distinct ethnicity. Lp(a) should be determined preconceptionally or, if this is not feasible, from early pregnancy onwards until 3–6 months after delivery. An apo(a) independent method should be used to measure Lp(a) concentrations and apo(a) isoform determination should be performed to estimate the influence of apo(a) isoform size on the study results. From the studies comparing Lp(a) concentrations between women with (a history of) preeclampsia and controls, only two studies were in agreement with four of the criteria [31,37], but from one study it was unclear whether the assay used to measure Lp(a) was apo(a) independent or not [37]. For the rest of the studies fewer than four criteria were fulfilled. The study by Leerink et al. [31] found no difference in Lp(a) concentrations between women with a history of preeclampsia and women with a history of uneventful pregnancy. The apo(a) isoform distribution was not different between the two study groups. The study population was predominantly Negroid,
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which questions its validity for Caucasian populations. Furthermore, this study provides no information on Lp(a) concentrations during manifest preeclampsia. The study by Ogunyemi et al. [37] found that a higher percentage of women with obstetrical complications, such as preeclampsia, had Lp(a) concentrations over 300 mg/L, as compared to healthy pregnant women. No information was given on the absolute values of Lp(a) and no separate comparison was made between the subgroup of women with preeclampsia and healthy pregnant controls. To summarize, the relationship between Lp(a) concentrations and preeclampsia remains unclear. Therefore, a prospective case control study is needed. In such a study a group of women with preeclampsia according to strict criteria should be compared with healthy controls with an uneventful pregnancy, with no ethnic differences between the two groups. Both study groups should contain at least 50 individuals, or apo(a) isoform determination should be performed. An apo(a) independent method should be used to measure Lp(a) concentrations. There are only three articles on Lp(a) and IUGR [7,36,37]. Two of these are case control studies [36,37] and the third paper is a case report [7]. The two case control studies addressed the relationship of Lp(a) with a range of pregnancy complications, including IUGR. In both studies the Lp(a) concentration was higher in women with complicated pregnancy as compared to women with uncomplicated pregnancy. No subanalysis for IUGR only was done. Neither of these studies used an apo(a) isoform independent method to measure Lp(a). More prospective data are needed to decide whether there is a difference in Lp(a) concentration between women with pregnancy complicated by IUGR and women with uncomplicated pregnancies. For such studies we propose a similar design as for studies on the relationship of Lp(a) with preeclampsia.
4. Conclusions and recommendations The literature up to now is divided on whether Lp(a) increases during normal pregnancy and on the possible association of Lp(a) with preeclampsia and IUGR. Studies performed on Lp(a) in normal and complicated pregnancy, thus far yielded very diverse results. These differences are likely to be due to differences in methods to measure Lp(a) concentrations, differences in sample size, differences in study design and differences between and heterogeneity of (ethnicity of) study populations. Therefore, comparison of the results of the different studies is hazardous, if justifiable at all. The changes of Lp(a) in normal and complicated pregnancy are not clarified. Recommendations for future studies include: (1) the use of an apo(a) independent method for measuring Lp(a); (2) consideration of apo(a) isoforms to avoid misinterpretation of results, which are caused by random deviation, especially
in case control studies; (3) inclusion of sufficient numbers (at least 50) of patients and controls in case control studies; (4) the use of a longitudinal study design when the objective is to study the changes of Lp(a) during pregnancy; and (5) selection of a study population that is ethnically representative for the general population.
Acknowledgements We thank Dr. M.L. Bots of the Julius Center for Health Sciences and Primary Care, for his critical comments and his help with statistical issues.
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