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Extracardiac anomalies in the heterotaxy syndromes with focus on anomalies of midline-associated structures
Extracardiac Anomalies in the Heterotaxy Syndromes With Focus on Anomalies of Midline-Associated Structures Baruch S. Ticho,
MD, PhD,
Allan M. Goldstein,
MD,
and Richard Van Praagh,
MD
The extracardiac defects in patients with heterotaxy have not been examined as extensively as cardiac defects. We found a high incidence of midline-associated defects in 160 autopsied cases of heterotaxy (asplenia, polysplenia, or single right-sided spleen). Fifty-two percent of patients with left-sided polysplenia had a midline-associated defect, as did 45% of those with asplenia. Most common were musculoskeletal or genitourinary anomalies, as well as cleft palate. Fused adrenal glands and anal stenosis or atresia occurred exclusively among patients with asplenia. A midline anomaly was twice as likely to be detected on complete
autopsy than from clinical findings alone. Linkage studies should take into account that affected subjects may have isolated subclinical midline defects. The high incidence of midline-associated defects supports the theory that the midline plays a critical role in establishing leftright asymmetry in the body. Comparison of these defects with mouse models of laterality defects suggests that mutations that disrupt the transforming growth factor  pathway may result in heterotaxy. 䊚2000 by Excerpta Medica, Inc. (Am J Cardiol 2000;85:729 –734)
atients with heterotaxy constitute a heterogenous group, some of whom have severe and complex P congenital heart disease and extracardiac anomalies.
and other data derived from experimental embryology.
1
Heterotaxy is described as the discordance of the orientation of organs with respect to each other or isomerism of a normally asymmetric organ.1,2 When all organs are concordant with respect to their situs, but in a mirror-image reversal of normal, the situation is described as situs inversus totalis. Most patients with heterotaxy have abnormalities of the spleen, such as asplenia, polysplenia, or single right-sided spleen (also known as isolated levocardia).3 Patients with heterotaxy, especially asplenia, continue to have relatively high mortality rates compared with most other congenital heart diseases.4 Elucidation of the mechanisms involved in causing heterotaxies will be aided by thorough evaluations of the anatomic anomalies present in these patients. Previous reviews of postmortem cases of heterotaxy included extensive evaluation of the cardiac lesions, and limited evaluation of noncardiac lesions.5,6 We present a complete anatomic description of human heterotaxy syndromes based on 160 postmortem cases and correlate our findings with the phenotypes of previously described mouse models From the Departments of Pediatrics and Surgery, Massachusetts General Hospital; the Departments of Pathology and Cardiology, Children’s Hospital, and the Departments of Pediatrics, Surgery, and Pathology, Harvard Medical School, Boston, Massachusetts. This study was supported in part by Clinical Investigator Award KO8HL02380-03 from the National Institutes of Health, Bethesda, Maryland. Manuscript received June 14, 1999; revised manuscript received and accepted October 18, 1999. Address for reprints: Baruch S. Ticho MD, PhD, VBK 615, Massachusetts General Hospital, Fruit Street, Boston, Massachusetts 02114. E-mail: [email protected]. ©2000 by Excerpta Medica, Inc. All rights reserved. The American Journal of Cardiology Vol. 85 March 15, 2000
METHODS We reviewed the medical records at the cardiac registry at Children’s Hospital, Boston, to identify patients with asplenia, polysplenia, or right-sided single spleen for the period from 1927 to 1998. All cases of heterotaxy listed in the cardiac registry had a cardiac lesion. There were 120 cadiac registry cases of patients who died at Children’s Hospital, and 40 consult cases of patients who had died elsewhere. In the consult cases, the autopsies were performed at another institution and the autopsy report was provided, and only the heart (and attached organs, if any) were examined at Children’s Hospital. Patients were included in this review if they had visceroatrial heterotaxy and definitive documentation of spleen status, either by direct visualization at necropsy or spleen scan, which confirmed a diagnosis of asplenia, polysplenia, or single right-sided spleen. Cases in which accessory splenules were identified were excluded. For each of the 160 patients, the medical record was reviewed for all clinical, radiologic, operative, or autopsy diagnoses. Data were collected for cardiac position (dextrocardia, mesocardia, levocardia), visceral and atrial situs (solitus, inversus), ventricular loop (D-loop, L-loop), common atrioventricular (AV) canal, and lung morphology. All noncardiac defects were noted. Midline defects were defined as defects that either have a midline origin, or bilateral and symmetric structures influenced by midline primordia or morphogenetic events at the midline (such as kidneys) as suggested by Opitz and Gilbert.7 0002-9149/00/$–see front matter PII S0002-9149(99)00849-8
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TABLE I Visceral and Pulmonary Situs of Patients With Heterotaxy
No. of patients Male Female Visceral situs R-sided stomach L-sided stomach Lung lobation Solitus Inversus Bilateral unilobed Bilateral bilobed Bilateral trilobed Bilateral tetralobed Unknown/other
Asplenia (%)
Polysplenia (%)
91 55 (60) 36 (40)
58 22 (38) 36 (62)
11 9 (82) 2 (18)
39 (43) 52 (57)
29 (50) 29 (50)
11 (100) 0 (0)
3 4 1 3 76 1 3
8 5 2 34 4 0 5
(3) (4) (1) (3) (84) (1) (3)
(14) (9) (3) (58) (7) (0) (9)
Single R-Sided spleen (%)
1 2 0 3 5 0 0
(9) (18) (0) (27) (45) (0) (0)
Statistical analysis was done using chi-square or Fisher’s exact test.
RESULTS
Patients: A total of 160 eligible heterotaxy patients were included in this review. All patients underwent autopsy, 80% (127 of 160) of which were complete; the remaining autopsies were limited to the chest only. There were 91 patients with asplenia, 58 with polysplenia, and 11 with a single right-sided spleen (Table I). Among the polysplenia group, in 27 cases the spleens were located on the left side, in 29 on the right side, and in 2 cases the autopsy report stated the diagnosis, but did not specify which side the spleen was on. Most polysplenia subjects were female (36 of 58, 62%). Males predominated the asplenia group (55 of 91, 60%) and the right-sided spleen group (9 of 11, 82%) (Table II). Cardiac defects: A thorough description of the cardiac defects found in 109 of the subjects was published previously.5 For this review we noted the cardiac position, atrial situs, and ventricular loop, and had adequate documentation for all in 132 patients. In addition, the presence of an AV canal defect was noted, as a measure of the severity of cardiac disease. Levocardia was present in 64% (84 of 132), dextrocardia in 32% (42 of 132), and mesocardia in 4% (6 of 132). Sixty-nine percent (91 of 132) had D-loop ventricles, and 31% (41 of 132) had L-looping. Figures 1A and 1B display the relation between cardiac looping and the position of the heart in the chest. In patients with D-loop ventricles, 19% (17 of 91) had either dextrocardia or mesocardia. L-looping was associated with dextrocardia in 71% (29 of 41) of cases (p ⬍0.0001). We also examined the correlation of atrial situs and visceral situs. Of the patients with known atrial situs, 62% (74 of 120) had solitus atria. Sixty-one percent (45 of 74) of subjects with solitus atria had levoposition (situs solitus) of the stomach (Figures 1C and 1D). Similarly, inverted atria were associated with dextroposed (situs inversus) stomachs in 67% (31 of 46) of cases (p ⬍0.01). Patients with left-sided polysplenia tended to have 730 THE AMERICAN JOURNAL OF CARDIOLOGY姞
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less severe cardiac disease, with 30% (7 of 23) having common AV canal compared with 44% (12 of 27) with right-sided polysplenia, 64% (7 of 11) of patients with solitary right-sided spleen, and 80% (72 of 91) (p ⬍0.01) with asplenia. A similar incidence of common AV canal in asplenia and polysplenia has been reported.1 Central nervous system: Seven percent of patients (12 of 160) had midline central nervous system defects. These included meningomyelocele, porencephalic cyst, cerebellar agenesis, encephalocele, Dandy-Walker cyst, holoprosencephaly, diplomyelia, and hydromyelia (Table II). Craniofacial: Nine percent (14 of 160) of all cases had ⱖ1 midline-associated craniofacial defect. Nine cases of cleft lip and palate were noted in 5.6% of patients. Agnathia or micrognathia were noted in several patients, whereas others had choanal atresia, high arched palate, laryngeal cleft, or cyclopia (Table II). Endocrine: Only 1 significant structural anomaly of the endocrine glands was noted, this being a fusion of the adrenal glands across the midline. Although rare in the general population, this defect occurred in 10% of patients with asplenia (9 of 91), and was not observed in other forms of heterotaxy (Table II) (p ⫽ 0.01). Gastrointestinal: Virtually all patients had some type of gastrointestinal anomaly. Dextroposition of the stomach was present in 40% (36 of 91) of asplenic patients, 45% (26 of 58) of polysplenic patients, and 100% of those with single right spleen (11 of 11) (Table I). Malrotation of the intestines was found in 33% (52 of 160), occurring almost equivalently among the 3 subtypes, in 37% of patients with asplenia (33 of 91), 27% of patients with polysplenia (16 of 58), and 27% of those with right-sided spleen (3 of 11). Six percent of patients (10 of 160) had a midline gastrointestinal defect, including tracheoesophageal fistula or atresia, and omphalocele (Table II). Anal stenosis or atresia occurred exclusively in subjects with asplenia syndrome (p ⫽ 0.07). Absence of the gallbladder and biliary atresia occurred in patients with polysplenia. Genitourinary: Genitourinary anomalies were found in 14% (22 of 160) of the cases. These included hypoplastic kidneys, hypospadias, absent kidney, duplicated vagina, horseshoe kidney, urethral duplication, duplicated uterus, bicornuate uterus, unicornuate uterus, and vaginal atresia (Table II). Musculoskeletal: Musculoskeletal abnormalities involving midline structures were present in 13% (21 of 160) of patients. Severe kyphosis or scoliosis occurred most often in 5.5% of patients, which was congenital in at least half of these cases (presenting before surgery, or in the first month of life). Hemivertebrae, fused vertebras, pectus deformity, vertebral anomalies, bifid sacrum, and sacral agenesis were also noted (Table II). Three patients had caudal regression syndrome. Pulmonary: Ninety-two percent of patients with heterotaxy had abnormal lung morphology. Symmetrically trilobed lungs with bilateral eparterial bronchi MARCH 15, 2000
TABLE II Types and Incidence of Midline-Associated Anomalies in Cases of Heterotaxy Polysplenia Asplenia (n ⫽ 91) Craniofacial Cleft lip/palate Agnathia/micrognathia Choanal atresia High arched palate Laryngeal cleft Cyclopia Endocrine Fused adrenals Gastrointestinal Tracheoesophageal fistula or atresia Anal stenosis/atresia Omphalocele Genitourinary Hypoplastic kidney Hypospadias Absent kidney Duplicated vagina Horseshoe kidney Urethral duplication Duplicated uterus Bicornuate uterus Unicornuate uterus Vaginal atresia Nervous system Meningomyelocele Cerebellar agenesis Porencephalic cyst Encephalocele Dandy-Walker Holoprosencephaly Diplomyelia Hydromyelia Skeletal Kyphosis/scoliosis Hemivertebrae Caudal regression Fused vertebrae Pectus carinatum Absent vertebra Bifid sacrum Sacral agenesis Totals
Left (n ⫽ 27)
Right (n ⫽ 29)
Unknown (n ⫽ 2)
Single Right spleen (n ⫽ 11)
Totals (n ⫽ 160)
Percent of Total patients
7 4 0 1 0 0
2 1 1 0 0 1
0 1 0 0 1 0
0 0 0 0 0 0
0 0 0 0 0 0
9 6 1 1 1 1
5.6 3.7 0.6 0.6 0.6 0.6
9
0
0
0
0
9
5.6
3
3
0
0
0
6
3.7
5 2
0 0
0 0
0 0
0 0
5 2
3.1 1.2
5 1 2 1 1 0 1 1 0 1
2 2 0 2 1 1 1 0 0 0
0 2 1 0 0 1 0 0 1 0
0 0 0 0 0 0 0 0 0 0
0 1 0 0 0 0 0 0 0 0
7 6 3 3 2 2 2 1 1 1
4.4 3.7 1.8 1.8 1.2 1.2 1.2 0.6 0.6 0.6
2 0 1 1 0 0 1 0
0 0 0 0 1 1 0 1
1 2 0 0 0 0 0 0
1 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0
4 2 1 1 1 1 1 1
2.5 1.2 0.6 0.6 0.6 0.6 0.6 0.6
4 2 1 1 1 1 0 0
2 1 1 1 0 1 1 1
2 1 1 0 1 0 0 0
1 0 0 0 0 0 0 0
0 1 0 0 0 0 0 0
9 5 3 2 2 2 1 1
5.5 3.1 1.8 1.2 1.2 1.2 0.6 0.6
59
28
15
2
2
106
were present in 76 of the 91 patients with asplenia (83%) (Table I). Among those with polysplenia, 34 of 58 (58%) had bilaterally bilobed lungs with hyparterial bronchi. Of the remaining patients with polysplenia, 8 (14%) had solitus lobation, 5 (9%) had inversus, 4 (7%) were bilaterally trilobed, and 2 (3%) were bilaterally unilobed (Table I). Incidence of midline-associated anomalies: A total of 106 midline-associated anomalies were found in this collection of patients with heterotaxy. Defects most often occurred in the genitourinary (28 of 106 [26%]) and musculoskeletal (25 of 106 [24%]) systems (Table IIIA). Nearly one fifth of the lesions (19 of 106) involved craniofacial structures and 11% (12 of 106) occurred in the nervous system. Overall, 38% of patients with heterotaxy had a midline-associated defect (60 of 160) (Table IIIB). The incidence varied among heterotaxy subtype (Table IIIB) and was most reliable in subjects who had a complete autopsy (see below). More than one half
(52%, 11 of 21) of the complete autopsy cases of left-sided polysplenia had a midline defect, which differed significantly from cases of right-sided polysplenia or single right-sided spleen, of which 29% (6 of 21) and 18% (2 of 11), respectively, had midline defects (p ⫽ 0.05). In 45% of asplenia cases (33 of 73) a midline defect was noted. Type of autopsy: The observation of midline-associated defects was affected by the type of autopsy (Table IIIB). In 42% (53 of 127) of the cases where a complete autopsy was performed, a midline defect was noted; however, in cases when the autopsy was incomplete a midline defect was described only in 21% (7 of 33) (p ⫽ 0.02). Multiple anomalies: A total of 106 midline defects were found in 60 patients, indicating that some patients had ⬎1 defect. Patients with a complete autopsy were analyzed for the presence of a single midline defect or multiple midline defects. Of the 20 patients with polysplenia and a midline defect, 25% (5 of 20)
CONGENITAL HEART DISEASE/MIDLINE DEFECTS IN HETEROTAXY
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these cases. Extracardiac defects involved almost all systems. Particular note was made of midline-associated defects (Table II). The incidence of midline defects depends on the subtype of heterotaxy:
There was a significant difference in the incidence of midline defects occurring in patients with left-sided polysplenia (52%), asplenia (45%), right-sided polysplenia (29%), and single rightsided spleen (18%) (Table IIIB). The argument has been put forth that asplenia and polysplenia may not be completely distinct entities,1 especially because there is genetic and phenotypic evidence that some crossover exists between these 2 diseases. Our results argue that, based on the incidence of midline defects, not only are polysplenia and asplenia distinct, but left-sided polysplenia is distinct from right-sided polysplenia. Similarly, the cardiovascuFIGURE 1. A, cardiac sidedness with D-loop ventricles; B, cardiac sidedness with lar defects differ significantly between L-loop ventricles; C, gastric situs in patients with solitus atria; D, gastric situs in polysplenia and asplenia (R. Van patients with inverted atria. Praagh and S. Litvosky, unpublished results). These data may point to unique, or at least separable, mechanisms for the etiologies of these syndromes. TABLE IIIA Frequency of Midline-Associated Defects in Patients With Heterotaxy by Organ System Genitourinary Musculoskeletal Craniofacial Gastrointestinal Nervous system Endocrine
28/106 25/106 19/106 13/106 12/106 9/106
(26%) (24%) (18%) (12%) (11%) (8%)
TABLE IIIB Prevalence of Midline-Associated Defects by Heterotaxy Subtype and by Type of Autopsy Asplenia (complete autopsy) Polysplenia-left (complete autopsy) Polysplenia-right (complete autopsy) Single right spleen Any associated midline defect Complete autopsy Incomplete autopsy
33/73 (45%) 11/21 (52%) 6/21 (29%) 2/11 (18%) 62/160 (39%) 53/127 (42%) 7/33 (21%)
had a single defect and 75% (15 of 20) had ⬎1 defect. This differed significantly from the asplenia cases where 58% (19 of 33) had an isolated midline defect and 42% (14 of 33) had multiple midline anomalies (p ⫽ 0.01). There was no association between visceral or atrial situs and the number of anomalies. Four patients with multiple anomalies had an identified syndrome or complex; 1 each had Turner, Goldenhaar, Trisomy 13, or Meckel-Gruber syndromes.
DISCUSSION The emphasis of this review of autopsied heterotaxy cases was on the noncardiac anomalies present in 732 THE AMERICAN JOURNAL OF CARDIOLOGY姞
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Patients with heterotaxy have a high incidence of midline-associated defects: Overall, we found that 38%
of patients with heterotaxy had at least 1 midlineassociated defect (Table IIIB). The frequency of midline defects in this heterotaxy population ranged from 10 to 60 times greater than the respective frequency of each lesion in the general population,8 suggesting that midline defects occur in heterotaxy patients much more often than would be expected by chance alone. A recent population study of infants with midline defects found that 95% of nonsyndromic infants had a single isolated midline defect.8 We find that in heterotaxy patients with midline defects, nearly one half of asplenics (44%) and most polysplenics (78%) have ⬎1 defect present. These data point to the need for further refinement in the study of the association of midline and laterality defects, particularly with respect to embryologic and pathogenetic mechanisms. The association of lateralization defects and midline anomalies has been previously described.9 –12 The incidence of extracardiac defects in our study was higher than that reported in another large review of extracardiac defects in asplenia cases.13 The latter review included nonautopsied cases and thus likely underrepresented the true incidence (see below). An earlier report of 29 necropsied asplenia cases from our institution6 found a similar rate of extracardiac defects as reported here. The relatively high incidence of midline-associated defects in patients with heterotaxy supports the theory that the midline plays a critical role in establishing normal left-right asymmetry.14 –16 According to Yost,17 cells in the midline, such as notochord cells, may act as sources of signals that MARCH 15, 2000
unpublished results]), and associated midline defects involving the vertebras,28 reminiscent of human aspleMouse Mouse nia syndrome. The results presented Human Mouse ActR ActRIIA ⫹/⫺ Nodal ⫹/⫺ in this study demonstrate the marked Asplenia IIB⫺/⫺ B⫺/⫺ ActR IIB⫺/⫺ similarity between the phenotypes of Cardiac defect 99% 49% 100% 100% patients with asplenia and the mice Dextrocardia 27% 16% 42% 35% with selective disruption of the acRespiratory tivin receptor-type II gene (Table Right lung isomerism 83% 100% 100% 100% Abnormal spleen 100% 34% 100% 100% IV). These similarities suggest that Craniofacial this molecule, or others in the TGF- Cleft lip/palate 8% 2% 50% 0 pathway, may be mutated in patients Agnathia 4% 0 10% 0 with asplenia. Gastrointestinal The genetics of human heterotaxy Stomach dextroposition 35% 0 5% 0 Omphalocele 2% 0 10% 0 are beginning to be determined. SevGenitourinary eral lines of evidence point to single Hypoplastic kidney 5% 19% 12% 0 gene defects.29,30 A mutation in the Absent kidney 2% 38% 79% 0 gene Zic3 has been linked to a subset Skeletal Abnormal ribs 6% 100% 100% 0 of patients with X-linked heteroSacral agenesis 0 0 58% 0 taxy.31 Recently 2 mutations in the gene LEFTY A were identified in ActR IIA ⫽ activin receptor IIA; Actr IIB ⫽ activin receptor IIB; ⫹/⫺ ⫽ heterozygous knockout; ⫺/⫺ ⫽ homozygous knockout. patients with left lung isomerism.32 Mutations in the human activin receptor-type IIB gene have been redirect left-right asymmetry, and may act as a barrier to ported in 3 patients with heterotaxy.33 Future studies maintain a gradient of left-right signaling molecules. are likely to reveal additional genes associated with Recent evidence demonstrates that the leftward flow human disorders of laterality. of extraembryonic fluid at the embryonic node is Cardiac position does not always predict ventricular essential for left-right determination.18 Disruption of looping: We found that heart-tube looping does not the midline barrier separating left from right may always correlate with the position of the heart in the permit a mixing of molecules that are normally asym- chest. Twenty percent of the heterotaxy patients with metrically distributed in early embryos, resulting in D-looped ventricles had dextrocardia (Figure 1A). abnormal organ sidedness later in development. Conversely, 30% of L-looped ventricles presented as The midline developmental field concept7 may ex- either meso- or levocardia (Figure 1B). These results plain the occurrence of apparently unrelated malfor- have particular pertinence for studies in other organmations in the same patient. Deficiencies in the mid- isms in which cardiac position or sidedness in the line field are likely to arise from single gene defects, thorax has been assumed to correlate with the direcas has been observed in mouse models of lateraliza- tion of ventricular looping.34 tion defects.19 A mutation in a gene, or combination of Atrial situs does not correlate with visceral situs: The genes, that is fundamental to midline development atrial situs can be unambiguously determined in most may cause a specific constellation of defects that over- cases, based on the lateralization of pulmonary and lap, with combinations of defects resulting from a systemic venous return. Although isomerization of the mutation in another gene in the same mechanistic atrial appendages or pectinate muscles can occur ocpathway. We must await definitive genotype-pheno- casionally in heterotaxy, they are not consistent findtype correlation to classify these disorders appropri- ings, and do not represent atrial isomerism (R. Van ately. Praagh, unpublished results). Our findings support the The phenotype of mice lacking the activin receptor IIB hypothesis that atrial situs is not necessarily associated is similar to human asplenia syndrome: Several mouse with visceral situs. In 62% of heterotaxy patients with models of heterotaxy have been investigated.19 normal atrial situs, the stomach had normal situs. Among these is the iv mouse that arises due to a When the atria were inverted, the stomach was inmutation of the gene left-right dynein,20,21 and the inv verted in 67% of the cases. Although our data show mouse, which has a defect in an ankyrin-repeat pro- that there is no obligatory link between cardiac looping and cardiac position, or between atrial situs and tein.22–25 Members of the transforming growth factor  visceral situs, they do argue against the notion that (TGF-) family function in a molecular cascade that these processes are completely randomized in heteroestablishes organ asymmetry early in develop- taxy. D-looping, normal atrial situs, and normal visment.14,26 Activin receptor IIB is thought to be a ceral situs each occurred in approximately two thirds receptor for 2 of these TGF- family members, nodal of patients, rather than in half of the population. Simiand activin.27 Mice with a targeted mutation of the larly, levocardia occurred in two thirds of patients, inactivin receptor IIB gene display abnormal spleens, stead of occurring in equal distribution with meso- and right pulmonary isomerism, cardiac anomalies (in- dextrocardia. One could argue that the incidence may cluding common AV canal [B. Ticho, S.P. Oh, E. Li, appear more random if all fetuses were included, since TABLE IV Similarity Between Asplenia Syndrome and Mouse Disruption of Activin Receptors
CONGENITAL HEART DISEASE/MIDLINE DEFECTS IN HETEROTAXY
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fetuses with more disordered left-right asymmetry would be less likely to survive to birth. This did not appear to be the case for the small number of fetuses examined in this review (100% [3/3] had levocardia). The frequency of central nervous system defects implies effects on central nervous system situs: The finding
of central nervous system defects among heterotaxy patients supports the concept that early embryonic events may affect cerebral structural asymmetry and midline central nervous system structures. A recent study35 supports the existence of a cerebral situs. Magnetic resonance imaging brain scans demonstrate that cerebral situs can be determined by anatomic asymmetry and that it is reversed in patients with situs inversus totalis. It appears that factors that dictate visceral and thoracic situs also affect cerebral situs. Implications for the genetic analysis of heterotaxy syndromes: Efforts at identifying the genes responsible
for aberrant asymmetry require a complete characterization of these disorders, including abnormalities of organ positioning and all associated congenital defects. Such a thorough phenotypic characterization allows precise classification of patients as well as a clearer understanding of animal models in the context of human disorders of asymmetry. Some of the midline-associated defects noted in heterotaxy patients were subclinical and were found only at autopsy. Table III shows that in the patients who had incomplete autopsies the noted incidence of these defects was half that of patients in whom a complete autopsy was performed. This discrepancy is most likely explained by the fact that in these cases, data on midline defects were mostly dependent on clinical records. This implies that these defects may not be noted in up to half of the cases of affected persons. As a result, pedigree studies using linkage analysis to identify loci associated with heterotaxy syndromes may incorrectly identify family members as unaffected when based solely on usual clinical examinations. Assessment of disease status in living patients would have to rely on magnetic resonance imaging, or other imaging modalities, to identify midline-associated defects in these otherwise unaffected persons. Implications for the clinical management of heterotaxy patients: The relatively high frequency of urogenital
anomalies (26%), including those with potentially significant medical impact, such as hypoplastic or absent kidney, implies that a thorough evaluation for renal anomalies is indicated in patients with heterotaxy. Acknowledgment: We thank Stella Van Praagh and Silvio Litovsky for their assistance. 1. Gutgesell, HP. Cardiac malposition and heterotaxy. In: Garson A, Bricker JT, Fisher DJ, Neish SR. eds. The Science and Practice of Pediatric Cardiology. Baltimore: Williams & Wilkins, 1998:1539 –1562. 2. Splitt MR, Burn J, Goodship J. Defects in the determination of left-right asymmetry. J Med Genet 1996;33:498 –503. 3. Liberthson RR, Hastreiter AR, Sinha SN, Bharati S, Novak GM, Lev M. Levocardia with visceral heterotaxy-isolated levocardia: pathologic anatomy and its clinical implications. Am Heart J 1973;85:40 –54. 4. Hashmi A, Abu-Sulaiman R, McCrindle BW, Smallhorn JF, Williams WG, Freedom RM. Management and outcomes of right atrial isomerism: a 26-year experience. J Am Coll Cardiol 1998;31:1120 –1126. 5. Van Praagh S, Santini F, Sanders SP. Cardiac malpositions with special emphasis
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Clinical/epidemiological analysis of alteration of laterality (normal body symmetry and asymmetry). Am J Med Gen 1995;56:382–388. 11. Casey B, Devoto M, Jones KL, Ballabio A. Mapping a gene for familial situs abnormalities to human chromosome Xq24-q27.1. Nat Genet 1993;5:403– 406. 12. Ferrero GB, Gebbia M, Pilia G, Witte D, Peier A, Hopkin RJ, Craigen WJ, Shaffer LG, Schlessinger D, Ballabio A, Casey B. A submicroscopic deletion in Xq26 associated with familial situs ambiguus. Am J Hum Genet 1997;61:395– 401. 13. Phoon CK, Neill CA. Asplenia syndrome: insight into embryology through an analysis of cardiac and extracardiac anomalies. Am J Cardiol 1994;73:581–587. 14. Hyatt BA, Yost HJ. Left-right development in Xenopus and zebrafish. Semin Cell Dev 1998;9:61– 66. 15. Danos MC, Yost HJ. Role of notochord in specification of cardiac left-right orientation in zebrafish and Xenopus. Dev Biol 1996;177:96 –103. 16. Lohr JL, Danos MC, Yost HJ. Left-right asymmetry of a nodal-related gene is regulated by dorsoanterior midline structures during Xenopus development. Dev 1997;124:1465–1472. 17. Yost HJ. Left-right development in Xenopus and zebrafish. Semin Cell Dev Biol 1998;9:61– 66. 18. Nonaka S, Tanaka Y, Okada Y, Takeda S, Harada A, Kanai Y, Kido M, Hirokawa, N. Randomization of left-right asymmetry due to loss of nodal cilia generating leftward flow of extraembryonic fluid in mice lacking KIF3B motor protein. Cell 1998;95:829 – 837. 19. Supp DM, Brueckner M, Potter SS. Handed asymmetry in the mouse: understanding how things go right (or left) by studying how they go wrong. Semin Cell Dev Biol 1998;9:77– 87. 20. Supp DM, Witte DP, Potter SS, Brueckner M. Mutation of an axonemal dynein in the left-right asymmetry mouse mutant inversus viscerum. Nature 1997;389:963–966. 21. Layton WM. Random determination of a developmental process. Reversal of normal visceral asymmetry in the mouse. J Hered 1976;67:336 –338. 22. Mochizuki T, Saijoh Y, Tsuchiya K, Shirayoshi Y, Takai S, Taya C, Yonekawa H, Yamada K, Nihei H, Nakatsuji N, Overbeek PA, Hamada H, Yokoyama T. Cloning of inv, a gene that controls left/right asymmetry and kidney development. Nature 1998;395:177–181. 23. Morgan D, Turnpenny L, Goodship J, Dai W, Majumder K, Matthews L, Gardner A, Schuster G, Vien L, Harrison W, Elder FF, Penman-Splitt M, Overbeek P, Strachan T. Inversin, a novel gene in the vertebrate left-right axis pathway, is partially deleted in the inv mouse. Nat Genet 1998;20:149 –156. 24. Yokoyama T, Copeland NG, Jenkins NA, Montgomery CA, Elder FFB, Overbeek PA. Reversal of left-right asymmetry: a situs inversus mutation. Science 1993;260:679 – 682. 25. Lowe LA, Supp DM, Sampath K, Yokoyama T, Wright CVE, Potter SS, Overbeek P, Kuehn MR. Conserved left-right asymmetry of nodal expression and alterations in murine situs inversus. Nature 1996;381:158 –161. 26. Hyatt BA, Lohr JL, Yost HJ. Initiation of vertebrate left-right axis formation by maternal Vg1. Nature 1996;384:62– 65. 27. Nomura M, Li E. Smad2 role in mesoderm formation, left-right patterning and craniofacial development. Nature 1998;393:786 –90. 28. Oh SP, Li E. The signaling pathway mediated by the type IIB activin receptor controls axial patterning and lateral asymmetry in the mouse. Genes Dev 1997; 11:1812–1826. 29. Kosaki K, Casey B. Genetics of human left-right axis malformations. Semin Cell Dev Biol 1998;9:89 –99. 30. Bowers PN, Brueckner M, Yost HJ. The genetics of left-right development and heterotaxia. Semin Perinatol 1996;20:577–588. 31. Gebbia M, Ferrero GB, Pilia G, Bassi M T, Aylsworth AS, Penman-Splitt M, Bird LM, Bamforth JS, Burn J, Schlessinger D, Nelson DL, Casey B. X-linked situs abnormalities result from mutations in ZIC3. Nature Genet 1997;17:305–308. 32. Kosaki K, Bassi MT, Kosaki R, Lewin M, Belmont J, Schauer G, Casey B. Characterization and mutation analysis of human LEFTY A and LEFTY B, homologues of murine genes implicated in left-right axis development. Am J Hum Genet 1999;64:712–721. 33. Kosaki R, Gebbia M, Kosaki K, Lewin M, Bowers P, Towbin JA, Casey B. Left-right axis malformations associated with mutations in ACVR2B, the gene for human activin receptor type IIB. Am J Med Genet 1999;82:70 –76. 34. Levin M. Left-right asymmetry and the chick embryo. Semin Cell Dev Biol 1998;9:67–76. 35. Kennedy D, O’Craven, KM, Ticho, BS, Goldstein, AM, Henson JW. Structural and functional asymmetries of the brain are dissociated in situs inversus. Neurology, 1999;53:1260 –1265.
MARCH 15, 2000
MD, PhD,
Allan M. Goldstein,
MD,
and Richard Van Praagh,
MD
The extracardiac defects in patients with heterotaxy have not been examined as extensively as cardiac defects. We found a high incidence of midline-associated defects in 160 autopsied cases of heterotaxy (asplenia, polysplenia, or single right-sided spleen). Fifty-two percent of patients with left-sided polysplenia had a midline-associated defect, as did 45% of those with asplenia. Most common were musculoskeletal or genitourinary anomalies, as well as cleft palate. Fused adrenal glands and anal stenosis or atresia occurred exclusively among patients with asplenia. A midline anomaly was twice as likely to be detected on complete
autopsy than from clinical findings alone. Linkage studies should take into account that affected subjects may have isolated subclinical midline defects. The high incidence of midline-associated defects supports the theory that the midline plays a critical role in establishing leftright asymmetry in the body. Comparison of these defects with mouse models of laterality defects suggests that mutations that disrupt the transforming growth factor  pathway may result in heterotaxy. 䊚2000 by Excerpta Medica, Inc. (Am J Cardiol 2000;85:729 –734)
atients with heterotaxy constitute a heterogenous group, some of whom have severe and complex P congenital heart disease and extracardiac anomalies.
and other data derived from experimental embryology.
1
Heterotaxy is described as the discordance of the orientation of organs with respect to each other or isomerism of a normally asymmetric organ.1,2 When all organs are concordant with respect to their situs, but in a mirror-image reversal of normal, the situation is described as situs inversus totalis. Most patients with heterotaxy have abnormalities of the spleen, such as asplenia, polysplenia, or single right-sided spleen (also known as isolated levocardia).3 Patients with heterotaxy, especially asplenia, continue to have relatively high mortality rates compared with most other congenital heart diseases.4 Elucidation of the mechanisms involved in causing heterotaxies will be aided by thorough evaluations of the anatomic anomalies present in these patients. Previous reviews of postmortem cases of heterotaxy included extensive evaluation of the cardiac lesions, and limited evaluation of noncardiac lesions.5,6 We present a complete anatomic description of human heterotaxy syndromes based on 160 postmortem cases and correlate our findings with the phenotypes of previously described mouse models From the Departments of Pediatrics and Surgery, Massachusetts General Hospital; the Departments of Pathology and Cardiology, Children’s Hospital, and the Departments of Pediatrics, Surgery, and Pathology, Harvard Medical School, Boston, Massachusetts. This study was supported in part by Clinical Investigator Award KO8HL02380-03 from the National Institutes of Health, Bethesda, Maryland. Manuscript received June 14, 1999; revised manuscript received and accepted October 18, 1999. Address for reprints: Baruch S. Ticho MD, PhD, VBK 615, Massachusetts General Hospital, Fruit Street, Boston, Massachusetts 02114. E-mail: [email protected]. ©2000 by Excerpta Medica, Inc. All rights reserved. The American Journal of Cardiology Vol. 85 March 15, 2000
METHODS We reviewed the medical records at the cardiac registry at Children’s Hospital, Boston, to identify patients with asplenia, polysplenia, or right-sided single spleen for the period from 1927 to 1998. All cases of heterotaxy listed in the cardiac registry had a cardiac lesion. There were 120 cadiac registry cases of patients who died at Children’s Hospital, and 40 consult cases of patients who had died elsewhere. In the consult cases, the autopsies were performed at another institution and the autopsy report was provided, and only the heart (and attached organs, if any) were examined at Children’s Hospital. Patients were included in this review if they had visceroatrial heterotaxy and definitive documentation of spleen status, either by direct visualization at necropsy or spleen scan, which confirmed a diagnosis of asplenia, polysplenia, or single right-sided spleen. Cases in which accessory splenules were identified were excluded. For each of the 160 patients, the medical record was reviewed for all clinical, radiologic, operative, or autopsy diagnoses. Data were collected for cardiac position (dextrocardia, mesocardia, levocardia), visceral and atrial situs (solitus, inversus), ventricular loop (D-loop, L-loop), common atrioventricular (AV) canal, and lung morphology. All noncardiac defects were noted. Midline defects were defined as defects that either have a midline origin, or bilateral and symmetric structures influenced by midline primordia or morphogenetic events at the midline (such as kidneys) as suggested by Opitz and Gilbert.7 0002-9149/00/$–see front matter PII S0002-9149(99)00849-8
729
TABLE I Visceral and Pulmonary Situs of Patients With Heterotaxy
No. of patients Male Female Visceral situs R-sided stomach L-sided stomach Lung lobation Solitus Inversus Bilateral unilobed Bilateral bilobed Bilateral trilobed Bilateral tetralobed Unknown/other
Asplenia (%)
Polysplenia (%)
91 55 (60) 36 (40)
58 22 (38) 36 (62)
11 9 (82) 2 (18)
39 (43) 52 (57)
29 (50) 29 (50)
11 (100) 0 (0)
3 4 1 3 76 1 3
8 5 2 34 4 0 5
(3) (4) (1) (3) (84) (1) (3)
(14) (9) (3) (58) (7) (0) (9)
Single R-Sided spleen (%)
1 2 0 3 5 0 0
(9) (18) (0) (27) (45) (0) (0)
Statistical analysis was done using chi-square or Fisher’s exact test.
RESULTS
Patients: A total of 160 eligible heterotaxy patients were included in this review. All patients underwent autopsy, 80% (127 of 160) of which were complete; the remaining autopsies were limited to the chest only. There were 91 patients with asplenia, 58 with polysplenia, and 11 with a single right-sided spleen (Table I). Among the polysplenia group, in 27 cases the spleens were located on the left side, in 29 on the right side, and in 2 cases the autopsy report stated the diagnosis, but did not specify which side the spleen was on. Most polysplenia subjects were female (36 of 58, 62%). Males predominated the asplenia group (55 of 91, 60%) and the right-sided spleen group (9 of 11, 82%) (Table II). Cardiac defects: A thorough description of the cardiac defects found in 109 of the subjects was published previously.5 For this review we noted the cardiac position, atrial situs, and ventricular loop, and had adequate documentation for all in 132 patients. In addition, the presence of an AV canal defect was noted, as a measure of the severity of cardiac disease. Levocardia was present in 64% (84 of 132), dextrocardia in 32% (42 of 132), and mesocardia in 4% (6 of 132). Sixty-nine percent (91 of 132) had D-loop ventricles, and 31% (41 of 132) had L-looping. Figures 1A and 1B display the relation between cardiac looping and the position of the heart in the chest. In patients with D-loop ventricles, 19% (17 of 91) had either dextrocardia or mesocardia. L-looping was associated with dextrocardia in 71% (29 of 41) of cases (p ⬍0.0001). We also examined the correlation of atrial situs and visceral situs. Of the patients with known atrial situs, 62% (74 of 120) had solitus atria. Sixty-one percent (45 of 74) of subjects with solitus atria had levoposition (situs solitus) of the stomach (Figures 1C and 1D). Similarly, inverted atria were associated with dextroposed (situs inversus) stomachs in 67% (31 of 46) of cases (p ⬍0.01). Patients with left-sided polysplenia tended to have 730 THE AMERICAN JOURNAL OF CARDIOLOGY姞
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less severe cardiac disease, with 30% (7 of 23) having common AV canal compared with 44% (12 of 27) with right-sided polysplenia, 64% (7 of 11) of patients with solitary right-sided spleen, and 80% (72 of 91) (p ⬍0.01) with asplenia. A similar incidence of common AV canal in asplenia and polysplenia has been reported.1 Central nervous system: Seven percent of patients (12 of 160) had midline central nervous system defects. These included meningomyelocele, porencephalic cyst, cerebellar agenesis, encephalocele, Dandy-Walker cyst, holoprosencephaly, diplomyelia, and hydromyelia (Table II). Craniofacial: Nine percent (14 of 160) of all cases had ⱖ1 midline-associated craniofacial defect. Nine cases of cleft lip and palate were noted in 5.6% of patients. Agnathia or micrognathia were noted in several patients, whereas others had choanal atresia, high arched palate, laryngeal cleft, or cyclopia (Table II). Endocrine: Only 1 significant structural anomaly of the endocrine glands was noted, this being a fusion of the adrenal glands across the midline. Although rare in the general population, this defect occurred in 10% of patients with asplenia (9 of 91), and was not observed in other forms of heterotaxy (Table II) (p ⫽ 0.01). Gastrointestinal: Virtually all patients had some type of gastrointestinal anomaly. Dextroposition of the stomach was present in 40% (36 of 91) of asplenic patients, 45% (26 of 58) of polysplenic patients, and 100% of those with single right spleen (11 of 11) (Table I). Malrotation of the intestines was found in 33% (52 of 160), occurring almost equivalently among the 3 subtypes, in 37% of patients with asplenia (33 of 91), 27% of patients with polysplenia (16 of 58), and 27% of those with right-sided spleen (3 of 11). Six percent of patients (10 of 160) had a midline gastrointestinal defect, including tracheoesophageal fistula or atresia, and omphalocele (Table II). Anal stenosis or atresia occurred exclusively in subjects with asplenia syndrome (p ⫽ 0.07). Absence of the gallbladder and biliary atresia occurred in patients with polysplenia. Genitourinary: Genitourinary anomalies were found in 14% (22 of 160) of the cases. These included hypoplastic kidneys, hypospadias, absent kidney, duplicated vagina, horseshoe kidney, urethral duplication, duplicated uterus, bicornuate uterus, unicornuate uterus, and vaginal atresia (Table II). Musculoskeletal: Musculoskeletal abnormalities involving midline structures were present in 13% (21 of 160) of patients. Severe kyphosis or scoliosis occurred most often in 5.5% of patients, which was congenital in at least half of these cases (presenting before surgery, or in the first month of life). Hemivertebrae, fused vertebras, pectus deformity, vertebral anomalies, bifid sacrum, and sacral agenesis were also noted (Table II). Three patients had caudal regression syndrome. Pulmonary: Ninety-two percent of patients with heterotaxy had abnormal lung morphology. Symmetrically trilobed lungs with bilateral eparterial bronchi MARCH 15, 2000
TABLE II Types and Incidence of Midline-Associated Anomalies in Cases of Heterotaxy Polysplenia Asplenia (n ⫽ 91) Craniofacial Cleft lip/palate Agnathia/micrognathia Choanal atresia High arched palate Laryngeal cleft Cyclopia Endocrine Fused adrenals Gastrointestinal Tracheoesophageal fistula or atresia Anal stenosis/atresia Omphalocele Genitourinary Hypoplastic kidney Hypospadias Absent kidney Duplicated vagina Horseshoe kidney Urethral duplication Duplicated uterus Bicornuate uterus Unicornuate uterus Vaginal atresia Nervous system Meningomyelocele Cerebellar agenesis Porencephalic cyst Encephalocele Dandy-Walker Holoprosencephaly Diplomyelia Hydromyelia Skeletal Kyphosis/scoliosis Hemivertebrae Caudal regression Fused vertebrae Pectus carinatum Absent vertebra Bifid sacrum Sacral agenesis Totals
Left (n ⫽ 27)
Right (n ⫽ 29)
Unknown (n ⫽ 2)
Single Right spleen (n ⫽ 11)
Totals (n ⫽ 160)
Percent of Total patients
7 4 0 1 0 0
2 1 1 0 0 1
0 1 0 0 1 0
0 0 0 0 0 0
0 0 0 0 0 0
9 6 1 1 1 1
5.6 3.7 0.6 0.6 0.6 0.6
9
0
0
0
0
9
5.6
3
3
0
0
0
6
3.7
5 2
0 0
0 0
0 0
0 0
5 2
3.1 1.2
5 1 2 1 1 0 1 1 0 1
2 2 0 2 1 1 1 0 0 0
0 2 1 0 0 1 0 0 1 0
0 0 0 0 0 0 0 0 0 0
0 1 0 0 0 0 0 0 0 0
7 6 3 3 2 2 2 1 1 1
4.4 3.7 1.8 1.8 1.2 1.2 1.2 0.6 0.6 0.6
2 0 1 1 0 0 1 0
0 0 0 0 1 1 0 1
1 2 0 0 0 0 0 0
1 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0
4 2 1 1 1 1 1 1
2.5 1.2 0.6 0.6 0.6 0.6 0.6 0.6
4 2 1 1 1 1 0 0
2 1 1 1 0 1 1 1
2 1 1 0 1 0 0 0
1 0 0 0 0 0 0 0
0 1 0 0 0 0 0 0
9 5 3 2 2 2 1 1
5.5 3.1 1.8 1.2 1.2 1.2 0.6 0.6
59
28
15
2
2
106
were present in 76 of the 91 patients with asplenia (83%) (Table I). Among those with polysplenia, 34 of 58 (58%) had bilaterally bilobed lungs with hyparterial bronchi. Of the remaining patients with polysplenia, 8 (14%) had solitus lobation, 5 (9%) had inversus, 4 (7%) were bilaterally trilobed, and 2 (3%) were bilaterally unilobed (Table I). Incidence of midline-associated anomalies: A total of 106 midline-associated anomalies were found in this collection of patients with heterotaxy. Defects most often occurred in the genitourinary (28 of 106 [26%]) and musculoskeletal (25 of 106 [24%]) systems (Table IIIA). Nearly one fifth of the lesions (19 of 106) involved craniofacial structures and 11% (12 of 106) occurred in the nervous system. Overall, 38% of patients with heterotaxy had a midline-associated defect (60 of 160) (Table IIIB). The incidence varied among heterotaxy subtype (Table IIIB) and was most reliable in subjects who had a complete autopsy (see below). More than one half
(52%, 11 of 21) of the complete autopsy cases of left-sided polysplenia had a midline defect, which differed significantly from cases of right-sided polysplenia or single right-sided spleen, of which 29% (6 of 21) and 18% (2 of 11), respectively, had midline defects (p ⫽ 0.05). In 45% of asplenia cases (33 of 73) a midline defect was noted. Type of autopsy: The observation of midline-associated defects was affected by the type of autopsy (Table IIIB). In 42% (53 of 127) of the cases where a complete autopsy was performed, a midline defect was noted; however, in cases when the autopsy was incomplete a midline defect was described only in 21% (7 of 33) (p ⫽ 0.02). Multiple anomalies: A total of 106 midline defects were found in 60 patients, indicating that some patients had ⬎1 defect. Patients with a complete autopsy were analyzed for the presence of a single midline defect or multiple midline defects. Of the 20 patients with polysplenia and a midline defect, 25% (5 of 20)
CONGENITAL HEART DISEASE/MIDLINE DEFECTS IN HETEROTAXY
731
these cases. Extracardiac defects involved almost all systems. Particular note was made of midline-associated defects (Table II). The incidence of midline defects depends on the subtype of heterotaxy:
There was a significant difference in the incidence of midline defects occurring in patients with left-sided polysplenia (52%), asplenia (45%), right-sided polysplenia (29%), and single rightsided spleen (18%) (Table IIIB). The argument has been put forth that asplenia and polysplenia may not be completely distinct entities,1 especially because there is genetic and phenotypic evidence that some crossover exists between these 2 diseases. Our results argue that, based on the incidence of midline defects, not only are polysplenia and asplenia distinct, but left-sided polysplenia is distinct from right-sided polysplenia. Similarly, the cardiovascuFIGURE 1. A, cardiac sidedness with D-loop ventricles; B, cardiac sidedness with lar defects differ significantly between L-loop ventricles; C, gastric situs in patients with solitus atria; D, gastric situs in polysplenia and asplenia (R. Van patients with inverted atria. Praagh and S. Litvosky, unpublished results). These data may point to unique, or at least separable, mechanisms for the etiologies of these syndromes. TABLE IIIA Frequency of Midline-Associated Defects in Patients With Heterotaxy by Organ System Genitourinary Musculoskeletal Craniofacial Gastrointestinal Nervous system Endocrine
28/106 25/106 19/106 13/106 12/106 9/106
(26%) (24%) (18%) (12%) (11%) (8%)
TABLE IIIB Prevalence of Midline-Associated Defects by Heterotaxy Subtype and by Type of Autopsy Asplenia (complete autopsy) Polysplenia-left (complete autopsy) Polysplenia-right (complete autopsy) Single right spleen Any associated midline defect Complete autopsy Incomplete autopsy
33/73 (45%) 11/21 (52%) 6/21 (29%) 2/11 (18%) 62/160 (39%) 53/127 (42%) 7/33 (21%)
had a single defect and 75% (15 of 20) had ⬎1 defect. This differed significantly from the asplenia cases where 58% (19 of 33) had an isolated midline defect and 42% (14 of 33) had multiple midline anomalies (p ⫽ 0.01). There was no association between visceral or atrial situs and the number of anomalies. Four patients with multiple anomalies had an identified syndrome or complex; 1 each had Turner, Goldenhaar, Trisomy 13, or Meckel-Gruber syndromes.
DISCUSSION The emphasis of this review of autopsied heterotaxy cases was on the noncardiac anomalies present in 732 THE AMERICAN JOURNAL OF CARDIOLOGY姞
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Patients with heterotaxy have a high incidence of midline-associated defects: Overall, we found that 38%
of patients with heterotaxy had at least 1 midlineassociated defect (Table IIIB). The frequency of midline defects in this heterotaxy population ranged from 10 to 60 times greater than the respective frequency of each lesion in the general population,8 suggesting that midline defects occur in heterotaxy patients much more often than would be expected by chance alone. A recent population study of infants with midline defects found that 95% of nonsyndromic infants had a single isolated midline defect.8 We find that in heterotaxy patients with midline defects, nearly one half of asplenics (44%) and most polysplenics (78%) have ⬎1 defect present. These data point to the need for further refinement in the study of the association of midline and laterality defects, particularly with respect to embryologic and pathogenetic mechanisms. The association of lateralization defects and midline anomalies has been previously described.9 –12 The incidence of extracardiac defects in our study was higher than that reported in another large review of extracardiac defects in asplenia cases.13 The latter review included nonautopsied cases and thus likely underrepresented the true incidence (see below). An earlier report of 29 necropsied asplenia cases from our institution6 found a similar rate of extracardiac defects as reported here. The relatively high incidence of midline-associated defects in patients with heterotaxy supports the theory that the midline plays a critical role in establishing normal left-right asymmetry.14 –16 According to Yost,17 cells in the midline, such as notochord cells, may act as sources of signals that MARCH 15, 2000
unpublished results]), and associated midline defects involving the vertebras,28 reminiscent of human aspleMouse Mouse nia syndrome. The results presented Human Mouse ActR ActRIIA ⫹/⫺ Nodal ⫹/⫺ in this study demonstrate the marked Asplenia IIB⫺/⫺ B⫺/⫺ ActR IIB⫺/⫺ similarity between the phenotypes of Cardiac defect 99% 49% 100% 100% patients with asplenia and the mice Dextrocardia 27% 16% 42% 35% with selective disruption of the acRespiratory tivin receptor-type II gene (Table Right lung isomerism 83% 100% 100% 100% Abnormal spleen 100% 34% 100% 100% IV). These similarities suggest that Craniofacial this molecule, or others in the TGF- Cleft lip/palate 8% 2% 50% 0 pathway, may be mutated in patients Agnathia 4% 0 10% 0 with asplenia. Gastrointestinal The genetics of human heterotaxy Stomach dextroposition 35% 0 5% 0 Omphalocele 2% 0 10% 0 are beginning to be determined. SevGenitourinary eral lines of evidence point to single Hypoplastic kidney 5% 19% 12% 0 gene defects.29,30 A mutation in the Absent kidney 2% 38% 79% 0 gene Zic3 has been linked to a subset Skeletal Abnormal ribs 6% 100% 100% 0 of patients with X-linked heteroSacral agenesis 0 0 58% 0 taxy.31 Recently 2 mutations in the gene LEFTY A were identified in ActR IIA ⫽ activin receptor IIA; Actr IIB ⫽ activin receptor IIB; ⫹/⫺ ⫽ heterozygous knockout; ⫺/⫺ ⫽ homozygous knockout. patients with left lung isomerism.32 Mutations in the human activin receptor-type IIB gene have been redirect left-right asymmetry, and may act as a barrier to ported in 3 patients with heterotaxy.33 Future studies maintain a gradient of left-right signaling molecules. are likely to reveal additional genes associated with Recent evidence demonstrates that the leftward flow human disorders of laterality. of extraembryonic fluid at the embryonic node is Cardiac position does not always predict ventricular essential for left-right determination.18 Disruption of looping: We found that heart-tube looping does not the midline barrier separating left from right may always correlate with the position of the heart in the permit a mixing of molecules that are normally asym- chest. Twenty percent of the heterotaxy patients with metrically distributed in early embryos, resulting in D-looped ventricles had dextrocardia (Figure 1A). abnormal organ sidedness later in development. Conversely, 30% of L-looped ventricles presented as The midline developmental field concept7 may ex- either meso- or levocardia (Figure 1B). These results plain the occurrence of apparently unrelated malfor- have particular pertinence for studies in other organmations in the same patient. Deficiencies in the mid- isms in which cardiac position or sidedness in the line field are likely to arise from single gene defects, thorax has been assumed to correlate with the direcas has been observed in mouse models of lateraliza- tion of ventricular looping.34 tion defects.19 A mutation in a gene, or combination of Atrial situs does not correlate with visceral situs: The genes, that is fundamental to midline development atrial situs can be unambiguously determined in most may cause a specific constellation of defects that over- cases, based on the lateralization of pulmonary and lap, with combinations of defects resulting from a systemic venous return. Although isomerization of the mutation in another gene in the same mechanistic atrial appendages or pectinate muscles can occur ocpathway. We must await definitive genotype-pheno- casionally in heterotaxy, they are not consistent findtype correlation to classify these disorders appropri- ings, and do not represent atrial isomerism (R. Van ately. Praagh, unpublished results). Our findings support the The phenotype of mice lacking the activin receptor IIB hypothesis that atrial situs is not necessarily associated is similar to human asplenia syndrome: Several mouse with visceral situs. In 62% of heterotaxy patients with models of heterotaxy have been investigated.19 normal atrial situs, the stomach had normal situs. Among these is the iv mouse that arises due to a When the atria were inverted, the stomach was inmutation of the gene left-right dynein,20,21 and the inv verted in 67% of the cases. Although our data show mouse, which has a defect in an ankyrin-repeat pro- that there is no obligatory link between cardiac looping and cardiac position, or between atrial situs and tein.22–25 Members of the transforming growth factor  visceral situs, they do argue against the notion that (TGF-) family function in a molecular cascade that these processes are completely randomized in heteroestablishes organ asymmetry early in develop- taxy. D-looping, normal atrial situs, and normal visment.14,26 Activin receptor IIB is thought to be a ceral situs each occurred in approximately two thirds receptor for 2 of these TGF- family members, nodal of patients, rather than in half of the population. Simiand activin.27 Mice with a targeted mutation of the larly, levocardia occurred in two thirds of patients, inactivin receptor IIB gene display abnormal spleens, stead of occurring in equal distribution with meso- and right pulmonary isomerism, cardiac anomalies (in- dextrocardia. One could argue that the incidence may cluding common AV canal [B. Ticho, S.P. Oh, E. Li, appear more random if all fetuses were included, since TABLE IV Similarity Between Asplenia Syndrome and Mouse Disruption of Activin Receptors
CONGENITAL HEART DISEASE/MIDLINE DEFECTS IN HETEROTAXY
733
fetuses with more disordered left-right asymmetry would be less likely to survive to birth. This did not appear to be the case for the small number of fetuses examined in this review (100% [3/3] had levocardia). The frequency of central nervous system defects implies effects on central nervous system situs: The finding
of central nervous system defects among heterotaxy patients supports the concept that early embryonic events may affect cerebral structural asymmetry and midline central nervous system structures. A recent study35 supports the existence of a cerebral situs. Magnetic resonance imaging brain scans demonstrate that cerebral situs can be determined by anatomic asymmetry and that it is reversed in patients with situs inversus totalis. It appears that factors that dictate visceral and thoracic situs also affect cerebral situs. Implications for the genetic analysis of heterotaxy syndromes: Efforts at identifying the genes responsible
for aberrant asymmetry require a complete characterization of these disorders, including abnormalities of organ positioning and all associated congenital defects. Such a thorough phenotypic characterization allows precise classification of patients as well as a clearer understanding of animal models in the context of human disorders of asymmetry. Some of the midline-associated defects noted in heterotaxy patients were subclinical and were found only at autopsy. Table III shows that in the patients who had incomplete autopsies the noted incidence of these defects was half that of patients in whom a complete autopsy was performed. This discrepancy is most likely explained by the fact that in these cases, data on midline defects were mostly dependent on clinical records. This implies that these defects may not be noted in up to half of the cases of affected persons. As a result, pedigree studies using linkage analysis to identify loci associated with heterotaxy syndromes may incorrectly identify family members as unaffected when based solely on usual clinical examinations. Assessment of disease status in living patients would have to rely on magnetic resonance imaging, or other imaging modalities, to identify midline-associated defects in these otherwise unaffected persons. Implications for the clinical management of heterotaxy patients: The relatively high frequency of urogenital
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