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Intrauterine growth restriction—genetic causes and consequences
Seminars in Fetal & Neonatal Medicine (2004) 9, 371e378
www.elsevierhealth.com/journals/siny
Intrauterine growth restrictiondgenetic causes and consequences David Monk, Gudrun E. Moore) Institute of Developmental and Reproductive Biology, Imperial College London, Hammersmith Hospital, Du Cane Road, London W12 0NN, UK
KEYWORDS Intrauterine growth restriction; Small for gestational age; Fetal programming; Genomic imprinting; Uniparental disomy
Summary Intrauterine growth restriction is known to be associated with many medical problems for the baby, both before and after delivery. The mechanisms involved in fetal growth are not well understood, with an increasing range of metabolic diseases being implicated. Several key genes involved in normal embryonic and fetal growth and development are now known to be imprinted. Disruption of this parent-specific mono-allelic expression causes phenotypic changes, many of which are important for growth and development. Two growth disorders, Beckwithe Wiedemann syndrome and SilvereRussell syndrome, are discussed in detail as they represent well-characterized phenotypes that arise as a consequence of disrupted imprinting. These human models will allow us to elucidate key genes and mechanisms important in normal fetal growth. ª 2004 Elsevier Ltd. All rights reserved.
Introduction Each year around 20% of the 4 million babies born in the USA are born at the high and low extremes of fetal growth. Classically, low birth weight is defined as less than 2500 g. However, using these criteria, babies born prematurely are also included in this classification.1 The National Institute of Health estimates that 6e10% of the babies with birth weights below 2500 g are born at term, having suffered abnormal intrauterine growth. As will be discussed in this review, these babies are at increased risk of developing both chronic and fatal adult diseases through the effects of fetal intrauterine programming.2 Approximately 10% of births in the USA ) Corresponding author. Tel.: C44-20-759-42125; fax: C4420-759-42129. E-mail address: [email protected] (G.E. Moore).
have a birth weight above 4000 g and are termed macrosomic; however, unless these babies suffer from a defined genetic syndrome, they are not associated with any increased morbidity or mortality.
Normal fetal growth Normal human fetal growth patterns can be subdivided into three consecutive stages. The first stage of fetal growth occurs throughout the first trimester, up to 20 weeks’ gestation. This initial phase is associated with rapid mitosis, and therefore increasing cell number. The second phase, corresponding to the second trimester, includes both hyperplasia and hypertrophy. From 28 weeks’ gestation to term, there is a rapid increase in cell size with fat, muscle and connective tissue accumulation. The corresponding fetal growth rates during
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372 these three phases are 5 g/day at 15 weeks, 15e20 g/day at 24 weeks and 30e35 g/day at 34 weeks. Therefore, most fetal weight gain occurs in the last 20 weeks of gestation.3
Intrauterine growth restriction Intrauterine growth restriction (IUGR) is recognized as the failure of an infant to achieve his or her genetic growth potential in utero, and therefore failure to grow along a consistent centile is more important than absolute size. In other words, a fetus with an abdominal circumference on the 90th centile at 28 weeks’ gestation and the 50th centile at 36 weeks’ gestation is more likely to be growth restricted than a fetus which is on the 5th centile throughout pregnancy. The latter baby is termed ‘small for gestational age’ (SGA). The terms ‘IUGR’, ‘SGA’ and ‘low birth weight’ are not synonymous but there is considerable overlap between them. Low birth weight is defined by the World Health Organization as a birth weight below 2500 g but does not correct for gestational age. SGA describes a fetus or baby with growth parameters below a defined centile (normally the 10th) for gestational age that is not pathologically growth restricted; these babies are small simply because of constitutional factors. An IUGR baby is an SGA baby but not all SGA babies are growth restricted. IUGR can be subdivided into symmetric and asymmetric fetal growth, and this can give more insight into the onset and aetiology of the causative fetal insult. Head-to-abdominal circumference ratios (HC/AC) have been used to differentiate fetuses into subtypes based on those that are proportionally small (symmetrical) or those with relative head sparing (asymmetrical). Approximately 75% of all IUGR cases show asymmetric growth patterns, and these pregnancies are at significantly higher risk of severe pre-eclampsia, fetal distress, operative intervention and low neonatal Apgar scores compared with their symmetrical counterparts. Symmetrical IUGR is likely to result from an early fetal insult due to chemical exposure (e.g. nicotine from cigarette smoking reduces birth weight by 175e300 g), viral infection or inherent developmental abnormalities resulting from aneuploidy. These will result in a proportionate reduction in fetal measurements. Conversely, asymmetrical IUGR will lead to disproportionate reduction in fetal measurements due to uteroplacental insufficiency with preferential shunting of blood to the fetal brain.3
D. Monk, G.E. Moore
Implications of IUGR Fetuses that suffer IUGR have a perinatal mortality 12 times higher than normal-birth weight babies.3 These babies also have the immediate problems of hypothermia, hypoglycaemia, pulmonary haemorrhage and encephalopathy compared with babies with birth weights appropriate for gestational age. Later in life, IUGR babies that show catch-up growth are also at greater risk of heart disease, hypertension and type 2 diabetes.4 Epidemiological studies by Barker2 suggested that a range of metabolic diseases may be associated with the fetal environment. The mechanisms underlying these are not clear. Research using animal models is currently being undertaken to address this. Extensive work in which pregnant female rats are fed a diet of moderately restricted proteins results in reduction in birth weight of all pups by approximately 15%. Male growth-restricted rats that were cross-fostered shortly after birth to normal lactating females exhibited catch-up growth, but showed reduced longevity.5 These data back the human epidemiological studies suggesting that catch-up growth is potentially beneficial in the short term, but may be detrimental to long-term fitness and survival.
Aetiology of the extremes of fetal growth Fetal growth is a complex, dynamic process controlled by a wide range of factors of maternal, placental and fetal origin (Table 1). As a result, the aetiology of IUGR and macrosomia is often multifactorial, involving both genetic and environmental factors to a varying degree. Although many factors have been implicated in the process of fetal growth, the precise molecular and cellular mechanisms by which normal fetal growth occurs are still not well understood. In early fetal life, the major determinant of growth is the fetal genome, but later in pregnancy, environmental, nutritional and hormonal influences become increasingly important. For the remainder of this review, we will concentrate on the genetic control of intrauterine growth.
Fetal chromosomal abnormalities There is strong association between IUGR, chromosome aberrations and congenital malformations. Fetuses with chromosome disorders, including the common trisomies of chromosomes 13, 18 and 21,
Genetic causes and consequences of IUGR Table 1
373
The known clinical, genetic and environmental factors associated with IUGR
Fetal
Maternal
Placental
Chromosomal abnormality e Aneuploidy
Substance abuse e Smoking e Alcohol e Drugs Chronic disease Constitutionally small mother Lack of second-trimester weight gain
Partial placental separation
Multifactorial congenital malformations Multiple fetus pregnancy Infection e Malaria Aberrant genomic imprinting e Uniparental disomy e Epimutations
Placental infarction Placenta previa Uteroplacental insufficiency
High-altitude pregnancy e Hypoxia
Small placenta
Malnutrition
Chronic vascular disease Pre-eclampsia
are frequently growth restricted, and suboptimal growth is also reported for many autosomal abnormalities such as duplications, deletions and ring chromosomes. It is thought that an abnormal fetal karyotype is responsible for approximately 20% of all IUGR fetuses, and the percentage is substantially higher if growth failure is detected before 26 weeks’ gestation.6 It is likely that the compromised karyotype impairs normal cell division leading to a reduction in cell number and fetal growth.
Genomic imprinting The vast majority of autosomal genes are expressed from both parental alleles, which are functionally equivalent. However, for a small group of genes that are imprinted, only one of the alleles is expressed in a parent-of-origin-dependent manner. The first clues to the existence of genomic imprinting in mammals came from a small number of key experiments carried out in the mid-1980s. Firstly, nuclear transfer experiments in mice made it possible to produce embryos carrying chromosomes derived solely from one parent. Gynogenetic embryos were created by replacing the paternal pronucleus with a maternal pronucleus, whereas replacing the female pronucleus with paternally derived material resulted in androgenetic embryos. Even though the two types of conceptuses contained the normal diploid amount of DNA, neither were viable at term. The critical observation from these reconstructed diploid embryos was that androgenetic embryos were associated with relatively well-formed extra-embryonic tissue with little or no embryo, whereas gynogenetic embryos showed poorer extra-embryonic tissue development and a more apparent embryo.7,8 Naturally occurring examples have been found in humans. Ovarian
teratomas that occur at the rate of one in 10 000 ovulations contain only maternal chromosomes, and are thought to result from oocytes activated in a pathogenetic fashion. Teratomas are classically embryonic in origin containing components from all three germ layers. Teratomas have been found to contain skin, hair and bone-like structures, but have no extra-embryonic components. In contrast, hydatidiform moles, which arise from two sperms in an empty sac, result in an abundance of placental tissue but lack a fetal component. These paternal uniparental hydatiform moles show aggressive invasive properties, indicating that placental development depends on the expression of specific genes on the paternal chromosomes whereas the development of the embryo requires the maternal genome.9 Subsequent experiments by Cattanach and Kirk10 and Searle and Beechey11 extended these findings by demonstrating that the hypothetical imprinted genes could be localized to specific chromosomal regions. Using mice that were heterozygous for Robertsonian translocations, and taking advantage of non-disjunction and gamete complementation, mice were produced that had both chromosomes derived from a single parent, a phenomenon called uniparental disomy (UPD). Intercrosses of heterozygotes with reciprocial translocations were used to narrow these imprinted whole disomic chromosomes down further, to define subchromosomal regions of the chromosomes that displayed specific phenotypic anomalies. This is in contrast to the nuclear transfer experiments where all chromosomes were uniparentally derived. In the mouse studies, severe phenotypic differences were observed with maternal vs paternal origin of the UPD. These phenotypes are often manifested as overgrowth or growth restriction, as well as behavioural problems. However, not all chromosomes produced abnormal phenotypes
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when presented as UPD, suggesting that imprinted genes are not uniformly distributed throughout the genome ( for a comprehensive review of imprinting phenotypes, see Ref.12).
UPD in humans UPD is probably the most common phenomenon involved in exposing imprinting effects in humans. The theory that emerged in 1980 was that UPD probably arose as a consequence of the high rate of germ cell aneuploidy found in man; this can be as high as 50% per conception (see section on mechanism below).13 Each individual normally inherits a chromosome from each parent, which were themselves inherited from each grandparent. UPD, arising from one parent alone, may result in isodisomy, in which the uniparental pairs consist of two copies of a single parental or grandparental homologue. This may lead to an abnormal phenotype by increasing the risk of recessive disorders by reduction to homozygosity and also through disrupted imprinting effects. Alternatively, the UPD may be contributed to by both grandparental homologues known as heterodisomy.13 In heterodisomy, the abnormal chromosome pair may cause disruption of genomic imprinted gene expression alone resulting in distinct phenotypes (Table 2). There are several mechanisms that could give rise to UPD, but most common is trisomic rescue involving the loss of a supernumerary chromosome from a trisomic conceptus, leaving two homologues from the same gametes ( for an overview of mechanisms leading to UPD, see Robinson14). Trisomic rescue would account for the finding of confined placental mosaicism (CPM) where the karyotype in part of the placenta is different to the fetus. This finding is not exceptional, as CPM is found in approximately 1e2% of routine chorionic villous biopsies (CVS). Trisomy 16 is the most common trisomy found in CPM associated with IUGR, and as a Table 2
consequence, chromosome 16 was one of the first candidate chromosomes to be studied with respect to IUGR. If CPM is found during routine CVS for prenatal diagnosis that involves a chromosome known to harbour imprinted genes, it is important to check for UPD in the fetus. However, it is not always clear whether the observed IUGR is due to a dysfunctional mosaic placenta or as a consequence of fetal UPD and aberrant imprinted gene expression. For example, in a systematic study of 35 idiopathic IUGR cases, two cases of UPD16 associated with CPM 16 were found, both with an imperforate anus.15 However, placental trisomy for chromosome 16 affects intrauterine development of both UPD16 and biparental diploid fetuses (normal inheritance of both chromosome 16s) in a similar way, causing severe IUGR and making it difficult to divide up the effect of imprinting from having a mosaic placenta. The higher the proportion of trisomic cells in the placenta, the more severe the effects on the fetus.16 In contrast, there is evidence that intrauterine growth is not affected by the presence of a trisomic cell line for chromosome 7 in the placenta, since biparental chromosome 7 diploid fetuses with a CPM 7 placenta exhibit normal growth parameters. Significant growth restriction is only seen in cases with CPM7 and maternal UPD7.17 This, therefore, makes it unlikely that CPM is responsible for the significant growth restriction seen in cases with mUPD7.
Fetal growth, genomic imprinting and UPD Observations from UPD in humans and mice imply that genomic imprinting has relevance to fetal growth and its associated abnormalities. The early studies on mouse imprinting phenotypes showed that imprinted genes play a major role in growth. The majority of subchromosomal regions showing imprinting phenotypes are related to fetal growth, placental size and early embryonic, neonatal or
Selected human uniparental disomy (UPD) phenotypes
Chromosome
Parental origin
Syndrome
6 7 11 (segmental) 14 15
Paternal Maternal Paternal Maternal Maternal Paternal Maternal
Transient neonatal diabetes mellitus Severe IUGR; SilvereRussell syndrome BeckwitheWiedemann syndrome Hypotonia, motor delay, IUGR PradereWilli syndrome Angelman syndrome Severe IUGR Albrights hereditary osteodystrophy (phenotype modified depending on parental origin)
16 (associated with CPM16) 20 IUGR, intrauterine growth restriction.
Genetic causes and consequences of IUGR postnatal lethality, with paternal disomies promoting growth and maternal disomies inhibiting growth.12,18 Furthermore, chimeric embryos formed from normal and gynogenetic embryos tend to be smaller than normal litter mates. Conversely, androgenetic chimeras are larger than expected.19 Over 50 imprinted genes have now been identified.12 A large number of these are involved in growth regulation,20 although behavioural phenotypes are also becoming apparent.21,22 Imprinted genes such as insulin (Ins2), cyclin-dependent kinase inhibitor 1C ( p57kip2), guanine nucleotide binding protein (Gnas) and achaete-scute homologue 2 (Mash2) are known to regulate fetal growth or cell-cycle progression.23,24 The effect of several other imprinted genes on the fetal growth axis is mediated by insulin-like growth factor II (Igf2). These genes include Igf2 itself, Igf2 receptor (Igf2r), H19 and growth factor receptor bound protein 10 (Grb10). There has been much speculation surrounding the biological significance of the imprinting of Igf2 and Igf2r, since both these products interact physically. Both genes are reciprocally imprinted in that Igf2 is expressed from the paternal allele, whereas the Igf2r transcripts are derived from the maternal allele. Targeted mutations of both genes have been shown to result in abnormal fetal growth.25e27 The balance between the two gene products is obviously important as the lethal maternal phenotype associated with proximal mouse chromosome 17 that results from a lack of Igf2r is known to be rescued by a second mutation in Igf2 that is paternally inherited.28 Based on their known functions (Igf2 as a growth enhancer and Igf2r as a growth regulator, via Igf2 binding and degradation), such reciprocal imprinting and mechanical interaction makes evolutionary sense if selection forces apply. This complementary imprinted expression, in which growth enhancers are paternally expressed and growth regulators are maternally expressed, underpins the basis of the Haig parental conflict hypothesis.29 This conflict theory suggests that the paternal genome wants to enhance fetal growth at the expense of the mother whereas the mother wants to minimize fetal growth to protect her own survival to reproduce again. The father can reproduce again with a different mother.
375 the SilvereRussell syndrome (SRS), which leads to growth restriction.
BeckwitheWiedemann syndrome The mechanisms that lead to in utero overgrowth prove as elusive as those for IUGR. However, with the generation of experimental mouse models, and detailed clinical and molecular genetic analysis of children with overgrowth syndromes, the genetic mechanisms are slowly being revealed. Transgenic mice with manipulated expression of genes for insulin-like growth factor, insulin and their receptors have highlighted this pathway as being essential for normal growth, and fetal somatic overgrowth often occurs when the ligands are present in excess. Igf2 is normally a paternally expressed imprinted gene;30 however, re-activation of the maternal allele (i.e. loss of imprinting) results in BWS.31 BWS is characterized by pre- and postnatal overgrowth, macroglossia, abnormal digits, organomegaly, hemihypertrophy and a predisposition to childhood cancers such as Wilms tumour.32 Whilst the majority of cases are sporadic, familial inheritance patterns have been reported in the literature. Rare cytogenetic re-arrangements have given valuable clues to the exact regions involved, with 20% of BWS patients presenting with segmental pUPD1133 and 2% having small duplications of chromosome 11p, invariably derived from the father, which resulted in overexpression of Igf2. Maternally inherited re-arrangements localize to distinct breakpoint cluster regions and are indicative of an imprinting effect and growth restriction. Loss of imprinting of Igf2 is the most common molecular defect seen in sporadic patients. However, in 40% of familial cases, the only identifiable alteration is maternal mutations of p57kip2.34 A phenotypically related overgrowth syndrome, SimpsoneGolabieBehmel syndrome, is caused by mutations in the X-linked glypican-3 gene (Gpc3).35 The glypican-3 protein is thought to interact with the gene products important in BWS. Information gathered from targeted deletions of p57kip2 and Gpc3 and overexpression of Igf2 in the mouse has helped to characterize the molecular basis for these disorders.36
Specific growth phenotypes associated with human syndromes
SilvereRussell syndrome
Examples of well-characterized phenotypes that arise as a consequence of disrupted imprinting are the BeckwitheWiedemann syndrome (BWS), which is associated with fetal overgrowth, and
In contrast to the fetal overgrowth seen in BWS, SRS is a condition where IUGR and reduced postnatal growth are associated with other dimorphic features, including characteristic facies, limb and
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truncal asymmetry, striking lack of subcutaneous fat and relative macrocephaly. SRS cases are sporadic, but there are several published pedigrees that support autosomal-dominant and -recessive inheritance in some families. Recently, approximately 10% of SRS cases have been associated with mUPD737 without any underlying fetal mosaicism.38 A definite role for an imprinting error and not the unmasking of a recessive allele was demonstrated by the lack of a consistent region of isodisomy across five mUPD7 SRS patients.39 Further evidence for an involvement of human chromosome 7 in SRS comes from the reports of numerous SRS patients with cytogenetic disruptions of 7p11.2ep13, including several duplications that are of maternal origin.40 A second SRS candidate gene region on human chromosome 7 has been mapped to 7q32-qter, as a result of a single patient with segmental mUPD7 for this region.41 Interestingly, both domains correspond with chromosomal regions in the mouse that are known to contain imprinted genes. When these genes are maternally inherited, this results in severe growth restriction that mimics the human SRS phenotype.
Mouse models for IUGR The most valuable source of information about the function of imprinted genes has been from work on Table 3 Imprinted gene
targeted mutations in the mouse. Fetal growth restriction in these circumstances may be the result of lack of expression of the targeted gene in the fetus or placenta or a combination of both. Therefore, data relating to gene function must be carefully interpreted. Recent work in the mouse has indicated that the roles of imprinted genes in the fetal and placental compartments can be genetically separated, and that in the placenta, these genes regulate both growth and specific nutrient transfer.41 Of the 200 expected genes in the eutherine genome, a substantial proportion are predicted to be involved in the control of fetal growth, in which paternally expressed imprinted genes enhance growth, whereas maternally expressed genes suppress it. This paternal vs maternal genome tug-of-war is the basis of the genetic conflict hypothesis, where paternally derived genes influence nutrient acquisition so selecting more nutrients from the mother, and maternally derived genes try and balance the provision of nutrients to the current fetus with that of potential future pregnancies and protect their own survival to reproduce again.29 In fitting with the Haig hypothesis of genetic conflict between parental genomes, knockouts of paternally expressed genes such as Igf2, mesoderm-specific transcript (Peg1/Mest), Peg3 and insulin all result in IUGR, whereas the reverse
Placental and embryonic expression of known mouse imprinted genes Knockout growth phenotype
Paternally expressed Igf2 P0 C Peg1 C Peg3 C Insulin C Ata3 Maternally expressed Grb10 P57kip2 C Igf2r H19 C Ipl C Mash2 Slc22a2 Slc22a3 Obph1 Napl14 Dcn Tssc4 Tnfrh1 Gatm
Placental expression
Knockout placental phenotype
Spongiotrophoblast
Labyrinthine trophoblast
C
C
C
C
C C C
C C C C
C
C C C C C C C C C
C
C C
C
C
Physiological role
C
C
C C C
C
Genetic causes and consequences of IUGR situation is observed for the null mice for H19 and Igf2r which are maternally expressed and result in overgrowth of the fetus.24 Recently, it has been shown that many imprinted genes are expressed in the placenta and fetus, and that a subset are only imprinted in the placenta and are biallelic and therefore not imprinted in the fetus (Table 3, see Reik et al.42 for review of literature). The most studied and best-characterized imprinted gene is probably Igf2. The Igf2 protein has been shown to be a major modulator of fetal and placental size. In the mouse, expression of Igf2 is regulated by four distinct promoters. Promoters P1e3 are classified as fetal promoters. The Igf2 gene also has a placental-specific isoformdP0. Targeted deletions of the P0 promoter, by removing the promoter and first exon, result in a P0 null; however, all the other promoters are unaffected and transcriptionally active. The P0 null mice present with a phenotype virtually indistinguishable from the Igf2 total null, each having 30% growth restriction compared with wild-type litter mates. The P0 null animals have a smaller than normal placenta at Day E16 of gestation, which reduces the nutrient transfer capacity of the placenta. However, once the placental constraints on growth are lifted at birth, the P0 knockout mice show catch-up growth,43 a scenario with the highest risk of metabolic disorders as outlined by the intrauterine fetal programming hypothesis.2,5 The key observation outlined in the elegant murine model described above may also relate to humans. Mutations or epigenetic aberrations resulting in disrupted placental-specific imprinted gene expression may result in IUGR. After birth, when placental constraints on growth are lifted, catch-up growth would be expected, again leading to an increased risk of metabolic syndromes such as diabetes and hypertension that are only prevalent later in life.
Research agenda Find the genes responsible for macrosomia and IUGR, and ascertain the molecular mechanisms involved. Develop accurate early diagnosis for aberrant fetal growth. Develop early in utero therapies for the prevention of IUGR. Define the role of placental-specific imprinting in normal placental development and IUGR.
377
References 1. Available from: http://www.modimes.org/. 2. Barker DJ. The fetal and infant origins of disease. Eur J Epidemiol 1995;25:457e63. 3. Cunningham FG, Gant NF, Leveno KJ. Fetal growth disorders. In: Cunningham FG, Gant NF, Leveno KJ, editors. Williams obstetrics. 21st ed. New York: Appleton and Lange; 1997. p. 743e64. 4. Reece AE, Hagay Z. Medicine of the fetus and mother. In: Reece AE, Hobbins JC, editors. Prenatal diagnosis of deviant fetal growth. Philadelphia: Lippincourt-Raven; 1999. 5. Hales CN, Ozanne SE. For debate: fetal and early postnatal growth restriction lead to diabetes, the metabolic syndrome and renal failure. Diabetologia 2003;46:1013e9. 6. Snijders RJ, Sherrod C, Gosden CM, Nicolaides KH. Fetal growth retardation: associated malformations and chromosomal abnormalities. Am J Obstet Gynecol 1993;168:547e55. 7. Surani MA, Barton SC, Norris ML. Development of reconstituted mouse eggs suggests imprinting of the genome during gametogenesis. Nature 1984;308:548e50. 8. Mcgrath J, Solter D. Completion of mouse embryogenesis requires both the maternal and paternal genomes. Cell 1984;37:179e83. 9. Lindor NM, Ney JA, Gaffey TA, Jenkins RB, Thibodeau SN, Dewald GW. A genetic review of complete and partial hydatidiform moles and nonmolar triploidy. Mayo Clin Proc 1992;67:791e9. 10. Cattanach BM, Kirk M. Differential activity of maternally and paternally derived chromosome regions in mice. Nature 1985;315:496e8. 11. Searle AG, Beechey CV. Genome imprinting phenomena on mouse chromosome 7. Genet Res 1990;56:237e44. 12. Available from: http://www.mgu.har.mrc.ac.uk/imprinting/ imprinting.html. 13. Engel E. A new genetic concept: the uniparental disomy and its potential effect, the isodisomy. J Genet Hum 1980;28: 11e22. 14. Robinson WP. Mechanisms leading to uniparental disomy and their clinical consequences. Bioessays 2000;22:452e9. 15. Moore GE, Ali Z, Khan RU, Blunt S, Bennett PR, Vaughan JI. The incidence of uniparental disomy associated with intrauterine growth retardation in a cohort of thirty-five severely affected babies. Am J Obstet Gynecol 1997;176: 294e9. 16. Kalousek DK, Langlois S, Barrett I, Yam I, Wilson DR, HowardPeebles PN. Uniparental disomy for chromosome 16 in humans. Am J Hum Genet 1993;52:8e16. 17. Kalousek DK, Langlois S, Robinson WP, Telenius A, Bernard L, Barrett IJ. Trisomy 7 CVS mosaicism: pregnancy outcome, placental and DNA analysis in 14 cases. Am J Hum Genet 1994;65:348e52. 18. Cattanach BM, Shibata H, Hayashizaki Y, Townsend KM, Ball S, Beechey CV. Association of a redefined proximal mouse chromosome 11 imprinting region and U2afbp-rs/U2af1-rs1 expression. Cytogenet Cell Genet 1998;80:41e7. 19. Barton SC, Ferguson-Smith AC, Fundele R, Surani MA. Influence of paternally imprinted genes on development. Development 1991;113:679e87. 20. Morison IM, Reeve AE. A catalogue of imprinted genes and parent-of-origin effects in humans and animals. Hum Mol Genet 1998;7:1599e609. 21. Lefebvre L, Viville S, Barton SC, Ishino F, Keverne EB, Surani MA. Abnormal behaviour and growth retardation associated with loss of the imprinted gene Mest. Nat Genet 1998;20: 163e9.
378 22. Murphy SK, Wylie AA, Jirtle RL. Imprinting of PEG3, the human homologue of a mouse gene involved in nurturing behavior. Genomics 2001;71:110e7. 23. Reik W, Walter J. Evolution of imprinting mechanisms: the battle of the sexes begins in the zygote. Nat Genet 2001;27: 255e60. 24. Tycko B, Morison IM. Physiological functions of imprinted genes. J Cell Physiol 2002;192:245e58. 25. DeChiara TM, Efstratiadis A, Robertson EJ. A growthdeficiency phenotype in heterozygous mice carrying an insulin-like growth factor II gene is disrupted by targeting. Nature 1990;345:78e80. 26. Lau MM, Stewart CE, Liu Z, Bhatt H, Rotwein P, Stewart CL. Loss of the imprinted IGF2/cation-independent mannose 6-phosphate receptor results in fetal overgrowth and perinatal lethality. Genes Dev 1994;8:2953e63. 27. Leighton PA, Ingram RS, Eggenschwiler J, Efstratiadis A, Tilghman SM. Disruption of imprinting caused by a deletion of the H19 region in mice. Nature 1995;375:34e9. 28. Filson AJ, Louvi A, Efstratiadis A, Robertson EJ. Rescue of the T-associated maternal effect in mice carrying null mutations in Igf-2 and Igf2r, two reciprocally imprinted genes. Development 1993;118:731e6. 29. Moore T, Haig D. Genomic imprinting in mammalian development: a parental tug of war. Trends Genet 1991;7:45e9. 30. Giannoukakis N, Deal C, Paquette J, Goodyer CG, Polychronakos C. Parental genomic imprinting of the human IGF2 gene. Nat Genet 1993;4:98e101. 31. Weksberg R, Shen DR, Fei YL, Song QL, Squire J. Disruption of insulin-like growth factor-2 in Beckwith Wiedemann syndrome. Nat Genet 1993;5:143e9. 32. Engel JR, Smallwood A, Harper A, Higgins MJ, Oshimura M, Reik W. Epigenotypeephenotype correlations in Beckwithe Wiedemann syndrome. J Med Genet 2002;37:921e6. 33. Henry I, Bonaiti-Pellie C, Chehensse V, Beldjord C, Schwartz C, Uterman G. Uniparental disomy in a genetic cancerpredisposing syndrome. Nature 1991;351:665e7.
D. Monk, G.E. Moore 34. Maher ER, Reik W. BeckwitheWiedemann syndrome: imprinting in clusters revisited. J Clin Invest 2000;105: 247e52. 35. Pilia G, Hughes-Benzie RM, MacKenzie A, Baybayan P, Chen EY, Huber R. Mutations in GPC3, a glypican gene, cause the SimpsoneGolabieBehmel overgrowth syndrome. Nat Genet 1996;12:241e7. 36. Li M, Squire JA, Weksberg R. Overgrowth syndromes and genomic imprinting: from mouse to man. Clin Genet 1998; 53:165e70. 37. Kotzot D, Schmitt S, Bernasconi F, Robinson WP, Lurie IW, Ilyina H. Uniparental disomy 7 in SilvereRussell syndrome and primordial growth retardation. Hum Mol Genet 1995;4: 583e7. 38. Monk D, Hitchins M, Russo S, Preece M, Stanier P, Moore GE. No evidence for mosaicism in SilvereRussell syndrome. J Med Genet 2001;38:E11. 39. Preece MA, Abu-Amero SN, Ali Z, Abu-Amero KK, Wakeling EL, Stanier P. An analysis of the distribution of hetero- and isodisomic regions of chromosome 7 in five mUPD7 Silvere Russell syndrome probands. J Med Genet 1999;36:457e60. 40. Monk D, Bentley L, Hitchins M, Myler RA, Clayton-Smith J, Ismail S. Chromosome 7p disruptions in SilvereRussell syndrome: delineating an imprinted candidate gene region. Hum Genet 2002;111:376e87. 41. Hannula K, Lipsanen-Nyman M, Kontiokari T, Kere J. A narrow segment of maternal uniparental disomy of chromosome 7q31-qter in SilvereRussell syndrome delimits a candidate gene region. Am J Hum Genet 2001;68:247e53. 42. Reik W, Constancia M, Fowden A, Anderson N, Dean W, Ferguson-Smith A. Regulation of supply and demand for maternal nutrients in mammals by imprinted genes. J Physiol 2003;547:35e44. 43. Constancia M, Hemberger M, Hughes J, Dean W, FergusonSmith A, Fundele R. Placental-specific IGF-II is a major modulator of placental and fetal growth. Nature 2002;417: 945e8.
www.elsevierhealth.com/journals/siny
Intrauterine growth restrictiondgenetic causes and consequences David Monk, Gudrun E. Moore) Institute of Developmental and Reproductive Biology, Imperial College London, Hammersmith Hospital, Du Cane Road, London W12 0NN, UK
KEYWORDS Intrauterine growth restriction; Small for gestational age; Fetal programming; Genomic imprinting; Uniparental disomy
Summary Intrauterine growth restriction is known to be associated with many medical problems for the baby, both before and after delivery. The mechanisms involved in fetal growth are not well understood, with an increasing range of metabolic diseases being implicated. Several key genes involved in normal embryonic and fetal growth and development are now known to be imprinted. Disruption of this parent-specific mono-allelic expression causes phenotypic changes, many of which are important for growth and development. Two growth disorders, Beckwithe Wiedemann syndrome and SilvereRussell syndrome, are discussed in detail as they represent well-characterized phenotypes that arise as a consequence of disrupted imprinting. These human models will allow us to elucidate key genes and mechanisms important in normal fetal growth. ª 2004 Elsevier Ltd. All rights reserved.
Introduction Each year around 20% of the 4 million babies born in the USA are born at the high and low extremes of fetal growth. Classically, low birth weight is defined as less than 2500 g. However, using these criteria, babies born prematurely are also included in this classification.1 The National Institute of Health estimates that 6e10% of the babies with birth weights below 2500 g are born at term, having suffered abnormal intrauterine growth. As will be discussed in this review, these babies are at increased risk of developing both chronic and fatal adult diseases through the effects of fetal intrauterine programming.2 Approximately 10% of births in the USA ) Corresponding author. Tel.: C44-20-759-42125; fax: C4420-759-42129. E-mail address: [email protected] (G.E. Moore).
have a birth weight above 4000 g and are termed macrosomic; however, unless these babies suffer from a defined genetic syndrome, they are not associated with any increased morbidity or mortality.
Normal fetal growth Normal human fetal growth patterns can be subdivided into three consecutive stages. The first stage of fetal growth occurs throughout the first trimester, up to 20 weeks’ gestation. This initial phase is associated with rapid mitosis, and therefore increasing cell number. The second phase, corresponding to the second trimester, includes both hyperplasia and hypertrophy. From 28 weeks’ gestation to term, there is a rapid increase in cell size with fat, muscle and connective tissue accumulation. The corresponding fetal growth rates during
1744-165X/$ - see front matter ª 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.siny.2004.03.002
372 these three phases are 5 g/day at 15 weeks, 15e20 g/day at 24 weeks and 30e35 g/day at 34 weeks. Therefore, most fetal weight gain occurs in the last 20 weeks of gestation.3
Intrauterine growth restriction Intrauterine growth restriction (IUGR) is recognized as the failure of an infant to achieve his or her genetic growth potential in utero, and therefore failure to grow along a consistent centile is more important than absolute size. In other words, a fetus with an abdominal circumference on the 90th centile at 28 weeks’ gestation and the 50th centile at 36 weeks’ gestation is more likely to be growth restricted than a fetus which is on the 5th centile throughout pregnancy. The latter baby is termed ‘small for gestational age’ (SGA). The terms ‘IUGR’, ‘SGA’ and ‘low birth weight’ are not synonymous but there is considerable overlap between them. Low birth weight is defined by the World Health Organization as a birth weight below 2500 g but does not correct for gestational age. SGA describes a fetus or baby with growth parameters below a defined centile (normally the 10th) for gestational age that is not pathologically growth restricted; these babies are small simply because of constitutional factors. An IUGR baby is an SGA baby but not all SGA babies are growth restricted. IUGR can be subdivided into symmetric and asymmetric fetal growth, and this can give more insight into the onset and aetiology of the causative fetal insult. Head-to-abdominal circumference ratios (HC/AC) have been used to differentiate fetuses into subtypes based on those that are proportionally small (symmetrical) or those with relative head sparing (asymmetrical). Approximately 75% of all IUGR cases show asymmetric growth patterns, and these pregnancies are at significantly higher risk of severe pre-eclampsia, fetal distress, operative intervention and low neonatal Apgar scores compared with their symmetrical counterparts. Symmetrical IUGR is likely to result from an early fetal insult due to chemical exposure (e.g. nicotine from cigarette smoking reduces birth weight by 175e300 g), viral infection or inherent developmental abnormalities resulting from aneuploidy. These will result in a proportionate reduction in fetal measurements. Conversely, asymmetrical IUGR will lead to disproportionate reduction in fetal measurements due to uteroplacental insufficiency with preferential shunting of blood to the fetal brain.3
D. Monk, G.E. Moore
Implications of IUGR Fetuses that suffer IUGR have a perinatal mortality 12 times higher than normal-birth weight babies.3 These babies also have the immediate problems of hypothermia, hypoglycaemia, pulmonary haemorrhage and encephalopathy compared with babies with birth weights appropriate for gestational age. Later in life, IUGR babies that show catch-up growth are also at greater risk of heart disease, hypertension and type 2 diabetes.4 Epidemiological studies by Barker2 suggested that a range of metabolic diseases may be associated with the fetal environment. The mechanisms underlying these are not clear. Research using animal models is currently being undertaken to address this. Extensive work in which pregnant female rats are fed a diet of moderately restricted proteins results in reduction in birth weight of all pups by approximately 15%. Male growth-restricted rats that were cross-fostered shortly after birth to normal lactating females exhibited catch-up growth, but showed reduced longevity.5 These data back the human epidemiological studies suggesting that catch-up growth is potentially beneficial in the short term, but may be detrimental to long-term fitness and survival.
Aetiology of the extremes of fetal growth Fetal growth is a complex, dynamic process controlled by a wide range of factors of maternal, placental and fetal origin (Table 1). As a result, the aetiology of IUGR and macrosomia is often multifactorial, involving both genetic and environmental factors to a varying degree. Although many factors have been implicated in the process of fetal growth, the precise molecular and cellular mechanisms by which normal fetal growth occurs are still not well understood. In early fetal life, the major determinant of growth is the fetal genome, but later in pregnancy, environmental, nutritional and hormonal influences become increasingly important. For the remainder of this review, we will concentrate on the genetic control of intrauterine growth.
Fetal chromosomal abnormalities There is strong association between IUGR, chromosome aberrations and congenital malformations. Fetuses with chromosome disorders, including the common trisomies of chromosomes 13, 18 and 21,
Genetic causes and consequences of IUGR Table 1
373
The known clinical, genetic and environmental factors associated with IUGR
Fetal
Maternal
Placental
Chromosomal abnormality e Aneuploidy
Substance abuse e Smoking e Alcohol e Drugs Chronic disease Constitutionally small mother Lack of second-trimester weight gain
Partial placental separation
Multifactorial congenital malformations Multiple fetus pregnancy Infection e Malaria Aberrant genomic imprinting e Uniparental disomy e Epimutations
Placental infarction Placenta previa Uteroplacental insufficiency
High-altitude pregnancy e Hypoxia
Small placenta
Malnutrition
Chronic vascular disease Pre-eclampsia
are frequently growth restricted, and suboptimal growth is also reported for many autosomal abnormalities such as duplications, deletions and ring chromosomes. It is thought that an abnormal fetal karyotype is responsible for approximately 20% of all IUGR fetuses, and the percentage is substantially higher if growth failure is detected before 26 weeks’ gestation.6 It is likely that the compromised karyotype impairs normal cell division leading to a reduction in cell number and fetal growth.
Genomic imprinting The vast majority of autosomal genes are expressed from both parental alleles, which are functionally equivalent. However, for a small group of genes that are imprinted, only one of the alleles is expressed in a parent-of-origin-dependent manner. The first clues to the existence of genomic imprinting in mammals came from a small number of key experiments carried out in the mid-1980s. Firstly, nuclear transfer experiments in mice made it possible to produce embryos carrying chromosomes derived solely from one parent. Gynogenetic embryos were created by replacing the paternal pronucleus with a maternal pronucleus, whereas replacing the female pronucleus with paternally derived material resulted in androgenetic embryos. Even though the two types of conceptuses contained the normal diploid amount of DNA, neither were viable at term. The critical observation from these reconstructed diploid embryos was that androgenetic embryos were associated with relatively well-formed extra-embryonic tissue with little or no embryo, whereas gynogenetic embryos showed poorer extra-embryonic tissue development and a more apparent embryo.7,8 Naturally occurring examples have been found in humans. Ovarian
teratomas that occur at the rate of one in 10 000 ovulations contain only maternal chromosomes, and are thought to result from oocytes activated in a pathogenetic fashion. Teratomas are classically embryonic in origin containing components from all three germ layers. Teratomas have been found to contain skin, hair and bone-like structures, but have no extra-embryonic components. In contrast, hydatidiform moles, which arise from two sperms in an empty sac, result in an abundance of placental tissue but lack a fetal component. These paternal uniparental hydatiform moles show aggressive invasive properties, indicating that placental development depends on the expression of specific genes on the paternal chromosomes whereas the development of the embryo requires the maternal genome.9 Subsequent experiments by Cattanach and Kirk10 and Searle and Beechey11 extended these findings by demonstrating that the hypothetical imprinted genes could be localized to specific chromosomal regions. Using mice that were heterozygous for Robertsonian translocations, and taking advantage of non-disjunction and gamete complementation, mice were produced that had both chromosomes derived from a single parent, a phenomenon called uniparental disomy (UPD). Intercrosses of heterozygotes with reciprocial translocations were used to narrow these imprinted whole disomic chromosomes down further, to define subchromosomal regions of the chromosomes that displayed specific phenotypic anomalies. This is in contrast to the nuclear transfer experiments where all chromosomes were uniparentally derived. In the mouse studies, severe phenotypic differences were observed with maternal vs paternal origin of the UPD. These phenotypes are often manifested as overgrowth or growth restriction, as well as behavioural problems. However, not all chromosomes produced abnormal phenotypes
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when presented as UPD, suggesting that imprinted genes are not uniformly distributed throughout the genome ( for a comprehensive review of imprinting phenotypes, see Ref.12).
UPD in humans UPD is probably the most common phenomenon involved in exposing imprinting effects in humans. The theory that emerged in 1980 was that UPD probably arose as a consequence of the high rate of germ cell aneuploidy found in man; this can be as high as 50% per conception (see section on mechanism below).13 Each individual normally inherits a chromosome from each parent, which were themselves inherited from each grandparent. UPD, arising from one parent alone, may result in isodisomy, in which the uniparental pairs consist of two copies of a single parental or grandparental homologue. This may lead to an abnormal phenotype by increasing the risk of recessive disorders by reduction to homozygosity and also through disrupted imprinting effects. Alternatively, the UPD may be contributed to by both grandparental homologues known as heterodisomy.13 In heterodisomy, the abnormal chromosome pair may cause disruption of genomic imprinted gene expression alone resulting in distinct phenotypes (Table 2). There are several mechanisms that could give rise to UPD, but most common is trisomic rescue involving the loss of a supernumerary chromosome from a trisomic conceptus, leaving two homologues from the same gametes ( for an overview of mechanisms leading to UPD, see Robinson14). Trisomic rescue would account for the finding of confined placental mosaicism (CPM) where the karyotype in part of the placenta is different to the fetus. This finding is not exceptional, as CPM is found in approximately 1e2% of routine chorionic villous biopsies (CVS). Trisomy 16 is the most common trisomy found in CPM associated with IUGR, and as a Table 2
consequence, chromosome 16 was one of the first candidate chromosomes to be studied with respect to IUGR. If CPM is found during routine CVS for prenatal diagnosis that involves a chromosome known to harbour imprinted genes, it is important to check for UPD in the fetus. However, it is not always clear whether the observed IUGR is due to a dysfunctional mosaic placenta or as a consequence of fetal UPD and aberrant imprinted gene expression. For example, in a systematic study of 35 idiopathic IUGR cases, two cases of UPD16 associated with CPM 16 were found, both with an imperforate anus.15 However, placental trisomy for chromosome 16 affects intrauterine development of both UPD16 and biparental diploid fetuses (normal inheritance of both chromosome 16s) in a similar way, causing severe IUGR and making it difficult to divide up the effect of imprinting from having a mosaic placenta. The higher the proportion of trisomic cells in the placenta, the more severe the effects on the fetus.16 In contrast, there is evidence that intrauterine growth is not affected by the presence of a trisomic cell line for chromosome 7 in the placenta, since biparental chromosome 7 diploid fetuses with a CPM 7 placenta exhibit normal growth parameters. Significant growth restriction is only seen in cases with CPM7 and maternal UPD7.17 This, therefore, makes it unlikely that CPM is responsible for the significant growth restriction seen in cases with mUPD7.
Fetal growth, genomic imprinting and UPD Observations from UPD in humans and mice imply that genomic imprinting has relevance to fetal growth and its associated abnormalities. The early studies on mouse imprinting phenotypes showed that imprinted genes play a major role in growth. The majority of subchromosomal regions showing imprinting phenotypes are related to fetal growth, placental size and early embryonic, neonatal or
Selected human uniparental disomy (UPD) phenotypes
Chromosome
Parental origin
Syndrome
6 7 11 (segmental) 14 15
Paternal Maternal Paternal Maternal Maternal Paternal Maternal
Transient neonatal diabetes mellitus Severe IUGR; SilvereRussell syndrome BeckwitheWiedemann syndrome Hypotonia, motor delay, IUGR PradereWilli syndrome Angelman syndrome Severe IUGR Albrights hereditary osteodystrophy (phenotype modified depending on parental origin)
16 (associated with CPM16) 20 IUGR, intrauterine growth restriction.
Genetic causes and consequences of IUGR postnatal lethality, with paternal disomies promoting growth and maternal disomies inhibiting growth.12,18 Furthermore, chimeric embryos formed from normal and gynogenetic embryos tend to be smaller than normal litter mates. Conversely, androgenetic chimeras are larger than expected.19 Over 50 imprinted genes have now been identified.12 A large number of these are involved in growth regulation,20 although behavioural phenotypes are also becoming apparent.21,22 Imprinted genes such as insulin (Ins2), cyclin-dependent kinase inhibitor 1C ( p57kip2), guanine nucleotide binding protein (Gnas) and achaete-scute homologue 2 (Mash2) are known to regulate fetal growth or cell-cycle progression.23,24 The effect of several other imprinted genes on the fetal growth axis is mediated by insulin-like growth factor II (Igf2). These genes include Igf2 itself, Igf2 receptor (Igf2r), H19 and growth factor receptor bound protein 10 (Grb10). There has been much speculation surrounding the biological significance of the imprinting of Igf2 and Igf2r, since both these products interact physically. Both genes are reciprocally imprinted in that Igf2 is expressed from the paternal allele, whereas the Igf2r transcripts are derived from the maternal allele. Targeted mutations of both genes have been shown to result in abnormal fetal growth.25e27 The balance between the two gene products is obviously important as the lethal maternal phenotype associated with proximal mouse chromosome 17 that results from a lack of Igf2r is known to be rescued by a second mutation in Igf2 that is paternally inherited.28 Based on their known functions (Igf2 as a growth enhancer and Igf2r as a growth regulator, via Igf2 binding and degradation), such reciprocal imprinting and mechanical interaction makes evolutionary sense if selection forces apply. This complementary imprinted expression, in which growth enhancers are paternally expressed and growth regulators are maternally expressed, underpins the basis of the Haig parental conflict hypothesis.29 This conflict theory suggests that the paternal genome wants to enhance fetal growth at the expense of the mother whereas the mother wants to minimize fetal growth to protect her own survival to reproduce again. The father can reproduce again with a different mother.
375 the SilvereRussell syndrome (SRS), which leads to growth restriction.
BeckwitheWiedemann syndrome The mechanisms that lead to in utero overgrowth prove as elusive as those for IUGR. However, with the generation of experimental mouse models, and detailed clinical and molecular genetic analysis of children with overgrowth syndromes, the genetic mechanisms are slowly being revealed. Transgenic mice with manipulated expression of genes for insulin-like growth factor, insulin and their receptors have highlighted this pathway as being essential for normal growth, and fetal somatic overgrowth often occurs when the ligands are present in excess. Igf2 is normally a paternally expressed imprinted gene;30 however, re-activation of the maternal allele (i.e. loss of imprinting) results in BWS.31 BWS is characterized by pre- and postnatal overgrowth, macroglossia, abnormal digits, organomegaly, hemihypertrophy and a predisposition to childhood cancers such as Wilms tumour.32 Whilst the majority of cases are sporadic, familial inheritance patterns have been reported in the literature. Rare cytogenetic re-arrangements have given valuable clues to the exact regions involved, with 20% of BWS patients presenting with segmental pUPD1133 and 2% having small duplications of chromosome 11p, invariably derived from the father, which resulted in overexpression of Igf2. Maternally inherited re-arrangements localize to distinct breakpoint cluster regions and are indicative of an imprinting effect and growth restriction. Loss of imprinting of Igf2 is the most common molecular defect seen in sporadic patients. However, in 40% of familial cases, the only identifiable alteration is maternal mutations of p57kip2.34 A phenotypically related overgrowth syndrome, SimpsoneGolabieBehmel syndrome, is caused by mutations in the X-linked glypican-3 gene (Gpc3).35 The glypican-3 protein is thought to interact with the gene products important in BWS. Information gathered from targeted deletions of p57kip2 and Gpc3 and overexpression of Igf2 in the mouse has helped to characterize the molecular basis for these disorders.36
Specific growth phenotypes associated with human syndromes
SilvereRussell syndrome
Examples of well-characterized phenotypes that arise as a consequence of disrupted imprinting are the BeckwitheWiedemann syndrome (BWS), which is associated with fetal overgrowth, and
In contrast to the fetal overgrowth seen in BWS, SRS is a condition where IUGR and reduced postnatal growth are associated with other dimorphic features, including characteristic facies, limb and
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truncal asymmetry, striking lack of subcutaneous fat and relative macrocephaly. SRS cases are sporadic, but there are several published pedigrees that support autosomal-dominant and -recessive inheritance in some families. Recently, approximately 10% of SRS cases have been associated with mUPD737 without any underlying fetal mosaicism.38 A definite role for an imprinting error and not the unmasking of a recessive allele was demonstrated by the lack of a consistent region of isodisomy across five mUPD7 SRS patients.39 Further evidence for an involvement of human chromosome 7 in SRS comes from the reports of numerous SRS patients with cytogenetic disruptions of 7p11.2ep13, including several duplications that are of maternal origin.40 A second SRS candidate gene region on human chromosome 7 has been mapped to 7q32-qter, as a result of a single patient with segmental mUPD7 for this region.41 Interestingly, both domains correspond with chromosomal regions in the mouse that are known to contain imprinted genes. When these genes are maternally inherited, this results in severe growth restriction that mimics the human SRS phenotype.
Mouse models for IUGR The most valuable source of information about the function of imprinted genes has been from work on Table 3 Imprinted gene
targeted mutations in the mouse. Fetal growth restriction in these circumstances may be the result of lack of expression of the targeted gene in the fetus or placenta or a combination of both. Therefore, data relating to gene function must be carefully interpreted. Recent work in the mouse has indicated that the roles of imprinted genes in the fetal and placental compartments can be genetically separated, and that in the placenta, these genes regulate both growth and specific nutrient transfer.41 Of the 200 expected genes in the eutherine genome, a substantial proportion are predicted to be involved in the control of fetal growth, in which paternally expressed imprinted genes enhance growth, whereas maternally expressed genes suppress it. This paternal vs maternal genome tug-of-war is the basis of the genetic conflict hypothesis, where paternally derived genes influence nutrient acquisition so selecting more nutrients from the mother, and maternally derived genes try and balance the provision of nutrients to the current fetus with that of potential future pregnancies and protect their own survival to reproduce again.29 In fitting with the Haig hypothesis of genetic conflict between parental genomes, knockouts of paternally expressed genes such as Igf2, mesoderm-specific transcript (Peg1/Mest), Peg3 and insulin all result in IUGR, whereas the reverse
Placental and embryonic expression of known mouse imprinted genes Knockout growth phenotype
Paternally expressed Igf2 P0 C Peg1 C Peg3 C Insulin C Ata3 Maternally expressed Grb10 P57kip2 C Igf2r H19 C Ipl C Mash2 Slc22a2 Slc22a3 Obph1 Napl14 Dcn Tssc4 Tnfrh1 Gatm
Placental expression
Knockout placental phenotype
Spongiotrophoblast
Labyrinthine trophoblast
C
C
C
C
C C C
C C C C
C
C C C C C C C C C
C
C C
C
C
Physiological role
C
C
C C C
C
Genetic causes and consequences of IUGR situation is observed for the null mice for H19 and Igf2r which are maternally expressed and result in overgrowth of the fetus.24 Recently, it has been shown that many imprinted genes are expressed in the placenta and fetus, and that a subset are only imprinted in the placenta and are biallelic and therefore not imprinted in the fetus (Table 3, see Reik et al.42 for review of literature). The most studied and best-characterized imprinted gene is probably Igf2. The Igf2 protein has been shown to be a major modulator of fetal and placental size. In the mouse, expression of Igf2 is regulated by four distinct promoters. Promoters P1e3 are classified as fetal promoters. The Igf2 gene also has a placental-specific isoformdP0. Targeted deletions of the P0 promoter, by removing the promoter and first exon, result in a P0 null; however, all the other promoters are unaffected and transcriptionally active. The P0 null mice present with a phenotype virtually indistinguishable from the Igf2 total null, each having 30% growth restriction compared with wild-type litter mates. The P0 null animals have a smaller than normal placenta at Day E16 of gestation, which reduces the nutrient transfer capacity of the placenta. However, once the placental constraints on growth are lifted at birth, the P0 knockout mice show catch-up growth,43 a scenario with the highest risk of metabolic disorders as outlined by the intrauterine fetal programming hypothesis.2,5 The key observation outlined in the elegant murine model described above may also relate to humans. Mutations or epigenetic aberrations resulting in disrupted placental-specific imprinted gene expression may result in IUGR. After birth, when placental constraints on growth are lifted, catch-up growth would be expected, again leading to an increased risk of metabolic syndromes such as diabetes and hypertension that are only prevalent later in life.
Research agenda Find the genes responsible for macrosomia and IUGR, and ascertain the molecular mechanisms involved. Develop accurate early diagnosis for aberrant fetal growth. Develop early in utero therapies for the prevention of IUGR. Define the role of placental-specific imprinting in normal placental development and IUGR.
377
References 1. Available from: http://www.modimes.org/. 2. Barker DJ. The fetal and infant origins of disease. Eur J Epidemiol 1995;25:457e63. 3. Cunningham FG, Gant NF, Leveno KJ. Fetal growth disorders. In: Cunningham FG, Gant NF, Leveno KJ, editors. Williams obstetrics. 21st ed. New York: Appleton and Lange; 1997. p. 743e64. 4. Reece AE, Hagay Z. Medicine of the fetus and mother. In: Reece AE, Hobbins JC, editors. Prenatal diagnosis of deviant fetal growth. Philadelphia: Lippincourt-Raven; 1999. 5. Hales CN, Ozanne SE. For debate: fetal and early postnatal growth restriction lead to diabetes, the metabolic syndrome and renal failure. Diabetologia 2003;46:1013e9. 6. Snijders RJ, Sherrod C, Gosden CM, Nicolaides KH. Fetal growth retardation: associated malformations and chromosomal abnormalities. Am J Obstet Gynecol 1993;168:547e55. 7. Surani MA, Barton SC, Norris ML. Development of reconstituted mouse eggs suggests imprinting of the genome during gametogenesis. Nature 1984;308:548e50. 8. Mcgrath J, Solter D. Completion of mouse embryogenesis requires both the maternal and paternal genomes. Cell 1984;37:179e83. 9. Lindor NM, Ney JA, Gaffey TA, Jenkins RB, Thibodeau SN, Dewald GW. A genetic review of complete and partial hydatidiform moles and nonmolar triploidy. Mayo Clin Proc 1992;67:791e9. 10. Cattanach BM, Kirk M. Differential activity of maternally and paternally derived chromosome regions in mice. Nature 1985;315:496e8. 11. Searle AG, Beechey CV. Genome imprinting phenomena on mouse chromosome 7. Genet Res 1990;56:237e44. 12. Available from: http://www.mgu.har.mrc.ac.uk/imprinting/ imprinting.html. 13. Engel E. A new genetic concept: the uniparental disomy and its potential effect, the isodisomy. J Genet Hum 1980;28: 11e22. 14. Robinson WP. Mechanisms leading to uniparental disomy and their clinical consequences. Bioessays 2000;22:452e9. 15. Moore GE, Ali Z, Khan RU, Blunt S, Bennett PR, Vaughan JI. The incidence of uniparental disomy associated with intrauterine growth retardation in a cohort of thirty-five severely affected babies. Am J Obstet Gynecol 1997;176: 294e9. 16. Kalousek DK, Langlois S, Barrett I, Yam I, Wilson DR, HowardPeebles PN. Uniparental disomy for chromosome 16 in humans. Am J Hum Genet 1993;52:8e16. 17. Kalousek DK, Langlois S, Robinson WP, Telenius A, Bernard L, Barrett IJ. Trisomy 7 CVS mosaicism: pregnancy outcome, placental and DNA analysis in 14 cases. Am J Hum Genet 1994;65:348e52. 18. Cattanach BM, Shibata H, Hayashizaki Y, Townsend KM, Ball S, Beechey CV. Association of a redefined proximal mouse chromosome 11 imprinting region and U2afbp-rs/U2af1-rs1 expression. Cytogenet Cell Genet 1998;80:41e7. 19. Barton SC, Ferguson-Smith AC, Fundele R, Surani MA. Influence of paternally imprinted genes on development. Development 1991;113:679e87. 20. Morison IM, Reeve AE. A catalogue of imprinted genes and parent-of-origin effects in humans and animals. Hum Mol Genet 1998;7:1599e609. 21. Lefebvre L, Viville S, Barton SC, Ishino F, Keverne EB, Surani MA. Abnormal behaviour and growth retardation associated with loss of the imprinted gene Mest. Nat Genet 1998;20: 163e9.
378 22. Murphy SK, Wylie AA, Jirtle RL. Imprinting of PEG3, the human homologue of a mouse gene involved in nurturing behavior. Genomics 2001;71:110e7. 23. Reik W, Walter J. Evolution of imprinting mechanisms: the battle of the sexes begins in the zygote. Nat Genet 2001;27: 255e60. 24. Tycko B, Morison IM. Physiological functions of imprinted genes. J Cell Physiol 2002;192:245e58. 25. DeChiara TM, Efstratiadis A, Robertson EJ. A growthdeficiency phenotype in heterozygous mice carrying an insulin-like growth factor II gene is disrupted by targeting. Nature 1990;345:78e80. 26. Lau MM, Stewart CE, Liu Z, Bhatt H, Rotwein P, Stewart CL. Loss of the imprinted IGF2/cation-independent mannose 6-phosphate receptor results in fetal overgrowth and perinatal lethality. Genes Dev 1994;8:2953e63. 27. Leighton PA, Ingram RS, Eggenschwiler J, Efstratiadis A, Tilghman SM. Disruption of imprinting caused by a deletion of the H19 region in mice. Nature 1995;375:34e9. 28. Filson AJ, Louvi A, Efstratiadis A, Robertson EJ. Rescue of the T-associated maternal effect in mice carrying null mutations in Igf-2 and Igf2r, two reciprocally imprinted genes. Development 1993;118:731e6. 29. Moore T, Haig D. Genomic imprinting in mammalian development: a parental tug of war. Trends Genet 1991;7:45e9. 30. Giannoukakis N, Deal C, Paquette J, Goodyer CG, Polychronakos C. Parental genomic imprinting of the human IGF2 gene. Nat Genet 1993;4:98e101. 31. Weksberg R, Shen DR, Fei YL, Song QL, Squire J. Disruption of insulin-like growth factor-2 in Beckwith Wiedemann syndrome. Nat Genet 1993;5:143e9. 32. Engel JR, Smallwood A, Harper A, Higgins MJ, Oshimura M, Reik W. Epigenotypeephenotype correlations in Beckwithe Wiedemann syndrome. J Med Genet 2002;37:921e6. 33. Henry I, Bonaiti-Pellie C, Chehensse V, Beldjord C, Schwartz C, Uterman G. Uniparental disomy in a genetic cancerpredisposing syndrome. Nature 1991;351:665e7.
D. Monk, G.E. Moore 34. Maher ER, Reik W. BeckwitheWiedemann syndrome: imprinting in clusters revisited. J Clin Invest 2000;105: 247e52. 35. Pilia G, Hughes-Benzie RM, MacKenzie A, Baybayan P, Chen EY, Huber R. Mutations in GPC3, a glypican gene, cause the SimpsoneGolabieBehmel overgrowth syndrome. Nat Genet 1996;12:241e7. 36. Li M, Squire JA, Weksberg R. Overgrowth syndromes and genomic imprinting: from mouse to man. Clin Genet 1998; 53:165e70. 37. Kotzot D, Schmitt S, Bernasconi F, Robinson WP, Lurie IW, Ilyina H. Uniparental disomy 7 in SilvereRussell syndrome and primordial growth retardation. Hum Mol Genet 1995;4: 583e7. 38. Monk D, Hitchins M, Russo S, Preece M, Stanier P, Moore GE. No evidence for mosaicism in SilvereRussell syndrome. J Med Genet 2001;38:E11. 39. Preece MA, Abu-Amero SN, Ali Z, Abu-Amero KK, Wakeling EL, Stanier P. An analysis of the distribution of hetero- and isodisomic regions of chromosome 7 in five mUPD7 Silvere Russell syndrome probands. J Med Genet 1999;36:457e60. 40. Monk D, Bentley L, Hitchins M, Myler RA, Clayton-Smith J, Ismail S. Chromosome 7p disruptions in SilvereRussell syndrome: delineating an imprinted candidate gene region. Hum Genet 2002;111:376e87. 41. Hannula K, Lipsanen-Nyman M, Kontiokari T, Kere J. A narrow segment of maternal uniparental disomy of chromosome 7q31-qter in SilvereRussell syndrome delimits a candidate gene region. Am J Hum Genet 2001;68:247e53. 42. Reik W, Constancia M, Fowden A, Anderson N, Dean W, Ferguson-Smith A. Regulation of supply and demand for maternal nutrients in mammals by imprinted genes. J Physiol 2003;547:35e44. 43. Constancia M, Hemberger M, Hughes J, Dean W, FergusonSmith A, Fundele R. Placental-specific IGF-II is a major modulator of placental and fetal growth. Nature 2002;417: 945e8.