New chromosome 11p15 epigenotypes identified in male monozygotic twins with Beckwith-Wiedemann syndrome

Phenotypically concordant but epigenetically discordant monozygotic dichorionic diamniotic twins with Beckwith-Wiedemann syndrome

Beckwith-Wiedemann syndrome (BWS) is an imprinting disorder caused by (epi)genetic alterations. The incidence of monozygotic (MZ) twins in BWS is higher than in the general population. Most MZ twins with BWS are female and have phenotypical discordance: one twin is clinically diagnosed with BWS, while the other shows a mild or normal phenotype. The most frequent (epi)genetic alteration in MZ twins is loss of methylation of imprinting control region 2 (ICR2-LOM) at 11p15.5. Intriguingly, ICR2-LOM is usually found in the peripheral blood leukocytes (PBL) of both twins, even if they are clinically discordant. Here, we present a rare pair of MZ dichorionic diamniotic female twins with BWS and concordant phenotypes (a Beckwith-Wiedemann spectrum score of 5 in each twin). Molecular analysis of genomic DNA from PBL revealed ICR2-LOM in one twin but not the other. Our analyses suggest that ICR2-LOM occurred between days 1 and 3 after fertilization, followed by twinning. We speculate that during embryogenesis, ICR2-LOM cells were distributed to the hematopoietic stem cells in different ratios in the two fetuses, and also to commonly affected tissues, such as the tongue, in similar ratios, although we were unable to analyze any tissues other than PBL.

Preclinical and Clinical Epigenetic-Based Reconsideration of Beckwith-Wiedemann Syndrome

Epigenetics has achieved a profound impact in the biomedical field, providing new experimental opportunities and innovative therapeutic strategies to face a plethora of diseases. In the rare diseases scenario, Beckwith-Wiedemann syndrome (BWS) is a pediatric pathological condition characterized by a complex molecular basis, showing alterations in the expression of different growth-regulating genes. The molecular origin of BWS is associated with impairments in the genomic imprinting of two domains at the 11p15.5 chromosomal region. The first domain contains three different regions: insulin growth like factor gene (IGF2), H19, and abnormally methylated DMR1 region. The second domain consists of cell proliferation and regulating-genes such as CDKN1C gene encoding for cyclin kinase inhibitor its role is to block cell proliferation. Although most cases are sporadic, about 5-10% of BWS patients have inheritance characteristics. In the 11p15.5 region, some of the patients have maternal chromosomal rearrangements while others have Uniparental Paternal Disomy UPD(11)pat. Defects in DNA methylation cause alteration of genes and the genomic structure equilibrium leading uncontrolled cell proliferation, which is a typical tumorigenesis event. Indeed, in BWS patients an increased childhood tumor predisposition is observed. Here, we summarize the latest knowledge on BWS and focus on the impact of epigenetic alterations to an increased cancer risk development and to metabolic disorders. Moreover, we highlight the correlation between assisted reproductive technologies and this rare disease. We also discuss intriguing aspects of BWS in twinning. Epigenetic therapies in clinical trials have already demonstrated effectiveness in oncological and non-oncological diseases. In this review, we propose a potential "epigenetic-based" approaches may unveil new therapeutic options for BWS patients. Although the complexity of the syndrome is high, patients can be able to lead a normal life but tumor predispositions might impair life expectancy. In this sense epigenetic therapies should have a supporting role in order to guarantee a good prognosis.

Diagnosis and management of the phenotypic spectrum of twins with Beckwith-Wiedemann syndrome

Beckwith-Wiedemann syndrome (BWS) is an overgrowth disorder with a heterogeneous phenotypic spectrum. There is an increased prevalence of monozygotic twinning in BWS. Given the epigenetic nature and phenotypic spectrum that defines BWS, twins are often discordant for clinical features, and clinicians are faced with the challenge of diagnosing and managing these twins. We present a cohort of multiple pregnancies in which one or more child from each pregnancy was diagnosed with BWS. We conducted a chart review of monochorionic and dichorionic gestations. Clinical scores for monochorionic twins demonstrated phenotypic discordance between the proband and twin. Based on linear regression analysis, a higher clinical score in the proband correlated with larger phenotypic discordance between twin siblings. Despite phenotypic discordance, however, we observed a consistent additive clinical score for a pregnancy (proband's plus twin's scores from a pregnancy). This idea of a finite degree of affectedness for a pregnancy implies a finite number of epigenetically affected cells. This further corroborates the idea that timing of monozygotic monochorionic twinning correlates with the disruption of establishment and/or maintenance of imprinting. The difference in clinical score between a proband and their twin may be due to diffused mosaicism, whereby there is an asymmetric distribution of affected cells among the multiple fetuses in a monozygotic monochorionic pregnancy, leading to a spectrum of variably affected phenotypes. Based on these findings, we recommend an algorithm for a conservative approach to clinically evaluate all children in a monozygotic multiple gestation affected by BWS.

Esophageal atresia and Beckwith-Wiedemann syndrome in one of the naturally conceived discordant newborn twins: first report

Recent studies report a high incidence of monozygotic twinning in Beckwith–Wiedemann syndrome. A phenotypical discordance in monozygotic twins is rare. Twinning and Beckwith–Wiedemann syndrome show higher incidence in children born after assisted reproductive techniques. We report on the first observation of esophageal atresia and Beckwith–Wiedemann syndrome in one of the naturally conceived discordant monozygotic twins.

Silver-Russell Syndrome and Beckwith-Wiedemann Syndrome: Opposite Phenotypes with Heterogeneous Molecular Etiology

Silver-Russell syndrome (SRS) and Beckwith-Wiedemann syndrome (BWS) are 2 clinically opposite growth-affecting disorders belonging to the group of congenital imprinting disorders. The expression of both syndromes usually depends on the parental origin of the chromosome in which the imprinted genes reside. SRS is characterized by severe intrauterine and postnatal growth retardation with various additional clinical features such as hemihypertrophy, relative macrocephaly, fifth finger clinodactyly, and triangular facies. BWS is an overgrowth syndrome with many additional clinical features such as macroglossia, organomegaly, and an increased risk of childhood tumors. Both SRS and BWS are clinically and genetically heterogeneous, and for clinical diagnosis, different diagnostic scoring systems have been developed. Six diagnostic scoring systems for SRS and 4 for BWS have been previously published. However, neither syndrome has common consensus diagnostic criteria yet. Most cases of SRS and BWS are associated with opposite epigenetic or genetic abnormalities in the 11p15 chromosomal region leading to opposite imbalances in the expression of imprinted genes. SRS is also caused by maternal uniparental disomy 7, which is usually identified in 5-10% of the cases, and is therefore the first imprinting disorder that affects 2 different chromosomes. In this review, we describe in detail the clinical diagnostic criteria and scoring systems as well as molecular causes in both SRS and BWS.

(Epi)genotype-phenotype correlations in Beckwith-Wiedemann syndrome: a paradigm for genomic medicine

Beckwith-Wiedemann syndrome (BWS) is the commonest overgrowth cancer predisposition disorder and represents a model for human imprinting dysregulation and tumorigenesis. BWS features can variably combine and present a widely variable range of severity in the phenotypic expression. This wide spectrum is paralleled at molecular level by complex (epi)genetic defects on chromosome 11p15.5 leading to disrupted expression of imprinted genes controlling growth and cellular proliferation. In this review, we outline the spectrum of clinical manifestations of BWS analyzing their (epi)genotype-phenotype correlations. The differences observed in the phenotypic profiles of BWS molecular subtypes allow a composite view of this syndrome with implications on clinical care, diagnosis, follow-up, and management, and provide directions for future disease monitoring.

Loss of methylation at GNAS exon A/B is associated with increased intrauterine growth

CONTEXT

GNAS is one of few genetic loci that undergo allelic-specific methylation resulting in the parent-specific expression of at least four different transcripts. Due to monoallelic expression, heterozygous GNAS mutations affecting either paternally or maternally derived transcripts cause different forms of pseudohypoparathyroidism (PHP), including autosomal-dominant PHP type Ib (AD-PHP1B) associated with loss of methylation (LOM) at exon A/B alone or sporadic PHP1B (sporPHP1B) associated with broad GNAS methylation changes. Similar to effects other imprinted genes have on early development, we recently observed severe intrauterine growth retardation in newborns, later diagnosed with pseudopseudohypoparathyroidism (PPHP) because of paternal GNAS loss-of-function mutations.

OBJECTIVES

This study aimed to determine whether GNAS methylation abnormalities affect intrauterine growth.

PATIENTS AND METHODS

Birth parameters were collected of patients who later developed sporPHP1B or AD-PHP1B, and of their healthy siblings. Comparisons were made to newborns affected by PPHP or PHP1A.

RESULTS

As newborns, AD-PHP1B patients were bigger than their healthy siblings and well above the reference average; increased sizes were particularly evident if the mothers were unaffected carriers of STX16 deletions. SporPHP1B newborns were slightly above average for weight and length, but their overgrowth was less pronounced than that of AD-PHP1B newborns from unaffected mothers.

CONCLUSION

LOM at GNAS exon A/B due to maternal STX16 deletions and the resulting biallelic A/B expression are associated with enhanced fetal growth. These findings are distinctly different from those of PPHP patients with paternal GNAS exons 2-13 mutations, whose birth parameters are almost 4.5 z-scores below those of AD-PHP1B patients born to healthy mothers.

Maternal Hypomethylation of KvDMR in a Monozygotic Male Twin Pair Discordant for Beckwith-Wiedemann Syndrome

Beckwith-Wiedemann syndrome (BWS; OMIM 130650) is a heterogeneous overgrowth syndrome characterized by visceromegaly, macroglossia, tumor predisposition, and other congenital abnormalities. BWS is usually associated with abnormalities of chromosome 11p15, including (epi)genetic changes, paternal disomy and point mutations. A number of identical twin pairs, mostly female, have been reported to be clinically discordant for BWS. Studies of monozygotic twins discordant for BWS provide more information about failure in the DNA methylation maintenance machinery during very early embryonic development. Here, we report a case of monozygotic male twins discordant for BWS phenotype. Methylation analysis of the 2 imprinted domains at 11p15.5 (H19DMR and KvDMR) was performed by methylation-specific MLPA and pyrosequencing of DNA extracted from peripheral blood and buccal swabs of both twins. Hypomethylation at KvDMR was identified in both cell types of the affected twin, whereas his healthy brother presented hypomethylation only in blood cells and a normal methylation profile in buccal swab. For diagnostic purposes, it is important to remember that twins can share fetal circulation and possibly share hematopoietic stem cells early in development; therefore, the affected and unaffected twins can share an epigenotype that will resemble partial hypomethylation. If a patient is a twin, it is valuable to obtain a sample from a tissue other than blood.

Molecular findings in Beckwith-Wiedemann syndrome

Our understanding of Beckwith-Wiedemann syndrome (BWS) has recently been enhanced by advances in its molecular characterization. These advances have further delineated intricate (epi)genetic regulation of the imprinted gene cluster on chromosome 11p15.5 and the role of these genes in normal growth and development. Studies of the molecular changes associated with the BWS phenotype have been instrumental in elucidating critical molecular elements in this imprinted region. This review will provide updated information on the multiple new regulatory elements that have been recently found to contribute to in cis or in trans control of imprinted gene expression in the chromosome 11p15.5 region and the clinical expression of the BWS phenotype.

Epigenetic discordance at imprinting control regions in twins

Imprinting control regions are differentially methylated in a parent-of-origin-dependent manner and this methylation state is inherited through the germline. These regions control parent-specific monoallelic expression of their target genes. Genetically identical organisms show considerable variation in their epigenomes owing to environmental and stochastic influences creating fluctuations in phenotype. Monozygotic twin pairs discordant for imprinting disorders due to epigenetic changes at imprinting control regions are an example of phenotypic variation caused by extreme variations of the epigenome. Here, we discuss the within-pair epigenetic discordance at imprinted loci, both in phenotypically concordant and discordant monozygotic twin pairs.

The value of twins in epigenetic epidemiology

During past decades, twin studies have played an important role in genetic epidemiology studies of complex traits. The strength of twin studies lies in the ability to disentangle genetic and environmental factors that contribute to a phenotype, by comparing genetically identical monozygotic twins to dizygotic twins, who share on average 50% of genetic variants. Twin studies now offer the opportunity to study epigenetic variation across the genome with two aims. First, twin studies can improve our understanding of the factors regulating epigenetic variability by assessing the heritability of epigenetic variants. Secondly, the use of twins in epigenetic research is increasingly recognized as an important approach to help unravel the complexities associated with human development and disease. The strategic use of identical twins discordant for complex disease has revealed the importance of linking epigenetic disruption to the disease-associated risk in humans. Lastly, we also discuss the possibility that epigenetic effects on disease may in part explain some of the missing heritability in genome-wide association studies. The study of human epigenetic factors in twins can inform the role of genetics, as well as in utero and postnatal environments to the establishment, maintenance and functional consequences of human epigenome variation.

DNA methylation studies on imprinted loci in a male monozygotic twin pair discordant for Beckwith-Wiedemann syndrome

Beckwith–Wiedemann syndrome (BWS) is one of the most prevalent congenital disorders predominantly caused by epigenetic alterations. Here we present an extensive case study of a monozygotic monochorionic male twin pair discordant for BWS. Our analysis allows to correlate BWS symptoms, like a protruding tongue, indented ears and transient neonatal hypoglycaemia, to an abnormal methylation at the KvDMR1. DNAs extracted from peripheral blood, skin fibroblasts, saliva and buccal swab of both twins, their sister and parents were analysed at 11 differentially methylated regions (DMRs) including all four relevant DMRs of the BWS region. The KvDMR1 was exclusively found to be hypomethylated in all cell types of the affected BWS twin, while the unaffected twin and the relatives showed normal methylation in fibroblasts, buccal swab and saliva DNA. Interestingly, the twins share a common blood-specific hypomethylation phenotype most probably caused by a feto-fetal transfusion between both twins. Because microsatellite analysis furthermore revealed a normal biparental karyotype for chromosome 11, our results point to an exclusive correlation of the observed BWS symptoms to locally restricted epimutations at the KvDMR1 of the maternal chromosome.

Exclusion of the GNAS locus in PHP-Ib patients with broad GNAS methylation changes: evidence for an autosomal recessive form of PHP-Ib?

Most patients with autosomal dominant pseudohypoparathyroidism type Ib (AD-PHP-Ib) carry maternally inherited microdeletions upstream of GNAS that are associated with loss of methylation restricted to GNAS exon A/B. Only few AD-PHP-Ib patients carry microdeletions within GNAS that are associated with loss of all maternal methylation imprints. These epigenetic changes are often indistinguishable from those observed in patients affected by an apparently sporadic PHP-Ib form that has not yet been defined genetically. We have now investigated six female patients affected by PHP-Ib (four unrelated and two sisters) with complete or almost complete loss of GNAS methylation, whose healthy children (11 in total) showed no epigenetic changes at this locus. Analysis of several microsatellite markers throughout the 20q13 region made it unlikely that PHP-Ib is caused in these patients by large deletions involving GNAS or by paternal uniparental isodisomy or heterodisomy of chromosome 20 (patUPD20). Microsatellite and single-nucleotide variation (SNV) data revealed that the two affected sisters share their maternally inherited GNAS alleles with unaffected relatives that lack evidence for abnormal GNAS methylation, thus excluding linkage to this locus. Consistent with these findings, healthy children of two unrelated sporadic PHP-Ib patients had inherited different maternal GNAS alleles, also arguing against linkage to this locus. Based on our data, it appears plausible that some forms of PHP-Ib are caused by homozygous or compound heterozygous mutation(s) in an unknown gene involved in establishing or maintaining GNAS methylation.

Beckwith-Wiedemann-Syndrom

Das Beckwith-Wiedemann-Syndrom (BWS) ist ein pädiatrisches Überwuchssyndrom mit variablem klinischem Erscheinungsbild. Obwohl die betroffenen Kinder mit zunehmendem Alter immer normaler aussehen, ist es wichtig, die Diagnose BWS zu stellen. Gründe sind mögliche syndromspezifische Komplikationen, insbesondere ein 400-fach erhöhtes Risiko der Patienten, an bestimmten embryonalen Tumoren – Nephroblastome (Wilms-Tumoren), Hepatoblastome u. a. – innerhalb der ersten Lebensjahre zu erkranken. Klinisch überlappt das BWS mit anderen Krankheitsentitäten, sodass eine eindeutige molekulargenetische Diagnostik zur Risikoabschätzung und adäquaten Therapie wünschenswert ist. Molekular ist das BWS mit der Chromosomenregion 11p15.5 assoziiert, einer Region in der es 2 Cluster von Genen gibt, die dem genomischen Imprinting unterliegen. Bei den Patienten lassen sich Sequenzabweichungen in bestimmten Genen finden, die Mehrzahl weisen aber DNA-Methylierungsveränderungen auf, welche die Gendosis der funktionell zur Verfügung stehenden, monoallelisch aktiven 11p15.5-Gene pathogen beeinflussen. Zurzeit existiert nur eine sehr unvollständige Genotyp-Phänotyp-Korrelation. Aktuelle Forschungsarbeiten liefern Ansätze, die Ätiopathogenese des Syndroms molekular besser zu verstehen. So werden beispielsweise Interaktionspartner identifiziert, die das Imprinting der 11p15.5-Gene modifizieren und epigenetisch regulieren.

Beckwith-Wiedemann syndrome

Beckwith-Wiedemann syndrome (BWS) is an imprinting disorder characterized by overgrowth, tumor predisposition, and congenital malformations. Approximately 85% of reported BWS cases are sporadic, while the remaining 15% are familial. BWS is caused by epigenetic or genomic alterations which disrupt genes in one or both of the two imprinted domains on chromosome 11p15.5. In each domain, an imprinting center regulates the expression of imprinted genes in cis. Normally in domain 1, insulin-like growth factor 2 (IGF2) and the untranslated mRNA H19 are monoallelically expressed. In BWS, increased expression of IGF2 occurs via several mechanisms. In domain 2, CDKN1C, a growth repressor, and an untranslated RNA, KCNQ1OT1, are normally expressed monoallelically. In cases of BWS, several mechanisms result in reduced expression of CDKN1C. Recent reports of BWS cases have identified mutations outside the chromosome 11p15.5 critical region, thereby broadening the challenges in the diagnosis and genetic counseling of individuals and families with BWS.

Identical but not the same: the value of discordant monozygotic twins in genetic research

Monozygotic (MZ) twins show remarkable resemblance in many aspects of behavior, health, and disease. Until recently, MZ twins were usually called "genetically identical"; however, evidence for genetic and epigenetic differences within rare MZ twin pairs has accumulated. Here, we summarize the literature on MZ twins discordant for Mendelian inherited disorders and chromosomal abnormalities. A systematic literature search for English articles on discordant MZ twin pairs was performed in Web of Science and PubMed. A total number of 2,016 publications were retrieved and reviewed and 439 reports were retained. Discordant MZ twin pairs are informative in respect to variability of phenotypic expression, pathogenetic mechanisms, epigenetics, and post-zygotic mutagenesis and may serve as a model for research on genetic defects. The analysis of single discordant MZ twin pairs may represent an elegant approach to identify genes in inherited disorders.

Beckwith-Wiedemann syndrome

Beckwith-Wiedemann syndrome (BWS) is a model disorder for the study of imprinting, growth dysregulation, and tumorigenesis. Unique observations in this disorder point to an important embryonic developmental window relevant to the observations of increased monozygotic twinning and an increased rate of epigenetic errors after subfertility/assisted reproduction.

Lessons from BWS twins: complex maternal and paternal hypomethylation and a common source of haematopoietic stem cells

The Beckwith-Wiedemann syndrome (BWS) is a growth disorder for which an increased frequency of monozygotic (MZ) twinning has been reported. With few exceptions, these twins are discordant for BWS and for females. Here, we describe the molecular and phenotypic analysis of 12 BWS twins and a triplet; seven twins are MZ, monochorionic and diamniotic, three twins are MZ, dichorionic and diamniotic and three twins are dizygotic. Twelve twins are female. In the majority of the twin pairs (11 of 13), the defect on chromosome 11p15 was hypomethylation of the paternal allele of DMR2. In 5 of 10 twins, there was additional hypomethylation of imprinted loci; in most cases, the loci affected were maternally methylated, but in two cases, hypomethylation of the paternally methylated DLK1 and H19 DMRs was detected, a novel finding in BWS. In buccal swabs of the MZ twins who share a placenta, the defect was present only in the affected twin; comparable hypomethylation in lymphocytes was detected in both the twins. The level of hypomethylation reached levels below 25%. The exchange of blood cells through vascular connections cannot fully explain the degree of hypomethylation found in the blood cell of the non-affected twin. We propose an additional mechanism through which sharing of aberrant methylation patterns in discordant twins, limited to blood cells, might occur. In a BWS-discordant MZ triplet, an intermediate level of demethylation was found in one of the non-affected sibs; this child showed mild signs of BWS. This finding supports the theory that a methylation error proceeds and possibly triggers the twinning process.

Isolated fetal omphalocele, Beckwith-Wiedemann syndrome, and assisted reproductive technologies

OBJECTIVE

To assess the frequency of Beckwith-Wiedemann syndrome (BWS) among fetuses with an ultrasound diagnosis of isolated omphalocele and to examine relevant clinical variables, in particular route of conception.

MATERIALS AND METHODS

An ultrasound and consultation database (1988-2007) was searched and cases were included that met the following criteria: omphalocele, no additional major structural anomaly or autosomal aneuploidy, and either newborn examination or molecular diagnostic studies for BWS. Medical records were reviewed and a nested case-control analysis matching day of birth assessed the route of conception, given the changing prevalence of assisted reproduction during the study period.

RESULTS

Thirty cases of isolated omphalocele were identified. Beckwith-Wiedemann syndrome was diagnosed in six cases (6/30 [20.0%]); four by newborn examination (4/6) and two on prenatal molecular studies (2/6). Of note, one case of BWS had a karyotype of 47,XXX, the remainder was euploid. Compared with isolated omphaloceles, fetuses with BWS more often were twins (3/6 vs. 1/24; p < 0.001), had polyhydramnios (4/6 vs. 2/24; p < 0.001), were macrosomic at birth (3/6 vs. 4/24 p < 0.001), and had been conceived by assisted reproduction (3/6 vs. 2/22; p = 0.04). When compared with normal controls matched by date of birth, conception by assisted reproductive technique among BWS births was highly significant, (p < 0001).

CONCLUSIONS

Beckwith-Wiedemann syndrome is present in a noteworthy portion of fetuses with isolated omphalocele on ultrasound, and prenatal molecular studies are warranted. Even among this small cohort, a recurring theme of conception by assisted reproduction exists.

Syndromes and Disorders Associated with Omphalocele (I): Beckwith–Wiedemann Syndrome

Beckwith-Wiedemann syndrome (BWS, OMIM 130650) is characterized by macrosomia, macroglossia, visceromegaly, hemihypertrophy, abdominal wall defects, ear creases/pits, neonatal hypoglycemia, polyhydramnios, placentomegaly, placental mesenchymal dysplasia, cardiac defects, nevus flammeus, hemangiomata, and an increased frequency of embryonal tumors. This article provides an overview of BWS including the genetics, genetic diagnosis, genotype/epigenotype-phenotype correlations, association with assisted reproductive technology, and prenatal diagnosis. Omphalocele is an important sonographic marker for BWS. Prenatal detection of omphalocele, fetal overgrowth, polyhydramnios, increased abdominal circumference, placentomegaly and/or placental mesenchymal dysplasia should alert one to the possibility of BWS and prompt a genetic investigation and counseling for BWS.

Epigenetics in reproductive medicine

Imprinted genes comprise a small subset of the genome whose epigenetic reprogramming in the germ line is necessary for subsequent normal embryonic development. This reprogramming and resetting of the imprints, through an erasure/acquisition/maintenance cycle, is a subtle and tightly orchestrated phenomenon, involving specific genomic regions and methylation enzymes. Dysregulation of imprinted genes has indeed been shown to lead to several human disorders as well as to affect placental and fetal growth. There have been numerous and conflicting studies assessing the possible association of imprinting disorders with assisted reproductive techniques. This work analyzes all relevant and available reports with regard to the association between assisted reproductive techniques and imprinting disorders. It also discusses whether this possibly increased risk of imprinting disorders may be linked to specific steps of these reproductive techniques or already present in the gametes of infertile patients. A better understanding of epigenetic reprogramming in the germ line is absolutely necessary both to assess the safety of these methods and of the use of impaired spermatogenesis gametes for assisted reproduction.

Atypical patterns of inheritance

The incomplete prediction of clinical phenotype from genotype in monogenic disorders assumes other complex mechanisms are responsible. Recent examples derived from well-known human diseases will be discussed in this review in the context of the roles of modifier genes, digenic and triallelic inheritance, and the consequence of imprinting and opposite transcripts in known human genetic disorders. Specifically, this review will focus on cystic fibrosis, Huntington's disease, sensory neural deafness due to Connexin gene mutations, Bardet-Biedl syndrome, and the Beckwith-Wiedemann syndrome as there is evidence that complex inheritance is responsible for at least part of the phenotypic variability that is not explainable by the genotype alone. This review is meant to extend and complement the other topics in this issue as the concept of atypical inheritance is explored in more detail.

Imprinted genes and their role in human fetal growth

Growth is defined as the progressive increase in size and is listed as one of the eight main characteristics of life. In human gestation the most rapid growth phase is from 16 to 32 weeks when first there is both cell number and size increase and then from 32 weeks onwards there is continued size increase (Pollack and Divon, 1992). The mechanism of growth in utero is of fundamental interest to clinicians and scientists because of its implications for neonatal health. Growth is multifactorial in origin with both genetics and environment contributing equally large parts. Despite this complexity analysis of the candidate genes involved is possible using simple tissue biopsies at the relevant stages of development. Of particular interest in understanding fetal growth is the analysis of a group of genes that show a parent-of-origin effect known as genomic imprinting. Imprinted genes are not only found in eutherian (placental) and metatherian (marsupial) mammals but surprisingly also in plants. Nevertheless, their evolution in mammals appears to be linked primarily to placentation. It is thought to result from a potential conflict between the parents in terms of the drive to successfully propagate their own separate genes and the mother's added drive for her survival through the pregnancy to reproduce again. This means that the mother wants to restrict fetal growth and the father to enhance it.