Environmental effects on genomic imprinting in mammals

Genomic imprinting is an epigenetic marking mechanism by which certain genes become repressed on one of the two parental alleles. Imprinting plays important roles in mammalian development, and in humans its deregulation may result in disease and carcinogenesis. During different medical, technological and scientific interventions, pre-implantation embryos and cells are taken from their natural environment and subjected to culture in artificial media. Studies in the mouse demonstrate that environmental stress, such as in vitro culture, can affect the somatic maintenance of epigenetic marks at imprinted loci. These effects are associated with aberrant growth and morphology at fetal and perinatal stages of development.

Culture of preimplantation mouse embryos affects fetal development and the expression of imprinted genes

Culture of preimplantation mammalian embryos and cells can influence their subsequent growth and differentiation. Previously, we reported that culture of mouse embryonic stem cells is associated with deregulation of genomic imprinting and affects the potential for these cells to develop into normal fetuses. The purpose of our current study was to determine whether culture of preimplantation mouse embryos in a chemically defined medium (M16) with or without fetal calf serum (FCS) can affect their subsequent development and imprinted gene expression. Only one third of the blastocysts that had been cultured from two-cell embryos in M16 medium complemented with FCS developed into viable Day 14 fetuses after transfer into recipients. These M16 + FCS fetuses were reduced in weight as compared with controls and M16 fetuses and had decreased expression of the imprinted H19 and insulin-like growth factor 2 genes associated with a gain of DNA methylation at an imprinting control region upstream of H19. They also displayed increased expression of the imprinted gene Grb10. The growth factor receptor binding gene Grb7, in contrast, was strongly reduced in its expression in most of the M16 + FCS fetuses. No alterations were detected for the imprinted gene MEST: Preimplantation culture in the presence of serum can influence the regulation of multiple growth-related imprinted genes, thus leading to aberrant fetal growth and development.

A dominant modifier of transgene methylation is mapped by QTL analysis to mouse chromosome 13

The single-copy hepatitis B virus transgene in the E36 transgenic mouse strain undergoes methylation changes in a parent-of-origin, tissue, and strain-specific fashion. In a C57BL/6 background, the paternally transmitted transgene is methylated in 30% of cells, whereas it is methylated in more than 80% of cells in (BALB/c x C57BL/6) F1 mice. We established previously that several genetic factors were likely to contribute to the transgene methylation profile, some with demethylating and some with de novo methylating activities. Using quantitative trait loci (QTL) mapping, we have now localized one major modifier locus on chromosome 13 (Mod13), which explains a 30% increase in the methylation level of this transgene with no effect on the flanking endogenous sequences. No other QTL could be identified, except for a demethylating activity of low significance located on chromosome 12. Recombinant inbred mice containing a BALB/c allele of Mod13 were then used to show that the presence of Mod13 is sufficient to induce de novo methylation. A segregation between de novo methylation and repression of transgene expression was uncovered, suggesting that this genetic system is also useful for the identification of factors that interpret methylation patterns in the genome.

Genomic imprinting: parental influence on the genome

Genomic imprinting affects several dozen mammalian genes and results in the expression of those genes from only one of the two parental chromosomes. This is brought about by epigenetic instructions--imprints--that are laid down in the parental germ cells. Imprinting is a particularly important genetic mechanism in mammals, and is thought to influence the transfer of nutrients to the fetus and the newborn from the mother. Consistent with this view is the fact that imprinted genes tend to affect growth in the womb and behaviour after birth. Aberrant imprinting disturbs development and is the cause of various disease syndromes. The study of imprinting also provides new insights into epigenetic gene modification during development.

Epigenotype-phenotype correlations in Beckwith-Wiedemann syndrome

Beckwith-Wiedemann syndrome (BWS) is a model imprinting disorder resulting from mutations or epigenetic events involving imprinted genes at chromosome 11p15.5. Thus, germline mutations in CDKN1C, uniparental disomy (UPD), and loss of imprinting of IGF2 and other imprinted genes have been implicated. Many familial BWS cases have germline CDKN1C mutations. However, most BWS cases are sporadic and UPD or putative imprinting errors predominate in this group. We have identified previously a subgroup of sporadic cases with loss of imprinting (LOI) of IGF2 and epigenetic silencing of H19 proposed to be caused by a defect in a distal 11p15.5 imprinting control element (designated BWSIC1). However, many sporadic BWS patients show biallelic IGF2 expression in the presence of normal H19 methylation and expression patterns. This and other evidence suggested the existence of a further imprinting control element (BWSIC2) at 11p15. 5. Recently, we showed that a subgroup of BWS patients have loss of methylation (LOM) at a differentially methylated region (KvDMR1) within the KCNQ1 gene centromeric to the IGF2 and H19 genes. We have now analysed a large series of sporadic cases to define the frequency and phenotypic correlates of epigenetic abnormalities in BWS. LOM at KvDMR1 was detected by Southern analysis or a novel PCR based method in 35 of 69 (51%) sporadic BWS without UPD. LOM at KvDMR1 was often, but not invariably associated with LOI of IGF2. KvDMR1 LOM was not detected in BWS patients with putative BWSIC1 defects and cases with KvDMR1 LOM (that is, putative BWSIC2 defects) invariably had a normal H19 methylation pattern. The incidence of exomphalos in putative BWSIC2 defect patients was not significantly different from that in patients with germline CDKN1C mutations (20/29 and 13/15 respectively), but was significantly greater than that in patients with putative BWSIC1 defects (0/5, p=0.007) and UPD (0/22, p<0.0001). These findings are consistent with the hypothesis that LOM of KvDMR1 (BWSIC2 defect) results in epigenetic silencing of CDKN1C and variable LOI of IGF2. BWS patients with embryonal tumours have UPD or a BWSIC1 defect but not LOM of KvDMR1. This study has further shown how (1) variations in phenotypic expression of BWS may be linked to specific molecular subgroups and (2) molecular analysis of BWS can provide insights into mechanisms of imprinting regulation.

Sequence and functional comparison in the Beckwith-Wiedemann region: implications for a novel imprinting centre and extended imprinting

The clustered organization of most imprinted genes in mammals suggests coordinated genetic and epigenetic control mechanisms. Comparisons between human and mouse will help in elucidating these mechanisms by identifying structural and functional similarities. Previously we reported on such a comparison in the central part of the mouse imprinting cluster on distal chromosome 7 with the homologous Beckwith-Wiedemann syndrome (BWS) gene cluster on human chromosome 11p15.5. Here we focus on the adjacent sequences of 0.5 Mb including the KCNQ1/Kcnq1 and CDKN1C/Cdkn1c genes, which are implicated in BWS, and on one of the proposed boundary regions of the imprinting cluster. As in the previously analysed central region, this part of the cluster exhibits a highly conserved arrangement and structure of genes. The most striking similarity is found in the 3' part of the KCNQ1/Kcnq1 genes in large stretches of mostly non-coding sequences. The conserved region includes the recently identified KCNQ1OT1/Kcnq1ot1 antisense transcripts, flanked by a strikingly conserved cluster of LINE/Line elements and a CpG island which we show to carry a maternal germline methylation imprint. This region is likely to be the proposed second imprinting centre (IC2) in the BWS cluster. We also identified several novel genes inside and outside the previously proposed boundaries of the imprinting cluster. One of the genes outside the cluster, Obph1, is imprinted in mouse placenta indicating that at least in extra-embryonic tissues the imprinting cluster extends into a larger domain.

Deletion of a silencer element in Igf2 results in loss of imprinting independent of H19

Igf2 and H19 are closely linked, reciprocally imprinted genes on mouse distal chromosome 7. The paternally expressed Igf2 encodes a potent fetal growth factor and the maternally expressed H19 encodes a non-coding RNA (refs 1,2). Shared endoderm-specific enhancers 3' to H19 are necessary for transcription of the maternal copy of H19 and the paternal copy of Igf2 (ref. 3), a chromatin boundary upstream of H19 preventing access of the enhancers to the maternal Igf2 promoters. Mesoderm-specific control elements have not been identified, and the role of differentially methylated regions (DMRs) in Igf2 has not been addressed. Two DMRs in Igf2 are methylated on the active paternal allele, suggesting that they contain silencers. Here we have deleted the DMR1 region in Igf2. Maternal transmission of the deletion results in biallelic expression of Igf2 in most mesodermally derived tissues without altering H19 imprinting or expression. Paternal or maternal transmission leads to continued postnatal transcription of Igf2, in contrast to the wild-type allele, which is silenced soon after birth. These results reveal a mesodermal silencer, which may be regulated by methylation and which has a major role in H19-independent expression and imprinting control of Igf2. Our results establish a new mechanistic principle for imprinted genes whereby epigenetically regulated silencers interact with enhancers to control expression, and suggest a new mechanism for loss of imprinting (LOI) of Igf2, which may be important in a number of diseases.

Sequence conservation and variability of imprinting in the Beckwith-Wiedemann syndrome gene cluster in human and mouse

In human and mouse most imprinted genes are arranged in chromosomal clusters. This linked organization suggests coordinated mechanisms controlling imprinted expression. We have sequenced 250 kb in the centre of the mouse imprinting cluster on distal chromosome 7 and compared it with the orthologous Beckwith-Wiedemann gene cluster on human chromosome 11p15.5. This first comparative imprinting cluster analysis revealed a high structural and functional conservation of the six orthologous genes identified. However, several striking differences were also discovered. First, compared with the mouse the human sequence is approximately 40% longer, mostly due to insertions of two large repetitive clusters. One of these clusters encompasses an additional gene coding for a homologue of the ribosomal protein L26. Second, pronounced blocks of unique direct repeats characteristic of imprinted genes were only found in the human sequence. Third, two of the orthologous gene pairs Tssc4/TSSC4 and Ltrpc5/LTRPC5 showed apparent differences in imprinting between human and mouse, whereas others like Tssc6/TSSC6 were not imprinted in either organism. Together these results suggest a significant functional and structural variability in the centre of the imprinting cluster. Some genes escape imprinting in both organisms whereas others exhibit tissue- and species-specific imprinting. Hence the control of imprinting in the cluster appears to be a highly dynamic process under fast evolutionary adaptation. Intriguingly, whereas imprinted genes within the cluster contain CpG islands the non-imprinted Ltrpc5 and Tssc6/TSSC6 do not. This and additional comparisons with other imprinted and non-imprinted regions suggest that CpG islands are key features of imprinted domains.

Igf2 imprinting in development and disease

Igf2 is one of the first imprinted genes discovered and occupies a centre stage in the study of imprinting. This is because it has dramatic effects on the control of fetal growth, it is involved in growth disorders and in cancer, it interacts with products of other imprinted genes, and its imprinting status is under complex regulation in a cluster of tightly linked imprinted genes. Here we review briefly the key features of Igf2 imprinting in normal development and in disease, and hope to show what a fascinating subject of study this gene and its biology provides.

Active demethylation of the paternal genome in the mouse zygote

DNA methylation is essential for the control of a number of biological mechanisms in mammals [1]. Mammalian development is accompanied by two major waves of genome-wide demethylation and remethylation: one during germ-cell development and the other after fertilisation [2] [3] [4] [5] [6] [7]. Most previous studies have suggested that the genome-wide demethylation observed after fertilisation occurs passively, that is, by the lack of maintenance methylation following DNA replication and cell division [6] [7], although one other study has reported that replication-independent demethylation may also occur during early embryogenesis [8]. Here, we report that genes that are highly methylated in sperm are rapidly demethylated in the zygote only hours after fertilisation, before the first round of DNA replication commences. By contrast, the oocyte-derived maternal alleles are unaffected by this reprogramming. They either remain methylated after fertilisation or become further methylated de novo. These results provide the first direct evidence for active demethylation of single-copy genes in the mammalian zygote and, moreover, reveal a striking asymmetry in epigenetic methylation reprogramming. Whereas paternally (sperm)-derived sequences are exposed to putative active demethylases in the oocyte cytoplasm, maternally (oocyte)-derived sequences are protected from this reaction. These results, whose generality is supported by findings of Mayer et al. [9], have important implications for the establishment of biparental genetic totipotency after fertilisation, the establishment and maintenance of genomic imprinting, and the reprogramming of somatic cells during cloning.

A maternally methylated CpG island in KvLQT1 is associated with an antisense paternal transcript and loss of imprinting in Beckwith-Wiedemann syndrome

Loss of imprinting at IGF2, generally through an H19-independent mechanism, is associated with a large percentage of patients with the overgrowth and cancer predisposition condition Beckwith-Wiedemann syndrome (BWS). Imprinting control elements are proposed to exist within the KvLQT1 locus, because multiple BWS-associated chromosome rearrangements disrupt this gene. We have identified an evolutionarily conserved, maternally methylated CpG island (KvDMR1) in an intron of the KvLQT1 gene. Among 12 cases of BWS with normal H19 methylation, 5 showed demethylation of KvDMR1 in fibroblast or lymphocyte DNA; whereas, in 4 cases of BWS with H19 hypermethylation, methylation at KvDMRl was normal. Thus, inactivation of H19 and hypomethylation at KvDMR1 (or an associated phenomenon) represent distinct epigenetic anomalies associated with biallelic expression of IGF2. Reverse transcription-PCR analysis of the human and syntenic mouse loci identified the presence of a KvDMR1-associated RNA transcribed exclusively from the paternal allele and in the opposite orientation with respect to the maternally expressed KvLQT1 gene. We propose that KvDMR1 and/or its associated antisense RNA (KvLQT1-AS) represents an additional imprinting control element or center in the human 11p15.5 and mouse distal 7 imprinted domains.

Analysis of germline CDKN1C (p57KIP2) mutations in familial and sporadic Beckwith-Wiedemann syndrome (BWS) provides a novel genotype-phenotype correlation

Beckwith-Wiedemann syndrome (BWS) is a human imprinting disorder with a variable phenotype. The major features are anterior abdominal wall defects including exomphalos (omphalocele), pre- and postnatal overgrowth, and macroglossia. Additional less frequent complications include specific developmental defects and a predisposition to embryonal tumours. BWS is genetically heterogeneous and epigenetic changes in the IGF2/H19 genes resulting in overexpression of IGF2 have been implicated in many cases. Recently germline mutations in the cyclin dependent kinase inhibitor gene CDKN1C (p57KIP2) have been reported in a variable minority of BWS patients. We have investigated a large series of familial and sporadic BWS patients for evidence of CDKN1C mutations by direct gene sequencing. A total of 70 patients with classical BWS were investigated; 54 were sporadic with no evidence of UPD and 16 were familial from seven kindreds. Novel germline CDKN1C mutations were identified in five probands, 3/7 (43%) familial cases and 2/54 (4%) sporadic cases. There was no association between germline CDKN1C mutations and IGF2 or H19 epigenotype abnormalities. The clinical phenotype of 13 BWS patients with germline CDKN1C mutations was compared to that of BWS patients with other defined types of molecular pathology. This showed a significantly higher frequency of exomphalos in the CDKN1C mutation cases (11/13) than in patients with an imprinting centre defect (associated with biallelic IGF2 expression and H19 silencing) (0/5, p<0.005) or patients with uniparental disomy (0/9, p<0.005). However, there was no association between germline CDKN1C mutations and risk of embryonal tumours. No CDKN1C mutations were identified in six non-BWS patients with overgrowth and Wilms tumour. These findings (1) show that germline CDKN1C mutations are a frequent cause of familial but not sporadic BWS, (2) suggest that CDKN1C mutations probably cause BWS independently of changes in IGF2/H19 imprinting, (3) provide evidence that aspects of the BWS phenotype may be correlated with the involvement of specific imprinted genes, and (4) link genotype-phenotype relationships in BWS and the results of murine experimental models of BWS.

Overexpression of insulin-like growth factor-II in transgenic mice is associated with pancreatic islet cell hyperplasia

We have used an insulin-like growth factor (IGF)-II transgenic mouse model in which mouse IGF-II is widely overexpressed, resulting in increased fetal size and selective organ overgrowth, to investigate the effects on the development of the endocrine pancreas. Fetuses examined on day 19.5-20 of gestation had significantly elevated circulating levels of IGF-II, compared with control mice. The pancreatic islets in transgenic animals were of irregular shape and had a mean area five times greater than in controls, whereas the mean number of islets per tissue section was not altered. The size of individual endocrine cells was not altered. Although the islets in animals expressing the IGF-II transgene were considerably larger, immunohistochemistry for insulin and glucagon showed that the relative proportion of beta-cells was significantly less, and that of alpha-cells was higher. Normal islet morphology was disrupted, with alpha-cells appearing in small groups within the islets, as well as on the periphery, whereas beta-cells were often seen at the edge of the islets. Twice as many islet cells (21.9% vs. 11.4%) were involved in cell replication, detected by the presence of immunoreactive proliferating cell nuclear antigen, in pancreata from transgenic mice vs. controls, whereas the number of cells undergoing apoptosis was significantly reduced. Abundant IGF-II messenger RNAwas found within the islets of transgenic animals by in situ hybridization, and the relative area of islets demonstrating immunoreactive IGF-II was significantly greater. Immunoreactive IGF-I was much less abundant and was further reduced in islets of transgenic animals. The area of islets immunopositive for IGF binding protein-2 was unaltered. Despite the presence of islet hyperplasia, circulating insulin levels and serum glucose levels were not significantly different between transgenic and control mice. These results show that an overexpression of IGF-II in fetal life has a profound effect on islet morphology and causes islet hyperplasia while reducing the attrition of islet cells by apoptosis.

Imprinting mechanisms

A number of recent studies have provided new insights into mechanisms that regulate genomic imprinting in the mammalian genome. Regions of allele-specific differential methylation (DMRs) are present in all imprinted genes examined. Differential methylation is erased in germ cells at an early stage of their development, and germ-line-specific methylation imprints in DMRs are reestablished around the time of birth. After fertilization, differential methylation is retained in core DMRs despite genome-wide demethylation and de novo methylation during preimplantation and early postimplantation stages. Direct repeats near CG-rich DMRs may be involved in the establishment and maintenance of allele-specific methylation patterns. Imprinted genes tend to be clustered; one important component of clustering is enhancer competition, whereby promoters of linked imprinted genes compete for access to enhancers. Regional organization and spreading of the epigenotype during development is also important and depends on DMRs and imprinting centers. The mechanism of cis spreading of DNA methylation is not known, but precedent is provided by the Xist RNA, which results in X chromosome inactivation in cis. Reading of the somatic imprints could be carried out by transcription factors that are sensitive to methylation, or by methyl-cytosine-binding proteins that are involved in transcriptional repression through chromatin remodeling.

Syntenic organization of the mouse distal chromosome 7 imprinting cluster and the Beckwith-Wiedemann syndrome region in chromosome 11p15.5

In human and mouse, most imprinted genes are arranged in chromosomal clusters. Their linked organization suggests co-ordinated mechanisms controlling imprinting and gene expression. The identification of local and regional elements responsible for the epigenetic control of imprinted gene expression will be important in understanding the molecular basis of diseases associated with imprinting such as Beckwith-Wiedemann syndrome. We have established a complete contig of clones along the murine imprinting cluster on distal chromosome 7 syntenic with the human imprinting region at 11p15.5 associated with Beckwith-Wiedemann syndrome. The cluster comprises approximately 1 Mb of DNA, contains at least eight imprinted genes and is demarcated by the two maternally expressed genes Tssc3 (Ipl) and H19 which are directly flanked by the non-imprinted genes Nap1l4 (Nap2) and Rpl23l (L23mrp), respectively. We also localized Kcnq1 (Kvlqt1) and Cd81 (Tapa-1) between Cdkn1c (p57(Kip2)) and Mash2. The mouse Kcnq1 gene is maternally expressed in most fetal but biallelically transcribed in most neonatal tissues, suggesting relaxation of imprinting during development. Our findings indicate conserved control mechanisms between mouse and human, but also reveal some structural and functional differences. Our study opens the way for a systematic analysis of the cluster by genetic manipulation in the mouse which will lead to animal models of Beckwith-Wiedemann syndrome and childhood tumours.

Altered imprinted gene methylation and expression in completely ES cell-derived mouse fetuses: association with aberrant phenotypes

In vitro manipulation of preimplantation mammalian embryos can influence differentiation and growth at later stages of development. In the mouse, culture of embryonic stem (ES) cells affects their totipotency and may give rise to fetal abnormalities. To investigate whether this is associated with epigenetic alterations in imprinted genes, we analysed two maternally expressed genes (Igf2r, H19) and two paternally expressed genes (Igf2, U2af1-rs1) in ES cells and in completely ES cell-derived fetuses. Altered allelic methylation patterns were detected in all four genes, and these were consistently associated with allelic changes in gene expression. All the methylation changes that had arisen in the ES cells persisted on in vivo differentiation to fetal stages. Alterations included loss of methylation with biallelic expression of U2af1-rs1, maternal methylation and predominantly maternal expression of Igf2, and biallelic methylation and expression of Igf2r. In many of the ES fetuses, the levels of H19 expression were strongly reduced, and this biallelic repression was associated with biallellic methylation of the H19 upstream region. Surprisingly, biallelic H19 repression was not associated with equal levels of Igf2 expression from both parental chromosomes, but rather with a strong activation of the maternal Igf2 allele. ES fetuses derived from two of the four ES lines appeared developmentally compromised, with polyhydramnios, poor mandible development and interstitial bleeding and, in chimeric fetuses, the degree of chimerism correlated with increased fetal mass. Our study establishes a model for how early embryonic epigenetic alterations in imprinted genes persist to later developmental stages, and are associated with aberrant phenotypes.

Imprinting mechanisms in mammals

Imprinting is a genetic mechanism that determines expression or repression of genes according to their parental origin. Some imprinted genes occur in clusters in the genome. Recent work using transgenic mice shows that multiple cis-acting sequences are needed for correct imprinting. Mutation analysis in a normal chromosomal context reveals the importance of imprinting centres for regional establishment or maintenance of imprinting in a cluster. Elements that contribute to the function of imprinting centres and regional propagation of the imprints are CpG-rich differentially methylated regions (that during development retain germline imposed methylation or demethylation), direct repeat clusters, and unusual RNAs (antisense, non-translated etc.). The interaction of these cis elements with transacting factors such as methylase and chromatin factors establishes a hierarchical control system with local and regional effects.

Analysis and identification of imprinted genes

Genomic imprinting in mammals results in the unequal expression of the two parental alleles of specific genes. The existence of imprinting in the mouse emerged from nuclear transplantation studies and from the abnormal phenotypes associated with uniparental inheritance of particular chromosome segments. Over the past 5 years, 20 or so imprinted genes have been identified. This has emphasized the important roles played by some imprinted genes in development, permitted a description of the epigenetic properties associated with imprinting, and provided the first insights into the regulation of imprinting. In this article, we discuss the generation of experimental material in which imprinting effects can be analyzed, review the properties of imprinted genes, and discuss how to examine them using state-of-the-art techniques. Finally, we consider the means by which new imprinted genes can be identified.

Multiple imprinted sense and antisense transcripts, differential methylation and tandem repeats in a putative imprinting control region upstream of mouse Igf2

The mouse insulin-like growth factor 2 (Igf2) locus is a complex genomic region that produces multiple transcripts from alternative promoters. Expression at this locus is regulated by parental imprinting. However, despite the existence of putative imprinting control elements in the Igf2 upstream region, imprinted transcriptional repression is abolished by null mutations at the linked H19 locus. To clarify the extent to which the Igf2 upstream region contains autonomous imprinting control elements we have performed functional and comparative analyses of the region in the mouse and human. Here we report the existence of multiple, overlapping imprinted (maternally repressed) sense and antisense transcripts that are associated with a tandem repeat in the mouse Igf2 upstream region. Regions flanking the repeat exhibit tissue-specific parental allelic methylation patterns, suggesting the existence of tissue-specific control elements in the upstream region. Studies in H19 null mice indicate that both parental allelic methylation and monoallelic expression of the upstream transcripts depends on an intact H19 gene acting in cis. The homologous region in human IGF2 is structurally conserved, with the significant exception that it does not contain a tandem repeat. Our results support the proposal that tandem repeats act to target methylation to imprinted genetic loci.

Transactivation of Igf2 in a mouse model of Beckwith-Wiedemann syndrome

The gene IGF2, which encodes a fetal insulin-like growth factor, is imprinted, so only one of two parental copies of the gene is expressed. The altered expression of IGF2 has been implicated in Beckwith-Wiedemann syndrome, a human fetal overgrowth syndrome, which is characterized by overgrowth of several organs and an increased risk of developing childhood tumours. We have introduced Igf2 transgenes into the mouse genome by using embryonic stem cells, which leads to transactivation of the endogenous Igf2 gene. The consequent overexpression of Igf2 results in most of the symptoms of Beckwith-Wiedemann syndrome, including prenatal overgrowth, polyhydramnios, fetal and neonatal lethality, disproportionate organ overgrowth including tongue enlargement, and skeletal abnormalities. These phenotypes establish Igf2 overexpression as a key determinant of Beckwith-Wiedemann syndrome.