Duplication of 7p11.2-p13, including GRB10, in Silver-Russell syndrome

Silver-Russell syndrome (SRS) is characterized by pre- and postnatal growth failure and other dysmorphic features. The syndrome is genetically heterogeneous, but maternal uniparental disomy of chromosome 7 has been demonstrated in approximately 7% of cases. This suggests that at least one gene on chromosome 7 is imprinted and involved in the pathogenesis of SRS. We have identified a de novo duplication of 7p11.2-p13 in a proband with features characteristic of SRS. FISH confirmed the presence of a tandem duplication encompassing the genes for growth factor receptor-binding protein 10 (GRB10) and insulin-like growth factor-binding proteins 1 and 3 (IGFBP1 and -3) but not that for epidermal growth factor-receptor (EGFR). Microsatellite markers showed that the duplication was of maternal origin. These findings provide the first evidence that SRS may result from overexpression of a maternally expressed imprinted gene, rather than from absent expression of a paternally expressed gene. GRB10 lies within the duplicated region and is a strong candidate, since it is a known growth suppressor. Furthermore, the mouse homologue (Grb10/Meg1) is reported to be maternally expressed and maps to the imprinted region of proximal mouse chromosome 11 that demonstrates prenatal growth failure when it is maternally disomic. We have demonstrated that the GRB10 genomic interval replicates asynchronously in human lymphocytes, suggestive of imprinting. An additional 36 SRS probands were investigated for duplication of GRB10, but none were found. However, it remains possible that GRB10 and/or other genes within 7p11.2-p13 are responsible for some cases of SRS.     

Biparental expression of IGFBP1 and IGFBP3 renders their involvement in the etiology of Silver-Russell syndrome unlikely.

Maternal uniparental disomy (UPD) of chromosome 7 has recently been reported in about 10% of Silver-Russell (SRS) patients. It can therefore be concluded that at least one gene on chromosome 7 is imprinted and mutations in this gene/these genes might contribute to the phenotype of the disease. Two genes which are involved in growth and localised in 7p12-13 are the insulin-like growth factor binding proteins 1 and 3 (IGFBP1; IGFBP3). Comparison to the mouse genome shows that the syntenic region on mouse chromosome 11 is imprinted, UPD of this region leads to deviations in growth in mice. In the present study we investigated whether the genes for IGFBP1 and IGFBP3 might be involved in the etiology of SRS: after exclusion of SRS specific mutations we could demonstrate biparental expression of both genes in lymphocytes of an SRS patient without UPD7 as well as expression in a patient with maternal UPD7. Our results as well as those from other groups show biparental expression of IGFBP1 in fetal tissues and expression of IGFBP3 in nearly every tissue during puberty and adult life. Thus, no evidence is given for an involvement of the two genes in SRS.     

A narrow segment of maternal uniparental disomy of chromosome 7q31-qter in Silver-Russell syndrome delimits a candidate gene region

Maternal uniparental disomy of chromosome 7 (matUPD7), the inheritance of both chromosomes from only the mother, is observed in approximately 10% of patients with Silver-Russell syndrome (SRS). It has been suggested that at least one imprinted gene that regulates growth and development resides on human chromosome 7. To date, three imprinted genes-PEG1/MEST, gamma2-COP, and GRB10-have been identified on chromosome 7, but their role in the etiology of SRS remains uncertain. In a systematic screening with microsatellite markers, for matUPD7 cases among patients with SRS, we identified a patient who had a small segment of matUPD7 and biparental inheritance of the remainder of chromosome 7. Such a pattern may be explained by somatic recombination in the zygote. The matUPD7 segment at 7q31-qter extends for 35 Mb and includes the imprinted gene cluster of PEG1/MEST and gamma2-COP at 7q32. GRB10 at 7p11.2-p12 is located within a region of biparental inheritance. Although partial UPD has previously been reported for chromosomes 6, 11, 14, and 15, this is the first report of a patient with SRS who has segmental matUPD7. Our findings delimit a candidate imprinted region sufficient to cause SRS.     

Differentially methylated regions in maternal and paternal uniparental disomy for chromosome 7

DNA methylation is a hallmark of genomic imprinting and differentially methylated regions (DMRs) are found near and in imprinted genes. Imprinted genes are expressed only from the maternal or paternal allele and their normal balance can be disrupted by uniparental disomy (UPD), the inheritance of both chromosomes of a chromosome pair exclusively from only either the mother or the father. Maternal UPD for chromosome 7 (matUPD7) results in Silver-Russell syndrome (SRS) with typical features and growth retardation, but no gene has been conclusively implicated in SRS. In order to identify novel DMRs and putative imprinted genes on chromosome 7, we analyzed eight matUPD7 patients, a segmental matUPD7q31-qter, a rare patUPD7 case and ten controls on the Infinium HumanMethylation450K BeadChip with 30?017 CpG methylation probes for chromosome 7. Genome-scale analysis showed highly significant clustering of DMRs only on chromosome 7, including the known imprinted loci GRB10, SGCE/PEG10, and PEG/MEST. We found ten novel DMRs on chromosome 7, two DMRs for the predicted imprinted genes HOXA4 and GLI3 and one for the disputed imprinted gene PON1. Quantitative RT-PCR on blood RNA samples comparing matUPD7, patUPD7, and controls showed differential expression for three genes with novel DMRs, HOXA4, GLI3, and SVOPL. Allele specific expression analysis confirmed maternal only expression of SVOPL and imprinting of HOXA4 was supported by monoallelic expression. These results present the first comprehensive map of parent-of-origin specific DMRs on human chromosome 7, suggesting many new imprinted sites.     

Differentially methylated regions in maternal and paternal uniparental disomy for chromosome 7

DNA methylation is a hallmark of genomic imprinting and differentially methylated regions (DMRs) are found near and in imprinted genes. Imprinted genes are expressed only from the maternal or paternal allele and their normal balance can be disrupted by uniparental disomy (UPD), the inheritance of both chromosomes of a chromosome pair exclusively from only either the mother or the father. Maternal UPD for chromosome 7 (matUPD7) results in Silver-Russell syndrome (SRS) with typical features and growth retardation, but no gene has been conclusively implicated in SRS. In order to identify novel DMRs and putative imprinted genes on chromosome 7, we analyzed eight matUPD7 patients, a segmental matUPD7q31-qter, a rare patUPD7 case and ten controls on the Infinium HumanMethylation450K BeadChip with 30 017 CpG methylation probes for chromosome 7. Genome-scale analysis showed highly significant clustering of DMRs only on chromosome 7, including the known imprinted loci GRB10, SGCE/PEG10, and PEG/MEST. We found ten novel DMRs on chromosome 7, two DMRs for the predicted imprinted genes HOXA4 and GLI3 and one for the disputed imprinted gene PON1. Quantitative RT-PCR on blood RNA samples comparing matUPD7, patUPD7, and controls showed differential expression for three genes with novel DMRs, HOXA4, GLI3, and SVOPL. Allele specific expression analysis confirmed maternal only expression of SVOPL and imprinting of HOXA4 was supported by monoallelic expression. These results present the first comprehensive map of parent-of-origin specific DMRs on human chromosome 7, suggesting many new imprinted sites.     

Imprinted methylation profiles for proximal mouse Chromosomes 11 and 7 as revealed by methylation-sensitive representational difference analysis

Proximal mouse Chromosome (Chr) 11 shares regions of orthology with the candidate gene region for the imprinting growth disorder Silver-Russell syndrome (SRS) on human Chr 7p. It has previously been shown that mice with two maternal or two paternal copies (duplications, Dp) of proximal Chr 11 exhibit reciprocal growth phenotypes. Those with two paternal copies show fetal and placental overgrowth, while those with two maternal copies are growth retarded. The growth retardation observed in the latter is reminiscent of the intrauterine growth restriction (IUGR) observed in SRS patients with maternal uniparental disomy for Chr 7 (mUPD7). We have carried out a methylation-sensitive representational difference analysis (Me-RDA) screen to look for regions of differential methylation (DMRs) associated with imprinted genes. For these experiments, we have used mouse embryos with uniparental duplications of Chrs 11 and 7 proximal to the breakpoint of the reciprocal translocation T(7;11)40Ad. Two previously known imprinted loci associated with paternal allele hypomethylation were recovered on proximal mouse Chr 11, U2af1-rs1 and Meg1/Grb10. These two genes map 15 cM apart, so it seems likely that they are within separate imprinted domains that do not contain additional DMRs. The known imprinted gene Peg3, located on mouse proximal Chr 7, was also detected in our screen. The finding that Peg3 was differentially methylated in embryos with uniparental inheritance of proximal Chr 7 confirms that Peg3 is located proximal to the breakpoint of T40Ad in G-band 7A2. Because GRB10 has previously been reported to be a candidate gene for SRS, we analysed 22 patients for epimutations of the GRB10 differentially methylated region that could lead to the altered expression of this gene. No such mutations were found.     

Imprinting of human GRB10 and its mutations in two patients with Russell-Silver syndrome

Documentation of maternal uniparental disomy of chromosome 7 in 10% of patients with Russell-Silver syndrome (RSS), characterized by prenatal and postnatal growth retardation and dysmorphic features, has suggested the presence of an imprinted gene on chromosome 7 whose mutation is responsible for the RSS phenotype. Human GRB10 on chromosome 7, a homologue of the mouse imprinted gene Grb10, is a candidate, because GRB10 has a suppressive effect on growth, through its interaction with either the IGF-I receptor or the GH receptor, and two patients with RSS were shown to have a maternally derived duplication of 7p11-p13, encompassing GRB10. In the present study, we first demonstrated that the GRB10 gene is also monoallelically expressed in human fetal brain tissues and is transcribed from the maternally derived allele in somatic-cell hybrids. Hence, human GRB10 is imprinted. A mutation analysis of GRB10 in 58 unrelated patients with RSS identified, within the N-terminal domain of the protein, a P95S substitution in two patients with RSS. In these two cases, the mutant allele was inherited from the mother. The fact that monoallelic GRB10 expression was observed from the maternal allele in this study suggests but does not prove that these maternally transmitted mutant alleles contribute to the RSS phenotype.     

Transienter neonataler Diabetes und Hypomethylierungssyndrome

Transient neonatal diabetes (TNDM) is manifested before the age of 6 weeks and typically resolves within 18 months. Main clinical features include intrauterine growth retardation, hyperglycemia and dehydration with absent ketoacidosis. Causes of TNDM are heterogeneous but 70% are due to a chromosomal aberration in the region 6q24 which contains the imprinted genes PLAGL1/ZAC and HYMAI. Paternal uniparental disomy 6 (upd(6)pat) or paternal duplications of the imprinted region as well as imprinting defects of the maternal allele all result in an overexpression of the paternally expressed gene PLAGL1. Imprinting defects in 6q24 can occur as isolated events or can affect more than one locus (hypomethylation syndrome). Hypomethylation at multiple loci has so far been observed in patients with TNDM, Silver-Russell syndrome (SRS) and Beckwith-Wiedemann syndrome (BWS).The risk of recurrence depends on the underlying cause of TNDM. Chromosomal aberrations in the parents affecting chromosome 6 increase the risk for UPD or duplication of the imprinted locus in 6q24. Nevertheless, UPD and duplication 6q24 are mostly de novo occurrences.     

Absence of a paternally inherited FOXP2 gene in developmental verbal dyspraxia

Mutations in FOXP2 cause developmental verbal dyspraxia (DVD), but only a few cases have been described. We characterize 13 patients with DVD--5 with hemizygous paternal deletions spanning the FOXP2 gene, 1 with a translocation interrupting FOXP2, and the remaining 7 with maternal uniparental disomy of chromosome 7 (UPD7), who were also given a diagnosis of Silver-Russell Syndrome (SRS). Of these individuals with DVD, all 12 for whom parental DNA was available showed absence of a paternal copy of FOXP2. Five other individuals with deletions of paternally inherited FOXP2 but with incomplete clinical information or phenotypes too complex to properly assess are also described. Four of the patients with DVD also meet criteria for autism spectrum disorder. Individuals with paternal UPD7 or with partial maternal UPD7 or deletion starting downstream of FOXP2 do not have DVD. Using quantitative real-time polymerase chain reaction, we show the maternally inherited FOXP2 to be comparatively underexpressed. Our results indicate that absence of paternal FOXP2 is the cause of DVD in patients with SRS with maternal UPD7. The data also point to a role for differential parent-of-origin expression of FOXP2 in human speech development.     

Parental Origin of Interstitial Duplications at 15q11.2-q13.3 in Schizophrenia and Neurodevelopmental Disorders

Duplications at 15q11.2-q13.3 overlapping the Prader-Willi/Angelman syndrome (PWS/AS) region have been associated with developmental delay (DD), autism spectrum disorder (ASD) and schizophrenia (SZ). Due to presence of imprinted genes within the region, the parental origin of these duplications may be key to the pathogenicity. Duplications of maternal origin are associated with disease, whereas the pathogenicity of paternal ones is unclear. To clarify the role of maternal and paternal duplications, we conducted the largest and most detailed study to date of parental origin of 15q11.2-q13.3 interstitial duplications in DD, ASD and SZ cohorts. We show, for the first time, that paternal duplications lead to an increased risk of developing DD/ASD/multiple congenital anomalies (MCA), but do not appear to increase risk for SZ. The importance of the epigenetic status of 15q11.2-q13.3 duplications was further underlined by analysis of a number of families, in which the duplication was paternally derived in the mother, who was unaffected, whereas her offspring, who inherited a maternally derived duplication, suffered from psychotic illness. Interestingly, the most consistent clinical characteristics of SZ patients with 15q11.2-q13.3 duplications were learning or developmental problems, found in 76% of carriers. Despite their lower pathogenicity, paternal duplications are less frequent in the general population with a general population prevalence of 0.0033% compared to 0.0069% for maternal duplications. This may be due to lower fecundity of male carriers and differential survival of embryos, something echoed in the findings that both types of duplications are de novo in just over 50% of cases. Isodicentric chromosome 15 (idic15) or interstitial triplications were not observed in SZ patients or in controls. Overall, this study refines the distinct roles of maternal and paternal interstitial duplications at 15q11.2-q13.3, underlining the critical importance of maternally expressed imprinted genes in the contribution of Copy Number Variants (CNVs) at this interval to the incidence of psychotic illness. This work will have tangible benefits for patients with 15q11.2-q13.3 duplications by aiding genetic counseling.     

Chromosome 7p disruptions in Silver Russell syndrome: delineating an imprinted candidate gene region

Silver-Russell syndrome (SRS) is characterised by pre- and postnatal growth restriction (PNGR) and additional dysmorphic features including body asymmetry and fifth finger clinodactyly. The syndrome is genetically heterogeneous, with a number of chromosomes implicated. However, maternal uniparental disomy for chromosome 7 has been demonstrated in up to 10% of all cases. Three SRS probands have previously been described with a maternally inherited duplication of 7p11.2-p13, defining this as a candidate region. Over-expression of a maternally transcribed, imprinted gene with growth-suppressing activity located within the duplicated region, or breakpoint disruption of genes or regulatory sequences, may account for the phenotype in these cases. Here we describe two additional SRS patients and four probands with PNGR with a range of cytogenetic disruptions of 7p, including duplications, pericentric inversions and a translocation. An incomplete contig consisting of 80 PACs and BACs from the centromere to 7p14 was constructed. Individual clones from this contig were used as FISH probes to map the breakpoints in the six new cases and the three duplication probands previously described. Our data provide further evidence for a candidate SRS region at 7p11.1-p14. A common breakpoint region was identified within 7p11.2 in all nine cases, pinpointing this specific interval. The imprinting status of genes within the 7p11.1-p14 region flanked by the most extreme breakpoints have been analysed using both somatic cell hybrids containing a single full-length maternally or paternally derived chromosome 7 and expressed single nucleotide polymorphisms in paired fetal and maternal samples.     

Mutation analysis of UBE3A in Angelman syndrome patients.

Angelman syndrome (AS) is caused by chromosome 15q11-q13 deletions of maternal origin, by paternal uniparental disomy (UPD) 15, by imprinting defects, and by mutations in the UBE3A gene. UBE3A encodes a ubiquitin-protein ligase and shows brain-specific imprinting. Here we describe UBE3A coding-region mutations detected by SSCP analysis in 13 AS individuals or families. Two identical de novo 5-bp duplications in exon 16 were found. Among the other 11 unique mutations, 8 were small deletions or insertions predicted to cause frameshifts, 1 was a mutation to a stop codon, 1 was a missense mutation, and 1 was predicted to cause insertion of an isoleucine in the hect domain of the UBE3A protein, which functions in E2 binding and ubiquitin transfer. Eight of the cases were familial, and five were sporadic. In two familial cases and one sporadic case, mosaicism for UBE3A mutations was detected: in the mother of three AS sons, in the maternal grandfather of two AS first cousins, and in the mother of an AS daughter. The frequencies with which we detected mutations were 5 (14%) of 35 in sporadic cases and 8 (80%) of 10 in familial cases.     

Unexpected Inheritance of a Balanced Homologous translocation t(22q;22q) from father to a phenotypically normal daughter

Rearrangements between homologous chromosomes are extremely rare and manifest mainly as monosomic or trisomic offsprings. There are remarkably few reports of balanced homologous chromosomal translocation t (22q; 22q) and only two cases of transmission of this balanced homohologous rearrangement from mother to normal daughter are reported. Robersonian translocation carriers in non-homologous chromosomes have the ability to have an unaffected child. However, it is not possible to have an unaffected child in cases with Robersonian translocations in homologous chromosomes. Carriers of homologous chromosome 22 translocations with maternal uniparental disomy do not have any impact on their phenotype. We are presenting a family with a history of multiple first trimester miscarriages and an unexpected inheritance of balanced homologous translocation of chromosome 22 with paternal uniparental disomy. There are no data available regarding the impact of paternal UPD 22 on the phenotype. We claim this to be the first report explaining that paternal UPD 22 does not impact the phenotype.     

Maternal and paternal chromosomes 7 show differential methylation of many genes in lymphoblast DNA

Genomic imprinting, the differential expression of paternal and maternal alleles, involves many chromosomal regions and plays a role in development and growth. Differential methylation of maternal and paternal alleles is a hallmark of imprinted genes, and thus methylation assays are widely used to support the identification of novel imprinted genes. Either blood or lymphoblast DNAs are most often used in these assays, even though methylation levels may change in cell culture. We undertook a systematic survey of parent-of-origin-specific methylation of chromosome 7 genes and ESTs by comparing DNA samples from cases of maternal and paternal uniparental disomy for chromosome 7 using DNA from fresh blood and lymphoblast cell lines. Our results revealed that up to 41% of genes and ESTs show parent-of-origin-specific methylation differences in lymphoblast DNA after only a short time in culture, whereas methylation differences were not seen in blood DNA. The methylation changes occurred most commonly on paternal chromosome 7, whereas alterations on maternal chromosome 7 were more infrequent and weaker. These findings indicate that methylation patterns may change significantly during cell culture in a parent-of-origin-dependent manner and suggest that methylation is maintained differently on maternal and paternal chromosomes 7.     

Retinal Dystrophy Due to Paternal Isodisomy for Chromosome 1 or Chromosome 2, with Homoallelism for Mutations in RPE65 or MERTK, Respectively

Uniparental disomy (UPD) is a rare condition in which a diploid offspring carries a chromosomal pair from a single parent. We now report the first two cases of UPD resulting in retinal degeneration. We identified an apparently homozygous loss-of-function mutation of RPE65 (1p31) in one retinal dystrophy patient and an apparently homozygous loss-of-function mutation of MERTK (2q14.1) in a second retinal dystrophy patient. In both families, the gene defect was present in the patient's heterozygous father but not in the patient's mother. Analysis of haplotypes in each nuclear kindred, by use of DNA polymorphisms distributed along both chromosomal arms, indicated the absence of the maternal allele for all informative markers tested on chromosome 1 in the first patient and on chromosome 2 in the second patient. Our results suggest that retinal degeneration in these individuals is due to apparently complete paternal isodisomy involving reduction to homoallelism for RPE65 or MERTK loss-of-function alleles. Our findings provide evidence for the first time, in the case of chromosome 2, and confirm previous observations, in the case of chromosome 1, that there are no paternally imprinted genes on chromosomes 1 and 2 that have a major effect on phenotype.     

Formation of uniparental disomy 7 delineated from new cases and a UPD7 case after trisomy 7 rescue. Presentation of own results and review of the literature

Maternal uniparental disomy for the entire chromosome 7 (matUPD7) has been reported several times in Silver-Russell syndrome (SRS) and growth-restricted patients. Here we present our results from the analysis of an abortion with confined placental mosaicism (CPM) for trisomy 7 which showed a maternal meiotic origin of the trisomy in the placenta and rescue to maternal UPD7 in foetal membrane. Furthermore, two newly detected SRS cases with maternal UPD7 revealed isodisomy and partial heterodisomy, respectively. Summarising these results with those published previously on the origin of UPD7, similar numbers of isodisomy (n=11) and cases with complete or partial heterodisomy (n=12) have been reported. In respect to the different formation mechanisms of UPD, complete isodisomy should be the result of a post-zygotic mitotic segregation error, whereas heterodisomic UPDs should be caused by trisomic rescue after meiotic non-disjunction events. In maternal UPD7, 50% of cases seem to be caused by post-zygotic mitotic segregation errors, which is similar to the situation in trisomy 7. This result corresponds to the situation in trisomy 8 but is in contrast to observations in the frequent aneuploidies. Thus, the different findings in these aberrations reflect the presence of multiple factors that act to ensure normal segregation, varying in importance for each chromosome.