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

Diagnostic proceeding in Silver-Russell syndrome

BACKGROUND

Silver-Russell syndrome (SRS) describes a uniform malformation syndrome characterized by pre- and postnatal growth restriction (<3rd percentile) and a typical craniofacial gestalt. The basic defect of SRS is currently unknown, and the number of meaningful genetic tests available is therefore limited. Different chromosomal aberrations have been identified, including in the chromosomal region 7p12-p14. Detailed analyses of numerous candidate genes have not revealed any relevant insights with respect to the etiology of the disease.However, maternal uniparental disomy (UPD) of chromosome 7 (matUPD7), the inheritance of both homologues of chromosome 7 only from the mother, is observed in approximately 10% of SRS patients. Here, we report on our experiences of UPD testing in patients referred to our laboratory with the clinical diagnosis of SRS. A diagnostic algorithm for SRS is suggested.

METHODS

Eighty-six patients with the clinical diagnosis of SRS were screened for matUPD7 by microsatellite typing. In 13 cases, the clinical data were consistent with the diagnosis of SRS. The other 73 patients were referred for UPD testing with the suspected diagnosis of SRS, but clinical data were scarce.

RESULTS

In total, we identified three new cases of matUPD7: one patient belonged to the cohort of 13 clinically characterized patients; the other two patients were referred with the suspected diagnosis of SRS but initially without detailed reports. DNA studies revealed uniparental heterodisomy 7 in two patients, while the results in the third case were consistent with uniparental isodisomy.

CONCLUSIONS

MatUPD7 is predominantly detectable in patients showing SRS features, and testing should therefore be restricted to this group of growth-restricted patients. Generally, a combination of cytogenetic and molecular genetic tests can be offered in SRS, aiming at the detection of chromosomal rearrangements and matUPD7 in >10% of SRS patients.

Chromosome 7 aberrations in a young girl with myelodysplasia and hepatoblastoma: an unusual association

We report a 30-month-old female with intrauterine growth retardation, postnatal failure to thrive, pancytopoenia and myelodysplasia with monosomy 7 in the marrow. The child succumbed to overwhelming sepsis, following a bone marrow transplant to facilitate chemotherapy for metastatic hepatoblastoma--a tumour that has not been previously reported in myelodysplasia syndromes. Cytogenetic, molecular and microarray analysis of peripheral blood, skin fibroblasts and bone marrow revealed unusual results, suggestive of somatic chromosome instability. A normal peripheral blood karyotype was documented in infancy. Monosomy 7 was found in the bone marrow. Molecular (microsatellite marker) results for a later peripheral blood specimen were suggestive of partial maternal isodisomy 7q, and this was supported by microarray data on single-nucleotide polymorphisms. Microarray data on gene copy number, collected for the same blood specimen, indicated cryptic mosaicism for the monosomy 7 cell line, with the monosomic line lacking the paternal copy. In fibroblasts, cytogenetic data showed mosaic partial trisomy for distal 7p.

Grb10 and Grb14: enigmatic regulators of insulin action--and more?

The Grb proteins (growth factor receptor-bound proteins) Grb7, Grb10 and Grb14 constitute a family of structurally related multidomain adapters with diverse cellular functions. Grb10 and Grb14, in particular, have been implicated in the regulation of insulin receptor signalling, whereas Grb7 appears predominantly to be involved in focal adhesion kinase-mediated cell migration. However, at least in vitro, these adapters can bind to a variety of growth factor receptors. The highest identity within the Grb7/10/14 family occurs in the C-terminal SH2 (Src homology 2) domain, which mediates binding to activated receptors. A second well-conserved binding domain, BPS [between the PH (pleckstrin homology) and SH2 domains], can act to enhance binding to the IR (insulin receptor). Consistent with a putative adapter function, some non-receptor-binding partners, including protein kinases, have also been identified. Grb10 and Grb14 are widely, but not uniformly, expressed in mammalian tissues, and there are various isoforms of Grb10. Binding of Grb10 or Grb14 to autophosphorylated IR in vitro inhibits tyrosine kinase activity towards other substrates, but studies on cultured cell lines have been conflicting as to whether Grb10 plays a positive or negative role in insulin signalling. Recent gene knockouts in mice have established that Grb10 and Grb14 act as inhibitors of intracellular signalling pathways regulating growth and metabolism, although the phenotypes of the two knockouts are distinct. Ablation of Grb14 enhances insulin action in liver and skeletal muscle and improves whole-body tolerance, with little effect on embryonic growth. Ablation of Grb10 results in disproportionate overgrowth of the embryo and placenta involving unidentified pathways, and also impacts on hepatic glycogen synthesis, and probably on glucose homoeostasis. This review discusses the extent to which previous studies in vitro can account for the observed phenotype of knockout animals, and considers evidence that aberrant function of Grb10 or Grb14 may contribute to disorders of growth and metabolism in humans.

Trisomy 7 mosaicism, maternal uniparental heterodisomy 7 and Hirschsprung's disease in a child with Silver–Russell syndrome

Prenatal trisomy 7 is usually a cell culture artifact in amniocytes with normal diploid karyotype at birth and normal fetal outcome. In the same way, true prenatal trisomy 7 mosaicism usually results in a normal child except when trisomic cells persist after birth or when trisomy rescue leads to maternal uniparental disomy, which is responsible for 5.5-7% of patients with Silver-Russell syndrome (SRS). We report here on the unusual association of SRS and Hirschsprung's disease (HSCR) in a patient with maternal uniparental heterodisomy 7 and trisomy 7 mosaicism in intestine and skin fibroblasts. HSCR may be fortuitous given its frequency, multifactorial inheritance and genetic heterogeneity. However, the presence of the trisomy 7 mosaicism in intestine as well as in skin fibroblasts suggests that SRS and HSCR might possibly be related. Such an association might result from either an increased dosage of a nonimprinted gene due to trisomy 7 mosaicism in skin fibroblasts (leading to SRS) and in intestine (leading to HSCR), or from an overexpression, through genomic imprinting, of maternally expressed imprinted allele(s) in skin fibroblasts and intestine or from a combination of trisomy 7 mosaicism and genomic imprinting. This report suggests that the SRS phenotype observed in maternal uniparental disomy 7 (mUPD(7)) patients might also result from an undetected low level of trisomy 7 mosaicism. In order to validate this hypothesis, we propose to perform a conventional and molecular cytogenetic analysis in different tissues every time mUPD7 is displayed.

Global analysis of uniparental disomy using high density genotyping arrays

BACKGROUND

Uniparental disomy (UPD), the inheritance of both copies of a chromosome from a single parent, has been identified as the cause for congenital disorders such as Silver-Russell, Prader-Willi, and Angelman syndromes. Detection of UPD has largely been performed through labour intensive screening of DNA from patients and their parents, using microsatellite markers.

METHODS

We applied high density single nucleotide polymorphism (SNP) microarrays to diagnose whole chromosome and segmental UPD and to study the occurrence of continuous or interspersed heterodisomic and isodisomic regions in six patients with Silver-Russell syndrome patients who had maternal UPD for chromosome 7 (matUPD7).

RESULTS

We have devised a new high precision and high-throughput computational method to confirm UPD and to localise segments where transitions of UPD status occur. Our method reliably confirmed and mapped the matUPD7 regions in all patients in our study.

CONCLUSION

Our results suggest that high density SNP arrays can be reliably used for rapid and efficient diagnosis of both segmental and whole chromosome UPD across the entire genome.

Paternal uniparental disomy of chromosome 14 and unique exchange of chromosome 7 in cases of spontaneous abortion

To investigate the involvement of uniparental disomies (UPDs) in spontaneous abortion, the polymorphic patterns of microsatellites on each chromosome were analyzed in 164 cases of abortion. Eighty-three of the 164 cases had chromosomal abnormalities. In 79 of the remaining 81 cases with normal karyotypes, the microsatellite analysis revealed that biparental patterns were present in the informative microsatellites in all chromosomes. In one of the remaining two cases, however, the polymorphic patterns of chromosome 14 appeared to be both of paternal origin. The patterns of the distal of the long arm were homozygous, and those of the remaining region were heterozygous. That is, this fetus had paternal UPD 14, originating from meiosis I nondisjunction. In the other case, the polymorphic patterns of the distal one third of the long arm of chromosome 7 were uniparental (maternal) in origin whereas those of the remaining region of this chromosome were biparental. These findings thus suggested that this chromosome might have originated from chromatid exchange between the long arms of paternal and maternal chromosome 7 at the first mitotic division. Microsatellite analysis, however, produced no evidence of duplication or deletion of any segments. The findings also suggest the possibility that some UPDs may cause spontaneous abortion.

Uniparental disomy (UPD) other than 15: Phenotypes and bibliography updated

Uniparental disomy (UPD) describes the inheritance of a pair of chromosomes from only one parent. The concept was introduced in Medical Genetics by Engel (1980); Am J Med Genet 6:137-143. Aside UPD 15, which is the most frequent one, up to now (February 2005) 197 cases with whole chromosome maternal UPD other than 15 (124 X heterodisomy, 59 X isodisomy, and 14 cases without information of the mode of UPD) and 68 cases with whole chromosome paternal UPD other than 15 (13 X heterdisomy, 53 X isodisomy, and 2 cases without information of the mode of UPD) have been reported. In this review we discuss briefly the problems associated with UPD and provide a comprehensive clinical summary with a bibliography for each UPD other than 15 as a guide for genetic counseling.

Two sisters with Silver-Russell phenotype

Silver-Russell syndrome (SRS) is a well recognizable syndrome, but the etiology of SRS seems to be heterogeneous. SRS is listed in Mendelian Inheritance in Man as an autosomal dominant disorder because most described cases have been of sporadic occurrence, and most likely were caused by de novo autosomal dominant mutation, and because families with apparent dominant transmission of a SRS phenotype have been described. Still, in a few families, autosomal recessive inheritance has been suggested. We describe two sisters who meet the criteria for SRS proposed by Price et al. [1999]. The parents had normal facial features, normal height, and normal post-natal growth. This is the second well-documented case of familial recurrence of SRS that resembles an autosomal recessive inheritance pattern. Since sib recurrence is so rare in SRS, other modes of inheritance should be considered. The finding of maternal uniparental disomy 7 (mUPD7) in 10% of SRS cases suggests that lack of paternally expressed imprinted gene(s) or overexpression of maternal imprinted gene(s) on chromosome 7 cause SRS. The recurrence in sibs could be caused by a mutation in the imprinted gene or imprinting center carried by one parent. Alternatively, recurrence in sibs could represent germ line mosaicism for a dominant mutation in one of the parents.

Intrauterine growth restriction--genetic causes and consequences

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, Beckwith-Wiedemann syndrome and Silver-Russell 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.

Paternal reciprocal translocation t(11;16)(p13;q24.3) in a Silver-Russel syndrome patient

We describe a 7-month-old male child with Silver-Russel syndrome (SRS) phenotype, presented with two major clinical features: low birth weight, short stature, and minor features, such as macrocephaly, clinodactyly, essential for the diagnosis of SRS. Routine cytogenetic studies with GTG-banding showed 46,XY,t(11;16)(p13;q24.3). Fluorescence in situ hybridisation (FISH) with single copy probes BAC (11p13) and PAC (16q24.3), showed a reciprocal translocation. Chromosomal analysis of the mother was normal and the phenotypically normal father had apparently identical translocation t(11;16)(p13;q24.3). The disruption of growth factor genes at 11p and 16q breakpoint regions due to reciprocal translocation in the father might have caused SRS phenotype in the child.

Phenotypical variation in cousins with the identical partial trisomy 9 (pter-q22.2) and 7 (q35-qter) at 16 and 23 weeks gestation

From the study of numerical and structural chromosomal abnormalities, there is convincing evidence and accumulating information of a direct karyotype to phenotype correlation. Knowledge of phenotypic consequences of a specific chromosomal imbalance is important for genetic counseling and prenatal diagnosis. However, for unbalanced non-Robertsonian translocations a precise karyotype to phenotype correlation is difficult to predict for several reasons: (I) unbalanced non-Robertsonian translocations are rare, (II) the published case reports are often not age-matched, (III) varying breakpoints result in different lengths of the monosomic and trisomic segments and therefore the phenotype will depend on additional genes present or the loss of coding regions, and (IV) the combination of the same trisomy with different monosomies, or vice versa, can result in diverging phenotypes. Therefore, the study of the karyotype to phenotype correlation in affected relatives of the same age and the identical unbalanced translocation provides a good model to investigate phenotypic consequences of a specific genetic imbalance. We report of two second trimester fetuses with the identical major partial trisomy 9 (9pter-9q22.2) and minor partial trisomy 7 (q35-qter) resulting from a familial translocation (7;9)(q35;q22.2)mat. One fetus presented with a Dandy-Walker malformation, polymicrogyria, and mild dysmorphic features, whereas the other fetus showed unilateral cleft lip and palate without cerebral anomalies. Potential mechanisms for this different phenotypic expression of the same unbalanced translocation resulting in partial trisomy 9 and 7 in the two cousins and possible consequences for genetic counseling and prenatal diagnosis are discussed.

Segmental maternal heterodisomy of the proximal part of chromosome 15 in an infant with Prader–Willi syndrome

Uniparental disomy (UPD) 15, detected in patients with Prader-Willi (PWS) and Angelman syndromes, has to date always involved the entire chromosome 15. We report the first case of segmental maternal uniparental heterodisomy confined to a proximal part of chromosome 15 in a child with clinical features of PWS. This unusual finding can be explained by the rare combination of three consecutive events: a trisomy 15 zygote caused by a maternal meiosis I error, early postzygotic mitotic recombination between maternal and paternal chromatids, and, finally, trisomy rescue by the loss of the rearranged chromosome 15 containing the paternal 15q11-q13 segment.

Imprinting analysis of 10 genes and/or transcripts in a 1.5-Mb MEST-flanking region at human chromosome 7q32

MEST is one of the imprinted genes in human. With the assistance of our integration map and the complete sequence in the registry, we mapped a total of 16 genes/transcripts at the 1.5-Mb MEST-flanking region at 7q32. This region has been suggested to form an imprinted gene cluster, because MEST and its three flanking genes/transcripts (MESTIT1, CPA4, and COPG2IT1) were reported to be imprinted, although two (TSGA14 and COPG2) were shown to escape imprinting. In this study, 10 other genes/transcripts were examined for their imprinting status in human fetal tissues. The results indicated that 8 genes/transcripts (NRF1, UBE2H, HSPC216, KIAA0265, FLJ14803, CPA2, CPA1, and DKFZp667F0312) were expressed biallelically. The imprinting status of two (TSGA13 and CPA5) was not conclusive, because of their weak and/or tissue-specific expression and inconstant results. These findings provided evidence that only 4 of the 16 genes/transcripts located to the region show monoallelic expression, while others are not involved in imprinting. Therefore, it is less likely that the MEST-flanking 7q32 region forms a large imprinted domain.

A child with Angelman syndrome and trisomy 13 findings due to associated paternal UPD 15 and segmental UPD 13

A child with Angelman syndrome, cutis aplasia, cleft palate, and congenital microform cleft lip, born to a father with a Robertsonian translocation 13;15 is described. Molecular studies using polymorphic markers on chromosomes 15 and 13 showed paternal uniparental disomy (UPD) 15 and segmental UPD 13.

Myoclonus in a patient with a deletion of the epsilon-sarcoglycan locus on chromosome 7q21

Autosomal dominant myoclonus-dystonia syndrome (MDS) is characterized by myoclonic and/or dystonic movements with onset as early as infancy. In most families, MDS is caused by mutations in the gene SGCE, which encodes epsilon -sarcoglycan and is located on chromosome 7q21. Data from several sources, including multi-generation pedigrees revealing parent-of-origin effects on MDS penetrance, suggest that SGCE is maternally imprinted. We present a 32-month-old patient with an interstitial deletion affecting chromosome 7q21, and a phenotype including myoclonus, microcephaly, short stature, dysmorphic face and language delay. We used fluorescence in situ hybridization (FISH) to estimate the size of our patient's deletion (9.0-15 Mbp) and to confirm absence of SGCE on the affected chromosome. Polymerase chain reaction (PCR) analysis of polymorphic markers in the region revealed that the paternally inherited chromosome contained the deletion, consistent with a model of maternal SGCE imprinting. Our patient is the first case of MDS caused by complete deletion of SGCE, and represents a new contiguous gene disorder. The case underscores the need to consider chromosomal deletions in patients whose phenotypes are more complex than the classic presentation of a known disease.

The imprinted region on human chromosome 7q32 extends to the carboxypeptidase A gene cluster: an imprinted candidate for Silver-Russell syndrome

Imprinted gene(s) on human chromosome 7q32-qter have been postulated to be involved in intrauterine growth restriction associated with Silver-Russell syndrome (SRS) as 7-10% of patients have mUPD(7). Three imprinted genes, MEST, MESTIT1, and COPG2IT1 on chromosome 7q32, are unlikely to cause SRS since epigenetic and sequence mutation analyses have not shown any changes. One hundred kilobases proximal to MEST lies a group of four carboxypeptidase A (CPA) genes. Since most imprinted genes are found in clusters, this study focuses on analysing these CPAs for imprinting effects based on their proximity to an established imprinted domain. Firstly, a replication timing study across 7q32 showed that an extensive genomic region including the CPAs, MEST, MESTIT1, and COPG2IT1 replicates asynchronously. Subsequently, SNP analysis by sequencing RT-PCR products of CPA1, CPA2, CPA4, and CPA5 indicated preferential expression of CPA4. Pyrosequencing was used as a quantitative approach, which confirmed predominantly preferential expression of the maternal allele and biallelic expression in brain. CPA5 expression levels were too low to allow reliable evaluation of allelic expression, while CPA1 and CPA2 both showed biallelic expression. CPA4 was the only gene from this family in which an imprinting effect was shown despite the location of this family of genes next to an imprinted cluster. As CPA4 has a potential role in cell proliferation and differentiation, two preferentially expressed copies in mUPD patients with SRS syndrome would result in excess expression and could alter the growth profiles of these subjects and give rise to intrauterine growth restriction.

The novel imprinted carboxypeptidase A4 gene ( CPA4) in the 7q32 imprinting domain

By a search for novel human imprinted genes in the vicinity of the imprinted gene MEST, at chromosome 7q32, we identified the carboxypeptidase A4 gene ( CPA4) in a gene cluster of the carboxypeptidase family, 200 kb centromeric to MEST. Because CPA4 was originally identified as a protein induced in a prostate cancer cell line (PC-3) by histone deacetylase inhibitors, and was located at the putative prostate cancer-aggressiveness locus at 7q32, we investigated its imprinting status in fetal tissues and in adult benign hypertrophic prostate (BPH). RT-PCR using four intragenic polymorphisms as markers showed that CPA4 was expressed preferentially from the maternal allele in the fetal heart, lung, liver, intestine, kidney, adrenal gland, and spleen, but not in the fetal brain. It was also preferentially expressed in the BPH. These findings support that CPA4 is imprinted and may become a strong candidate gene for prostate cancer-aggressiveness. As a Silver-Russell syndrome (SRS) locus has been proposed to be located to a region near MEST and to be involved in imprinting, CPA4 would have been a candidate gene for SRS. However, analysis of ten SRS patients revealed no mutations in CPA4.

The epsilon-sarcoglycan gene (SGCE), mutated in myoclonus-dystonia syndrome, is maternally imprinted

Myoclonus-dystonia syndrome (MDS) is a non-degenerative neurological disorder that has been described to be inherited in an autosomal dominant mode with incomplete penetrance. MDS is caused by loss of function mutations in the epsilon-sarcoglycan gene. Reinvestigation of MDS pedigrees provided evidence for a maternal imprinting mechanism. As differential methylated regions (DMRs) are a characteristic feature of imprinted genes, we studied the methylation pattern of CpG dinucleotides within the CpG island containing the promoter region and the first exon of the SGCE gene by bisulphite genomic sequencing. Our findings revealed that in peripheral blood leukocytes the maternal allele is methylated, while the paternal allele is unmethylated. We also showed that most likely the maternal allele is completely methylated in brain tissue. Furthermore, CpG dinucleotides in maternal and paternal uniparental disomy 7 (UPD7) lymphoblastoid cell lines show a corresponding parent-of-origin specific methylation pattern. The effect of differential methylation on the expression of the SGCE gene was tested in UPD7 cell lines with only a weak RT-PCR signal observed in matUPD7 and a strong signal in patUPD7. These results provide strong evidence for a maternal imprinting of the SGCE gene. The inheritance pattern in MDS families is in agreement with such an imprinting mechanism with the exception of a few cases. We investigated one affected female that inherited the mutated allele from her mother. Surprisingly, we found the paternal wild type allele expressed whereas the mutated maternal allele was not detectable in peripheral blood cDNA.

X-linked thrombocytopenia in a girl

We report X-linked thrombocytopenia (XLT) in a 6-year-old girl with petechiae and thrombocytopenia from the age of 3 months. Her 2-year-old brother was also diagnosed with XLT. The Wiskott-Aldrich syndrome protein (WASP) gene was detected as a replacement of +5th G to Aon intron 6 using sequence analysis, and the WASP expression levels in this patient were one-third those of a healthy control. The X-inactivation analysis of the patients lymphocytes showed a random pattern of X-chromosome inactivation. To our knowledge, this is the first confirmed report of XLT in a female.

Genetic screening for maternal uniparental disomy of chromosome 7 in prenatal and postnatal growth retardation of unknown cause

OBJECTIVE

Many short-statured children lack an etiologic explanation for their retarded growth. Recently, uniparental disomy (UPD), the inheritance of both chromosomes of a chromosome pair from only 1 parent, has been associated with short stature for many chromosomes. Silver-Russell syndrome (SRS) represents an extreme syndrome of intrauterine growth retardation (IUGR) and slight dysmorphic signs, and maternal UPD of human chromosome 7 (matUPD7) has been observed in approximately 10% of SRS cases. In addition, matUPD7 has been reported in patients with only slight dysmorphic features and prenatal or postnatal growth retardation. The objectives of this study were to study the role of matUPD7 in growth failure of unknown cause and in cases of SRS, and to evaluate the efficiency of genetic testing for matUPD7 as a diagnostic tool.

METHODS

DNA samples were studied from 205 children, 92 girls and 113 boys, with short stature of unknown cause and their parents. The patient cohort included 39 cases of SRS, 91 patients with IUGR and subsequent postnatal short stature, and 75 patients with postnatal growth retardation only. MatUPD7 was screened for by genotyping DNA samples from the patient, mother, and father with 13 chromosome-7-specific polymorphic microsatellite markers.

RESULTS

Six (3%) of 205 matUPD7 cases were observed exclusively among 39 (15%) SRS patients studied. Patients with IUGR and/or postnatal growth retardation and with dysmorphic features did not reveal cases of matUPD7.

CONCLUSIONS

Our results indicate that matUPD7 cases are predominantly observed among patients meeting the criteria of SRS, and matUPD7 is not a common cause for growth retardation. Genetic screening for cases of matUPD7 among growth-retarded patients should be focused on patients with severe IUGR and features of SRS. In addition, matUPD7 screening is advisable in individuals with cystic fibrosis and other recessive disorders mapped to chromosome 7 who have unusually short stature.

Molecular genetic studies of human chromosome 7 in Russell-Silver syndrome

Russell-Silver syndrome (RSS) is a form of congenital short stature characterized by severe growth retardation and variable dysmorphic features. In some RSS individuals, alterations in imprinted genes may be involved because approximately 7% of sporadic patients have been observed to have maternal uniparental disomy (mUPD) of chromosome 7. RSS patients with structural abnormalities of chromosome 7 have also been described. In these individuals the chromosome rearrangement could disrupt the balance of imprinted genes, contribute to a recessive form of RSS, or lead to haploinsufficiency of a crucial developmental gene product. Because the mechanism and molecular defects on chromosome 7 causing RSS are still unknown, we tested our collection of 77 RSS families for mUPD7 and were able to identify three new cases. We also characterized two RSS patients with de novo cytogenetic abnormalities involving the short arm of chromosome 7. One had a partial duplication [46, XX, dup(7)(p12 p14)] and the second contained a paracentric inversion [46, XY, inv(7)(p14 p21)]. Fluorescence in situ hybridization (FISH) mapping revealed that the breakpoints on 7p14 were localized to the same novel gene, C7orf10, which encompasses >700 kb of DNA. We also identified other transcription units from this immediate region, but all seem to be biallelically expressed when using a somatic cell hybrid assay.

Evidence from skewed X inactivation for trisomy mosaicism in Silver-Russell syndrome

The finding of maternal uniparental disomy for chromosome 7 (matUPD7) in approximately 7% of Silver-Russell syndrome (SRS) cases has lead to the assumption that imprinted gene(s) on chromosome 7 are responsible for at least some cases. However, the observation in a familial case that both maternal and paternal inheritance of proximal 7p results in an SRS-like phenotype suggests that the causative genes may not be imprinted, and that an extra copy of genes within this region cause SRS. As all cases of complete matUPD7 could have arisen by trisomy rescue, it is possible that undetected trisomy 7 mosaicism contributes towards the phenotype of SRS, and that the matUPD7 seen in some cases is a consequence of trisomy rescue. Previous studies in cases of trisomy rescue for a number of autosomes have shown a strong association with skewed X inactivation in diploid tissues. Thus, we hypothesised that if trisomy mosaicism was involved in SRS, the frequency of skewed X inactivation should be increased in a population of non-matUPD7 SRS patients. Consistent with this hypothesis, results showed a significant increase in the frequency of completely skewed X inactivation in SRS patients (three of 29) when compared to controls (three of 270), suggesting the possible presence of undetected trisomy 7 in SRS patients and/or their placentas.

Silver-Russell syndrome: a dissection of the genetic aetiology and candidate chromosomal regions

The main features of Silver-Russell syndrome (SRS) are pre- and postnatal growth restriction and a characteristic small, triangular face. SRS is also accompanied by other dysmorphic features including fifth finger clinodactyly and skeletal asymmetry. The disorder is clinically and genetically heterogeneous, and various modes of inheritance and abnormalities involving chromosomes 7, 8, 15, 17, and 18 have been associated with SRS and SRS-like cases. However, only chromosomes 7 and 17 have been consistently implicated in patients with a strict clinical diagnosis of SRS. Two cases of balanced translocations with breakpoints in 17q23.3-q25 and two cases with a hemizygous deletion of the chorionic somatomammatropin gene (CSH1) on 17q24.1 have been associated with SRS, strongly implicating this region. Maternal uniparental disomy for chromosome 7 (mUPD(7)) occurs in up to 10% of SRS patients, with disruption of genomic imprinting underlying the disease status in these cases. Recently, two SRS patients with a maternal duplication of 7p11.2-p13, and a single proband with segmental mUPD for the region 7q31-qter, were described. These key patients define two separate candidate regions for SRS on both the p and q arms of chromosome 7. Both the 7p11.2-p13 and 7q31-qter regions are subject to genomic imprinting and the homologous regions in the mouse are associated with imprinted growth phenotypes. This review provides an overview of the genetics of SRS, and focuses on the newly defined candidate regions on chromosome 7. The analyses of imprinted candidate genes within 7p11.2-p13 and 7q31-qter, and gene candidates on distal 17q, are discussed.

Post-zygotic origin of complete maternal chromosome 7 isodisomy and consequent loss of placental PEG1/MEST expression

Maternal UPD of chromosome 7 is associated with pre- and postnatal growth retardation (IUGR, PNGR) and Silver-Russell syndrome (SRS [MIM 180860]). We report a case of IUGR in a newborn with SRS stigmata. Using combined haplotyping and cytogenetic-FISH studies we characterized the lymphocytes, umbilical cord and four placental cotyledons. The results are consistent with complete maternal isodisomy 7 and trisomy 7 mosaicism of post-zygotic origin. The trisomic cell line was prevalent in trophoblast cells from two placental cotyledons. Trisomy 7 of post-zygotic origin is a frequent finding, but maternal isodisomy 7, due to trisomic rescue has never been reported. PEG1/MEST expression was evaluated on placenta cDNA and a specific transcript was revealed only in the cotyledons with a high percentage of trisomic cells and the presence of the paternal chromosome 7 contribution, but not in the placental biopsies with maternal isodisomy 7. The histological features of the four placental fragments revealed that isodisomy 7 correlates with a pattern of cotyledonary hyper-ramification due to an increase of the branching angiogenesis, which could be the result of a defect of angiogenesis caused by the absence of PEG1 product. The severe hypo-ramification of the two cotyledons, showing trisomy 7 mosaicism, may be due to the triplicate dosage of genes on chromosome 7. The delayed fetal growth could be the phenotypic effect of the imbalance between imprinted and non-imprinted genes on chromosome 7 in the fetus or the result of abnormal placental function during pregnancy.

Evidence against GRB10 as the gene responsible for Silver-Russell syndrome

Recent evidence shows that Silver-Russell syndrome (SRS), the major functional deficit of which is limited growth, both intrauterine and postnatal, is due to a double dose of a gene within 7p11.2-p13 that is normally expressed exclusively from the maternal copy. Of the several growth-related genes in this chromosomal region, only GRB10 has been demonstrated to be imprinted; however, imprinting was limited to brain and muscle and was incomplete. Using reverse-transcript PCR, we now confirm GRB10 imprinting in these two tissues is isoform-specific and, more importantly, demonstrate absence of imprinting in growth plate cartilage, the tissue most directly involved in linear growth. Thus, it is unlikely that GRB10 is the gene responsible for SRS.

Complex and segmental uniparental disomy (UPD): review and lessons from rare chromosomal complements

OBJECTIVE

To review all cases with segmental and/or complex uniparental disomy (UPD), to study aetiology and mechanisms of formation, and to draw conclusions.

DESIGN

Searching published reports in Medline.

RESULTS

The survey found at least nine cases with segmental UPD and a normal karyotype, 22 cases with UPD of a whole chromosome and a simple or a non-homologous Robertsonian translocation, eight cases with UPD and two isochromosomes, one of the short arm and one of the long arm of a non-acrocentric chromosome, 39 cases with UPD and an isochromosome of the long arm of two homologous acrocentric chromosomes, one case of UPD and an isochromosome 8 associated with a homozygous del(8)(p23.3pter), and 21 cases with UPD of a whole or parts of a chromosome associated with a complex karyotype. Segmental UPD is formed by somatic recombination (isodisomy) or by trisomy rescue. In the latter mechanism, a meiosis I error is associated with meiotic recombination and an additional somatic exchange between two non-uniparental chromatids. Subsequently, the chromatid that originated from the disomic gamete is lost (iso- and heterodisomy). In cases of UPD associated with one isochromosome of the short arm and one isochromosome of the long arm of a non-acrocentric chromosome and in cases of UPD associated with a true isochromosome of an acrocentric chromosome, mitotic complementation is assumed. This term describes the formation by misdivision at the centromere during an early mitosis of a monosomic zygote. In cases of UPD associated with an additional marker chromosome, either mitotic formation of the marker chromosome in a trisomic zygote or fertilisation of a gamete with a marker chromosome formed in meiosis by a disomic gamete or by a normal gamete and subsequent duplication are possible.

CONCLUSIONS

Research in the field of segmental and/or complex UPD may help to explain undiagnosed non-Mendelian disorders, to recognise hotspots for meiotic and mitotic recombinations, and to show that chromosomal segregation is more complex than previously thought. It may also be helpful to map autosomal recessively inherited genes, genes/regions of genomic imprinting, and dysmorphic phenotypes. Last but not least it would improve genetic counselling.