Complex rearrangements of chromosome 15 in two patients with mild/atypical Prader Willi syndrome

Multiple mechanisms are responsible for the development of Prader Willi syndrome (PWS), the most common genetic cause of obesity in childhood. Molecular findings are usually deletions and uniparental disomy (UPD) of the 15q11-13 region. Rarely, structural rearrangements of the pericentromeric region of chromosome 15 are also detected. Two cases with mild PWS phenotype and complex maternal UPD identified by microsatellite analysis are described: the first patient had uniparental iso and heterodisomy and the second displayed biallelic inheritance and uniparental isodisomy.     

A Somatic Origin of Homologous Robertsonian Translocations and Isochromosomes

One t(14q14q), three t(15q15q), two t(21q21q), and two t(22q22q) nonmosaic, apparently balanced, de novo Robertsonian translocation cases were investigated with polymorphic markers to establish the origin of the translocated chromosomes. Four cases had results indicative of an isochromosome: one t(14q14q) case with mild mental retardation and maternal uniparental disomy (UPD) for chromosome 14, one t(15q15q) case with the Prader-Willi syndrome and UPD(15), a phenotypically normal carrier of t(22q22q) with maternal UPD(22), and a phenotypically normal t(21q21q) case of paternal UPD(21). All UPD cases showed complete homozygosity throughout the involved chromosome, which is supportive of a postmeiotic origin. In the remaining four cases, maternal and paternal inheritance of the involved chromosome was found, which unambiguously implies a somatic origin. One t(15q15q) female had a child with a ring chromosome 15, which was also of probable postmeiotic origin as recombination between grandparental haplotypes had occurred prior to ring formation. UPD might be expected to result from de novo Robertsonian translocations of meiotic origin; however, all de novo homologous translocation cases, so far reported, with UPD of chromosomes 14, 15, 21, or 22 have been isochromosomes. These data provide the first direct evidence that nonmosaic Robertsonian translocations, as well as isochromosomes, are commonly the result of a mitotic exchange.     

Normal phenotype with paternal uniparental isodisomy for chromosome 21.

Uniparental disomy (UPD) involving several different chromosomes has been described in several cases of human pathologies. In order to investigate whether UPD for chromosome 21 is associated with abnormal phenotypes, we analyzed DNA polymorphisms in DNA from a family with de novo Robertsonian translocation t(21q;21q). The proband was a healthy male with 45 dup(21q) who was ascertained through his trisomy 21 offspring. No phenotypic abnormalities were noted in the physical exam, and his past medical history was unremarkable. We obtained genotypes for the proband and his parents' leukocyte DNAs from 17 highly informative short sequence repeat polymorphisms that map in the pericentromeric region and along the entire length of 21q. The order of the markers has been previously determined through the linkage and physical maps of this chromosome. For the nine informative markers there was no maternal allele contribution to the genotype of the proband; in addition, there was always reduction to homozygosity of a paternal allele. These data indicated that there was paternal uniparental isodisomy for chromosome 21 (pUPiD21). We conclude that pUPiD21 is not associated with abnormal phenotypes and that there are probably no imprinted genes on chromosome 21.     

Mitotic recombination and uniparental disomy in Beckwith-Wiedemann syndrome

Beckwith-Wiedemann syndrome (BWS) is a model human imprinting disorder resulting from altered activity of one or more genes in the 11p15.5 imprinted gene cluster. Approximately 20% of BWS cases have uniparental disomy (UPD) of chromosome 11. Such cases appear to result from mitotic recombination occurring in early embryogenesis and offer a rare opportunity to study mitotic recombination in nonneoplastic cells. We analyzed a cohort of 52 children with BWS and UPD using a panel of microsatellite markers for chromosome 11. All cases demonstrated mosaic paternal isodisomy, and IGF2 and H19 were included in the segment of UPD in all cases. However, the extent of segmental disomy was variable, with no evidence of clustering of the proximal UPD breakpoint. In most cases (92% of those informative) UPD did not involve 11q, but 4 patients demonstrated UPD for the whole of chromosome 11. In contrast to meiotic recombination, the mitotic recombination frequency did not decline near the centromere.     

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.     

A case of segmental paternal isodisomy of chromosome 14

Uniparental disomy of chromosome 14 (UPD 14) results in one of two distinct abnormal phenotypes, depending upon the parent of origin. This discordance may result from the reciprocal over-expression and/or under-expression of one or more imprinted genes. We report a case of segmental paternal isodisomy for chromosome 14 with features similar to those reported in other paternal disomy 14 cases. Microsatellite marker analysis revealed an apparent somatic recombination event in 14q12 leading to proximal biparental inheritance, but segmental paternal uniparental isodisomy distal to this site. Analysis of monochromosomal somatic cell hybrids containing either the paternally inherited or the maternally inherited chromosome 14 revealed no deletion of the maternally inherited chromosome 14 and demonstrated the presence of paternal sequences from D14S121 to the telomere on both chromosomes 14. Thus, the patient has paternal isodisomy for 14q12-14qter. Because the patient shows most of the features associated with paternal disomy 14, this supports the presence of the imprinted domain(s) distal to 14q12 and suggests that the proximal region of chromosome 14 does not contain imprinted genes that contribute significantly to the paternal UPD 14 phenotype.     

The impact of imprinting: Prader-Willi syndrome resulting from chromosome translocation, recombination, and nondisjunction.

Prader-Willi syndrome (PWS) is most often the result of a deletion of bands q11.2-q13 of the paternally derived chromosome 15, but it also occurs either because of maternal uniparental disomy (UPD) of this region or, rarely, from a methylation imprinting defect. A significant number of cases are due to structural rearrangements of the pericentromeric region of chromosome 15. We report two cases of PWS with UPD in which there was a meiosis I nondisjunction error involving an altered chromosome 15 produced by both a translocation event between the heteromorphic satellite regions of chromosomes 14 and 15 and recombination. In both cases, high-resolution banding of the long arm was normal, and FISH of probes D15S11, SNRPN, D15S10, and GABRB3 indicated no loss of this material. Chromosome heteromorphism analysis showed that each patient had maternal heterodisomy of the chromosome 15 short arm, whereas PCR of microsatellites demonstrated allele-specific maternal isodisomy and heterodisomy of the long arm. SNRPN gene methylation analysis revealed only a maternal imprint in both patients. We suggest that the chromosome structural rearrangements, combined with recombination in these patients, disrupted normal segregation of an imprinted region, resulting in uniparental disomy and PWS.     

Angelman syndrome in an inbred family

Angelman syndrome (AS) is characterized by severe mental retardation, absent speech, puppet-like movements, inappropriate laughter, epilepsy, and abnormal electroencephalogram. The majority of AS patients ( 65%) have a maternal deficiency within chromosomal region 15q11–q13, caused by maternal deletion or paternal uniparental disomy (UPD). Approximately 35% of AS patients exhibit neither detectable deletion nor UPD, but a subset of these patients have abnormal methylation at several loci in the 15q11–q13 interval. We describe here three patients with Angelman syndrome belonging to an extended inbred family. High resolution chromosome analysis combined with DNA analysis using 14 marker loci from the 15q11-q13 region failed to detect a deletion in any of the three patients. Paternal UPD of chromosome 15 was detected in one case, while the other two patients have abnormal methylation atD15S9, D15S63, andSNRPN. Although the three patients are distantly related, the chromosome 15q11-q13 haplotypes are different, suggesting that independent mutations gave rise to AS in this family.     

A case of autism and uniparental disomy of chromosome 1

We report a male child with autism found to have maternal uniparental disomy (UPD) of chromosome 1. The child met diagnostic criteria for the three symptom domains of autism: language impairment, deficient social communication and excessively rigid and repetitive behaviours. He also had a variety of features often associated with autism, including mild mental retardation, small head circumference, hyperactivity, poor fine motor skills, slightly dysmorphic facial features and a heightened interest in olfactory stimulation. His brother, who did not have chromosome 1 UPD, was also autistic. The mother, but not the father, had a history of psychiatric illness and a number of personality and social traits similar to the core features of autism. The discovery of the cytogenetic abnormality was made during the course of a genome-wide linkage screen, wherein genotypes at 6 out of 17 chromosome 1 markers were non-Mendelian and all transmissions were consistent with UPD. Further genotyping (a total of 54 markers) revealed alternating regions of heterodisomy and isodisomy. Whereas chromosome 1 UPD has not been shown to cause disease by effects on imprinting, numerous reports exist of the abnormality unmasking recessive disease-causing mutations. In agreement with this, one of the regions of isodisomy overlaps an emerging chromosome 1 region of interest in autism located at 150–160 Mb.     

Small supernumerary marker chromosomes (SMCs): genotype-phenotype correlation and classification

Small supernumerary marker chromosomes (SMCs) are present in about 0.05% of the human population. In approximately 30% of SMC carriers (excluding the ~60% SMC derived from one of the acrocentric chromosomes), an abnormal phenotype is observed. The clinical outcome of an SMC is difficult to predict as they can have different phenotypic consequences because of (1) differences in euchromatic DNA-content, (2) different degrees of mosaicism, and/or (3) uniparental disomy (UPD) of the chromosomes homologous to the SMC. Here, we present 35 SMCs, which are derived from all human chromosomes, apart from chromosome 6, as demonstrated by the appropriate molecular cytogenetic approaches, such as centromere-specific multicolor fluoresence in situ hybridization (cenM-FISH), multicolor banding (MCB), and subcentromere-specific multicolor FISH (subcenM-FISH). In nine cases without an aberrant phenotype, neither partial proximal trisomies nor UPD could be detected. Abnormal clinical findings, such as psychomotoric retardation and/or craniofacial dysmorphisms, were associated with seven of the cases in which subcentromeric single-copy probes were proven to be present in three copies. Conversely, in eight cases with a normal phenotype, proximal euchromatic material was detected as partial trisomy. UPD was studied in 12 cases and subsequently detected in two of the cases with SMC (partial UPD 4p and maternal UPD 22 in a der(22)-syndrome patient), indicating that SMC carriers have an enhanced risk for UPD. At present, small proximal trisomies of 1p, 1q, 2p, 6p, 6q, 7q, 9p, and 12q seem to lead to clinical manifestations, whereas partial proximal trisomies of 2q, 3p, 3q, 5q, 7p, 8p, 17p, and 18p may not be associated with significant clinical symptoms. With respect to clinical outcome, a classification of SMCs is proposed that considers molecular genetic and molecular cytogenetic characteristics as demonstrated by presently available methods.Electronic database information: accession numbers and URLs for the data in this article are as follows:ENSEMBL-database, National Center for Biotechnology Information (NCBI), Genome Database (GDB), OMIM (Online Mendelian Inheritance in Man) Database,     

Scanning for telomeric deletions and duplications and uniparental disomy using genetic markers in 120 children with malformations

We screened 120 children with sporadic multiple congenital anomalies and either growth or mental retardation for uniparental disomy (UPD) or subtelomeric deletions. The screening used short tandem repeat polymorphisms (STRP) from the subtelomeric regions of 41 chromosome arms. Uninformative marker results were reanalyzed by using the next available marker on that chromosome arm. In total, approximately 25,000 genotypes were generated and analyzed for this study. Subtelomeric deletions of 1 Mb in size were excluded for 27 of 40 chromosome arms. Among the 120 subjects none was found to have UPD, but five subjects (4%, 95% confidence interval 1-9%) were found to have a deletion or duplication of one or more chromosome arms. We conclude that UPD is not a frequent cause of undiagnosed multiple congenital anomaly syndrome. In addition, we determined that 9p and 7q harbor chromosome length variations in the normal population. We conclude that subtelomeric marker analysis is effective for the detection of subtelomeric duplications and deletions, although it is labor intensive. Given a detection rate that is similar to prior studies and the large workload imposed by STRPs, we conclude that STRPs are an effective, but impractical, approach to the determination of segmental aneusomy given current technology.     

Combined cytogenetic and molecular analyses for the diagnosis of Prader-Willi/Angelman syndromes

Prader-Willi (PWS) and Angelman (AS) are syndromes of developmental impairment that result from the loss of expression of imprinted genes in the paternal (PWS) or maternal (AS) 15q11-q13 chromosome. Diagnosis on a clinical basis is difficult in newborns and young infants; thus, a suitable molecular test capable of revealing chromosomal abnormalities is required. We used a variety of cytogenetic and molecular approaches, such as, chromosome G banding, fluorescent in situ hybridization, a DNA methylation test, and a set of chromosome 15 DNA polymorphisms to characterize a cohort of 27 PWS patients and 24 suspected AS patients. Molecular analysis enabled the reliable diagnosis of 14 PWS and 7 AS patients, and their classification into four groups: (A) 6 of these 14 PWS subjects (44 %) had deletions of paternal 15q11-q13; (B) 4 of the 7 AS patients had deletions of maternal 15q11-q13; (C) one PWS patient (8 %) had a maternal uniparental disomy (UPD) of chromosome 15; (D) the remaining reliably diagnoses of 7 PWS and 3 AS cases showed abnormal methylation patterns of 15q11-q13 chromosome, but none of the alterations shown by the above groups, although they may have harbored deletions undetected by the markers used. This study highlights the importance of using a combination of cytogenetic and molecular tests for a reliable diagnosis of PWS or AS, and for the identification of genetic alterations.     

Clinical outcome and follow-up of the first reported case of Russell-Silver syndrome with the unique combination of maternal uniparental heterodisomy 7 and mosaic trisomy 7

BACKGROUND: Russell-Silver syndrome (RSS) has been associated with maternal uniparental disomy (UPD) for chromosome 7 although the etiology of the syndrome is still unknown. Cases of RSS associated with maternal UPD7 have involved isodisomies, heterodisomies, and mixed isodisomy with heterodisomy simultaneously. This publication is a follow-up report of the postnatal clinical outcome of the first prenatally suspected case of combined mosaic trisomy 7 with maternal uniparental disomy of chromosome 7 (UPD7). CASE: The diagnosis of RSS in the proband was suspected prenatally because trisomy 7 mosaicism (47,XX,+7[13]/46,XX[19]) and maternal uniparental heterodisomy 7 were both found in amniotic fluid cells. Cord blood karyotype analysis showed only disomic cells (46,XX[50]), whereas postpartum chorionic villus analysis was completely trisomic for chromosome 7 (47,XX,+7[19]). Postnatally, the diagnosis of RSS was confirmed by physical findings, her trisomy 7 mosaicism was confirmed by cytogenetic analysis of her skin biopsy (47,XX,+7[9]/46,XX[20]) and her UPD7 was confirmed on both peripheral blood and skin biopsy using microsatellite markers. During infancy, the proband experienced growth deficiency, persistent hypoglycemia, and psychomotor developmental delay. CONCLUSIONS: Trisomic rescue as a life-saving mechanism, with subsequent chromosomal mosaicism in combination with UPD may occur more frequently in RSS than has been reported. Systematic testing of cases suspected prenatally or postnatally would be informative regarding the individual contribution of each factor. Imprinting, loss of heterozygosity for recessive genes, and mosaicism may explain the short stature, asymmetry, and the variable expression of the phenotype. The contribution of these mechanisms to the syndrome should be evaluated in these cases.     

Uniparentale Disomien und Mosaike

Of the various mechanisms of formation of uniparental disomy (UPD) discussed in the literature, the mechanism of trisomy rescue is mostly prone to mosaicism from a trisomy cell line and from a disomy 46, XN uniparental cell line. Therefore, low level or undetected mosaicism has been assumed for a significant number of UPD cases. The clinical consequences of trisomy/UPD mosaicism probably depend on the chromosome involved and the proportional content in individual tissues. As the trisomy cell line of some mosaics might have a disadvantage in biological selection it might not be detected in routine lymphocyte investigations. For evaluation of the clinical relevance in the case of an imprinted chromosome the associated imprinting disorder must also be considered. In a postnatal setting analysis of UPD is indicated in the case of clinical, cytogenetic and molecular data. In the prenatal setting genetic counseling of the parents should be offered prior to any laboratory testing. In total, the impact of mosaicism associated with UPD has to consider the affected chromosome, the associated phenotype, the mechanism of formation and the laboratory method used.     

Origin of uniparental disomy 6: presentation of a new case and review on the literature

Paternal uniparental disomy (UPD) of chromosome 6 has been reported several times in patients with (transient) neonatal diabetes mellitus ((T)NDM). Here we present our short tandem repeat typing results in a new patient with NDM, revealing a paternal isodisomy (UPiD). Summarising these data with those published previously on complete paternal (n=13) and maternal (n=2) UPD6, all cases show isodisomy. In general, several modes of UPD formation have been suggested: While a meiotic origin of UPD mainly results in a uniparental heterodisomy (UPhD), UPiD is probably the result of a post-zygotic mitotic error. This mode of formation consists of a mitotic nondisjunction in a disomic zygote, followed by either a trisomic rescue or a reduplication. Endoduplication in a monosomic zygote is another possible but less probable mechanism, taking into consideration that monosomic zygotes are not viable. The exclusive finding of isodisomy in case of chromosome 6 therefore gives strong evidence that segregational errors of this chromosome are mainly influenced by postzygotic factors. This hypothesis is supported by the observation of two cases with partial paternal UPiD6 originating from mitotic recombination events. The influence of mitotic segregational errors in UPD6 formation is in agreement with the results in trisomy/UPD of other chromosomes of the C group (7 and 8), and is in remarkable contrast to the findings in studies on the origin of the frequent aneuploidies. Multiple factors ensure normal segregation and we speculate that they vary in importance for each chromosome.     

Uniparental disomy and the phenotype of mosaic trisomy 20: a new case and review of the literature

Mosaic trisomy 20 is one of the most commonly reported chromosome abnormalities detected prenatally, but is rare postnatally. Many studies have hypothesized that uniparental disomy (UPD) may play a role in phenotype variability, but this has not been widely studied. Here we report an additional case of mosaic trisomy 20 with altered pigmentation, in which UPD was not found, and we review the literature.     

Uniparental disomy analysis in trios using genome-wide SNP array and whole-genome sequencing data imply segmental uniparental isodisomy in general populations

Whole chromosomal and segmental uniparental disomy (UPD) is one of the causes of imprinting disorder and other recessive disorders. Most investigations of UPD were performed only using cases with relevant phenotypic features and included few markers. However, the diagnosis of cases with segmental UPD requires a large number of molecular investigations. Currently, the accurate frequency of whole chromosomal and segmental UPD in a normal developing embryo is not well understood. Here, we present whole chromosome and segmental UPD analysis using single nucleotide polymorphism (SNP) microarray data of 173 mother-father-child trios (519 individuals) from six populations (including 170 HapMap trios). For two of these trios, we also investigated the possibility of shorter segmental UPD as a consequence of homologous recombination repair (HR) for DNA double strand breaks (DSBs) during the early developing stage using high-coverage whole-genome sequencing (WGS) data from 1000 Genomes Project. This could be overlooked by SNP microarray. We identified one obvious segmental paternal uniparental isodisomy (iUPD) (8.2 mega bases) in one HapMap sample from 173 trios using Genome-Wide Human SNP Array 6.0 (SNP6.0 array) data. However, we could not identify shorter segmental iUPD in two trios using WGS data. Finally, we estimated the rate of segmental UPD to be one per 173 births (0.578%) based on the UPD screening for 173 trios in general populations. Based on the autosomal chromosome pairs investigated, we estimate the rate of segmental UPD to be one per 3806 chromosome pairs (0.026%). These data imply the possibility of hidden segmental UPD in normal individuals.     

Supernumerary marker chromosome 5 diagnosed by M-FISH in a child with congenital heart defect and unusual face

We describe a female patient with a small supernumerary marker chromosome (sSMC) present in mosaic and characterized in detail by fluorescence in situ hybridization (FISH) using all 24 human whole chromosome painting probes, multicolor banding (MCB) and subcentromere specific multicolor FISH (subcenM-FISH). The sSMC was demonstrated to be derived from chromosome 5 and the karyotype of our patient was as follows: 47,XX,+mar.ish r(5)(::p13.2 approximately p13.3-->q11.2::) [60%]/46,XX [40%]. Partial trisomy for the proximal 5p and q chromosomal regions is a rare event. A critical region exists at 5p13 for the phenotype associated with duplication 5p. As far as we know, eight similar cases have been published up to now. We describe a new case which, to our knowledge, is the first characterized in such detail. The role of uniparental disomy (UPD) in cases of SMC is also discussed.     

Parental origin-specific developmental defects in mice with uniparental disomy for chromosome 12

Genetic analysis has shown that the distal portion of mouse chromosome 12 is imprinted; however, the developmental roles of imprinted genes in this region are not known. We have therefore generated conceptuses with uniparental disomy for chromosome 12, in which both copies of chromosome 12 are either paternally or maternally derived (pUPD12 and mUPD12, respectively). Both types of UPD12 result in embryos that are non-viable and that exhibit distinct developmental abnormalities. Embryos with pUPD12 die late in gestation, whereas embryos with mUPD12 can survive to term but die perinatally. The mUPD12 conceptuses are invariably growth-retarded while pUPD12 conceptuses exhibit placentomegaly. Skeletal muscle maturation defects are evident in both types of UPD12. In addition, embryos with paternal UPD12 have costal cartilage defects and hypo-ossification of mesoderm-derived bones. In embryos with mUPD12, the development of the neural crest-derived middle ear ossicles is defective. Some of these anomalies are consistent with those seen with uniparental disomies of the orthologous chromosome 14 region in humans. Thus, imprinted genes on chromosome 12 are essential for viability, the regulation of prenatal growth, and the development of mesodermal and neural crest-derived lineages.     

Partial monosomy and partial trisomy for different segments of chromosome 13 in several individuals of the same family.

A reciprocal translocation, 46,XX,rcp(13;17)(q13;p13), was found to be segregating in a family. Two children have duplication of the distal portion of the long arm of chromosome 13, 46,XX,der(17),rcp(13;17)(q13;p13)mat. They are mentally retarded, have long philtra and postaxial hexadactyly. A maternal half-uncle has a duplication of the short arm and proximal portion of the long arm of chromosome 13, 47,XY,+der(13),rcp (13;17)(q13;p13)mat. He is mentally retarded, has scalp and skull defects and a very short philtrum. A fetus was found, on analysis of amniotic fluid cells, to have a deletion of the distal portion of the long arm of chromosome 13, 46,XX,der,(13),rcp(13;17)(q13;p13)mat. The fetus had multiple internal abnormalities and only 4 fingers on each hand.