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.     

Down syndrome due to de novo Robertsonian translocation t(14q;21q): DNA polymorphism analysis suggests that the origin of the extra 21q is maternal.

Down syndrome is rarely due to a de novo Robertsonian translocation t(14q;21q). DNA polymorphisms in eight families with Down syndrome due to de novo t(14q;21q) demonstrated maternal origin of the extra chromosome 21q in all cases. In seven nonmosaic cases the DNA markers showed crossing-over between two maternal chromosomes 21, and in one mosaic case no crossing-over was observed (this case was probably due to an early postzygotic nondisjunction). In the majority of cases (five of six informative families) the proximal marker D21S120 was reduced to homozygosity in the offspring with trisomy 21. The data can be best explained by chromatid translocation in meiosis I and by normal crossover and segregation in meiosis I and meiosis II.     

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.     

Inherited pericentric inversion of chromosome no. 2 with Robertsonian translocation (13q 14q) resulting in trisomy for chromosome 13q

This report includes a patient with an inherited pericentric inversion of chromosome No. 2 in addition to a Robertsonian translocation resulting in trisomy for chromosome 13q. The chromosomal constitution of the proband was 46,XX,inv(2) (pter leads to p11 : : q14 leads to p11 : : q14 leads to qter); t(13,14) (13qter leads to 13p11 : : 14q11 leads to 14qter). Sequential QFQ, RFA and GTG banding techniques were employed on the chromosomes of all family members. The chromosomal constitutions of the father and his first child were normal while the mother had an inversion of chromosome No. 2 [46,XX,inv(2) (pter leads to p11 : : q14 leads to p11 : : q14 leads to qter)]. The proband inherited this abnormal chromosome. In addition, she had a de novo Robertsonian translocation involving chromosomes 13q and 14q resulting in trisomy of chromosome 13q.     

Uniparental heterodisomy for chromosome 14 in a phenotypically abnormal familial balanced 13/14 Robertsonian translocation carrier.

A 9-year-old mentally retarded girl with multiple congenital anomalies was found to carry a balanced 13/14 Robertsonian translocation [45,XX,t(13q14q)] which was also present in her father. Her mother carried a balanced reciprocal translocation between chromosomes 1 and 14 [46,XX,t(1;14) (q32;q32)]. Both of her parents were phenotypically normal. Molecular studies were carried out to determine the parental origin of chromosomes 1, 13, and 14 in the patient. Using probes for D14S13 and D14S22, we could show that the patient inherited both chromosomes 14 from her father and none from her mother. Similar studies using probes for chromosomes 1 (D1S76) and 13 (D13S37) loci showed the presence of both maternal and paternal alleles in the patient. Our findings indicate that paternal uniparental heterodisomy for chromosome 14 most likely accounts for the phenotypic abnormalities observed in our patient. It is suggested that uniparental disomy may be the basis for abnormal development in at least some phenotypically abnormal familial balanced-translocation carriers.     

Supernumerary small marker chromosome (SMC) and uniparental disomy 22 in a child with confined placental mosaicism of trisomy 22: trisomy rescue due to marker chromosome formation

Trisomy rescue is one of various proposed mechanisms in formation of supernumerary small marker chromosomes (SMC) and uniparental disomy (UPD). In the present report a small de novo marker chromosome derived from chromosome 14 or 22 was diagnosed at prenatal diagnosis due to maternal age. Follow up investigations at birth revealed mosaicism 47,XX,+mar/46,XX. Using FISH, the marker was positive for the probe D14/22Z1, but negative for the probes midi 54 and D22Z4. Using three informative markers both chromosomes 22 were shown to be inherited from the mother (UPDmat). The results are consistent with nondisjunction at maternal meiosis I. The girl is 18 months old now and phenotypically normal. Cardiac and abdominal malformations were excluded by sonographic examinations. Motor and mental development is according to or ahead of developmental milestones (free walking with 10 months, first words at 12 months). The case confirms that maternal UPD 22 most likely is not associated with clinical abnormalities. According to FISH results, UPD 22, and 47,XX,+22 in the placenta, we conclude that the SMC was derived from alpha satellite sequences of chromosome 22. This case for the first time gives evidence that early postzygotic reduction of a chromosome to a small marker chromosome is a real existing mechanism to rescue a conceptus with trisomy.     

Molecular cytogenetic study of Robertsonian translocation 13;14 and Down syndrome in a 3-year-old infant

We describe here a rare case of Robertsonian translocation 13;14 of maternal origin combined with regular trisomy 21 (46,XX,der(13;14)(q10;q10)mat,+21) with Down syndrome phenotype. Molecular cytogenetic studies allowed us to determine the maternal origin of additional chromosome 21 and the non-disjunction of chromosome 21 to occur in meiosis I. On the basis of data obtained we discuss the possible involvement of structural alterations of chromosomes 13 and 14 in the chromosome 21 non-disjunction.     

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.     

Molecular cytogenetic characterization of 17 rob(13q14q) Robertsonian translocations by FISH, narrowing the region containing the breakpoints.

We have characterized 17 rob(13q14q) Robertsonian translocations, using six molecular probes that hybridize to the repetitive sequences of the centromeric and shortarm regions of the five acrocentric chromosomes by FISH. The rearrangements include six de novo rearrangements and the chromosomally normal parents, five maternally and three paternally inherited translocations, and three translocations of unknown origin. The D21Z1/D13Z1 and D14Z1/D22Z1 centromeric alpha-satellite DNA probes showed all rob(13q14q) chromosomes to be dicentric. The rDNA probes did not show hybridization on any of the 17 cases studied. The pTRS-47 satellite III DNA probe specific for chromosomes 14 and 22 was retained around the breakpoints in all cases. However, the pTRS-63 satellite III DNA probe specific for chromosome 14 did not show any signals on the translocation chromosomes examined. In 16 of 17 translocations studied, strong hybridization signals on the translocations were detected with the pTRI-6 satellite I DNA probe specific for chromosome 13. All parents of the six de novo rob(13q14q), including one whose pTRI-6 sequence was lost, showed strong positive hybridization signals on each pair of chromosomes 14 and 13, with pTRS-47, pTRS-63, and pTRI-6. Therefore, the translocation breakpoints in the majority of rob(13q14q) are between the pTRS-47 and pTRS-63 sequences in the p11 region of chromosome 14 and between the pTRI-6 and rDNA sequences within the p11 region of chromosome 13.     

Molecular characterization of de novo secondary trisomy 13.

Unbalanced Robertsonian translocations are a significant cause of mental retardation and fetal wastage. The majority of homologous rearrangements of chromosome 21 in Down syndrome have been shown to be isochromosomes. Aside from chromosome 21, very little is known about other acrocentric homologous rearrangements. In this study, four cases of de novo secondary trisomy 13 are presented. FISH using alpha-satellite sequences, rDNA, and a pTRI-6 satellite I sequence specific to the short arm of chromosome 13 showed all four rearrangements to be dicentric and apparently devoid of ribosomal genes. Three of four rearrangements retained the pTRI-6 satellite I sequence. Case 1 was the exception, showing a deletion of this sequence in the rearrangement, although both parental chromosomes 13 had strong positive hybridization signals. Eleven microsatellite markers from chromosome 13 were also used to characterize the rearrangements. Of the four possible outcomes, one maternal Robertsonian translocation, two paternal isochromosomes, and one maternal isochromosome were observed. A double recombination was observed in the maternally derived rob(13q13q). No recombination events were detected in any isochromosome. The parental origins and molecular chromosomal structure of these cases are compared with previous studies of de novo acrocentric rearrangements.     

Chromosome 14 maternal uniparental disomy in the euploid cell line of a fetus with mosaic 46,XX/47,XX,+14 karyotype

We investigated the parental origin of the extra chromosome 14 and of the two chromosomes 14 of the euploid cell line, in a case of fetal mosaicism 46,XX/47,XX+14 diagnosed at amniocentesis. Molecular analysis of five polymorphic loci of the short tandem repeat type was performed. Markers D14S43 and D14S49 showed the presence of maternal uniparental disomy of chromosome 14 in the apparently normal cell line. The distribution of the markers analysed along the chromosome suggests maternal heterodisomy with a large isodisomic segment in the telomeric region, possibly caused by meiotic crossing-over.     

Tertiary trisomy of 10p15.pter and 14pter.ql3 due to maternal translocation t(10;14)(p15;q13)

Double partial trisomy resulting from 3:1 segregation of the respective chromosomal segments of the chromosomes involved in a balanced translocation in meiosis is rarely reported in the literature. We present here a first patient with multiple congenital malformations associated with double partial trisomy of 10pter-p15 and 14pter-q13 resulting from 3:1 segregation of maternal balanced translocation t(10;14)(p15;q13). Proximal partial trisomy of chromosome 14 and subterminal trisomy of the short arm of the chromosome 10 are rare. The present case is the first case with double partial trisomy of these segments resulting from 3:1 segregation of a maternal balanced translocation.     

Robertsonian translocations: mechanisms of formation,aneuploidy, and uniparental disomy and diagnostic considerations

Robertsonian translocations (ROBs) are rearrangements of the acrocentric chromosomes 13-15 and 21-22. Cytologically, ROBs between homologous chromosomes cannot be distinguished from isochromosomes that originate through duplication of a single homologue. Both types of rearrangements can be involved in aneuploidy. A conceptus with a trisomy or a monosomy can be rescued, and in a proportion of cases, a uniparental disomy (UPD) would result. If there are regions of genome imprinting on a uniparental chromosome pair, phenotypic consequences can result. Chromosomes 14 and 15 are imprinted, and UPD of these are known to result in abnormalities. Thus, prenatal testing should be considered in all pregnancies when one of the parents is a balanced carrier of a ROB because of the risk for aneuploidy, and UPD testing should be considered in fetuses found to carry a balanced ROB or isochromosome that involves chromosomes 14 or 15. Additionally, infants or children with congenital anomalies who carry a ROB should also be considered for UPD testing.     

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.     

A chromosome 14-specific human satellite III DNA subfamily that shows variable presence on different chromosomes 14.

We describe a new subfamily of satellite III DNA (pTRS-63), which, by a combination of in situ hybridization to human metaphase chromosomes and analysis of a panel of somatic cell hybrids, is shown to be specific for human chromosome 14. This DNA has a basic 5-bp repeating unit of diverged GGAAT which is tandemly repeated and organized into either one of two distinct higher-order structures of 5 kb (designated the "L" form) or 4.8 kb (designated the "S" form). In addition, a third (Z) form, representing no detectable levels of this satellite III subfamily, is found. Results from five somatic cell hybrid lines and from a number of informative human individuals suggest that, on any one chromosome 14, only one of the three forms may exist. Subchromosomally, this sequence has been mapped to the p11 region and is distal to the domain occupied by another previously described satellite III subfamily (pTRS-47) found on chromosome 14. The pTRS-63 sequence described adds to the understanding of the structural organization of the short arm of human chromosome 14 and should be useful for the investigation of the molecular etiology of the frequently occurring t(13q14q) and t(14q21q) Robertsonian translocations.     

Identification of uniparental disomy following prenatal detection of Robertsonian translocations and isochromosomes

Rearrangements of the acrocentric chromosomes (Robertsonian translocations and isochromosomes) are associated with an increased risk of aneuploidy. Given this, and the large number of reported cases of uniparental disomy (UPD) associated with an acrocentric rearrangement, carriers are presumed to be at risk for UPD. However, an accurate risk estimate for UPD associated with these rearrangements is lacking. A total of 174 prenatally identified acrocentric rearrangements, including both Robertsonian translocations and isochromosomes, were studied prospectively to identify UPD for the chromosomes involved in the rearrangements. The overall goal of the study was to provide an estimate of the risk of UPD associated with nonhomologous Robertsonian translocations and homologous acrocentric rearrangements. Of the 168 nonhomologous Robertsonian translocations studied, one showed UPD for chromosome 13, providing a risk estimate of 0.6%. Four of the six homologous acrocentric rearrangements showed UPD, providing a risk estimate of 66%. These cases have also allowed delineation of the mechanisms involved in producing UPD unique to Robertsonian translocations. Given the relatively high risk for UPD in prenatally identified Robertsonian translocations and isochromosomes, UPD testing should be considered, especially for cases involving the acrocentric chromosomes 14 and 15, in which UPD is associated with adverse clinical outcomes.     

Maternal uniparental disomy 22 has no impact on the phenotype.

A 25-year-old normal healthy male was karyotyped because five of his wife's pregnancies terminated in spontaneous abortions at 6-14 wk of gestation. Cytogenetic investigation disclosed a de novo balanced Robertsonian t(22q;22q) translocation. Molecular studies revealed maternal only inheritance for chromosome 22 markers. Reduction to homozygosity for all informative markers indicates that the rearranged chromosome is an isochromosome derived from one of the maternal chromosomes 22. Except for the possibility of homozygosity for recessive mutations, maternal uniparental disomy 22 does not seem to have an adverse impact on the phenotype, apart from causing reproductive failure. It can be concluded that no maternally imprinted genes with major effect map to chromosome 22.     

Identification of DNA sequences flanking the breakpoint of human t(14q21q) Robertsonian translocations.

We have employed molecular probes and in situ hybridization to investigate the DNA sequences flanking the breakpoint of a group of t(14q21q) Robertsonian translocations. In all the families studied, the probands were patients with Down syndrome who carried a de novo t(14q21q) translocation. The DNA probes used were two alphoid sequences, alphaRI and alphaXT, which are specific for the centromeres of chromosomes 13 and 21 and of chromosomes 14 and 22, respectively; a satellite III sequence, pTRS-47, which is specific for the proximal p11 region of chromosomes 14 and 22; and a newly defined satellite III DNA, pTRS-63, which is specific for the distal p11 region of chromosome 14. The two alphoid probes detected approximately the same amount of autoradiographic signal on the translocated chromosomes as was expected for chromosomes 14 and 21 of the originating parent, suggesting that there has been no loss of these centromeric sequences during the translocation events. Results with the two satellite III probes indicated that the domain corresponding to pTRS-47 was retained in the translocated chromosomes, whereas the domain for pTRS-63 was lost. These results have allowed us to place the translocation breakpoint between the pTRS-47 and pTRS-63 domains within the p11 region of chromosome 14.     

46,XX,t(15;21)/47,XX,15p-,+21 mosaicism in a child with Down's syndrome

We report here the first case of a mosaic Down's syndrome in which both clones are trisomic for chromosome 21, one of them (90%) by a Robertsonian translocation (15;21) appearing de novo, and the other (10%) by an additional chromosome 21. Three hypotheses can explain the appearance of such a mosaic: that of a chimera formed by the fusion of two trisomy 21 zygotes, one of which had a Robertsonian translocation, the other an additional trisomy 21 zygote; that of a fusion between a chromosome 15 and a chromosome 21 in one of the early segmentation blastomeres of a trisomy 21 zygote; the more probable hypothesis of the occurrence of a fission at the break-attachment point of a Robertsonian translocation (15;21) in one of the cells arising from the early postzygotic divisions of a zygote which was a trisomy 21 by Robertsonian translocation (15;21).     

The consequences of uniparental disomy and copy number neutral loss-of-heterozygosity during human development and cancer

UPD (uniparental disomy) describes the inheritance of a pair of chromosomes from only one parent. Mechanisms that lead to UPD include trisomy rescue, gamete complementation, monosomy rescue and somatic recombination. Most of these mechanisms can involve aberrant chromosomes, particularly isochromosomes and Robertsonian translocations. In the last decade, the number of UPD cases reported in the literature has increased exponentially. This is partly due to the advances in genomic technologies that have allowed for high‐resolution SNP (single nucleotide polymorphism) studies, which have complemented traditional methods relying on polymorphic microsatellite markers. In this review, we discuss aberrant cellular mechanisms leading to UPD and their impact on gene expression. Special emphasis is placed on the unmasking of mutant recessive alleles and the disruption of imprinted gene dosage, which give rise to specific and recurrent imprinting phenotypes. Finally, we discuss how copy number maps determined from SNP array datasets have helped identify not only deletions and duplications but also recurrent copy number neutral regions of loss‐of‐heterozygosity, which have been reported in many cancer types and that may constitute an important driving force in cancer. These tiny regions of UPD also alter imprinted gene dosage, which may have cumulative tumourgenic effects in addition to that of unmasking homozygous cancer‐associated mutations.