Uniparental isodisomy for paternal 7p and maternal 7q in a child with growth retardation.

Uniparental isodisomy resulting from the simultaneous presence of isochromosomes of the p and q arms of a chromosome and absence of a normal homologue is an exceptionally rare event. We have observed a growth-retarded female infant in whom the normal chromosome 7 homologues were replaced by what appeared cytogenetically to be isochromosomes of 7p and 7q. Polymorphic microsatellite loci spanning the length of 7p and 7q were analyzed in the proband and her parents to ascertain the parental origin and extent of heterozygosity of the proband's rearranged chromosomes. These studies demonstrated that the 7p alleles of the proband were derived only from the father, the 7q alleles were derived only from the mother, and there was homozygosity for all chromosome 7 loci analyzed. The mechanisms leading to the formation of the proband's isochromosomes could reflect abnormalities of cell division occurring at meiosis, postfertilization mitosis, or both. We believe that the present case may result from incomplete mitotic interchange in the pericentromeric regions of chromosome 7 homologues, with resolution by sister-chromatid reunion in an early, if not first, zygotic division. Phenotypically, our proband resembled three previously reported cases of maternal isodisomy for chromosome 7, suggesting that lack of paternal genes from 7q may result in a phenotype of short stature and growth retardation.     

Determining the origin of human X isochromosomes by use of DNA sequence polymorphisms and detection of an apparent i(Xq) with Xp sequences

Summary The parental origin of five X isochromosomes were determined using 11 DnA markers. The isochromosome was derived from a maternal X chromosome in three cases and from a paternal X chromosome in two. Unexpected heterozygosity was detected for the proximal Xp region in one individual in whom the i(Xq) chromosome was paternally derived. This was confirmed by in situ hybridisation. A mode of formation of isochromosomes by breakage and reunion between the sister chromatids of the arms of an X chromosome is proposed to account for this. Sister chromatid breakage and reunion can be considered as a significant mechanism for the origin of i(Xq) chromosomes.     

Normal phenotype with maternal isodisomy in a female with two isochromosomes: i(2p) and i(2q)

A 36-year-old normal healthy female was karyotyped because all of her five pregnancies had terminated in spontaneous abortions during the first 3 mo. Cytogenetic investigation disclosed a female karyotype with isochromosomes of 2p and 2q replacing the two normal chromosomes 2. Her husband and both of her parents had normal karyotypes. Molecular studies revealed maternal only inheritance for chromosome 2 markers. Reduction to homozygosity of all informative markers indicated that the isochromosomes derived from a single maternal chromosome 2. Except for the possibility of homozygosity for recessive mutations, maternal uniparental disomy 2 appears to have no adverse impact on the phenotype. Our data indicate that no maternally imprinted genes with major effect map to chromosome 2.     

Formation of supernumerary euchromatic short arm isochromosomes: parent and cell stage of origin in new cases and review of the literature.

In order to get insight in the formation of isochromosomes we analysed different supernumerary euchromatic short arm isochromosomes for the parent and cell stage of origin. After cytogenetic detection and confirmation by fluorescence-in-situ hybridization we performed short tandem repeat typing in a child with i(9p), three with i(12p) and three with i(18p). The extra chromosomes were monocentric in each case, the i(9p) and i(12p) constitutions were found in mosaic with normal cell lines. Our results and those of other groups indicate a strong role of maternal meiosis in isochromosome formation: in one i(8p), 4 out of 5 i(9p), 7 out of 12 i(12p) and 18 out of 23 i(18p) families a maternal meiotic nondisjunction had occurred prior to the centromere misdivision. For chromosome 18, the majority of isochromosomes originated from a maternal meiosis II error (16/18). For the other tetrasomic constitutions the isochromosomes could be delineated from paternal as well as from maternal origin, the short tandem repeat typing patterns being consistent with meiotic or mitotic cell stages of formation. Thus, independently of the chromosomal origin, in the majority of cases with additional euchromatic isochromosomes maternal meiosis nondisjunction is the initial step followed by centromeric misdivision. Postzygotic nondisjunction as suggested previously due to mosaics observed in tetrasomies 9p and 12p seems to be of minor importance. The observed origin of isochromosomes 18 corresponds to that of trisomy 18, where the majority of cases can be delineated from maternal meiosis II errors.     

Analysis of DNA polymorphisms suggests that most de novo dup(21q) chromosomes in patients with Down syndrome are isochromosomes and not translocations.

Down syndrome is rarely due to a de novo duplication of chromosome 21 [dup(21q)]. To investigate the origin of the dup(21q) and the nature of this chromosome, we used DNA polymorphisms in 10 families with Down syndrome due to de novo dup(21q). The origin of the extra chromosome 21q was maternal in six cases and paternal in four cases. Furthermore, the majority (eight of 10) of dup(21q) chromosomes were isochromosomes i(21q) (four were paternal in origin, and four were maternal in origin); however, in two of 10 families the dup(21q) chromosome appeared to be the result of a Robertsonian translocation t(21q;21q) (maternal in origin in both cases).     

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.     

Partial trisomy 14q due to maternal t(4;14)(p16;q32) in a dysmorphic newborn

Partial Trisomy 14q is a rare chromosomal disorder that mostly results from a parental translocation. We report here a newborn boy with partial trisomy 14q and dysmorphic features that are compatible with previously reported cases. Conventional cytogenetic analysis revealed an extra chromosomal segment at the end of the short arm of chromosome 4. In order to determine the origin of this chromosome region we used subtelomeric FISH technique. Based on the results of these cytogenetic studies and the physical examination, this dysmorphic case was diagnosed as partial trisomy of 14q and his karyotype determined as 46 XY, der(4)t(4;14)(p16;q32) resulting from a balanced maternal translocation identified as 46,XX, t(4;14)(p16;q32).     

A molecular genetic approach to the identification of isochromosomes of chromosome 21

Summary The largest class of de novo chromosomal rearrangements in Down syndrome are rea(21q21q). Classically, these rearrangements have been termed Robertsonian translocations, implying an attachment of two different chromosome 21 homologues. Additionally, a Robertsonian translocation between two chromosomes 21 cannot be distinguished from an isochromosome composed of genetically identical arms by cytogenetic analyses. Therefore, we have used molecular techniques to differentiate between true Robertsonian translocations and isochromosomes. Samples were obtained from 12 probands, ascertained for de novo rearrangements between homologous chromosomes 21 [11 rea(21q21q) and 1 rea (21;21)(q22;q22)], their parents (n = 24) and available siblings (n = 7). The parental origins of the de novo rearrangements were assigned using molecular and cytogenetic analyses. Although not statistically significant, there was a two-fold increase in the number of paternally derived de novo rearrangements (n = 8) as compared with maternally derived rearrangements (n = 4). To distinguish between rob(21q21q) and i(21q), we used restriction fragment length polymorphisms (RFLPs) spanning the length of chromosome 21. Using all informative and partially informative RFLPs, we used the method of maximum likelihood to assign the most likely rearrangement definition (i or rob) and parental origin in each family. The maximum likelihood estimates indicated that all rearrangements tested (n = 8) were isochromosomes. C-banding revealed two centromeres in three cases indicating that a U-type exchange occurred between sister chromatids in these rearrangements. Our results suggest that the majority of de novo rea(21q21q) are isochromosomes derived from a single parental chromosome 21.     

Preferential maternal derivation in inv dup(15)

Summary Eight patients are reported with a de nov extra inverted duplicated chromosome 15. The abnormal chromosome was considered to be the same in all cases, but its precise delineation remained uncertain and was defined as either 15qter15q12::15q1215pter or 15pter15q11::15q1315pter. Analysis with various techniques of the satellite regions of the bisatellited chromosomes demonstrated maternal derivation in six and paternal derivation in one of the seven families. A nonsister chromatid exchange between the two homologous chromosomes 15 is considered a likely origin of the inv dup(15) in the cases with maternal derivation; in the only case of paternal derivation, however, the abnormal chromosome originated from one single chromosome 15. The clinical findings confirm that patients with inv dup(15) have mental and developmental retardation and are frequently affected by seizures, while severe physical malformations are absent.     

Trisomy 21 (Down syndrome): studying nondisjunction and meiotic recombination by using cytogenetic and molecular polymorphisms that span chromosome 21.

By combining molecular and cytogenetic techniques, we demonstrated the feasibility and desirability of a comprehensive approach to analysis of nondisjunction for chromosome 21. We analyzed the parental origin and stage of meiotic errors resulting in trisomy 21 in each of five families by successfully using cytogenetic heteromorphisms and DNA polymorphisms. The 16 DNA fragments used to detect polymorphisms spanned the length of the long arm and detected recombinational events on nondisjoined chromosomes in both maternal meiosis I and maternal meiosis II errors. The meiotic stage at which errors occurred was determined by sandwiching the centromere between cytogenetic heteromorphisms on 21p and an informative haplotype constructed using two polymorphic DNA probes that map to 21q just below the centromere. This study illustrates the necessity of combining cytogenetic polymorphisms on 21p with DNA polymorphisms spanning 21q to determine (1) the source and stage of meiotic errors that lead to trisomy 21 and (2) whether an association exists between nondisjunction and meiotic recombination.     

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.     

Chromosome 15 uniparental disomy is not frequent in Angelman syndrome.

Genetic imprinting has been implicated in the etiology of two clinically distinct but cytogenetically indistinguishable disorders--Angelman syndrome (AS) and Prader-Willi syndrome (PWS). This hypothesis is derived from two lines of evidence. First, while the molecular extents of de novo cytogenetic deletions of chromosome 15q11q13 in AS and PWS patients are the same, the deletions originate from different parental chromosomes. In AS, the deletion occurs in the maternally inherited chromosome 15, while in PWS the deletion is found in the paternally inherited chromosome 15. The second line of evidence comes from the deletion of an abnormal parental contribution of 15q11q13 in PWS patients without a cytogenetic and molecular deletion. These patients have two maternal copies and no paternal copy of 15q11q13 (maternal uniparental disomy) instead of one copy from each parent. By qualitative hybridization with chromosome 15q11q13 specific DNA markers, we have now examined DNA samples from 10 AS patients (at least seven of which are familial cases) with no cytogenetic or molecular deletion of chromosome 15q11q13. Inheritance of one maternal copy and one paternal copy of 15q11q13 was observed in each family, suggesting that paternal uniparental disomy of 15q11q13 is not responsible for expression of the AS phenotype in these patients.     

Asynchronous DNA replication between 15q11.2q12 homologs: cytogenetic evidence for maternal imprinting and delayed replication

DNA replication kinetics of Prader-Willi/Angelman syndrome region of 15q11.2q12 was studied without synchronization in five human amniotic cell and five skin fibroblast strains with a marker 15 chromosome, i.e., 15p+ or der(15), as cytological marker to distinguish between the two homologs. BrdU-33258 Hoechst-Giemsa techniques were used to analyze and compare the late replication patterns in the 15q11.2q12 region between the homologs. Asynchronous replication between the homologs was observed in both amniocytes and fibroblasts. From cells of a marker 15 of known parental origin, the paternal 15q11.2q12 replicated earlier than that of the maternal 15 in 92%–95% of asynchronous metaphases. The remaining 5%–8% of asynchronous metaphases displayed maternal early/paternal late replication. This mosaic pattern of replication in the 15q11.2q12 region may be due to methylation mosaicism of genomic imprinting or a relative lack of self-control of replication. These results provide cytogenetic evidence of maternal imprinting and delayed replication in the 15q11.2q12 region.     

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.     

A molecular study of X isochromosomes: parental origin, centromeric structure, and mechanisms of formation.

Fourteen individuals with an i(Xq) or idic(Xq) were studied using RFLP analysis in order to determine both parental origin and extent of heterozygosity of the isochromosome and to search for the presence of short-arm material. In five cases the isochromosome was paternally derived, while nine patients had a maternal i(Xq). The analysis of heterozygosity of the nine maternally derived isochromosomes by using Xq markers showed heterozygosity in two cases, suggesting an origin from two homologous X chromosomes. Homozygosity was found at all informative loci in seven cases, which therefore are probably the product of either centromere misdivision or sister-chromatid exchange. Presence of Xp markers was seen both in the three i(Xq) chromosomes which appeared dicentric by cytogenetic analysis and in three additional cytogenetically monocentric cases. Mean parental ages were greater for the maternally derived cases as compared with the paternally derived cases.     

Two complementary recombinant chromosomes 5 in a healthy woman

We report a healthy woman with two abortions who is a carrier for a rare heterozygous double recombinant of an inv(5) chromosome, karyotype 46,XX,rec(5)dup(5p) inv(5)(p13q22),rec(5)dup(5q)inv(5)(p13q22). Her father had a 46,XY,inv(5)(p13q22) karyotype; his consanguineous wife had died. Molecular investigation of 11 highly polymorphic markers spanning chromosome 5 revealed biparental inheritance for two markers (D5S406, D5S681) on 5p15.3 and 5q13.1, and an allele constellation not compatible with paternal heterodisomy for marker D5S623 on 5q11.2. Eight markers were not informative. Three mechanisms of formation are proposed: First, fertilization of a normal oocyte by a sperm carrying the two recombinant chromosomes 5, followed by postzygotic recombination between the normal maternal homologue and the rec(5)dup(5p), and by loss of the mitotically recombined maternal homologue, leading to segmental paternal heterodisomy 5q13-->qter (trisomic rescue). Second, postzygotic recombination in a 46,XX,inv(5)(p13q22) zygote resulting in the 46,XX,rec(5)dup(5p)inv(5)(p13q22),rec(5) dup(5q)inv(5)(p13q22) karyotype, followed by absence of the original cell line in lymphocytes. Third and most likely, both parents were inv(5) carriers and complementary recombinations in maternal and paternal meiosis resulted in a zygote with two recombinant chromosomes 5. Our patient refused any further studies but later reported the birth of a phenotypically normal child. This is the first report known to us of complementation by two non-homologous recombinant chromosomes in a phenotypically normal woman, and the first example of a child born to a carrier of complementary recombinant chromosomes.     

Cytogenetic investigation of six patients with X isochromosomes,i(Xq), and of two subjects with isodicentric X chromosomes,idic(Xq)

Summary The detailed cytogenetic study of six Xq isochromosomes, i(Xq), and two isodicentric Xq chromosomes revealed that both their banding and their inactivation patterns differ as a result of differences in their mechanisms of origin. The arms of Xq isochromosomes may be expected to be mirror images in respect of their morphologic pattern and DNA replication sequence only in dicentric Xq isochromosomes.     

Molecular studies of trisomy 18.

We have determined the parental origin of 50 cases of trisomy 18. In 48 cases the additional chromosome was maternal in origin, and in 2 cases it was paternal in origin. Seven cases, including both those with an additional paternal chromosome, appeared to be the result of postzygotic error. In contrast to the situation in nondisjunction involving chromosomes 21 and X, there was no evidence for nullochiasmate nondisjunction.     

Masked complex chromosome rearrangement in a child thought to have del(8qter) as the sole cytogenetic abnormality

Cytogenetic analyses of constitutional diseases have disclosed several chromosomal rearrangements. At the molecular level, these rearrangements often result in the breakage of genes or alteration of genome architecture. Fluorescence in situ hybridization (FISH) and molecular investigations of a patient showing hypotonia and dysmorphic traits revealed a masked complex chromosome abnormality previously detected by G-banding as a simple 8qter deletion. To characterize the genetic rearrangements panels of bacterial artificial chromosomes (BACs) covering 8q24.22-->qter were constructed, and short tandem repeats (STRs) were used to refine the localization of the breakpoints and to assess the parental origin of the defect. Chromosome 8 displayed the breakpoint at 8q24.22 and an unexpected distal breakpoint at 8q24.23 resulting in unbalanced translocation of a small 8q genomic region on the chromosome 16qter. The study of the 16qter region revealed that the 16q subtelomere was retained and the translocated material of distal 8q was juxtaposed. Moreover, molecular analyses showed that part of the translocated 8qter segment on der(16) was partially duplicated, inverted and that the rearrangement arose in the paternal meiosis. These findings emphasize the complexity of some only apparently simple chromosomal rearrangements and suggest a subtelomeric FISH approach to enhance diagnostic care when a cytogenetic terminal deletion is found.     

Nondisjunction and transmission ratio distortion ofChromosome 2 in a (2.8) Robertsonian translocation mouse strain

Aneuploidy results from nondisjunction of chromosomes in meiosis and is the leading cause of developmental disabilities and mental retardation in humans. Therefore, understanding aspects of chromosome segregation in a genetic model is of value. Mice heterozygous for a (2.8) Robertsonian translocation were intercrossed with chromosomally normal mice and Chromosome 2 was genotyped for number and parental origin in 836 individuals at 8.5 dpc. The frequency of nondisjunction of this Robertsonian chromosome is 1.58%. Trisomy of Chromosome 2 with two maternally derived chromosomes is the most developmentally successful aneuploid karyotype at 8.5 dpc. Trisomy of Chromosome 2 with two paternally derived chromosomes is developmentally delayed and less frequent than the converse. Individuals with maternal or paternal uniparental disomy of Chromosome 2 were not detected at 8.5 dpc. Nondisjunction events were distributed randomly across litters, i.e., no evidence for clustering was found. Transmission ratio distortion is frequently observed in Robertsonian chromosomes and a bias against the transmission of the (2.8) Chromosome was detected. Interestingly, this was observed for female and male transmitting parents.