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.     

Transmission ratio distortion in offspring of mouse heterozygous carriers of a (7.18) Robertsonian translocation

Transmission ratio distortion (TRD) is defined as a significant departure from expected Mendelian ratios of inheritance of an allele or chromosome. TRD is observed among specific regions of the mouse and human genome and is frequently associated with chromosome rearrangements such as Robertsonian (Rb) chromosomes. We intercrossed mice heterozygous for a (7.18) Rb translocation and genotyped chromosomes 7 and 18 in 1812 individuals, 47% of which were informative for chromosome segregation. We substantiated previous findings that females were less likely than expected to transmit the Rb chromosome to their offspring. Surprisingly, however, we report that heterozygous males transmitted the Rb translocation chromosome significantly more frequently than the acrocentrics. The transmission of the Rb chromosome was not significantly influenced by either the sex of the Rb grandparent or the strain of the Rb.     

Evaluation of segregation patterns of 21;21 Robertsonian translocation along with sex chromosomes and interchromosomal effects in sperm nuclei of carrier by FISH technique

Meiotic segregation patterns of carriers of Robertsonian translocations (RT) are important for assessing the risk of unbalanced forms. We investigated the ratio of sperm with t(21;21) to sperm with nullisomy for chromosome 21; the segregation of the t(21;21) along with sex chromosomes, and also interchromosomal effects on chromosome 10 by using three color fluorescence in situ hybridization (FISH) with telomere specific (Tel 21q) and centromere-specific alpha satellite probes for chromosomes X, Y, and 10. The percentage of cosegregation of t(21;21) with sex chromosomes (49.50%) and without sex chromosomes (46.98%) was not significant. There are no significant differences between the percentages of cosegregation of t(21;21) with chromosome X (23.36%) and with chromosome Y (26.16%). No evidence of an interchromosomal effect on chromosome 10 was detected, the percentage of chromosome 10 aneuploidy being similar to that in controls. In addition, the frequency of diploid sperm nuclei was not significantly higher in the carrier (0.32%) than in the controls (0.44%) (P > 0.05). The sex ratio was similar within the carrier and the controls and between the carrier and the control. Three color-FISH analysis, using different probe combinations, seems a rapid and accurate tool for direct analysis of meiotic segregation product.     

Analysis of centromeric activity in Robertsonian translocations: implications for a functional acrocentric hierarchy

Approximately 90% of human Robertsonian translocations occur between nonhomologous acrocentric chromosomes, producing dicentric elements which are stable in meiosis and mitosis, implying that one centromere is functionally inactivated or suppressed. To determine if this suppression is random, centromeric activity in 48 human dicentric Robertsonian translocations was assigned by assessment of the primary constrictions using dual color fluorescence in situ hybridzation (FISH). Preferential activity/constriction of one centromere was observed in all except three different rearrangements. The activity is meiotically stable since intrafamilial consistency of a preferentially active centromere existed in members of six families. These results support evidence for nonrandom centromeric activity in humans and, more importantly, suggest a functional hierarchy in Robertsonian translocations with the chromosome 14 centromere most often active and the chromosome 15 centromere least often active.     

De novo microdeletion on an inherited Robertsonian translocation chromosome: a cause for dysmorphism in the apparently balanced translocation carrier.

Robertsonian translocations are usually ascertained through abnormal children, making proposed phenotypic effects of apparently balanced translocations difficult to study in an unbiased way. From molecular genetic studies, though, some apparently balanced rearrangements are now known to be associated with phenotypic abnormalities resulting from uniparental disomy. Molecular explanations for other cases in which abnormality is seen in a balanced translocation carrier are being sought. In the present paper, an infant is described who has retarded growth, developmental delay, gross muscular hypotonia, slender habitus, frontal bossing, micrognathia, hooked nose, abundant wispy hair, and blue sclerae. Cytogenetically, she appeared to be a carrier of a balanced, paternally derived 14;21 Robertsonian translocation. Analysis of DNA polymorphisms showed that she had no paternal allele at the D14S13 locus (14q32). Study of additional DNA markers within 14q32 revealed that her previously undescribed phenotype results from an interstitial microdeletion within 14q32. Fluorescent in situ hybridization was used to show that this microdeletion had occurred de novo on the Robertsonian translocation chromosome. These observations may reactivate old suspicions of a causal association between Robertsonian translocations and de novo rearrangements in offspring; a systematic search for similar subcytogenetic rearrangements in other families, in which there are phenotypically abnormal children with apparently balanced translocations, may be fruitful. The clinical and molecular genetic data presented also define a new contiguous gene syndrome due to interstitial 14q32 deletion.     

Parental origin-dependent, male offspring-specific transmission-ratio distortion at loci on the human X chromosome.

We have analyzed the transmission of maternal alleles at loci spanning the length of the X chromosome in 47 normal, genetic disease-free families. We found a significant deviation from the expected Mendelian 1:1 ratio of grandpaternal:grandmaternal alleles at loci in Xp11.4-p21.1. The distortion in inheritance ratio was found only among male offspring and was manifested as a strong bias in favor of the inheritance of the alleles of the maternal grandfather. We found no evidence for significant heterogeneity among the families, which implies that the major determinant involved in the generation of the non-Mendelian ratio is epigenetic. Our analysis of recombinant chromosomes inherited by male offspring indicates that an 11.6-cM interval on the short arm of the X chromosome, bounded by DXS538 and DXS7, contains an imprinted gene that affects the survival of male embryos.     

Increased risk of trisomy 21 in offspring of carriers of balanced non-contributing autosomal rearrangements is not explained by interchromosomal effect

Only relatively recently the suggestion that interchromosomal effect (ICE) may be present in man had stopped to be argued. At once it became evident that this phenomenon is inherent to a proportion of balanced chromosome rearrangement carriers, predominantly to patients with fertility problems. It is important to establish whether ICE operates in genome of fertile rearrangement carriers and to determine what kind of rearrangement and how far increases a risk of aneuploidy offspring. Using own and literature data 1) we have assessed rates of inherited non-contributing balanced rearrangements in patients with trisomy 21 (T21) and rates of both mutant and inherited non-contributing balanced rearrangements in parents of offspring with T21 and 2) we have analyzed a parental origin of T21 in affected offspring of carriers of balanced rearrangement. We have found that carriers of balanced reciprocal translocation or inversion, but not robertsonian translocation, are at increased risk of T21 offspring. However these data do not support the existence of ICE in its common sense, i.e. as an effect of rearrangement on other chromosome’s segregation at the carrier’s meiosis. Probably the data obtained suggest an effect of paternal rearrangements on maternal chromosomes segregation after fertilization.     

Chromosome segregation in mice heterozygous for Robertsonian translocations. I. The effect of mutation in chromosome 17 on the disjunction of homologs in females

The effects of mutations on chromosome 17 upon the segregation of the metacentric and acrocentric homologues in the progeny of female mice heterozygous for Robertsonian translocations Rb(8.17) 1Iem and Rb(16.17) 7Bnr were studied. Genetic analysis indicated that the ratio of Rb to non-Rb (normal karyotype) progeny from mothers heterozygous for mutations tf, qk, t12 were not significantly different from 1:1 expected. Introduction of mutations T, Ki, Fu, t6 into the female genotype caused strong distortion of segregation and an increase in the proportion of progeny with normal karyotype (65-70%). From the data on embryonic mortality and cytogenetic observations, it is concluded that distortion of equal transmission arises before MII of meiosis. Consequently, preferential distribution of the metacentric chromosome into the polar body during the first meiotic division is relevant to the segregation distortion observed.     

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.     

Pachytene analysis in a 17;21 reciprocal translocation carrier: role of the acrocentric chromosomes in male sterility

Summary Pachytene analysis was undertaken in an infertile male, heterozygous for a 17;21 reciprocal translocation. The quadrivalent was identified by its configuration and chromomere pattern. A non-random association was found between the quadrivalent and the sex vesicle in 77% of the pachytene nuclei analysed. In 13.1% of the cells the contact with the sex vesicle was established by the terminal chromomere of the two chromosomes 21; in 63.9% of the cells, the entire region of the breakpoints was completely hidden by the sex vesicle. In some nuclei asynapsis was found in the region of the breakpoints. The nature of the contact between the quadrivalent and the sex vesicle is discussed in this paper. It is proposed that the acrocentric chromosome favours the contact between the quadrivalent and the sex vesicle, and increases the risk of sterility in male carriers of Robertsonian translocations and of reciprocal translocations involving one acrocentric chromosome.     

Chromosome Segregation Analysis in Human Embryos Obtained from Couples Involving Male Carriers of Reciprocal or Robertsonian Translocation

The objective of this study was to investigate the frequency and type of chromosome segregation patterns in cleavage stage embryos obtained from male carriers of Robertsonian (ROB) and reciprocal (REC) translocations undergoing preimplantation genetic diagnosis (PGD) at our reproductive center. We used FISH to analyze chromosome segregation in 308 day 3 cleavage stage embryos obtained from 26 patients. The percentage of embryos consistent with normal or balanced segregation (55.1% vs. 27.1%) and clinical pregnancy (62.5% vs. 19.2%) rates were higher in ROB than the REC translocation carriers. Involvement of non-acrocentric chromosome(s) or terminal breakpoint(s) in reciprocal translocations was associated with an increase in the percent of embryos consistent with adjacent 1 but with a decrease in 3∶1 segregation. Similar results were obtained in the analysis of nontransferred embryos donated for research. 3∶1 segregation was the most frequent segregation type in both day 3 (31%) and spare (35%) embryos obtained from carriers of t(11;22)(q23;q11), the only non-random REC with the same breakpoint reported in a large number of unrelated families mainly identified by the birth of a child with derivative chromosome 22. These results suggest that chromosome segregation patterns in day 3 and nontransferred embryos obtained from male translocation carriers vary with the type of translocation and involvement of acrocentric chromosome(s) or terminal breakpoint(s). These results should be helpful in estimating reproductive success in translocation carriers undergoing PGD.     

Chromosome translocations: study of 232 cases originating from 144 families

Between 1974 and 1987, 232 translocation carriers have been detected in our Center; they belong to 144 different families. Indications for chromosome analysis were the following: familial studies in relation with a patient suggesting a chromosome anomaly (25.4%); mental retardation with or without malformations (24.6%); 2 or more spontaneous abortions (17.2%); infertility problems, mainly male (16.4%); genetic counseling for a non-chromosomal disease (9.5%); prenatal diagnosis in risk pregnancies (6.9%). The chromosome anomalies detected were the following; balanced Robertsonian fusions (114 cases = 49.1%); balanced translocations (74 cases = 31.9%); unbalanced translocations, Robertsonian fusions included (44 cases = 19%). Two groups may be distinguished: the first one confirms data already known, such as high frequency of balanced translocations in couples with multiple abortions, or in infertile males. The second group on the contrary shows more unusual observations: 4 cases of standard trisomy 21 born to young parents carriers of a balanced translocation not involving chromosome 21; 5 cases of trisomy 13 with 46 chromosomes and a Robertsonian fusion, born to parents carriers of a t(13q; Dq) (twice the mother and thrice the father); 14 cases of apparently balanced translocations, however with an abnormal phenotype; and finally 22 cases of balanced translocations incidentally detected during the course of investigations in patients with a genetic problem generally not associated with a chromosome defect.     

Male-offspring-specific, haplotype-dependent, nonrandom cosegregation of alleles at loci on two mouse chromosomes

F(1) backcrosses involving the DDK and C57BL/6 inbred mouse strains show transmission ratio distortion at loci on two different chromosomes, 11 and X. Transmission ratio distortion on chromosome X is restricted to female offspring while that on chromosome 11 is present in offspring of both sexes. In this article we investigate whether the inheritance of alleles at loci on one chromosome is independent of inheritance of alleles on the other. A strong nonrandom association between the inheritance of alleles at loci on both chromosomes is found among male offspring, while independent assortment occurs among female offspring. We also provide evidence that the mechanism by which this phenomenon occurs involves preferential cosegregation of nonparental chromatids of both chromosomes at the second meiotic division, after the ova has been fertilized by a C57BL/6 sperm bearing a Y chromosome. These observations confirm the influence of the sperm in the segregation of chromatids during female meiosis, and indicate that a locus or loci on the Y chromosome are involved in this instance of meiotic drive.     

Chromosome segregation in mice heterozygous for Robertsonian translocations. II. Changes in the structure of homologs as a cause of abnormal disjunction in females

It was demonstrated that mutations T, Fu, Ki, t6 of chromosome 17 cause preferential transmission of the acrocentric homologues to the progeny from female Rb heterozygotes. The results indicate that the effects of these mutations on segregation are restricted to the Robertsonian translocations involving chromosome 17. Substitution of the parts of chromosome 17 distal or proximal to the T-locus did not alter the effect, of this chromosome on the transmission rate of the homologue. The transmissions effects of these mutations, whether cis or trans with Rb, were the same. It was observed that mothers Rb7/T43H transmitted the chromosome with the reciprocal translocation T43H to 70.9% of their progeny. Data were obtained supporting the idea that structural changes of the chromosomes caused by mutations affect segregation of the homologues in Rb heterozygous females. The possible mechanism of this influence is discussed.     

Breakpoints in Robertsonian translocations are localized to satellite III DNA by fluorescence in situ hybridization.

We characterized 21 t(13;14) and 3 t(14;21) Robertsonian translocations for the presence of DNA derived from the short arms of the translocated acrocentric chromosomes and identified their centromeres. Nineteen of these 24 translocation carriers were unrelated. Using centromeric alpha-repeat DNA as chromosome-specific probe, we found by in situ hybridization that all 24 translocation chromosomes were dicentric. The chromatin between the two centomeres did not stain with silver, and no hybridization signal was detected with probes for rDNA or beta-satellite DNA that flank the distal and proximal ends of the rDNA region on the short arm of the acrocentrics. By contrast, all 24 translocation chromosomes gave a distinct hybridization signal when satellite III DNA was used as probe. This result strongly suggests that the chromosomal rearrangements leading to Robertsonian translocations occur preferentially in satellite III DNA. We hypothesize that guanine-rich satellite III repeats may promote chromosomal recombination by formation of tetraplex structures. The findings localize satellite III DNA to the short arm of the acrocentric chromosomes distal to centromeric alpha-repeat DNA and proximal to beta-satellite DNA.     

Segregation of chromosomes in sperm of Robertsonian translocation carriers

Robertsonian translocations are the most frequent structural chromosomal abnormalities in humans and can affect fertility, with various degrees of sperm alterations in men; or the pregnancy outcome of the carriers. The studies on meiotic segregation of chromosomes in sperm of Robertsonian translocation males find a majority of normal or balanced spermatozoa for the chromosomes related to the translocation (mean 85.42%; range 60-96.60%). Furthermore, recent studies suggest an interchromosomal effect. Studies on spermatozoa from translocation carriers, and in mouse models help the comprehension of the meiotic segregation mechanisms. Results of meiotic segregation analysis in man could be integrated in genetic counselling especially when assisted reproductive technology is required.     

Meiotic drive favors Robertsonian metacentric chromosomes in the common shrew (Sorex araneus, Insectivora, mammalia)

Meiotic drive has attracted much interest because it concerns the robustness of Mendelian segregation and its genetic and evolutionary stability. We studied chromosomal meiotic drive in the common shrew (Sorex araneus, Insectivora, Mammalia), which exhibits one of the most remarkable chromosomal polymorphisms within mammalian species. The open question of the evolutionary success of metacentric chromosomes (Robertsonian fusions) versus acrocentrics in the common shrew prompted us to test whether a segregation distortion in favor of metacentrics is present in female and/or male meiosis. Performing crosses under controlled laboratory conditions with animals from natural populations, we found a clear trend toward a segregation distortion in favor of metacentrics during male meiosis, two chromosome combinations (gm and jl) being significantly preferred over their acrocentric homologs. Apart for one Robertsonian fusion (hi), this trend was absent in female meiosis. We propose a model based on recombination events between twin acrocentrics to explain the difference in transmission ratios of the same metacentric in different sexes and unequal drive of particular metacentrics in the same sex. Pooled data for female and male meiosis revealed a trend toward stronger segregation distortion for larger metacentrics. This is partially in agreement with the frequency of metacentrics occurring in natural populations of a chromosome race showing a high degree of chromosomal polymorphism.     

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 15 anomalies and the Prader-Willi syndrome: Cytogenetic analysis

Summary The behaviour of chromosome 15 is very different from that of the other acrocentric chromosomes. The cytogenetic characteristics of rearrangements associated with Prader-Willi syndrome (PWS) are analyzed as similar rearrangements irrespective of the associated phenotype (reciprocal translocations of chromosome 15, small bisatellited additional chromosomes, Robertsonian translocations, interstitial deletions, pericentric inversions). This study suggests that: (1) The proximal (15q) region and PWS seem to be indissociable; (2) chromosome 15 has an indisputable cytogenetic originality which could be related to its histochemical properties. Chromosome 15 constitutive heterochromatin usually contains much 5-methylcytosine-rich DNA and a large amount of each of the four satellite DNAs. Furthermore the existence in the proximal (15q) region of one or several palindromic sequences could be postulated to explain the great lability of this region of chromosome 15.     

Putative non-Mendelian transmission of retinoblastoma in males: a phenotypic segregation analysis of 150 pedigrees

In a previous genotypic study of eight families, we discribed paternal segregation distortion favoring the transmission of mutant alleles at the retinoblastoma gene locus (RB1). In the current study, we reviewed all published retinoblastoma pedigrees with defined ascertainment (n = 150), to determine whether the phenotypic segregation frequency at the RB1 locus is in general influenced by the sex of the transmitting parent. Segregation analysis under complete ascertainment revealed that 49.1% of the offspring of male transmitters were affected, while 44.3% of the offspring of female transmitters were affected. While this difference is not statistically significant, it is consistent with the previous findings. No significant sex distortion could be detected among the progeny of carrier fathers and mothers. In order to quantify the transmission ratio more precisely further prospective molecular genetic analysis is warranted. We propose a biological mechanism to account for a putative segregation distortion, namely that genetic recombination creates clones of spermatogonia that are homozygous for the mutant RB1 allele leading to a non-Mendelian ratio of sperm. This model can be experimentally tested using amplification of DNA from single sperm cells.