Molecular studies of the parental origin and nature of human X isochromosomes

Large inverted repeats within Xp11.2 are present at the breakpoints of isodicentric X chromosomes in Turner syndrome

Turner syndrome (TS) results from whole or partial monosomy X and is mediated by haploinsufficiency of genes that normally escape X-inactivation. Although a 45,X karyotype is observed in half of all TS cases, the most frequent variant TS karyotype includes the isodicentric X chromosome alone [46,X,idic(X)(p11)] or as a mosaic [46,X,idic(X)(p11)/45,X]. Given the mechanism of idic(X)(p11) rearrangement is poorly understood and breakpoint sequence information is unknown, this study sought to investigate the molecular mechanism of idic(X)(p11) formation by determining their precise breakpoint intervals. Karyotype analysis and fluorescence in situ hybridization mapping of eight idic(X)(p11) cell lines and three unbalanced Xp11.2 translocation lines identified the majority of breakpoints within a 5 Mb region, from approximately 53 to 58 Mb, in Xp11.1-p11.22, clustering into four regions. To further refine the breakpoints, a high-resolution oligonucleotide microarray (average of approximately 350 bp) was designed and array-based comparative genomic hybridization (aCGH) was performed on all 11 idic(X)(p11) and Xp11.2 translocation lines. aCGH analyses identified all breakpoint regions, including an idic(X)(p11) line with two potential breakpoints, one breakpoint shared between two idic(X)(p11) lines and two Xp translocations that shared breakpoints with idic(X)(p11) lines. Four of the breakpoint regions included large inverted repeats composed of repetitive gene clusters and segmental duplications, which corresponded to regions of copy-number variation. These data indicate that the rearrangement sites on Xp11.2 that lead to isodicentric chromosome formation and translocations are probably not random and suggest that the complex repetitive architecture of this region predisposes it to rearrangements, some of which are recurrent.

Parental origin of normal X chromosomes in Turner syndrome patients with various karyotypes: implications for the mechanism leading to generation of a 45,X karyotype

The parental origin of the X chromosome of 45,X females has been the subject of many studies, and most of them have shown that the majority (60-80%) of the X chromosomes are maternal in origin. However, studies on the parental origin of normal X chromosomes are relatively limited for Turner syndrome (TS) females with sex chromosome aberrations. In this study, we used PCR-based typing of highly polymorphic markers and an assay of methylation status of the androgen receptor gene to determine the parental origin of normal X chromosomes in 50 unbiased TS females with a variety of karyotypes. Our results showed a higher paternal meiotic error rate leading to the generation of abnormal sex chromosomes, especially in the case of del(Xp) and abnormal Y chromosomes. Isochromosome Xq and ring/marker X chromosomes, on the other hand, were equally likely the result of both maternal and paternal meiotic errors. A thorough review of previous results, together with our data suggests, that the majority of TS karyotype are caused by paternal meiotic errors that generate abnormal sex chromosomes, and that most 45,X cells are generated by mitotic loss of these abnormal sex chromosomes, resulting in maternal X dominance in these cells.

Microsatellite analysis in Turner syndrome: parental origin of X chromosomes and possible mechanism of formation of abnormal chromosomes

Turner syndrome is a chromosomal disorder in which all or part of one X chromosome is missing. The meiotic or mitotic origin of most cases remains unknown due to the difficulty in detecting hidden mosaicism and to the lack of meiotic segregation studies. We analyzed 15 Turner patients, 10 with a 45,X whereas the rest had a second cell line with abnormal X-chromosomes: a pseudodicentric, an isochromosome, one large and one small ring, and the last with a long arm deletion. Our aims were: to detect X cryptic mosaicism in patients with a 45,X constitution; to determine the parental origin of the abnormality; to infer the zygotic origin of the karyotype and to suggest the timing and mechanism of the error(s) leading to the formation of abnormal X chromosomes from maternal origin. Molecular investigation did not revealed heterozygosity for any microsatellite, excluding X mosaicism in the 45,X cases. Parental origin of the single X chromosome was maternal in 90% of these patients. Three of the structurally abnormal Xs were maternally derived whereas the other two were paternal. These results allowed us to corroborate breakpoints in these abnormal X chromosomes and suggest that the pseudodicentric chromosome originated from post-zygotic sister chromatid exchange, whereas the Xq deleted chromosome probably arose after a recombination event during maternal meiosis.

Parental origin of the X chromosome, X chromosome mosaicism and screening for "hidden" Y chromosome in 45,X Turner syndrome ascertained cytogenetically

Our study confirms the finding that about 85% of X chromosomes in Turner girls are maternally derived. A new observation is the detection of a high frequency of mosaicism (15%) in Turner girls who by cytogenetic analysis were thought to have a pure 45,X karyotype. DNA examination of the material was done by hybridization with digoxigenin labelled, non-radioactive probes, and PCR products for microsatellite analysis were run on polyacrylamide gels. We screened for the presence of "hidden" Y chromosome mosaicism, using the primers SRY, ZFY, DYZ3, DYZ1 and DYS132. Contrary to other reports using the PCR technique to unravel "hidden" Y chromosome mosaics, we did not find any positive cases. A precise technical protocol for these new techniques is given, and the advantages are discussed.

Isodicentric X chromosome and mosaicism: report on two cases of 45,X/46,X,idic(Xq)/47,X,idic(Xq),idic(Xq) and review of the literature

We present 2 instances of Ullrich-Turner syndrome with mosaicism 45,X/46,X,idic(Xq)/47, X,idic(Xq),idic(Xq) and X-isochromosomes with 2 C-bands. The mosaicism with the 3 cell lines points to the presence of the isodicentric chromosome in the zygote and a subsequent nondisjunction event.

Paternal uniparental disomy in a child with a balanced 15;15 translocation and Angelman syndrome

Chromosome 15 (15q11-q13) abnormalities cause two distinct conditions, Angelman syndrome (AS) and Prader-Willi syndrome (PWS). We present the first case of a child with a balanced 15;15 translocation and AS in whom molecular studies were crucial in confirming a diagnosis. DNA polymorphisms demonstrated paternal uniparental disomy for chromosome 15, consistent with the diagnosis of AS. The molecular studies also showed the patient to be homozygous at all loci for which the father was heterozygous, suggesting that the structural rearrangement was an isochromosome 15q and not a Robertsonian translocation.

Survival of fetuses with 45,X: an instructive case and an hypothesis

The high (greater than 95%) fetal loss rate of 45,X embryos and fetuses has led to the suggestion that fetal survival with this karyotype requires the presence of mosaicism. However, in many instances, even given a "mild Ullrich-Turner syndrome" phenotype, mosaicism is not detected. In a pregnancy studied for advanced maternal age, CVS cultured cells showed 65% 45,X and 35% 46,X,r(X). After termination, 2 fetal tissues showed 95% 45,X cells. It is suggested that infants with the 45,X karyotype likely had mosaicism for a structurally abnormal X or Y chromosome during embryogenesis, but the abnormal cell line disappeared prior to birth.

Molecular and cytogenetic analysis of a familial microdeletion of Xq

Cytogenetic analysis of a male infant referred for poor neurological development and failure to thrive showed a microdeletion of the X chromosome, his karyotype being 46,Y,del(X)(pter----q21.1:: q21.2----qter). His mother and grandmother were also found to carry the deletion. DNA probes were used to define the deletion molecularly and it was shown to span intervals 2 to 6 of Cremers et al, a portion of Xq that contains the TCD gene and genes whose absence is associated with deafness and mental retardation. RFLP analysis together with X inactivation studies using the probe M27 beta verified the carrier status of the female relatives and showed non-random X inactivation in the heterozygous females.

Analysis of the origin of Turner's syndrome using polymorphic DNA probes

Thirty-four families with a child or fetus with Turner's syndrome were studied using a series of polymorphic DNA probes. Analysis of the origin of the normal X chromosome was possible in all cases. In 16 families with 45,X (four fetuses and 12 livebirths), the observed X was maternal in each case, indicating a preferential loss of the paternal sex chromosome at, or before, conception. In the remaining 18 families with a variety of karyotypes, but especially in those where the child had an isochromosome of Xq or a ring X, there was again a strong tendency for the normal X to be maternal. Analysis of parental ages was performed with known origin of each abnormality, but no evidence for an increased or decreased parental age effect was detected.

A cytogenetic and molecular reappraisal of a series of patients with Turner's syndrome

The results of a cytogenetic and molecular reinvestigation of a series of 52 patients with Turner's syndrome are reported. No evidence of Y chromosome material was found among the patients with a 45,X constitution but two patients were found to have a cell line with a r(Y) chromosome which was previously thought to be a r(X). The parental origin of the single X in the 45,X patients was maternal in 69% and paternal in 31%, a similar ratio to that seen among spontaneously aborted 45,X conceptuses. This suggests that X-chromosome imprinting is not responsible for the two grossly different phenotypes associated with a 45,X chromosome constitution. Approximately half of the structurally abnormal X chromosomes were maternal in origin and half paternal. This observation is consistent with either a meiotic or post-zygotic mitotic origin and at variance with the predominantly paternal origin reported for autosome structural abnormalities.

Molecular genetics of Turner's syndrome

Recombinant DNA technology now allows an analysis of sex chromosomal abnormalities at the molecular level. X chromosomes differ in respect of their pattern of cutting sites for particular restriction enzymes; these differences can be used to determine which X chromosomes or which part of an X chromosome has been inherited from a parent. Furthermore, these techniques can be used to define the presence or absence of particular regions of the X chromosomes with a higher level of resolution than it is possible to achieve using light microscopy. Thirty-eight families in which there was a child or fetus with Turner's syndrome were studied using a series of DNA probes that detect differences (restriction fragment length polymorphisms) among X chromosomes. Analysis of the origin of the normal X chromosome was possible in 27 families. In 14 families with 45,X the observed X was maternal in each, whereas in 13 children with other karyotypes (46,X,i(Xq); 45,X/46,X,i(Xq); 45,X/46,XX; 45,X/46,X,r(X)) the origin of the normal X was paternal in six and maternal in seven patients. In two other families, mosaicism, which was unsuspected at chromosomal analysis, was revealed. DNA probes studied in the remaining nine families were uninformative. These results suggest that different pathogenic mechanisms operate for pure 45,X when compared with the other karyotypes associated with Turner's syndrome. The presence of unsuspected mosaicism in some of these families has clinical implications in relation to prognosis and management.