Centromere-telomere (12;8p) fusion, telomeric 12q translocation, and i(12p) trisomy

Isochromosome Yp and jumping translocation of Yq resulting in five cell lines in an infertile male: a case report and review of the literature

Background

Jumping translocations are a rare type of mosaicism in which the same portion of one donor chromosome is translocated to several recipient chromosomes. Constitutional forms of jumping translocations are rare, and the 48 cases reported to date have been associated with both normal and abnormal phenotypes. Concurrence of isochromosome (i) of one arm and translocation of the other is also rare, with seven reported cases. We describe a unique case involving concurrence of i(Yp) and a jumping translocation of Yq to the telomere of chromosomes 12q and 17q, which resulted in five cell lines.

Case presentation

The patient, an otherwise healthy 35-year-old man, was referred for cytogenetic studies because of absolute azoospermia. He had elevated levels of follicle stimulating hormone and luteinizing hormone, consistent with abnormal spermatogenesis, and decreased levels of free testosterone and inhibin B. G-banded chromosome analysis revealed a mosaic male karyotype involving five abnormal cell lines. One of the cell lines showed loss of chromosome Y and presence of i(Yp) as the sole abnormality. Three cell lines exhibited jumping translocation: two involved 17qter, and the other involved 12qter as the recipient and Yq as the common donor chromosome. One of the cell lines with der(17) additionally showed i(Yp). The other der(17) and der(12) cell lines had a missing Y chromosome. All five cell lines were confirmed by FISH. Subtelomric FISH study demonstrated no loss of chromosome material from the recipient chromosomes at the translocation junctions.

Conclusions

We postulate that a postzygotic pericentromeric break of the Y chromosome led to formation of isochromosome Yp, whereas Yq formed a jumping translocation through recombination between its internal telomere repeats and telomeric repeats of recipient chromosomes. This in turn led to either pairing or an exchange at the complimentary sequences. Such translocation junctions appear to be unstable and to result in a jumping translocation. Cryptic deletion or disruption of AZF (azoospermic factor) genes at Yq11 during translocation or defective pairing of X and Y chromosomes during meiosis, with abnormal sex vesicle formation and consequent spermatogenetic arrest, might be the main cause of the azoospermia in our patient.

Centromere fission, not telomere erosion, triggers chromosomal instability in human carcinomas

The majority of sporadic carcinomas suffer from a kind of genetic instability in which chromosome number changes occur together with segmental defects. This means that changes involving intact chromosomes accompany breakage-induced alterations. Whereas the causes of aneuploidy are described in detail, the origins of chromosome breakage in sporadic carcinomas remain disputed. The three main pathways of chromosomal instability (CIN) proposed until now (random breakage, telomere fusion and centromere fission) are largely based on animal models and in vitro experiments, and recent studies revealed several discrepancies between animal models and human cancer. Here, we discuss how the experimental systems translate to human carcinomas and compare the theoretical breakage products to data from patient material and cancer cell lines. The majority of chromosomal defects in human carcinomas comprises pericentromeric breaks that are captured by healthy telomeres, and only a minor proportion of chromosome fusions can be attributed to telomere erosion or random breakage. Centromere fission, not telomere erosion, is therefore the most probably trigger of CIN and early carcinogenesis. Similar centromere–telomere fusions might drive a subset of congenital defects and evolutionary chromosome changes.

A 9p13-->p24 duplication coupled with a whole 22q translocation onto 9p24

We report on a 3-year-old girl with a typical 9p trisomy syndrome, whose 45-chromosome karyotype includes a 9p+. As assessed by G, C and Ag-NOR bands, the rearranged chromosome resulted from a 9p13-->p24 direct duplication coupled with a translocation of the whole 22q onto 9pter, had heterochromatin at the junction site, lacked both nucleolar organizing regions (NORs) and centromere dots at the unconstricted fusion point, and was present in all metaphases scored. FISH results: a 9p subtelomere probe gave a diminished signal on the 9p+ precisely at the duplication junction 9p24::9p13, but no labeling was observed at the 9;22 translocation site; a pancentromeric alphoid probe labeled all centromeres, and gave a distinct signal at the 9pter;22cen junction. Hence, her karyotype was 45,XX,rea(9;22)(9qter-->9p24::9p13-->9p24::22p10-->22qter).ish rea(9;22) (9psubtel+dim,pancen+). Parental chromosomes were normal. The distinctiveness of the present centromere-telomere fusion rests on the coupling of an intrachromosomal distal duplication with a whole-arm translocation including alphoid DNA onto the duplicated segment. The centromeric inertia of the residual alphoid DNA in the present case compares with the variable functional status of the chromosome 22 centromere in true heterodicentrics involving such a chromosome.

Jumping translocations

Jumping translocations (JT) are uncommon constitutional or acquired chromosome rearrangements involving one donor and several recipient chromosomes. They occur in various pathologic conditions and the mechanism of their formation remains elusive. A review of the literature showed that the major localizations of the breakpoints of JTs in human samples are nonrandomly located in pericentromeric and telomeric regions of chromosomes. Interestingly, comparison of the localization of the chromosomal breakpoints and of presence of interstitial DNA repeats showed differences between constitutional and acquired JTs suggesting differences in the mechanisms for the genesis of JTs and their consequences.

A constitutional telomeric translocation showing meiotic instability

Constitutional telomeric translocations are rare chromosome rearrangements. They are thought to occur as a result of chromosome breakage and subsequent ligation with the telomeric sequence of a different chromosome. Most frequently they occur as de novo events and, depending on the donor chromosome breakpoint, may be associated with an abnormal phenotype. We report a case of an unbalanced translocation involving the long arm of chromosome 15 and the short arm of chromosome 8 [45,XY, der(8)t(8;15)(p23.3;q11.2),-15], diagnosed prenatally; the father carried an unbalanced translocation of the long arm of chromosome 15 and the short arm of chromosome 2 [45,XY,der(2)t(2;15)(p25.3;q11.2),-15]. Both translocations were shown to have telomere repeat sequences at the translocation breakpoints. There was no apparent imbalance of euchromatic material in either translocation, and no associated abnormal phenotype.

A patient with isochromosome 18q, radial-thumb aplasia, thrombocytopenia, and an unbalanced 10;18 chromosome translocation

We report on the clinical and cytogenetic findings in a newborn with a de novo isochromosome 18q. Radial/thumb aplasia and thrombocytopenia were significant features in addition to multiple congenital anomalies. Comparison with reported cases suggests that the genes for such features are located on the 18q arm. An additional finding of a non-reciprocal translocation between chromosome 18p telomere and chromosome 10q telomere was also observed in a majority of cells examined. This additional rearrangement likely has minimal phenotypic consequences, but does raise the possibility that cryptic translocations of telomeric ends of the deleted arm in isochromosome cases may be more common than appreciated.

A new patient with pure trisomy 4p resulting from isochromosome formation and whole arm translocation

Short arm isochromosome formation with translocation of the entire long arm of the same chromosome is an unusual constitutional abnormality that has been observed, to our knowledge, in 18 cases. Only one of these previously reported cases involved chromosome 4, resulting in pure trisomy 4p. Pure trisomy 4p has been reported in a number of cases, the majority of them due to familial chromosome rearrangements, and is associated with a distinct pattern of abnormal findings. We report here a second case of a de novo chromosome 4 whole arm translocation with short-arm isochromosome formation, which we have delineated further by FISH studies.

Ring chromosome 18q and jumping translocation 18p in an adult male with hypergonadotrophic hypogonadism

Constitutional jumping translocations (JT) are rare, especially in phenotypically normal individuals. We report on an adult male with partial hypogonadism as the sole phenotypic abnormality with an unusual chromosome abnormality. In this patient, centric fission of chromosome 18 lead to formation of a ring 18q chromosome, while 18p formed a JT through centromere-telomere fusion with chromosome 8q (66%) or 20q (13%). In 21% of cells, the 18p fragment was missing. Fluorescent in situ hybridization revealed the presence of interstitial telomeres at the junction site of the fusion and unequal distribution of the alphoid sequences through the centric fission, leaving a small, yet functional centromere within the ring. We discuss the phenotype of the patient in light of this unusual karyotype.

Ring 21 chromosome and a satellited 1p in the same patient: novel origin for an ectopic NOR

Nucleolus organizer regions (NORs) are present on the satellite stalks located on the short arms of the acrocentric chromosomes. NORs present on non-acrocentric chromosomes (ectopic NORs) are rare and were reported in both phenotypically normal and abnormal individuals. We describe a patient, ascertained prenatally, with an ectopic NOR on 1p and a ring 21 chromosomes. Amniocentesis was performed at 27-weeks gestation on a 19-year-old woman after identification of intrauterine growth retardation (IUGR) by ultrasound. Cytogenetic analysis of amniocytes from the fetus showed a mos 46,XX,1ps,r(21) (p11.2q22.3)[44]/45,XX,1ps,-21[6] karyotype. Parental karyotypes were normal, indicating a de novo origin for these rearrangements in the fetus. Molecular cytogenetic characterization of the 1ps showed no loss of euchromatin and retention of the telomeric repeats. Characterization of the r(21) using array comparative genomic hybridization (CGH) identified that the deletion was approximately 5 Mb encompassing most of chromosome band 21q22.3. The ectopic NOR (1ps) was most likely derived from the acentric 21p fragment generated by the chromosome breakage event that lead to formation of the r(21) chromosome. This represents a novel mechanism for the origin of ectopic NORs. In addition, this study illustrates the importance of FISH analysis with telomeric and subtelomeric probes for characterization of chromosomes with ectopic NORs.

Homologous telomere association of 19q in a female with premature ovarian failure

Premature ovarian failure (POF) may be due to a variety of genetic mechanisms. We report here, for the first time, telomere association of the long arms of chromosome 19, identified at low frequency (1%) in the peripheral blood cultures of a 30-year-old female with POF. Repeat cultures identified, in addition, the presence of 16q and 22q associations at a lower frequency (0.5%). These consistent observations are suggestive of a non-random event. Their association with POF may just be coincidental or may hypothetically explain it by an abnormal mechanism of chromosome separation, a constitutional telomere anomaly or an unidentified chromosome instability disorder.