A stable ring chromosome 21 was found in an azoospermic man with an otherwise normal phenotype. Meiotic studies in another known azoospermic male with r(21) had indicated that breakdown of spermatogenesis resulted from pairing failure of chromosome 21, followed by degenerative changes in the chromosomes, before the cells had completed the first meiotic division. While primary sterility was a constant feature in the three adult males, eight healthy females with r(21) were fertile. However, they were at risk for Down syndrome and spontaneous abortions.

Ring chromosome 21 in healthy persons: different consequencies in females and in males

Constitutional chromosome aberrations as pathogenetic events in hematologic malignancies.

Two cases of growth failure, microcephaly, facial dysmorphism, muscular hypertonia, and severe psychomotor retardation are described. At birth, both cases had cytogenetic mosaicism in lymphocytes and skin fibroblasts, in case 1 ring chromosome 21 and monosomy 21 and in case 2, deletion of chromosome 21 and monosomy 21. At a later age the lymphocyte karyotype changed almost completely to 46,XX, but the fibroblast karyotype remained as before. DNA polymorphism analysis described elsewhere indicated that the 46,XX lymphocytes contained two identical chromosomes 21 (isodisomy), in case 1 inherited from the father and in case 2 from the mother. The isodisomy was the result of duplication of a chromosome in mitosis after the loss of the homologous abnormal chromosome ("compensatory isodisomy"). We report here that this cytogenetic mechanism can result in false normal cytogenetic findings. The phenotypes were attributed to the cells with monosomy 21 in case 1 and to the deletion and monosomy of chromosome 21 in case 2.

https://jmg.bmj.com/content/jmedgenet/31/7/534.full.pdf

"Compensatory" uniparental disomy of chromosome 21 in two cases.

Though reported from a wide variety of human neoplasias, ring chromosomes, in general, are a rare finding in these diseases. The majority were detected by chance when tumors were screened for chromosomal aberrations. In most cases they are a part of highly complex karyotypic alterations and therefore part of unfavourable prognostic factors. However, in some tumor entities (e.g. tumors of mesenchymal origin) they are of such high prevalence (up to 70% of these tumors) and of such extraordinary specificity that they can even serve as cytogenetic hallmarks for differential diagnosis and for prognostic purposes. The well-known technical problems in malignant cells of achieving high banding quality to define all single chromosomal alterations have severely hampered clear identification of the chromosomes involved in rings until recently. Substantial progress of ring identification could only be achieved when molecular cytogenetic techniques became available. By these techniques it could not only be shown that certain breakpoint regions nonrandomly contribute to ring rearrangements which – at least in certain malignancies – are of basic importance, but also the molecular consequences of these changes could be defined in some cases. The present review summarizes a great number of reports on a total of 760 ring chromosomes in human neoplasias at different sites, but includes only cases with clearly identified rings. In addition, the molecular consequences of ring formation are addressed wherever pertinent information has recently been presented in the literature.

/pdf/ring-chromosomes-in-human-neoplasias-54w3gtrgoa.pdf

Ring chromosomes in human neoplasias.

Constitutional abnormalities of chromosome 21 predispose to iAMP21-acute lymphoblastic leukaemia

Acute lymphoblastic leukemia in two children with a congenital chromosome anomaly: familial inv(11)(p15q13) in one and ring chromosome No. 21 in the other.

Dorothy Pitmon

Papers

Acute lymphoblastic leukemia in two children with a congenital chromosome anomaly: familial inv(11)(p15q13) in one and ring chromosome No. 21 in the other.