Ring chromosome 12 and latent centromeres

Abstract

A ring 12 chromosome was found in a male child with minor phenotypic alterations. No obvious loss of chromosome material was detected. Since there is no other case of a ring 12 in the literature, it was not possible to determine whether the phenotype was due to (invisible) terminal deletions or to karyotypic variation. In lymphocyte cultures 9% of the cells had either no or two rings, but the patient’s RBC had normal activities of the enzymes lactate dehydrogenase B and peptidase B, whose loci are located on the proximal portions of 12p and 12q, respectively. Dicentric rings were found in 37 cells, and all had two active centromeres, in contrast with the relatively frequent finding of latent centromeres in translocated dicentric autosomes. Two latent centromeres were found in one tricentric “rod-shaped” open ring 12, probably derived from a tetracentric ring. It is postulated that latent centromeres are rare in ring chromosomes because, if consistent suppression of centromeres in excess of one took place at each duplication, rings with four, eight, or more centromeres would be formed rather frequently, which is not the case.

A case of ring chromosome 3, 46,XX,-3,+r(3)(p26q29)

“…Dicentric, rod-shaped and interlocked ring abnormalities found in our case have been frequently reported in other ring chromosomes (Ledbetter et al, 1980, Zuffardi et al, 1980, Wyandt et al, 1982. Our case is the first to be described to have aneuploidy of ring chromosome 3.…”

Characterization of the first supernumerary tricentric ring chromosome 1 mosaicism by conventional and molecular cytogenetic techniques

“…In some cases, single cells carrying rings with two or more centromeric regions have been described (Zuffardi et al, 1980;Nishi et al, 1982). Henegariu et al (1997) described a patient with different polycentric ring chromosomes of the same origin.…”

Chromosome 20 Ring: A Chromosomal Disorder Associated with a Particular Electroclinical Pattern

“…However, because PAC 204A16, specific for distal 20q deletion, does not contain subtelomeric repeats (16), a very distal 20q deletion cannot be ruled out, with the broken arm linked to the (TTAGGG)n repeats of 20p. Conversely, the patients' phenotype is engendered by the dynamic mosaicism inherent to all ring chromosomes (21)(22)(23), possibly resulting in mendelian syndromes caused by haploinsufficiency. An example of this is the case of a patient with r(10) associated with Hirshsprung's disease (25).…”