Acrocentric cryptic translocation associated with nondisjunction of chromosome 21.

Down syndrome is the most frequent autosome aneuploidy in live newborns. It was recently proposed that pericentromeric cryptic translocations might be a cause of chromosome nondisjunction. We describe here a phenotypically normal subject with a cryptic translocation involving the short arms of chromosomes 13 or 21 and 22, who had a son with Down syndrome. Fluorescent in situ hybridization (FISH) on paternal metaphase chromosomes showed a chromosome 22 centromere positive for both 13/21 and 14/22 centromeric probes. The same probes hybridized on different and contiguous sites of chromatin fibers, eliminating cross-hybridization artifacts. This confirmed the presence of a cryptic translocation generating a dicentric chromosome 22: fib ish dic(21;22)(21 pter --> 21q10::22q10 --> 22 qter)(D13/21Z1+;D14/22Z1+). Microsatellite STR segregation analysis confirmed the paternal origin of the additional chromosome 21 in the Down syndrome patient. To determine whether the father showed a higher-than-normal frequency of chromosome 21 nondisjunction, FISH analysis of spermatozoa was performed using a sequence specific probe (21q22.13-q22.2). The frequency of disomy 21 spermatozoa was twofold higher in the cryptic translocation carrier as compared to normal subjects (P < 0.014), suggesting that the rearrangement favored the nondisjunction of chromosome 21. This is the first report associating a pericentromeric cryptic translocation of acrocentric chromosomes with the generation of aneuploidy, supporting the hypothesis that this type of rearrangement may contribute to abnormal chromosomal segregation.

Duplication 2p and monosomy 8p in mosaicism: clinical, molecular cytogenetic and molecular markers of a unique case.

We report on a female patient, with a de novo mosaicism for a structural rearrangement producing trisomy 2p21-->pter and monosomy 8p21-pter. GTG bands and fluorescence in situ hybridization (FISH) in lymphocytes identified: mos 46,XX,der(8)(8qter-->8p21::p21::2p21-->pter),9qh +[52]/ 46,XX,9qh+[82]. Fibroblasts showed the same cell lines in 15 and 12 cells respectively. DNA profiling with fourteen autosomal STR markers, did not reveal a chimerism status in our patient. She did not present the classical phenotype described for trisomy 2p and for monosomy 8p probably due to approximately 60% of the patient's cells being normal. The abnormality probably arose in a very early stage of development during the first post-fertilization divisions with a non-sister chromatid exchange event between chromosomes 2 and 8 producing three cellular clones: a normal clone, one with trisomy 2p and monosomy 8p and a third with monosomy 2p and trisomy 8p. Only the first two cell lines were found in both lymphocytes and fibroblasts of hypopigmented skin; the third may have been lost or limited to other tissues.