Manual electrophoretic methods for genotyping amplified STR loci

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.