RESUMEN
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.
Asunto(s)
Cromosomas Humanos Par 21/genética , Reordenamiento Génico/genética , No Disyunción Genética , Aneuploidia , Estudios de Casos y Controles , Centrómero/genética , Cromosomas Humanos Par 22/genética , Sondas de ADN , Síndrome de Down/genética , Femenino , Marcadores Genéticos , Humanos , Hibridación Fluorescente in Situ , Masculino , Repeticiones de Microsatélite , Linaje , Espermatozoides/citología , Espermatozoides/metabolismoRESUMEN
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.