MOLECULAR CHARACTERIZATION OF DE-NOVO SECONDARY TRISOMY-13

Unbalanced Robertsonian translocations are a significant cause of ment al retardation and fetal wastage. The majority of homologous rearrange ments of chromosome 21 in Down syndrome have been shown to be isochrom osomes. Aside from chromosome 21, very little is known about other acr ocentric homologous rearrangements. In this study, four cases of de no vo secondary trisomy 13 are presented. FISH using alpha-satellite sequ ences, rDNA, and a pTRI-6 satellite I sequence specific to the short a rm of chromosome 13 showed all four rearrangements to be dicentric and apparently devoid of ribosomal genes. Three of four rearrangements re tained the pTRI-6 satellite I sequence. Case 1 was the exception, show ing a deletion of this sequence in the rearrangement, although both pa rental chromosomes 13 had strong positive hybridization signals. Eleve n microsatellite markers from chromosome 13 were also used to characte rize the rearrangements. Of the four possible outcomes, one maternal R obertsonian translocation, two paternal isochromosomes, and one matern al isochromosome were observed. A double recombination was observed in the maternally derived rob(13q13q). No recombination events were dete cted in any isochromosome. The parental origins and molecular chromoso mal structure of these cases are compared with previous studies of de novo acrocentric rearrangements.