MOLECULAR STUDIES OF TRANSLOCATIONS AND TRISOMY INVOLVING CHROMOSOME-13

Twenty-four cases of trisomy 13 and one case with disomy 13, but a de novo dic(13,13) (p12p12) chromosome, were examined with molecular mark ers to determine the origin of the extra (or rearranged) chromosome. T wenty-one of 23 informative patients were consistent with a maternal o rigin of the extra chromosome. Lack of a third allele at any locus in both paternal origin cases indicate a somatic duplication of the pater nal chromosome occurred. Five cases had translocation trisomy: one de novo rob (13q14q), one paternally derived rob(13q14q), two de novo t(1 3q13q), and one mosaic de novo t(13q13q)/r(13). The patient with a pat ernal rob(13q14q) had a maternal meiotic origin of the trisomy; thus, the paternal inheritance of the translocation chromosome was purely co incidental. Since there is not a significantly increased risk for unba lanced offspring of a t(13q14q) carrier and most trisomies are materna l in origin, this result should not be surprising; however, it illustr ates that one cannot infer the origin of translocation trisomy based o n parental origin of the translocation. Lack of a third allele at any locus in one of the three t(13q13q) cases indicates that it was most l ikely an isochromosome of postmeiotic origin, whereas the other two ca ses showed evidence of recombination. One balanced (non-trisomic) case with a nonmosaic 45, -13,-13, +t(13;13) karyotype was also investigat ed and was determined to be a somatic Robertsonian translocation betwe en the maternal and paternal homologues, as has been found for all bal anced homologous Robertsonian translocations so far investigated. Thus , it is also incorrect to assume in de novo translocation cases that t he two involved chromosomes are even from the same parent. Despite a m aternal origin of the trisomy, we cannot therefore infer anything abou t the parental origin of the chromosomes 13 and 14 involved in the tra nslocation in the de novo t(13q14q) case nor for the two t(13;13) chro mosomes showing a meiotic origin of the trisomy. (C) 1996 Wiley-Liss, Inc.