Translocation trisomy dup(21q) and free trisomy 21 can be distinguished byinterphase-FISH

The possibility of distinguishing in routine diagnostics translocation tris omy dup(21q) from disomy 21 as well as from free trisomy 21 using interphas e fluorescence in situ hybridization (FISH) with a single copy probe (LSI 2 1) localized on chromosome 21q22.13-q22.2 is described. In free trisomy 21 and translocation trisomy dup(21q) 94%-98% of the nuclei exhibit 3 specific signals, while in disomy 21 only up to 6% of them have 3 false positive si gnals. Furthermore, reliable differentiation between free and translocation trisomy dup(21q) can be achieved by evaluating the percentage of nuclei wi th one single and two co-localized chromosome 21q22.13-q22.2 specific signa ls in 50-100 interphase nuclei. While in translocation trisomy 75+/-4.3% ar e co-localized due to a chromosomal rearrangement, in free trisomy 21 only 40+/-2.83% of the nuclei have two co-localized signals by chance. No differ ences in interphase signal distribution could be detected in two cases with a dicentric chromosome dup(21q) compared to one case with a monocentric on e, a comparison not previously carried out. In addition, the single copy pr obe LSI 21 was compared with the alphoid probe D13Z1/D21Z1 which was found to be unsuitable for such assays due to polymorphisms in the cc satellite r egions of chromosome 21.