Mapping of members of the low-copy-number repetitive DNA sequence family chAB4 within the p arms of human acrocentric chromosomes: characterization of Robertsonian translocations

Members of the long-range, low-copy-number repetitive DNA sequence family c hAB4 are located on nine different human chromosome pairs and the Y chromos ome, i.e. on the short arms of all the acrocentrics. To localize the chAB4 sequences more precisely on the acrocentrics, chAB4-specific probes togethe r with rDNA and a number of satellite sequences were hybridized to metaphas e chromosomes of normal probands and of carriers of Robertsonian translocat ions of the frequent types rob(13q14q) and rob(14q21q). The results demonst rate that chAB4 is located on both sides of the rDNA on all the acrocentric s; the exact location, however, may be chromosome specific. Chromosome 22, most probably, is the only chromosome where chAB4 is found in the direct ne ighbourhood of the centromere. Fluorescence in situ hybridization analyses of metaphase chromosomes of carriers of rob(21q22q) revealed breakpoint div ersity for this rare type of Robertsonian translocation chromosome. A direc t involvement of chAB4 sequences in recombination processes leading to the Robertsonian translocations analysed in this study can be excluded.