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Low level of DNA repair in human chromosome 1 heterochromatin

Band 1q12 is a breakage prone region commonly involved in chromosome 1 rearrangements found in human cancer. We have investigated whether a lack of DNA repair can account for the observed 1q12 fragility. Ethyl methanesulfonate (EMS)‐induced repair sites originating in G0/G1 phase of the cell cycle were converted to chromosome breaks by blocking the re‐synthesis step of excision repair with cytosine arabinoside. Thus, unfilled repair gaps reached S‐phase and were detected as chromosome breaks after cell division. Chromosome breakage in the overall genome was monitored by scoring of micronuclei. Breaks in 1q12 were detected in interphase lymphocytes by in situ hybridization with tandem probes targeting the chromosome 1 centromere (D1Z5) and the adjacent heterochromatic band 1q12 (pUC1.77). Mitomycin‐C (0.12 and 0.24 μg/ml) and X‐rays (1 and 2 Gy) were used as positive controls to check the suitability of the tandem labelling approach in detecting 1q12 clastogenicity. Our data indicated that, although the methodology was suitable to detect 1q12 breakage and a high level of overall genome DNA repair occurred, there was a low level of EMS‐induced DNA repair sites in 1q12 converted to hromosome breaks. This finding was consistent throughout G1 phase, suggesting that there is a relatively low level of DNA excision repair in human chromosome 1 heterochromatin. Genes Chromosomes Cancer 20:173–184, 1997. © 1997 Wiley‐Liss, Inc.