The repair of X-ray induced chromosomal damage in trisomy 2-and normal diploid lymphocytes.

The frequency of chromosomal aberrations produced by X-rays is greater in lymphocytes cultured from trisomy 21 patients (Down's syndrome) than from normal diploid donors. This increase, which can be detected by a micronucleus assay for chromosomal damage, was postulated by us to result from a defect in the rejoining system which repairs chromosomal breaks. The postulated defect would result in a longer rejoining time, therapy permitting more movement of broken ends and thus enhancing the frequency of exchanges. To test this possibility, the time required for the rejoining (repair) of chromosome breaks was measured in lymphocytes from five Down's syndrome (four trisomy 21 and one D/G translocation partial trisomy 21) donors, from a monosomy 21 donor, and from five diploid donors. The rejoining time was reduced in the Down's syndrome lymphocytes in comparison to the normal diploid and monosomy 21 lymphocytes. Thus the repair of chromosome breaks, far from being defective as evidenced by a longer rejoining time in Down's syndrome cells, occurred more rapidly than in normal cells. A mechanism is proposed by which reduced rejoining times would increase aberration frequencies as a consequence of competition between a (hypothetical) error-free repair system and the error-prone repair system that generates chromosomal aberrations. We suggest that the alteration in the rejoining of chromosomal aberrations may underlie the increased susceptibility of people with Down's syndrome to leukemia.