Evolution of chromosomal abnormalities in sequential cytogenetic studies of ataxia telangiectasia.

Sequential cytogenetic studies of four patients with ataxia telangiectasia showed the progressive development of lymphocyte clones, each marked with a rearranged chromosome 14. Initial studies had shown random chromosomal breaks and rearrangements. Later studies in all patients showed nonrandom rearrangement of chromosome 14 with a breakpoint at 14q12 and with the distal segment translocated to either chromosome 14 or 7. The proportion of circulating lymphocytes carrying the marker tended to increase with time, accounting for the majority of the lymphocytes eventually in one case. The marked lymphocyte clones evolved further, as a result of loss of the small centric portions of the rearranged chromosome 14 (14pter leads to 14q12). Perhaps the abnormal clones in ataxia telangiectasia escape immunologic surveillance and flourish in an immunologically impaired environment. Subsequent to the loss of the centric portion of the rearranged chromosome 14, the cells may acquire additional capabilities that enhance malignant transformation.