We have examined various aspects of lymphocyte chromosomal instability in three families comprised of five individuals affected with ataxiatelangiectasia (AT), their obligate heterozygous parents, and their unaffected sibs. We found that neither baseline sister chromatid exchanges (SCEs) nor mitomycin-C-induced increments in SCEs showed any significant differences among family members or between AT heterozygotes or homozygotes. Chromosome breakage in first-division metaphases was found to be moderately elevated in three of the five AT homozygotes (range 5-12%); breakage in the six AT obligate heterozygotes was within normal limits (0-4%). Analysis of Giemsa-banded metaphases indicated the presence of a clone bearing a paracentric inversion of chromosome #14 in addition to other chromosome #14 abnormalities in one AT homozygote. The same inversion was also found in this individual's affected sister and his obligate heterozygous father. A discussion regarding the relationship of the specificity of breakage and reunion of bands q12 and q32 on chromosome #14 and the high incidence of malignancy in AT is included.
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