X‐Irradiation Induces Up‐regulation of ATM Gene Expression in Wild‐type Lymphoblastoid Cell Lines, but Not in Their Heterozygous or Homozygous Ataxia‐telangiectasia Counterparts

Ataxia‐telangiectasia (AT) is an autosomal recessive disease. The relevant gene has been cloned and designated ATM. We studied the expression of both ATM mRNA and the ATM protein in unirradi‐ated and X‐irradiated EBV (Epstein‐Barr virus)‐transformed lymphoblastoid cell lines (LCLs) derived from donors who were normal (ATM+/+), AT heterozygotes (ATM+/−), or AT homozy‐gotes (ATM−/−), respectively. In ATM+/+ LCLs, the levels of ATM mRNA were found to have increased by approximately 1.5‐fold within 1 h of exposure to 10 Gy of X‐rays, while the ATM protein levels had increased by 1.5‐ to 2.0‐fold within 2 to 3 h of irradiation. The wild‐type mRNA and protein levels both returned to their basal values fairly quickly after this tune. The results obtained with the ATM+/− LCLs were quite different, however: neither the mRNA nor protein levels were found to have increased as a consequence of X‐irradiation in any ATM+/− LCL. Twelve of the mutations in the ATM−/− LCLs we used were truncating mutations, and we suspected that the corresponding truncated ATM proteins would be too labile to be detected by western blot analysis. However, five of the ATM−/− LCLs produced mutant ATM proteins that were identical in molecular weight to the wild‐type ATM protein. When cells from three of these five clones were exposed to X‐rays, transcription of the mutant ATM genes appeared to reduce somewhat, as were the levels of protein being produced. These results suggest that the normal ATM gene responds to ionizing radiation by up‐regulating its activity, whereas none of the mutant ATM genes we studied were able to respond in this way.