Ataxia-telangiectasia: Phenotype/genotype studies of ATM protein expression, mutations, and radiosensitivity

Previous studies on a Limited number of ataxia-telangiectasia (A-T) patient s with detectable levels of intracellular ATM protein have suggested a geno type/ phenotype correlation. We sought to elucidate this possible correlati on by comparing ATM protein levels with mutation types, radiosensitivity, a nd clinical phenotype. Ttl this study, Western blot analysis was used to me asure,ATM protein in lysates of lymphoblastoid cell lines (LCLs) from 123 u nrelated A-T patients, 10 A-T heterozygotes, and 10 patients with phenotype s similar to A-T. Our Western blot protocol can detect the presence of ATM protein in as Little as 1 mu g of total protein; at least 25 mu g of protei n was tested for each individual. ATM protein was absent in 105 of the 123 patients (85%); most of these patients had truncating mutations. The remain ing subset of 18 patients (15%) had reduced levels of normal-sized ATM prot ein; missense mutations were more common in this subset. We used a colony s urvival assay to characterize the phenotypic response of the LCLs to radiat ion exposure; patients with or without detectable ATM protein were typicall y radiosensitive. Nine of 10 A-T heterozygotes also had reduced expression of ATM, indicating that both alleles contribute to ATM protein production. These data suggest that although ATM-specific mRNA is abundant in A-T cells , the abnormal ATM protein is unstable and is quickly targeted for degradat ion. We found little correlation between level of ATM protein and the type of underlying mutation, the clinical phenotype, or the radiophenotype, (C) 2000 Academic Press.