Cells were irradiated with 10 Gy and were incubated for 45 min at 37°C before fixation on coverslips. ATM pS1981 IRIFs were not observed in untreated cells; maximum intensity of staining was seen in the nuclei of cells exposed to 75 g͞ml. Nonirradiated cells, untreated or exposed to comparable concentrations of geneticin, showed only occasional foci. A taxia-telangiectasia (A-T) is an autosomal recessive neurodegenerative disorder with onset in early childhood, resulting from mutations in the A-T mutated (ATM) gene (1). The ATM protein is a hierarchical serine-threonine kinase, phosphorylating many substrates involved in repair of doublestranded DNA breaks, control of cell cycle checkpoints, and responses to oxidative stress, as well as in radiosensitivity, cancer susceptibility, immune function, and neurological development (2). ATM protein levels are undetectable in the cells of most A-T patients by conventional testing. However a few patients, often with a milder phenotype, have some detectable ATM protein (3,4). This finding encouraged us to seek compounds, such as aminoglycosides, that have the potential to read through premature termination codons and restore ATM protein function (5, 6).To test these effects, we selected 13 lymphoblastoid cell lines (LCLs) with primary premature termination codon (PTC) mutations from a repository of Ͼ400 lines derived from A-T patients. Herein, we show representative data from 5 of the 13 LCLs. Using protein truncation testing (PTT) driven by plasmid templates containing PTC mutations in the ATM gene (7), we observed in vitro read-through effects of various magnitudes with three of four aminoglycosides tested. Full-length ATM protein was also documented ex vivo by immunoprecipitation. Correction of radioresistant DNA synthesis and radiosensitivity, as well as autophosphorylation of ATM, suggested that this readthrough produces functional ATM protein.
Experimental ProceduresCell Lines. Lymphoblastoid cell lines were maintained in RPMI medium 1640 (Invitrogen) with 15% FBS (HyClone) and 1% penicillin͞streptomycin (Invitrogen) at 37°C and 5% CO 2 . In ex vivo experiments, aminoglycosides were added daily into culture media at the indicated doses and times. Because streptomycin is also an aminoglycoside that can perturb proofreading by binding to another ribosomal site, ex vivo experiments were performed with or without streptomycin; no differences were noted.Mutations. We selected only PTC mutations that resulted directly from the mutation, i.e., primary PTC mutations, and not those caused indirectly by upstream mutations or aberrant splicing, because the latter would have no potential therapeutic benefits. The LCLs carried the following mutations (www.benaroyaresearch.org͞bri investigators͞atm.htm): AT185LA, homozygous 3673C 3 T (a UAA G stop codon in PTT fragment 4); TAT51, homozygous 5623C 3 T (a UGA C stop codon in PTT fragment 5); AT187LA, homozygous 5908C 3 T (a UAA G stop codon in PTT fragment 6); AT153LA, homozygous 8977C 3 T (a UGA A stop codon in fragment 8); and AT...