ATM protein and p53-serine 15 phosphorylation in ataxia-telangiectasia (AT) patients and at heterozygotes

ATM (ataxia-telangiectasia mutated) gene plays a central role in the DNA-da mage response pathway. We characterized the ATM protein expression in immor talized cells from AT and AT-variant patients, and heterozygotes and correl ated it with two ATM-dependent radiation responses, Gf checkpoint arrest an d p53-Ser 15 phosphorylation. On Western blots, the full-length ATM protein was detected in eight of 18 AT cases, albeit at 1-32% of the normal levels , whereas a truncated ATM protein was detected in a single case, despite th e prevalence among cases of truncation mutations. Of two ataxia without tel angiectasia [A-(T)] cases, one expressed 20% and the other similar to 70% o r the normal ATM levers. Noteworthy, among ten asymptomatic heterozygous ca rriers for AT, normal amounts of ATM protein were found in one and reduced by 40-50% in the remaining cases. The radiation-induced phosphorylation of p53 protein at serine 15, largely mediated by ATM kinase, was defective in AT, A(-T) and in 2/4 heterozygous carriers, while the G1 cell cycle checkpo int was disrupted in all AT and A(-T) cases, and in 3/10 AT heterozygotes. Altogether, our study shows that AT and A(-T) cases bearing truncation muta tions of the ATM gene can produce modest amounts of full-length (and only r arely truncated) ATM protein. However, this limited expression of ATM prote in provides no benefit regarding the ATM-dependent responses related to G1 arrest and p53-ser15 phosphorylation. Our study additionally shows that the majority of AT heterozygotes express almost halved revels of ATM protein, sufficient in most cases to normally regulate the ATM-dependent DNA damage- response pathway. (C) 2000 Cancer Research Campaign.