Missense mutation and defective function of ATM in a childhood acute leukemia patient with MLL gene rearrangement

The possible involvement of germline mutation of the ataxia telangiectasia mutated (ATM) gene in childhood acute leukemia with mixed lineage leukemia (MLL) gene rearrangement (MLL⁺) was investigated. Of the 7 patients studied, 1 showed a germline missense ATM mutation (8921C>T; Pro2974Leu), located in the phosphatidylinositol-3 (PI-3) kinase domain. In reconstitution assays, the ATM mutant 8921T could only partially rescue the radiosensitive phenotype of AT fibroblasts, and in an in vitro kinase assay, it showed a defective phosphorylation of p53-Ser15. Furthermore, the introduction of 8921T in U2OS cells, characterized by a normal ATM/p53 signal transduction, caused a significant reduction of in vivo p53-Ser15 phosphorylation, suggesting a dominant-negative effect of the mutant ATM over the wild-type protein. Our finding in this patient suggests that altered function of ATM plays some pathogenic roles in the development of MLL⁺ leukemia.