Functional analysis of ATM with relevance for primary immunodeficiency and tumor formation

Abstract

Ataxia-telangiectasia is a rare autosomal recessive disorder characterized by cerebellar degeneration with ataxia, ocular and cutaneous telangiectasias, radiosensitivity, chromosomal instability, immunodeficiency, and cancer predisposition in both patients and heterozygous carriers of the gene. The gene responsible for the disorder, ATM, has been identified as a member of a family of phosphatidylinositol 3-kinaserelated genes and is one of the master controllers of the networks that are involved in cell cycle control and response to DNA damage.

In an attempt to investigate the functions of ATM, we have first identified the Swedish patients with the disorder and studied the Nordic A-T families for mutations in the ATM gene. We have also studied the role of ATM mutations in the pathogenesis of B-cell chronic lymphocytic leukemia and found that ATM inactivation is not a common or essential event in the pathogenesis of the vast majority of patients.

The immunopathogenic mechanism linking ATM dysfunction and immunodeficiency has not yet been established. The fact that the mechanisms involved in maintaining the genome stability are also utilized during lymphocyte development to create immune diversity, lead us to study class switch recombination in patients with ataxia-telangiectasia and related disorders Nijmegen breakage syndrome, ataxiatelangiectasia like disorder and Seckel syndrome, with mutations in NBS1, MRE11 and ATR, respectively.

We performed a detailed analysis of breakpoints resulting from in vivo switching from µ to alpha in the patients to study the nonhomologous end joining during class switch recombination. The switch junctions in all patient groups showed a trend toward increased microhomology usage. We also found that ATM and ATR are likely to act in the same pathway, but during different phases of the process, and both are involved in generation of mutations at or close to the junctions, and the upstream Sµ region. In addition, the absence of NBS1 or MRE11, that together with RAD50 form the Mre11 complex, clearly reduces switching in patients. These findings suggest that ATM, ATR and the Mre 11 complex are involved in nonhomologous end joining during class switch recombination and are likely to have both common and independent roles in the process.