Causative mechanisms and clinical impact of immunoglobulin deficiencies in Ataxia Telangiectasia.

BACKGROUND: Ataxia Telangiectasia (AT) is characterized by cerebellar ataxia, telangiectasia, immunodeficiency, and increased cancer susceptibility, caused by mutations in the ataxia telangiectasia mutated (ATM) gene. The immunodeficiency comprises predominantly immunoglobulin (Ig)-deficiency, mainly IgA and IgG2, with a variable severity. So far, the exact mechanisms underlying the Ig-deficiency, especially the variable severity, remain unelucidated.

OBJECTIVE: To characterize the clinical impact of Ig-deficiencies in AT and to elucidate the mechanisms of Ig-deficiencies in AT.

METHODS: We analyzed long-term Ig-levels, immunophenotyping, and survival time in our cohort (n=87, median age 16 years; maximum 64 years). Somatic hypermutation (SHM) and class-switch junctions in B-cells were analyzed by next-generation sequencing. Furthermore, an in vitro class-switching induction assay was performed followed by RNAseq, to assess the effect of ATM-inhibition.

RESULTS: Only the hyper-IgM AT phenotype significantly worsened survival time, while IgA or IgG2-deficiencies did not. The Ig-levels showed predominantly decreased IgG2 and IgA. Moreover, flowcytometric analysis demonstrated reduced naïve B- and T-lymphocytes and a deficiency of class-switched IgG2 and IgA memory B-cells. SHM frequencies were lowered in IgA and IgG2-deficient patients, indicating a hampered germinal center (GC) reaction. In addition, the microhomology of switch junctions was elongated, suggesting alternative end-joining during class-switch DNA-repair. The in vitro class-switching and proliferation were negatively affected by ATM-inhibition. RNA-sequencing analysis showed that ATM-inhibitor influenced expression of GC reaction genes.

CONCLUSION: Ig-deficiency in AT is caused by disturbed development of class-switched memory B-cells. This study suggests that ATM-deficiency affects both the GC reaction and the choice of DNA-repair pathways in class-switching.