Antibody deficiency in patients with ataxia telangiectasia is caused by disturbed B- and T-cell homeostasis and reduced immune repertoire diversity.

BACKGROUND: Ataxia telangiectasia (AT) is a multisystem DNA-repair disorder caused by mutations in the ataxia telangiectasia mutated (ATM) gene. Patients with AT have reduced B- and T-cell numbers and a highly variable immunodeficiency. ATM is important for V(D)J recombination and immunoglobulin class-switch recombination (CSR); however, little is known about the mechanisms resulting in antibody deficiency severity. OBJECTIVE: We sought to examine the immunologic mechanisms responsible for antibody deficiency heterogeneity in patients with AT. METHODS: In this study we included patients with classical AT plus early-onset hypogammaglobulinemia (n = 3), classical AT (n = 8), and variant AT (late onset, n = 4). We studied peripheral B- and T-cell subsets, B-cell subset replication history, somatic hypermutation frequencies, CSR patterns, B-cell repertoire, and ATM kinase activity. RESULTS: Patients with classical AT lacked ATM kinase activity, whereas patients with variant AT showed residual function. Most patients had disturbed naive B-cell and T-cell homeostasis, as evidenced by low cell numbers, increased proliferation, a large proportion CD21(low)CD38(low) anergic B cells, and decreased antigen receptor repertoire diversity. Impaired formation of T cell-dependent memory B cells was predominantly found in patients with AT plus hypogammaglobulinemia. These patients had extremely low naive CD4(+) T-cell counts, which were more severely reduced compared with those seen in patients with classical AT without hypogammaglobulinemia. Finally, AT deficiency resulted in defective CSR to distal constant regions that might reflect an impaired ability of B cells to undergo multiple germinal center reactions. CONCLUSION: The severity of the antibody deficiency in patients with AT correlates with disturbances in B- and T-cell homeostasis resulting in reduced immune repertoire diversity, which consequently affects the chance of successful antigen-dependent cognate B-T interaction.

Presence of ATM protein and residual kinase activity correlates with the phenotype in ataxia-telangiectasia: a genotype-phenotype study.

Ataxia-telangiectasia (A-T) is an autosomal recessive neurodegenerative disorder with multisystem involvement and cancer predisposition, caused by mutations in the A-T mutated (ATM) gene. To study genotype-phenotype correlations, we evaluated the clinical and laboratory data of 51 genetically proven A-T patients, and additionally measured ATM protein expression and kinase activity. Patients without ATM kinase activity showed the classical phenotype. The presence of ATM protein, correlated with slightly better immunological function. Residual kinase activity correlated with a milder and essentially different neurological phenotype, absence of telangiectasia, normal endocrine and pulmonary function, normal immunoglobulins, significantly lower X-ray hypersensitivity in lymphocytes, and extended lifespan. In these patients, cancer occurred later in life and generally consisted of solid instead of lymphoid malignancies. The genotypes of severely affected patients generally included truncating mutations resulting in total absence of ATM kinase activity, while patients with milder phenotypes harbored at least one missense or splice site mutation resulting in expression of ATM with some kinase activity. Overall, the phenotypic manifestations in A-T show a continuous spectrum from severe classical childhood-onset A-T to a relatively mild adult-onset disorder, depending on the presence of ATM protein and kinase activity. Each patient is left with a tremendously increased cancer risk.

ATM germline heterozygosity does not play a role in chronic lymphocytic leukemia initiation but influences rapid disease progression through loss of the remaining ATM allele.

Ataxia telangiectasia patients, with constitutional bi-allelic ATM mutations, have a marked risk of lymphoid tumors and ATM mutation carriers have a smaller risk of cancer. Sporadic ATM mutations occur in 10-20% of chronic lymphocytic leukemia and are often associated with chromosome 11q deletions which cause loss of an ATM allele. The role of constitutional ATM mutations in the pathogenesis of chronic lymphocytic leukemia is unknown. Here we investigated the frequency of constitutional ATM mutations in either of two chronic lymphocytic leukemia cohorts, those with and without a chromosome 11q deletion. We found that in comparison to controls, constitutional pathogenic ATM mutations were increased in patients with chromosome 11q deletions (6 of 140 vs. 0 of 281, P = 0.001) but not in those without 11q deletions (2 of 178 vs. 0 of 281, P = 0.15). These results suggest that ATM germline heterozygosity does not play a role in chronic lymphocytic leukemia initiation but rather influences rapid disease progression through ATM loss.

Neuropathology in classical and variant ataxia-telangiectasia.

Ataxia-telangiectasia (A-T) is classically characterized by progressive neurodegeneration, oculocutaneous telangiectasia, immunodeficiency and elevated α-fetoprotein levels. Some patients, classified as variant A-T, exhibit a milder clinical course. In the latter patients extrapyramidal symptoms, instead of cerebellar ataxia, tend to be the dominating feature and other classical disease hallmarks, like telangiectasia, appear later or even may be absent. Some patients with variant disease have clinically pronounced anterior horn cell degeneration. Neuropathological studies of genetically proven A-T patients are lacking. The aims of our study were to describe the neuropathology of three A-T patients; in two of them the diagnosis was genetically confirmed. The neuropathological findings were compared with those of all known published autopsy findings in A-T patients up to now. Two classical A-T patients aged 19 and 22 and a 33-year-old patient with variant disease were autopsied. In line with previous reports, our patients had severe cerebellar atrophy, less pronounced degeneration of the dentate nucleus and inferior olive, degeneration of the posterior columns and neurogenic muscular atrophy. In addition, all three had anterior horn cell degeneration, which was most prominent at the lumbar level. Compared to the literature, the degenerative changes in the brain stem of the variant A-T patient were somewhat less than anticipated for his age. Degenerative changes in the cerebellum and spinal cord were comparable with those in the literature. Progeric changes were lacking. In conclusion, compared to classical A-T, the variant A-T patient showed essentially the same, only slightly milder neuropathological abnormalities, except for anterior horn degeneration.