ATM deficiency impairs thymocyte maturation because of defective resolution of T cell receptor α locus coding end breaks

Abstract

The ATM (ataxia telangiectasia mutated) protein plays a central role in sensing and responding to DNA double-strand breaks. Lymphoid cells are unique in undergoing physiologic doublestrand breaks in the processes of Ig class switch recombination and T or B cell receptor V(D)J recombination, and a role for ATM in these processes has been suggested by clinical observations in ataxia telangiectasia patients as well as in engineered mice with mutations in the Atm gene. We demonstrate here a defect in thymocyte maturation in ATM-deficient mice that is associated with decreased efficiency in V-J rearrangement of the endogenous T cell receptor (TCR)␣ locus, accompanied by increased frequency of unresolved TCR J␣ coding end breaks. We also demonstrate that a functionally rearranged TCR␣␤ transgene is sufficient to restore thymocyte maturation, whereas increased thymocyte survival by bcl-2 cannot improve TCR␣ recombination and T cell development. These data indicate a direct role for ATM in TCR gene recombination in vivo that is critical for surface TCR expression in CD4 ؉ CD8 ؉ cells and for efficient thymocyte selection. We propose a unified model for the two major clinical characteristics of ATM deficiency, defective T cell maturation and increased genomic instability, frequently affecting the TCR␣ locus. In the absence of ATM, delayed TCR␣ coding joint formation results both in a reduction of ␣␤ TCR-expressing immature cells, leading to inefficient thymocyte selection, and in accumulation of unstable open chromosomal DNA breaks, predisposing to TCR␣ locus-associated chromosomal abnormalities.ataxia telangiectasia mutated ͉ recombination ͉ T cell development T he ATM (ataxia telangiectasia mutated) protein, first identified in patients with ataxia telangiectasia syndrome, plays a critical role in sensing and responding to chromatin changes such as DNA double-strand breaks induced by ionizing irradiation (1, 2). ATM is a member of a family of phosphatidylinositol 3-kinase-related proteins that function in cell cycle regulation, monitoring of telomere length, meiotic recombination, and DNA repair (1, 2). After activation by as-yet-incompletely understood signals associated with changes in DNA structure, ATM mediates an early step in the damage response, by phosphorylating a variety of protein targets and activating multiple signal transduction pathways (3-5). The downstream outcomes of these events can include cell cycle arrest and apoptotic cell death.In addition to the well characterized role of ATM in responses to DNA damage caused by environmental insults such as ionizing radiation, both clinical and laboratory observations have indicated that ATM also plays an important role in the unique physiological instances of DNA recombination that occur during development and differentiation of T and B lymphocytes (6-8). The generation of diversity in the antigen-specific receptors of T and B lymphocytes is mediated by induction of DNA breaks by recombinationactivating gene recombinase followed by repair through the nonhomol...

Programmed DNA breaks in lymphoid cells: repair mechanisms and consequences in human disease

“…This is an autosomal recessive disorder characterized by progressive cerebellar degeneration and an increased incidence of cancer. Importantly, ATM deficiency leads to a defect in thymocyte maturation that is associated with decreased efficiency in V‐J rearrangement of the endogenous TCRα locus, accompanied by increased frequency of unresolved TCR‐Jα coding end breaks 55. It was also shown that residual ATM kinase activity correlates with the severity of immune defects 56…”

Aberrantly resolved RAG-mediated DNA breaks in Atm-deficient lymphocytes target chromosomal breakpoints incis

“…In this regard, deficiency in the ataxia telangiectasia mutated (ATM) serine threonine kinase in both mice and humans leads to an increased incidence of lymphoid tumors with RAG-dependent translocations involving antigen receptor loci (17)(18)(19)(20)(21). In addition, mature, nontransformed Atm-deficient lymphocytes have an increased frequency of translocations and other chromosomal aberrations involving antigen receptor loci (18,19,(22)(23)(24).…”

“…Inhibition of the abl kinase with STI571 leads to G 1 cell cycle arrest, induction of RAG expression, and robust rearrangement of the endoge-nous Ig (Ig) light (L) chain genes and chromosomally integrated retroviral recombination substrates (25)(26)(27). Induction of V(D)J recombination by STI571 treatment of Atm Ϫ/Ϫ abl pre-B cells leads to an accumulation of un-repaired coding ends and to the aberrant resolution of these coding ends, analogous to what is observed in Atm-deficient lymphocytes in vivo (19,(22)(23)(24)(25). Here, we describe an experimental approach that permits high-throughput cloning of breakpoint targets from aberrantly resolved RAG DSBs generated during V(D)J recombination in Atm Ϫ/Ϫ abl pre-B cells.…”

Insights into the Transcriptional Regulation of the Unrearranged and Rearranged Tcra and Tcrd Genes

“…Other important aberrant translocations involving the Tcra/Tcrd locus include those that result in ataxia-telangiectasia mutations in the A-T mutated kinase (ATM) that cause chromosome instability and defects in DNA repair [69]. An important percent of A-T syndrome patients develop the disease due to translocations and inversions involving specific breakpoints at the Tcra/Tcrd locus and most of all ATM -/-mice die due to thymic lymphomas derived from and incorrect repair of the breaks that result from V(D)J recombination and aberrant Tcra/Tcrd locus translocations [70][71][72][73][74][75]. The knowledge of the precise mechanisms by which the Tcra/Tcrd locus transcription and recombination are regulated is important to understand the defective control of these processes that results in disease.…”