In vitro and in vivo studies on the generation of the primary T-cell receptor repertoire

Genetic and epigenetic regulation of Tcrb gene assembly

Vertebrate development requires the formation of multiple cell types from a single genetic blueprint, an extraordinary feat that is guided by the dynamic and finely tuned reprogramming of gene expression. The sophisticated orchestration of gene expression programs is driven primarily by changes in the patterns of covalent chromatin modifications. These epigenetic changes are directed by cis elements, positioned across the genome, which provide docking sites for transcription factors and associated chromatin modifiers. Epigenetic changes impact all aspects of gene regulation, governing association with the machinery that drives transcription, replication, repair and recombination, a regulatory relationship that is dramatically illustrated in developing lymphocytes. The program of somatic rearrangements that assemble antigen receptor genes in precursor B and T cells has proven to be a fertile system for elucidating relationships between the genetic and epigenetic components of gene regulation. This chapter describes our current understanding of the cross-talk between key genetic elements and epigenetic programs during recombination of the Tcrb locus in developing T cells, how each contributes to the regulation of chromatin accessibility at individual DNA targets for recombination, and potential mechanisms that coordinate their actions.

ATM and the Mre11 complex combine to recognize and signal DNA double-strand breaks

The recognition and repair of DNA double-strand breaks (DSBs) is a complex process that draws upon a multitude of proteins. This is not surprising since this is a lethal lesion if left unrepaired and also contributes to genome instability and the consequential risk of cancer and other pathologies. Some of the key proteins that recognize these breaks in DNA are mutated in distinct genetic disorders that predispose to agent sensitivity, genome instability, cancer predisposition and/or neurodegeneration. These include members of the Mre11 complex (Mre11/Rad50/Nbs1) and ataxia-telangiectasia (A-T) mutated (ATM), mutated in the human genetic disorder A-T. The mre11 (MRN) complex appears to be the major sensor of the breaks and subsequently recruits ATM where it is activated to phosphorylate in turn members of that complex and a variety of other proteins involved in cell-cycle control and DNA repair. The MRN complex is also upstream of ATM and ATR (A-T-mutated and rad3-related) protein in responding to agents that block DNA replication. To date, more than 30 ATM-dependent substrates have been identified in multiple pathways that maintain genome stability and reduce the risk of disease. We focus here on the relationship between ATM and the MRN complex in recognizing and responding to DNA DSBs.