TCR Translocations at the Normal-malignant T Cell Interface

Hematopoiesis is the process leading to production and maturation of peripheral blood cells. All blood cells are derived from hematopoietic stem cells (HSCs) which reside in hematopoietic organs. In mammals, the site of hematopoiesis changes during development, which is sequentially taking place in different organs starting with primitive erythrocytes in the yolk sac, the aorta-gonad mesonephros (AGM) region, the fetal lever, and finally the bone marrow (BM) during adulthood. Blood cells are short-lived, and with a daily demand for more than a billion new hematopoietic cells, a continuous replenishment of progenitor cells committed to specific hematopoietic lineages is required. HSCs are at the top of the hematopoietic hierarchy, and are the only source of progenitors. HSCs comprise 0.005-0.01% of the bone marrow, and their unique properties, i.e. the ability of self-renewal and multi-lineage differentiation potential in combination with a specific stem cell microenvironment/ niche, enable these cells to sustain the hematopoietic system. These cells differentiate into progenitor cells, either into common lymphoid progenitors (CLP) or common myeloid progenitors (CMP), which in due course differentiate into mature blood cells, providing cells to the myeloid or lymphoid system respectively 6. CLPs carry the potential to give rise to B cells, T cells (via the thymus) and NK cells, whereas CMPs have the potential to differentiate into erythrocytes, megakaryocytes, macrophages, and granulocytes. Dendritic cells can arise from both progenitor types. The process of hematopoietic lineage determination is tightly regulated by the BM microenvironment’s extrinsic factors, such as growth factors and cytokines mediated by cell-cell interactions, which sustain survival and proliferation of committed cells. Equally important in determining cell fate are the lineage- and cell-type-specific gene expression signatures (intrinsic factors). These signatures are based on the up and down regulation of transcription factors apparently regulated by the epigenetic-micro RNAs regulatory circuit. The strict regulation of both extrinsic and intrinsic signals is of utmost importance, as deregulation of the expression of these factors could result in hematopoietic malignancies such as leukemia or lymphoma. Such deregulation of gene expression is usually caused by irreversible molecular-cytogenetic changes introduced into the genomic DNA sequence. These changes can be caused by mutations, translocations and deletions concerning genes involved in cell cycle, differentiation, proliferation, and self-renewal processes. During the last decade it has become evident that, next to genetic aberrations, epigenetic alterations can also contribute to tumorigenesis, for example through gene silencing due to aberrant methylation.

Breakpoint sites disclose the role of the V(D)J recombination machinery in the formation of T-cell receptor (TCR) and non-TCR associated aberrations in T-cell acute lymphoblastic leukemia

Aberrant recombination between T-cell receptor genes and oncogenes gives rise to chromosomal translocations that are genetic hallmarks in several subsets of human T-cell acute lymphoblastic leukemias. The V(D)J recombination machinery has been shown to play a role in the formation of these T-cell receptor translocations. Other, non-T-cell receptor chromosomal aberrations, such as SIL-TAL1 deletions, have likewise been recognized as V(D)J recombination associated aberrations. Despite the postulated role of V(D)J recombination, the extent of the V(D)J recombination machinery involvement in the formation of T-cell receptor and non-T-cell receptor aberrations in T-cell acute lymphoblastic leuke

Genetic and epigenetic determinants mediate proneness of oncogene breakpoint sites for involvement in TCR translocations

T-cell receptor (TCR) translocations are a genetic hallmark of T-cell acute lymphoblastic leukemia and lead to juxtaposition of oncogene and TCR loci. Oncogene loci become involved in translocations because they are accessible to the V(D)J recombination machinery. Such accessibility is predicted at cryptic recombination signal sequence (cRSS) sites ('Type 1') as well as other sites that are subject to DNA double-strand breaks (DSBs) ('Type 2') during early stages of thymocyte development. As chromatin accessibility markers have not been analyzed in the context of TCR-associated translocations, various genetic and epigenetic determinants of LMO2, TAL1 and TLX1 translocation breakpoint (BP) sites and BP cluster regions (BCRs) were examined in human thymocytes to establish DSB proneness and heterogeneity of BP site involvement in TCR translocations. Our data show that DSBs in BCRs are primarily induced in the presence of a genetic element of sequence vulnerability (cRSSs, transposable elements), whereas breaks at single BP sites lacking such elements are more likely induced by chance or perhaps because of patient-specific genetic vulnerability. Vulnerability to obtain DSBs is increased by features that determine chromatin organization, such as methylation status and nucleosome occupancy, although at different levels at different BP sites