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Mechanisms of aberrant DNA double-strand break repair leading to lymphoid malignancy

Chun-Chin Chen

Chun-Chin Chen


Massachusetts Institute of Technology

Project Term: July 1, 2019 - June 30, 2022

The aim of this project is to address why Down syndrome children are prone to leukemia. We establish a model system using stem cells derived from Down syndrome patients to study early development of blood cells and identify the genomic aberrations in these processes. We hope to help understand why the genetic alterations in Down syndrome promote mutations in blood cells that drive leukemia development.

Lay Abstract

An essential function of the immune system is the ability to identify and react to substances foreign to the body, called antigens. Antigens are recognized by antibodies and antigen receptors, which resemble antibodies but are anchored to the cell. These are intricate multi-protein complexes that are generated by a process of gene recombination. Recombination happens by breaking the DNA and then repairing it in new configurations. When these repair processes do not work properly, they can cause mutations, including translocations of segments of chromosomes, which may lead to lymphoma. This can happen during early lymphocyte development when DNA breaks are generated by the RAG protein. The DNA breaks provide lymphocytes an opportunity to introduce various modifications in antigen receptor genes via the DNA repair machinery, which greatly increases the diversity of immune repertoire. Developing lymphocytes preferentially use a DNA repair pathway called canonical non-homologous end joining (c-NHEJ), instead of a more error-prone pathway called alternative end joining (alt-EJ) that is more likely to cause mutations. It remains unclear how developing lymphocytes normally utilize c-NHEJ while suppressing alt-EJ in response to DNA breaks. To study how developing lymphocytes maintain DNA repair fidelity, we use B cell lines derived from mouse bone marrow that mimic characteristics of early-stage B cells. One possibility of suppressing mutagenic alt-EJ repair at RAG-induced DNA breaks is through an antagonism with factors of c-NHEJ repair, namely the DNA end-binding protein Ku. We will perform genetic analyses to identify the roles of Ku and related proteins in protecting DNA ends from improper processing that could allow for alt-EJ and translocations. In addition, we plan to take an unbiased approach by performing a genome-wide screening using state-of-the-art CRISPR genome editing tools to identify novel factors involved in preventing mutagenic alt-EJ in lymphocytes. These results will help elucidate how lymphocytes regulate the choice of DNA repair pathways to resolve physiological DNA breaks correctly. Defects in these protective mechanisms contribute to genome instability, such as mutations and translocations, that can drive lymphoma progression. This study will help expand our fundamental knowledge about the regulation of antigen receptor gene recombination and the impact of mutagenic DNA repair activity during lymphocyte development.

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