Lig4 R278H Mutation Impacts on LT-HSC Functions, Cancer Development and Aging

Abstract 1350

DNA Ligase IV (Lig4) is an essential component of the non-homologous end-joining (NHEJ) repair pathway, essential for the end-ligation of broken DNA ends generated during V(D)J recombination and IgH Class Switch Recombination (CSR) in B lymphocytes. Mutations in Lig4 underlie the DNA Lig4 Syndrome, a rare herited immunodeficiency disease. We have modeled one such mutation in mice, a homozygous arginine to histidine 278 (R278H) change with the enzyme active site, which others and we have shown significantly impairs the catalytic end-ligation activity of the LigIV protein. These mice show significant immune-specific defects, including decreased T and B lymphocytes, in part associated with defects in development, increased cell death, and/or autophagy. Bypass of B lymphocyte development leads to increased numbers of total peripheral B lymphocytes that can undergo robust CSR at near normal levels, which however contain high levels of IgH specific DNA breaks and translocations. Both young and older mice display significant reduction in LT-HSCs, which may underlie their B and T lymphocyte associated defects, which in LT-HSC reconstitution experiments in SCID mice, fail to engraft. The decreased R278H mice LT-HSCs observed are in contrast to previous reported findings for Ku80 and DNA-PKcs deficient mice, and also ATM deficiency, which we further are investigating to elucidate the normal and pathological functions of aberrant Lig4. We hypothesize the cause of the LT-HSC defects to be caused by changes in DNA damage, cell cycle checkpoint responses and genomic stability maintenance associated with the R278H mutation, that promote the restriction or selection of LT-HSC cell populations that are prone to genomic instability during differentiation. As such, the majority of relatively, young adult R278H mice routinely spontaneously develop T cell lymphomas, and mice that do not succumb to lymphomas subsequently go on to develop other solid tumors and other pathological consequences which are characteristic of premature aging. Our findings implicate Lig4 is required for genomic stability maintenance and proliferative expansion of earliest HSCs and other stem cell populations. The implications of our findings in the context of impacts of the R278H mutation on genomic instability and DNA damage response in the context of HSCs, cancer development and aging in the DNA Lig4 Syndrome, and its relation to other human immunodeficiency and progeroid syndromes will be discussed.