Primary immunodeficiency can be caused by reduced immune cell number or function. Lymphocytes are essential components of the adaptive immune system. Mutations in DNA repair factors compromise lymphocyte development and contribute to three of the eight classes of human primary immunodeficiencies recognized by the International Union of Immunological Societies. XLF (also called Cernuous or NHEJ1) is a member of the non-homologous end joining (NHEJ) family of DSB repair genes. Human patients with loss of function mutations in the XLF gene develop a primary immunodeficiency characterized by age-progressive lymphocytopenia and occasional myeloproliferative diseases. The NHEJ pathway as a whole is required during lymphocyte development at the step of V(D)J recombination that assembles the antigen receptor gene products in developing B and T cells. While the role of XLF in V(D)J recombination, was thought to explain the lymphocytopenia in XLF-deficient patients, we and others reported that XLF is NOT required for chromosomal V(D)J recombination in developing murine lymphocytes, underscoring the need for an alternative mechanism. In this proposal, we investigate an alternative and novel mechanism that might contribute to the XLF related immunodeficiency. Specifically, we hypothesize that XLF-mediated NHEJ is essential for the maintenance of genomic stability in hematopoietic stem cells (HSC). XLF-/- HSCs recapitulate the phenotype of aged HSCs - i.e. loss of self-renewal function, impaired lymphocyte (versus myeloid) differentiation and a relative expansion of the myeloid compartment. The lymphocytopenia observed in XLF-/- mice and in human patients is therefore secondary to HSC dysfunction and is accelerated with age. The deficiencies in HSC function and differentiation in the XLF-deficient patients or mice could be due to defects in the HSCs (cell autonomous) or the microenvironment (cell non-autonomous). HSCs are known to reside in bone marrow "niches" defined by osteoblasts, mesenchymal cells and endothelial cells, all of which regulate HSC function. Notably, prior studies by the co-investigator- Dr. Siddhartha Mukherjee, revealed that genetic alterations in osteoblasts can impair HSC differentiation. To test of hypothesis, we will determine (Aim 1) the cell autonomous function of XLF and DNA repair in HSC maintenance, function and B cell differentiation and (Aim 2) how XLF deficiency in the bone marrow niche environments affects HSC renewal, function and B cell differentiation. In particular, the later study will provide information on the long term therapeutic effects of bone marrow transplantation, the only curative therapy that is currently available for patients with primary immunodeficiency associated with DNA repair defects. The completion of this proposal will establish a link between HSC and primary immunodeficiency that will have general implication in other genomic instability syndromes.
Our proposed study addresses 1) the cell autonomous function of XLF by which XLF contributes to hematopoietic stem cell maintenance and function 2) the molecular mechanism by which XLF deficiency in bone marrow niche environment affects hematopoietic stem cell maintenance and function to determine how mutations in XLF lead to progressive primary immunodeficiency in human patients.
|Avagyan, Serine; Churchill, Michael; Yamamoto, Kenta et al. (2014) Hematopoietic stem cell dysfunction underlies the progressive lymphocytopenia in XLF/Cernunnos deficiency. Blood 124:1622-5|