New Insights into the Role of DNA Damage and Repair in Age-Related Changes of Hematopoietic Stem Cells

Normal hematopoiesis and peripheral blood counts are maintained throughout life. However, observations in mice indicate that lymphopoiesis declines with age and hematopoietic stem cells (HSCs) undergo numerical and qualitative changes, including telomere shortening, accumulation of DNA double-strand breaks, increased frequencies of long-term reconstituting HSCs (LT-HSCs), and reduced ability to self-renew under the proliferative stress of multiple serial transplantations. To better understand the mechanisms responsible for these alterations, Nijnik et al. and Rossi et al. characterized HSC subpopulations from young and old mice with defects in DNA repair pathways. The mouse strain used by Nijnik et al. contains a hypomorphic mutation in DNA ligase IV (Lig4^Y288C ), which specifically impairs non-homologous end-joining (NHEJ) repair of DNA double-strand breaks and recapitulates many features of the human DNA ligase IV syndrome. The Rossi group studied three genetically engineered strains of mice deficient in NHEJ repair (Ku80^-/-), nucleotide excision repair (XPD^TTD), or telomere maintenance (late-generation [G3] mTR^-/-). With aging, the absolute numbers of LT-HSCs, short-term HSCs, and multipotent progenitors in Lig4^Y288C mice decreased significantly, in contrast to the stable cell numbers maintained in older wild-type (WT) control animals. Marrow cell counts and erythroid cell percentages were also reduced in older compared with younger mutant mice. HSCs from Lig4^Y288C mice competed poorly in co-transplantation models, and progenitor growth in cobblestone-area-forming cell cultures was greatly compromised. These changes were not due to defective Lig4^Y288Cmarrow stromal cells or macrophages. Double-stranded DNA breaks were more frequent in progenitors from 18- week-old Lig4^Y288C mice than in age-matched WT controls. Rossi et al. found that the numbers of LT-HSCs in older Ku80^-/-, XPD^TTD, and G₃mTR^-/- mice were equivalent to WT controls. However, diminished frequencies of multipotent and oligopotent progenitor populations were observed in an age-independent pattern. Serial transplantation and competitive repopulation experiments revealed severe, age-dependent deficits in the self-renewal and proliferative capacities of LT-HSCs from each of these mouse strains; this was associated with increased apoptotic activity in cultured progenitors. Age-related increases in DNA damage were observed in normal mouse HSCs and early multipotent progenitors, with the greatest increases in the LT-HSC population, thus suggesting that the more primitive, quiescent stem cells are less efficient at repairing genomic damage and/or better able to tolerate unrepairable lesions.