Rescue of the HSC Maintenance Defects in Ku70-Deficient Mice by Overexpression of Bcl2 Reveals a Novel Role of Bcl2 in HSC

Abstract 1235

Ku70 is a key component of the non-homologous end joining (NHEJ) pathway; Ku70-deficient mice are hypersensitive to radiation and show a leaky SCID phenotype. We find that HSC from Ku70-deficient mice are severely impaired in maintenance, defective in self-renewal, competitive repopulation and BM hematopoietic niche occupancy. HSCs from Ku70-deficient mice are not prone to spontaneous apoptosis, however, they display more active proliferation, and fewer cells in a quiescent state than HSC obtained from WT mice. Further, gene expression profiling showed that multiple HSC quiescence- related genes such as c-MPL and p57 were significantly downregulated in Ku70-deficient HSCs. These data suggested that loss of quiescence results in the dramatic defect in the maintenance of Ku70-deficient HSC.

Bcl2 plays important roles in both anti-apoptosis and anti-proliferation, while the anti-apoptosis function of Bcl2 has been extensively studied and well established in various settings, including HSCs, the anti-proliferation function of Bcl2 in HSC has yet to be investigated. To exam whether overexpression of Bcl2 can rescue the HSC defect in Ku70-deficient mice, H2K-Bcl2 transgenic/Ku70-deficient double mutant mice were generated. Though overexpression of Bcl2 does not rescue the SCID phenotype in Ku70-deficient mice, overexpression of Bcl2 in Ku70-deficient HSCs almost completely rescued the impaired HSC quiescence, repopulation and BM hematopoietic niche occupancy capacities. At the transcriptional level, overexpression of Bcl2 restored the expression of c-MPL and p57 comparable to WT levels. However, additional deletion of Bax, a pro-apoptosis protein in the Bcl2 family, in Ku70-deficient mice could not rescue the HSC repopulation defect, suggesting that apoptosis is not the main mediator for the impaired HSC maintenance in Ku70-deficient mice.

Together, our data indicate that the HSC maintenance defect of Ku70-deficient mice is due to the loss of HSC quiescence populations, whereas overexpression of Bcl2 rescues the HSC defect in Ku70-deficient mice by restoration of quiescence. Our study uncovers a novel role of Bcl2 in hematopoietic stem cell quiescence regulation.