Normal Hematopoietic Stem Cell Functioning in p21^−/− Mice.

Recently, several studies have suggested that the family of cyclin-dependent kinase inhibitors plays a crucial role in regulating hematopoietic stem and progenitor pool size. However, due to a lack of appropriate transplantation models, competitive repopulation assays have not been performed. In the present study we have backcrossed a p21 null allele from mice with a mixed genetic background to inbred C57BL/6 mice. As expected, mouse embryonic fibroblasts (MEFs) derived from B6p21^−/− mice failed to undergo senescence, whereas B6p21^+/+ MEFs show a normal senescent phenotype. Moreover, B6p21^−/− CFU-GM were more resistant to radiation compared to B6p21^+/+. In contrast, homozygous deletion of the p21 allele did not affect the percentage of Lin⁻ Sca-1⁺ c-kit⁺ cells in S-phase when measured by 7-AAD staining, and did not result in any alterations of in vitro cobblestone area forming cell activity. In a competitive repopulating assay different ratios of Ly5.2 BM cells from B6p21^−/− or B6p21^+/+ littermates were competed with 2 x 10⁶ Ly5.1 B6 BM cells. Assuming similar repopulating capacity of both cell populations, expected chimerism was calculated. Surprisingly, observed and expected chimerism were identical, strongly suggesting that B6p21^−/− stem cells had completely normal competitive repopulating activity for up to 1 year after transplant. Our data argue against an important role of p21 in maintaining stem cell function during steady-state hematopoiesis.