Defective long-term repopulating ability in hematopoietic stem cells lacking the Polycomb-group gene rae28.

The rae28 gene (rae28) is a member of a Polycomb-group (PcG) complex 1, which is known to help maintain transcription states once these have been initiated, by generating heritable higher-order chromatin structures. In this study, we examined the capacity of rae28-deficient (rae28-/-) hematopoietic stem cells (HSCs) to generate long-term marrow reconstitution. rae28-/- fetal liver cells containing 20 competitive repopulation units (CRUs) were able to support the survival of lethally irradiated congenic mice for as long as 6 months. The marrow reconstituted with the rae28-/- cells, however, could not increase HSCs efficiently. This was evidenced by its inability to reconstitute marrow in serial transplantation experiments, as well as by the reduction in HSC-enriched Lin- c-kit+ Sca-1high+ subpopulation in the bone marrow cells. Moreover, the reconstituted marrow produced less than half of the peripheral blood cells in each of the lineages examined. We also monitored the mean stem cell activity (MAS). MAS of rae28-/- CRUs was progressively reduced after transplantation, and after 12 months it was reduced to one-tenth of that of the wild-type. These in vivo results clearly indicate that rae28 is indispensable for the long-term repopulating ability of HSCs. We further referred to the plausible mechanisms underlying defective long-term repopulating ability of rae28-deficient HSCs and argued for its involvement in maintenance of cell proliferation capability as well as that in self-renewal ability.