Impaired Repopulating Activity of Fus-Deficient Hematopoietic Stem Cells.

RNA-binding protein FUS (also known as TLS) was originally identified in chromosomal translocation in human myxoid liposarcoma. The FUS gene is also translocated with the transcription factor gene ERG in human myeloid leukemia with recurrent chromosomal translocation t(16;21). Multiple data suggest that wild-type FUS is also involved in the development of leukemia as one of the downstream targets for oncoproteins including BCR-ABL. However, little is known about the role of FUS in the normal hematopoiesis. The previous report demonstrated that Fus-deficient (Fus^−/−) newborn mice, which die shortly after birth because they cannot suckle, have a non-cell-autonomous defect in B lymphocyte development. No cell-autonomous defect of Fus^−/− hematopoietic cells has been documented. Here we report the detailed analyses of the Fus^−/− fetal liver hematopoietic stem cells (HSCs). Fus^−/− fetal livers at embryonic day 14.5 were smaller in size and exhibited a significant reduction in hematopoietic cell numbers by 60% compared with the wild type (WT). Nonetheless, no significant difference was observed in the proportion of stem/progenitor cell fraction (lineage-marker-c-Kit⁺Sca-1⁺; KSL) as well as colony-forming cells between WT and Fus^−/− fetal livers. Fus^−/− KSL cells proliferated and differentiated almost normally in vitro. To examine in vivo repopulating activity, we transplanted fetal liver cells to lethally irradiated CD45.1 recipients with competitor bone marrow cells. Fus^−/− fetal liver donor cells reconstituted recipients’ hematopoiesis for the long term and contributed to all cell lineages including B lymphocytes. In contrast to the in vitro results, however, the chimerism of donor-derived cells was significantly lower in recipients receiving Fus^−/− fetal liver cells compared with WT controls (approximately 2-fold reduction). This trend was reproducible with both unfractionated and purified KSL fetal liver test cells. Our data demonstrated that the proto-oncogene Fus is involved in the maintenance of normal HSC functions. Detailed analyses on the underlying mechanisms are in progress.