Polycomb group gene mel-18 modulates the self-renewal activity and cell cycle status of hematopoietic stem cells

Objective. Mel-18 is a member of the mammalian Polycomb group (PcG) genes. This family of genes regulates global gene expression in many biologic processes, including hematopoiesis and anterior-posterior axis formation by manipulating specific target genes, including members of the Hox family. Here, we demonstrate that mel-18 negatively regulates the self-renewal activity of hematopoietic stem cells (HSCs). Materials and Methods. Long-term reconstitution activity was evaluated by competitive repopulating unit (CRU) and mean activity of the stem cells (MAS) assays in vivo in bone marrow cells (BMCs) derived from mel-18(-/-) and mel-18 tg mice. The expression levels of mel-18 and Hoxb4 were measured by quantitative real-time reverse transcription polymerase chain reaction. Results. The Hoxb4 gene was highly expressed in HSCs derived from mel-18(-/-) mice. The observed CRUs were 3.21, 4.77, 3.32, and 1.64 CRU per 10(5) BMCs in mel-18(+/+), mel-18(-/-), C57BL/6, and mel-18 tg, respectively. MAS was 0.58, 0.18, 0.41, and 5.89 in mel-18(+/+), mel-18(-/-), C57BL/6, and mel-18 tg, respectively. The percentage in GO phase HSCs (lin(-)flk2(-)c-Kit(+) Scal(+) cells) was increased in mel-18(-/-) mice and decreased in mel-18 tg mice. Conclusion. Loss or knockdown of inel-18 leads to the expression of Hoxb4, an increase in the proportion of HSCs in Go phase, and the subsequent promotion of HSC self-renewal. These findings will enable us to develop new approaches for controlling HSC activity for hematopoietic transplantations based on ex vivo expansion of HSCs. (C) 2004 International Society for Experimental Hematology. Published by Elsevier Inc.