STAGE-SPECIFIC EXPRESSION OF POLYCOMB GROUP GENES IN HUMAN BONE-MARROW CELLS

Mammalian Polycomb group (Pc-G) genes, constituting some 5 subfamilies based on their identity to the Drosophila genes Pc, Psc, ph, esc, and E(z),appear to play critical roles in maintaining the transcriptional repression state of Hox/HOM-C genes during development. Despite incre asing evidence of the important role of Hox genes in both normal hemat opoiesis and leukemic transformation, little is known about the expres sion and possible function played by Pc-G genes in hematopoietic cells . To address this, we first examined the expression of Pc genes in pur ified CD34(+) human bone marrow cells by reverse transcriptase-polymer ase chain reaction (RT-PCR), using degenerate primers that specificall y amplify the majority of Pc genes. This analysis showed the expressio n of 8 different Pc genes in CD34(+) bone marrow cells, including HP1( HS alpha), HP1(Hs gamma), the heterochromatin p25 protein, the human h omologue of the murine M32 gene, and 4 novel members of this family. T o assess whether Pc-G mRNA levels change during differentiation of bon e marrow cells, a quantitative RT-PCR method was used to amplify the t otal cDNA originating from three purified subpopulations of CD34(+) bo ne marrow cells known to differ in their ability to grow in long-term or semisolid cultures. In sharp contrast to Hox gene expression, which is highest in the most primitive bone marrow cells, these studies sho w that the expression level of 8 of the 9 Pc-G genes studied (ie, HP1( H5 alpha), HP1(H5 gamma), M31, M32, M33, Mel-18, Mph1/Rae-28, and ENX- 1) markedly increases with differentiation of bone marrow cells. Inter estingly, BMI-1 exhibits a strikingly different pattern of expression, with high expression levels in primitive cells and very little expres sion in mature CD34(-) cells. Together, these results document for the first time that differentiation of human bone marrow cells is accompa nied by profound changes in Pc-G gene expression levels. (C) 1998 by T he American Society of Hematology.