Differential expression of human polycomb group proteins in various tissues and cell types

Polycomb group proteins are involved in the maintenance of cellular identit y. As multimeric comp teres they repress cell type-specific sets of target genes. One model predicts that the composition of Polycomb group complexes determines the specificity for their target genes. To study this hypothesis , we analyzed the expression of Polycomb group genes in various human tissu es using Northern blotting and immunohistochemistry. We found that Polycomb group expression varies greatly among tissues and even among specific cell types within a particular tissue. Variations in mRNA expression ranged fro m expression of all analyzed Polycomb group genes in the heart and testis t o no detectable Polycomb group expression at all in bone marrow. Furthermor e, each Polycomb group gene was expressed in a different number of tissues. RING1 was expressed in practically all tissues, while HPH1 was expressed i n only a few tissues. Also within one tissue the level of Polycomb group ex pression varied greatly. Cell type-specific Polycomb group expression patte rns were observed in thyroid, pancreas, and kidney. Finally, in various dev elopmental stages of fetal kidney, different Polycomb group expression patt erns were observed. We conclude that Polycomb group expression can vary dep ending on the tissue, cell type, and development stage. Polycomb group comp lexes can only be composed of the Polycomb group proteins that are expresse d. This implies that with cell type-specific Polycomb group expression patt erns, cell type-specific Polycomb group complexes exist. The fact that ther e are cell type-specific Polycomb group targets and cell type-specific Poly comb group complexes fits well with the hypothesis that the composition of Polycomb group complexes may determine their target specificity. (C) 2001 W iley-Liss, Inc.