Distinct histone lysine methylation marks are involved in transcriptional repression linked to the formation and maintenance of facultative heterochromatin, although the underlying mechanisms remain unclear. We demonstrate that the malignant-brain-tumor (MBT) protein L3MBTL1 is in a complex with core histones, histone H1b, HP1gamma, and Rb. The MBT domain is structurally related to protein domains that directly bind methylated histone residues. Consistent with this, we found that the L3MBTL1 MBT domains compact nucleosomal arrays dependent on mono- and dimethylation of histone H4 lysine 20 and of histone H1b lysine 26. The MBT domains bind at least two nucleosomes simultaneously, linking repression of transcription to recognition of different histone marks by L3MBTL1. Consistently, L3MBTL1 was found to negatively regulate the expression of a subset of genes regulated by E2F, a factor that interacts with Rb.

Transforming growth factor β (TGFβ) is one of few known negative regulators of hematopoiesis, yet the mechanisms by which it affects cell cycle arrest and stem cell quiescence are poorly understood. Induction of the cyclin-dependent kinase inhibitors, p15INK4b (p15) and p21WAF1 (p21) is important for TGFβ-mediated cytostasis in epithelial cells but not in hematopoietic cells. Using primary human hematopoietic cells and microarray analysis, we identified p57KIP2 (p57) as the only cyclin-dependent kinase inhibitor induced by TGFβ. Up-regulation of p57 mRNA and protein occurs before TGFβ-induced G 1 cell cycle arrest, requires transcription, and is mediated via a highly conserved region of the proximal p57 promoter. The up-regulation of p57 is essential for TGFβ-induced cell cycle arrest in these cells, because two different small interfering RNAs that prevent p57 up-regulation block the cytostatic effects of TGFβ on human hematopoietic cells. Reduction of basal p57 expression by this approach also allows hematopoietic cells to proliferate more readily in the absence of TGFβ. p57 is a putative tumor suppressor gene whose expression is frequently silenced by promoter hypermethylation in hematologic malignancies. Our studies identify a molecular pathway by which TGFβ mediates its cytostatic effects on human hematopoietic cells and suggests an explanation for the frequent silencing of p57 expression.

Flow cytometric analysis is superior to the Ham test and permits concomitant diagnosis of PNH in about 20% of patients with myelodysplasia (a rate similar to that seen in patients with aplastic anemia). The presence of GPI-anchored protein-deficient cells in myelodysplasia predicts responsiveness to immunosuppressive therapy. Early emergence of GPI-anchored protein-deficient hematopoiesis in a patient with marrow failure may point to an underlying immune pathogenesis.