Histone deacetylases (HDACs) regulate gene expression by deacetylating histones and also modulate the acetylation of a number of nonhistone proteins, thus impinging on various cellular processes. Here, we analyzed the major class I enzymes HDAC1 and HDAC2 in primary mouse fibroblasts and in the B-cell lineage. Fibroblasts lacking both enzymes fail to proliferate in culture and exhibit a strong cell cycle block in the G1 phase that is associated with up-regulation of the CDK inhibitors p21 WAF1/CIP1 and p57 Kip2 and of the corresponding mRNAs. This regulation is direct, as in wild-type cells HDAC1 and HDAC2 are bound to the promoter regions of the p21 and p57 genes. Furthermore, analysis of the transcriptome and of histone modifications in mutant cells demonstrated that HDAC1 and HDAC2 have only partly overlapping roles. Next, we eliminated HDAC1 and HDAC2 in the B cells of conditionally targeted mice. We found that B-cell development strictly requires the presence of at least one of these enzymes: When both enzymes are ablated, B-cell development is blocked at an early stage, and the rare remaining pre-B cells show a block in G1 accompanied by the induction of apoptosis. In contrast, elimination of HDAC1 and HDAC2 in mature resting B cells has no negative impact, unless these cells are induced to proliferate. These results indicate that HDAC1 and HDAC2, by normally repressing the expression of p21 and p57, regulate the G1-to-S-phase transition of the cell cycle.[Keywords: Histone deacetylases; cell cycle control; p21; p57; B-cell development] Supplemental material is available at http://www.genesdev.org.

Class I Histone deacetylases (HDACs) play a central role in controlling cell cycle regulation, cell differentiation, and tissue development. These enzymes exert their function by deacetylating histones and a growing number of non-histone proteins, thereby regulating gene expression and several other cellular processes. Class I HDACs comprise four members: HDAC1, 2, 3, and 8. Deletion and/or overexpression of these enzymes in mammalian systems has provided important insights about their functions and mechanisms of action which are reviewed here. In particular, unique as well as redundant functions have been identified in several paradigms. Studies with small molecule inhibitors of HDACs have demonstrated the medical relevance of these enzymes and their potential as therapeutic targets in cancer and other pathological conditions. Going forward, better understanding the specific role of individual HDACs in normal physiology as well as in pathological settings will be crucial to exploit this protein family as a useful therapeutic target in a range of diseases. Further dissection of the pathways they impinge on and of their targets, in chromatin or otherwise, will form important avenues of research for the future.

The retinoblastoma tumor suppressor protein (pRB) and related p107 and p130 "pocket proteins" function together with the E2F transcription factors to repress gene expression during the cell cycle and development. Recent biochemical studies have identified the multisubunit DREAM pocket protein complexes in Drosophila melanogaster and Caenorhabditis elegans in regulating developmental gene repression. Although a conserved DREAM complex has also been identified in mammalian cells, its physiological function in vivo has not been determined. Here we addressed this question by targeting Lin9, a conserved core subunit of DREAM. We found that LIN9 is essential for early embryonic development and for viability of adult mice. Loss of Lin9 abolishes proliferation and leads to multiple defects in mitosis and cytokinesis because of its requirement for the expression of a large set of mitotic genes, such as Plk1, Aurora A, and Kif20a. While Lin9 heterozygous mice are healthy and normal, they are more susceptible to lung tumorigenesis induced by oncogenic c-Raf than wild-type mice. Together these experiments provide the first direct genetic evidence for the role of LIN9 in development and mitotic gene regulation and they suggest that it may function as a haploinsufficient tumor suppressor.The retinoblastoma tumor suppressor protein (pRB) and related p107 and p130 "pocket proteins" function together with the E2F transcription factors to regulate gene expression during the cell cycle (7). The identification of evolutionary conserved pocket protein/E2F complexes in Drosophila melanogaster has provided new insights into E2F-mediated gene regulation (21,24). These multisubunit complexes, alternatively called dREAM or Myb-MuvB (MMB), consist of at least eight subunits, including the repressor dE2F2 and one of the two retinoblastoma-related proteins, RBF1 or RBF2. In addition, the complex also contains Drosophila dMYB and three Myb-interacting proteins. RNA interference (RNAi)-mediated depletion of several subunits of the complex demonstrated a role in stable repression of developmental genes, although more recent genome-wide studies have found that dREAM/ MMB also functions in activation of genes involved in G 2 and mitosis (2,13,21,24).Remarkably, all subunits of dREAM/MMB, except for dMYB, are related to the Caenorhabditis elegans synMuv class B genes that antagonize RAS-induced vulva differentiation (3,9). Indeed, several synMuv proteins form a multisubunit complex that is highly related to dREAM/MMB (14). Therefore, in analogy to dREAM/MMB, it has been suggested that DRM mainly functions in gene repression during development (14).We and others recently identified a complex in human cells that is closely related to dREAM and DRM (20,25,31,40). The human complex, alternatively called LINC or human DREAM, consists of a five-protein core module that binds in quiescent cells to the repressors p130 and E2F4. In S phase this binding is lost and B-MYB associates with the complex. The high degree of conservation of the DREAM-like complex...

During gametogenesis, germ cells must undergo meiosis in order to become viable haploid gametes. Successful completion of this process is dependent upon the expression of genes whose protein products function specifically in meiosis. Failure to express these genes in meiotic cells often results in infertility, whereas aberrant expression in somatic cells may lead to mitotic catastrophe. The mechanisms responsible for regulating the timely expression of meiosis-specific genes have not been fully elucidated. Here we demonstrate that E2F6, a member of the E2F family of transcription factors, is essential for the repression of the newly identified meiosis-specific gene, Slc25a31 (also known as Ant4, Aac4), in somatic cells. This discovery, along with previous studies, prompted us to investigate the role of E2F6 in the regulation of meiosis-specific genes in general. Interestingly, the core E2F6-binding element (TCCCGC) was highly conserved in the proximal promoter regions of 19 out of 24 (79.2%) meiosis-specific genes. This was significantly higher than the frequency found in the promoters of all mouse genes (15.4%). In the absence of E2F6, only a portion of these meiosis-specific genes was derepressed in somatic cells. However, endogenous E2F6 bound to the promoters of these meiosis-specific genes regardless of whether they required E2F6 for their repression in somatic cells. Further, E2F6 overexpression was capable of reducing their transcription. These findings indicate that E2F6 possesses a broad ability to bind to and regulate the meiosis-specific gene population.