The CUL4-DDB1 ubiquitin ligase complex controls adult and embryonic stem cell differentiation and homeostasis

Little is known on post-transcriptional regulation of adult and embryonic stem cell maintenance and differentiation. Here we characterize the role of Ddb1, a component of the CUL4-DDB1 ubiquitin ligase complex. Ddb1 is highly expressed in multipotent hematopoietic progenitors and its deletion leads to abrogation of both adult and fetal hematopoiesis, targeting specifically transiently amplifying progenitor subsets. However, Ddb1 deletion in non-dividing lymphocytes has no discernible phenotypes. Ddb1 silencing activates Trp53 pathway and leads to significant effects on cell cycle progression and rapid apoptosis. The abrogation of hematopoietic progenitor cells can be partially rescued by simultaneous deletion of Trp53. Conversely, depletion of DDB1 in embryonic stem cell (ESC) leads to differentiation albeit negative effects on cell cycle and apoptosis. Mass spectrometry reveals differing protein interactions between DDB1 and distinct DCAFs, the substrate recognizing components of the E3 complex, between cell types. Our studies identify CUL4-DDB1 complex as a novel post-translational regulator of stem and progenitor maintenance and differentiation

Regulation of transcriptional elongation in pluripotency and cell differentiation by the PHD-finger protein Phf5a

Pluripotent embryonic stem cells (ESCs) self-renew or differentiate into all tissues of the developing embryo and cell-specification factors are necessary to balance gene expression. Here we delineate the function of the PHD-finger protein 5a (Phf5a) in ESC self-renewal and ascribe its role in regulating pluripotency, cellular reprogramming, and myoblast specification. We demonstrate that Phf5a is essential for maintaining pluripotency, since depleted ESCs exhibit hallmarks of differentiation. Mechanistically, we attribute Phf5a function to the stabilization of the Paf1 transcriptional complex and control of RNA polymerase II elongation on pluripotency loci. Apart from an ESC-specific factor, we demonstrate that Phf5a controls differentiation of adult myoblasts. Our findings suggest a potent mode of regulation by the Phf5a in stem cells, which directs their transcriptional program ultimately regulating maintenance of pluripotency and cellular reprogramming

Contrasting roles of histone 3 lysine 27 demethylases in acute lymphoblastic leukaemia

T-cell acute lymphoblastic leukaemia (T-ALL) is a haematological malignancy with a dismal overall prognosis, including a relapse rate of up to 25%, mainly because of the lack of non-cytotoxic targeted therapy options. Drugs that target the function of key epigenetic factors have been approved in the context of haematopoietic disorders, and mutations that affect chromatin modulators in a variety of leukaemias have recently been identified; however, ‘epigenetic’ drugs are not currently used for T-ALL treatment. Recently, we described that the polycomb repressive complex 2 (PRC2) has a tumour-suppressor role in T-ALL. Here we delineated the role of the histone 3 lysine 27 (H3K27) demethylases JMJD3 and UTX in T-ALL. We show that JMJD3 is essential for the initiation and maintenance of T-ALL, as it controls important oncogenic gene targets by modulating H3K27 methylation. By contrast, we found that UTX functions as a tumour suppressor and is frequently genetically inactivated in T-ALL. Moreover, we demonstrated that the small molecule inhibitor GSKJ4 (ref. 5) affects T-ALL growth, by targeting JMJD3 activity. These findings show that two proteins with a similar enzymatic function can have opposing roles in the context of the same disease, paving the way for treating haematopoietic malignancies with a new category of epigenetic inhibitors.National Institutes of Health (U.S.) (Grant R37-HD04502

Opposing functions of H2BK120 ubiquitylation and H3K79 methylation in the regulation of pluripotency by the Paf1 complex

Maintenance of stem cell plasticity is determined by the ability to balance opposing forces that control gene expression. Regulation of transcriptional networks, signaling cues and chromatin-modifying mechanisms constitute crucial determinants of tissue equilibrium. Histone modifications can affect chromatin compaction, therefore co-transcriptional events that influence their deposition determine the propensities toward quiescence, self-renewal, or cell specification. The Paf1 complex (Paf1C) is a critical regulator of RNA PolII elongation that controls gene expression and deposition of histone modifications, however few studies have focused on its role affecting stem cell fate decisions. Here we delineate the functions of Paf1C in pluripotency and characterize its impact in deposition of H2B ubiquitylation (H2BK120-ub) and H3K79 methylation (H3K79me), 2 fundamental histone marks that shape transcriptional regulation. We identify that H2BK120-ub is increased in the absence of Paf1C on its embryonic stem cell targets, in sharp contrast to H3K79me, suggesting opposite functions in the maintenance of self-renewal. Furthermore, we found that core pluripotency genes are characterized by a dual gain of H2BK120-ub and loss of H3K79me on their gene bodies. Our findings elucidate molecular mechanisms of cellular adaptation and reveal novel functions of Paf1C in the regulation of the self-renewal network

Therapeutic Targeting of the Cyclin D3:CDK4/6 Complex in T Cell Leukemia

SummaryD-type cyclins form complexes with cyclin-dependent kinases (CDK4/6) and promote cell cycle progression. Although cyclin D functions appear largely tissue specific, we demonstrate that cyclin D3 has unique functions in lymphocyte development and cannot be replaced by cyclin D2, which is also expressed during blood differentiation. We show that only combined deletion of p27Kip1 and retinoblastoma tumor suppressor (Rb) is sufficient to rescue the development of Ccnd3−/− thymocytes. Furthermore, we show that a small molecule targeting the kinase function of cyclin D3:CDK4/6 inhibits both cell cycle entry in human T cell acute lymphoblastic leukemia (T-ALL) and disease progression in animal models of T-ALL. These studies identify unique functions for cyclin D3:CDK4/6 complexes and suggest potential therapeutic protocols for this devastating blood tumor