To gain insights into the regulatory mechanisms of hematopoietic stem cells (HSCs), we employed a genome-wide RNAi screen in human cord-blood derived cells and identified candidate genes whose knockdown maintained the HSC phenotype during culture. A striking finding was the identification of members of the cohesin complex (STAG2, RAD21, STAG1, and SMC3) among the top 20 genes from the screen. Upon individual validation of these cohesin genes, we found that their knockdown led to an immediate expansion of cells with an HSC phenotype in vitro. A similar expansion was observed in vivo following transplantation to immunodeficient mice. Transcriptome analysis of cohesin-deficient CD34(+) cells showed an upregulation of HSC-specific genes, demonstrating an immediate shift toward a more stem-cell-like gene expression signature upon cohesin deficiency. Our findings implicate cohesin as a major regulator of HSCs and illustrate the power of global RNAi screens to identify modifiers of cell fate.

We report on a forward RNAi screen in primary human hematopoietic stem and progenitor cells, using pooled lentiviral shRNA libraries deconvoluted by next generation sequencing. We identify

The cultured skin fibroblasts from three patients with lacticacidemia were found to have low rates of 1-1[4Cjpyruvate oxidation in the face of normal pyruvate-dehydrogenase activity. After incubation with 1 mM glucose, these three cell strains also exhibited lactate/pyruvate ratios which were three times greater than those of controls. In two of the patients, both ATP and oxygen consumption in fibroblast mitochondrial preparations was deficient with NAD-linked substrates but normal with succinate and

Key Points Chemokine (C-C motif) ligand 28 (CCL28) is a novel growth factor for human hematopoietic stem and progenitor cells. CCL28 supports the in vitro and in vivo functional integrity of cultured primitive hematopoietic cells.

Disease relapse sometimes occurs after acute promyelocytic leukemia (APL) therapy with all-trans retinoic acid (ATRA). Among the diagnostic parameters predicting relapse, heterogeneity in the in vitro differentiation rate of blasts is an independent factor. To identify biologic networks involved in resistance, we conducted pharmacogenomic studies in APL blasts displaying distinct ATRA sensitivities. Although the expression profiles of genes invested in differentiation were similarly modulated in low-and high-sensitive blasts, low-sensitive cells showed higher levels of transcription of ATRA-target genes, transcriptional regulators, chromatin remodelers, and transcription factors. In opposition, only high-sensitive blasts expressed the CYP26A1 gene, encoding the p450 cytochrome which is known to be involved in retinoic acid catabolism. In NB4 cells, ATRA treatment activates a novel signaling pathway, whereby interleukin-8 stimulates the expression of the homeobox transcription factor HOXA10v2, an effective enhancer of CYP26A1 transcription. These data were corroborated in primary APL cells, as maturation levels correlated with CYP26A1 expression. IntroductionHematopoiesis can be described as a balance between proliferation and differentiation. In 90% of acute promyelocytic leukemia (APL) cases, the t(15;17) translocation and consequent expression of the PML (promyelocytic leukemia protein)/RAR␣ (retinoic acid receptor ␣) fusion protein blocks differentiation at the promyelocytic stage. RAR␣ belongs to the nuclear receptor superfamily and functions as a ligand-dependent transcription factor, acting to establish the granulocytic lineage. Under normal conditions, binding of the ligand (all-trans retinoic acid, or ATRA) to the receptor provokes a conformational change in the receptor resulting in the disruption of a co-repression complex, recruitment of coactivators, binding to the retinoic acid response element (RARE), and finally the transcription of ATRA-dependent genes. 1 The PML portion of the PML/RAR␣ fusion protein has a high affinity for the corepression complex, preventing RAR␣ transcriptional activity. To induce complete remission in a high proportion of patients, a pharmacologic dose of ATRA is required to destabilize this interaction and reestablish the differentiation program. Combining ATRA with chemotherapy results in a significant prolongation of disease-free and overall survival. [2][3][4][5][6] Despite the general efficacy of ATRA-based therapy, 10% to 20% of patients with APL still relapse. Several mechanisms have been proposed to explain the involved retinoid resistance arising during cancer therapy. With the exception of primary resistance, which is extremely rare in PML/RAR␣ cells, 7,8 APL relapse has been suggested to be related to various processes observed in cell lines. These include variations in nuclear ATRA concentrations due to, for example, cytoplasm retinoid sequestration, 9,10 increased intracellular metabolism, 11 alterations in ATRA transcriptional efficiency via mutations in the P...