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Hypoxia-inducible factor (HIF) regulates the major transcriptional cascade central to the response of all mammalian cells to alterations in oxygen tension. Expression arrays indicate that many hundreds of genes are regulated by this pathway, controlling diverse processes that in turn orchestrate both oxygen delivery and utilization. However, the extent to which HIF exerts direct versus indirect control over gene expression together with the factors dictating the range of HIF-regulated genes remains unclear. Using chromatin immunoprecipitation linked to high throughput sequencing, we identify HIF-binding sites across the genome, independently of gene architecture. Using gene set enrichment analysis, we demonstrate robust associations with the regulation of gene expression by HIF, indicating that these sites operate over long genomic intervals. Analysis of HIF-binding motifs demonstrates sequence preferences outside of the core RCGTG-binding motif but does not reveal any additional absolute sequence requirements. Across the entire genome, only a small proportion of these potential binding sites are bound by HIF, although occupancy of potential sites was enhanced approximately 20-fold at normoxic DNAse1 hypersensitivity sites (irrespective of distance from promoters), suggesting that epigenetic regulation of chromatin may have an important role in defining the response to hypoxia.

Hypoxia results in adaptive changes in the transcription of a range of genes including erythropoietin. An important mediator is hypoxia-inducible factor-1 (HIF-1), a DNA binding complex shown to contain at least two basic helix-loop-helix PAS-domain (bHLH-PAS) proteins, HIF-1α and aryl hydrocarbon nuclear receptor translocator (ARNT). In response to hypoxia, HIF-1α is activated and accumulates rapidly in the cell. Endothelial PAS domain protein 1 (EPAS-1) is a recently identified bHLH-PAS protein with 48% identity to HIF-1α, raising the question of its role in responses to hypoxia. We developed specific antibodies and studied expression and regulation of EPAS-1 mRNA and protein across a range of human cell lines. EPAS-1 was widely expressed, and strongly induced by hypoxia at the level of protein but not mRNA. Comparison of the effect of a range of activating and inhibitory stimuli showed striking similarities in the EPAS-1 and HIF-1α responses. Although major differences were observed in the abundance of EPAS-1 and HIF-1α in different cell types, differences in the inducible response were subtle with EPAS-1 protein being slightly more evident in normoxic and mildly hypoxic cells. Functional studies in a mutant cell line (Ka13) expressing neither HIF-1α nor EPAS-1 confirmed that both proteins interact with hypoxically responsive targets, but suggest target specificity with greater EPAS-1 transactivation (relative to HIF-1α transactivation) of the VEGF promoter than the LDH-A promoter.

Context : Germline mutations in genes involved in the hypoxia sensing pathway (VHL, PHD2/EGLN1, HIF2A/EPAS1) predispose patients to erythrocytosis associated with normal or high serum erythropoietin level. The more frequent mutation, VHL-R200W (R200W), has been identified in homozygous carriers with a congenital erythrocytosis named Chuvash polycythemia. Survival in the Chuvash patients was found to be reduced compared to control groups due to higher rates of arterial and venous thromboses, and to haemorrhagic events. Noteworthily, a characteristic of these patients and their parents, heterozygous for the mutation, is the total absence of tumor development as the opposite of heterozygous carriers of other VHL mutations. Indeed, VHL is a tumor suppressor gene and heterozygous carriers of VHLmutations have von Hippel-Lindau disease and are at high risk of multiple tumors (e.g. CNS hemangioblastomas, pheochromocytoma, renal cell carcinoma). The absence of tumor development in patients with congenital erythrocytosis remained unexplained. The R200W mutation is transmitted within a 340-kilobases haplotype inherited 14,000 to 62,000 years ago from a single founder event. The absence of tumor development in patients carrying the R200W mutation raised the possibility of the presence of a protective element within this core haplotype. Objective: The purpose of this study was to determine the mechanisms underlying the absence of tumor development in patients carrying the R200W mutation, healthy (heterozygous carriers) or with congenital erythrocytosis (homozygous). Methods: We report here one particular family carring a heterozygous R200W mutation associated, for the first time, with a typical von Hippel-Lindau disease (with pheochromocytoma, hemangioblastoma and renal cell carcinoma) (Olswang et al., 1998). We performed a comprehensive study with genotyping, phenotypic analysis, structural modeling, functional and transcriptomic studies of the R200W mutant in comparison to classical mutants involved in von Hippel-Lindau disease. For the first time, the use of inducible expression vectors encoding untagged VHL in the functional studies allowed the observation of very subtle effects. Results: We show that the R200W mutation alone is definitely not associated with the von Hippel Lindau disease because this particular family actually carries another pathogenic mutation, VHL-R161Q, on the same VHL allele, and the double mutated allele segregates with disease. Our genotyping showed that the doubled mutations lies on the Chuvash core haplotype transmitted with the R200W mutation. We can thus conclude that this haplotype does not contain any protective elements against cancer development and that the VHLmutations are, by themselves, predominantly responsible for manifestations of the von Hippel-Lindau disease. Functional studies demonstrate that the function of the R200W mutation is close to the wild type protein and the severity of disease manifestations observed in VHL syndrome is perfectly correlated with a gradient of pVHL dysfunction in hypoxia signaling pathways. Conclusion: We show that the R200W mutation associated with congenital erythrocytosis is at the bottom of an oncogenic gradient. These results validate for the first time in humans the recently proposed “continuum” model of tumor suppression, which differs from the classical Knudson’s two-hit model and implies that subtle gene dosage-sensitivity may underlie tumor predisposition. This model may be of major importance in our understanding of tumor risk in patients with erythrocytosis carrying germline mutations in other genes belonging the HIF pathway. Indeed, in rare cases, PHD2 and HIF2A pathogenic mutations also predispose patients to pheochromocytomas or paragangliomas development. Disclosures No relevant conflicts of interest to declare.

Haematopoietic stem cells (HSCs) reside in hypoxic niches in the bone marrow (BM) and sustain long-life haematopoiesis. HSCs are largely quiescent, self-renew, undergo apoptosis and generate progenitor cells, which differentiate to multiple blood lineages. The strict regulation of the balance between these fate decisions is essential for haematopoiesis and their dysregulation in HSCs and progenitor cells can result in leukaemic transformation. HSCs and leukemic stem cells (LSCs) are suggested to share the same niche and are in need to adapt to hypoxic conditions. Hypoxia-inducible-factor-1α (HIF-1α) is a key mediator of cellular responses to hypoxia and is important for the maintenance of HSC functions under stressful conditions. Furthermore, in chronic myeloid leukaemia (CML) and acute myeloid leukaemia (AML) HIF-1α is essential for LSC maintenance and ablation or knockdown of HIF-1α leads to exhaustion of established LSCs. The aim of this study was to investigate the requirement for HIF-1α in the generation of pre-LSCs and the establishment of LSCs. To investigate the role of HIF-1α in the generation of pre-LSCs we retrovirally transduced haematopoietic stem and progenitor cells (HSPCs) from either WT or HIF1-αfl/fl Vav-iCre with MLL-ENL retroviruses. Next we performed serial re-plating assays under normoxic and hypoxic conditions to generate pre-LSCs. Surprisingly, WT and HIF-1α deficient HSPCs generated comparable numbers of colonies in normoxia and hypoxia (Fig. 1a). In addition no significant difference was found in the immunophenotypic profile of colonies (Figure 1b). Furthermore, microscopic examination indicated that colonies of all genotypes were dense consistent with their transformed shape (Fig. 1c). WT and HIF-1α-deficient pre-LSCs cultured under normoxia and hypoxia had similar cloning efficiency, which is known to directly correlate with the numbers of LSCs in vivo (Fig. 2). These results indicate that HIF-1α is dispensable for the generation of pre-LSCs. To test the role of HIF-1α in establishment of LSCs from pre-LSCs we transplanted pre-LSCs into lethally irradiated mice together with support BM and monitored the mice for disease development. No significant difference was found in disease latency (Fig. 3a) or frequency of LSCs in peripheral blood, bone marrow or spleens (Fig. 3b) indicating that pre-LSCs lacking HIF-1α can efficiently generate LSCs that cause aggressive AML. In conclusion, we provide genetic evidence that HIF-1α is dispensable for the generation of pre-LSCs and the establishment of LSCs from pre-LSCs. These surprising findings, together with published results indicating that HIF-1α is essential for maintenance of LSCs, imply that HIF-1α has different roles at different stages of leukaemic transformation. Further studies are required to explain the distinct roles of HIF-1α in different stages of leukaemogenesis. Disclosures: Ratcliffe: RedOx: Founder Other. Holyoake:Novartis: Membership on an entity’s Board of Directors or advisory committees; Bristol Myers Squibb: Membership on an entity’s Board of Directors or advisory committees; Ariad: Membership on an entity’s Board of Directors or advisory committees.

[Figure][1] Molecular genetic analysis of inherited human disease has an exemplary record of informing our understanding of complex biologic processes. In keeping with this, recent studies of inherited erythrocytosis have provided important insights into physiologic mechanisms that govern