HIF-1α Is Not Essential For The Establishment Of MLL-Leukaemic Stem Cells
Authors: Gezer Deniz , Guitart Amelie V , Vukovic Milica , Subramani Chithra , Dunn Karen , Pollard Patrick , Ratcliffe Peter J , Holyoake Tessa L. , Kranc Kamil 
Published: October 2019

Abstract
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.