It is established that hematopoietic stem cells (HSC) in the hypoxic bone marrow have adapted their metabolism to oxygen-limiting conditions. This adaptation includes suppression of mitochondrial activity, induction of anerobic glycolysis, and activation of hypoxia-inducible transcription factor 1α (Hif1α)-dependent gene expression. During progression of hematopoiesis, a metabolic switch towards mitochondrial oxidative phosphorylation is observed, making this organelle essential for determining cell fate choice in bone marrow. However, given that HSC metabolism is essentially oxygen-independent, it is still unclear whether functional mitochondria are absolutely required for their survival. To assess the actual dependency of these undifferentiated cells on mitochondrial function, we have performed an analysis of the hematopoiesis in a mouse mutant, named SDHD-ESR, with inducible deletion of the mitochondrial protein-encoding SdhD gene. This gene encodes one of the subunits of the mitochondrial complex II (MCII). In this study, we demonstrate that, in contrast to what has been previously established, survival of HSC, and also myeloid and B-lymphoid progenitors, depends on proper mitochondrial activity. In addition, gene expression analysis of these hematopoietic lineages in SDHD-ESR mutants calls into question the proposed activation of Hif1α in response to MCII dysfunction.

Mutations in mitochondrial complex II (MCII; succinate dehydrogenase, Sdh) genes cause familiar pheochromocytoma/paraganglioma tumors. Several mechanisms have been proposed to account for Sdh-mutation-induced tumorigenesis, the most accepted of which is based on the constitutive expression of the hypoxia-inducible factor 1α (Hif1α) at normal oxygen tension, a theory referred to as “pseudo-hypoxic drive”. Other molecular processes, such as oxidative stress, apoptosis, or chromatin remodeling have been also proposed to play a causative role. Nevertheless, the actual contribution of each of these mechanisms has not been definitively established. Moreover, the biological factors that determine the tissue-specificity of these tumors have not been identified. In this work, we made use of the inducible SDHD-ESR mouse, a conditional mutant in the SdhD gene, which encodes the small subunit of MCII, and that acts as a tumor suppressor gene in humans. The analysis of the Hif1α pathway in SDHD-ESR tissues and in two newly derived cell lines after complete SdhD loss -a requirement for hereditary paraganglioma type-1 tumor formation in humans- partially recapitulated the “pseudo-hypoxic” response and rendered inconsistent results. Therefore, we performed microarray analysis of adrenal medulla and kidney in order to identify other early gene expression changes elicited by SdhD deletion. Our results revealed that each mutant tissue displayed different variations in their gene expression profiles affecting to different biological processes. However, we found that the Cdkn1a gene was up-regulated in both tissues. This gene encodes the cyclin-dependent kinase inhibitor p21WAF1/Cip1, a factor implicated in cell cycle, senescence, and cancer. The two SDHD-ESR cell lines also showed accumulation of this protein. This new and unprecedented evidence for a link between SdhD dysfunction and p21WAF1/Cip1 will open new avenues for the study of the mechanisms that cause tumors in Sdh mutants. Finally, we discuss the actual role of Hif1α in tumorigenesis.