Differential HIF2α Protein Expression in Human Carotid Body and Adrenal Medulla under Physiologic and Tumorigenic Conditions

SIMPLE SUMMARY: Thoraco-abdominal paraganglioma and pheochromocytoma (PPGL) are pathogenically linked to mutations in SDH genes. Loss-of-function of SDH has been associated with stabilization of HIF2α, which otherwise would be degraded via oxygen-sensitive mechanisms. SDH dysfunction also predispose to the development of paragangliomas arising at the carotid body or other head and neck paraganglia (HNPGL). Although PPGL and HNPGL share similar features, they have certain clinical and genetic peculiarities. By comparison of HIF2α expression in HNPGL and PPGL, we found that functional HIF2α is overexpressed in 80% of PPGLs, including those with SDH mutations as compared with non-tumor tissue. However, HIF2α is already highly expressed in the carotid body under physiologic conditions, and it is not overexpressed in HNPGL. These data suggest that selective HIF2α inhibitors already in clinical trials may benefit a wide spectrum of PPGL. ABSTRACT: Hypoxia-inducible factors (HIF) 2α and 1α are the major oxygen-sensing molecules in eukaryotic cells. HIF2α has been pathogenically linked to paraganglioma and pheochromocytoma (PPGL) arising in sympathetic paraganglia or the adrenal medulla (AM), respectively. However, its involvement in the pathogenesis of paraganglioma arising in the carotid body (CB) or other parasympathetic ganglia in the head and neck (HNPGL) remains to be defined. Here, we retrospectively analyzed HIF2α by immunohistochemistry in 62 PPGL/HNPGL and human CB and AM, and comprehensively evaluated the HIF-related transcriptome of 202 published PPGL/HNPGL. We report that HIF2α is barely detected in the AM, but accumulates at high levels in PPGL, mostly (but not exclusively) in those with loss-of-function mutations in VHL and genes encoding components of the succinate dehydrogenase (SDH) complex. This is associated with upregulation of EPAS1 and the HIF2α-regulated genes COX4I2 and ADORA2A. In contrast, HIF2α and HIF2α-regulated genes are highly expressed in CB and HNPGL, irrespective of VHL and SDH dysfunctions. We also found that HIF2α and HIF1α protein expressions are not correlated in PPGL nor HNPGL. In addition, HIF1α-target genes are almost exclusively overexpressed in VHL-mutated HNPGL/PPGL. Collectively, the data suggest that involvement of HIF2α in the physiology and tumor pathology of human paraganglia is organ-of-origin-dependent and HIF1α-independent.