Congenital polycythemias have diverse etiologies, including mutations in the hypoxia sensing pathway leading to increased levels of hypoxia inducible factors (HIF) and erythropoietin. These include mutations of negative regulators of HIFs, such as germline von Hippel-Lindau (VHL) gene heterozygous mutations (Chuvash polycythemia), HIF-prolyl hydroxylase 2 (PHD2) gene mutations, and gain-of-function mutations of HIF-2-alpha (HIF2A) (exon 12). Mutation of the PHD2 gene in one family was associated with polycythemia and recurrent pheochromocytoma/paraganglioma (PHEO/PGL) a neuroendocrine tumor commonly found in so called VHL tumor predisposition syndrome. Over the past two decades, we have studied six unrelated patients with sporadic congenital polycythemia who subsequently developed PHEO/PGLs without till now discernible molecular basis. We now report on these three patients, two with different tumor somatic codon A530 HIF2A mutations in exon 12 and one with germline mutation F374Y inherited from his mother, in a unique domain (exon 9) of HIF2A, all three cases later developed multiple recurrent neuroendocrine tumors and were subjects of our study.
In search of species homology, codons F374 and A530 of the HIF2A are highly conserved among man, chimpanzee, mouse, horse, cattle, chicken and zebrafish, suggesting this serves an important function for the gene. These mutations were identified in the vicinity of the primary hydroxylation site (exon 12) and novel domain (exon 9) of the HIF2A protein which affects VHL protein binding, Functional studies of the HIF2A mutants shown that three HIF2A variants have increased half-life consistent with gain-of-function of the HIF2A due to disruption in the VHL binding to these mutant residues impairing ubiquitination and proteasomal degradation. This results in increased transcription of genes downstream of HIF2A including erythropoietin. Further examination did not reveal any evidence of loss-of-heterozygosity, nor an additional mutation of HIF2A, or other HIF-pathway genes in their tumor tissues. The fact that two patients with polycythemia and PHEO/PGL had somatic and one germline HIF2A mutations, albeit at different locations, underscore the PHEO/PGL promoting potential of gain-of-function mutations of HIF2A that alone, either as somatic or germline mutations can contribute to, but are not sufficient for PHEO/PGL development but is sufficient for inducing polycythemic phenotype.
No relevant conflicts of interest to declare.
Asterisk with author names denotes non-ASH members.