VHL gene methylation contributes to excessive erythrocytosis in chronic mountain sickness rat model by upregulating the HIF-2α/EPO pathway

Hypoxia-inducible factors (HIFs) play important roles in the pathogenesis of erythrocytosis in chronic mountain sickness (CMS). von Hippel-Lindau (VHL) is a key regulator of hypoxia that can direct the poly-ubiquitylation and degradation of HIFs. Epigenetic mechanisms are believed to contribute toward adaption to chronic hypoxia. Here, we investigated the contribution and mechanism of VHL methylation in rats with erythrocytosis in CMS. The methylation status of VHL was measured via bisulfite sequencing PCR, while VHL, DNMT1, DNMT3α, and DNMT3β expression were assessed using real-time reverse transcription PCR and western blotting. HIF-2α and EPO expression levels in bone marrow were determined via immunohistochemical staining, and erythroid hyperplasia in bone marrow sections were observed with hematoxylin and eosin staining. We found that chronic hypoxia triggered erythroid hyperplasia in the bone marrow and increased the quantity of peripheral red blood cells in CMS rats. Chronic hypoxia significantly induced methylation at the CpG site in the VHL promoter, decreased VHL expression, and increased HIF-2α and EPO expression. Chronic hypoxia increased DNMT3α and DNMT3β expression, consistent with the decrease in VHL expression. The DNA methyltransferase inhibitor 5-azacytidine reduced chronic hypoxia-induced erythroid proliferation in the bone marrow of rats with CMS by suppressing VHL methylation and DNMTs expression. Our study suggests that VHL methylation contributes toward excessive erythrocytosis in CMS by upregulating the HIF-2α/EPO pathway in the bone marrow of rats. We demonstrated that the DNMT inhibitor 5-azacytidine can attenuate erythroid hyperplasia in the bone marrow by demethylating the VHL promoter.