Yak response to high-altitude hypoxic stress by altering mRNA expression and DNA methylation of hypoxia-inducible factors.

Hypoxia-inducible factors (HIFs) are oxygen-dependent transcriptional activators, which play crucial roles in tumor angiogenesis and mammalian development, and regulate the transcription of genes involved in oxygen homeostasis in response to hypoxia. However, information on HIF-1α and HIF-2α in yak (Bos grunniens) is scarce. The complete coding region of yak HIF-2α was cloned, its mRNA expression in several tissues were determined, and the expression levels were compared with those of closely related low-altitude cattle (Bos taurus), and the methylation status of promoter regions were analyzed to better understand the roles of HIF-1α and HIF-2α in domesticated yak. The yak HIF-2α cDNA was cloned and sequenced in the present work reveals the evolutionary conservation through multiple sequence alignment, although 15 bases changed, resulting in 8 amino acid substitutions in the translated proteins in cattle. The tissue-specific expression results showed that HIF-1α is ubiquitously expressed, whereas HIF-2α expression is limited to endothelial tissues (kidney, heart, lung, spleen, and liver) and blood in yak. Both HIF-1α and HIF-2α expressions were higher in yak tissues than in cattle. The HIF-1α expression level is much higher in yak than cattle in these organs, except for the lung (P < 0.05), but the HIF-2α gene is significantly different in the heart, spleen, and kidney (P < 0.05). Furthermore, the methylation levels in the 5' flanking regulatory regions of HIF-1α and HIF-2α in yak kidney were significantly decreased than cattle counterparts (P < 0.05). Identifying these genes and the comparison of different expressions facilitates the understanding of the biological high-altitude hypoxic stress response mechanism and may assist current medical research to understand hypoxia-related diseases.