Knockout of the fih gene by CRISPR/Cas9 enhances the hypoxia tolerance in grass carp (Ctenopharyngodon idella)

Grass carp (Ctenopharyngodon idella), a key freshwater fish species in China, is threatened by hypoxia in aquaculture due to high stocking densities and climate change. Factor inhibiting hypoxia-inducible factor (FIH), an asparagine hydroxylase that represses the expression of hypoxia-inducible factor (HIF), inhibits the transcription of hypoxia-responsive genes by blocking interactions between HIF and the coactivators CBP and p300. In this study, we used CRISPR-Cas9 genome editing to generate fih-deficient heterozygotes of grass carp. Our study demonstrates that individuals with a deletion of the fih gene exhibit significantly enhanced hypoxia tolerance compared to wild-type grass carp, as evidenced by the critical oxygen tension at which equilibrium is lost (LOEcrit). Furthermore, although the hypoxia tolerance of grass carp has increased, fih gene knockout did not have a significant impact on their growth and development. We performed real-time quantitative PCR and TUNEL staining to assess hypoxic responses in fih-deficient grass carp, and accordingly found that compared with the wild grass carp, certain hypoxia-responsive genes, including hif1 alpha, phd3, and glut1, were upregulated in fihdeficient individuals. Additionally, we established that, under hypoxic conditions, levels of apoptosis in the gills and brains of fih-deficient fish were significantly lower than those in their wild counterparts. These findings indicate that fih plays an essential role in hypoxia tolerance of grass carp and providing candidate genes for the genetic breeding of hypoxia-tolerant varieties.