Hypothalamic expression analysis of m⁶A RNA methylation associated genes suggests a potential role of epitranscriptomics in sexual maturation of Atlantic salmon

Better understanding the molecular processes contributing to variation in maturation timing is important for Atlantic salmon (Salmo salar) aquaculture, as early maturation causes considerable financial losses. The m6A RNA methylation is a highly conserved and dynamically reversible mechanism controlling gene expression in a myriad of biological processes. The role of m6A methylation in sexual maturation, however, has remained largely unexplored and has never been studied in Atlantic salmon. While the maturation process is known to be affected by many genetic and environmental factors, the molecular mechanisms causing variation in the timing of maturation are still poorly understood. Hence, investigation of whether a widespread mechanism like m6A methylation could be involved in controlling of the maturation timing is warranted. In Atlantic salmon, two genes, also associated with age at maturity in humans, vgll3 and six6, have been shown to play an important role in maturation timing. In this study, we investigated the expression of 16 genes involved in the regulation of m6A RNA methylation in the hypothalamus of Atlantic salmon with different homozygous combinations of late (L) and early (E) alleles for vgll3 and six6 genes. We found differential expression of ythdf2.2, an m6A reader promoting mRNA degradation, with higher expression in six6*LL compared to other genotypes as well as in immature compared to mature males. In addition, we found that the expression levels of genes coding for an eraser, alkbh5, and for a reader, ythdf1, were higher in the hypothalamus of females than in males across all the different genotypes studied. However the total m6A levels between the whole hypothalamus of males and females were similar. Our results indicate a potential role of the m6A methylation process in sexual maturation of Atlantic salmon, and therefore, provide the first evidence for such regulatory mechanism in the hypothalamus of any vertebrate.