A homozygous mutation in the human selenocysteine tRNA gene impairs UGA recoding activity and selenoproteome regulation by selenium

The selenocysteine (Sec) tRNA (tRNA([Ser]Sec)) governs Sec insertion into selenoproteins by the recoding of a UGA codon, typically used as a stop codon. A homozygous point mutation (C65G) in the human tRNA([Ser]Sec) acceptor arm has been reported by two independent groups and was associated with symptoms such as thyroid dysfunction and low blood selenium levels; however, the extent of altered selenoprotein synthesis resulting from this mutation has yet to be comprehensively investigated. In this study, we used CRISPR/Cas9 technology to engineer homozygous and heterozygous mutant human cells, which we then compared with the parental cell lines. This C65G mutation affected many aspects of tRNA([Ser]Sec) integrity and activity. Firstly, the expression level of tRNA([Ser]Sec) was significantly reduced due to an altered recruitment of RNA polymerase III at the promoter. Secondly, selenoprotein expression was strongly altered, but, more surprisingly, it was no longer sensitive to selenium supplementation. Mass spectrometry analyses revealed a tRNA isoform with unmodified wobble nucleotide U34 in mutant cells that correlated with reduced UGA recoding activities. Overall, this study demonstrates the pleiotropic effect of a single C65G mutation on both tRNA phenotype and selenoproteome expression.