Tumourigenesis and cancer progression require enhanced global protein translation^1-3. Such enhanced translation is caused by oncogenic and tumour-suppressive events that drive the synthesis and activity of translational machinery^4,5. Here we report the surprising observation that leucyl-tRNA synthetase (LARS) becomes repressed during mammary cell transformation and in human breast cancer. Monoallelic genetic deletion of LARS in mouse mammary glands enhanced breast cancer tumour formation and proliferation. LARS repression reduced the abundance of select leucine tRNA isoacceptors, leading to impaired leucine codon-dependent translation of growth suppressive genes, including epithelial membrane protein 3 (EMP3) and gamma-glutamyltransferase 5 (GGT5). Our findings uncover a tumour-suppressive tRNA synthetase and reveal that dynamic repression of a specific tRNA synthetase-along with its downstream cognate tRNAs-elicits a downstream codon-biased translational gene network response that enhances breast tumour formation and growth.

The human genome contains 61 codons encoding 20 amino acids. Synonymous codons representing a given amino acid are decoded by a set of transfer RNAs (tRNAs) called isoacceptors. We report the surprising observation that two isoacceptor tRNAs that decode synonymous codons become modulated in opposing directions during breast cancer progression. Specifically, tRNAIleUAU became upregulated, whereas tRNAIleGAU became repressed as breast cancer cells attained enhanced metastatic capacity. Functionally, tRNAIleUAU promoted and tRNAIleGAU suppressed metastatic colonization in mouse xenograft models. These tRNAs mediated opposing effects on codon-dependent translation of growth-promoting genes, consistent with genomic enrichment or depletion of their cognate codons in mitotic genes. Our findings uncover a specific isoacceptor tRNA pair that act in opposition, divergently impacting growth-regulating genes and a disease phenotype. Degeneracy of the genetic code can thus be biologically exploited by human cancer cells via tRNA isoacceptor shifts that causally facilitate the transition toward a growth-promoting state.