Esteban A. Orellana

Several transfer RNAs (tRNA) that differ only in their body sequence decode the same codon, yet their levels and behavior vary across cell types and disease contexts. In a landmark 2014 study, a mutation in the arginine isodecoder Arg-TCT-4 was linked to neurodegeneration in mice. In 2021, increased expression of the Arg-TCT family, especially Arg-TCT-4, was unexpectedly linked to oncogenic transformation in multiple cancer models. A recent publication by Kim and colleagues reports that loss of a single tRNA isodecoder, Arg-TCT-1, drives lineage plasticity and resistance to androgen receptor (AR) pathway inhibitors in prostate cancer by reducing the translation of AGA-rich mRNAs. Together, these findings raise a broader question: What do differences in isodecoder sequence actually do? They may influence tRNA processing, charging, interactions with elongation factors and the ribosome, or the modifications tRNAs receive. This commentary places the new findings in the context of prior work and considers potential mechanisms behind the functional diversity of Arg-TCT isodecoders and their role in cancer.

TRNAs play a critical role in protein synthesis, influencing mRNA translation dynamics to shape proteomes. Emerging evidence links dysregulated tRNA activity to cancer progression, with tRNA-Arg-TCT identified as an oncogenic driver when ectopically overexpressed in non-malignant cells. The requirement of endogenous tRNA-Arg-TCT in cancer biology, however, remains untested. Moreover, considering that the tRNA-Arg-TCT family comprises six genes in humans, the importance of an individual tRNA isodecoder in cancer remains unknown. Here, we find elevated levels of tRNA-Arg-TCT-4-1 isodecoder are associated with poor patient prognosis across multiple cancer types. We demonstrate that, using different antisense RNA strategies, specific inhibition of tRNA-Arg-TCT-4-1 suppresses the growth of glioblastoma (GBM) and liposarcoma (LPS) cancer cells. Mechanistically, we find that tRNA-Arg-TCT-4-1 inhibition leads to a codon-biased remodeling of mRNA translation and the proteome, preferentially suppressing expression of growth-promoting genes and pathways encoded by mRNAs enriched in arginine AGA codons. Strikingly, intratumoral delivery of an antisense oligonucleotide (ASO) targeting tRNA-Arg-TCT-4-1 suppresses tumor growth and extends survival in mouse xenograft experiments performed using either a human LPS cell line or a patient-derived soft tissue sarcoma model. This study provides a foundation for targeting tRNA dysregulation as a novel therapeutic approach for cancer. One Sentence Summary: This study identifies tRNA-Arg-TCT-4-1 as a new anti-cancer therapeutic target and demonstrates that an antisense oligonucleotide (ASO) targeting this tRNA effectively suppresses tumorigenesis and extends survival in mouse xenograft models.

Chemical modifications of RNA have key roles in many biological processes^1,2,3. N⁷-methylguanosine (m⁷G) is required for integrity and stability of a large subset of tRNAs^4,5,6,7. The methyltransferase 1–WD repeat-containing protein 4 (METTL1–WDR4) complex is the methyltransferase that modifies G46 in the variable loop of certain tRNAs, and its dysregulation drives tumorigenesis in numerous cancer types^{8,9,10,11,12,13,14}. Mutations in WDR4 cause human developmental phenotypes including microcephaly^15,16,17. How METTL1–WDR4 modifies tRNA substrates and is regulated remains elusive¹⁸. Here we show, through structural, biochemical and cellular studies of human METTL1–WDR4, that WDR4 serves as a scaffold for METTL1 and the tRNA T-arm. Upon tRNA binding, the αC region of METTL1 transforms into a helix, which together with the α6 helix secures both ends of the tRNA variable loop. Unexpectedly, we find that the predicted disordered N-terminal region of METTL1 is part of the catalytic pocket and essential for methyltransferase activity. Furthermore, we reveal that S27 phosphorylation in the METTL1 N-terminal region inhibits methyltransferase activity by locally disrupting the catalytic centre. Our results provide a molecular understanding of tRNA substrate recognition and phosphorylation-mediated regulation of METTL1–WDR4, and reveal the presumed disordered N-terminal region of METTL1 as a nexus of methyltransferase activity.

TRNAs are key adaptor molecules that decipher the genetic code during translation of mRNAs in protein synthesis. In contrast to the traditional view of tRNAs as ubiquitously expressed housekeeping molecules, awareness is now growing that tRNA-encoding genes display tissue-specific and cell type-specific patterns of expression, and that tRNA gene expression and function are both dynamically regulated by post-transcriptional RNA modifications. Moreover, dysregulation of tRNAs, mediated by alterations in either their abundance or function, can have deleterious consequences that contribute to several distinct human diseases, including neurological disorders and cancer. Accumulating evidence shows that reprogramming of mRNA translation through altered tRNA activity can drive pathological processes in a codon-dependent manner. This Review considers the emerging evidence in support of the precise control of functional tRNA levels as an important regulatory mechanism that coordinates mRNA translation and protein expression in physiological cell homeostasis, and highlights key examples of human diseases that are linked directly to tRNA dysregulation.

Efforts to search for better treatment options for cancer have been a priority, and due to these efforts, new alternative therapies have emerged. For instance, clinically relevant tumor-suppressive microRNAs that target key oncogenic drivers have been identified as potential anti-cancer therapeutics. MicroRNAs are small non-coding RNAs that negatively regulate gene expression at the posttranscriptional level. Aberrant microRNA expression, through misexpression of microRNA target genes, can have profound cellular effects leading to a variety of diseases, including cancer. While altered microRNA expression contributes to a cancerous state, restoration of microRNA expression has therapeutic benefits. For example, ectopic expression of microRNA-34a (miR-34a), a tumor suppressor gene that is a direct transcriptional target of p53 and thus is reduced in p53 mutant tumors, has clear effects on cell proliferation and survival in murine models of cancer. MicroRNA replacement therapies have recently been tested in combination with other agents, including other microRNAs, to simultaneously target multiple pathways to improve the therapeutic response. Thus, we reasoned that other microRNA combinations could collaborate to further improve treatment. To test this hypothesis miR-34a was used in an unbiased cell-based approach to identify combinatorial microRNA pairs with enhanced efficacy over miR-34a alone. This approach identified a subset of microRNAs that was able to enhance the miR-34a antiproliferative activity. These microRNA combinatorial therapeutics could offer superior tumor-suppressive abilities to suppress oncogenic properties compared to a monotherapeutic approach. Collectively these studies aim to address an unmet need of identifying, characterizing, and therapeutically targeting microRNAs for the treatment of cancer.