Mitochondrial tRNA fragment, mt-tRF-Tyr-GTA-001 (tRF-21-X3OJI8EWB), in breast cancer and its potential clinical implications.

Background: Transfer RNA (tRNA) fragments (tRFs) are a group of small non-coding RNAs with biological functions. The involvement of tRNAs in cancer has also been recognized, but most studies focused on nuclear tRFs, very few on mitochondrial tRFs.

Methods: We analyzed the TCGA microRNAseq data to identify differentially expressed mitochondrial tRFs (mt-tRFs) in breast tumors and evaluated their associations with the disease outcome. Cox proportional hazards regression was used to determine the associations between mt-tRFs and patient survival while adjusting for clinicopathological variables. Quantitative RT-PCR was developed to measure a specific tRF expression in a validation study.

Results: Our analysis of 1,060 tumor samples from TCGA revealed that mt-tRF-Tyr-GTA-001 (tRF-21-X3OJI8EWB or t00018104) expression, a tRF from mitochondrial tRNA with tyrosine anticodon GTA (mt-tRNA-Tyr-GTA), was significantly lower in breast tumors than the adjacent tissues (p< 0.0001). Patients with low expression had significantly higher risk of death (HR = 1.69, p = 0.0018) regardless of their age at diagnosis, disease stage, tumor grade, and hormone receptor status. This survival association was replicated in an independent study where mt-tRF-Tyr-GTA-001 expression was measured with qRT-PCR. Further analysis suggested that the mt-tRF expression was correlated with ribonuclease ANG and RNase 4 known to cleave tRNAs and upregulated under hypoxia. IPA interrogation of the mt-tRF-Tyr-GTA-001 expression signature indicated the inhibitory effects of mt-tRF-Tyr-GTA-001 on malignant transformation, tumor growth, and cell invasion. In silico analysis showed that the binding targets of mt-tRF-Tyr-GTA-001 included several oncogenic transcription factors (E2Fs, CCNE1, FOXM1). We also found the mt-tRF correlated with the abundances of M0 macrophages and resting mast cells, two of the immune cells known for innate immunity.

Conclusions: In summary, our study suggests that mt-tRF-Tyr-GTA-001, a mitochondrial tRF, may suppress breast cancer progression through its involvement in regulation of cell phenotype and tumor immunity.