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.