Identification of two new small ncRNAs, 3' and 5' Glu-tRNA-derived fragments: their possible role as biomarkers for breast cancer PhD

Over the last decades, small non-coding RNAs (ncRNAs), ~13-40 nucleotides long, have been discovered. Many classes of this kind of molecules have been described: microRNAs (miRNAs), small interfering RNAs (siRNAs) and piwi-interacting RNAs (piRNAs) are the most studied. However, new kinds of small ncRNAs are progressively being discovered, like tRNA-derived fragments, which have now been recognized to be the major RNA species in human cells. For a long time, small ncRNAs were considered as by-products from random degradation; nonetheless, there are increasing evidences that they are functional molecules with precise sequence structure, specific expression patterns and critical roles in gene regulation. Dysregulation of genes involved in cell proliferation, differentiation and/or apoptosis is associated with tumorigenic processes. Such as gene regulators, a wide variety of small ncRNAs have been demonstrated to be involved in tumor initiation and progression controlling and modulating cancer-related gene expression. Interestingly, besides cellular small ncRNAs, they have been also discovered in body fluids as stable circulating small ncRNAs carried within extracellular microvesicles (exosomes) or associated with Ago proteins. Numerous of them have been studied as promising non-invasive biomarkers for several kinds of diseases like cancer. Currently, breast cancer is the most common cancer and leading cause of cancer death among women worldwide. The HER2+ -breast cancer subtype has been described as one with the most aggressive phenotype. Albeit new therapies have been developed, more efforts to improve diagnosis, treatment and prognosis are needed. Here, we report the identification of two new circulating tRNA-derived fragments, 3'Glu-tRF-M and 5'Glu-tRF-M. Cloning experiments and in silico analyses suggest that they are processed by Dicer enzyme from mature Glutamic tRNA. We assessed their expression in different healthy and HER2+ -breast cancer samples from mice and humans by semi-quantitative PCR, Northern Blot and quantitative real-time PCR. Moreover cytoplasmatic location of both, 3'Glu-tRF-M and 5'Glu-tRF-M, was proved by an immunofluorescence assay. A significative decrease of 3'Glu-tRF-M expression was observed in both, mice and human individuals with HER2+ -breast cancer. We conclude hypothesizing that the neoplasia could regulate, directly or indirectly, 3'Glu-tRF-M expression, leading to its decrease. Since this variation seems to depend on tumor development status, outcome that will be confirmed in further studies, 3'Glu-tRF-M could be considered a future non-invasive biomarker for HER2+ -breast cancer.