miR-20a and miR-290, multi-faceted players with a role in tumourigenesis and senescence.

Expression of microRNAs changes markedly in tumours and evidence indicates that they are causatively related to tumourigenesis, behaving as tumour suppressor microRNAs or onco microRNAs; in some cases they can behave as both depending on the type of cancer. Some tumour suppressor microRNAs appear to be an integral part of the p53 and Retinoblastoma network, the main regulatory pathways controlling senescence, a major tumour suppressor mechanism. The INK4a/ARF locus which codifies for two proteins, p19ARF and p16INK4a,plays a central role in senescence by controlling both p53 and Retinoblastoma. Recent evidence shows that the proto-oncogene leukaemia/lymphoma related factor, a p19ARF specific repressor, is controlled by miRNAs and that miRNAs, in particular miR-20a and miR-290, are causatively involved in mouse embryo fibroblasts senescence in culture.Intriguingly, both miR-20a, member of the oncogenic miR-17-92 cluster, and miR-290, belonging to the miR-290-295 cluster, are highly expressed in embryonic stem cells. The prosenescence role of miR-20a and miR-290 in mouse embryo fibroblast is apparently in contrast with their proliferative role in tumour and embryonic stem cells. We propose that miRNAs may exert opposing functions depending on the miRNAs repertoire as well as target/s level/s present in different cellular contexts, suggesting the importance of evaluating miRNAsactivity in diverse genetic settings before their therapeutic use as tumour suppressors.