microRNA expression patterns across seven cancers are highly correlated and dominated by evolutionarily ancient families

microRNAs (miRNAs) are involved in almost all normal and pathogenic eukaryotic cell processes. One area in which the influence of miRNAs is most prominent is cancer. Numerous expression surveys and more focused studies have revealed miRNA involvement in carcinogenesis, cellular pathology, cell behavior and prognosis. Large-scale comparisons of miRNA expression in varioius types of cancer have not been previously possible. However, The Cancer Genome Atlas (TCGA), an extensive multi-centered effort to characterize the genomes of hundreds of types of cancer, has enabled such comparisons. In the present study, the expression patterns of hundreds of miRNAs in thousands of tumors covering seven types of cancer: uterine corpus adenocarcinoma, ovarian serous adenocarcinoma, breast adenocarcinoma, prostate adenocarcinoma, pancreatic adenocarcinoma, colorectal adenocarcinoma, and lung adenocarcinoma were analyzed. The results showed that miRNA expression patterns among these cancer types are highly correlated (0.874>ρ>0.974) and that miRNA expression in all seven cancer types is dominated by miRNAs belonging to the most evolutionarily ancient miRNA families. This raises the possibility that more ancient miRNAs are involved in the fundamental cell processes that are central to tumor evolution.