Small RNA sequencing of colorectal cancer to reveal differences in microRNA expression between normal and tumor tissue.
506 Background: MicroRNAs (miRNAs) are promising biomarkers and therapeutic agents for colorectal cancer (CRC). However, much remains to be known about the biology of miRNAs and miRNA variants in tumor tissue and why the expression differs from normal tissue. Methods: RNA from 48 normal and 48 cancer tissue samples were sequenced using the Illumina small RNA sequencing protocol. Samples were taken from both male and female patients with tumor stage 1-4. Results: We show that almost 50% of microRNAs (miRNAs) are differentially expressed between normal and cancer tissue. When analyzing pre-miRNAs that map to genomic clusters, so-called clustered miRNAs, we show that miRNAs in the same cluster often have similar expression profiles. This shows that common changes in co-expressed miRNAs seem to explain some of the miRNA expression changes between normal and tumor samples and indicates that altered pri-miRNA transcription is the mechanism for at least some of the miRNA changes. Analyses of miRNA variants, so-called isomiRs, revealed large variations between tumor and normal samples, similar to those observed for the canonical miRNAs. Different isomiR types show different expression profiles in tumor vs normal samples, indicating that miRNA processing is altered in tumor cells. Finally, we show that several canonical miRNAs and isomiRs correlate with genetic signatures in the tumor samples, including KRAS and BRAF mutation, Microsatellite instability and MLH1 methylation, indicating that tumor subtypes have different miRNA expression. Conclusions: Our results show that miRNA expression predicts tumor subtypes with different genetic signatures. We show that some of the changes in miRNA expression are due to different transcription programs in cancer vs normal tissue.