Widespread 8-oxoguanine modifications of miRNA seeds differentially regulate redox-dependent cancer development.

Oxidative stress contributes to tumourigenesis by altering gene expression. One accompanying modification, 8-oxoguanine (o8G) can change RNA-RNA interactions via o8G•A base pairing, but its regulatory roles remain elusive. Here, on the basis of o8G-induced guanine-to-thymine (o8G > T) variations featured in sequencing, we discovered widespread position-specific o8Gs in tumour microRNAs, preferentially oxidized towards 5' end seed regions (positions 2-8) with clustered sequence patterns and clinically associated with patients in lower-grade gliomas and liver hepatocellular carcinoma. We validated that o8G at position 4 of miR-124 (4o8G-miR-124) and 4o8G-let-7 suppress lower-grade gliomas, whereas 3o8G-miR-122 and 4o8G-let-7 promote malignancy of liver hepatocellular carcinoma by redirecting the target transcriptome to oncogenic regulatory pathways. Stepwise oxidation from tumour-promoting 3o8G-miR-122 to tumour-suppressing 2,3o8G-miR-122 occurs and its specific modulation in mouse liver effectively attenuates diethylnitrosamine-induced hepatocarcinogenesis. These findings provide resources and insights into epitranscriptional o8G regulation of microRNA functions, reprogrammed by redox changes, implicating its control for cancer treatment.

Position-specific oxidation of miR-1 encodes cardiac hypertrophy.

In pathophysiology, reactive oxygen species oxidize biomolecules that contribute to disease phenotypes1. One such modification, 8-oxoguanine2 (o8G), is abundant in RNA3 but its epitranscriptional role has not been investigated for microRNAs (miRNAs). Here we specifically sequence oxidized miRNAs in a rat model of the redox-associated condition cardiac hypertrophy4. We find that position-specific o8G modifications are generated in seed regions (positions 2-8) of selective miRNAs, and function to regulate other mRNAs through o8G•A base pairing. o8G is induced predominantly at position 7 of miR-1 (7o8G-miR-1) by treatment with an adrenergic agonist. Introducing 7o8G-miR-1 or 7U-miR-1 (in which G at position 7 is substituted with U) alone is sufficient to cause cardiac hypertrophy in mice, and the mRNA targets of o8G-miR-1 function in affected phenotypes; the specific inhibition of 7o8G-miR-1 in mouse cardiomyocytes was found to attenuate cardiac hypertrophy. o8G-miR-1 is also implicated in patients with cardiomyopathy. Our findings show that the position-specific oxidation of miRNAs could serve as an epitranscriptional mechanism to coordinate pathophysiological redox-mediated gene expression.