MicroRNAs (miRNAs) are now recognized to be key regulators of development and cell differentiation. Unsurprisingly, therefore, evidence is emerging that mi-RNAs can be mutated or abnormally expressed in human cancers. Steven M. Johnson et al. now provide a link between certain miRNAs and the notorious RAS oncogenic signalling pathway, suggesting a mechanism by which these miRNAs might regulate tumorigenesis.

Animal miRNAs are proposed to exert their regulatory effects by binding to complementary sequences in the 3′ untranslated regions (UTRs) of their target mRNA, thereby controlling translation. One of the first miRNAs to be discovered was let-7 in the worm Caenorhabditis elegans, and there are now four let-7 relatives known in these creatures. let-7 controls cell fate in hypodermal seam cells, which are a type of epithelial cell, but the functions of the other family members are unknown. Johnson et al. searched the C. elegans genome for potential target genes, looking for sites complimentary to a consensus let-7 family sequence in the 3′ UTRs of their genes. The highest-scoring candidate was let-60, the C. elegans homologue of human RAS.

The authors found that let-7 and let-60 are both expressed in hypodermal seam cells, but in a reciprocal manner, so that during developmental stages when let-7 is high, let-60 is low, and vice versa. Moreover, the let-60 3′ UTR is sufficient to downregulate a reporter gene at the same developmental stage when let-60 expression normally drops off. Notably, in worms with let-7 loss-of-function mutations, there was no decline in reporter-gene expression. All of which indicates that the let-60 mRNA is a functional target of let-7 in worms.

Moving into humans, Johnson et al. found several potential let-7 complimentary sites in the 3′ UTRs of the three RAS genes. Transfecting HepG2 cells with a let-7 miRNA reduced the expression of RAS by ∼70%. Moreover, the 3′ UTRs of NRAS and KRAS repressed the expression of a luciferase reporter in cells that have endogenous let-7 (HeLa cells). Co-transfection of an antisense inhibitor of let-7 relieved this repression.

Several human homologues of let-7 reside in genomic regions that are deleted in cancer, and Johnson et al. theorized that the loss of regulation by let-7 miRNA could cause overexpression of RAS and contribute to tumorigenesis. They analysed levels of let-7 in a range of cancers from 21 patients. In lung cancers, let-7 was reduced by an average of half, compared with normal adjacent tissue, and northern and western analyses revealed the reciprocal correlation between expression of let-7 miRNA and RAS protein. However, the amount of RAS protein did not seem to reflect the amount of NRAS mRNA, indicating that expression of human RAS protein is regulated significantly at the level of translation — consistent with a control mechanism where the miRNA binds to the 3′ UTR. The authors concluded that let-7 is strongly implicated as a tumour suppressor in lung tissue.