MicroRNAs are regulatory, non-coding RNAs about 22 nucleotides in length: over 200 have been identified in humans, and their functions are beginning to be pinned down. It has been suggested that like other regulatory molecules they might be involved in tumour formation, and three papers in this issue confirm that this is the case. One cluster of microRNAs, known as mir-17–92, is shown to be a potential oncogene by its action in an in vivo model of human B-cell lymphoma. A cluster of microRNAs on human chromosome 13 has been found to be regulated by c-Myc, an important transcription factor that is overexpressed in many human cancers. And analysis of microRNA expression in over 300 individuals shows that microRNA profiles could be of value in cancer diagnosis. There is a global downregulation of microRNA in tumours, and the microRNA profile also reflects the origin and differentiation state of the tumours. To date, more than 200 microRNAs have been described in humans; however, the precise functions of these regulatory, non-coding RNAs remains largely obscure. One cluster of microRNAs, the mir-17–92 polycistron, is located in a region of DNA that is amplified in human B-cell lymphomas1. Here we compared B-cell lymphoma samples and cell lines to normal tissues, and found that the levels of the primary or mature microRNAs derived from the mir-17–92 locus are often substantially increased in these cancers. Enforced expression of the mir-17–92 cluster acted with c-myc expression to accelerate tumour development in a mouse B-cell lymphoma model. Tumours derived from haematopoietic stem cells expressing a subset of the mir-17–92 cluster and c-myc could be distinguished by an absence of apoptosis that was otherwise prevalent in c-myc-induced lymphomas. Together, these studies indicate that non-coding RNAs, specifically microRNAs, can modulate tumour formation, and implicate the mir-17–92 cluster as a potential human oncogene.

The highly aggressive character of melanoma makes it an excellent model for probing the mechanisms underlying metastasis, which remains one of the most difficult challenges in treating cancer. We find that miR-182, member of a miRNA cluster in a chromosomal locus (7q31–34) frequently amplified in melanoma, is commonly up-regulated in human melanoma cell lines and tissue samples; this up-regulation correlates with gene copy number in a subset of melanoma cell lines. Moreover, miR-182 ectopic expression stimulates migration of melanoma cells in vitro and their metastatic potential in vivo, whereas miR-182 down-regulation impedes invasion and triggers apoptosis. We further show that miR-182 over-expression promotes migration and survival by directly repressing microphthalmia-associated transcription factor-M and FOXO3, whereas enhanced expression of either microphthalmia-associated transcription factor-M or FOXO3 blocks miR-182's proinvasive effects. In human tissues, expression of miR-182 increases with progression from primary to metastatic melanoma and inversely correlates with FOXO3 and microphthalmia-associated transcription factor levels. Our data provide a mechanism for invasion and survival in melanoma that could prove applicable to metastasis of other cancers and suggest that miRNA silencing may be a worthwhile therapeutic strategy.