The microRNA pathway and cancer

Abstract

MicroRNAs (miRNAs) are ∼22nt long, non-coding RNAs that guide post-transcriptional gene silencing of their target genes and regulate diverse biological processes including cancer. miRNAs do not act alone, but require assembly into RNA-induced silencing complex (RISC). In this review, we summarize how miRNAs are produced, assembled into RISC, and regulate target mRNAs, and discuss how the miRNA pathway is involved in cancer.

Figure 1

Biogenesis of microRNA (miRNA). miRNAs are encoded in the genome and upon transcription give rise to primary miRNAS (pri‐miRNAs). The Drosha/DiGeorge syndrome critical region 8 protein (DGCR8) complex or the spliceosome catalyzes pri‐ to precursor miRNA (pre‐miRNA) conversion. Exportin‐5/RanGTP complex exports pre‐miRNAs from the nucleus to the cytoplasm. Dicer interacts with TAR RNA‐binding protein (TRBP) or PKR activating protein (PACT) and cleaves pre‐miRNAs into miRNA/miRNA* duplexes. These duplexes are assembled into Argonaute (Ago)1–4, and eventually form mature RNA‐induced silencing complexes (RISCs), which contain only single‐stranded guides.

Figure 2

RNA‐induced silencing complex (RISC) assembly can be divided into at least two steps, RISC loading and unwinding. The former prefers central mismatches in microRNA (miRNA)/miRNA* duplexes. Perfectly complementary siRNA duplexes can also be loaded into Argonaute (Ago)1–4, but duplexes without central mismatches are not favored for RISC loading. The RISC loading of small RNA duplexes is ATP‐dependent and facilitated by Hsc70/Hsp90 chaperone machinery. Mismatches in the seed region and/or the middle of the 3′ region enhance the slicer‐independent unwinding efficiency of all four Ago proteins. In contrast, only Ago2 can efficiently unwind perfectly complementary duplexes by passenger strand cleavage.

Figure 3

RNA‐induced silencing (RISC) regulates gene expression through various mechanisms: (a) cleavage; (b) deadenylation; (c) degradation; and (d) translational repression of target mRNA. (1–6) Proposed mechanisms of translational repression. Mechanisms 1–3 target the initiation step of translation; mechanisms 4 and 5 target the steps after initiation. AAA..., poly(A) tail; Ago, Argonaute; C, C‐terminal domain of GW182 protein: silencing domain; CAF1, CCR4 associated factor 1; CCR4, carbon catabolite repression 4; N, N‐terminal domain of GW182 protein: Ago binding domain; NOT1, negative on TATA‐less 1; PABPC, cytoplasmic poly(A) binding protein; P‐body, processing body; XRN1, 5′‐exoribonuclease 1.

Figure 4

MicroRNA (miRNA) pathway and cancer. (a) Genomic abnormalities, such as chromosomal translocations and point mutations, can attenuate or stimulate miRNA transcription leading to an increase or decrease of primary miRNA (pri‐miRNA). (b) Mutations in a miRNA gene can lead to RNA‐induced silencing (RISC) assembly abnormalities. For example, when nucleotides that establish the thermodynamic asymmetry are substituted, this can lead to “flipped” strand selection, resulting in the miRNA* strand, instead of the miRNA strand, being favored for RISC incorporation. (c) Furthermore, single nucleotide polymorphisms or mutations in either the miRNA or the target RNA can abrogate proper target recognition, especially when located within the seed region. Consequently, genes might escape being regulated by RISCs or perhaps even become differently regulated as a result of erroneous targeting. (d) Genomic abnormalities can also result in aberrant miRNA processing. Some pri‐miRNAs require additional proteins for efficient conversion. p53 mutants affect the interaction between p68/p72 RNA helicases and Drosha and this decreases pri‐ to precursor miRNA (pre‐miRNA) conversion of a subset of miRNAs. Additionally, p68 associates with receptor‐regulated Smads. This interaction has been shown to be important for efficient pri‐ to pre‐miRNA conversion of another subset of miRNAs. In the cytoplasm, TAR RNA‐binding protein (TRBP) is phosphorylated by MAPK/Erk signaling. The phosphorylation of TRBP increases pre‐miRNA to miRNA/miRNA* conversion for general miRNAs, but it decreases conversion for let‐7 family miRNAs. AAA..., poly(A) tail; Ago, Argonaute; DGCR8, DiGeorge syndrome critical region 8 protein.