Micro-RNAs as diagnostic or prognostic markers in human epithelial malignancies

Micro-RNAs (miRs) are important regulators of mRNA and protein expression which play important yet complex roles in human cancers [1]. Their biogenesis and biological networks are complex (Figure 1); they are first synthesized as large RNA precursors, processed in the nucleus into approximately 70 nt pre-miRs, folded into imperfect stem-loop structures, transported to the cytoplasm, whereupon they are incorporated into RISC (RNA-induced silencing complex) (reviewed in [2]). Cleavage by Argonaute-2, then Dicer, results in an approximately 22-nt mature miR duplex; the "guide" strand is retained within the RISC; the "passenger" strand is degraded. Through the seed region (nt 2 to 8), the miR can then bind to the 3'UTR of target mRNA sequences, preventing protein translation, leading to mRNA degradation. More recently, miRs have also been described to target 5'UTR, and even coding regions of transcripts [3]. The current miRDatabase (http://www.mirbase.org webcite) has catalogued more than 1,300 human sequences. Given their ability to target mRNA with imperfect complementarity, and predicted to regulate the expression of approximately one-third of all human transcripts [4], miRs are considered to be among the largest class of gene regulators [5,6].Multiple mechanisms can mediate miR dysregulations in human cancers, including chromosomal gains or losses [7], mutations of miR located loci [8], or epigenetic aberrations [8]. Any misstep in miR biogenesis (Figure 1) can also affect miR expression [9,10], exemplified by the down-regulation of Drosha and Dicer being associated with worse survival in ovarian, lung, and breast cancers [11]. MiRs can be either over- or under-expressed, functioning as tumour suppressors or oncogenes, depending on their downstream target genes [12]. MiR-15a and miR-16-1 are two of the first described down-regulated miRs in chronic lymphocytic leukemia [13], both target Bcl-2 [14]; thus their absence inhibits apoptosis. Alternatively, miR-21, one of