Espinosa and Slack: The Role of MicroRNAs in Cancer 133 tion of cells into different tissues. For exam- ple, miR-273 and the miRNA encoded by lys-6 have been shown to play important roles in the patterning of the C. elegans nervous system [30,31], and miR-430 has been implicated in brain development [3]. The importance of miRNA in development has been shown also in mammalian systems, examples of which are miR-181 in the dif- ferentiation of hematopoietic cells toward the B-cell lineage [33], miR-374 in pancre- atic islet-cell development [34], miR-143 in adipocyte differentiation [35], miR-196 in limb patterning by SHH [36], and miR-1 in heart development [37]. miRNAS AS TUMOR SUPPRESSORS miR-15a and miR-16-1 in CLL The first study directly suggesting that dysregulation of miRNA played an important role for miRNAs in tumorigenesis came from Calin et al. [38] It was known that loss of chromosomal region 13q14 was strongly as- sociated with B cell chronic lymphocytic leukemia (CLL), the most common form of leukemia in the Western hemisphere; how- ever, no gene or genes had been definitely di- rectly linked with CLL. After refining the critical region within 13q14 to about 30kb, Calin et al. failed to identify any protein cod- ing genes but recognized that two miRNAs, miR-15a and miR-16-1, were clustered within this chromosomal locus. Indeed, expression of these miRNAs was found to be diminished or completely ablated in > 65 percent of CLL cases examined. Further studies revealed a germline C-to-T mutation seven base pairs downstream of the miR-16-1 hairpin in two out of 75 CLL patients. This mutation was not found in any of 160 control individuals. Be- cause the mechanisms for miRNA biogenesis are not yet fully understood, it is unclear what the precise effect of this mutation is; however, it was found that the mutation correlated with diminished expression of this miRNA in pa- tient-derived CLL cells, while preventing the processing of the miRNA into its mature form when expressed in heterologous cell systems [39]. The common loss of miR-15a and miR- 16-1 in CLL, as well as the loss of 13q14 in mantle cell lymphoma (50 percent of cases), multiple myeloma (16 to 40 percent) and prostate cancer (60 percent) [38,40], strongly suggests that these two miRNAs act as tumor suppressor genes. While their full target complement is unknown, they appear to me- diate their effects largely by down-regulating the anti-apoptotic protein BCL2. This protein is often found expressed at high levels in CLL and is thought to be important for the survival of the malignant cells [41]. Consis- tent with a role for these miRNAs in the down-regulation of BCL2, loss of miR-15a and miR-16-1 correlates with ele- vated levels of BCL2, while heterologous ex- pression of these miRNAs results in decreased levels of the endogenous proteins [42]. The 3’UTR of the Bcl2 transcript has been found to contain binding sites for miR-15a and miR-16-1, and reporter con- structs containing the Bcl2 3’UTR are down- regulated upon co-expression with miR-15a and miR-16-1. Moreover, expression of these miRNAs is capable of inducing apoptosis in a leukemia cell line. Further research is nec- essary to better evaluate whether miR-15a and miR-16-1 exert their effects mainly through BCL2 or target additional transcripts in CLL. Nonetheless, the evidence available supports an important role for miR-15a and miR-16-1 in the prevention of lymphomage- nesis and leukaemogenesis. The let-7 family can negatively regulate Ras As noted earlier, the miRNAs of the let- 7 family were among the first to be de- scribed, and experiments performed in C. elegans provided the first indication that they may be involved in cancer. Early stud- ies showed that loss-of-function let-7 mu- tants were defective in the transition from late larval to adult stage. Upregulation of let- 7 in adult seam cells, on the other hand, is necessary to induce cell-cycle exit and ter- minal differentiation. If this miRNA is ab- sent, these cells fail to differentiate, undergoing additional rounds of division, a phenomenon often observed in cancer cells [43].