The Role Of miRNAs In Cancer Diagnosis And Drug Discovery

miRNAs have distinct expression patterns among tissues and cells in different differentiation stages (34, 35). Lim et al. (36) showed that over-expression of miR-124, a brain-specific miRNA, shifted the gene expression profile of HeLa cells toward that of the brain. Similarly, over-expression of muscle-specific miR-1 shifted the expression profile toward that of muscle. These results indicate that miRNAs play important roles in cell differentiation and characterization. Therefore, miRNAs are considered to have a significant influence on various disorders.

Recently, it has been reported that the expression of several miRNAs are altered in a variety of human cancers, suggesting potential roles of miRNAs in tumorigenesis (37). Calin et al. (38) showed that more than 50% of miRNAs were located in cancer-associated genomic regions or in fragile sites. In fact, miR-15a and miR-16 genes exist as a cistronic cluster at 13q14, which is deleted or down-regulated in most cases (~68%) of B-cell chronic lymphocytic leukemias (39).

Cimmino et al. (40) found that both these miRNAs negatively regulate the expression of B-cell lymphoma 2 (BCL2), which inhibits apoptosis and is present in many types of cancers including leukemias. In fact, overexpression of miR-15 and miR-16 in the MEG-01 cell line induces apoptotic cell death.

Alterations in the gene copy number of miRNAs are detected in a variety of human cancers (41,42,43). Zhang et al. (41) showed that miRNAs exhibited high-frequency ge-nomic alterations in human ovarian, breast cancer, and melanoma using high-resolution array-based comparative genomic hybridization.

Hayashita et al. (42) found that the expression and gene copy number of the miR-17-92 cluster—composed of seven miRNAs—is increased in lung cancer cell lines, especially with small-cell lung cancer histology. Enforced expression of miRNAs included in this polycistronic cluster enhances cell proliferation in a lung cancer cell line. The increase in expression and gene copy number of miR-17-92 cluster was also found in B-cell lymphomas (43). The expression of miRNAs in this cluster is upregulated by c-Myc, whose expression and/or function is one of the most common abnormalities in human cancers, and miR-17-5p and miR-20a in this miR-17-92 cluster negatively regulate the expression of the transcriptional factor E2F1 (44).

Furthermore, it was indicated that miR-17-19b cluster included in miR-17-92 cluster inhibited apoptotic cell death, and accelerated c-Myc-induced lymphomagenesis in mice reconstituted with miR-17-19b cluster-over-expressed haematopoietic stem cells (43). In addition, the miR-17-92 cluster has been reported to augment angiogenesis in vivo by down-regulation of anti-angiogenic thrombospondin-1 and connective tissue growth factor in Ras-transformed colonocytes (45).

miR-155was identified as a miRNA whose copy number and expression were up-regulated in several types of B-cell lymphomas (46). The miR-155gene is located in the final exon of the B-cell integration cluster (BIC) non-coding gene, which is shown to accelerate the pathogenesis of c-Myc-associated lymphomas and leukemias, suggesting potential roles of miR-155 in B-cell lymphomas (47). Indeed, transgenic mice with miR-155 driven by the B-cell-specific E^ enhancer rapidly develop a polyclonal B-cell malignancy (48).

The up-regulated expression of miR-155 is also reported in breast, lung, colon, and thyroid cancer (49,50). However, the actual molecular mechanism of miR-155 remains unknown, although it is reported that miR-155 down-regulates the expression of the angiotensin II type I receptor (51).

As an antiapoptotic miRNA, miR-21 was recently identified to be up-regulated in human breast tumor tissues, glioblastoma tumor tissues, and malignant cholangiocytes (52, 53, 54). Inhibition of miR-21 by antisense oligonucleotides causes activation of caspases and induction of apoptotic cell death in a human breast cancer cell line and glioblastoma cell line (52,53). Furthermore, miR-21 inhibits gemcitabine-induced apoptotic cell death in cholangiocarcinoma cell lines by down-regulation of PTEN (phos-phatase and tensin homolog deleted on chromosome 10), which positively regulates apoptosis via inhibition of PI 3-kinase signaling activation (54). The expression of miR-141 andmiR-200b are also up-regulated in malignant cholangiocytes. Inhibitions of these miRNAs using miRNA-specific antisense oligonucleotides decreased proliferation of a malignant cholangiocyte cell line. The up-regulated expression of miR-21 is also detected in human colon, lung, pancreas, prostate, and stomach cancer (49,55), suggesting the possibility that miR-21 inhibits apoptotic cell death in these cancers.

Interestingly, it has been reported that the expression level of let-7 is reduced in human lung cancers (56). This result suggests that let-7 might act as a tumor suppressor gene in lung cancer. In fact, regardless of disease stage, lung cancer patients with down-regulation of let-7 had shortened post-operative survival (56). Furthermore, Johnson et al. (57) found that let-7 negatively regulated the expression of human RAS family members, which possess potent oncogenic activity. Actually, RAS protein levels are inversely correlated with let-7 expression levels in human lung cancers, suggesting a possible mechanism for let-7 in lung cancer

To identify novel miRNAs involved in cellular transformation, Voorhoeve et al. (58) performed functional genetic screens using a library of vectors expressing human miR-NAs and in vitro neoplastic transformation assays. They showed that miR-372 and miR-373 accelerate proliferation and tumorigenic development in primary human cells that express oncogenic RAS and tumor suppressor p53, possibly through suppression of p53-mediated CDK inhibition by down-regulation of large tumor suppressor homolog 2 (LATS2) (58,59). Furthermore, miR-372 was found to be exclusively over-expressed in most human testicular germ cell tumors that rarely exhibit loss of p53 function, suggesting contribution of miR-372 to the development of human testicular germ cell tumors by inhibition of the p53 pathway (58).

Recent evidence indicates that polymorphisms and genetic variation in germ line as well as somatic cells have a critical role in cancer predisposition and malignancy (60,61). However, in spite of comprehensive scanning of protein coding genes, the molecular basis of familial cancers remains largely unknown. Recently, a germ line mutation of the miR-16-1-miR-15a primary precursor, which impaired mature miRNA expressions, was identified in B-cell chronic lymphocytic leukemia patients (62). Furthermore, germ line or somatic mutations of miRNAs were found in 11 of 75 patients with B-cell chronic lymphocytic leukemia, but none of these mutations were found in 160 persons without cancer (62). These results suggest that genetic variation of miRNAs in a germ line may play important roles in cancer predisposition and malignancy. In addition, germ line mutation in miRNA-target sites of mRNA 3' UTR were found in KIT and slit and trk-like family member 1 (SLITRK1), suggesting genetic variation of miRNA-target sites in a germ line may also play significant roles in disease predisposition (50,63).

Human cytochrome P450 (CYP) 1B1, which is abundantly expressed in malignant tumor tissues, is a member of drug-metabolizing enzymes and catalyzes the metabolic activation of various procarcinogens. Recently, it was found that CYP1B1 expression was post-transcriptionally inhibited by miR-27b (64). Furthermore, decrease of miR-27b expression and increase of CYP1B1 expression in most breast cancer tissues was detected (64). These results indicate that miRNAs may play important roles in not only physiologic events but also drug metabolism and production of carcinogens.

Global expression profiling analysis of protein coding genes is known to be useful for cancer diagnoses and prognosis predictions (65). Recently, Lu et al. (37) indicated that miRNA expression profiles can successfully classify poorly differentiated cancers that cannot be classified by mRNA expression profiles. Accordingly, miRNA expression profiles are more accurately correlated with clinical severity of cancer malignancy than

Table 1

Canser-Associated miRNAs


Cancer types





BCL, lung

CTGF, E2F1,Tsp1



breast, cholangiocyte, colon,


49, 52-55

glioblastoma, lung, pancreas,

prostate, stomach


BCL, breast, colon, lung, thyroid


46, 49-51


testicular germ cell



Tumor suppressor gene


breast, lung






39, 40

a Target genes identified by the biological experiments are listed. Abbreviations: AT1R, angiotensin II type I receptor; B-CLL, B-cell chronic lymphocytic leukemia; BCL, B-cell lymphoma; BCL2, B-cell lymphoma 2; CTGF, connective tissue growth factor; LATS2, large tumor suppressor homolog 2; PTEN, phosphatase and tensin homolog deleted on chromosome 10; Tspl, thrombospondin-1.

a Target genes identified by the biological experiments are listed. Abbreviations: AT1R, angiotensin II type I receptor; B-CLL, B-cell chronic lymphocytic leukemia; BCL, B-cell lymphoma; BCL2, B-cell lymphoma 2; CTGF, connective tissue growth factor; LATS2, large tumor suppressor homolog 2; PTEN, phosphatase and tensin homolog deleted on chromosome 10; Tspl, thrombospondin-1.

protein-coding gene expression profiles. This result indicates the potential of miRNA expression profiles in cancer classification and prognosis prediction (37).

Because miRNAs act as oncogenes or tumor suppressor genes (Table 1), miRNAs are potential targets of therapeutic strategies. Recently, Krutzfeldt et al. (66) indicated that chemically engineered oligonucleotides, called antagomirs, efficiently inhibited miRNAs in vivo. Additionally, it is reported that introduction of 2'-O-methoxyethyl phosphorothioate antisense oligonucleotide of miR-122 (abundant in the liver and regulates cholesterol and fatty-acid metabolism) decreases plasma cholesterol levels and improves liver steatosis in mice with diet-induced obesity (67). These findings indicate that antisense oligonucleotides are also potential targets for drug discovery, suggesting the possibility that intractable cancers may become curable by over-expression and/or inhibition of miRNAs. However, for miRNAs to be used in gene therapy, further improvement is required to make miRNAs more effective and less toxic than other cancer therapy.

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