Significance of microRNAs in predictive oncology: experimental and clinical observations in colorectal cancer, breast cancer and glioblastoma

MicroRNAs (miRNAs) are short (18-25 nucleotides in length) noncoding RNA molecules that post-transcriptionally regulate gene expression. Bioinformatic tools predict that miRNAs are able to regulate approximately one-third of mammalian genes, including a significant number of oncogenes, tumor suppressor genes and genes associated with the invasion, dissemination and chemoresistance of tumors. In our colorectal cancer study, we examined by real-time PCR expression levels of miR-21, miR-31, miR-143, miR-145 and let-7a-1 in bioptic samples of 35 colorectal cancer (CRC) patients including 5 cases of IUCC Stage I, 13 of Stage II, 8 of Stage III, 9 of Stage IV. For 6 cases of CRC samples adjacent non-tumor tissue was also analyzed. The expression levels of all tested miRNAs significantly differ in tumor and normal mucosa, miR-21 (p = 0.0001) and miR-31 (p = 0.0006) were up-regulated and miR-143 (p = 0.013) and miR-145 (p = 0.018) were down-regulated in tumors. miR-21 was also correlated with CRC stage. High expression of miR-21 was associated with lymph node positivity (p = 0.025), development of distant metastases (p = 0.009) and also with poor survival (long-rank p = 0.043) in CRC patients. Tumors with down-regulated miR-143 and miR-145 were larger and more frequent (not significantly) in proximal CRC. In the case of glioblastoma, we examined the expression levels of miR-21, miR-221, miR-222, miR-181a-c, miR-125b and miR-128a in 22 primary glioblastomas and six specimens of adult brain tissue by the Real-Time PCR method. In addition, we examined the methylation status of MGMT (O6-methylguanine-DNA methyltransferase) promoter by methylation-specific RT-PCR, as this has previously been shown to be a predictive marker in glioblastomas. MGMT status and microR-NA expression levels were tested for any association with the patient´s response to concomitant chemoradiotherapy with temozolomide (RT/TMZ). MGMT methylation status did not correlate with age, gender, performance status, expression level of any microRNA analyzed and, most importantly, with response to RT/ TMZ. Patients who responded to RT/TMZ tended to have lower expression levels of microRNA-181 family members than those with progressive disease. MiR-181 b and miR-181c were significantly down-regulated in patients with a response to treatment (p = 0.016; p = 0.047, respectively) in comparison to patients with progressive disease. The "basal-like" mammary carcinomas occur in 15 % of breast cancer with higher frequency in patients with altered BRCA1. Typical invasive "basal-like" carcinoma is characterized by triple negative phenotype (ER, PR and HER2 negative) and higher frequency of mutations in tumor suppressor p53. It seems probable that alterations in the apoptotic p53-signaling pathways are one of the causal events in the pathogenesis of this molecular subtype of breast cancer. In our study of the "basal-like" mammary carcinoma we focused on the significance of miR-34a-c family known to be under direct transcriptional control of p53. The results of this study will be part of our presentation.

This work was supported by grant IGA MZ NS 9814-4/2008, NR/9076 - 4 of the Czech Ministry of Health and Project No. MZ0MOU2005 of the Czech Ministry of Health.