Figure provided by Dr. Grady.
Colorectal cancer (CRC) is the second leading cause of cancer-related deaths in the U.S. More than half of the patients with CRC will develop metastasis. Several methods are available to detect the likelihood of relapse including the initial level of carcinoembryonic antigen (CEA) in the serum. CEA is a glycoprotein normally found in embryonic epithelium but which is also found to be increased in several types of cancer including CRC. However, none of these methods are optimal predictors of CR recurremce. Detecting CRC metastasis as early as possible is key for more effective care and thus improved survival.
An innovative and non-invasive way to predict or detect cancer metastasis is the detection of circulating biomarkers in the blood, such as proteins, DNA or microRNA (miRNA). miRNAs are 19 to 22 nucleotides long noncoding strands of RNA that can control protein expression at the post-transcriptional level by binding to complementary mRNA molecules thus triggering their degradation. Thousands of unique microRNAs have been detected to date. In healthy tissues miRNAs are found inside cells. In several types of cancer, it was observed that certain species of miRNA are highly abundant both in tumor tissues and in the blood.
Previous studies identified several miRNAs as predictors for CRC metastasis, however because of variability between these reports, these miRNAs have been generally considered unreliable for use in clinical care. In a recent study published in the British Journal of Cancer, Drs. Zixu Yuan and William Grady (Clinical Research Division, Fred Hutch) selected the most robust protocols available to date to resolve these issues and characterize reliable miRNA biomarkers for CRC metastasis that have high potential to be eventually used in the clinic.
miRNA levels and patient survival were evaluated in 122 individuals from an active CRC cohort called the ColoCare study. Newly diagnosed patients were recruited and serial primary tumor samples were collected. Follow-up data was analysed at multiple time points post-surgery. For the purpose of this study, all stages of CRC were included. Several miRNA were selected for further characterization. According to Dr. Grady, “A subset of the miRNAs have been shown to be elevated in other cancer. These miRNAs were selected for study because of their known low levels in blood and their elevated levels in colorectal cancer (CRC) tissue.” Of the miRNAs selected, miR-31 and miR-203 showed increased levels the blood in the presence of CRC metastasis. Six more that have previously been associated with CRC prognosis or progression when their levels are elevated in plasma (miR-29a, -92a, 17-3p, 125b, -200b, -141) were also studied and miR-16 was used as a plasma control. miRNAs were isolated from the plasma and quantified by quantitative reverse transcription PCR (qRT-PCR).
Increased plasma levels of mir-29a were observed in stage IV CRC patients relative to stages II or III, and relative to healthy individuals and patients with polyps. Recurrence-free survival was observed more frequently in individuals with lower levels of miR-29a, -31, -203 and -200b before surgery. At 6, 12 and 24 months following surgery, only miR-31 showed a significant association with tumor recurrence. Indeed, higher levels at these time points correlated with three-fold increased risk for tumor recurrence. MiR-29a and -200b also showed associations as well but these were not statistically significant. A post-surgery decrease in mir-141 at 12 and 24 months was observed in recurrence-free patients, suggesting favorable outcome when miR-131 is not elevated. Overall, these data indicate that some subsets of miRNA biomarkers might have potential to be robust predictors of CRC recurrence.
Characterizing the mechanisms associated with the increase of these miRNAs in the blood could help to understand their role in the metastatic process. To this end, the authors noted that some of the miRNAs identified as predictive biomarkers (miR-29a, -31, and -203) were also highly expressed in CRC tissues relative to normal tissues from the same patients. This was confirmed by consultation of a publicly available online dataset demonstrating that circulating exosomal miRNAs miR-29a, 200b and -31 isolated from CRC patients were also increased relative to samples from healthy donors. Dr. Grady explained, “These miRNAs have been presumed to be shed from tumor cells and to reflect tumor burden in the patient. They are most likely shed in exosomes. There is data in preclinical models to suggest that at least some circulating miRNAs may promote tumorigenesis and metastatic spread by fusing to normal host cells and inducing oncogenic behavior in those cells”.
From this study, several miRNAs represent potential candidates for surveillance of CRC recurrence. Dr. Grady concluded, “We are most excited about the potential of the miRNA assays to be used for the early detection of metastatic CRC. Once considered incurable, using modern management protocols 10%-30% of patients can be cured of their metastatic cancer if it is caught early. We are planning to do further studies to validate these markers in the future for their use in the clinic in monitoring for recurrent CRC. On a more speculative level, it is also conceivable that directing therapies against these miRNAs may decrease the risk of cancer. This is an exciting possibility that deserves further study.”
Funding for this study was provided by the National Institutes of Health, RACE Charities, Burroughs Wellcome Fund Translational Research Award for Clinician Scientist, Listwin Family Foundation, Fred Hutchinson Cancer Research Center and the International Program Fund for PhD. Candidates of Chinese Sun Yat-Sen University, National Natural Science Foundation of China, National Key Technology R&D Program for the 12th Five-Year Plan of China.
Yuan Z,Baker K,Redman MW,Wang L,Adams SV,Yu M,Dickinson B,Makar K,Ulrich N,Bohm J,Wurscher M,Westerhoff M,Medwell S,Moonka R,Sinanan M,Fichera A,Vickers K,Grady WM. 2017. Dynamic plasma microRNAs are biomarkers for prognosis and early detection of recurrence in colorectal cancer. British Journal of Cancer. doi: 10.1038/bjc.2017.266.