Exploring fingerprint ctDNA as a stratifying factor in colorectal cancer.

Abstract

e16101 Background: Colorectal cancer (CRC) ranks near the top in tumor-related deaths. Its standard-of-care is surgical resection plus adjuvant chemotherapy, which can extend patient’s live and disease-free survival for up to several years. However, choosing an appropriate chemotherapy agent is difficult because CRC is a heterogenous disease. It is believed that molecular stratification can help to improve treatment accuracy and this has been proven by the consensus molecular subtypes (CMS) classification. However, the CMS classification relies on expensive and complex gene-expression profiling, and more cumbersomely, is limited to early stage patients without prior chemotherapy, radiation and metastasis. This situation denies many CRC patients from the benefit of CMS classification. Circulating tumor DNA (ctDNA) is a minimally invasive way to monitor disease progression and treatment response in solid tumors but its clinical utility in CRC remains to be validated. We explore the potential of using patient’s fingerprint ctDNA (defined below) as a stratifying factor in CRC patients to assist clinical decision. Methods: WES is performed on tumor and matched blood samples from 149 CRC patients. Based on the WES result, 20-30 tumor specific genes are selected for each patient and form their ctDNA fingerprints. The patients are grouped according to their level of fingerprint ctDNA (high- vs. low-ctDNA). Results: The two groups of patients show significant difference in treatment responses and mutational profiles. The low-ctDNA group in general respond to treatment well and have good prognosis. The high-ctDNA group, in contrast, often experience relapse or recurrence. Interestingly, the low-ctDNA group is dominated by point and small indel mutations in the top mutated genes while the high-ctDNA group is dominated by gene copy variations (CNV). SMAD4 deficit and DCC amplification are well known CNV in CRC, but they only appear in the high-ctDNA group. Similarly, CNV of AGO2, ACTG1, MYC, and BRD3 are only associated with the high-ctDNA group but not the low-ctDNA group. Conclusions: Our result indicates a strong correlation of fingerprint ctDNA level to both tumor mutational profile and prognosis. This suggests fingerprint ctDNA level may be used as an effective stratifying factor in CRC. At the same time, possibly a common molecular cause is driving persistent ctDNA production in CRC patients, including a large portion of them who have received surgical and adjuvant chemotherapy. Further perspective study, however, is needed to test whether fingerprint ctDNA can be a predictive biomarker.