The relationship between the degree of aberrant methylation in colorectal cancer tissue and appearance of tumor-derived DNA in blood.

Abstract

579 Background: We have developed a blood test for colorectal cancer (CRC) detecting circulating tumor DNA (ctDNA) by measuring the presence of methylated BCAT1 and IKZF1. This study aimed to compare the levels of methylated BCAT1/IKZF1 in tissue from patients with colorectal cancer to those in blood before and after tumor resection. Methods: 80 people with invasive CRC had blood collected prior to resection. Tumor and adjacent non-tumor colorectal tissue was collected at surgery. Five patients had tissue collected from secondary cancers in the liver. Additional blood samples were collected within a year of surgery from 45 of these patients. DNA was extracted from all samples and assayed for methylated BCAT1 and IKZF1 DNA. Results: The median % BCAT1 and IKZF1 methylation in colorectal tissues were 45.7% and 59.9% (tumor), and 4.8% and 0% (non-tumor), p < 0.001. Median methylation levels in secondary tumors of the liver were 4.9% for BCAT1 and 29.5% for IKZF1, compared to 0% for both biomarkers in non-tumor liver tissue (p < 0.01). The methylation levels did not differ across CRC stages (p > 0.05), whereas the detection of ctDNA in the blood was significantly associated with tumor staging (p < 0.01). Of the 21 ctDNA positive CRC patients, 18 (86%) tested negative post resection. Of the 3 that remained blood positive post resection, further cancers were confirmed in two (one with recurrence in the lung, one with thyroid cancer). Conclusions: Aberrant BCAT1 and IKZF1 methylation is an early event in CRC development and is localised to the tumor tissue. Methylated BCAT1 and IKZF1 in blood are dependent on tumor stage and measuring minimal residual disease by detecting ctDNA based on methylated BCAT1 and IKZF1 may inform completeness of tumor resection.