Nanoparticle-enabled highly sensitive detection of DNA methylation in single tube

Abstract

Abstract Background: One of the often occurred epigenetic events associated to cancer is inheritable transcriptional silencing of the tumor suppressor genes by aberrant methylation of cytosines at the promoter regions. Current approaches to detect promoter DNA methylation commonly involve three separate independent processes, including DNA extraction, bisulfite conversion and methylation detection through PCR amplification, such as methylation specific PCR (MSP). This method includes many disconnected steps with loss of genetic materials, potentially reducing the sensitivity required for analysis of challenging clinical samples. Methods and Results: To address this problem, we developed a new technique named Methylation-on-Beads (MOB). MOB uses the silica superparamagnetic nanobeads (SSB) to combine DNA extraction, bisulfite treatment (Bst) and PCR into a single tube. The single-tube scheme minimizes the DNA loss during the tube transfers and improves the analytical sensitivity for methylation detection. Further enhancement in sensitivity is achieved by combining MOB with quantum dot-enhanced MSP detection. The pre-PCR DNA yields using MOB were compared with those carried out with conventional organic solvent extraction and ethanol precipitation (PC). 15 serum samples from lung cancer patients (7 Stage I, 3 Stage II, 5 Stage III) were analyzed using both MOB and PC. The DNA recovery using MOB was higher than PC for each patient serum sample with a median increase of 6.61 fold. To demonstrate the versatile applicability of the new method, we further analyzed 10 samples to include fresh tissue and paraffin embedded tissue from normal patients, fresh tumors from cancer patients and sputum samples. Median DNA yield increase of 7.8, 5.3, 6.4 and 7.5 respectively was determined using MOB when compared to the conventional method. We examined the outcome of bisulfite conversion in the presence of SSBs using real-time methylation specific PCR (MSP) to analyze p16INK4a promoter methylation. A set of triplicate reactions were examined with bisulfite-treated DNA of varying treatment durations (0h, 1h, 3h, 4h, 8h), and compared to the control using 16 hrs of conventional bisulfite treatment. Results indicated that 4h of bisulfite treatment was sufficient for conversion, and that the presence of beads did not alter the conversion process. Assessment of methylation in serum or plasma can be a useful tool for early detection of cancer. However, extending the DNA methylation analysis to clinically usable serum/ blood-based tests has been limited by the lack of sensitivity of conventional methods. In order to address whether improving DNA yields could enhance methylation detection, we compared methylation of p16INK4a promoter in 49 patient serum samples (18 normal and 31 cancer) in a blinded study using both MOB and PC/Bst/MSP. The 31 tumor samples were pre-selected from patients diagnosed with lung cancer who were also methylated for p16INK4a promoter in corresponding tumors. While p16INK4a methylation was detected in 14/31 patients with lung cancer using conventional approach; using MOB, we were able to detect p16INK4a methylation in 23/31of these patients. When samples used for methylation analysis contained large amounts of DNA (cell lines, tumors etc), a single-tube analysis of entire input amount may be unnecessary. Instead, multiple reactions in parallel were feasible by directly splitting the SSB into several different tubes. Conclusion: MOB successfully combined three processes required for DNA methylation analysis into a single-tube using SSB thereby allowing for ease in handling and increased throughput in detection. Increased pre-PCR yield in MOB allowed for efficient, diagnostically sensitive methylation detection.