AbstractThe global DNA methylation degree may be a ubiquitous and early biomarker to distinguish cancer cells from benign cells. However, its usefulness in clinical diagnosis was scarcely demonstrated, because the cancer cells isolated from patients were usually very rare. Even if 10 mL of peripheral blood was sampled from a patient, only tens of cancer cells could be isolated. So a method to quantify DNA methylation from small number of cells was needed to apply DNA methylation in clinical environment. In this study, we found that normal breast cell line MCF10A and breast cancer cell line MCF7 cells present significantly different percentage of genomic 5-methylcytosine (p < 0.02, n = 8), it could be a potential indicator for rapid discrimination of rare cancer cells from normal cells. However, conventional mass spectrometry needs usually ~10⁶ cells to quantify DNA methylation degree, which was too large to be applied in clinical diagnosis. Here we developed a fast mass spectrometry-based method capable of analyzing the DNA methylation degree from only ~100 human cells. Our method could reveal the different DNA methylation degree between MCF10A and MCF7 cells in less than two hours, having the potential to provide reliable information for clinical application.