Death-associated protein kinase promoter hypermethylation in normal human lymphocytes

A high frequency of death-associated protein kinase (DAPK) promoter hypermethylation has been noted in B-cell malignancies, head and neck cancers, and other solid tumors, and it has been used as a tumor marker in molecular detection strategies. Low levels of DAPK promoter hypermethylation, ranging from 0.003 to 1.181%, were detected in peripheral blood cells from 75 of 143 (52%) normal subjects by quantitative methylation-specific PCR (Q-MSP). In 10 of 10 selected patients, MSP amplification of a portion of the DAPK promoter followed by PCR product sequencing confirmed dense hypermethylation of the CpG island in their peripheral blood cells. Q-MSP analysis of fluorescence-activated cell-sorted peripheral blood cells from three of these patients demonstrated that a significantly greater proportion of B cells (1.074-6.026%) were DAPK hypermethylated than were T cells, monocytes, or neutrophils, which were <0.06% hypermethylated. Further analysis after sorting of one subject's B cells into IgM+, IgM-, IgG+, and IgG- subpopulations demonstrated that DAPK hypermethylation was predominantly present in the IgM- compared with IgM+ B cells (3.338% versus 0.436%). DAPK promoter hypermethylation was found in IgM- B cells in normal individuals. The same hypermethylation identified in B-cell malignancies may reflect a clonal outgrowth of B cells arising from this compartment and may indicate a susceptibility to neoplastic transformation in a subset of B cells. Normal circulating lymphocytes with DAPK promoter hypermethylation may act as confounding factors in tumor detection based on DAPK hypermethylation.