Ascorbic Acid Promotes Demethylation during Early NK Cell Differentiation.

Variegated expression of killer Ig-like receptors (KIR) in human NK cells is a stochastic process exclusive to subsets of mature NK cells and CD8 T cells. Allele-specific expression is maintained by DNA methylation within the proximal promoter regions. Because genes are densely methylated in NK cell progenitors, there is an implied stage of human NK cell development in which DNA demethylation takes place to allow for active transcription. When and how this process occurs is unknown. In this study, we show that proximal promoters are densely methylated in less mature CD56 NK cells and are progressively demethylated in CD56 NK cells as they mature and acquire KIR. We hypothesized that ten-eleven translocation (TET) enzymes, which oxidize 5mC on DNA could mediate promoter demethylation. The catalytic efficiency of TET enzymes is known to be enhanced by ascorbic acid. We found that the addition of ascorbic acid to ex vivo culture of sorted CD56 NK cells increased the frequency of KIR expression in a dose-dependent manner and facilitated demethylation of proximal promoters. A marked enrichment of the transcription factor Runx3 as well as TET2 and TET3 was observed within proximal promoters in CD56 NK cells cultured with ascorbic acid. Additionally, overexpression of TET3 and Runx3 promoted KIR expression in CD56 NK cells and NK-92 cells. Our results show that promoter demethylation can be induced in CD56, and this process is facilitated by ascorbic acid.