Apoptosis genes, such as TP53 and p16/CDKN2A, that mediate responses to 
cytotoxic chemotherapy, are frequently nonfunctional in melanoma. 
Differentiation may be an alternative to apoptosis for inducing melanoma cell 
cycle exit. Epigenetic mechanisms regulate differentiation, and DNA methylation 
alterations are associated with the abnormal differentiation of melanoma cells. 
The effects of the deoxycytidine analogue decitabine (5-aza-2'-deoxycytidine), 
which depletes DNA methyl transferase 1 (DNMT1), on melanoma differentiation 
were examined. Treatment of human and murine melanoma cells in vitro with 
concentrations of decitabine that did not cause apoptosis inhibited 
proliferation accompanied by cellular differentiation. A decrease in promoter 
methylation, and increase in expression of the melanocyte late-differentiation 
driver SOX9, was followed by increases in cyclin-dependent kinase inhibitors 
(CDKN) p27/CDKN1B and p21/CDKN1A that mediate cell cycle exit with 
differentiation. Effects were independent of the TP53, p16/CDKN2A and also the 
BRAF status of the melanoma cells. Resistance, when observed, was pharmacologic, 
characterized by diminished ability of decitabine to deplete DNMT1. Treatment of 
murine melanoma models in vivo with intermittent, low-dose decitabine, 
administered sub-cutaneously to limit high peak drug levels that cause 
cytotoxicity and increase exposure time for DNMT1 depletion, and with 
tetrahydrouridine to decrease decitabine metabolism and further increase 
exposure time, inhibited tumor growth and increased molecular and tumor stromal 
factors implicated in melanocyte differentiation. Modification of decitabine 
dose, schedule and formulation for differentiation rather than cytotoxic 
objectives inhibits the growth of melanoma cells in vitro and in vivo.