MMP9 and diabetic retinopathy: In the pathogenesis of diabetic retinopathy, MMP-9 promoter, in addition to histone modifications, also undergoes DNA methylation-hydroxymethylation.11,20,39 Although DNA methylation and histone modification are mediated by different sets of enzymes, these epigenetic modifications appear to be biologically interrelated, and the relationship can work in either direction.