DNA methylation does not stably lock gene expression but instead serves as a molecular mark for gene silencing memory.
DNA methylation is commonly thought of as a "molecular lock" that leads to permanent gene silencing. To investigate this notion, we tested 24 different histone deacetylase inhibitors (HDACi) on colon cancer cells that harbor a GFP locus stably integrated and silenced by a hypermethylated cytomegalovirus (CMV) promoter. We found that HDACi efficiently reactivated expression of GFP and many other endogenous genes silenced by DNA hypermethylation. After treatment, all promoters were marked with active chromatin, yet DNA hypermethylation did not change. Thus, DNA methylation could not prevent gene reactivation by drug-induced resetting of the chromatin state. In evaluating the relative contribution of DNA methylation and histone modifications to stable gene silencing, we followed expression levels of GFP and other genes silenced by DNA hypermethylation over time after treatment with HDACi or DNA-demethylating drugs. Reactivation of methylated loci by HDACi was detectable for only 2 weeks, whereas DNA-demethylating drugs induced permanent epigenetic reprogramming. Therefore, DNA methylation cannot be considered as a lock for gene expression but rather as a memory signal for long-term maintenance of gene silencing. These findings define chromatin as an important druggable target for cancer epigenetic therapy and suggest that removal of DNA methylation signals is required to achieve long-term gene reactivation.
Department of Leukemia, The University of Texas MD Anderson Cancer Center, Houston, Texas, USA.
Noël J-M
NJ
Raynal
Jiali
J
Si
Rodolphe F
RF
Taby
Vazganush
V
Gharibyan
Saira
S
Ahmed
Jaroslav
J
Jelinek
Marcos R H
MR
Estécio
Jean-Pierre J
JP
Issa
D045744:Cell Line, Tumor; D042002:Chromatin Assembly and Disassembly; D003110:Colonic Neoplasms / Q000235:genetics*; D019175:DNA Methylation / Q000187:drug effects; D047630:Depsipeptides / Q000494:pharmacology; D015972:Gene Expression Regulation, Neoplastic; D020868:Gene Silencing; D049452:Green Fluorescent Proteins / Q000235:genetics; D056572:Histone Deacetylase Inhibitors / Q000494:pharmacology; D006801:Humans; D011401:Promoter Regions, Genetic; D015533:Transcriptional Activation / Q000187:drug effects
D047630:Depsipeptides; D056572:Histone Deacetylase Inhibitors; D049452:Green Fluorescent Proteins