Epigenetic silencing of MIR203 in multiple myeloma

Epigenetic inactivation of tumour suppressor microRNAs has been implicated in carcinogenesis. We studied the promoter methylation of MIR203 in eight normal marrow controls, eight multiple myeloma (MM) cell lines, 20 monoclonal gammopathy of undetermined significance (MGUS), 123 diagnostic MM and 19 relapsed MM samples by methylation-specific polymerase chain reaction. Promoter of MIR203 was unmethylated in normal controls but homozygously methylated in 25% MM cell lines. Treatment with 5-Aza-2'-deoxycytidine led to promoter demethylation and MIR203 re-expression. Cyclic AMP responsive element binding protein 1 (CREB1) mRNA was predicted as a MIR203 direct target. Luciferase activity was reduced in constructs carrying wild-type CREB1 3'UTR upon MIR203 expression but not in those carrying mutant CREB1 3'UTR. Moreover, restoration of MIR203 led to downregulation of CREB1 protein and inhibition of myeloma cell proliferation. In primary samples, MIR203 methylation occurred in 25.0% MGUS, 23.6% diagnostic MM, and 21.1% relapsed MM samples. In conclusion, MIR203 methylation is disease-specific with reversible gene silencing in MM. MIR203 is a tumour suppressor microRNA inhibiting cellular proliferation by targeting CREB1 mRNA in MM. Comparable occurrence of MIR203 methylation in MGUS and MM at diagnosis or relapse suggested that MIR203 methylation may be an early event in myelomagenesis instead of being acquired during disease progression.