Methylation and silencing of the retinoic acid receptor-β2 gene in cervical cancer

BACKGROUND: Expression of the retinoic acid receptor β2 (RAR-β2), a putative tumor suppressor gene, is reduced in various human cancers, including squamous cell carcinomas (SCC) of the uterine cervix. The mechanism of the inhibition of RAR-β2 expression remains obscure. We examined whether methylation of RAR-β2 gene could be responsible for this silencing in cervical SCC. METHODS: Expression of RAR-β2 mRNA and methylation status of the 5' region of RAR-β2 gene were examined in 20 matched specimens from patients with cervical SCC and in three cervical cancer cell lines by Northern blot analysis and methylation-specific PCR (MSP) assay or Southern blot analysis respectively. RESULTS: In 8 out 20 cervical SCC (40%) the levels of RAR-β2 mRNA were decreased or undetectable in comparison with non-neoplastic cervix tissues. All 8 tumors with reduced levels of RAR-β2 mRNA expression showed methylation of the promoter and the first exon expressed in the RAR-β2 transcript. The RAR-β2 gene from non-neoplastic cervical tissues was mostly unmethylated and expressed, but methylated alleles of the gene were found in three samples of the morphologically normal tissues adjacent to the tumors. Three cervical cancer cell lines with extremely low level of RAR-β2 mRNA expression, SiHA, HeLA and CaSki, also showed methylation of this region of the RAR-β2 gene. CONCLUSIONS: These findings suggest that methylation of the 5' region of RAR-β2 gene may contribute to gene silencing and that methylation of this region may be an important and early event in cervical carcinogenesis. These findings may be useful to make retinoids more effective as preventive and therapeutic agents in combination with inhibitors of DNA methylation.