BACKGROUND. Allelotype studies have suggested that chromosome 1p is frequently lost in thyroid cancers, thus suggesting that there is an important tumor suppressor at this location. RIZ1 (PRDM2), located on 1p36, is a recently described tumor suppressor gene and is a member of the protein methyltransferase superfamily. RIZ1 expression is lost in a variety of tumors, primarily by means of epigenetic mechanisms that involve promoter hypermethylation.METHODS. RIZ1 expression was examined in a panel of thyroid tumor cell lines and primary thyroid tissues (14 normal, 19 benign, and 31 cancerous) by using real-time polymerase chain reaction (PCR). Methylation status of the RIZ1 promoter was studied using bisulfite sequencing and methylation-specific PCR. RESULTS.The authors demonstrated that RIZ1 expression is lost in thyroid tumor cell lines and is also significantly reduced in thyroid carcinomas, when compared with normal thyroid tissues (P < .0001) and benign tumors (P ¼ .0003). The current study results also showed that loss of RIZ1 is mediated by aberrant cytosine methylation of the RIZ1 promoter. One hundred percent of carcinomas were methylated, compared with 33% of normal thyroid tissues (P ¼ .001). RIZ1 mRNA expression was significantly higher (P ¼ .02) in unmethylated (1.22 6 1.2, mean 6 standard deviation [SD]), compared with methylated tissues (0.37 6 0.42, mean 6 SD). Last, treatment with a DNA methyltransferase inhibitor led to reactivation of RIZ1 expression in cell lines that had negligible RIZ1 expression at baseline. CONCLUSIONS.The current study suggested an important role for RIZ1 expression in thyroid tumorigenesis and identified a potential novel therapeutic target for tumors unresponsive to other therapies.

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Increased 14-3-3σ expression has been observed by immunohistochemistry in papillary and anaplastic tumors, but not follicular thyroid cancers. 14-3-3σ mRNA expression and methylation status was examined in tumor cell lines and primary thyroid tissues using real-time RT-PCR, bisulfite sequencing and methylation-specific PCR. Most of the 27 CpG's in the gene's CpG island were methylated in normal thyroid, TPC-1, NPA, FTC-238 and 2-7, which did not express 14-3-3σ. In contrast, they were unmethylated in KAK-1 and anaplastic lines KAT4 and DRO-90. 14-3-3σ expression was not increased in thyroid carcinomas, the majority of which had a methylated CpG island. In addition, 5-aza-dC treatment increased 14-3-3σ expression in the FTC-238 and NPA cell lines, which had low baseline expression. We conclude 14-3-3σ expression in thyroid carcinomas is regulated by CpG island hypermethylation.

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