RASSF1A and NORE1A methylation and BRAFV600E mutations in thyroid tumors.

We analyzed RASSF1A and NORE1A methylation and BRAF mutation in 89 thyroid tumors, 42 non-neoplastic thyroid tissues and three thyroid tumor cell lines using polymerase chain reaction (PCR), methylation-specific PCR, Western blotting and DNA sequencing in order to study thyroid tumor pathogenesis and progression. RASSF1A promoter methylation was present in all three thyroid cell lines and in 27/78 (35%) of benign and malignant thyroid tumors. We showed for the first time that there was generally good agreement between RASSF1A methylation status and RASSF1A protein expression. We also examined for the first time NORE1A promoter region methylation in thyroid cell lines and primary tumors and showed that two of three thyroid cell lines were methylated in the NORE1A promoter region, while all primary thyroid tumors analyzed (n=51) were unmethylated. BRAF mutation was present in 38% of papillary thyroid carcinomas (PTC), including 20% of PTC with a follicular variant pattern and 67% of the tall cell variant of PTC. Hyalinizing trabecular tumors (n=23), which had nuclear features similar to PTC, did not have BRAF mutations, indicating that the presence of BRAF mutations can help to separate these two tumor types. Phospho-MEK expression was increased in the NPA cell line, which had a BRAF mutation, supporting the importance of the BRAF pathway alterations in PTC pathogenesis. These results indicate that RASSF1A epigenetic changes are an early event in thyroid tumor pathogenesis and progression and that NORE1A methylation is uncommon in primary thyroid tumors. BRAF mutation occurs later in thyroid tumor progression and is restricted mainly to PTC and anaplastic thyroid carcinoma.