Hypermethylation of the RASSF1A tumor suppressor gene in Japanese clear cell renal cell carcinoma

Hypermethylation associated inactivation of RASSF1A tumor suppressor gene at chromosome 3p21.3 has been observed in several human malignancies. Relatively high (91%) or low (23%) frequencies were reported in the methylation status of promoter region of the RASSF1A gene in clear cell renal carcinoma (RCC) depending on the country the report was from. To clarify exact contribution of the hypermethylation of RASSF1A gene in the development of RCC in Japan, we analyzed the methylation status of the RASSF1A promoter region in 50 Japanese clear cell RCC and RCC cell lines. Although relatively high frequency of hypermethylation in RASSF1A promoter (39 of 50 tumors, 78%) was observed, most of matched proximal normal tissue DNA also showed weak methylation. By comparison with methylation level of adapted normal kidney tissue DNA, tumor preferential hypermethylation in RASSF1A promoter was recognized as 40% (20/50 matched sets) of primary clear cell RCCs. Hypermethylation in RASSF1A promoter was observed in 36% (15/42) and 64% (5/8) of stage I-II or III-IV tumors, and also observed in 42% (11/26) and 38% (9/24) of our tumor samples with pathological grade I or II, respectively. In addition, 16 of 19 RCC cell lines (84%) showed complete or partial methylation of RASSF1A promoter region. There was no association between the frequency of RASSF1A methylation and inactivation of VHL tumor suppressor gene in either primary RCCs or RCC cell lines. Our results showed tumor specific RASSF1A promoter hypermethylation in up to 40% of low grade or low stage clear cell RCCs. It is essential to compare the methylation status of RASSF1A promoter in tumor with normal tissue to understand tumor specific hypermethylation. Since considerable cases of normal kidney are hypermethylated, contribution of the RASSF1A for the development and progression of kidney cancer may be more complex than expected.