Epigenetic inactivation of tumor suppressor genes is a fundamental event in the pathogenesis of malignant tumors. Here we report that
RASSF2, a member of the Ras Association Domain family, is frequently silenced in primary thyroid tumor and thyroid cancer cell lines. To the best of our knowledge this is the first report that shows that the epigenetic inactivation of
RASSF2 is a common event in thyroid carcinogenesis. We also show that RASSF2 suppressed growth of thyroid cancer cells. The
RASSF2 transcript can be detected in most normal tissues [
20]. Down-regulation of
RASSF2A by promoter hypermethylation has been shown in different tumor cell lines and primary tumors, including lung cancer, colon cancer, gastric cancer, nasopharyngeal carcinoma, head and neck cancer [
20,
26‐
33]. It has been reported that primary colorectal cancers, which showed
KRAS or
BRAF mutations, also frequently showed
RASSF2 methylation, and inactivation of
RASSF2 enhanced KRAS-induced oncogenic transformation [
28,
30]. RASSF2 binds directly to K-Ras in a GTP-dependent manner via the Ras effector domain [
20]. In addition, patients with methylated
RASSF2A promoter presented a higher frequency of lymph node metastasis [
27]. Methylation of
RASSF1A and
RASSF2 was associated with poor outcome after radiotherapy in oral squamous cell carcinoma [
34]. Here we report that the degree of
RASSF2 methylation was associated with the age of thyroid cancer patients. It has been suggested that aberrant methylation of
RASSF2 in plasma of colorectal cancer patients may serve as a cancer biomarker [
35]. Thus, it will be interesting to determine hypermethylation of
RASSF2 in blood samples of thyroid cancer patients.
Regarding RASSF2's interaction with other RASSF members, it was shown to associate with RASSF3 and RASSF5 [
29]. Our yeast two hybrid assay confirmed these bindings and additionally binding of RASSF1A and RASSF2 was observed. However these interactions were not observed in pulldown (Tab.
2). In general only the strong yeast two hybrid interactions could be confirmed by pulldowns. Since yeast two hybrid assays are prone to false positive results, we rather agree with the pull down interactions. RASSF2 also binds both proapoptotic kinases MST1 and MST2 in yeast two hybrid assays and this was confirmed for RASSF2 and MST1 by pulldown. It has been suggested that binding of the classical RASSFs, MST1, MST2 and WW45 with each other is conducted by their SARAH domains [
21]. Deletion of the SARAH domain of RASSF2 abolished binding with MST1 and MST2 [
36,
37]. It is interesting to note that all RASSFs and WW45 bind MST1, however only certain RASSFs (e.g. RASSF1A and RASSF5C) bind each other in pulldowns. This result suggests that SARAH domains show different binding properties.
RASSF2 exhibits several tumor suppressor properties, like inhibition of cell growth and induction of apoptosis [
20,
27]. Here we show that RASSF2 also acts as a novel tumor suppressor in thyroid carcinogenesis. It has been shown that RASSF2 binds and regulates the proapoptotic kinases MST1 and MST2 [
36,
37]. RASSF2 also engages the JNK pathway and induces apoptosis in an MST1-independent manner [
37]. Other findings suggest that MST2 and RASSF2 form a complex, in which RASSF2 is maintained in a phosphorylated state by MST2 and protects MST2 from degradation and turnover [
36]. However, the exact phosphorylation site of RASSF2 by MST2 was not revealed.
Additionally, we have found that
RASSF5A is frequently hypermethylated in thyroid tissues, however methylation was not tumor specific since it was also observed in controls. Epigenetic silencing of
RASSF5 in thyroid cancer has been analyzed previously, but was also not tumor specific [
11,
12]. Interestingly,
RASSF5A and
RASSF2 methylation was reduced in MTC compared to other cancers. It has been reported that RASSF5A expression is suppressed in FTC carrying PAX8-PPARγ fusions [
11]. Methylation of the other investigated CpG island promoters of RAS effectors (
RASSF3, RASSF4,
MST1,
MST2 and
WW45) was uncommon.