Aberrant promoter methylation, expression and function of RASSF1A gene in a series of Italian parathyroid tumors

Aberrant epigenetic features occurring in parathyroid tumors are key events involved in parathyroid tumorigenesis. They include DNA methylation, histone methylation, and non-coding RNAs. Ras Association Domain Family Protein1 Isoform A (RASSF1A) and Adenomatous Polyposis of Colon (APC) are frequently downregulated in human cancers and they have an emerging role in parathyroid neoplasms. Here, investigating the methylation of RASSF1A and APC promoters in a series of parathyroid adenomas from Italian patients with primary hyperparathyroidism (PAds, n=80), we reported that RASSF1A and APC promoter methylation occurred in about 90% of samples. Parathyroid carcinomas (PCas, n=9) displayed RASSF1A promoter methylation, while APC promoter was methylated only in 2 samples. Of note, RASSF1A promoter methylation negatively correlated with PAds tumor size. However, RASSF1A transcript and protein levels were reduced in PAds and PCas compared with parathyroid normal glands. Investigating the potential mechanism involved in RASSF1A promoter methylation, we found that DNA methyltransferases (DNMTs) activity was variable in PAds and inversely correlated with RASSF1A protein levels. In addition, the RASSF1A promoter methylation negatively correlated with long-non-coding Antisense Intronic Noncoding RASSF1A (ANRASSF1A) mRNA levels, excluding the involvement of ANRASSF1 in RASSF1A regulation. In HEK293A cells transfected with the calcium sensing receptor (CASR), loss of RASSF1A increased basal phosphorylated Extracellular signal-regulated kinase (pERK/ERK) levels blunting the CASR-induced increases. In conclusion, RASSF1A and APC promoter methylation is a hallmark of parathyroid tumors; deregulation of DNMTs activity contributes to modulation of RASSF1A expression. Loss of RASSF1A may be involved in the tuning of ERK pathway in parathyroid tumors