NORE1B, a candidate tumor suppressor, is epigenetically silenced in human hepatocellular carcinoma

Area of Science:

• Oncology
• Molecular Biology
• Epigenetics

Background:

• Hepatocellular carcinoma (HCC) rarely exhibits mutations in the ras proto-oncogene.
• Investigated potential inactivation of tumor suppressors NORE1A, NORE1B, and RASSF1A in HCC.

Purpose of the Study:

• To determine if NORE1A, NORE1B, and RASSF1A are inactivated in HCC through mutation or epigenetic gene silencing via promoter-CpG hypermethylation.

Main Methods:

• Single-strand conformation polymorphism (SSCP) analyses, sequencing, and methylation-specific PCR were employed.
• Samples included 28 fibrotic/cirrhotic livers and 40 HCCs, as well as hepatocarcinoma cell lines.

Main Results:

• No sequence deviations were found in NORE1A/B. RASSF1A showed a non-silent polymorphism in ~10% and a missense mutation in one HCC, potentially affecting its growth-inhibiting function.
• Epigenetic inactivation of NORE1B via promoter methylation occurred in 62% of HCCs and cell lines.
• RASSF1A promoter methylation was also detected in HCCs and cell lines, leading to combined epigenetic silencing of NORE1B and/or RASSF1A in 97% of HCCs.

Conclusions:

• Candidate tumor suppressor genes NORE1B and RASSF1A are frequently epigenetically down-regulated in HCC (62% individually, 97% in combination).
• This frequent epigenetic silencing is a critical event in hepatocarcinogenesis, enabling HCCs to evade growth control despite an unaltered Ras.
• These findings highlight the role of epigenetic alterations in HCC development, independent of Ras mutations.