Quantitative evaluation of CpG island methylation in hyperplastic polyps

Area of Science:

• Oncology
• Gastroenterology
• Molecular Biology

Background:

• Microsatellite unstable (MSI) cancers, common in sporadic colon cancers, often exhibit MLH1 promoter methylation and CIMP.
• Proximal hyperplastic polyps are suspected precursors but lack consistent MLH1 methylation evidence.
• Discrepancies in MLH1 methylation status between MSI cancers and hyperplastic polyps require investigation.

Purpose of the Study:

• To quantitatively assess MLH1 promoter methylation and CIMP in proximal hyperplastic polyps.
• To resolve the discrepancy regarding MLH1 methylation in potential MSI cancer precursors.
• To investigate the role of BRAF V600E mutations in hyperplastic polyps.

Main Methods:

• Quantitative real-time PCR was used to determine MLH1 promoter methylation and CIMP.
• Immunohistochemistry assessed MLH1 protein expression.
• BRAF V600E mutation status was analyzed.
• Samples included proximal hyperplastic polyps, distal hyperplastic polyps, and MSI colon cancers.

Main Results:

• Only 1 of 29 proximal hyperplastic polyps showed positive MLH1 methylation; none had MLH1 protein loss.
• All 11 MSI cancers displayed high MLH1 methylation (PMR >30).
• CIMP was found in 48% of proximal and 4% of distal hyperplastic polyps, and 10/11 MSI cancers, with lower CIMP marker methylation in polyps.

Conclusions:

• Quantitative MLH1 methylation analysis does not support its presence in proximal hyperplastic polyps.
• Proximal hyperplastic polyps show CIMP methylation at lower levels than MSI cancers.
• MLH1 methylation and increased CIMP likely occur later in the carcinogenesis of sporadic MSI colon cancers.