B-RAF mutation and accumulated gene methylation in aberrant crypt foci (ACF), sessile serrated adenoma/polyp (SSA/P) and cancer in SSA/P.

Background: Sessile serrated adenomas/polyps (SSA/Ps) are a putative precursor of colon cancer with microsatellite instability (MSI). However, the developmental mechanism of SSA/P remains unknown. We performed genetic analysis and genome-wide DNA methylation analysis in aberrant crypt foci (ACF), SSA/P, and cancer in SSA/P specimens to show a close association between ACF and the SSA/P-cancer sequence. We also evaluated the prevalence and number of ACF in SSA/P patients.

Methods: ACF in the right-side colon were observed in 36 patients with SSA/Ps alone, 2 with cancers in SSA/P, and 20 normal subjects and biopsied under magnifying endoscopy. B-RAF mutation and MSI were analysed by PCR-restriction fragment length polymorphism (RFLP) and PCR-SSCP, respectively, in 15 ACF, 20 SSA/P, and 2 cancer specimens. DNA methylation array analysis of seven ACF, seven SSA/P, and two cancer in SSA/P specimens was performed using the microarray-based integrated analysis of methylation by isochizomers (MIAMI) method.

Results: B-RAF mutations were frequently detected in ACF, SSA/P, and cancer in SSA/P tissues. The number of methylated genes increased significantly in the order of ACF
Conclusions: Our results suggest that ACF are precursor lesions of the SSA/P-cancer sequence in patients with SSA/P, where ACF arise by B-RAF mutation and methylation of some of the six identified genes and develop into SSA/Ps through accumulated methylation of these genes.