DNA damage response activation and cell cycle dysregulation in infiltrative ameloblastomas: A proposed model for ameloblastoma tumor evolution

Ameloblastoma is an odontogenic tumor that frequently harbour the BRAFV600E mutation. The encapsulated unicystic (UA) and infiltrative multicystic (MA) variants are interesting models to study neoplastic progression. Although the transition between these types has not been proved, infiltrative areas appear along the evolution of UA. An oncogene-induced DNA damage system has been proposed to foster tumor evolution and a connection between DNA damage response (DDR) and the early stages of carcinogenesis has been evidenced in some tumor types. We aimed to compare DDR activation and abnormal activity of cell cycle in MA and UA. We evaluated the phospho-Histone-H2AX and phospho-Chk2 DDR molecular markers by immunohistochemistry in 13 FFPE samples. qPCR arrays were used to investigate the expression of 44 DDR genes in 4 samples of MA, 4 UA and 4 normal oral mucosa and the expression of 84 cell cycle genes in 3 MA and 3 UA. As oncogenic B-raf might trigger DDR, we assessed the BRAFV600E mutation status. BRAFV600E mutation were found in 11/13 ameloblastoma samples. Nuclear DDR molecular markers staining was found in the infiltrative areas of ameloblastoma samples. Upregulation of CCND1, CDK6, CCNG2, CDK5RAP1, SKP2, MAD2L2, MCM5, CHEK2, and NTHL1 genes was found in MA compared to UA. MA showed higher levels of CHEK2 and NTHL1, while UA showed GADD45G upregulation, compared to normal oral mucosa. In conclusion, ameloblastomas with histologic infiltrative areas showed cell cycle genes transcriptional dysregulation and an activated DDR. These alterations might be related to oncogenic B-raf activation, influencing the development of tumor aggressiveness over time.