Clonality and K-ras mutation analyses of epithelia in intraductal papillary mucinous tumor and mucinous cystic tumor of the pancreas

Histological criteria for subclassification of intraductal papillary mucinous tumor (IPMT) and mucinous cystic tumor (MCT) of the pancreas remain ambiguous in the absence of apparent invasion or metastasis. To elucidate this issue, we evaluated clonality and K-ras mutations in 11 cystic tumors of the pancreas from female patients, including 7 IPMTs and 4 MCTs. The analyses were performed on DNA from laser microdissected epithelia showing different degrees of atypia as well as normal-appearing epithelia (NAE) in the individual tumors. The grades of atypia were classified into three groups on conventional hematoxylin-eosin staining. Clonality was assessed using the methylation-induced polymorphic inactivation of the X-linked phosphoglycerate kinase gene. The incidence of monoclonality increased with the grades of atypia: 27% for NAE, 43% for grade 1, and 100% for grades 2 and 3. In three of four MCTs, foci of NAE were polyclonal, while monoclonality was seen in each one of grades 1 and 2. The frequency of K-ras mutation depended on the grades of atypia: 0% for NAE, 29% for grade 1, 50% for grade 2, and 75% for grade 3. Polyclonal epithelia were devoid of K-ras mutation in 92% of sites, while monoclonality was associated with both wild and mutational types in an approximately equal ratio. Both IPMT and MCT seem to arise from polyclonal epithelia and to be replaced by monoclonal neoplastic cells as they undergo dysplastic changes and K-ras mutation. These data suggest that the monoclonal expansion precedes K-ras mutation.