Advances in the molecular characterization of pancreatic cancer and pre-malignant lesions

Intraductal papillary mucinous neoplasm

The intraductal papillary mucinous neoplasm (IPMN) is a well-accepted clinical and pathologic entity (see Fig. 9D.4 D; see Chapters 59 and 60 ). IPMNs typically produce radiographically identifiable pancreatic ductal dilation, which may predominantly involve the main pancreatic ducts (main duct type IPMN), the secondary ducts (branch duct type IPMN), or both types of ducts (mixed type) (see Chapter 17 ). The distinction between the branch duct type and main duct type IPMNs is important, because the former are more likely to involve the head and uncinate process of the pancreas and are associated with lower-grade dysplasia and fewer invasive carcinomas. Approximately 30% to 40% of resected IPMNs harbor an invasive adenocarcinoma, and adenocarcinoma is most strongly associated with main duct IPMNs. Approximately half of invasive carcinomas arising within IPMNs are so-called colloid (mucinous) carcinomas, and most of the remainder are tubular adenocarcinomas; the latter is histologically indistinguishable from invasive ductal adenocarcinomas that arise in the setting of PanINs. Colloid carcinomas associated with IPMNs have a relatively good prognosis compared with other pancreatic carcinomas of the ductal type and have a 5-year survival of 60% (see Chapters 61 and 62 ).

PanINs and IPMNs show some overlapping features (see Chapter 59 ). For example, both are inherently intraductal lesions composed predominantly of columnar, mucin-producing cells that may grow in a flat configuration or may produce papillae; these lesions show a range of cytologic and architectural atypia and can give rise to invasive adenocarcinomas of the pancreas (see Fig. 9D.4 E). An important feature that distinguishes the two lesions is that PanINs are microscopic lesions and IPMNs are macroscopic. Nevertheless, recognition of an IPMN and its distinction from a PanIN lesion is important for two reasons: (1) IPMN-associated colloid carcinomas have a significantly better prognosis than either PanIN- or IPMN-associated tubular adenocarcinomas and (2) IPMNs have a propensity to be multifocal lesions, therefore, patients who undergo partial pancreatectomy and are left with a remnant pancreas need to be followed for life, even when the lesion originally resected was a noninvasive IPMN.

Genetic analyses of IPMNs have disclosed abnormalities in many of the same genes altered in conventional ductal adenocarcinoma, including mutations in the KRAS2 , TP53 , and CDKN2A genes, although the frequency and stage of neoplastic progression at which these alterations occur in IPMNs differ from PanINs. ^, For example, in contrast to PanINs, IPMNs harbor KRAS mutations in only half of analyzed cases. Moreover, abnormalities in SMAD4, which are present in 30% of PanIN-3 and 55% of PDA, are rare in IPMNs. IPMNs may also contain genetic alterations of genes that are specific to this form of neoplasia. Activating mutations in GNAS have been found in more than 70% of IPMNs, and a subset shows genetic inactivation of RNF43. Interestingly, correlations between phenotypic differentiation of IPMNs (described later) and mutations have been identified: GNAS mutations are more common in gastric and intestinal type IPMNs than in pancreaticobiliary type IPMNs, whereas KRAS mutations are more common in gastric and pancreaticobiliary type IPMNs. Molecular testing of pancreatic cyst fluid for GNAS and KRAS mutations may help support a diagnosis of IPMN and distinguish it from other cystic lesions, including neuroendocrine tumors with cystic degeneration, benign pseudocysts, and solid and cystic pseudopapillary neoplasms. However, a negative result does not rule out a mucinous cystic neoplasm.

Another distinction between PanINs and IPMNs relates to the expression of the caudal differentiation factor CDX2, a marker of intestinal differentiation. Most IPMNs express CDX2, in particular IPMNs associated with an invasive colloid carcinoma, whereas this is uncommon both in PanINs and in the subset of IPMNs that give rise to invasive cancers resembling ductal adenocarcinomas. CDX2 expression in IPMNs is generally associated with expression of MUC2 , an intestinal epithelial apomucin, whereas the absence of CDX2 expression usually is associated with expression of MUC1 , a biliary apomucin, and concomitant lack of expression of MUC2 . These findings have suggested that there may be two divergent pathways of carcinogenesis within the pancreatic ducts. The first is a so-called intestinal pathway that gives rise to CDX2- and MUC2-expressing IPMNs that progress to colloid carcinomas, which have a better prognosis. The second is a pancreatobiliary pathway that gives rise to CDX2 -negative, MUC2-negative, and MUC1-expressing PanINs and a subset of IPMNs, both of which can progress to conventional ductal adenocarcinomas, which have a poorer prognosis.

MUC5AC, a gastric foveolar mucin, is a secretory product normally expressed by surface mucus cells in the stomach and bronchial tract. MUC5AC expression is typically absent in the normal pancreas and hyperplastic lesions, whereas MUC5AC has consistently been shown to be aberrantly expressed in PDAC and its associated premalignant lesions, including all subtypes of IPMNs. It has been suggested that MUC5AC expression in pancreatic tumors may play a role in the tumor cells evading the host immune response, as well as contributing to the invasive motility of pancreatic cancer cells. Overexpression of MUC5AC relays a poorer prognosis in PDAC.

Intraductal tubulopapillary neoplasm

Intraductal tubulopapillary neoplasm (ITPN) is an intraductal neoplasm of the pancreas with distinct genetic and immunophenotypic features that are different from IPMNs and conventional ductal adenocarcinoma (see Chapter 59 ). ITPNs were first described in 2009 and included as a distinct diagnostic entity in the 2010 World Health Organization (WHO) classification of tumors of the gastrointestinal tract. ITPNs account for 3% of intraductal neoplasms of the pancreas. They have a slight female predominance and typically present with nonspecific abdominal symptoms. Histologically, ITPNs are composed of nodules with back-to-back tubular glands lined by cuboidal cells with eosinophilic to amphophilic cytoplasm. Rare papillae may be seen, although the predominant architectural pattern is tubular. The glandular crowding results in the formation of large circumscribed cribriform structures with central comedo type necrosis. Intracellular mucin is typically minimal. An invasive carcinoma is identified in up to 70% of cases. The prognosis of ITPNs with an invasive carcinoma is regarded as better overall than that of ductal adenocarcinoma, with a 5-year overall survival rate of 71% in ITPN in contrast to a 5-year overall survival rate of 21% in PDAC.

The immunophenotype of ITPNs shares some overlap with pancreaticobiliary type IPMNs. Expression of MUC1 and MUC6 is commonly seen in both ITPNs and pancreaticobiliary type IPMNs, whereas MUC5AC expression, which is highly expressed in all subtypes of IPMNs, is rarely reported in ITPNs. MUC2 labeling, which is observed in the intestinal type IPMNs, is consistently not seen in ITPNs.

Genetic alterations characteristic of PDAC and IPMNs are typically absent in ITPNs. Mutations in KRAS, for example, are seen in 7.1% of ITPNs, whereas they are observed in 80% of IPMNs. GNAS mutations, which are found in more than 70% of IPMNs, have not been reported in ITPNs. Somatic mutations in PIK3CA have also been described in 27% of ITPNs, whereas these alterations are typically absent in IPMNs. Although both intraductal pancreatic neoplasms share morphologic and immunophenotypic characteristics, both are recognized as distinct intraductal entities based on the differences in their genetic alterations.

Intraductal oncocytic papillary neoplasm

Intraductal oncocytic papillary neoplasm (IOPNs) were formally considered as a variant of IPMNs; however, advancements in the understanding of the molecular and immunophenotypic features of IOPNs have led to the reclassification of these lesions as a distinct diagnostic entity in the 2018 WHO classification of tumors of the gastrointestinal tract (see Chapters 59 and 60 ). IOPNs account for 4.5% of all intraductal neoplasms of the pancreas. They typically present in female patients most commonly either as an incidental finding or with symptoms associated with tumoral mass effect. IOPNs have a unique histologic appearance characterized by complex arborizing papillae with delicate fibrovascular cores lined by cuboidal to columnar cells with granular, eosinophilic cytoplasm and a round, centrally located nucleus with a prominent eccentric nucleolus. Architecturally, the cells form cribriform spaces with intraluminal mucin. IOPNs are essentially regarded to exhibit high-grade dysplasia due to the degree of architectural and cellular complexity commonly seen.

Although there are overlapping clinicopathologic characteristics between IOPNs and IPMNs, IOPNs exhibit distinct molecular alterations. IOPNs are reported to harbor recurrent mutations in ARHGP26 , ASXL1 , EPHA8 , and ERBB4 , although none occur at a high enough frequency to be considered entity-defining genomic alterations. Mutations in KRAS and GNAS , by contrast, have rarely been reported. The recently described novel DNAJB1-PRKACA oncogenic fusion has been found to be a mutated subset of intraductal pancreatic neoplasms with oncocytic morphology. The novel fusion was first described in the fibrolamellar variant of hepatocellular carcinoma and had previously been considered to be diagnostic of this entity.

Immunohistochemically, IOPNs exhibit diffuse labeling for MUC1 and MUC6 in 50% and 29% of cases, respectively. Interestingly, 61% of IOPNs label for HepPar-1, a marker of hepatocellular differentiation. In situ hybridization for albumin, a more specific marker for hepatocellular differentiation, is consistently negative in these cases.

The distinction between IOPNs and IPMNs is critical as IOPNs are regarded to have a better overall prognosis. Invasive carcinoma is associated with IOPNs in about 30% of cases. The predominant morphology of invasive tumors is that of a tubular adenocarcinoma composed of oncocytic tumor cells similar to the preneoplastic lesion. Invasive carcinoma associated with IOPNs has a relatively good prognosis with a 5-year survival approaching 100%.