The gastrointestinal tract includes the luminal organs extending from the esophagus to the rectum in addition to the pancreas and the gall bladder.15 In addition to the sizeable cellular mass, the epithelium has a rapid turnover, laying the foundation for gastrointestinal cancers that are among the most frequent malignancies resulting in mortality.16 These tumors usually present late with metastatic disease. Though morphologically similar, adenocarcinomas of different sites in the gastrointestinal tract are clinically distinct with unique risk factors and dissimilar prognostic behavior.3–5
The determination of the primary site of these adenocarcinomas within the luminal GI tract in the setting of widely metastatic disease has remained challenging despite the advances in immunohistochemistry.5,6 The CK7-/CK20 + immune-profile has good specificity for differentiating colorectal adenocarcinoma from other gastrointestinal adenocarcinomas.7,17−18 Similarly, pancreatic ductal adenocarcinomas display a CK7+/CK20 + immune-profile along with substantially higher CK17 expression.19–20
But when it comes to the upper gastrointestinal adenocarcinomas including those of the esophagus, gastroesophageal junction, stomach, small intestine and gallbladder, immunohistochemical profiles are indistinct and overlapping. CK7 is usually positive in most of them while CK 20 is usually negative.7 Other markers such as CDX2, Villin, CK17 and MUCs do not display specific staining patterns and aren’t helpful in differentiating these adenocarcinomas when used individually or as a panel.5, 8–10,21
The emergence of high throughput technologies such as next generation sequencing technology has heralded the beginning of the genomic era. Novel genomic and epigenomic biomarkers and signatures are being discovered and developed for early detection and prognosis of gastrointestinal cancers.22 In addition, scientists are endeavoring to understand how different molecular events lead to varying biologic properties and clinical features of these cancers based on different cells and tissues of origin.23
Morphologically similar tumors with similar immunohistochemical profiles arising in different organs may be from identical preceding events driving the tumorigenesis. However, even morphologically identical tumors arising in different organs differ substantially not only in terms of the oncogenic threats and environmental risk factors but also in cellular dynamics and tumorigenic potential. These inter-tumor dissimilarities, when identified, can not only improve diagnostic accuracy but also identify therapeutic targets and further patient welfare using precision medicine.23
A comprehensive comparative analysis of genetic alterations identified by high throughput sequencing can potentially uncover tissue specific determinants in different gastrointestinal adenocarcinomas which can translate to differences in prognosis and may help direct therapeutic decisions. Presence or absence of certain gene mutations and/or varying mutational frequency result in tissue specific mutational signatures.23
Beta-catenin, a protein coded by the CTNNB1 gene, is integral in intercellular adhesion and signal transduction and its degradation is regulated largely by adenomatous polyposis coli (APC) gene. Mutation in either of these can cause aberrant accumulation of beta catenin leading to increased transcription of downstream target proteins of the wingless integration site family member (WNT) signaling pathway such as MYC and CCND1.24 The dysregulation of the APC/beta-catenin and WNT signaling pathway is an integral mechanism of tumorigenesis in several cancers, most prominently in colorectal carcinomas.25–27 Choi et al reported a very low frequency of APC/beta catenin mutations in their study analyzing esophageal and esophagogastric junction adenocarcinomas based on partial screening mutational analyses.27
In our study, APC gene mutations were found to be most frequent in esophageal adenocarcinomas followed by small intestinal adenocarcinomas and infrequent in gastric adenocarcinomas. Choi et al reported similar findings in esophageal adenocarcinomas in their study in 97 tumors.27 Salem et al reported similarly low frequency of APC mutations in gastric adenocarcinomas but found significantly lower frequency of APC mutations in esophageal adenocarcinomas.14 The reported frequency of APC mutations in gastric adenocarcinomas varies widely. Fang et al reported an APC mutation frequency of 25% in gastric adenocarcionmas.28 Rokutan et al in their study of 43 gastric intramucosal adenocarcinomas found a higher frequency of APC mutations than in our study.29 However, they reported that the APC and TP53 mutations were mutually exclusive. This is reflected in our study where 88.9% of the TP53 mutated adenocarcinomas were APC wild type though this finding did not reach statistical significance. This might be attributed to the low number of study subjects and might reach statistical significance if a higher number of cases were evaluated supporting the existence of TP53/APC molecular subsets. Similar to gastric adenocarcinomas, wide variations have been reported in APC gene mutations in small intestinal adenocarcinomas. Schrock et al found the APC mutational frequency to be around 26% in 317 small intestinal adenocarcinomas studied.30 Similar findings were reported by Hanninen et al.31 However, in a recent study, Ota et al reported a much higher frequency of APC mutations in small intestinal adenocarcinomas.32 Our findings are comparable to the APC mutational frequency reported by Ota et al. The difference in APC mutational frequency among different studies may be ascribed to locational differences. The reported incidence of APC mutation in duodenal adenocarcinomas is much lower as compared to other small intestinal locations.32The majority of adenocarcinomas in the study by Schrock et al were duodenal.30 However, this cannot be ascertained as the small intestinal adenocarcinomas were not further stratified based on location in our study due to the small number of small intestinal adenocarcinomas included. None of the gastroesophageal junction adenocarcinomas or gallbladder adenocarcinomas included in this study had any mutations in the APC gene and this low incidence is similar to those reported previously.27,33
PIK3 gene mutations were found to be relatively more frequent in small intestinal and gall bladder adenocarcinomas as compared to esophageal and gastric adenocarcinomas. All the PIK3 gene family mutations were limited to class 1 PIK3 genes with the majority localizing to PIK3CA as reported in previous studies.34–38 PIK3CA mutations are reported to occur in 8–10% cancers.39 Disturbances in the PIK3 signaling pathway and its regulation are known to underlie numerous human diseases. Activating mutations in the genes encoding the catalytic subunits of class IA PIK3 have been reported in several cancer types.40 The frequency of PIK3 mutations in esophageal and gastric adenocarcinomas has been reported to be low in multiple studies as reflected in our study.14,34−35
The previously reported frequency of PIK3 gene family mutations in gallbladder adenocarcinoma is much lower than found in our study.36,37 Though, this could represent a sampling bias due to the small number of gallbladder adenocarcinoma cases included in this study, a more plausible explanation for the higher than reported frequency of PIK3 gene family mutations is the inclusion of mutations in all the class 1 genes. The mutations in the PIK3 gene family in gallbladder adenocarcinomas were limited to the class 1 regulatory subunit 1 and 2 genes. All class IA catalytic subunits interact and are controlled by regulatory subunits, and mutations/deletions in these regulatory subunits have been identified in multiple cancers.40 Though the role of PIK3R1 and PIK3R2 mutations in gallbladder adenocarcinoma has so far not been described, they are known oncogenic drivers in endometrial adenocarcinoma where gain of function mutations in PIK3R2 results in oncogenesis via PTEN stabilization.41,42
Again, like for gallbladder adenocarcinoma, the frequency of PIK3 family mutations in small intestinal adenocarcinoma was found to be much higher in our study when compared to previously published data.30–31, 38 The majority of these mutations were in PIK3CA as previously reported however mutations were also found in PIK3CG and PIK3CB genes accounting for the higher than reported incidence. Though the incidence of PIK3CA mutation in intestinal adenocarcinomas is reportedly low, the PI3K/AKT pathway is the most mutated pathway, where at least one gene was mutated in the majority of small intestinal adenocarcinomas.31 Hare et al in their study showed that the most common PIK3CA mutation (Pik3caH1047R seen in colorectal carcinomas), when expressed at physiological levels, is insufficient to initiate intestinal tumorigenesis. However, when acting in tandem with APC loss, it promotes the development of invasive adenocarcinomas in the small intestine.43 This tandem effect is seen on the logistic regression model in small intestinal adenocarcinomas in our study, though the statistical significance is limited by the small study set.
In addition to the small study population that remains a limitation of this study, the association of different mutations with the histo-morphological types of adenocarcinomas was not assessed. Also, the association between the various mutations and presence or absence of precusor lesions was not assessed.