Esophagus-Gastric Cancer

Molecular Characteristics of Gastric Cancer

Several attempts to classify gastric cancer have been made over the past decades. The classification by Lauren using microscopic morphology alone is most widely used. The key subtypes include diffuse types, which encompass the signet ring–type gastric cancer, with a higher propensity for intraperitoneal metastasis and silencing of CDH1 ; intestinal-type distal gastric cancer, arising from precursor lesions in the setting of atrophic gastritis and chronic inflammation due to H. pylori ; and gastric cardia and GEJ cancer associated with inflammation resulting from gastric acid reflux and lifestyle factors, such as obesity and smoking.

A comprehensive molecular characterization of gastric cancer was published by TCGA. Historically, this disease has been viewed as a single entity, but data from TCGA redefined the disease into four major genomic subtypes that could more readily be applied in clinical care: Epstein-Barr virus (EBV)-infected tumors (9%), microsatellite instability (MSI) tumors (22%), genomically stable (GS) tumors (20%), and tumors with chromosomal instability (CIN, 50%). Each subtype was found throughout the stomach, but CIN tumors showed elevated frequency in the GEJ/cardia (65%, p = 0.012), whereas most EBV-positive tumors were present in the gastric fundus or body (62%). GS tumors were diagnosed at an earlier age (median age, 59 years), whereas MSI tumors were diagnosed at relatively older ages (median age, 72 years). MSI patients tended to be female (56%), but most EBV-positive cases were male (81%). No systematic differences in distribution of subtypes between patients of East Asian and Western origin were observed.

The EBV-positive tumors exhibited high prevalence of DNA promoter hypermethylation. All EBV-positive tumors displayed CDKN2A ( p16 ^INK4A ) promoter hypermethylation. In addition, a strong predilection for nonsilence PIK3CA mutations was observed in 80% of the subgroup. Notably, a novel recurrent amplification at 9p24.1 at the locus containing JAK2 , CD274 , and PDCD1LG2 was enriched in the EBV subgroup (15% of tumors). CD274 and PDCD1LG2 encode PD-L1 and PD-L2, immunosuppressant proteins currently being evaluated as targets to augment the endogenous antitumor immune response. This gene amplification is consistent with studies showing elevated PD-L1 expression in EBV-positive lymphoid cancers.

The MSI tumors, characterized by genomic instability due to a deficient DNA mismatch repair system, were enriched for MSI and hypermethylation, with patterns distinct from that observed of EBV-positive tumors, including hypermethylation at the MLH1 promoter region, leading to MLH1 silencing. Elevated somatic mutation rates were identified, including highly recurrent mutations of PIK3CA (42%) and ERBB3 (26%), and 12% of tumors having alterations of both genes. Of note, the BRAF ^V600E mutation commonly seen in MSI colorectal cancer was absent. However, gastric MSI tumors had a high rate of PD-L1 expression.

The CIN tumors were characterized by the presence of extensive somatic copy-number aberrations. They were noted to have marked aneuploidy and, although they lacked common mutations in PIK3CA and/or ERBB3 , the frequency of TP53 mutations (71%) and recurrent amplifications of receptor tyrosine kinases (RTKs) and cell cycle mediators were high, most notably HER2 (24%). Recurrent amplification of the gene-encoding ligand VEGFA was notable, as was the presence of frequent amplifications of cell cycle mediators ( CCNE , CCND , and CDK6 ).

The GS tumors were characterized by the elevated expression of molecules in the cell adhesion and angiogenesis-related pathways, mainly represented in the diffuse histological subtype (73%), with an absence of extensive somatic copy-number aberrations or elevated rates of mutation or hypermethylation. Loss of the tumor-suppressor gene CDH1 , encoding the cell adhesion molecule E-cadherin, was observed in the GS subtype (37%). The analysis by TCGA identified that 30% of GS tumors harbored novel alterations in components of the Rho signaling pathway, particularly somatic mutations of RHOA or fusion genes involving Rho-GTPase activating proteins CLDN18-ARHGAP6 or 26 . The CLDN18–ARHGAP fusions were mutually exclusive with RHOA mutations and were enriched in GS tumors. Given the role of RHOA in cell motility, alterations in either RHOA or CLDN18-ARHGAP6 might contribute to the disparate growth patterns and lack of cellular cohesion that are hallmarks of diffuse tumors.

Molecular Separation of Esophageal Squamous Cell Carcinoma and Adenocarcinoma

Although esophageal ACA and SCC share some recurring genomic alterations, substantial differences have been identified in the patterns of alterations between these two diseases. TCGA has confirmed the molecular distinctiveness of esophageal SCC and ACA. Strikingly, this study found that SCC arising in the esophagus were more similar to SCC of other organ types than to esophageal ACA, which more strongly resembled the CIN subtype of gastric cancer.

Genomic alterations in cell cycle mediators are frequent in both esophageal SCC and ACA. Inactivation of CDKN2A and amplification of CCND1 or CDK6 , or loss of RB1 were present in a majority of squamous tumors. Patterns of cell-cycle dysregulation differed in ACA, where CCND1 was amplified in only 15% of tumors and amplification of CCNE1 was more common. CDKN2A was inactivated in 76% of ACA by mutation, deletion, or epigenetic silencing. Esophageal ACA had a wider range of potentially oncogenic amplifications than SCC, most commonly ERBB2 , which was altered in 32% of ACA. Additional analysis identified dysregulation of the transforming growth factor-β (TGF-β) pathway and less frequent CTNNB1 (β-catenin) activation both to be more common in esophageal ACA than SCC. Therefore, although many of the same pathways were somatically altered in ACA and SCC, the specific genes affected were dissimilar, probably reflecting distinct pathophysiology and suggesting different therapeutic approaches. These data caution against performing clinical trials in mixed populations of esophageal ACA and SCC.

In esophageal ACA, TCGA analysis identified significant mutations in TP53 , CDKN2A , ARID1A , SMAD4 , and ERBB2 . These findings are consistent with the prominence of CDKN2A and TP53 mutations in dysplastic Barrett esophagus, a precursor to esophageal ACA. Of these mutations, amplifications containing VEGFA , ERBB2 , KRAS , EGFR , IGF1R , VEGFA , or GATA6 and deletion of SMAD4 were commonly detected only in ACA but absent in SCC.

Esophageal Adenocarcinoma in Relation to Gastric Cancer

TCGA network demonstrated that esophageal ACAs more closely resemble gastric cancer than other esophageal SCCs. When evaluated jointly with gastric cancers, esophageal ACA and CIN gastric cancer formed a group distinct from EBV, MSI, or GS tumors. Evaluating all gastroesophageal ACA, there was increasing prevalence of CIN moving proximally, with 71 of 72 esophageal ACAs classified as CIN. When further compared, TCGA found clear similarity between chromosomal aberrations in gastric CIN tumors and esophageal ACA. However, differences between esophageal ACA and gastric CIN tumors were also present. Esophageal ACA tumors harbor significantly higher DNA hypermethylation than among gastric CIN cancers. Moreover, esophageal ACA had higher rates of mutations of SMARCA4 and deletion of tumor suppressor RUNX1 but lower APC mutation rates relative to gastric tumors, suggesting a less prominent role for Wnt/beta-catenin. Overall, while some distinctions are apparent, the data suggests that gastric and esophageal CIN tumors lack absolute dichotomizing features and do not appear to be distinct tumor types.

Molecular Features of Esophageal Squamous Cell Carcinoma

In esophageal SCC, the TCGA study identified significant mutation rates in genes TP53 , NFE2L2 , MLL2 , ZNF750 , NOTCH1 , and TGFBR2 . Amplifications of TERT , FGFR1 , MDM2 , NKX2-1 , and deletion of RB1 are specifically found in esophageal SCC. These tumors also had amplifications of chromosome 3q, focused on the SOX2 locus. Genes that encode SOX2 or squamous transcription factor p63, also on chromosome 3p and essential for squamous epithelial cell differentiation, were amplified in 48% of esophageal SCC. These data suggest the presence of lineage-specific alterations that drive progression in esophageal SCC. Moreover, mutations in ZNF750 and NOTCH1 in esophageal SCCs may similarly modulate squamous cell maturation.

HER2

Human epidermal growth factor receptor (HER2, also known as ErbB2) is a transmembrane tyrosine kinase receptor encoded by proto-oncogene HER2 ( ERBB2 ), located on chromosome 17. Current clinical guidelines define tumor HER2 overexpression as 3+ immunohistochemical staining or fluorescence in situ hybridization (FISH) positivity. Cancers located at the GEJ have a higher rate of HER2 positivity than distal gastric cancers. Well and moderately differentiated tumors have a higher rate of HER2 positivity than poorly differentiated tumors. Although HER2 amplification seemed to be associated with a worse prognosis in one study, large investigations could not confirm HER2 protein expression or gene amplification as an independent prognostic factor. Trastuzumab is a humanized immunoglobulin G1 (IgG1) monoclonal antibody (mAb) directed against the extracellular domain of HER2, which prevents dimerization of the HER2 receptors. In a cohort of 3665 patient samples, the Trastuzumab for Gastric Cancer (ToGA) study revealed that 810 (22%) were HER2 positive. The ToGA trial showed a significant overall survival (OS) benefit for patients with HER2-positive advanced-stage gastric cancer treated with trastuzumab and cisplatin–fluoropyrimidine (fluorouracil or capecitabine) chemotherapy versus cisplatin-fluoropyrimidine alone. The development of tumor resistance against HER2-directed treatment is also an important question. Loss of HER2 expression occurs in approximately one-third of patients with HER2-positive gastric cancer treated with trastuzumab and presents a possible mechanism for trastuzumab resistance. Upon tumor progression, molecular alterations have been observed in EGFR (13%), TP53 (92%), cell cycle mediators, such as cyclin-dependent kinases (42%), and in the PI3K/AKT/mTOR axis (21%). These data suggest the need for repeat biopsies to accurately determine the appropriate use of HER2-directed therapy upon tumor progression.

Other Receptor Tyrosine Kinases

Targeting other RTKs with monoclonal antibodies or small molecule inhibitors has not led to compelling efficacy thus far in patients with gastroesophageal cancers. Anti-EGFR directed treatment with the mAbs cetuximab and panitumumab did not provide survival benefit in two large randomized Phase III studies, despite promising data from Phase II trials. However, retrospective biomarker analysis suggests that a subpopulation of patients with tumors harboring EGFR amplifications or copy-number gains might benefit from this therapy.

Negative results have also been reported regarding targeting the HGF/MET axis in two Phase III studies. Alternative biomarkers to guide patient selection other than immunohistochemistry (IHC) to determine MET status might be required for the promise of anti-HGF/MET treatments in gastric cancer to be realized. One such option is MET amplification, which might be a more appropriate biomarker to select for tumors that will respond to MET inhibitors. Preliminary data showed that some patients with MET amplification (which occurs in < 5% of gastric cancers) can achieve complete responses with MET inhibitors, such as crizotinib or AMG337.

In the TCGA gastric-cancer dataset, 9% of CIN tumors and 8% of GS tumors harbored FGFR amplification. In addition, activation of the FGFR2 pathway was found to be required to drive growth and survival of gastric cancers carrying FGFR2 amplifications, both in vitro and in vivo. Targeting FGFR in a Phase II trial showed no statistically significant difference in progression-free survival (PFS) in patients with FGFR2 -amplified or FGFR2-polysomy tumors selected by FISH. Reasons for the lack of efficacy could be the marked intratumor heterogeneity of FGFR2 amplification and the low concordance with elevated FGFR2 protein expression, as exploratory biomarker analysis revealed. Different biomarker-based patient selection strategies, such as FGFR2 copy-number in cell-free plasma DNA, should be tested in clinical practice.

Signaling Pathways in Angiogenesis

Vascular endothelial growth factor (VEGF) and its receptors are amplified and overexpressed in approximately 7% of all GEJ and gastric cancers. Ramucirumab is a recombinant humanized IgG1 mAb targeting VEGFR2. It was investigated in patients with advanced-stage gastric cancer following disease progression during or after first-9line chemotherapy. The combination of ramucirumab and paclitaxel seems to be the most effective current treatment regimen for patients with advanced-stage gastric cancer in whom disease progresses following first-line therapy.

Immune Checkpoint Molecules

The presence of viral antigens such as EBV (a hallmark of 9% of gastric cancers) has been shown to result in increased neoepitope presentation, which might contribute to an antitumor immune response. Moreover, the strength of the interleukin-12 (IL-12)–mediated signaling signature suggests a robust immune cell presence and provides support for targeted immunotherapy when coupled with evidence of PD-L1/2 overexpression. PD-L1 may therefore represent promising targets in these tumors. Pembrolizumab and nivolumab are humanized mAbs directed against PD-1. They enhance the ability of the immune system to detect and destroy cancer cells by blocking the interaction between PD-1 and PD-L1/L2 and countering the tumor's immune-escaping tactic.

Other Potential Targets

Frequent amplifications of cell cycle mediators ( CCNE1 , CCND1 , and CDK6 ) suggest the potential for therapeutic inhibition of cyclin-dependent kinases. These data reveal a potential role for inhibitors of cell cycle kinases for treatment, especially in esophageal SCC. The discovery of mutations in the RHOA and CLDN18 gene of the GS gastric ACA subtype also could be exploited to develop new therapeutic strategies.

Summary

In recent years, the characterization of esophageal and gastric cancers into molecular subtypes has evolved and provides a roadmap for the next generation of potential targets. Through the development of new drugs and combinations, this emerging molecular understanding can be translated into therapeutic biomarkers and targets for clinical benefit. The TCGA classification will help identify the opportunities and challenges of developing strategies toward the goal of precision medicine in gastric and esophageal ACA. Large-scale international studies are ongoing, and more results will be reported soon. Hopefully, further insights into the molecular characteristics will stimulate the role of biomarker-driven targeted therapy in gastroesophageal cancer.

Esophagus

The esophagus is a hollow tube approximately 25 cm in length that extends from the pharynx to the stomach and consists of three parts: the cervical, thoracic, and distal esophagus. The cervical esophagus lies just left of the midline behind the larynx and trachea. The upper portion of the thoracic esophagus passes behind the tracheal bifurcation and left main stem bronchus. The lower thoracic esophagus runs behind the left atrium. The distal esophagus is 6 to 8 cm in length and merges into the gastroesophageal junction. Adenocarcinomas of the GEJ have been classified into three types by Siewert based on anatomic location and the epicenter of the tumor. The current classification system identifies tumors 1 to 5 cm above the GEJ as distal esophageal tumors (Siewert type 1); those tumors with the epicenter of mass located within 1 cm proximal and 2 cm distal to the GEJ are classified as Siewert type 2; and subcardial tumors with the epicenter located 2 to 5 cm distal to the GEJ are classified as Siewert type 3. The American Joint Committee on Cancer (AJCC) staging system specifies that Siewert types 1 and 2 tumors are classified as ACA of the esophagus for the purposes of staging, and those meeting criteria for Siewert type 3 are classified as gastric cancers.

In the esophagus, mucosal lymphatics merge with a submucosal plexus, which then merges with lymphatic channels in the muscularis. These channels communicate with an extensive network of lymphatics that extend longitudinally along the esophagus and eventually drain into nodal groups as cephalad as the internal jugular lymph nodes and as caudal as the celiac lymph nodes. The nodal groups at risk for involvement in cancers of the cervical esophagus include the low cervical, supraclavicular, and upper paratracheal nodes. The nodes at risk for gastroesophageal tumors include the pulmonary ligament, paracardial, left gastric, common hepatic, splenic, and celiac nodes. The esophagus lacks a serosal surface and is separated from adjacent structures only by a loose connective tissue, the adventitia. The adventitia provides little barrier to local spread; as a consequence, a tracheoesophageal or esophagobronchial fistula can develop in patients with cancers of the mid- and high esophagus.

Stomach

The stomach is surrounded by organs and structures that can be involved by direct tumor extension when a lesion has spread beyond the gastric wall. These include the omenta, pancreas, diaphragm, transverse colon or mesocolon, duodenum, jejunum, spleen, liver, superior mesenteric and celiac vessels, abdominal wall, left adrenal gland, and kidney. Although adherence from inflammatory conditions can mimic direct extension of tumor, all adhesions between a gastric carcinoma and adjacent structures should be regarded as malignant, and en bloc resection of involved structures is preferred, if feasible. Abundant lymphatic channels are present within the submucosal and subserosal layers of the gastric wall. Microscopic or subclinical spread beyond the visible gross lesions (intramural spread) occurs commonly via the lymphatic channels. Frozen sections of the gastric resection margins should therefore be obtained intraoperatively to ensure that margins of resection are uninvolved microscopically. The submucosal lymphatic plexus is prominent in the esophagus and the subserosal plexus in the duodenum, allowing both proximal and distal intramural tumor spread.

Because of the numerous pathways of lymphatic drainage in the stomach, it is difficult to perform a complete nodal dissection ( eFig. 54.3 ). Although initial drainage is usually to lymph nodes along the lesser and greater curvatures (perigastric or N1 nodes using the Japanese Research Society for Gastric Cancer designation), primary nodal drainage includes nodes along all three branches of the celiac (left gastric, common hepatic, splenic) and the celiac itself (Japanese N2 nodes). Node groups that are more distal include hepatoduodenal, peripancreatic, root of mesentery (Japan N3), periaortic, and middle colic (N4). When proximal gastric lesions extend into the distal esophagus, that nodal system is at risk.

For cancers that are limited to the gastric wall, venous drainage is primarily to the liver via the portal system, although spread can occur to virtually any organ. At initial evaluation, liver involvement is found in up to 30% of patients, predominantly resulting from hematogenous metastases but sometimes because of direct tumor extension. When cancers extend proximally to involve the esophagus or extend posteriorly, the risk of lung metastases increases.

Because the stomach is an intraperitoneal organ, peritoneal spread is possible when a lesion extends beyond the gastric wall to a free peritoneal (serosal) surface. Peritoneal involvement may initially be a localized process limited by surrounding organs and ligaments (gastrohepatic, gastrosplenic, and gastrocolic) but is most often a diffuse process.

Clinical Manifestations

More than 90% of patients with esophageal cancer present with progressive dysphagia and, often, significant weight loss. Other findings include odynophagia, chest pain, cough, and fever associated with possible respiratory fistulas, hoarseness associated with tumor involvement of the recurrent laryngeal nerve, and melena resulting from intraluminal bleeding. In contrast, for gastric cancer, the symptoms are nonspecific, including loss of appetite, abdominal discomfort, weight loss, weakness (as a result of anemia), nausea and vomiting, and melena. The duration of major symptoms is often fairly short before diagnosis.