An α-E-catenin (CTNNA1) mutation in hereditary diffuse gastric cancer

Hereditary diffuse gastric cancer - pathophysiology and clinical management

Hereditary Diffuse Gastric Cancer is an autosomal dominant inherited gastric cancer syndrome caused by germline alterations in CDH1 (E-cadherin) and CTNNA1 (alpha-E-catenin) genes. Germline CDH1 alterations encompass small frameshifts, splice-site, nonsense, and missense mutations, as well as large rearrangements. Most CDH1 truncating mutations are pathogenic, and several missense CDH1 mutations have a deleterious effect on E-cadherin function. CDH1 testing should be performed in probands. Screening of at-risk individuals is indicated from the age of consent following counselling with a multidisciplinary team. In mutation-positive individuals prophylactic gastrectomy is recommended. Endoscopic surveillance is an option for those refusing/postponing gastrectomy, those with mutations of undetermined significance, and in CDH1-negative families. Ongoing research focus on the search of genetic causes other than CDH1 or CTNNA1 germline defects; assessment of the pathogenicity and penetrance of CDH1 missense mutations and identification of somatic mechanisms behind the progression from early (indolent) lesions to invasive (lethal) carcinomas.

Genomic landscape and genetic heterogeneity in gastric adenocarcinoma revealed by whole-genome sequencing

Gastric cancer (GC) is the second most common cause of cancer-related deaths. It is known to be a heterogeneous disease with several molecular and histological subtypes. Here we perform whole-genome sequencing of 49 GCs with diffuse (N=31) and intestinal (N=18) histological subtypes and identify three mutational signatures, impacting TpT, CpG and TpCp[A/T] nucleotides. The diffuse-type GCs show significantly lower clonality and smaller numbers of somatic and structural variants compared with intestinal subtype. We further divide the diffuse subtype into one with infrequent genetic changes/low clonality and another with relatively higher clonality and mutations impacting TpT dinucleotide. Notably, we discover frequent and exclusive mutations in Ephrins and SLIT/ROBO signalling pathway genes. Overall, this study delivers new insights into the mutational heterogeneity underlying distinct histologic subtypes of GC that could have important implications for future research in the diagnosis and treatment of GC.

Genetics of gastric cancer

Gastric cancer remains highly prevalent and accounts for a notable proportion of global cancer mortality. This cancer is also associated with poor survival rates. Understanding the genetic basis of gastric cancer will offer insights into its pathogenesis, help identify new biomarkers and novel treatment targets, aid prognostication and could be central to developing individualized treatment strategies in the future. An inherited component contributes to <3% of gastric cancers; the majority of genetic changes associated with gastric cancer are acquired. Over the past few decades, advances in technology and high-throughput analysis have improved understanding of the molecular aspects of the pathogenesis of gastric cancer. These aspects are multifaceted and heterogeneous and represent a wide spectrum of several key genetic influences, such as chromosomal instability, microsatellite instability, changes in microRNA profile, somatic gene mutations or functional single nucleotide polymorphisms. These genetic aspects of the pathogenesis of gastric cancer will be addressed in this Review.

Prospective cohort study assessing outcomes of patients from families fulfilling criteria for hereditary diffuse gastric cancer undergoing endoscopic surveillance

BACKGROUND

Prophylactic total gastrectomy is performed in hereditary diffuse gastric cancer (HDGC) patients carrying the CDH1 mutation because endoscopic surveillance often fails to detect microscopic disease.

OBJECTIVE

The aim of this study was to determine the natural history and outcomes of patients with HDGC undergoing endoscopy.

DESIGN

Prospective, cohort observational study.

SETTINGS

Tertiary referral center.

PATIENTS

Patients fulfilling criteria for HDGC who opted to undergo endoscopy.

INTERVENTION

Research surveillance program using high-resolution white-light endoscopy with autofluorescence and narrow-band imaging combined with targeted and multiple random biopsies assessed by an expert histopathologist for the presence of signet ring cell carcinoma.

MAIN OUTCOME MEASUREMENTS

The primary endpoint was the endoscopic yield of microscopic signet ring cell carcinoma according to patient mutation status and subsequent decision to undergo surgery. The secondary endpoint was the additional yield of targeted biopsies compared with random biopsies.

RESULTS

Between September 2007 and March 2013, 29 patients from 17 families underwent 70 surveillance endoscopies. Signet ring cell carcinoma foci were identified in 14 of 22 (63.6%) patients with confirmed CDH1 germline mutations and 2 of 7 (28.6%) with no pathogenic mutation identified. Eleven of 16 (9 CDH1-positive) patients proceeded to gastrectomy in a median 5.7 months. Five patients delayed surgery. In 1 patient, advanced gastric cancer developed 40.2 months after the first endoscopic findings.

LIMITATIONS

No control group.

CONCLUSIONS

Careful white-light examination with targeted and random biopsies combined with detailed histopathology can identify early lesions and help to inform decision making with regard to gastrectomy. Autofluorescence and narrow-band imaging are of limited utility. Delaying gastrectomy in individuals with signet ring cell carcinoma foci carries a high risk and has to be weighed carefully.

Whole-genome sequencing and comprehensive molecular profiling identify new driver mutations in gastric cancer

Gastric cancer is a heterogeneous disease with diverse molecular and histological subtypes. We performed whole-genome sequencing in 100 tumor-normal pairs, along with DNA copy number, gene expression and methylation profiling, for integrative genomic analysis. We found subtype-specific genetic and epigenetic perturbations and unique mutational signatures. We identified previously known (TP53, ARID1A and CDH1) and new (MUC6, CTNNA2, GLI3, RNF43 and others) significantly mutated driver genes. Specifically, we found RHOA mutations in 14.3% of diffuse-type tumors but not in intestinal-type tumors (P < 0.001). The mutations clustered in recurrent hotspots affecting functional domains and caused defective RHOA signaling, promoting escape from anoikis in organoid cultures. The top perturbed pathways in gastric cancer included adherens junction and focal adhesion, in which RHOA and other mutated genes we identified participate as key players. These findings illustrate a multidimensional and comprehensive genomic landscape that highlights the molecular complexity of gastric cancer and provides a road map to facilitate genome-guided personalized therapy.

Unmet needs and challenges in gastric cancer: the way forward

Although the incidence of gastric cancer has fallen steadily in developed countries over the past 50 years, outcomes in Western countries remain poor, primarily due to the advanced stage of the disease at presentation. While earlier diagnosis would help to improve outcomes for patients with gastric cancer, better understanding of the biology of the disease is also needed, along with advances in therapy. Indeed, progress in the treatment of gastric cancer has been limited, mainly because of its genetic complexity and heterogeneity. As a result, there is an urgent need to apply precision medicine to the management of the disease in order to ensure that individuals receive the most appropriate treatment. This article suggests a number of strategies that may help to accelerate progress in treating patients with gastric cancer. Incorporation of some of these approaches could help to improve the quality of life and survival for patients diagnosed with the disease. Standardisation of care across Europe through expansion of the European Registration of Cancer Care (EURECCA) registry - a European cancer audit that aims to improve quality and decrease variation in care across the region - may also be expected to lead to improved outcomes for those suffering from this common malignancy.

Involvement of Mediator complex in malignancy

Mediator complex (MED) is an evolutionarily conserved multiprotein, fundamental for growth and survival of all cells. In eukaryotes, the mRNA transcription is dependent on RNA polymerase II that is associated to various molecules like general transcription factors, MED subunits and chromatin regulators. To date, transcriptional machinery dysfunction has been shown to elicit broad effects on cell proliferation, development, differentiation, and pathologic disease induction, including cancer. Indeed, in malignant cells, the improper activation of specific genes is usually ascribed to aberrant transcription machinery. Here, we focus our attention on the correlation of MED subunits with carcinogenesis. To date, many subunits are mutated or display altered expression in human cancers. Particularly, the role of MED1, MED28, MED12, CDK8 and Cyclin C in cancer is well documented, although several studies have recently reported a possible association of other subunits with malignancy. Definitely, a major comprehension of the involvement of the whole complex in cancer may lead to the identification of MED subunits as novel diagnostic/prognostic tumour markers to be used in combination with imaging technique in clinical oncology, and to develop novel anti-cancer targets for molecular-targeted therapy.