**3. Molecular classification and pathogenies of GC**

Up to 90% of stomach malignancies are adenocarcinomas. Non-Hodgkin's lymphomas and gastrointestinal stromal tumor (GIST) make up most of the remaining 10% [31]. Even though infrequently, adenosquamous, squamous, and undifferentiated carcinomas also occur. In regard to clinical diagnosis, several pathological characterization varied from time to time. Several histological classification systems for gastric adenocarcinoma have been described, but the most frequently used are those of the World Health Organization (WHO) and Lauren [32]. In the World Health Organization (WHO) classification, there are 10 histological types [33]. The Lauren classification is commonly applied and it makes the distinction between intestinal and diffuse types. The intestinal GC consists of cohesive neoplastic cells forming gland-like structures while the diffuse type has lost cell cohesion and resulting in diffuse discohesive cellular infiltration [31]. Men and elderly are more likely to suffer intestinal type, whereas diffuse type carcinomas are relatively more common among the younger population with an equal male-tofemale ratio [32]. Recently, a project named The Cancer Genome Atlas (TCGA) has proposed a brand new classification, in which GC is grouped by four subtypes: EBV-positive (EBV), microsatellite instability (MSI), genomically stable [34], and chromosomal instability [35, 36].

According to previous studies, about 9% of GC cases are infected by EBV [37]. All the EBVpositive GCs harbor the property of CpG island methylator phenotype (CIMP) [36, 38, 39]. EBV-positive tumors exhibited a higher incidence of whole-genomic DNA hypermethylation than any molecular subtypes. The genes with promoter hypermethylation showed most differentially silenced expression in EBV-associated GC [36]. Moreover, PI(3)-kinase inhibition was also strongly detected in EBV-positive GC, which offered a new method for the evaluation of this subtype [36]. The most highly transcribed EBV viral, message RNAs (mRNAs) and microRNAs (miRNAs), fell within the BamH1A region of the viral genome and showed similar expression patterns across tumors [36]. The mutation rate of PIK3CA is exclusively high in EBV-positive gastric cancer compared with other molecular subtypes. The mutation rate of PIK3CA in this subtype is about 80 and 68% of the mutations belongs to recurrent mutation in this dataset. In contrast, in other molecular subtypes, the mutation rates of PIK3CA are from 3 to 42%. So, this result provides a hint that using PI3K inhibitor might have the clinical therapeutic potential for this kind of molecular subtype.

The next subtypes of GC are abundant in MSI, which display increased mutation rates (in major histocompatibility complex class I genes, including B2M and HLA-B) and hypermethylation (containing hypermethylation at the *MLH1* promoter). The most obvious difference between EBV-CIMP (CpG island methylator phenotype) and MSI-associated gastric-CIMP methylation profiles is that all EBV-positive gastric tumors show promoter hypermethylation of *CDKN2A* (*p16INK4A*), but the *MLH1* hypermethylation was only detected in MSI-associated CIMP [38].

In genomically stable subtype, *RHOA* mutation was detected [36]. When binding with Guanosine-5'-triphosphate (GTP), RHOA behaves through a great number of downstream effectors, such as ROCK1, mDIA, and protein kinase N. This will lead to actin-myosin-dependent cell contractility and cellular motility [40, 41] and activation of STAT3 to promote carcinogenesis [42, 43]. Except from activating mDia or ROCK1, the *RHOA* mutation Y42C has been confirmed to attenuate the activationof protein kinase N. Because RHOA is strongly associated with cell motility, the *RHOA* mutations might contribute to the invasive growth patterns. In diffuse type GC or genomically stable GC, the lack of cellular cohesion is a hallmark for this diffuse phenotype. Apart from RHOA mutation, an inter-chromosomal translocation called CLDN18-ARHGAP26 fusion gene was identified. ARHGAP26 is a GTPase-activating protein that converts GTP-RHO to GDP-RHO and it is been reported to facilitate cellular motility. CLDN18 is a tight junction component that involves in cell adhesion. This fusion gene thus was thought to correlate with cell metastasis in this kind of molecular subtype.

With the somatic copy-number aberrations (SCNAs), the last group of GC was clustered as CIN subtype. In this subtype, a bunch of genes shows dysregulated, such as *TP53* mutations (in 71% of tumors) as well as *CDH1* somatic mutations (enriched in the genomically stable subtype, about 37% of cases).
