Gastric mucin phenotype indicates aggressive biological behaviour in early differentiated gastric adenocarcinomas following endoscopic treatment

The mucin phenotype classification is based on the mucin marker expression profile. After year 2000, the gastric and intestinal mucin phenotypes were analysed by IHC [15]. The mucin markers MUC5AC, MUC6, MUC2 and CD10 were considered necessary, although there is no consensus on the number of markers that should be used to define a mucin phenotype or the percentage of tumour cells that must be stained [6‐8, 11, 15, 18]. MUC5AC is a secreted mucin expressed in the surface mucous epithelium of normal gastric mucosa. High expression of MUC6 is observed in fundic mucous neck cells and pyloric glands of gastric mucosa. CD10 is a marker for the brush border on the luminal surface of the small intestine. In the normal adult intestine, MUC2 expression is observed in the perinuclear areas of goblet cells.

We showed that the expression of MUC5AC, MUC6, MUC2 and CD10 was detected in 167 (64.98 %), 187 (72.76 %), 164 (63.81 %), and 112 (43.58 %) of the 257 EGCs, respectively. In previous studies, the expression percentages of MUC5AC, MUC6, MUC2 and CD10 in GCs were 55.1-67.5 %, 44.9-64 %, 35.4-49.3 % and 20.6-20.9 %, respectively [19, 20], and for EGCs, the expression of each mucin marker was 68.75-96.8 %, 19.6-71.58 %, 25-62.10 %, and 0-79 %, respectively [6, 11, 21].

Based on the combinations of expression of these markers, the 257 EGCs were classified into the G-type (21 %, 54/257), GI-type (56 %, 144/257), I-type (20 %, 51/257) and N-type (3 %, 8/257); in previous reports, the incidence percentages of each of these mucin phenotypes were found to be 15-41.1 %, 20.3-60.1 %, 18.5-46.6 %, and 3.7-31.6 %, respectively, in advanced GCs [3, 13, 14], and 7.9-36.8 %, 18.8-41.2 %, 15.4-55.56 %, and 0-11.1 %, respectively, in early- stage GCs [7, 11, 19‐25]. Our results were consistent with these studies. The reported expression ranges vary greatly among different investigators, and different markers, antibodies and case groups may account for this discrepancy. Koyama et al. reported that the incidence of G-type was 19.3 % [26], which was similar to that found in the present study (21 %); however, in his report, the incidence of I-type was much higher than that of G-type (43.8 % vs. 19.3 %), as was reported by Fabio et al. [11]. While Tajima et al. [22] reported the opposite result, since in their study, the incidence of the G-type was much higher than that of the I-type (36.8 % vs. 15.4 %). In our study, the incidence of G-type was almost the same as that of I-type (21 % vs. 20 %). Overall, based on our data, much more than half of these cases were classified as G- and GI-G type GC (61.09 %, 157/257), which is much higher than the incidence of I- and GI-I type GC (36.96 %,95/257). A previous report revealed that almost all intramucosal GC cases exhibited the gastric phenotype, including the GI phenotype [15].

The relationship between mucin phenotypes and clinicopathological features was investigated. We found that histology classification (both the JGCA and WHO classification) was closely related to the mucin phenotype. The incidence of I-type was greater than those of the G-, GI- and N-type (100.0 % vs. 79.7 %, 93.1 %, 87.5 %) in differentiated tubular adenocarcinoma. The G-type was histologically significantly correlated with the mixed type (with poorly differentiated/papillary carcinoma). Our data showed that the proportion of G-type carcinoma increased during the transition from solely differentiated type to mixed type carcinoma. Mixed-type early-stage carcinoma more frequently expressed G-type mucin, and G-type tumours were associated with a higher rate of undifferentiated-type tumours than I-type tumours [7, 22].

There were no significant differences between mucin phenotypes and other parameters, including sex, age, margin, colour, tumour size, gross type, depth of invasion, and lymphovascular invasion (P > 0.05). These results are consistent with those of other studies in the literatures [22, 24, 27], and there was no clear correlation between phenotypes and clinicopathological characteristics, including sex, age, tumour size, location, macroscopic features, lymphatic or venous invasion, or lymph node metastasis in the case of the differentiated type [22, 24, 27]. Koseki et al. [7] and Oya et al. [28] reported that the incidence of lymphatic invasion, venous invasion and lymph node metastasis in gastric phenotype carcinomas was significantly higher than that in intestinal phenotype carcinomas. In addition, G-type EGCs were correlated with some distinct macroscopic features, namely, a smaller tumour diameter [15], discoloured surface and non-wavy tumour margins [23, 29]. G-type differentiated adenocarcinomas showed a depressed type, indistinct margins and monotonous colour tone across the mucosal layer, whereas I-type adenocarcinomas had an elevated, distinct margin and a red mucosa [3, 4, 30]. The discrepancy of these results may have been due to heterogeneous components that contained poorly differentiated adenocarcinoma [27].

Intestinal metaplasia has been frequently observed surrounding GC, especially differentiated adenocarcinomas. IM has malignant potential and has been regarded as a precursor of gastric neoplasms. According to Laurén, intestinal-type adenocarcinoma is preceded by metaplastic changes, while diffuse-type adenocarcinoma arises in non-IM gastric mucosa [2]. In the current study, background mucosal IM was observed in 79.9 % of cases among the G-, GI- and I-type EGCs and 87.0 % of cases among the G-type EGCs. 25 % of I-type cases arose from the normal mucosa without IM. IM did not significantly differ among mucin phenotypes (P > 0.05). However, incomplete and complete IM significantly differed with respect to mucin phenotypes (P = 0.004, P = 0,018). A total of 77.78 % (42/54) of G-type and 70.6 % (36/51) of I-type patients had complete IM, which was higher than the rates among GI-type patients (83/114, 57.6 %). The expression of incomplete IM in GI-type EGCs was higher than in G- and I-type EGCs (21.5 % vs. 9.3 % vs. 3.9 %). Our results demonstrated a remarkable difference between mucin phenotypes and the background mucosa. Similar results have been reported by Kabashima et al. [31] and Matsuoka [23]. The mucin phenotype of the carcinoma was independent of mucin phenotypic changes in the surrounding mucosa, and the carcinoma may undergo individual intestinalization. The G-type may imitate the surrounding mucosa, and the carcinomas and the background mucosa have an unstable status, as they commonly possess the hybrid phenotype of the stomach and the small intestine [23, 31].

Mucin phenotypes can indicate biological behaviour in GCs. G-type GCs have increased potential for invasion and metastasis due to infiltrating of deeper layers or more surrounding structures, a higher rate of lymph node metastasis, and poorer prognosis [3, 12, 18, 21]. Even differentiated adenocarcinomas of the G-type had similar outcomes, focused on prognoses, as undifferentiated adenocarcinomas [7‐10]. In our research, six patients underwent additional gastrectomy, and there was no residual tumour or lymph node metastasis. All patients were under close follow-up, and neither recurrence nor metastasis was detected. The mixed type (mixed with poorly differentiated or papillary adenocarcinoma) was mainly of the G-type, which was significantly higher than that of purely differentiated tubular adenocarcinoma (P < 0.05), and the depth of infiltration was deeper (P < 0.05). The G-type group had the highest proportion (11/54, 20.37 %) with poorly differentiated/undifferentiated components, and almost all of them (19/22, 86.36 %) expressed the G- and GI-G types. The mixed type may represent a progressive loss of glandular structure during progression of the cancer from the mucosa to advanced stage, and those with submucosal invasion was a risk factor for lymph node metastasis [7, 22, 32]. Differentiated EGC of G-type frequently changed histologically into signet ring-cell carcinoma or poorly differentiated adenocarcinoma. These results may imply more aggressive biological behaviour and poorer prognosis.