Relationship between intratumor histological heterogeneity and genetic abnormalities in gastric carcinoma with microsatellite instability

The Role and Clinical Implications of the Retinoblastoma (RB)-E2F Pathway in Gastric Cancer

Gastric cancer is the most common malignant tumor in the digestive tract, with very high morbidity and mortality in developing countries. The pathogenesis of gastric cancer is a complex biological process mediated by abnormal regulation of proto-oncogenes and tumor suppressor genes. Although there have been some in-depth studies on gastric cancer at the molecular level, the specific mechanism has not been fully elucidated. RB family proteins (including RB, p130, and p107) are involved in cell cycle regulation, a process that largely depends on members of the E2F gene family that encode transcriptional activators and repressors. In gastric cancer, inactivation of the RB-E2F pathway serves as a core transcriptional mechanism that drives cell cycle progression, and is regulated by cyclins, cyclin-dependent kinases, cyclin-dependent kinase inhibitors, p53, Helicobacter pylori and some other upstream molecules. The E2F proteins are encoded by eight genes (i.e. E2F1 to E2F8), each of which may play a specific role in gastric cancer. Interestingly, a single E2F such as E2F1 can activate or repress transcription, and enhance or inhibit cell proliferation, depending on the cell environment. Thus, the function of the E2F transcription factor family is very complex and needs further exploration. Importantly, the presence of H. pylori in stomach mucosa may affect the RB and p53 tumor suppressor systems, thereby promoting the occurrence of gastric cancer. This review aims to summarize recent research progress on important roles of the complex RB-E2F signaling network in the development and effective treatment of gastric cancer.

Circulating tumor DNA analyses predict progressive disease and indicate trastuzumab-resistant mechanism in advanced gastric cancer

Background

Circulating tumor DNA (ctDNA) isolated from plasma contains genetic mutations that can be representative of those found in primary tumor tissue DNA. These samples can provide insights into tumoral heterogeneity in patients with advanced gastric cancer (AGC). Although trastuzumab has been shown to be effective in first-line therapy for patients with metastatic gastric cancer with overexpression of human epidermal growth factor receptor 2 (HER2), the mechanism of AGC resistance is incompletely understood.

Methods

In this prospective study, we used targeted capture sequencing to analyze 173 serial ctDNA samples from 39 AGC patients. We analyzed cancer cell fractions with PyClone to understand the clonal population structure in cancer, and monitored serial samples during therapy. Serial monitoring of ctDNA using the molecular tumor burden index (mTBI), identified progressive disease before imaging results (mean: 18 weeks).

Findings

We reconstructed the clonal structure of ctDNA during anti-HER2 treatment, and identified 32 expanding mutations potentially related to trastuzumab resistance. Multiple pathways activating in the same patients revealed heterogeneity in trastuzumab resistance mechanisms in AGC. In patients who received chemotherapy, mTBI was validated for the prediction of progressive disease, with a sensitivity of 94% (15/16). A higher mTBI (≥1%) in pretreatment ctDNA was also a risk factor for progression-free survival.

Conclusions

Analysis of ctDNA clones based on sequencing is a promising approach to clinical management, and may lead to improved therapeutic strategies for AGC patients.

Fund

This work was supported by grants from the National International Cooperation Grant (to J.X.; Project No. 2014DFB33160).

A protein and mRNA expression-based classification of gastric cancer

The overall survival of gastric carcinoma patients remains poor despite improved control over known risk factors and surveillance. This highlights the need for new classifications, driven towards identification of potential therapeutic targets. Using sophisticated molecular technologies and analysis, three groups recently provided genetic and epigenetic molecular classifications of gastric cancer (The Cancer Genome Atlas, 'Singapore-Duke' study, and Asian Cancer Research Group). Suggested by these classifications, here, we examined the expression of 14 biomarkers in a cohort of 146 gastric adenocarcinomas and performed unsupervised hierarchical clustering analysis using less expensive and widely available immunohistochemistry and in situ hybridization. Ultimately, we identified five groups of gastric cancers based on Epstein-Barr virus (EBV) positivity, microsatellite instability, aberrant E-cadherin, and p53 expression; the remaining cases constituted a group characterized by normal p53 expression. In addition, the five categories correspond to the reported molecular subgroups by virtue of clinicopathologic features. Furthermore, evaluation between these clusters and survival using the Cox proportional hazards model showed a trend for superior survival in the EBV and microsatellite-instable related adenocarcinomas. In conclusion, we offer as a proposal a simplified algorithm that is able to reproduce the recently proposed molecular subgroups of gastric adenocarcinoma, using immunohistochemical and in situ hybridization techniques.

Regional bias of intratumoral genetic heterogeneity of nucleotide repeats in colon cancers with microsatellite instability

Intratumoral heterogeneity (ITH) may produce regional biases in genotype and phenotype evaluation in a single tumor and may impede proper cancer diagnosis. To evaluate the extent of ITH in colorectal cancer (CRC) with microsatellite instability (MSI), we obtained 4-7 biopsies from 39 CRCs followed by MSI analysis either using the Bethesda MSI evaluation system or Promega system with 5 mononucleotide markers. We found decreased prevalence of MSI (+) by the Promega system compared to the Bethesda system. The overall discordance between the two systems was 54 %. In contrast to the previous studies that had shown discordance only in low MSI (MSI-L), our results showed the discordance not only in MSI-L, but also in high MSI (MSI-H) cases. Among the MSI (+) CRCs, ITH of MSI status was identified in 41.7 % of CRC by the Bethesda system and 22.2 % by the Promega system. In terms of MSI markers, the ITH originated from dinucleotide markers in most cases (69 %), but it originated from mononucleotide markers (31 %) as well. Pooling of DNA from a regional biopsy with MSI (+) with additional biopsies from stable MSI (MSS) showed that this approach was beneficial to increase the sensitivity of MSI detection. Our results indicate that ITH of MSI phenotype by the Bethesda system is more overestimated than previously identified. However, because there was considerable ITH of MSI subtypes and markers even by the Promega system, our data suggest that analysis of MSI status in multiple regional biopsies is needed for a better evaluation of MSI status in CRC.

Intratumoral DNA stem-line heterogeneity in superficial spreading melanoma

BACKGROUND

In primary melanomas, data on the degree of intratumoral heterogeneity to date have been lacking.

OBJECTIVE

Our purpose was to investigate intratumoral DNA stem-line heterogeneity in superficial spreading melanoma (SSM).

METHODS

Multiple measuring fields of 54 SSMs (tumor thickness median 1.60 mm) were studied by DNA image cytometry to obtain data on the number of DNA stem lines per tumor, their ploidy characteristics, and intratumoral distribution. Results were compared with standard histopathological criteria.

RESULTS

Twenty-three of 54 SSMs were found to have two or three distinct proliferating tumor cell stem lines (1.46 +/- 0.57 per tumor). Stem lines appeared spatially separated in 22 of 23 SMMs. At least 3 measuring fields per tumor were necessary to identify all stem lines with a likelihood of 95%. DNA heterogeneity correlated with tumor thickness, but occurred in 5 of 19 cases of pT1 melanoma.

CONCLUSIONS

Primary SSMs can be regarded as potentially clonally unstable with a tendency for spatial separation of tumor cell stem lines.

High frequency of microsatellite instability in intestinal-type gastric cancer in Korean patients

BACKGROUND

Although there have been some reports on microsatellite alterations in gastric cancer, findings are inconsistent regarding the associations between histological classification and microsatellite instability (MSI). In the present study, we attempted to determine whether Lauren's histological subtypes are related with MSI status.

METHODS

Paraffin-embedded tissue samples from 14 diffuse-type and 14 intestinal-type gastric adenocarcinomas were matched up according to patient gender and age. Mononucleotide markers (BAT25 and BAT26) and dinucleotide markers (D2S123, D5S346, and D175S250) were used for MSI analyses. Microsatellite genotypes were categorized in terms of high MSI incidence (MSI-H, >30% positive marker) or low MSI incidence (MSI-L, <30% positive marker). Losses of hMLH1 and hMSH2 protein expression were immunohistochemically studied.

RESULTS

MSI-H was observed in 11 cases (78%) of the 14 intestinal-type cases as compared to 3 (21%) of the 14 diffuse-type cases (p=0.007). In MSI-H tumors, 10 cases (71%) showed losses of hMLH1 protein expression, while 2 cases (14%) in MSI-L tumors showed losses of hMLH1 protein expression (p=0.006).

CONCLUSION

MSI-H tumors are more frequently found in intestinal-type gastric cancer, which suggests the possibility that there are different pathogenic pathways in gastric carcinogenesis according to histologic type.

Identification of a new target region by loss of heterozygosity at 5p15.33 in sporadic gastric carcinomas: genotype and phenotype related

Chromosome 5p, especially 5p15, involves in several cancers. To investigate its role in gastric cancer, we analyzed 46 intestinal-type and 34 diffuse-type gastric cancers by Loss of heterozygosity (LOH). We found a high frequent LOH at 5p15.33, and identified a minimal 2.7 cM candidate region of tumor suppressor gene, encompassing four loci (D5S417, D5S2849, D5S1492 and D5S2088). In total 80 cases, the highest LOH occurs at D5S2849 (35.19%). In intestinal-type cases, the highest LOH frequency is 50%, whereas in diffuse-type cases, the highest is only 16.67%. By statistical analysis we also observed an obvious genotype-phenotype correlation on 5p15.3 (P<0.01).

Prognostic implications for gastric carcinoma based on loss of heterozygosity genotypes correlation with clinicopathologic variables

In this study we used polymorphic DNA markers to examine 38 patients with gastric carcinoma for loss of heterozygosity (LOH) on five chromosomal arms. The aims were to compare LOH genotyping with the clinicopathologic variables and to identify some genetic differences between early (EGC) and advanced gastric carcinoma (AGC). The frequency of LOH was found in 27 of 38 (71.1%) cases with a low-level LOH in 17 (44.7%) and a high-level LOH (LOH-H) in 10 (26.3%). There was statistical significance found in the differentiation of cells (WD/MD vs. PD [well or moderately differentiated vs. poorly differentiated]), metastasis (absent vs. present), and tumor-node-metastasis stage (I/II vs. III/IV) based on LOH genotyping. The frequency of LOH in the markers of chromosome 6 revealed a significant difference between the early and advanced stages (P=0.043). However, there were no differences in each chromosome or in the number of affected chromosomes with an allelic loss between the histologic types EGC and AGC, except for the frequency of the markers on chromosome 22. These findings suggest that LOH genotyping may be another independent prognostic indicator in gastric carcinoma, that LOH-H, particularly the LOH on chromosome 6, could be associated with an unfavorable prognosis, while the LOH on chromosome 22 may be related to the histologic progression of gastric carcinoma.

Preoperative genetic diagnosis of gastric carcinoma based on chromosomal loss and microsatellite instability

The degree of chromosomal losses and the presence of microsatellite instability (MSI) in gastric carcinomas have been categorized into low-risk (low-level loss and MSI) and high-risk (baseline- and high-level losses) genotypes. With the aim of making a preoperative diagnosis, this study confirmed the stem line genotype that is common over an entire tumor as well as in a single biopsy specimen. Biopsy specimens were obtained from 91 gastric carcinoma patients and examined for their microsatellite genotypes using a panel of 41 microsatellite markers on 8 cancer-associated chromosomes. The genotype of the biopsy specimens was compared with that of a surgical specimen, which had been multifocally examined for its intratumoral heterogeneity. Of the 91 pairs of biopsy and surgical specimens, 87 (96%) containing either the same (60 cases) or a similar (17 cases) number of chromosomal losses were categorized into the same microsatellite genotype, and the remaining 4 pairs (4%) were classified into a different genotype. The surgical specimens showed that an extraserosal invasion and lymph node metastasis are frequently associated with either a high-level (4 or more) of chromosomal losses irrespective of the tumor size (73% and 85%) or the large carcinomas > 5 cm in diameter irrespective of the tumor genotype (76% and 83%). The status of the extraserosal invasion and lymph node metastasis (0.691 and 0.802 receiver operating characteristic areas, respectively) predicted by the biopsy genotype and the tumor size corresponded closely to the surgical pathology results. Therefore, the extent of chromosomal losses and the presence of an MSI determined on a biopsy specimen will provide valuable information for making a preoperative genetic diagnosis of a gastric carcinoma.

Genetic classification of intestinal-type and diffuse-type gastric cancers based on chromosomal loss and microsatellite instability

The stage of gastrointestinal cancers has been correlated with the loss of heterozygosity (LOH) and the presence of microsatellite instability (MSI). This study delineated the category of the extent of LOH and the presence of MSI for the genetic classification of the intestinal-type and diffuse-type gastric cancers that frequently exhibited intralesional heterogeneity. A total of 390 tumor foci from 116 gastric cancers were screened using a panel of 40 microsatellite markers on chromosomes 3p, 4p, 5q, 8p, 9p, 13q, 17p, and 18q. One MSI-positive gastric cancer accompanying a LOH-positive focus and 19 gastric cancers with an intralesional LOH heterogeneity with a similar extent were identified. One hundred and sixteen gastric cancers were categorized based on the presence of MSI (16 cases) and the extent of LOH (100 cases) in a representative focus. A large fraction of MSI-positive cases was found in the intestinal-type (94%), late-onset (mean age 68 years), early-stage (75%) diseases (P<0.05). The diffuse-type gastric cancers with a baseline-level loss involving zero or one chromosome showed a correlation with the earlier onset (mean age 45 years), advanced-stage (81%) diseases (P<0.0001). In both the intestinal-type and diffuse-type gastric cancers, a low-level loss involving 0-3 chromosomes (2-3 chromosomes in the diffuse type) and a high-level loss involving 4-7 chromosomes were predominant in the early (69%) and advanced (86%) stages, respectively (P<0.0001), at similar mean ages of onset (61 years and 65 years). Gastric cancers were categorized into low-risk (MSI and low-level LOH) and high-risk (baseline-level and high-level LOH) genotypes displaying cell-type- and age-dependent oncogenicity.

Lymphocyte-rich gastric cancer: associations with Epstein-Barr virus, microsatellite instability, histology, and survival

Lymphocyte-rich gastric carcinomas may have a better prognosis than cancers without a pronounced host inflammatory response. Two subsets of gastric cancer-Epstein-Barr virus-positive and microsatellite instability high-have been associated with a lymphocyte-rich phenotype. We assessed relationships between tumor-infiltrating lymphocytes, Epstein-Barr virus status, microsatellite instability status, and cancer-specific survival in 110 resected gastric cancers. Seven patients had Epstein-Barr virus-positive cancer, including 4 (3.7%) of 107 consecutive patients. Tumors from 17 patients (16%) were designated microsatellite instability high on the basis of negative immunohistochemical staining for MLH1; all tumors had intact expression of MSH2 and MSH6. Epstein-Barr virus-positive cancers had increased tumor-infiltrating lymphocytes compared with Epstein-Barr virus-negative cancers (median 450/10 HPF versus 21/10 HPF, P <.001). Microsatellite instability-high cancers also had increased tumor-infiltrating lymphocytes compared with non-microsatellite instability-high cancers (median 150/10 HPF versus 20/HPF, P <.001). Microsatellite instability-high cancers affected older patients and were more likely to be intestinal in the Lauren classification and expanding in the Ming classification. By univariate analysis, decreased risk of death from gastric cancer was significantly associated with low tumor stage, expanding growth pattern, increasing tumor-infiltrating lymphocyte count, and microsatellite instability-high status. High tumor-infiltrating lymphocyte count and microsatellite instability-high status retained statistical significance as favorable prognostic factors after adjustment for tumor stage in multivariate analysis. Tumor-infiltrating lymphocyte count retained statistical significance as a favorable prognostic factor after adjustment for microsatellite instability-high status; but microsatellite instability-high status did not remain a significant independent prognosticator after adjustment for tumor-infiltrating lymphocyte count. The association between microsatellite instability-high cancers and high tumor-infiltrating lymphocyte counts may account for the association of microsatellite instability-high gastric cancers with improved survival.

Gastric cancer and other organ cancer history in the parents of patients with gastric cancer

A case-control study in a population from the Eastern Black Sea region of Turkey was performed to evaluate the risk of gastric cancer development in patients with gastric cancer reporting gastric cancer or other organ cancer history in their parents. Gastric cancer and/or other organ cancer history in the parents were found in 215 of 1240 patients with gastric cancer versus 73 of 1240 controls (odds ratio (OR) 3.35, P<0.001). The frequency of gastric cancer history among the parents was significantly different between the patients and the controls (148 versus 25, respectively; OR 6.59, P<0.001). The frequency of other organ cancer history was not significantly different between the patients and the controls (63 versus 48, respectively; OR 1.33, P>0.05). The frequency of gastric cancer history was significantly higher than the frequency of other organ cancer history in the parents of the patients (OR 4.51, P<0.001). Gastric cancer history was significantly higher in first- to third-degree relatives of the patients reporting gastric cancer and/or other organ cancer history in their parents than in the controls (OR 14.72, P<0.001). Familial clustering of gastric cancer defined by the presence of at least four cancer cases in family members, including parents, was reported by 12% of the patients. Overall, the result of this study of gastric cancer and other organ cancer history in the parents of patients with gastric cancer may suggest a genetic susceptibility for gastric carcinoma. A high risk of gastric cancer occurred in subjects reporting a gastric cancer history in their parents, and there was a higher predisposition to gastric cancer compared with other organ cancer in relatives and a familial clustering of the disease.

Gene-specific oxidative DNA damage in Helicobacter pylori-infected human gastric mucosa

To study the status of oxidative DNA damage in Helicobacter pylori infection in more detail, we examined oxidative DNA damage to individual genes by determining the loss of PCR product of a targeted gene before and after gastric mucosal DNA was treated with 8-hydroxyguanine glycosylase, which cleaves DNA at the 8-hydroxyguanine residues. The results showed that, of the 5 genes tested, p53, insulin-like growth factor II receptor and transforming growth factor-beta receptor type II showed significant oxidative DNA damage in H. pylori-positive tissues and that the BAX and beta-ACTIN genes were relatively undamaged. These results suggest that in H. pylori infection, oxidative DNA damage does not occur homogeneously throughout the genomic DNA but, rather, in a gene-specific manner. We conclude that the progressive accumulation of preferential oxidative DNA damage in certain genes, such as p53, likely contributes to gastric carcinogenesis.

Microsatellite instability in synchronous gastric carcinomas

Synchronous gastric carcinomas are found in 4% to 10% of all gastric carcinomas, and the tumor multiplicity is believed to be related to genetic predisposition. To investigate the role of mismatch repair error in synchronous gastric carcinomas, we analyzed the microsatellite instability (MSI) status of 101 cancers from 48 gastrectomy specimens and compared them with 149 solitary gastric carcinomas. Multiple synchronous gastric carcinomas are characterized by slightly older age, predominance in males, early stage and lower lymph node metastasis. Among the 48 cases, 8 (18 lesions) were associated with a gastric adenoma (type I) and 40 (83 lesions) were not associated with a gastric adenoma (type II). The MSI+ rate was 50% in the type I and 8.4% in the type II synchronous gastric carcinomas (p < 0.001), while that of solitary gastric carcinomas was 9.4%. In addition, the frameshift mutation rates of the TGF-betaRII, BAX and hMSH3 genes in the type I synchronous carcinomas were higher than those in the type II synchronous carcinomas. These findings indicate that a defect in the mismatch repair system might play a role in the carcinogenesis of a minor subset of multiple gastric carcinomas associated with adenomas.

Microsatellite instability and alteration of E2F-4 gene in adenosquamous and squamous cell carcinomas of the stomach

Microsatellite instability (MSI) due to defective DNA mismatch repair (MMR) is a form of genomic instability underlying the tumorigenesis of various human neoplasms. To evaluate the roles of MSI in the pathogenesis of gastric carcinomas with squamous differentiation, 17 primary stomach cancer patients (15 adenosquamous and two squamous cell carcinomas) were examined for MSI frequency using five microsatellite markers and the criteria for MSI recommended by the National Cancer Institute Workshop. The molecular causes and consequences of MSI in these neoplasms were further researched through the immunohistochemistry of MMR proteins and the mutational analysis of cancer-associated genes targeted by MSI, respectively. Two of the 17 (12%) cases demonstrated MSI at the most examined loci and were classified as having high level MSI (MSI-H). These tumors also exhibited frame-shift mutations at mononucleotide repeats in the target genes, including TGFbetaRII, IGFIIR, BAX, and hMSH6. It is interesting to note that the mutations of the serine (AGC)13 repeats within the E2F-4 gene were found only in the squamous cell carcinoma portions of them, whereas such alterations were not detected in any of the adenocarcinomatous portions. This suggests that E2F-4 might be implicated in the transformation of adenocarcinoma into squamous cell carcinoma and further studies are needed to understand its role in squamous differentiation.

Prognostic implications of microsatellite genotypes in gastric carcinoma

Microsatellite alterations such as loss of heterozygosity (LOH) and microsatellite instability (MSI) are observed in most (70% to 80%) gastric carcinomas. To determine whether the microsatellite genotypes are correlated with clinicopathological features, 118 patients with gastric carcinomas were examined by using polymorphic microsatellite markers for LOH on 5 gastric cancer-associated chromosome arms and non-polymorphic BAT markers for MSI. Microsatellite genotypes were categorized as high-frequency MSI (MSI-H), high-level LOH (LOH-H), low-level LOH (LOH-L) and LOH non-detectable (LOH-N). A significant fraction of the MSI-H, LOH-H and LOH-L types was observed in intestinal-type gastric carcinomas, whereas the LOH-N type was highly associated with diffuse-type tumors (p = 0.00162). There was a close relationship between microsatellite genotype and TNM (tumor-node-metastasis) stage (p = 0. 001). Univariate analysis showed that patients of LOH-H or LOH-N types and those of MSI-H or LOH-L types correlated with poor and favorable survival, respectively, not only in all tumor stages (p = 0.0001) but also in stages II and III (p = 0.0271). It is likely that the major genotypes of gastric carcinomas can be placed into at least 4 microsatellite categories, thus allowing the construction of a comprehensive genetic classification useful for the prediction of diverse clinical courses.

Molecular mechanisms of inactivation of TGF-beta receptors during carcinogenesis

Signals from the TGF-betas are mediated by the TGF-beta receptors and their substrates, the Smad proteins. Inactivation of either of the two transmembrane serine/threonine kinases called the TGF-beta type I and type II receptors is now known to underlie a wide variety of human pathologies including, especially carcinogenesis. Numerous studies have now demonstrated that the TGF-beta receptor complex and its downstream signaling intermediates constitute a tumor suppressor pathway. We review here a specific pathway of mutational inactivation of the TGF-beta type II receptor resulting from microsatellite instability and demonstrate that, by contrast, the most common mechanism of loss of expression of the TGF-beta type II receptor involves transcriptional repression. This provides a new target for therapeutic intervention.