RAB32 hypermethylation and microsatellite instability in gastric and endometrial adenocarcinomas

The recently described gene, RAB32, is a ras proto-oncogene family member that encodes an A-kinase-anchoring protein. RAB32 has been found to be frequently hypermethylated in microsatellite instability-high (MSI-H) colon cancers. We sought to determine the prevalence of RAB32 hypermethylation in gastric and endometrial adenocarcinomas, the 2 other major tumor types in which MSI-H is common. Moreover, we delineated the association of RAB32 hypermethylation with microsatellite instability (MSI) and hMLH1 hypermethylation. MSI status and hypermethylation of the RAB32 and hMLH1 genes were studied in paired primary normal and tumor tissues from 48 patients with gastric cancer. An additional 80 endometrial cancer patients were studied for RAB32 methylation and MSI status. Thirteen (27%) of 48 gastric cancers demonstrated evidence of RAB32 hypermethylation. MSI status was determined in 46 of the tumors, with 7 (100%) of 7 MSI-H tumors, 1 (33%) of 3 MSI-low (MSI-L) tumors and 4 (11%) of 36 microsatellite-stable (MSS) tumors found to harbor RAB32 hypermethylation. RAB32 methylation was significantly associated with intestinal type histology and concomitant hMLH1 hypermethylation in gastric cancer. In contrast, RAB32 methylation occurred in only 1 of 80 endometrial cancers, including 20 MSI-H, 8 MSI-L and 52 MSS tumors. Hypermethylation of hMLH1 was noted in 16 (20%) of 80 endometrial tumors. We conclude that although RAB32 methylation is rare in endometrial cancers, it is strongly associated with hMLH1 hypermethylation and MSI in gastric adenocarcinomas. Given its similar involvement in colon cancer, RAB32 inactivation may represent a component of the oncogenic pathway of microsatellite-unstable gastrointestinal adenocarcinomas.     

Genetic and clinical features of human pancreatic ductal adenocarcinomas with widespread microsatellite instability

The incidences of microsatellite instability (MSI) and underlying DNA mismatch repair (MMR) defects in pancreatic carcinogenesis have not been well established. We analyzed 100 sporadic and 3 hereditary pancreatic ductal adenocarcinomas for MSI, and high-frequency MSI (MSI-H) and low-frequency MSI (MSI-L) tumors were further analyzed for frameshift mutations of possible target genes and for promoter methylation and mutation of DNA MMR genes, including hMLH1, hMSH2, hMSH3, and hMSH6 genes. Among the 100 sporadic tumors, 13 (13%) were MSI-H, 13 (13%) were MSI-L, and 74 (74%) were microsatellite stable (MSS) tumors. All of the three hereditary tumors from hereditary nonpolyposis colorectal cancer (HNPCC) patients were MSI-H. MSI-H tumors were significantly associated with poor differentiation and the presence of wild-type K-RAS and p53 genes. Patients with MSI-H tumors had a significantly longer overall survival time than did those with MSI-L or MSS tumors (P = 0.0057). Frameshift mutations of hMSH3, hMLH3, BRCA-2, TGF-beta type II receptor, and BAX genes were detected in MSI-H tumors. Hypermethylation of the hMLH1 promoter was observed in 6 (46%) of the 13 sporadic MSI-H tumors but not in any of the 3 hereditary MSI-H tumors or 13 MSI-L tumors. All of the 3 HNPCC cases had germ-line hMLH1 mutation accompanied by loss of heterogeneity or other mutation in the tumor. Our results suggest that pancreatic carcinomas with MSI-H represent a distinctive oncogenic pathway because they exhibit peculiar clinical, pathological, and molecular characteristics. Our results also suggest the principal involvement of epigenetic or genetic inactivation of the hMLH1 gene in the pathogenesis of pancreatic carcinoma with MSI-H.     

Microsatellite instability and promoter hypermethylation in colorectal cancer in India

Microsatellite instability (MSI) is an important factor in tumor development and is a hypermutable phenotype caused by the loss of DNA mismatch repair activity. It is important to identify tumors with microsatellite instability as the patients have a better prognosis and differ with response to chemotherapy. Limited data are available on the incidence of MSI in Indian colorectal cancers (CRCs). The objectives of this study were to identify the extent of MSI in Indian CRC patients below 50 years and to determine promoter methylation status of hMLH1 and hMSH2 in relation to MSI. A total of 450 patients were diagnosed with CRC, out of which 91 individuals were recruited as per Bethesda guidelines and were tested for instability by the NCI-recommended Bethesda panel (BAT25, BAT26, D2S123, D5S346, and D17S2720) using labeled primers. The fragments were separated and analyzed on a Beckman GeXP sequencer. Promoter methylation status was determined by restriction enzyme digestion and PCR. MSI (high and low) was seen in 48.4 % (44/91) of CRC patients, out of which microsatellite instability-high (MSI-H) was detected in 13.2 % (12/91) and microsatellite instability-low (MSI-L) in 35.2 % (32/91) and the rest were microsatellite stable (MSS), 51.6 % (47/91). Majority of the MSI-H tumors were adenocarcinomas (10/12), in the rectum (8/12), and moderately or poorly differentiated (12/12). Promoter hypermethylation was seen in 75 % of the MSI-H, 56.24 % of MSI-L, and only 23.4 % of MSS individuals. MSI (high and low) was associated with 48.4 % of CRC patients, and a significantly higher proportion of promoter hypermethylation of hMLH1 and hMSH2 genes was associated with instable tumors.     

Morphological and molecular heterogeneity within nonmicrosatellite instability-high colorectal cancer

Colorectal cancer (CRC) has traditionally been classified into two groups: microsatellite stable/low-level instability (MSS/MSI-L) and high-level MSI (MSI-H) groups on the basis of multiple molecular and clinicopathologic criteria. Using methylated in tumor (MINT) markers 1, 2, 12, and 31, we stratified 77 primary CRCs into three groups: MINT++ (>2), MINT+ (1-2), and MINT- (0 markers methylated). The MSS/MSI-L/MINT++ group was indistinguishable from the MSI-H/MINT++ group with respect to methylation of p16(INK4a), p14(ARF), and RIZ1, and multiple morphological features. The only significant difference between MSI-H and non-MSI-H MINT++ cancers was the higher frequency of K-ras mutation (P < 0.004) and lower frequency of hMLH1 methylation (P < 0.001) in the latter. These data demonstrate that the separation of CRC into two nonoverlapping groups (MSI-H versus MSS/MSI-L) is a misleading oversimplification.     

Distinct methylation pattern and microsatellite instability in sporadic gastric cancer.

Aberrant 5' CpG island methylation is an alternative mechanism of gene inactivation during the development of cancer as demonstrated for several tumor-suppressor genes. Also, marked relationship of microsatellite instability (MSI) and DNA methylation has been reported in sporadic colorectal cancer, which is a result of epigenetic inactivation of hMLH1 in association of promoter hypermethylation. In the present study, we investigated the 5' CpG island hypermethylation of hMLH1, E-cadherin and p16 in 61 primary gastric cancers (GCs) by using combined bisulfite restriction analysis (COBRA) and methylation-specific PCR (MSP), and their MSI status. Of 61 GCs investigated, 5 (8.1%) tumors presented hMLH1 methylation, 16 (26.2%) and 25 (40.9%) showed E-cadherin and p16 methylation respectively, and 8 (13.1%) presented high-frequency MSI (MSI-H). Of the 8 MSI-H patients, 5 presented hMLH1 methylation, whereas no low-frequency MSI (MSI-L) and microsatellite stable (MSS) cases exhibited hMLH1 methylation (5/8 vs. 0/43, p < 0.00001). Furthermore, these patients also presented E-cadherin and p16 hypermethylation. Our data showed a significant correlation between hMLH1 methylation and MSI in GC, and suggested that a common mechanism of aberrant de novo methylation can be postulated in these cancers.     

HMSH6 alterations in patients with microsatellite instability-low colorectal cancer

Two microsatellite instability (MSI) phenotypes have been described in colorectal cancer (CRC): MSI-H (instability at >30% of the loci examined) and MSI-L (MSI at 1-30% of the loci examined). The MSI-H phenotype, observed in both hereditary nonpolyposis colon cancer-associated CRC and approximately 15% of sporadic CRC, generally results from mutational or epigenetic inactivation of the DNA mismatch repair (MMR) genes hMSH2 or hMLH1. The genetic basis for the MSI-L phenotype, however, is unknown. Several other proteins, including hMSH3 and hMSH6, also participate in DNA MMR. Inactivating mutations of MSH6 in yeast and human tumor cell lines are associated with an impaired ability to repair single-base mispairs and small insertion-deletion loops but not large insertion-deletion loops. This suggests that hMSH6 mutations are more likely to be associated with a MSI-L phenotype than a MSI-H phenotype in CRC. To explore this possibility, we screened tumors from 41 patients with MSI-L CRC for hMSH6 mutations with conformation-sensitive gel electrophoresis (CSGE) and for hMSH6 protein expression by immunohistochemistry. Alterations found with CSGE were confirmed by DNA sequencing of normal and tumor tissue. One somatic (Asp389Asn) and 15 germ-line changes were found. Of the 15 germ-line changes, 9 were found in an intron (none involving splice junctions), and 6 were found in an exon (Gly39Glu, Leu395Val, and 4 silent alterations). Immunohistochemical staining for hMSH6 performed on 34 of the 41 tumors revealed strong nuclear hMSH6 expression in all of the cases. Overall, our results suggest that hMSH6 mutations do not play a major role in the development of sporadic CRC with a MSI-L phenotype.     

Detection of Frameshift Mutations of the Transforming Growth Factor β Receptor Ⅱ in Gastric Cancers with Microsatellite Instability

OBJECTIVE To study the relationship among microsatellite instability （MSI）, frameshift mutations （FM） of the transforming growth factor β receptor Ⅱ （TGF β R Ⅱ）, methylation state of the hMLH1 promoter and hMLH1 protein expression level in gastric cancers, and to explore their relationship to gastric carcinogenesis. METHODS DNA was isolated from 101 gastric specimens and 5 microsatellite loci were detected. PCR, electrophoresis on denatured polyacrylamide gels and silver staining were performed to detect the MSI. The FMs of TGFβR Ⅱ were also screened with the same method. HMLH1 methylation was analyzed by methylation specific PCR （MSP） and sequencing. HMLH1 protein expression was detected using immunohistochemistry. RESULTS The incidence of MSIs was 53.7% and 0% in the cancers and normal tissues, respectively, with the frequency of MSIs being significantly higher in the gastric cancers compared to the normal gastric tissues （P〈0.05）. The frequency of hMLH1 methylation was 41.5%（17/41） in the gastric cancers and 0%（0/60） in the normal group. Decreased hMLH1 expression was observed in 94.1%（16/17） of cases exhibiting methylation. FMs of TGFβR Ⅱ were identified in 5 （62.5%） of the 8 samples with MSIH. In contrast, FMs were not found in MSI-L or microsatellite stable （MSS） cases. CONCLUSION MSIs and FMs of TGFβR Ⅱ may play an important role in gastric carcinogenesis. HMLH1 methylation is an important modification of the DNA which results in inactivation of hMLH1 and mismatch repair defects which lead to MSls and FMs of TGFβR Ⅱ.     

High thymidylate synthase expression in colorectal cancer with microsatellite instability: implications for chemotherapeutic strategies.

Colon cancers displaying microsatellite instability (MSI) are clinically less aggressive. Based on in vitro studies and recent clinical data, cancers displaying MSI do not respond to 5-fluorouracil (5-FU). The reasons why MSI tumors are clinically less aggressive and do not respond to 5-FU-based therapies have not been fully elucidated. PURPOSE: We investigated biomolecular markers in an attempt to explain the different clinical behavior and chemotherapeutic responses of MSI and non-MSI colon cancers. EXPERIMENTAL DESIGN: One hundred ninety-two sporadic colon cancers were tested for MSI with five mononucleotide markers and methylation of the hMLH1 promoter. Slides were stained for thymidylate synthase (TS), p53, MDM2, p21(WAF1/CIP1), beta-catenin, vascular endothelial growth factor, hMLH1, hMSH2, and hMSH6. Tumors were regarded as having wild-type, functional p53 (Fp53) if reduced expression of p53 and positive MDM2 and p21(WAF1/CIP1) expressions were found. RESULTS: Of the cases, 12.5% were MSI-H (at least two markers mutated). Of MSI-H cases, 83.3% were characterized by a complete loss of at least one of the mismatch repair proteins, in particular loss of hMLH1 by promoter hypermethylation. MSI-H colon cancers showed higher expression of TS compared with MSS (no mutated markers)/MSI-L (one mutated marker) colon cancers (66.6% for MSI-H versus 14.8% MSS/MSI-L; P < 0.0001); 20.8% of MSI-H cases showed high expression of the vascular endothelial growth factor, compared with 45.8% MSS/MSI-L colon cancers (P = 0.0005); 45.8% MSI-H cases had Fp53 compared 11.9% MSS/MSI-L cases (P < 0.0001). CONCLUSIONS: About 12% of colon cancers display MSI mostly due to lack of hMLH1 resulting from promoter hypermethylation. These tumors have high expression of TS and retain fully functional p53 system. Thus, these data suggest why sporadic hMLH1-defective colon cancers often do not respond to 5-FU.     

Microsatellite instability in inflammatory bowel disease-associated neoplastic lesions is associated with hypermethylation and diminished expression of the DNA mismatch repair gene, hMLH1

Twelve to 15% of sporadic colorectal cancers display defective DNA mismatch repair (MMR), manifested as microsatellite instability (MSI). In this group of cancers, promoter hypermethylation of the MMR gene hMLH1 is strongly associated with, and believed to be the cause of, MSI. A subset of colorectal neoplastic lesions arising in inflammatory bowel disease (IBD) is also characterized by MSI. We wished to determine whether hMLH1 hypermethylation was associated with diminished hMLH1 protein expression and MSI in IBD neoplasms. We studied 148 patients with IBD neoplasms, defined as carcinoma or dysplasia occurring in patients with ulcerative colitis or Crohn's disease. MSI was evaluated using multiplex fluorescent PCR to amplify loci D2S123, BAT-25, BAT-26, D5S346, and D17S250 in all cases. Lesions were characterized as high-frequency MSI (MSI-H) if they manifested instability at two or more loci, low-frequency MSI (MSI-L) if unstable at only one locus, or MS-stable (MSS) if showing no instability at any loci. Methylation-specific PCR was performed to determine the methylation status of the hMLH1 promoter region. hMLH1 protein expression was also evaluated by immunohistochemistry. Thirteen (9%) of 148 neoplasms arising in IBD were MSI-H, comprising 11 carcinomas and 2 dysplastic lesions. Sixteen additional lesions (11%) were MSI-L, comprising 11 carcinomas and 5 dysplastic lesions. The remaining 118 neoplasms (80%) were MSS. Six (46%) of 13 MSI-H, 1 (6%) of 16 MSI-L, and 4 (15%) of 27 MSS lesions showed hMLH1 hypermethylation (P = 0.013). Diminished hMLH1 protein expression in neoplastic cell nuclei relative to surrounding normal cell nuclei was demonstrated immunohistochemically in four of four (100%) hypermethylated lesions tested. In IBD neoplasia, hMLH1 promoter hypermethylation occurs frequently in the setting of MSI, particularly MSI-H. Furthermore, hMLH1 hypermethylation and MSI are strongly associated with diminished hMLH1 protein expression in IBD neoplasms. These findings suggest that hMLH1 hypermethylation causes defective DNA MMR in at least a subset of IBD neoplasms.     

Sporadic Early Onset Colorectal Cancer in Pakistan: a Case- Control Analysis of Microsatellite Instability

Background: Early onset sporadic colorectal cancer (CRC) is a biologically and clinically distinct entity hypothesized to exhibit differences in histological features and microsatellite instability (MSI) as compared to typical onset CRC. This study compared the MSI status, mismatch repair enzyme deficiency and clinicopathological features of early onset (aged 45 years) with controls (>45 years). Materials and Methods: A total of 30 cases and 30 controls were analyzed for MSI status using the Bethesda marker panel. Using antibodies against hMLH1, hMSH2 and hMSH6, mismatch repair protein expression was assessed by immunohistochemistry. Molecular characteristics were correlated with clinicopathological features. Results: The early onset sporadic CRCs were significantly more poorly differentiated tumors, with higher N2 nodal involvement and greater frequency of signet ring phenotype than the typical onset cases. MSI was observed in 18/30 cases, with 12/18 designated as MSI-high (MSI-H) and 6/18 designated as MSI-low (MSI-L). In the control group, 14 patients exhibited MSI, with 7 MSI-H and 7 MSI-L. MSI tumors in both cases and controls exhibited loss of hMLH1, hMSH2 and hMSH6. MSS tumors did not exhibit loss of expression of MMR proteins, except hMLH1 protein in 3 controls. No statistically significant difference was noted in MSI status or expression of MMR proteins in cases versus controls. Conclusions: Microsatellite status is comparable between early and typical onset sporadic CRC patients in Pakistan suggesting that differences in clinicopathological features between these two subsets are attributable to other molecular mechanisms.     

Sporadic colorectal adenocarcinomas with high-frequency microsatellite instability

BACKGROUND: Widespread microsatellite instability (MSI) occurs in nearly 15% of sporadic colorectal cancers. Large bowel carcinomas with high-frequency MSI (MSI-H) (instability at > or = 30% of microsatellite loci) are believed to display distinctive pathologic features and to behave less aggressively than microsatellite-stable (MSS) tumors and carcinomas with low-frequency MSI (MSI-L) (instability at < 30% of microsatellite loci). The aim of the current study was to accurately define the clinicopathologic and biologic features of MSI-H sporadic colorectal carcinomas. METHODS: MSI status was evaluated in 216 large bowel adenocarcinomas using polymerase chain reaction (PCR) and 6 microsatellite markers. Tumors that showed instability with at least two microsatellite markers were classified as MSI-H, whereas the other tumors were classified as MSI-L (instability at one locus) or MSS (no instability). Expression of p53, hMLH1, and hMSH2 gene products was determined by immunohistochemistry, and DNA ploidy pattern was determined by flow cytometry. The prognostic significance of MSI status was assessed by univariate and multivariate survival analyses. RESULTS: The significantly different pathologic features of MSI-H carcinomas were proximal location; large size; mucinous and medullary histotype; poor differentiation; expanding pattern of growth; more frequent Crohn-like conspicuous lymphoid reaction; and low incidence of extramural vein invasion. Most MSI-H tumors were DNA diploid (33 of 40 tumors; 82.5%) and p53 negative (34 of 44 tumors; 77.3%). Conversely, DNA aneuploidy and p53 overexpression were observed in 82.3% (130 of 158 tumors; P < 0.0001) and 54.1% (93 of 172 tumors; P = 0.0002) of MSI-L/MSS tumors, respectively. Loss of hMLH1 or hMSH2 expression was detected in a high fraction of MSI-H carcinomas (86. 0%). Patients with MSI-H tumors showed a better clinical outcome than patients with MSI-L/MSS tumors (P = 0.0017). Furthermore, in multivariate analysis that included conventional clinicopathologic parameters, MSI status, and p53 expression as covariates, MSI status was a significant independent prognostic indicator of disease specific survival. CONCLUSIONS: Assessment of MSI status is an essential step in the genetic characterization of large bowel carcinomas and identifies a subset of tumors with distinct clinical, pathologic, and biologic features.     

胃癌的微卫星不稳定性与hMLH1基因启动子甲基化的关系

背景与目的:由于细胞错配修复功能缺陷而导致基因组微卫星序列高度不稳定是遗传性非息肉性大肠癌发生的主要原因。以往的研究表明胃癌组织也有错配修复蛋白表达的缺失,但错配修复基因突变频率却很低;而启动子的甲基化是肿瘤抑癌基因失活的主要途径,也可能是错配修复基因功能丧失的主要原因。本研究拟通过对胃癌组织的微卫星不稳定性的分析及对hMLH1基因启动子甲基化和蛋白表达的检测,对胃癌发病的分子机制进行探讨。方法:从52例胃癌患者的癌组织及其周围组织提取DNA,PCR扩增基因组的5个微卫星位点BAT-26、D17S261、D3S1283、D2S123和D3S1611,毛细管电泳后,判定胃癌组织的微卫星不稳定性;免疫组织化学方法检测hMLH1蛋白的表达;酶切法检测hMLH1基因启动子甲基化。结果:52例胃癌标本中微卫星高度不稳定13例,低度不稳定2例,稳定37例。微卫星高度不稳定的13例胃癌组织中,均检测到hMLH1基因启动子甲基化(100.0%);微卫星低度不稳定和微卫星稳定的39例胃癌组织中,hMLH1基因启动子甲基化仅1例(2.6%),前者发生率高于后者,两者之间有显著性差异(P<0.01)。微卫星高度不稳定的13例胃癌的癌旁组织中,hMLH1基因启动子甲基化6例(46.2%),而微卫星低度不稳定和微卫星稳定39例胃癌的癌旁肿瘤组织标本中,hMLH1基因启动子甲基     

Immunohistochemistry versus microsatellite instability testing in phenotyping colorectal tumors.

PURPOSE: To compare microsatellite instability (MSI) testing with immunohistochemical (IHC) detection of hMLH1 and hMSH2 in colorectal cancer. PATIENTS AND METHODS: Colorectal cancers from 1,144 patients were assessed for DNA mismatch repair deficiency by two methods: MSI testing and IHC detection of hMLH1 and hMSH2 gene products. High-frequency MSI (MSI-H) was defined as more than 30% instability of at least five markers; low-level MSI (MSI-L) was defined as 1% to 29% of loci unstable. RESULTS: Of 1,144 tumors tested, 818 showed intact expression of hMLH1 and hMSH2. Of these, 680 were microsatellite stable (MSS), 27 were MSI-H, and 111 were MSI-L. In all, 228 tumors showed absence of hMLH1 expression and 98 showed absence of hMSH2 expression: all were MSI-H. CONCLUSION: IHC in colorectal tumors for protein products hMLH1 and hMSH2 provides a rapid, cost-effective, sensitive (92.3%), and extremely specific (100%) method for screening for DNA mismatch repair defects. The predictive value of normal IHC for an MSS/MSI-L phenotype was 96.7%, and the predictive value of abnormal IHC was 100% for an MSI-H phenotype. Testing strategies must take into account acceptability of missing some cases of MSI-H tumors if only IHC is performed.     

hMLH1 and hMSH2 expression in human hepatocellular carcinoma

The role of microsatellite instability (MSI) in the pathogenesis of hepatocellular carcinoma (HCC) is incompletely defined. Although high-frequency MSI (MSI-H) is infrequently seen in HCC, some studies have suggested a role for MSI in HCC development. While MSI has been clearly defined for a subset of tumors, in particular colorectal, gastric and endometrial cancers, generally accepted criteria have not been developed for other tumors. Colorectal cancers (CRC) are classified as MSI-H if >30-40% of >5 microsatellite loci analyzed show instability. The MSI-H phenotype is associated with defective DNA mismatch repair (MMR) and is observed in the majority of tumors from patients with hereditary non-polyposis colon cancer (HNPCC) and also in 15% of sporadic CRCs. Inactivating mutations of the hMLH1 or hMSH2 genes lead to defects in MMR in HNPCC. In sporadic CRCs, MMR is usually due to hypermethylation of the hMLH-1 promoter. The role of defective MMR in hepatocellular carcinogenesis is controversial. Immunohistochemistry for hMLH1 and hMSH2 reliably indicates hMLH1 or hMSH2 loss in MSI-H CRC tumors. To investigate the role of defective MMR in HCC carcinogenesis, we performed immunohistochemistry for hMLH1 and hMSH2 on 36 HCCs. BAT26, a microsatellite marker that reliably predicts MSI-H was also examined. All 36 of the tumors stained positively for both hMLH1 and hMSH2, strongly suggesting an absence of either inactivating mutations of hMLH1 and hMSH2 or promoter hypermethylation of hMLH1. None of the tumors showed MSI at the BAT26 locus. These findings suggest that defective MMR does not contribute significantly to hepatocellular carcinogenesis.     

Proliferation, apoptosis, and survival in high-level microsatellite instability sporadic colorectal cancer.

Sporadic colorectal cancer (CRC) characterized by high-level DNA microsatellite instability (MSI-H) has a favorable prognosis. The reason for this MSI-H survival advantage is not known. The aim of this study was to correlate proliferation, apoptosis, and prognosis in CRC stratified by MSI status. The proliferative index (PI) was measured by immunohistochemical staining with the Ki-67 antibody in a selected series of 100 sporadic colorectal cancers classified according to the level of MSI as 31 MSI-H, 29 MSI-Low (MSI-L), and 40 microsatellite stable (MSS). The Ki-67 index was significantly higher in MSI-H cancers (P < 0.0001) in which the PI was 90.1 +/- 1.2% (mean +/- SE) compared with 69.5 +/- 3.1% and 69.5 +/- 2.3% in MSI-L and MSS subgroups, respectively. There was a positive linear correlation between the apoptotic index (AI) and PI (r = 0.51; P < 0.001), with MSI-H cancers demonstrating an increased AI:PI ratio indicative of a lower index of cell production. A high PI showed a trend toward predicting improved survival within MSI-H cancers (P = 0.09) but did not predict survival in MSI-L or MSS cancers. The AI was not associated with survival in any MSI subgroup. In conclusion, this is the first study to show that sporadic MSI-H cancers are characterized by a higher AI:PI ratio and increased proliferative activity compared with MSI-L and MSS cancers, and that an elevated PI may confer a survival advantage within the MSI-H subset.     

Clinicopathological features and microsatellite instability (MSI) in colorectal cancers from African Americans

African Americans (AAs) have a 1.5 times higher risk of colorectal carcinoma (CRC) than Caucasians. Gene silencing through CpG island hypermethylation has been associated with the genesis or progression of microsatellite instability (MSI) largely due to 1 target for hypermethylation being the DNA mismatch repair gene hMLH1; there is anecdotal evidence of an increased incidence of MSI among AAs. P16 and hMLH1 can be inactivated by hypermethylation of their respective promoter regions, abrogating the ability to regulate cell proliferation and repair processes. We studied such methylation, as well as hMHS2 expression in colorectal cancers from AA patients to determine if MSI is associated with epigenetic silencing. Experiments were conducted on matched normal and colon cancer tissues from AA patients (n = 51). A total of 5 microsatellite markers (D2S123, D5S346, D17S250, BAT25 and BAT26) were used to evaluate MSI status. P16 and hMLH1 promoter methylation status was determined following bisulfite modification of DNA and using methylation specific PCR, while immunohistochemistry (IHC) was used to examine expression of hMLH1 and hMSH2. A total of 22 (43%) cancers demonstrated microsatellite instability-high (MSI-H), while 27 were microsatellite stable (MSS) and 2 were microsatellite instability-low (MSH-L). Most of the MSI-H tumors were proximal, well differentiated and highly mucinous. Most patients in the MSI-H group were females (68%). The p16 promoter was methylated in 19 of 47 (40%) tumors. A total of 7 of these CRCs demonstrated MSI-H (33%). The hMLH1 promoter was methylated in 29 of 34 (85%) tumors, of which 13 CRCs demonstrated MSI-H (87%). hMLH1 and hMSH2 staining was observed in 66% and 38% of MSI-H tumors, respectively. Overall, the prevalence of MSI-H colorectal tumor was 2-3-fold higher, while the defect in the percentage expression of mismatch repair (MMR) genes (hMLH1 and hMSH2) was similar in AA patients compared to the U.S. Caucasian population. Similar numbers of AA MSS tumors with p16 and hMLH1 methylation likely indicate hemimethylation of genes that might reflect environmental or genetic influences that might be more common in the AA population.     

Microsatellite instability with promoter methylation and silencing of hMLH1 can regionally occur during progression of gastric carcinoma

Microsatellite instability (MSI) is known to result from inactivation of mismatch repair genes largely by promoter methylation. However, the methylation usually accumulates time-dependently. To know whether MSI can be acquired later in tumorigenesis, we examined intratumoral heterogeneity of MSI and promoter methylation of hMLH1 after immunohistochemical screening for heterogeneous expression of hMLH1 in 55 cases of gastric carcinomas. We demonstrated for the first time that MSI-H can develop from MSI-L or the absence of MSI due to time-dependent accumulation of DNA methylation during progression of early-stage gastric carcinomas. The resultant replication errors may play a role in enhancing invasive activity.     

Genetic instability caused by loss of MutS homologue 3 in human colorectal cancer

Microsatellite instability (MSI) is a hallmark of mismatch repair (MMR) deficiency. High levels of MSI at mononucleotide and dinucleotide repeats in colorectal cancer (CRC) are attributed to inactivation of the MMR genes, hMLH1 and hMSH2. CRC with low levels of MSI (MSI-L) exists; however, its molecular basis is unclear. There is another type of MSI--elevated microsatellite alterations at selected tetranucleotide repeats (EMAST)--where loci containing [AAAG](n) or [ATAG](n) repeats are unstable. EMAST is frequent in non-CRCs; however, the incidence of EMAST and its cause in CRC is not known. Here, we report that MutS homologue 3 (MSH3) knockdown or MSH3-deficient cells exhibit the EMAST phenotype and low levels of mutations at dinucleotide repeats. About 60% of 117 sporadic CRC cases exhibit EMAST. All of the cases defined as MSI-H (16 cases) exhibited high levels of EMAST. Among 101 non-MSI-H cases, all 19 cases of MSI-L and 35 of 82 cases of MSS exhibited EMAST. Although non-MSI-H CRC tissues contained MSH3-negative tumor cells ranging from 2% to 50% of the total tumor cell population, the tissues exhibiting EMAST contained more MSH3-negative cells (average, 31.5%) than did the tissues not exhibiting EMAST (8.4%). Taken together, our results support the concept that MSH3 deficiency causes EMAST or EMAST with low levels of MSI at loci with dinucleotide repeats in CRC.     

胃癌环氧化酶-2 、hMLH1 及hMSH2 微卫星不稳定与基因调控的关系

 目的 检测临床手术切除43例胃癌及相应正常组织COX-2、hMLH1和hMSH2微卫星不稳定状态MSI及三种基因启动子甲基化情况，并进一步探讨它们与胃癌发生的关系。方法 采用聚合酶链反应（PCR）技术检测5个位点的MSI状态；甲基化特异性PCR（Methylation specific PCR，MSP）方法检测胃癌及正常组织COX-2、hMLH1及hMSH2三种基因启动子CpG岛甲基化状态。结果 43例胃癌中MSI总检出率为48．84％（21／43），五个位点的MSI检出率无显著差别。COX-2和hMLH1基因启动子CpG岛甲基化在43例胃癌中分别有8例和13例，正常组织中未检测到。hMSH2基因启动子CpG岛在胃癌及正常组织中均未检测到甲基化。在MSI-H组hMLH1基因启动子CpG岛甲基化率显著高于MSS组（P〈0．01）；而在MSI-H和MSI-L组间以及MSI-L和MSS无差别。MSI-H组中COX-2基因启动子CpG岛甲基化8例，且有7例胃癌同时出现hMLH1和COX-2基因启动子CpG岛甲基化。结论 在MSI胃癌（尤其是MSI-H型胃癌）的发生、发展过程中可能同时出现了hMLH1和COX-2基因启动子CpG岛的甲基化（即表型遗传修饰）。MSI胃癌hMLH1基因启动子CpG岛甲基化率高于MSS胃癌，提示检测hMLH1基因启动子CpG岛甲基化对于判断肿瘤类型有一定意义。