Causes of microsatellite instability in colorectal tumors: implications for hereditary non-polyposis colorectal cancer screening

MLH1 Promotor Hypermethylation in Colorectal and Endometrial Carcinomas from Patients with Lynch Syndrome

Screening for Lynch syndrome (LS) in colorectal cancer (CRC) and endometrial cancer patients generally involves immunohistochemical staining of the mismatch repair (MMR) proteins. In case of MLH1 protein loss, MLH1 promotor hypermethylation (MLH1-PM) testing is performed to indirectly distinguish the constitutional MLH1 variants from somatic epimutations. Recently, multiple studies have reported that MLH1-PM and pathogenic constitutional MMR variants are not mutually exclusive. This study describes 6 new and 86 previously reported MLH1-PM CRCs or endometrial cancers in LS patients. Of these, methylation of the MLH1 gene promotor C region was reported in 30 MLH1, 6 MSH2, 6 MSH6, and 3 PMS2 variant carriers at a median age at diagnosis of 48.5 years [interquartile range (IQR), 39-56.75 years], 39 years (IQR, 29-51 years), 58 years (IQR, 53.5-67 years), and 68 years (IQR, 65.6-68.5 years), respectively. For 31 MLH1-PM CRCs in LS patients from the literature, only the B region of the MLH1 gene promotor was tested, whereas for 13 cases in the literature the tested region was not specified. Collectively, these data indicate that a diagnosis of LS should not be excluded when MLH1-PM is detected. Clinicians should carefully consider whether follow-up genetic MMR gene testing should be offered, with age <60 to 70 years and/or a positive family history among other factors being suggestive for a potential constitutional MMR gene defect.

Colorectal adenocarcinoma in Uganda: are right-sided and left-sided colon cancers two distinct disease entities?

INTRODUCTION

In Western countries, right-sided colon cancers (RSCC) present at an older age and advanced stage. Researchers believe that there is a difference between left-sided colon cancer (LSCC) and RSCC. In Uganda, however, it is unknown whether differences exist in the pathological profile between RSCC and LSCC. The aim of this study was to determine the differences in clinicopathological characteristics between RSCC and LSCC in Ugandan patients.

METHODOLOGY

A cross-sectional study was conducted in which colorectal adenocarcinoma formalin-fixed paraffin-embedded tissue (FFPE) blocks were obtained from 2008 to 2021. Colorectal specimens were obtained from prospectively recruited patients. In the retrospective study arm, FFPE blocks and data were obtained from the archives of pathology laboratory repositories. Parameters studied included age, sex, location of the tumour, grade, stage, lymphovascular (LVI) status, and histopathological subtype between LSCC and RSCC.

RESULTS

Patients with RSCC were not older than those with LSCC (mean age, 56.3 years vs 53.5 years; p = 0.170). There was no difference in the stage between RSCC and LSCC. Poorly differentiated tumours were more commonly found in RSCC than in LSCC (18.7% vs 10.1%; p = 0.038). Moderately and poorly differentiated colonic tumours were more common with RSCC (89.3%) than with LSCC (75.1%) (p = 0.007). Younger patients had more poorly differentiated tumours than older patients (19.6% versus 8.6%; p = 0.002). LVI was more common with RSCC than with LSCC (96.8% vs 85.3%; p = 0.014). Mucinous adenocarcinoma (MAC) was more common with RSCC (15.8%) compared with LSCC (8.5%) (p = 0.056) although statistical significance was borderline.

CONCLUSIONS

Clinicopathological features of RSCCs tend to be different from those of LSCCs. RSCCs tend to be associated with MAC, a higher grade and LVI status compared to LSCC. LSCC and RSCC present predominantly with an advanced stage; therefore, national screening programmes for the early detection of CRC are necessary to reduce mortality in our Ugandan population.

MBD4 frameshift mutation caused by DNA mismatch repair deficiency enhances cytotoxicity by trifluridine, an active antitumor agent of TAS-102, in colorectal cancer cells

Backgrounds

Trifluridine is an active antitumor component of TAS-102 that resembles 5-fluorouracil. Although patients with advanced colorectal cancer (CRC) exhibiting a mismatch repair (MMR) deficiency reportedly do not benefit from 5-fluorouracil-based chemotherapy and we previously reported that truncated methyl-CpG binding domain protein 4 (MBD4) enhances 5-fluorouracil cytotoxicity in MMR-deficient CRC cells, little is known regarding the effect of MMR deficiency on trifluridine cytotoxicity in CRC.

Aim

We investigated whether trifluridine induces cytotoxicity in a DNA MMR-dependent manner and evaluated how truncated MBD4 alters trifluridine cytotoxicity.

Methods

We utilized the human CRC cell lines HCT116 (hMLH1-deficient cells) and HCT116+ch3 (hMLH1-restored cells) and compared their sensitivities to trifluridine. And we established 5-fluorouracil-refractory hMLH1-deficient cells and analyzed trifluridine cytotoxicity. Finally, we established truncated MBD4 overexpressed CRC cell lines, and compared trifluridine sensitivity.

Associations of defect mismatch repair genes with prognosis and heredity in sporadic colorectal cancer

BACKGROUND

Microsatellite instability arises due to defect mismatch repair (MMR) and occurs in 10-20% of sporadic colorectal cancer. The purpose was to investigate correlations between defect MMR, prognosis and heredity for colorectal cancer in first-degree relatives.

MATERIAL AND METHODS

Tumour tissues from 318 patients consecutively operated for colorectal cancer were analysed for immunohistochemical expression of MLH1, MSH2 and MSH6 on tissue microarrays. Information on KRAS and BRAF mutation status was available for selected cases.

RESULTS

Forty-seven (15%) tumours displayed MSI. No correlation was seen between patients exhibiting MSI in the tumour and heredity (p = 0.789). Patients with proximal colon cancer and MSI had an improved cancer-specific survival (p = 0.006) and prolonged time to recurrence (p = 0.037). In a multivariate analysis including MSI status, gender, CEA, vascular and neural invasion, patients with MSS and proximal colon cancer had an impaired cancer-specific survival compared with patients with MSI (HR, 4.32; CI, 1.46-12.78). The same prognostic information was also seen in distal colon cancer; no recurrences seen in the eight patients with stages II and III distal colon cancer and MSI, but the difference was not statistically significant.

CONCLUSION

No correlation between MSI and heredity for colorectal cancer in first-degree relatives was seen. Patients with MSI tumours had improved survival.

The analysis of microsatellite instability in extracolonic gastrointestinal malignancy

Microsatellite instability (MSI) is a genetic feature of sporadic and familial cancers of multiple sites and is related to defective mismatch repair (MMR) protein function. Lynch syndrome (LS) is a familial form of MMR deficiency that may present with a spectrum of MSI positive cancers including gastrointestinal (GI) malignancies. The incidence of high level MSI (MSI-H) in colorectal carcinoma is well defined in both familial and sporadic cases and these tumours portend a better overall prognosis in colorectal carcinoma (CRC). There are certain morphological features that suggest MSI-H CRC and international guidelines have been established for the evaluation of MSI in CRC. The prevalence and morphological features of extracolonic GI MSI-H tumours are less well documented. Furthermore, it is unclear whether the guidelines for the assessment of MSI in CRC are appropriate for application to extracolonic GI malignancies. This review aims to summarise the recent literature on MSI in extracolonic LS-related GI tract malignancies with special attention to the assessment of the MMR system by evaluation of specific microsatellite markers and/or immunohistochemical evaluation of MMR protein expression. The reported prevalence of sporadic and LS-related MSI-H tumours along with their associated unique morphological patterns and related prognostic or therapeutic implications will be discussed.

Subsets of microsatellite-unstable colorectal cancers exhibit discordance between the CpG island methylator phenotype and MLH1 methylation status

Although the presence of MLH1 methylation in microsatellite-unstable colorectal cancer generally indicates involvement of the CpG island methylator phenotype (CIMP) in the development of the tumor, these two conditions do not always correlate. A minority of microsatellite-unstable colorectal cancers exhibit discordance between CIMP and MLH1 methylation statuses. However, the clinicopathological features of such microsatellite-unstable colorectal cancers with discrepant MLH1 methylation and CIMP statuses remain poorly studied. Microsatellite-unstable colorectal cancers (n=220) were analyzed for CIMP and MLH1 methylation statuses using the MethyLight assay. Based on the combinatorial CIMP and MLH1 methylation statuses, the microsatellite-unstable colorectal cancers were grouped into four subtypes (CIMP-high (CIMP-H) MLH1 methylation-positive (MLH1m+), CIMP-H MLH1 methylation-negative, CIMP-low/0 (CIMP-L/0) MLH1m+, and CIMP-L/0 MLH1 methylation-negative), which were compared in terms of their associations with clinicopathological and molecular features. The CIMP-L/0 MLH1 methylation-negative and CIMP-H MLH1m+ subtypes were predominant, comprising 63.6 and 24.1% of total microsatellite-unstable colorectal cancers, respectively. The discordant subtypes, CIMP-H MLH1 methylation-negative and CIMP-L/0 MLH1m+, were found in 5 and 7% of microsatellite-unstable colorectal cancers, respectively. The CIMP-H MLH1 methylation-negative subtype exhibited elevated incidence rates in male patients and was associated with larger tumor size, more frequent loss of MSH2 expression, increased frequency of KRAS mutation, and advanced cancer stage. The CIMP-L/0 MLH1m+ subtype was associated with onset at an earlier age, a predominance of MLH1 loss, and earlier cancer stage. None of the CIMP-L/0 MLH1m+ subtype patients succumbed to death during the follow-up. Our findings suggest that the discordant subtypes of colorectal cancers exhibit distinct clinicopathological and molecular features, although the proportion of discordant subtypes is low. The microsatellite-unstable colorectal cancers of the same CIMP status tended to exhibit different clinicopathological features depending on MLH1 methylation status.

MLH1 promoter methylation frequency in colorectal cancer patients and related clinicopathological and molecular features

Purpose

To describe the frequency of MLH1 promoter methylation in colorectal cancer (CRC); to explore the associations between MLH1 promoter methylation and clinicopathological and molecular factors using a systematic review and meta-analysis.

Methods

A literature search of the PubMed and Embase databases was conducted to identify relevant articles published up to September 7, 2012 that described the frequency of MLH1 promoter methylation or its associations with clinicopathological and molecular factors in CRC. The pooled frequency, odds ratio (OR) and 95% confidence intervals (95% CI) were calculated.

Results

The pooled frequency of MLH1 promoter methylation in unselected CRC was 20.3% (95% CI: 16.8–24.1%). They were 18.7% (95% CI: 14.7–23.6%) and 16.4% (95% CI: 11.9–22.0%) in sporadic and Lynch syndrome (LS) CRC, respectively. Significant associations were observed between MLH1 promoter methylation and gender (pooled OR = 1.641, 95% CI: 1.215–2.215; P = 0.001), tumor location (pooled OR = 3.804, 95% CI: 2.715–5.329; P<0.001), tumor differentiation (pooled OR = 2.131, 95% CI: 1.464–3.102; P<0.001), MSI (OR: 27.096, 95% CI: 13.717–53.526; P<0.001). Significant associations were also observed between MLH1 promoter methylation and MLH1 protein expression, BRAF mutation (OR = 14.919 (95% CI: 6.427–34.631; P<0.001) and 9.419 (95% CI: 2.613–33.953; P = 0.001), respectively).

Conclusion

The frequency of MLH1 promoter methylation in unselected CRC was 20.3%. They were 18.7% in sporadic CRC and 16.4% in LS CRC, respectively. MLH1 promoter methylation may be significantly associated with gender, tumor location, tumor differentiation, MSI, MLH1 protein expression, and BRAF mutation.

Humans accumulate microsatellite instability with acquired loss of MLH1 protein in hematopoietic stem and progenitor cells as a function of age

Hematopoietic stem and progenitor cells (HPCs) are necessary for long-term survival. Genomic instability and persistent DNA damage may cause loss of adult stem cell function. The mismatch repair (MMR) pathway increases replication fidelity and defects have been implicated in malignant hematopoietic diseases. Little, however, is known about the role MMR pathway failure plays in the aging process of human HPCs. We hypothesized that loss of MMR occurs in HPCs as a process of human aging. We examined microsatellite instability and expression of the MMR genes MutL homologue 1 (MLH1) and MutS homologue 2 (MSH2) in HPCs and colony-forming cell-derived clones (CFCs) from human donors aged 0 to 86 years. CFCs from donors > 45 years had a greater frequency of microsatellite instability and CD34(+) progenitors lacking MLH1 expression and protein than individuals ≤ 45 years. Loss of MSH2 did not correlate with age. Thus, a potentially early event in the normal human aging process is microsatellite instability accumulation in normal human HPCs associated with the loss of MLH1 protein expression.

A meta-analysis of the prevalence of somatic mutations in the hMLH1 and hMSH2 genes in colorectal cancer

AIM

The study aimed to understand better the somatic mutations in the human MutL Homolog 1 (hMLH1) and human MutS Homolog 2 (hMSH2) genes in colorectal cancer (CRC) and to investigate the differences derived from ethnicity, family history, detection method and microsatellite instability (MSI).

METHOD

The terms 'hMSH2' or 'hMLH1' and 'colorectal cancer' 'colorectal carcinoma' or 'colorectal tumour' were searched in the PubMed, Springer, Lippincott, Williams & Wilkins and HighWire Press databases for the publication period December 1993 to September 2010. The Comprehensive Meta Analysis V2 software (Biostat Inc.) was used to explore the prevalence and 95% confidence intervals.

RESULTS

The prevalence of somatic mutations in the hMLH1 and hMSH2 genes in CRC was 0.15 (95% CI 0.10-0.22) and 0.10 (95% CI 0.07-0.16), respectively. A higher prevalence of somatic mutations in hMSH2 was found in hereditary non-polyposis CRC than in sporadic CRC: 0.36 (95% CI 0.14-0.67) and 0.10 (95% CI 0.07-0.13) respectively. In addition, a higher prevalence of somatic mutations in the hMLH1 gene was observed relative to hMSH2 in the European group. The prevalence was higher in the high-level instability (MSI-H) group than in both the low-level instability (MSI-L) and the microsatellite stable (MSS) groups.

CONCLUSION

Somatic mutations in the hMLH1 and hMSH2 genes play a vital role in CRC and a high prevalence was found in this meta-analysis. Furthermore, more studies are needed which focus on somatic mutations in the American population and in patients with MSI-L and MSS.

Selection of patients with germline MLH1 mutated Lynch syndrome by determination of MLH1 methylation and BRAF mutation

Lynch syndrome is one of the most common hereditary colorectal cancer (CRC) syndrome and is caused by germline mutations of MLH1, MSH2 and more rarely MSH6, PMS2, MLH3 genes. Whereas the absence of MSH2 protein is predictive of Lynch syndrome, it is not the case for the absence of MLH1 protein. The purpose of this study was to develop a sensitive and cost effective algorithm to select Lynch syndrome cases among patients with MLH1 immunohistochemical silencing. Eleven sporadic CRC and 16 Lynch syndrome cases with MLH1 protein abnormalities were selected. The BRAF c.1799T> A mutation (p.Val600Glu) was analyzed by direct sequencing after PCR amplification of exon 15. Methylation of MLH1 promoter was determined by Methylation-Sensitive Single-Strand Conformation Analysis. In patients with Lynch syndrome, there was no BRAF mutation and only one case showed MLH1 methylation (6%). In sporadic CRC, all cases were MLH1 methylated (100%) and 8 out of 11 cases carried the above BRAF mutation (73%) whereas only 3 cases were BRAF wild type (27%). We propose the following algorithm: (1) no further molecular analysis should be performed for CRC exhibiting MLH1 methylation and BRAF mutation, and these cases should be considered as sporadic CRC; (2) CRC with unmethylated MLH1 and negative for BRAF mutation should be considered as Lynch syndrome; and (3) only a small fraction of CRC with MLH1 promoter methylation but negative for BRAF mutation should be true Lynch syndrome patients. These potentially Lynch syndrome patients should be offered genetic counselling before searching for MLH1 gene mutations.

Methotrexate induces oxidative DNA damage and is selectively lethal to tumour cells with defects in the DNA mismatch repair gene MSH2

Mutations in the MSH2 gene predispose to a number of tumourigenic conditions, including hereditary non-polyposis colon cancer (HNPCC). MSH2 encodes a protein in the mismatch repair (MMR) pathway which is involved in the removal of mispairs originating during replication or from damaged DNA. To identify new therapeutic strategies for the treatment of cancer arising from MMR deficiency, we screened a small molecule library encompassing previously utilized drugs and drug-like molecules to identify agents selectively lethal to cells lacking functional MSH2. This approach identified the drug methotrexate as being highly selective for cells with MSH2 deficiency. Methotrexate treatment caused the accumulation of potentially lethal 8-hydroxy-2'-deoxyguanosine (8-OHdG) oxidative DNA lesions in both MSH2 deficient and proficient cells. In MSH2 proficient cells, these lesions were rapidly cleared, while in MSH2 deficient cells 8-OHdG lesions persisted, potentially explaining the selectivity of methotrexate. Short interfering (si)RNA mediated silencing of the target of methotrexate, dihydrofolate reductase (DHFR), was also selective for MSH2 deficiency and also caused an accumulation of 8-OHdG. This suggested that the ability of methotrexate to modulate folate synthesis via inhibition of DHFR, may explain MSH2 selectivity. Consistent with this hypothesis, addition of folic acid to culture media substantially rescued the lethal phenotype caused by methotrexate. While methotrexate has been used for many years as a cancer therapy, our observations suggest that this drug may have particular utility for the treatment of a subset of patients with tumours characterized by MSH2 mutations.

MS-MLPA reveals progressive age-dependent promoter methylation of tumor suppressor genes and possible role of IGSF4 gene in colorectal carcinogenesis of microsatellite instable tumors

In 43 MSI-H colorectal cancers we searched for new targets of promoter methylation, inspected the nature of methylation process, and the influence of methylation at specific CpG site on gene expression. CpG methylation was detected in 12 tumor suppressor genes. Our findings suggest a potential role of IGSF4 gene in the development of colorectal tumors. According to the detected methylation pattern, two groups of tumors, significantly differing in age, exist in MSI-H colorectal cancers. Our study also suggests that methylation at a specific CpG island in the promoter could be the representative for gene silencing and therefore serve as a biomarker.

Ultradeep sequencing of a human ultraconserved region reveals somatic and constitutional genomic instability

Ultradeep sequencing of genomes permits the detection of very low-level genomic instability in non-neoplastic tissues of patients with the most common form of inherited colorectal cancer.

Early detection of cancer-associated genomic instability is crucial, particularly in tumour types in which this instability represents the essential underlying mechanism of tumourigenesis. Currently used methods require the presence of already established neoplastic cells because they only detect clonal mutations. In principle, parallel sequencing of single DNA filaments could reveal the early phases of tumour initiation by detecting low-frequency mutations, provided an adequate depth of coverage and an effective control of the experimental error. We applied ultradeep sequencing to estimate the genomic instability of individuals with hereditary non-polyposis colorectal cancer (HNPCC). To overcome the experimental error, we used an ultraconserved region (UCR) of the human genome as an internal control. By comparing the mutability outside and inside the UCR, we observed a tendency of the ultraconserved element to accumulate significantly fewer mutations than the flanking segments in both neoplastic and nonneoplastic HNPCC samples. No difference between the two regions was detectable in cells from healthy donors, indicating that all three HNPCC samples have mutation rates higher than the healthy genome. This is the first, to our knowledge, direct evidence of an intrinsic genomic instability of individuals with heterozygous mutations in mismatch repair genes, and constitutes the proof of principle for the development of a more sensitive molecular assay of genomic instability.

In hereditary non-polyposis colorectal cancer (HNPCC), a germline mutation in one allele of a gene responsible for repairing DNA damage predisposes the host to cancer, because subsequent somatic inactivation of the one wild-type allele leads to genomic instability that favours tumourigenesis. Nonneoplastic tissues of HNPCC individuals are believed to repair DNA normally, as they are heterozygous and thus are thought to be genomically stable. However, methods used to date are known to be incapable of detecting very low levels of genome instability. Here, we present a more sensitive procedure based on the resequencing of a HNPCC genomic region using next-generation sequencing technology. With this approach, we show that genomic instability is in fact detectable in nonneoplastic tissues of HNPCC patients compared with healthy donors. This constitutional instability may predispose them to acquiring the second somatic mutation event needed for cancer development.

Partial loss of heterozygosity events at the mutated gene in tumors from MLH1/MSH2 large genomic rearrangement carriers

Background

Depending on the population studied, large genomic rearrangements (LGRs) of the mismatch repair (MMR) genes constitute various proportions of the germline mutations that predispose to hereditary non-polyposis colorectal cancer (HNPCC). It has been reported that loss of heterozygosity (LOH) at the LGR region occurs through a gene conversion mechanism in tumors from MLH1/MSH2 deletion carriers; however, the converted tracts were delineated only by extragenic microsatellite markers. We sought to determine the frequency of LGRs in Slovak HNPCC patients and to study LOH in tumors from LGR carriers at the LGR region, as well as at other heterozygous markers within the gene to more precisely define conversion tracts.

Methods

The main MMR genes responsible for HNPCC, MLH1, MSH2, MSH6, and PMS2, were analyzed by MLPA (multiplex ligation-dependent probe amplification) in a total of 37 unrelated HNPCC-suspected patients whose MLH1/MSH2 genes gave negative results in previous sequencing experiments. An LOH study was performed on six tumors from LGR carriers by combining MLPA to assess LOH at LGR regions and sequencing to examine LOH at 28 SNP markers from the MLH1 and MSH2 genes.

Results

We found six rearrangements in the MSH2 gene (five deletions and dup5-6), and one aberration in the MLH1 gene (del5-6). The MSH2 deletions were of three types (del1, del1-3, del1-7). We detected LOH at the LGR region in the single MLH1 case, which was determined in a previous study to be LOH-negative in the intragenic D3S1611 marker. Three tumors displayed LOH of at least one SNP marker, including two cases that were LOH-negative at the LGR region.

Conclusion

LGRs accounted for 25% of germline MMR mutations identified in 28 Slovakian HNPCC families. A high frequency of LGRs among the MSH2 mutations provides a rationale for a MLPA screening of the Slovakian HNPCC families prior scanning by DNA sequencing. LOH at part of the informative loci confined to the MLH1 or MSH2 gene (heterozygous LGR region, SNP, or microsatellite) is a novel finding and can be regarded as a partial LOH. The conversion begins within the gene, and the details of conversion tracts are discussed for each case.

A review on the molecular diagnostics of Lynch syndrome: a central role for the pathology laboratory

Lynch syndrome (LS) is caused by mutations in mismatch repair genes and is characterized by a high cumulative risk for the development of mainly colorectal carcinoma and endometrial carcinoma. Early detection of LS is important since surveillance can reduce morbidity and mortality. However, the diagnosis of LS is complicated by the absence of a pre-morbid phenotype and germline mutation analysis is expensive and time consuming. Therefore it is standard practice to precede germline mutation analysis by a molecular diagnostic work-up of tumours, guided by clinical and pathological criteria, to select patients for germline mutation analysis. In this review we address these molecular analyses, the central role for the pathologist in the selection of patients for germline diagnostics of LS, as well as the molecular basis of LS.

Redundant DNA methylation in colorectal cancers of Lynch-syndrome patients

Lynch syndrome is an inherited disease resulting predominantly in colorectal cancer (CRC). The crucial cause is DNA mismatch repair (MMR) malfunction that is associated mostly with MLH1 or MSH2 germline mutations. A significant hallmark of repair defects is a high level of instability in microsatellites (MSI-H). In many sporadic unstable CRCs, the MLH1 gene is inactivated by promoter hypermethylation in addition to extensive promoter methylation in many tumor-suppressor genes known as CpG island methylation phenotype (CIMP). To investigate the possible role of epigenetic alterations in causing MMR deficiency and thereby Lynch syndrome, we evaluated the MLH1 specific and global hypermethylation in hereditary CRCs. Of 22 Lynch-syndrome-related CRCs, 18 (81.8%) demonstrated various levels of DNA methylation; of these, 14 (63.6%) and 4 (18.2%) were methylated in distal and both distal and proximal regions of the MLH1 promoter, respectively. However, only 7/18 (38.9%) of results were confirmed by bisulfite sequencing. Similar methylation patterns in tumors and frequently in matched normal DNA were found in twelve and four patients with MLH1 and MSH2 alterations documented by the absence of protein or presence of germline mutation, respectively. Moreover, the same results were observed in five stable CRCs. None of 22 Lynch-syndrome-related tumors presented CIMP in contrast to 3/10 (30%) stable carcinomas. The rather randomly distributed weak methylation patterns in hereditary CRCs indicate that epigenetic events are redundant in Lynch-syndrome etiology, in contrast to the widespread DNA methylation in sporadic unstable CRCs. These methylation-profile differences can lead to more effective molecular diagnosis of Lynch syndrome.

Common germline MDR1/ABCB1 functional polymorphisms and haplotypes modify susceptibility to colorectal cancers with high microsatellite instability

Altered expression of P-glycoprotein (P-gp) encoded by the multidrug resistance (MDR1/ABCB1) gene, as well as somatic mutations and hypermethylation in the MDR1/ABCB1 gene, were identified in a proportion of previously untreated colorectal cancers (CRC) exhibiting high microsatellite instability (MSI-H), which suggested that MDR1/ABCB1 acts as a candidate gene contributing to the initiation and progression of MSI-H tumors. Here we report germline functional single nucleotide polymorphisms (SNPs) and haplotypes in the MDR1/ABCB1 gene, which could contribute to genetic risk or increase susceptibility to MSI-H cancers. We have confirmed disease association by comparing the MDR1/ABCB1 genotype, allele, and haplotype frequencies between healthy controls and patients with MSI-H tumors. In particular, carriers of the T/T genotype in exon 12 (1236 C-->T) SNP and the T/T genotype in exon 21 (2677G-->T) SNP were most significantly associated with a higher risk for developing MSI-H CRC compared to controls (P=0.01, OR=3.182 and P=0.005, OR=3.594, respectively). The most significant MSI-H-associated risk haplotypes include the most frequent haplotype H1 (T-C-T-T) defined by SNPs in exon 12, intron 13 (rs2235035), exon 21, and exon 26 (3435 C-->T; P=0.004, OR= 0.476). Our results suggest that ABCB1/MDR1 is a novel low-penetrance gene for susceptibility to MSI-H tumors. The present study provides additional evidence for the role that the MDR1/ABCB1 gene plays in the initiation and progression of MSI-H CRC development.

Functional analysis of human MLH1 variants using yeast and in vitro mismatch repair assays

The functional characterization of nonsynonymous single nucleotide polymorphisms in human mismatch repair (MMR) genes has been critical to evaluate their pathogenicity for hereditary nonpolyposis colorectal cancer. We previously established an assay for detecting loss-of-function mutations in the MLH1 gene using a dominant mutator effect of human MLH1 expressed in Saccharomyces cerevisiae. The purpose of this study is to extend the functional analyses of nonsynonymous single nucleotide polymorphisms in the MLH1 gene both in quality and in quantity, and integrate the results to evaluate the variants for pathogenic significance. The 101 MLH1 variants, which covered most of the reported MLH1 nonsynonymous single nucleotide polymorphisms and consisted of one 3-bp deletion, 1 nonsense and 99 missense variants, were examined for the dominant mutator effect by three yeast assays and for the ability of the variant to repair a heteroduplex DNA with mismatch bases by in vitro MMR assay. There was diversity in the dominant mutator effects and the in vitro MMR activities among the variants. The majority of functionally inactive variants were located around the putative ATP-binding pocket of the NH(2)-terminal domain or the whole region of the COOH-terminal domain. Integrated functional evaluations contribute to a better prediction of the cancer risk in individuals or families carrying MLH1 variants and provide insights into the function-structure relationships in MLH1.

Mechanisms of inactivation of MLH1 in hereditary nonpolyposis colorectal carcinoma: a novel approach

Mutations in the DNA mismatch repair gene MLH1 are a major cause of hereditary nonpolyposis colorectal cancer (HNPCC). No mutant phenotype is observed before the wild-type (wt) allele is somatically inactivated in target tissue. We addressed the mechanisms of MLH1 inactivation in 25 colorectal (CRC) and 32 endometrial cancers (ECs) from MLH1 mutation carriers (Mut1, in-frame genomic deletion; Mut2, out-of-frame splice site mutation; Mut3, missense mutation). By a quantitative method, matrix-assisted laser desorption/ionization-time-of-flight (MALDI-TOF), utilizing four intragenic single nucleotide polymorphisms and mutations, loss of heterozygosity (LOH) was present in 31/57 (54.4%) of tumors. The wt allele displayed LOH more often than the mutant allele (23/57 vs 8/57, P=0.006). For Mut1, LOH was more frequent in CRC than EC (10/11 vs 1/13, P<0.0001), whereas Mut2 and Mut3 displayed opposite LOH pattern. Moreover, although wt LOH predominated in CRC irrespective of the predisposing mutation, LOH often affected the mutant allele in EC from Mut2 and Mut3 carriers (6/19, 31.6%). MLH1 promoter methylation, which reflected a more widespread hypermethylation tendency, occurred in 4/55 (7.3%) of tumors and was inversely associated with LOH. In conclusion, the patterns of somatic events (LOH and promoter methylation) differ depending on the tissue and germline mutation, which may in part explain the differential tumor susceptibility of different organs in HNPCC. MALDI-TOF provides a novel approach for the detection and quantification of LOH.

Epigenetic modification and gastrointestinal tumor

Carcinogenesis springs from the combined forces of both genetic and epigenetic events. The role of epigenetic modification in the progression of carcinoma has been suggested by a lot of evidences in recent years. An epigenetic event alters the activity of genes without changing their structure. Gastrointestinal tumor is the most common cancer in China, and the investigation of epigenetic modification is significant in cell immunity, phylaxis, cell differentiation and preventive therapy.

No evidence for dual role of loss of heterozygosity in hereditary non-polyposis colorectal cancer

Hereditary non-polyposis colorectal cancer (HNPCC) is caused by germline mutations in mismatch repair (MMR) genes, mostly MLH1 and MSH2. Somatic inactivation of the wild-type allele of the respective MMR gene is required for tumor development. Unexpectedly, a recent study utilizing DNA from paraffin-embedded tissue material detected frequent loss of the mutant MMR gene allele in HNPCC tumors. Dual role for loss of heterozygosity (LOH) was proposed. If somatic loss of the wild-type MMR gene allele had occurred through point mutation or promoter hypermethylation, frequent somatic deletions at the region of the MMR gene locus, perhaps targeting other relevant cancer genes, could quite commonly lead to loss of the mutant allele. To test this hypothesis, we studied a population-based series of 25 fresh-frozen HNPCC tumors with a germline mutation in MLH1 or MSH2 for LOH. Fourteen of the 25 tumors (56%) showed LOH at the respective locus, and all 14 losses targeted the wild-type allele (P=0.00006). These results strongly support the traditional two-hit model of HNPCC gene inactivation.

Use of microsatellite instability and immunohistochemistry testing for the identification of individuals at risk for Lynch syndrome

It is now generally recognized that a specific subset of those patients clinically defined as having hereditary non polyposis colon cancer (HNPCC) have germline mutations in any one of several genes involved in DNA mismatch repair (MMR). This important subset of HNPCC families is now defined as having Lynch syndrome. A considerable amount of data has shown that tumors from patients with Lynch syndrome have characteristic features resulting from the underlying molecular involvement of defective MMR, that is, the presence of microsatellite instability (MSI) and the absence of MMR protein expression by immunohistochemistry (IHC). As a result, identifying patients with Lynch syndrome can now be accomplished by testing tumors for these tumor-related changes. Together, MSI and IHC are powerful tools that help identify individuals at risk for having Lynch syndrome and to distinguish these cases from HNPCC cases with other hereditary gene defects. Furthermore, IHC analysis provides valuable clues as to which MMR gene is mutated, allowing for comprehensive mutational analyses of that gene. Here, we discuss the current and historical perspectives regarding MSI and IHC analyses in tumors from sporadic colon cancer and from patients with Lynch syndrome. Given this background, we also provide a testing strategy for the identification of patients at risk for Lynch syndrome and subsequent gene testing.

Dual role of LOH at MMR loci in hereditary non-polyposis colorectal cancer?

hMLH1 and hMSH2 can be considered tumor suppressor genes, as both alleles must be inactivated in order to lose the mismatch repair (MMR) function. In this regard, it has been proposed that LOH at MMR loci is a common Knudson's second-hit mechanism in HNPCC patients. However, experimental evidence supporting this view is scarcely found in the literature. We have performed a comprehensive analysis of LOH in 45 HNPCC tumors carrying a germline alteration in MMR loci. Overall, we have detected LOH at MMR loci in 56% of the cases. However, up to 40% of the LOH events targeted the mutant allele, arguing against a second-hit role in these tumors. Interestingly, the age at diagnosis was significantly older in these patients. To explain this and previous data, we propose a dual role for LOH at MMR loci in HNPCC.

The role of MLH1, MSH2 and MSH6 in the development of multiple colorectal cancers

There is increased incidence of microsatellite instability (MSI) in patients who develop multiple primary colorectal cancers (CRC), although the association with hereditary nonpolyposis colon cancer (HNPCC) is unclear. This study aims to evaluate the underlying genetic cause of MSI in these patients. Microsatellite instability was investigated in 111 paraffin-embedded CRCs obtained from 78 patients with metachronous and synchronous cancers, and a control group consisting of 74 cancers from patients with a single CRC. Tumours were classified as high level (MSI-H), low level (MSI-L) or stable (MSS). MLH1, MSH2 and MSH6 gene expression was measured by immunohistochemistry. Methylation of the MLH1 promoter region was evaluated in MSI-H cancers that failed to express MLH1, and mutational analysis performed in MSI-H samples that expressed MLH1, MSH2 and MSH6 proteins. The frequency of MSI-H was significantly greater in the multiple, 58 out of 111 (52%), compared to the single cancers, 10 out of 74 (13.5%), P < 0.01. Of the 32 patients from whom two or more cancers were analysed, eight (25%) demonstrated MSI-H in both cancers, 13 (41%) demonstrated MSI-H in one cancer and 11 (34%) failed to demonstrate any MSI-H. MSI-H single cancers failed to express MLH1 or MSH2 in seven out of nine (78%) cases and MSI-L/MSS cancers failed to express MLH1 or MSH2 in one out of 45 (2.2%) cases, all cancers expressed MSH6. MSI-H multiple cancers failed to express MLH1 or MSH2 in 21 out of 43 (48%) cases and MSI-L/MSS cancers failed to express MLH1 or MSH2 in four out of 32 (12.5%) cases. MSH6 expression was lost in five MSI-H multiple cancers, four of which also failed to express MLH1 or MSH2. Loss of expression of the same mismatch repair (MMR) gene was identified in both cancers from six out of 19 (31%) patients. Methylation was identified in 11 out of 17 (65%) multiple and three out of six (50%) single MSI-H cancers that failed to express MLH1. Mutational analysis of 10 MSI-H multiple cancers that expressed MLH1, MSH2 and MSH6 failed to demonstrate mutations in the MLH1 or MSH2 genes. We suggest that, although MSI-H is more commonly identified in those with multiple colorectal cancers, this does not commonly arise from a classical HNPCC pathway.

Characterization of hMLH1 and hMSH2 gene dosage alterations in Lynch syndrome patients

BACKGROUND & AIMS

A significant proportion of Lynch syndrome cases are believed to be due to large genomic alterations in the mismatch repair genes hMLH1 and hMSH2. However, previous studies have not adequately identified the frequency and scope of such mutations, and routine clinical Lynch syndrome testing often does not include analysis for these mutations. Our aim was to characterize hMLH1 and hMSH2 genomic rearrangements in a large population of suspected Lynch syndrome patients.

METHODS

A total of 365 samples from probands referred for genetic testing for Lynch syndrome were analyzed for the presence of large genomic alterations in hMLH1 or hMSH2 by using a combination of techniques. Samples with a deletion in exons 1-6 in hMSH2 were further characterized by polymerase chain reaction to establish the presence of the hMSH2 American founder deletion.

RESULTS

An hMLH1 or hMSH2 mutation was identified in 153 cases, and, of these, 12 of 67 (17.9%) and 39 of 86 (45.3%) had a large genomic alteration in hMLH1 and hMSH2, respectively. Overall, 6 different hMLH1 and 12 different hMSH2 deletions/duplications, including 10 novel mutations, were identified. Analysis of the hMSH2 exon 1-6 deletion samples showed that 13 of 18 (72.2%) had the American founder deletion.

CONCLUSIONS

These data show a high frequency and diverse spectrum of large genomic alterations in hMLH1 and hMSH2 in suspected Lynch syndrome patients. Thus, a comprehensive mutation identification strategy that includes the ability to detect large genomic rearrangements is imperative for the clinical genetic identification of Lynch syndrome patients and families.

Clinicopathologic features in colorectal cancer patients with microsatellite instability

The microsatellite instability (MSI) mutational pathway is critical to carcinogenesis in a small but significant proportion of colorectal cancers. While MSI is identified in most cancers in individuals with hereditary non-polyposis colorectal cancer, the majority of MSI tumors are found in individuals with sporadic disease. Colorectal cancers arising as a result of MSI have distinct clinicopathologic features distinguishing them from those with microsatellite stability. MSI colorectal cancers affect a larger percentage of women, are usually localized proximal to the splenic flexure, and have a higher incidence of synchronous and metachronous tumors. They are associated with a mucinous histology, tumor-infiltrating lymphocytes, a Crohn's-like inflammatory response, and a higher grade but lower stage. Overall survival is better in individuals with MSI. The benefit of chemotherapy in MSI colorectal cancers, with and without lymph node metastases, remains unclear.

Challenges and pitfalls in HNPCC screening by microsatellite analysis and immunohistochemistry

Hereditary non-polyposis colorectal cancer (HNPCC) accounts for approximately 2 to 4% of the total colorectal cancer burden. For economic reasons a diagnostic "stepladder" is recommended. After evaluation of the family history, diagnostic microsatellite instability (MSI) analysis has found its place as a valuable screening tool for HNPCC. Immunohistochemical analysis can help to pinpoint the affected gene. The detection of a mutation in one of the responsible mismatch repair gene confirmed the diagnosis HNPCC. Here we demonstrate our experience of some important pitfalls that will be discussed in this study. In MSI testing, one potential source for false-negative results is intralesional heterogeneity. We demonstrate examples of a flat adenoma and a carcinoma, which required laser microdissection to correctly determine the microsatellite status. In these lesions manual microdissection, the most frequently applied method, was not sufficient. However, the number of cells obtained by using laser microdisssection can fall below a necessary minimum, which can also cause false-negative results of MSI analysis, as shown here in a mucinous carcinoma. In addition, evaluation of immunohistochemically stained tissue slides requires experience to avoid false-positive or false-negative interpretation. A case with two synchronous colorectal cancers revealed loss of MSH2 expression in one carcinoma, whereas the second carcinoma stained positively leading to a false-negative interpretation. In some cases, false-positive results can be obtained, if a perinuclear-staining pattern is interpreted as positive. In summary, there are several potential pitfalls in the molecular screening for HNPCC. Therefore the importance of correct interpretation of clinical data, immunohistochemistry, and microsatellite analysis in combination, performed by a pathologist with experience in molecular genetics is essential. In addition, laser microdissection of tumor areas that have been chosen by a pathologist is highly recommended in cases that cannot be resolved with manual microdissection.

Two modes of microsatellite instability in human cancer: differential connection of defective DNA mismatch repair to dinucleotide repeat instability

Microsatellite instability (MSI) is associated with defective DNA mismatch repair in various human malignancies. Using a unique fluorescent technique, we have observed two distinct modes of dinucleotide microsatellite alterations in human colorectal cancer. Type A alterations are defined as length changes of < or =6 bp. Type B changes are more drastic and involve modifications of > or =8 bp. We show here that defective mismatch repair is necessary and sufficient for Type A changes. These changes were observed in cell lines and in tumours from mismatch repair gene-knockout mice. No Type B instability was seen in these cells or tumours. In a panel of human colorectal tumours, both Type A MSI and Type B instability were observed. Both types of MSI were associated with hMSH2 or hMLH1 mismatch repair gene alterations. Intriguingly, p53 mutations, which are generally regarded as uncommon in human tumours of the MSI+ phenotype, were frequently associated with Type A instability, whereas none was found in tumours with Type B instability, reflecting the prevailing viewpoint. Inspection of published data reveals that the microsatellite instability that has been observed in various malignancies, including those associated with Hereditary Non-Polyposis Colorectal Cancer (HNPCC), is predominantly Type B. Our findings indicate that Type B instability is not a simple reflection of a repair defect. We suggest that there are at least two qualitatively distinct modes of dinucleotide MSI in human colorectal cancer, and that different molecular mechanisms may underlie these modes of MSI. The relationship between MSI and defective mismatch repair may be more complex than hitherto suspected.

Promoter hypermethylation frequency and BRAF mutations distinguish hereditary non-polyposis colon cancer from sporadic MSI-H colon cancer

BACKGROUND

Colorectal cancers resulting from defective DNA mismatch repair can occur in both hereditary non-polyposis colon cancer (HNPCC) and in the sporadic setting. They are characterised by a high level of microsatellite instability (MSI-H) and superficially resemble each other in that they are frequently located in the proximal colon and share features such as circumscribed tumour margins and tumour-infiltrating lymphocytes. However, significant differences can be demonstrated at the molecular level including widespread promoter hypermethylation and BRAF -activating mutations which occur significantly less often in HNPCC.

AIMS

In this study, we sought to determine whether the presence of widespread promoter hypermethylation and BRAF mutations would exclude HNPCC.

MATERIALS AND METHODS

We investigated the methylation status of four methylated in tumour markers (MINTs 1,2,12 and 31), and the promoter regions of 5 genes hMLH1, HPP1, MGMT, p16INK4A and p14ARF, in 21 sporadic MSI-H colorectal cancers and compared these with 18 cancers from HNPCC patients. The methylation status of CpG islands were determined by either methylation specific PCR (MSP) or combined bisulfite restricton analysis (COBRA). In addition we considered the BRAF mutation status of 18 HNPCC tumours and 19 sporadic MSI-H cancers which had been previously determined by RFLP analysis and confirmatory sequencing.

RESULTS

Methylation of the promoter regions in target genes occurred less frequently within the HNPCC tumours (27% of analyses), compared with the sporadic MSI-H tumours (59% of analyses) (P < 0.001). Methylation of MINTs 1, 2, 12 and 31 occurred in 4% of analyses for HNPCC tumours contrasted with 73% for sporadic MSI-H tumours (P < 0.001). BRAF mutations were detected in 74% of sporadic tumours but none of the HNPCC cancers tested.

CONCLUSIONS

The total number of genes and MINTs methylated in HNPCC was lower than in MSI-H colorectal tumours. No HNPCC tumour showed evidence of widespread promoter hypermethylation or BRAF mutation suggesting this feature could be used as a discriminator between familial and sporadic cases.

The utility of immunohistochemical detection of DNA mismatch repair gene proteins

Since the development of monoclonal antibodies against the MSH2 protein by Leach et al. in 1996, a series of investigations has been undertaken to determine the utility of immunohistochemical detection of DNA mismatch repair (MMR) gene proteins in the identification of hereditary or sporadic colorectal tumors with microsatellite instability. These studies, however, have been performed with different aims and on different patient populations. Interpretation of these immunohistochemical data relies on a thorough understanding of the biological and technical factors that affect the detection of MMR proteins. In this review, we analyze the data from the published research studies, pointing out the various factors affecting immunohistochemical detection of MMR proteins and projecting the utility of immunohistochemistry in different clinical settings.

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Revised Bethesda Guidelines for hereditary nonpolyposis colorectal cancer (Lynch syndrome) and microsatellite instability

Hereditary nonpolyposis colorectal cancer (HNPCC), also known as Lynch syndrome, is a common autosomal dominant syndrome characterized by early age at onset, neoplastic lesions, and microsatellite instability (MSI). Because cancers with MSI account for approximately 15% of all colorectal cancers and because of the need for a better understanding of the clinical and histologic manifestations of HNPCC, the National Cancer Institute hosted an international workshop on HNPCC in 1996, which led to the development of the Bethesda Guidelines for the identification of individuals with HNPCC who should be tested for MSI. To consider revision and improvement of the Bethesda Guidelines, another HNPCC workshop was held at the National Cancer Institute in Bethesda, MD, in 2002. In this commentary, we summarize the Workshop presentations on HNPCC and MSI testing; present the issues relating to the performance, sensitivity, and specificity of the Bethesda Guidelines; outline the revised Bethesda Guidelines for identifying individuals at risk for HNPCC; and recommend criteria for MSI testing.

Value of histopathology in predicting microsatellite instability in hereditary nonpolyposis colorectal cancer and sporadic colorectal cancer

Identification of colorectal carcinomas with high levels of DNA microsatellite instability (MSI-H) is important because of the suggested prognostic and therapeutic significance associated with MSI. The role of histology in identifying MSI-H colorectal carcinomas has been suggested by some studies but not confirmed by others. Furthermore, previous studies assumed that hereditary nonpolyposis colorectal cancer (HNPCC)-associated MSI-H tumors and sporadic MSI-H tumors have similar histology. This assumption, however, has been challenged by more recent studies. In this report, we first analyzed the value of various histologic features in predicting MSI-H in a series of 218 colorectal carcinomas containing mixed HNPCC and sporadic cases [77 tumors (35%) were MSI-H by polymerase chain reaction (PCR) method]. Then, we evaluated the various histologic features comparatively in two groups extracted from the 218 cases. Group A was composed of 84 tumors from 82 patients obtained based on a strong family history (HNPCC/HNPCC-like group) (male to female ratio, 42:40; age range, 23-80 years, median, 53.5 years). Thirty-one of the 84 tumors (41.7%) were MSI-H by PCR, and all 31 cases were HNPCC by Amsterdam criteria. Group B was composed of 109 patients with no family history of colorectal cancer or HNPCC-associated cancer, obtained from surgical clinics (sporadic group) (male to female ratio, 65:69; age range, 31-84 years, median, 65 years). Thirty-five of the 109 tumors (32.1%) were MSI-H by PCR. Our results showed that, overall, poor tumor differentiation, medullary type, mucinous type, signet-ring cell component, histologic heterogeneity, and increased tumor-infiltrating lymphocytes (TILs) were features more commonly seen in MSI-H tumors than in non-MSI-H tumors. Comparative analyses showed that the overall TIL count was significantly higher in HNPCC/HNPCC-like group, and mucinous type appeared to be more frequent in HNPCC MSI-H tumors than in sporadic MSI-H tumors. However, there was no significant difference in the odds ratio for predicting MSI-H status for any of the analyzed histologic features between HNPCC/HNPCC-like group and sporadic group, indicating that differences between HNPCC and sporadic MSI-H tumors did not significantly impact on the informative value of histology in predicting MSI in the two different clinical settings. TIL counts followed by histologic heterogeneity provided the greatest sensitivity and specificity in predicting MSI status in both HNPCC/HNPCC-like and sporadic cases. Using a stepwise logistic regression model, a formula was generated that could be used to calculate the probability of a colorectal carcinoma being MSI-H based on morphologic features.

Germline hMLH1 promoter mutation in a Newfoundland HNPCC kindred

Hereditary non-polyposis colon cancer (HNPCC) is an autosomal dominant form of inherited predisposition to colorectal and other malignancies. It is associated with mutations in DNA mismatch-repair genes, especially hMSH2 and hMLH1. Management of HNPCC families is improved if the underlying mutation in each family can be discovered. We describe a Newfoundland kindred, meeting the Amsterdam Criteria for HNPCC, in which a mutation in the promoter region of the hMLH1 gene co-segregates with the disease phenotype. The -42C > T mutation is within a putative Myb proto-oncogene binding site. Using electrophoretic mobility shift assays, we demonstrated that the mutated Myb binding sequence is less effective in binding nuclear proteins than the wild-type promoter sequence. Using in vivo transfection experiments in HeLa cells, we further demonstrated that the mutated promoter has only 37% of the activity of the wild-type promoter in driving the expression of a reporter gene. The average age of onset in six family members affected with colorectal cancer is 62 years, which is substantially later than the typical age of onset in HNPCC families. This is consistent with a substantial decrease, but not total elimination, of mismatch repair function in affected members of this family. This is the first report of a heritable hMLH1 promoter mutation in any HNPCC family.

Identification of novel genes with somatic frameshift mutations within coding mononucleotide repeats in colorectal tumors with high microsatellite instability

We have systematically retrieved genes with coding mononucleotide repeats from sequence databases and analyzed them for mutations in tumors with high levels of microsatellite instability (MSI-H). We found somatic frameshift mutations in 7/13 genes previously not analyzed in MSI-H tumors. According to the frequency of mutations in MSI-H tumors, these genes could be divided into genes with high coding mononucleotide repeat instability (CMRI-H) and genes with low coding mononucleotide instability (CMRI-L). CMR-H genes were mutated in more than 9/38 and CMRI-L in less than 4/38 of MSI-H tumors. Four genes in our study were CMRI-H and could thus possibly play a role in the development of MSI-H tumors: TFE3 (9/38), TEF4 (12/38), RGS12 (11/38), and TCF1 (12/38). Our results suggest that systematic identification of genes with CMR in the sequence databases and determination of mutation frequency in MSI-H tumors might be a powerful tool for identification of new molecular targets in the development of MSI-H tumors.

Mutation analysis of APC gene in gastric cancer with microsatellite instability

AIM: To evaluate the role of APC mutation in gastric carcinogenesis and to correlate APC mutation with microsatellite instability (MSI) in gastric carcinomas.

METHODS: APC mutation was measured with multiplex PCR, denaturing gradient gel electrophoresis and DNA sequencing; and MSI was analyzed by PCR-based methods.

RESULTS: Sixty-eight cases of sporadic gastric carcinoma were studied for APC mutation at exon 15 and MSI. APC mutaions were detected in 15 (22.1%) gastric cancers. Frequence of APC mutation (33.3%) in intestinal type of gastric cancer was significantly higher than that in diffuse type (13.1%, P < 0.05). On the contrary, no association was observed between APC mutation and tumor size, differentiation, depth of invasion, metastasis or clinical stages. Using five microsatellite markers, MSI in at least one locus was detected in 17 of 68 (25%) of the tumors analyzed. APC mutations were all detected in MSI-L (only one locus, n = 9) or MSS (tumor lacking MSI or stable, n = 51), but no mutation was found in MSI-H (≥ 2 loci, n = 8).

CONCLUSION: APC mutation is involved in carcinogenesis of intestinal type of gastric cancer and is independent of MSI phenotype but related to the LOH pathway in gastric cancer.

Differential involvement of the hypermethylator phenotype in hereditary and sporadic colorectal cancers with high-frequency microsatellite instability

High-frequency microsatellite instability (MSI-H) due to defective DNA mismatch repair occurs in the majority of hereditary nonpolyposis colorectal cancers (HNPCCs) and in a subset of sporadic malignant tumors. Clinicopathologic and genotypic features of MSI-H colorectal tumors in HNPCC patients and those in sporadic cases are very similar but not identical. Correlation between the MSI phenotype and aberrant DNA methylation has been highlighted recently. A strong association between MSI and CpG island methylation has been well characterized in sporadic colorectal cancers with MSI-H but not in those of hereditary origin. To address the issue, we analyzed hereditary and sporadic colorectal cancers for aberrant DNA methylation of target genes using methylation-specific polymerase chain reaction. DNA methylation of the MLH1, CDKN2A, MGMT, THBS1, RARB, APC, and p14ARF genes was found in 0%, 23%, 10%, 3%, 73%, 53%, and 33% of 30 MSI-H cancers in HNPCC patients and in 80%, 55%, 23%, 23%, 58%, 35%, and 50% of 40 sporadic colorectal cancers with MSI-H, respectively. Cases showing methylation at three or more loci of six genes other than MLH1 were defined as CpG island methylator phenotype-positive (CIMP +), and 23% of HNPCC tumors and 53% of sporadic cancers with MSI-H were CIMP+ (P = 0.018). Differences in the extent of CpG island methylation, coupled with the differential involvement of several genes by methylation, in HNPCC tumors and sporadic MSI-H colorectal cancers may be associated with diverging developmental pathways in hereditary and sporadic cancers despite similar MSI-H phenotypes.

DNA mismatch repair defects: role in colorectal carcinogenesis

The inactivation of the DNA mismah repair (MMR) system, which is associated with the predisposition to the hereditary non-polyposis colorectal cancer (HNPCC), has also been documented in nearly 20% of the sporadic colorectal cancers. These tumors are characterized by a high frequency of microsatellite instability (MSI(+) phenotype), resulting from the accumulation of small insertions or deletions that frequently arise during replication of these short repeated sequences. A germline mutation of one of the two major MMR genes (hMSH2 or hMLH1) is found in half to two-thirds of the patients with HNPCC, whereas in sporadic cases hypermethylation of the hMLH1 promoter is the major cause of the MMR defect. Germline mutations in hMSH6 are rare and rather confer predisposition to late-onset familial colorectal cancer, and frequent extracolonic tumors. Yet, the genetic background of a number of HNPCC patients remains unexplained, indicating that other genes participate in MMR and play important roles in cancer susceptibility. The tumor-suppressor genes that are potential targets for the MSI-driven mutations because they contain hypermutable repeated sequences are likely to contribute to the etiology and tissue specificity of the MSI-associated carcinogenesis. Because the prognosis and the chemosensitivity of the MSI(+) colorectal tumors differ from those without instability, the determination of the MSI phenotype is expected to improve the clinical management of patients. This review gives an overview of various aspects of the biochemistry and genetics of the DNA mismah repair system, with particular emphasis in its role in colorectal carcinogenesis.

Epigenetics in colorectal cancer

Malignant transformation is now known to require a series of molecular alterations that disrupt a limited number of pathways including autocrine and paracrine responses to growth factors, cell-cycle control, senescence, motility, and invasion. Studies on hereditary cancers have established genetic changes as the primary driving force for these molecular alterations. Recently, however, it has been recognized that epigenetic changes, defined as clonal changes in gene expression without accompanying changes in primary DNA coding sequence, can also be a driving force in neoplastic transformation, for selected genes, and in specific tumors. DNA methylation within gene promoters and associated alterations in histone acetylation appear primary mediators of epigenetic inheritance in cancer cells. In the large intestine, aberrant DNA methylation arises very early, initially in normal-appearing mucosa, and may be part of the age-related field defect observed in sporadic colorectal neoplasia. Aberrant methylation also contributes to later stages of colon cancer formation and progression through a hypermethylator phenotype termed cytosine phosphoguanosine (CpG) island methylator phenotype (CIMP), which appears to be a defining event in approximately half of all sporadic tumors. In sporadic colon cancer, CIMP has distinct epidemiologic and clinical features and is responsible for most cases of microsatellite instability related to hMLH1 inactivation. The recognition of epigenetic changes as a driving force in colorectal neoplasia opens new areas of research in disease epidemiology, risk assessment, screening, and treatment.

Telomere erosion is independent of microsatellite instability but related to loss of heterozygosity in gastric cancer

AIM: To correlate the length of the telomere to microsatellite instability (MSI) and loss of heterozygosity (LOH) of APC, MCC and DCC genes in gastric carcinomas.

METHODS: Telomeric restriction fragment (TRF) length of gastric cancer was measured with Southern blot. LOH of APC, MCC and DCC genes, microsatellite instability (MSI) and frameshift mutation of hMSH6, TGF-βRII and BAX genes were analyzed by PCR-based methods.

RESULTS: Sixty-eight cases of sporadic gastric carcinoma were studied for MSI using five microsatellite markers. MSI in at least one locus was detected in 17 (25%) of 68 tumors analyzed. Frameshift mutations of hMSH6, TGF-βRII and BAX were detected in 2, 6 and 3 of gastric carcinomas respectively showing high MSI (≥ 2 loci, n = 8), but none was found in those showing low MSI (only one locus, n = 9) or MSS (tumor lacking MSI or stable, n = 51). Thirty-five cases, including all high MSI and low MSI, were studied for TRF. The mean TRF length was not correlated with clinicopathological parameters. No association was observed between TRF length and MSI or frameshift mutation. On the contrary, LOH at the DCC locus was related to telomere shortening (P < 0.01). This tendency was also observed in APC and MCC genes, although there was no statistical significance.

CONCLUSION: The development of gastric cancer can arise through two different genetic pathways. In high MSI gastric cancers, defective mismatch repair allows mutations to accumulate and generate the high MSI phenotype. In gastric cancers showing either low MSI or MSS, multiple deletions may represent the LOH pathway. Telomere erosion is independent of high MSI phenotype but related to the LOH pathway in gastric cancer.