Mutations in MLH1 are more frequent than in MSH2 in sporadic colorectal cancers with microsatellite instability

Loss of MSH2 and MSH6 due to heterozygous germline defects in MSH3 and MSH6

Lynch Syndrome (LS) is the most common dominantly inherited colorectal cancer (CRC) predisposition and is caused by a heterozygous germline defect in one of the DNA mismatch repair (MMR) genes MLH1, MSH2, MSH6, or PMS2. High microsatellite instability (MSI-H) and loss of MMR protein expression in tumours reflecting a defective MMR are indicators for LS, as well as a positive family history of early onset CRC. MSH2 and MSH6 form a major functional heterodimer, and MSH3 is an alternative binding partner for MSH2. So far, the role of germline MSH3 variants remains unclear, as to our knowledge heterozygous truncating variants are not regarded causative for LS, but were detected in patients with CRC, and recently biallelic MSH3 defects have been identified in two patients with adenomatous polyposis. By gene screening we investigated the role of MSH3 in 11 LS patients with truncating MSH6 germline variants and an unexplained MSH2 protein loss in their corresponding MSI-H tumours. We report the first two LS patients harbouring heterozygous germline variants c.1035del and c.2732T>G in MSH3 coincidentally with truncating variants in MSH6. In the patient with truncating germline variants in MSH3 and MSH6, two additional somatic second hits in both genes abrogate all binding partners for the MSH2 protein which might subsequently be degraded. The clinical relevance of MSH3 germline variants is currently under re-evaluation, and heterozygous MSH3 defects alone do not seem to induce a LS phenotype, but might aggravate the MSH6 phenotype in affected family members.

Excluding Lynch syndrome in a female patient with metachronous DNA mismatch repair deficient colon- and ovarian cancer

Patients synchronously or metachronously presenting with ovarian and colon cancer can pose diagnostic challenges. A primary colon carcinoma can metastasize to one or both ovaries, two independent primary tumors can arise or an ovarian carcinoma can metastasize to the colon. Clinical and immunohistochemical characterization can aid the diagnosis. Recently, we reported that in difficult cases finding pathogenic APC variants supports a colonic origin.In this case report we describe the clinical history of a female patient suspected for Lynch syndrome. She was diagnosed with a bilateral ovarian cancer at age 44, followed by the detection of a colon carcinoma 12.5 months later. Lesions of both sites showed a DNA mismatch repair deficiency with immunohistochemical loss of MLH1 and PMS2 expression without MLH1 promoter hypermethylation. In absence of germline MMR gene variants identical somatic MLH1 and CTNNB1 gene variants were found, indicating a clonal relation. MMR germline mosaicism was made unlikely by ultra deep sequencing of the MLH1 variant in DNA isolated from normal mucosa, blood, urine and saliva. Although initially being suspect for Lynch syndrome it was eventually concluded that a metachronously diagnosed colon carcinoma that metastasized to both ovaries was most likely.

A meta-analysis of MSI frequency and race in colorectal cancer

PURPOSE

African Americans (AA) are at a higher risk of colorectal cancer (CRC) and some studies report a higher frequency of microsatellite instability (MSI) in this population while others report lower frequency compared to Caucasians.

AIM

To determine and evaluate the association of race and clinical factors with MSI frequency through meta- analysis.

METHODS

Twenty-two studies out of 15,105 (1997-2015) were evaluated after a search in different literature databases, using keywords “colorectal cancer, microsatellite instability, African Americans, Caucasians and Hispanics”. We used random effect meta-analysis to calculate the MSI frequency in all studies as well as in African American and Caucasian samples. Meta-regression analysis was used to assess the univariate effect of race, gender, age, tumor location and stage on MSI frequency.

RESULTS

The overall MSI frequency among CRCs was 17% (95%CI: 15%-19%, I²=91%). In studies with available race data, The MSI rate among AAs, Hispanics and Caucasians were 12%, 12% and 14% respectively and was not significantly different. Sub-group analysis of studies with racial information indicates MSI OR of 0.78 for AAs compared to Caucasians.

CONCLUSION

CRCs demonstrate an overall MSI frequency of 17%. MSI frequency differences between AAs and Caucasians were not pronounced, suggesting that other factors contribute to the racial disparity. The methodological approaches and biological sources of the variation seen in MSI frequency between different studies need to be further investigated.

Prevalence of pathological germline mutations of hMLH1 and hMSH2 genes in colorectal cancer

The prevalence of pathological germline mutations in colorectal cancer has been widely studied, as germline mutations in the DNA mismatch repair genes hMLH1 and hMSH2 confer a high risk of colorectal cancer. However, because the sample size and population of previous studies are very different from each other, the conclusions still remain controversial. In this paper, Databases such as PubMed were applied to search for related papers. The data were imported into Comprehensive Meta-Analysis V2, which was used to estimate the weighted prevalence of hMLH1 and hMSH2 pathological mutations and compare the differences of prevalence among different family histories, ethnicities and related factors. This study collected and utilized data from 102 papers. In the Amsterdam-criteria positive group, the prevalence of pathological germline mutations of the hMLH1 and hMSH2 genes was 28.55% (95%CI 26.04%–31.19%) and 19.41% (95%CI 15.88%–23.51%), respectively, and the prevalence of germline mutations in hMLH1/hMSH2 was 15.44%/10.02%, 20.43%/13.26% and 15.43%/11.70% in Asian, American multiethnic and European/Australian populations, respectively. Substitution mutations accounted for the largest proportion of germline mutations (hMLH1: 52.34%, hMSH2: 43.25%). The total prevalence of mutations of hMLH1 and hMSH2 in Amsterdam-criteria positive, Amsterdam-criteria negative and sporadic colorectal cancers was around 45%, 25% and 15%, respectively, and there were no obvious differences in the prevalence of germline mutations among different ethnicities.

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.

Endometrial cancer as a familial tumor: pathology and molecular carcinogenesis (review)

Some cases of endometrial cancer are associated with a familial tumor and are referred to as hereditary nonpolyposis colorectal cancer (HNPCC or Lynch syndrome). Such tumors are thought to be induced by germline mutation of the DNA mismatch repair (MMR) gene, but many aspects of the pathology of familial endometrial cancer are unclear and no effective screening method has been established. However, the pathology of endometrial cancer with familial tumor has been progressively clarified in recent studies. At present, about 0.5% of all cases of endometrial cancers meet the clinical diagnostic criteria for HNPCC. A recent analysis of the three MMR genes (hMLH1, hMSH2 and hMSH6) revealed germline mutations in 18 of 120 cases (15.0%) of endometrial cancer with familial accumulation of cancer or double cancer, with a frameshift mutation of the hMSH6 gene being the most common. Many cases with mutation did not meet the current clinical diagnostic criteria for HNPCC, indicating that familial endometrial cancer is often not diagnosed as HNPCC. The results suggest that the hMSH6 gene mutation may be important in carcinogenesis in endometrial cancer and germline mutations of the MMR gene may be more prevalent in cases associated with familial accumulation of cancer. An international large-scale muticenter study is required to obtain further information about the pathology of endometrial cancer as a familial tumor.

The G67E mutation in hMLH1 is associated with an unusual presentation of Lynch syndrome

Germline mutations in the mismatch repair (MMR) genes are associated with Lynch syndrome, also known as hereditary non-polyposis colorectal cancer (HNPCC) syndrome. Here, we characterise a variant of hMLH1 that confers a loss-of-function MMR phenotype. The mutation changes the highly conserved Gly67 residue to a glutamate (G67E) and is reminiscent of the hMLH1-p.Gly67Arg mutation, which is present in several Lynch syndrome cohorts. hMLH1-Gly67Arg has previously been shown to confer loss-of-function (Shimodaira et al, 1998), and two functional assays suggest that the hMLH1-Gly67Glu protein fails to sustain normal MMR functions. In the first assay, hMLH1-Gly67Glu abolishes the protein's ability to interfere with MMR in yeast. In the second assay, mutation of the analogous residue in yMLH1 (yMLH1-Gly64Glu) causes a loss-of-function mutator phenotype similar to yMLH1-Gly64Arg. Despite these molecular similarities, an unusual spectrum of tumours is associated with hMLH1-Gly67Glu, which is not typical of those associated with Lynch syndrome and differs from those found in families carrying the hMLH1-Gly67Arg allele. This suggests that hMLH1 may have different functions in certain tissues and/or that additional factors may modify the influence of hMLH1 mutations in causing Lynch syndrome.

Microsatellite instability in sporadic colon carcinomas has no independent prognostic value in a Belgian study population

Pathological stage is currently the most important determinant of colorectal cancer prognosis. Hence, identification of additional prognostic markers is warranted. This study aimed to analyse the prognostic relevance of microsatellite instability (MSI) in 241 colon and 90 rectal tumours, using a mononucleotide loci multiplex PCR assay and immunohistochemistry. Thirty (12.4%) colon tumours and one rectal tumour showed MSI. Although MSI was associated with proximal location and poor differentiation, no survival benefit was observed. The prognostic value of stage and differentiation was confirmed in this study. Analysis by stage revealed a longer overall (stage II/III) and disease free survival (stage II) for patients with well differentiated tumours. In addition, age and distal localisation were related to longer overall survival in stage II tumours. In conclusion, our findings show an association of MSI in sporadic colon tumours and certain clinical features; however, they do not suggest a survival benefit for MSI tumours.

Accurate classification of MLH1/MSH2 missense variants with multivariate analysis of protein polymorphisms-mismatch repair (MAPP-MMR)

Lynch syndrome, also known as hereditary nonpolyposis colon cancer (HNPCC), is the most common known genetic syndrome for colorectal cancer (CRC). MLH1/MSH2 mutations underlie approximately 90% of Lynch syndrome families. A total of 24% of these mutations are missense. Interpreting missense variation is extremely challenging. We have therefore developed multivariate analysis of protein polymorphisms-mismatch repair (MAPP-MMR), a bioinformatic algorithm that effectively classifies MLH1/MSH2 deleterious and neutral missense variants. We compiled a large database (n>300) of MLH1/MSH2 missense variants with associated clinical and molecular characteristics. We divided this database into nonoverlapping training and validation sets and tested MAPP-MMR. MAPP-MMR significantly outperformed other missense variant classification algorithms (sensitivity, 94%; specificity, 96%; positive predictive value [PPV] 98%; negative predictive value [NPV], 89%), such as SIFT and PolyPhen. MAPP-MMR is an effective bioinformatic tool for missense variant interpretation that accurately distinguishes MLH1/MSH2 deleterious variants from neutral variants.

Classification of ambiguous mutations in DNA mismatch repair genes identified in a population-based study of colorectal cancer

Identification of germline mutations in DNA mismatch repair genes in colorectal cancer probands without an extensive family history can be problematic when ascribing relevance to cancer causation. We undertook a structured assessment of the disease-causing potential of sequence variants identified in a prospective, population-based study of 932 colorectal cancer patients, diagnosed at <55 years of age. Patient samples were screened for germline mutations in MLH1, MSH2, and MSH6. Of 110 carriers, 74 (67%) had one of 33 rare variants of uncertain pathogenicity (12 MLH1, 11 MSH2, and 10 MSH6). Pathogenicity was assessed by determining segregation in families, allele frequency in large numbers of unaffected controls, effect on mRNA for putative splice-site mutations, effect on protein function by bioinformatic analysis and tumor microsatellite instability (MSI) status and DNA mismatch repair protein expression by immunohistochemistry. Because of the ambiguous nature of these variants and lack of concordance between functional assays and control allele frequency, we devised a scoring system to rank the degree of support for a pathogenic role. MLH1 c.200G>A p.G67E, MLH1 c.2041G>A p.A681T, and MSH2 c.2634+5G>C were categorized as pathogenic through assimilation of all available data, while 14 variants were categorized as benign (seven MLH1, three MSH2, and four MSH6). Interestingly, there is tentative evidence suggesting a possible protective effect of three variants (MLH1 c.2066A>G pQ689R, c.2146G>A p.V716M, and MSH2 c.965G>A p.G322D). These findings support a causal link with colorectal cancer for several DNA mismatch repair gene variants. However, the majority of missense changes are likely to be inconsequential polymorphisms.

Clinicopathological features of CpG island methylator phenotype-positive colorectal cancer and its adverse prognosis in relation to KRAS/BRAF mutation

CpG island methylator phenotype (CIMP) is a recently described subset of colorectal cancers (CRC) with widespread methylation of multiple promoter CpG islands. But the prognostic implication of CIMP in CRC has not been clarified. Thus, the aim of the present study was to differentiate the unique characteristics of CIMP from those of microsatellite instability (MSI)-high CRC, especially with regard to prognosis. CIMP, MSI, and mutations of KRAS codons 12 and 13 and of BRAF codon 600 were evaluated in 134 sporadic CRC. Patient survival and other clinicopathological variables were correlated with CIMP or genetic changes. High CIMP, high MSI, and mutations in KRAS or BRAF were detected in 31.3%, 14.2%, 33.6%, and 4.5% of overall CRC, respectively. High CIMP was closely associated with MSI and BRAF mutation but not with KRAS mutation. CIMP-high, microsatellite-stable (MSS) CRC were significantly associated with proximal location and nodal metastasis and had close but non-significant associations with liver metastasis. A worse clinical outcome was found for CIMP-high, MSS CRC with KRAS/BRAF mutation but not for those lacking KRAS/BRAF mutation. The findings support the contention that CIMP-high CRC have distinct clinicopathological and epidemiological features and suggest that the alleged poor clinical outcome of CIMP-high CRC patients is closely associated with the presence of KRAS/BRAF mutation.

Analysis of hMLH1 missense mutations in East Asian patients with suspected hereditary nonpolyposis colorectal cancer

PURPOSE

Germ line mutations in the DNA mismatch repair gene hMLH1 are a frequent cause of hereditary nonpolyposis colorectal cancer and about one-third of these are missense mutations. Several missense mutations in hMLH1 have frequently been detected in East Asian patients with suspected hereditary nonpolyposis colorectal cancer, but their pathogenic role has not been extensively assessed. The aim of this study was to perform functional analyses of these variants and their association with gastrointestinal cancer in East Asians.

EXPERIMENTAL DESIGN

Altogether, 10 hMLH1 variants were analyzed by yeast two-hybrid and coimmunoprecipitation assays.

RESULTS

The carboxyl-terminal replacements Q542L, L549P, L574P, and P581L in hMLH1 resulted in complete loss of activity in both yeast two-hybrid and coimmunoprecipitation tests and thus might be considered as pathogenic. The amino-terminal variants S46I, G65D, G67R, and R217C did not affect complex formation with hPMS2 in coimmunoprecipitation, but partly or fully lost their activity in yeast two-hybrid assay, and we suggested that these variants might reduce the efficiency of the heterodimer to go into the nucleus and thus the mismatch repair function might be blocked or reduced. The V384D and the Q701K variant resulted in the interaction of hMLH1 with hPMS2 at reduced efficiency and might raise the gastrointestinal cancer risk of the mutation carriers.

CONCLUSIONS

This work availably evaluated the functional consequences of some missense mutations not previously determined in the hMLH1 gene and might be useful for the clinical diagnosis of hereditary gastrointestinal cancer, especially in East Asians.

Mutually exclusive promoter hypermethylation patterns of hMLH1 and O6-methylguanine DNA methyltransferase in colorectal cancer

Hypermethylation of CpG islands in gene promoter regions is an important mechanism of gene inactivation in cancer. Many cellular pathways, including DNA repair, are inactivated by this type of epigenetic lesion, resulting in proposed mutator phenotypes. Promoter hypermethylation of hMLH1 has been implicated in a subset of colorectal cancers that show microsatellite instability (MSI). Transcriptional silencing of O6-methylguanine DNA methyltransferase (MGMT) has also been described in a variety of neoplasms and has been associated with a consequent mutational spectrum. We investigated the relationship between hMLH1 promoter hypermethylation and MGMT promoter hypermethylation in 110 colorectal cancers using methylation-specific polymerase chain reaction. Expression of hMLH1 and MGMT was assessed by immunohistochemistry. MSI testing was performed using the National Cancer Institute consensus panel of five microsatellite markers. Promoter hypermethylation of hMLH1 was detected in 12% of tumors. This was significantly associated with the MSI-high phenotype (P < 0.01) and loss of hMLH1 expression (P < 0.01). Methylation of the MGMT promoter was detected in 43% of tumors, which were mostly microsatellite stable or MSI-low (P = 0.041) and showed loss of MGMT expression (P < 0.01). We demonstrated an inverse relationship between hMLH1 promoter hypermethylation and MGMT promoter hypermethylation (P = 0.041), suggesting that a number of distinct hypermethylation-associated pathways may exist in colorectal cancer.

Polymorphisms in the hMSH2 Gene and the Risk of Primary Lung Cancer

Polymorphisms in the DNA repair genes may be associated with differences in the capacity to repair DNA damage, and so this can influence an individual's susceptibility to lung cancer. To test this hypothesis, we investigated the association of hMSH2 -118T>C, IVS1+9G>C, IVS10+12A>G, and IVS12-6T>C genotypes and their haplotypes with the risk of lung cancer in a Korean population. The hMSH2 genotypes were determined in 432 lung cancer patients and in 432 healthy controls who were frequency matched for age and gender. The hMSH2 haplotypes were estimated based on a Bayesian algorithm using the Phase program. The presence of at least one IVS10+12G allele was associated with a significantly decreased risk of adenocarcinoma, as compared with the IVS10+12AA genotype [adjusted odds ratio (OR), 0.59; 95% confidence interval (95% CI), 0.40-0.88; P = 0.01], and the presence of at least one IVS12-6C allele was associated with a significantly increased risk of adenocarcinoma, as compared with the IVS12-6TT genotype (adjusted OR, 1.52; 95% CI, 1.02-2.27; P = 0.04). Consistent with the results of the genotyping analysis, the TGGT haplotype with no risk allele was associated with a significantly decreased risk of adenocarcinoma, as compared with the TCAC haplotype with two risk allele [i.e., IVS10+12A and IVS12-6C allele; adjusted OR, 0.49; 95% CI, 0.30-0.78; P = 0.003 and P(c) (Bonferroni corrected P value) = 0.012]. The effect of the hMSH2 haplotypes on the risk of adenocarcinoma was statistically significant in the never smokers and younger individuals (adjusted OR, 0.45; 95% CI, 0.27-0.75; P = 0.002 and P(c) = 0.004; and adjusted OR, 0.44; 95% CI, 0.23-0.85; P = 0.014 and P(c) = 0.028, respectively) but not in the ever-smokers and older individuals. These results suggest that the hMSH2 polymorphisms and their haplotypes may be an important genetic determinant of adenocarcinoma of the lung, particularly in never smokers.

Tumor suppressor genetics

The observation that mutations in tumor suppressor genes can have haploinsufficient, as well as gain of function and dominant negative, phenotypes has caused a reevaluation of the 'two-hit' model of tumor suppressor inactivation. Here we examine the history of haploinsufficiency and tumor suppressors in order to understand the origin of the 'two-hit' dogma. The two-hit model of tumor suppressor gene inactivation was derived from mathematical modeling of cancer incidence. Subsequent interpretations implied that tumor suppressors were recessive, requiring mutations in both alleles. This model has provided a useful conceptual framework for three decades of research on the genetics and biology of tumor suppressor genes. Recently it has become clear that mutations in tumor suppressor genes are not always completely recessive. Haploinsufficiency occurs when one allele is insufficient to confer the full functionality produced from two wild-type alleles. Haploinsufficiency, however, is not an absolute property. It can be partial or complete and can vary depending on tissue type, other epistatic interactions, and environmental factors. In addition to simple quantitative differences (one allele versus two alleles), gene mutations can have qualitative differences, creating gain of function or dominant negative effects that can be difficult to distinguish from dosage-dependence. Like mutations in many other genes, tumor suppressor gene mutations can be haploinsufficient, dominant negative or gain of function in addition to recessive. Thus, under certain circumstances, one hit may be sufficient for inactivation. In addition, the phenotypic penetrance of these mutations can vary depending on the nature of the mutation itself, the genetic background, the tissue type, environmental factors and other variables. Incorporating these new findings into existing models of the clonal evolution will be a challenge for the future.

Frequent Loss of Heterozygosity but Rare Microsatellite Instability in Oesophageal Cancer in Japanese and Chinese Patients

Reported frequencies for microsatellite instability (MSI) in oesophageal cancer differ widely in the literature, perhaps due to the high incidence of loss of heterozygosity (LOH) in this cancer. Using high-resolution fluorescent microsatellite analysis (HRFMA), we analysed microsatellite alterations in detail in 50 Japanese and 50 Chinese patients with squamous cell carcinoma in the oesophagus. In HRFMA, several devices have been developed to improve the detection characteristics, reproducibility of polymerase chain reaction and the migration accuracy of electrophoresis. All the alterations observed were separable into MSI, LOH and alterations ambiguous for both. MSI was rare in these panels of oesophageal carcinomas. The frequencies of MSI in the Japanese and Chinese subjects were 8 and 4%, respectively. All the alterations were mild (within 2 base pairs) and were observed in a limited number of markers. More drastic types of MSI, such as those typical in colorectal cancer, were not observed. On the other hand, the incidence of LOH was high, reaching 50% for the Japanese and 70% for the Chinese subjects. In many of these cases, LOH was observed in multiple microsatellite markers. The frequency of LOH in each marker was not apparently biased. Although in many cases MSI and LOH were clearly distinguished with use of the sensitive and quantitative fluorescent assay, theoretically indistinguishable patterns were noted in some cases. In conclusion, MSI is rare and LOH predominates in squamous cell carcinoma in the oesophagus.

Identification of germline MSH2 gene mutations in endometrial cancer not fulfilling the new clinical criteria for hereditary nonpolyposis colorectal cancer

Endometrial cancer is the second most common malignancy in patients with hereditary nonpolyposis colorectal cancer (HNPCC). This cancer is caused by germline mutations in one of the DNA mismatch repair (MMR) genes. The present study was undertaken to analyze the relation between microsatellite instability (MSI) and germline mutations of MMR genes. We analyzed MSI in 38 cases of endometrial cancer. MSI was present in one or more (out of 5 examined) regions in 11 (29%) cases. Furthermore, alterations in MLH1 and MSH2, two culprit genes representative of HNPCC, were examined in the 11 MSI-positive patients using polymerase chain reaction-single-strand conformation polymorphism and sequencing. Germline mutations, namely, 1) a missense mutation at codon 688 (ATG-->ATA, Met-->Ile) and 2) a missense mutation at codon 390 (CTT-->TTT, Leu-->Phe) of the MSH2 gene, were found in 2 of the 11 patients (18%). Although these two cases do not fulfill the new Amsterdam criteria, they had strong family histories of colorectal and endometrial carcinoma. Our results show that genetic testing is important in cases of endometrial cancer with a history suggestive of HNPCC even if the new Amsterdam criteria are not fulfilled.

Is reduced expression of mismatch repair genes MLH1 and MSH2 in patients with sporadic colorectal cancer related to their prognosis?

The majority of mutations in hereditary nonpolyposis colon carcinoma (HNPCC) patients affect the mismatch-repair genes (MMRG) MLHI and MSH2. In addition, mutations of these genes were found in about 15% of sporadic colorectal carcinomas which appear to be related to microsatellite instability (MSI). However, mutations in MMRG were not found in all MSI-positive carcinomas, but MMRG mutations may be relevant for the assessment of tumor characteristics and patients' prognosis. Therefore, we investigated the relationship between expression of MMRG, tumor biology and patients' survival. In 127 patients with sporadic colorectal carcinomas and a minimum of 5 years follow-up after curative surgery immunohistochemical detection of MLHI and MSH2 was analyzed semiquantitatively. Lost expression of MLHI has been found in tumor specimens from 10 patients, whereas MSH2 expression was missing in 5 patients. This reduced expression did not correlate with tumor stage, lymph node involvement, grading or tumor invasion into blood vessels. However, a significant correlation was found for lymphovascular invasion (P = 0.02) and localization within the colorectum (P = 0.003) in MLH1-negative carcinomas. In addition, although there was a clear tendency for longer overall survival (72 vs. 63 months) for patients with MLH1-negative carcinomas, significant differences for overall and recurrence-free survival were not seen. In conclusion of our results and a critical review of literature, the prognostic importance of the MMR genes in sporadic colorectal carcinomas remains controversial.

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.

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

Microsatellite instability (MSI) analysis was performed using a "reference panel" of microsatellite markers in 345 unselected primary colorectal cancers (CRC). Thirty-five (10%) tumors were classified as high MSI (MSI-H). We identified 6 (17%) MSI-H tumors with germline mutations in mismatch repair (MMR) genes (tumors from patients with hereditary non-polyposis colorectal cancer (HNPCC) syndrome) and 29 (83%) MSI-H tumors without germline MMR mutations (sporadic MSI-H tumors). Hypermethylation of the hMLH1 promoter was found in 26/29 (90%) sporadic MSI-H tumors but only in 1/6 (17%) HNPCC tumors (P<.001). Somatic alterations were identified in both MMR genes in HNPCC tumors but mainly in the hMSH2 gene in sporadic MSI-H tumors. LOH at MMR loci was detected in 3/6 (50%) HNPCC tumors and in 4/26 (15%) informative sporadic MSI-H tumors. These results together indicate different mode of inactivation of MMR genes in sporadic MSI-H tumors versus MSI-H tumors in HNPCC patients. We therefore propose that MSI analysis of newly diagnosed primary CRC followed by methylation analysis of hMLH1 promoter in MSI-H tumors and mutational analysis of MMR genes in MSI-H tumors lacking hMLH1 promoter methylation might be an efficient molecular genetic approach for HNPCC screening.

Optimization of experimental conditions for RNA-based sequencing of MLH1 and MSH2 genes

The most sensitive technique for the detection of germline mutations is exon by exon sequencing of the gene under investigation using genomic DNA as a template for analysis. This approach, however, has cost and sensitivity limitations that can, at least in part, be overcome by RNA-based analysis. Germline mutations of MLH1 and MSH2 are the most frequent cause of the inherited susceptibility to colorectal and other epithelial cancers known as hereditary non-polyposis colorectal cancer (HNPCC). We compared the analysis of the MLH1 and MSH2 genes using mRNA and genomic DNA as starting material from 21 HNPCC patients. All samples were investigated by RT-PCR, sequencing of cDNA and simultaneous sequencing of genomic DNA. The cDNA was generated using specific primers complementary to the ends of MLH1 and MSH2 genes, respectively. Mutations in MLH1 and MSH2 were detected in 11 out of 21 unrelated patients. In 10 out of 11 cases, mutations were detected independently of the type of primers used for reverse transcription (RT). One novel missense mutation (K751R) in MLH1 was detected using this method. One nonsense mutation (E205X) in MSH2 was only detectable when RT was performed using MSH2 gene-specific primers. Shorter PCR products indicative of alternatively spliced transcripts were not observed when MLH1 or MSH2 specific cDNA RT primers were employed to generate template, except in one case where exon skipping was observed for exons 9 and 10. In this report we demonstrate that primers specific for RT of MLH1 and MSH2 are crucial for increasing the sensitivity of cDNA analysis. DNA sequencing using RNA as a basis for template construction may be a valuable and economical alternative to genomic DNA sequencing.

Four new mutations in the DNA mismatch repair gene MLH1 in colorectal cancers with microsatellite instability. Mutations in brief no. 157. Online

Hereditary nonpolyposis colorectal cancer (HNPCC) is frequently associated with inherited mutation in one of four DNA mismatch repair genes. Somatic mutations in the same genes are also found in a subset of sporadic colorectal cancers. A defect in DNA mismatch repair results in a RER (replication error) tumor phenotype. We screened 110 archival and 11 prospectively acquired colorectal cancers for the RER phenotype. A total of 22 cancers were RER-positive. RER-positive tumors were investigated for mutations in the DNA mismatch repair gene MLH1 using single-strand-conformation-polymorphism (SSCP) analysis. We identified four previously undescribed mutations in four different samples. Three mutations were exonic: a point mutation at codon 69 (AGG-->AAG(arg-->lys]); a single base pair deletion at codon 42/43 (GCAAAATCC-->GCAAATCC) leading to a new stop codon downstream; and a point mutation at codon 757 (TAA-->TAT [termination-->tyr] which extend the MLH1 peptide by 36 ammino acids. The fourth mutation was a 1 base pair insertion six base pairs 5' to the start of exon 14 (tttgtttt-->tttggtttt). The mutations were not seen in the patients' constitutional DNA. The somatic MLHI mutations identified appear to be causally associated with the RER phenotype.

Value of pedigree/clinical data, immunohistochemistry and microsatellite instability analyses in reducing the cost of determining hMLH1 and hMSH2 gene mutations in patients with colorectal cancer

The aim of this study was to evaluate the significance of pedigree/clinical data, immunohistochemistry (IHC) and microsatellite instability (MI) analyses in the reduction of costs of constitutional hMLH1 and hMSH2 gene mutation diagnosis in patients with colorectal cancers (CRC). Pedigree/clinical data were evaluated on a series of 168 patients with CRC, including 43 consecutive sporadic late-onset and 25 consecutive, definitive or suspected hereditary non-polyposis colorectal cancer (HNPCC) cases, examined by IHC and MI analyses. In the latter group, 6/25 (24%) constitutional mutations were found. We detected no germline mutations in the sporadic late-onset patients. The lowest costs (880 Euro/mutation detected) were achieved by performing pedigree/clinical data (for exclusion of late-onset sporadic CRC) in conjuction with IHC only. In this model 1/6 (17%) mutations was missed. Additional preselection by IHC and MI analyses before sequencing was required to detect all mutations. In this approach, which seems to be the most effective in the search for hMLH1 and hMSH2 gene mutation, the cost was 1767 euro/mutation detected.

Differential prognosis of replication error phenotype and loss of heterozygosity in sporadic colorectal cancer

Several distinct genetic alterations have been associated with colorectal tumorigenesis. This study investigated the frequency of microsatellite instability, also known as replication error (RER), and loss of heterozygosity (LOH) at six chromosome regions in sporadic colorectal cancer (CRC). Eighty-six tumour and paired normal mucosa samples were included in the study. A polymerase chain reaction (PCR)-based technique was performed to analyse six (CA)n dinucleotide repeats located near or within regions containing important genes implicated in the complex process of colorectal tumorigenesis (chromosomes 2p, 3p, 5q, 11p, 17p and 18q). Overall, LOH frequency was higher in RER-tumours (25/46, 54.3%) compared with RER+ tumours (9/40, 22.5) (P = 0.04). To investigate prognostic implications, survival analysis was performed for 66 patients. Compared with RER- tumours, patients with RER+ tumours at 2p, 3p, 5q, 11p or 18q were found to have an improved prognosis (overall survival, P = 0.02 and disease-free survival (DFS) P = 0.005) this variable being an independent prognostic factor by multivariate analysis (P = 0.001). Overall survival of patients whose tumours were LOH+ was significantly shorter compared with those without LOH (overall survival, P = 0.008 and DFS, P = 0.01). Thus, tumours displaying RER+ and LOH+ phenotype, as established by microsatellite analysis, show a differential prognosis. These data indicate that this may be a useful tool for the identification of patients at different risks affected by CRC.

The "comparative growth assay": examining the interplay of anti-cancer agents with cells carrying single gene alterations

We have developed a "comparative growth assay" that complements current assays of drug effects based on cytotoxicity. A co-culture of two cell lines, one of which is fluorescently labeled, is exposed to a cytotoxic agent and the proportion of fluorescent cells is compared with that of a baseline unexposed co-culture. For demonstration purposes, two HCT116 cell lines (an hMLH1 homozygous and an hMLH1 heterozygous mutant), altered by insertion of vector alone or the same vector carrying an insert for the expression of enhanced green fluorescent protein (EGFP), were exposed to numerous "anti-cancer" agents. The assay was further validated in a system of two cell lines differing only in the expression of the breast cancer resistance protein (BRCP). The assay allowed the estimation of the duration of action of a particular agent. Assessment of the agent's differential activity over a given time in culture could be expressed as a selection rate, which we chose to describe on an "average selection per day" basis. We conclude that this assay: 1) provides insight into the differential dynamic effects of chemotherapeutic agents or radiation; and 2) allows, through the use of matched cell lines, the investigation of critical physiologic features that govern cell sensitivity.

Mutational analysis of the PMS2 gene in sporadic endometrial cancers with microsatellite instability

OBJECTIVE

Approximately 20% of endometrial tumors have a defect in DNA mismatch repair and exhibit microsatellite instability (MSI). We assessed the role of the PMS2 DNA mismatch repair gene in MSI-positive sporadic endometrial tumors.

METHODS

We examined 40 sporadic endometrial tumor specimens with MSI. All 15 exons of the PMS2 gene were investigated for sequence alterations by single-strand conformational variant analysis.

RESULTS

Twelve polymorphisms were identified, 8 of which were in the coding sequence. Four specimens revealed mutations in intronic sequences that are not predicted to affect the PMS2 mRNA. No mutations were detected within the coding region of the PMS2 gene.

CONCLUSION

We conclude that structural mutations in the PMS2 gene are not responsible for defective DNA mismatch repair in sporadic endometrial cancers with MSI. The identification of single nucleotide polymorphisms in the PMS2 locus may aid in the mapping and characterization of genetic diseases.

Mutator phenotypes of common polymorphisms and missense mutations in MSH2

Hereditary non-polyposis colorectal cancer (HNPCC) is associated with germline mutations in the DNA mismatch repair gene hMSH2 [1], the human homologue of the Escherichia coli MutS gene. These are mostly nonsense, frameshift or deletion mutations that result in loss of intact protein and complete inactivation of DNA mismatch repair. However, cancer is also associated with hMSH2 missense mutations that are merely inferred to be deleterious because they result in non-conservative substitutions of amino acids that are highly conserved among MutS family proteins. Moreover, sequence polymorphisms exist in hMSH2 that also change conserved amino acids but whose functional consequences and relationship to cancer are uncertain. Here, we show that yeast strains harboring putative equivalents of three hMSH2 polymorphisms have elevated mutation rates. Mutator effects were also observed for yeast equivalents of hMSH2 missense mutations found in HNPCC families and in an early onset colon tumor. Several distinct phenotypes were observed, indicating that these missense mutations have differential effects on MSH2 function(s). The results suggest that cancer may be associated with even partial loss of hMSH2 function and they are consistent with the hypothesis that polymorphisms in hMSH2 might predispose humans to disease.

Medullary-type poorly differentiated adenocarcinoma of the large bowel: a distinct clinicopathologic entity characterized by microsatellite instability and improved survival

PURPOSE

Recent studies suggest the existence of a distinct class of poorly differentiated large bowel adenocarcinomas, usually termed medullary-type adenocarcinomas (MTAs). The aim of the present study was to accurately define the clinical, histopathologic, biologic, and genetic features of this tumor type.

MATERIALS AND METHODS

Among 1,265 surgically resected sporadic colorectal carcinomas, 45 MTAs were identified on the basis of the following criteria: predominantly solid growth pattern (at least 70% of the tumor area) and lack of marked nuclear pleomorphism. The clinicopathologic, biologic, and genetic characteristics of MTAs were compared with those of a series of 457 common glandular colorectal adenocarcinomas.

RESULTS

The significantly different clinicopathologic features of MTAs were proximal location, large size, invasion into adjacent organs, expanding pattern of growth, low incidence of distant metastases, more frequent conspicuous peritumoral lymphocytic infiltration, and Crohn's-like lymphoid reaction. Furthermore, young patients with MTAs often demonstrated a family history highly suggestive of a hereditary background. Unlike glandular adenocarcinomas, the large majority of MTAs were DNA diploid by flow cytometric analysis (21 of 25, 84%) and p53 negative by immunohistochemistry (36 of 41, 87.8%). In addition, 18 of the 20 MTAs examined by DNA microsatellite analysis demonstrated widespread microsatellite instability (90% of cases). Patients with MTAs showed a better clinical outcome with respect to patients with common poorly differentiated adenocarcinomas (PDAs) (P <.0001) and well- or moderately differentiated adenocarcinomas (WMDAs) (P =.133). In particular, none of the 33 patients with completely resectable stage II and III MTAs developed tumor recurrence during the observation period. Conversely, 24.7% of patients with stage II and III WMDAs and 48.9% of patients with stage II and III PDAs, who had undergone curative surgical resection, died of recurrent disease (P =.01 and P <.0001, respectively).

CONCLUSION

All these data strongly indicate that MTAs represent a distinct pathologic entity, with specific histologic appearance and peculiar clinical and genetic features. These tumors need to be classified separately from other poorly differentiated colorectal carcinomas.

Influence of selection criteria on mutation detection in patients with hereditary nonpolyposis colorectal cancer

BACKGROUND

Hereditary nonpolyposis colorectal cancer (HNPCC) is linked genetically to mutations in DNA mismatch repair (MMR) genes. Because a deficiency in MMR does not predict a specific phenotype, the original selection criteria may be too restrictive in identifying additional families. The current study was performed to determine whether a relaxation of the Amsterdam criteria (AC) could be applied to identify more families associated with DNA MMR.

METHODS

Twenty-eight unrelated Swiss families (15 complying with the AC and 13 fulfilling extended criteria [EC] to include other tumors of the HNPCC spectrum as well) were screened for mutations in the MMR genes hMSH2 and hMLH1, using single-stranded conformation polymorphism and direct DNA sequencing. Microsatellite instability (MSI) was determined in 14 families. A comparison was made between the phenotypic characteristics of the mutation positive and mutation negative families.

RESULTS

Ten AC families (67%) harbored germline mutations in hMLH1 (6 kindreds) or hMSH2 (4 kindreds). In none of the EC kindreds could an unambiguous disease-causing mutation be identified. Seven of eight AC families were found to display MSI whereas all colorectal carcinomas (CRC) in eight EC kindreds were MSI stable. CRC patients from mutation positive families had an earlier age at diagnosis (44 years vs. 49 years) and appeared to have a better survival (11.1 years vs. 7.7 years).

CONCLUSIONS

Extending the AC to include extracolonic tumors of the HNPCC spectrum results in a very low mutation detection rate for hMSH2 and hMLH1. The EC families appear to represent an alternative genetic entity not necessarily related to DNA MMR gene mutations because they do not display MSI.

Mutated gene-specific phenotypes of dinucleotide repeat instability in human colorectal carcinoma cell lines deficient in DNA mismatch repair

Mutations in DNA mismatch repair (MMR) genes in hereditary non-polyposis colon cancer (HNPCC) patients revealed the importance of MMR deficiency as a risk for carcinogenesis. Since diverse mutations occur in several MMR genes, the instability of repeat sequences dispersed in the genome, which are also governed by the MMR system, is a well used marker. However, the relationship between repeat sequence instability and MMR gene mutation in human cells has not been well defined mainly because precise systems to analyse repeat sequences have not been available. Using our newly developed system, we analysed alteration of dinucleotide repeats in human cell lines which harbour mutations in MMR genes. Among 24 subclones of DLD-1 cells (hMSH6-) only one had a dinucleotide repeat alteration in only one microsatellite locus, while LoVo cells (hMSH2-/hMSH6-) exhibited marked dinucleotide repeat instability (DRI). HCT116 cells, a hMLH1-mutant, showed an ultimate DRI phenotype. Interestingly, SW48 cells lacking hMLH1 expression also demonstrated DRI, albeit the extent of diversity being significantly lower than HCT116. These data suggest that the DRI phenotype in human cells is highly dependent on mutated MMR genes and on forms of mutation. The results of DRI analyses used to detect MMR-deficiency should be interpreted with caution.

A specific CpG methylation pattern of the MGMT promoter region associated with reduced MGMT expression in primary colorectal cancers

The enzyme O6-methylguanine-DNA methyltransferase (MGMT) protects cells from the cytotoxic and mutagenic effects of alkylating agents. Approximately 20% of tumor cell lines lack MGMT activity and are highly sensitive to alkylating agents. In established cancer cell lines, MGMT expression appears to be correlated with methylation of residues in both the promoter and the body of the gene. The effect of methylation of the MGMT promoter on gene expression and carcinogenesis in primary tumors is unknown. We investigated methylation of the MGMT promoter region in primary colorectal cancers and normal colonic mucosa. We used five methylation-sensitive restriction enzymes (BssHII, SacII, Eagl, Nael, and Smal) and Southern blot analysis to assess methylation in 46 cancers and 22 controls. Methylation of Eagl and Nael sites was seen in 12 tumors but in none of the 22 normal colorectal mucosa specimens. This difference was statistically significant (P<0.01). Methylation-sensitive single-nucleotide primer extension analysis of four additional cytosine residues confirmed methylation of the promoter region in the tumors identified by Eagl and Nael digestions and served to further quantitate the extent of methylation. Western blot analysis of 21 tumors revealed statistically significant lower MGMT expression in the eight tumors with methylation of the Eagl and Nael sites and nt -128 than in the 13 tumors lacking the methylation pattern (P<0.05). MGMT activity was lower in tumors with methylation than in tumors that were not methylated. The difference was not, however, statistically significant. We conclude that a subset of colorectal tumors is characterized by a specific methylation pattern in the MGMT promoter associated with reduced MGMT expression.

Microsatellite instability in human cancer: a prognostic marker for chemotherapy?

The majority of tumors associated with the nonpolyposis form of familial colorectal cancer (HNPCC) shows a specific form of genetic instability which is manifested by length alterations of mono- or dinucleotide repeat sequences [e.g., (A)n or (CA)n]. This phenomenon was termed the RER+ (replication error-positive) phenotype, MSI or MIN (microsatellite instability), and found to result from defects in the cells' DNA mismatch repair system. This system recognizes and restores misincorporated bases or slippage errors which frequently occur during DNA replication. Loss of DNA mismatch repair therefore strongly accelerates the evolutionary process of mutagenesis and selection which underlies the development of cancer. In addition to mutation avoidance, DNA mismatch repair also plays a crucial role in the toxicity of a number of DNA-damaging drugs that are used in cancer chemotherapy. In experimental systems, mismatch-repair-deficient cells are highly tolerant to the methylating chemotherapeutic drugs streptozocin and temozolomide and, albeit to a lesser extent, to cisplatin and doxorubicin. These drugs are therefore expected to be less effective on mismatch-repair-deficient tumors in humans. MIN was also found in a substantial portion of sporadic (nonfamilial) human tumors. However, in many cases the extent of microsatellite instability was not as dramatic as found in HNPCC-related tumors and the underlying genetic defect is unclear. Therefore, while the mismatch repair status of tumors may become an important determinant in the choice of chemotherapeutic intervention, the significance of MIN in sporadic cancer remains elusive.

Differences in patterns of TP53 and KRAS2 mutations in a large series of endometrial carcinomas with or without microsatellite instability

BACKGROUND

The correlation between tumor microsatellite instability (MSI) and the accumulation of mutations in KRAS2 and TP53 is uncertain. The authors evaluated the TP53 and KRAS2 genes for mutations in sporadic endometrial carcinomas with microsatellite instability (MSI) and matched MSI negative controls to determine whether defective DNA mismatch repair impacts the patterns of mutations in two genes known to be involved in endometrial tumorigenesis.

METHODS

Twenty-five MSI positive endometrial tumors were matched for prognostic factors with 25 MSI negative tumors. Mutations in codon 12 and 13 of KRAS2 were assessed using a polymerase chain reaction (PCR) restriction assay. Mutations in codon 61 of KRAS2 and exons 5-8 of TP53 were evaluated using PCR amplification and single strand conformation variant (SSCV) analysis. All variants were subjected to direct DNA sequencing.

RESULTS

KRAS2 and TP53 mutations were identified with equal frequency in the MSI positive and MSI negative groups. For TP53, the authors identified 5 mutations (20%) in the MSI positive specimens compared with 8 (32%) in the MSI negative group. For KRAS2, there were identified 8 mutations (32%) in the MSI positive specimens compared with 7 (28%) in the MSI negative tumors. The mutational spectra evident from sequence analysis of TP53 and KRAS2 variants were similar between MSI negative and MSI positive tumors. MSI negative tumors were more likely to have mutations in both KRAS2 and TP53 than MSI positive tumors, which were rarely mutant in both genes (P=0.046).

CONCLUSIONS

Although the overall frequency of mutations in TP53 and KRAS2 is similar, MSI positive tumors are less likely to have mutations in both genes than MSI negative sporadic endometrial carcinomas. MSI positive and MSI negative endometrial carcinomas may arise through distinct genetic pathways.

Accumulated clonal genetic alterations in familial and sporadic colorectal carcinomas with widespread instability in microsatellite sequences

A subset of hereditary and sporadic colorectal carcinomas is defined by microsatellite instability (MSI), but the spectra of gene mutations have not been characterized extensively. Thirty-nine hereditary nonpolyposis colorectal cancer syndrome carcinomas (HNPCCa) and 57 sporadic right-sided colonic carcinomas (SRSCCa) were evaluated. Of HNPCCa, 95% (37/39) were MSI-positive as contrasted with 31% (18/57) of SRSCCa (P < 0.000001), but instability tended to be more widespread in SRSCCa (P = 0.08). Absence of nuclear hMSH2 mismatch repair gene product by immunohistochemistry was associated with germline hMSH2 mutation (P = 0.0007). The prevalence of K-ras proto-oncogene mutations was similar in HNPCCa and SRSCCa (30% (11/37) and 30% (16/54)), but no HNPCCa from patients with germline hMSH2 mutation had codon 13 mutation (P = 0.02), and two other HNPCCa had multiple K-ras mutations attributable to subclones. 18q allelic deletion and p53 gene product overexpression were inversely related to MSI (P = 0.0004 and P = 0.0001, respectively). Frameshift mutation of the transforming growth factor beta type II receptor gene was frequent in all MSI-positive cancers (85%, 46/54), but mutation of the E2F-4 transcription factor gene was more common in HNPCCa of patients with germline hMSH2 mutation than in those with germline bMLH1 mutation (100% (8/8) versus 40% (2/5), P = 0.04), and mutation of the Bax proapoptotic gene was more frequent in HNPCCa than in MSI-positive SRSCCa (55% (17/31) versus 13% (2/15), P = 0.01). The most common combination of mutations occurred in only 23% (8/35) of evaluable MSI-positive cancers. Our findings suggest that the accumulation of specific genetic alterations in MSI-positive colorectal cancers is markedly heterogeneous, because the occurrence of some mutations (eg, ras, E2F-4, and Bax genes), but not others (eg, transforming growth factor beta type II receptor gene), depends on the underlying basis of the mismatch repair deficiency. This genetic heterogeneity may contribute to the heterogeneous clinical and pathological features of MSI-positive cancers.

Comparative genomic hybridization reveals differences in DNA copy number changes between sporadic gastric carcinomas and gastric carcinomas from patients with hereditary nonpolyposis colorectal cancer

Comparative genomic hybridization was used to search for DNA copy number changes in samples of gastric cancer from 12 hereditary nonpolyposis colon cancer (HNPCC) patients and in samples of sporadic gastric carcinoma from 13 patients. The gastric cancer samples from HNPCC patients showed gains affecting 19q, Xp, and whole chromosome 22, each in a single patient. Neither high-level amplifications nor losses of DNA copy number were detected. On the other hand, 10 of the 13 (77%) sporadic gastric carcinoma samples had multiple DNA copy number changes. The most frequent gains occurred with minimal common overlapping regions at 1q22-q31, 8q23-qter, 17p11.2-q22, and 20q, all at a frequency of 31%. High-level amplifications were also seen at 17q21 in three cases (23%). Losses were rare, and the most frequent loss was with a minimal common overlapping region at 4q32 (23%). This suggests that multiple DNA copy number changes are needed for the development of sporadic gastric carcinoma but not for gastric carcinoma in HNPCC patients.

Mutational analysis of MLH1 and MSH2 in 25 prospectively-acquired RER+ endometrial cancers

Mutations in the DNA mismatch repair (MMR) genes MLH1 and MSH2 have been linked to several human cancers which display the replication error (RER) phenotype. Germline mutations in these two genes have been implicated in about 90% of families with hereditary nonpolyposis colorectal cancer (HNPCC). A significant proportion of endometrial cancers, the second most common malignancy of the HNPCC syndrome, also exhibit RER. We screened 125 primary endometrial adenocarcinomas with seven microsatellite markers and identified 25 specimens with RER (20%). We used single-strand conformation variant analysis to search for mutations in MLH1 and MSH2. Direct sequencing of variants revealed only one germline mutation in MLH1 and a single somatic mutation in MSH2. However, six previously unreported sequence polymorphisms in MLH1 were identified. Four of these polymorphisms show clear population-based differences in allele frequency. In addition, a highly informative marker for MLH1 was characterized. The low frequency of mutations in MLH1 and MSH2 in this large series of cancers suggests that other MMR genes are responsible for the RER phenotype in endometrial cancers.