Mutation of a mutL homolog in hereditary colon cancer

Identification of women at high genetic risk of breast cancer through the National Health Service Breast Screening Programme (NHSBSP)

Breast cancer is a multifactorial disease with an inherited predisposition being implicated in around 5% of all cases. Using previous epidemiological data assessing risks for the relatives of women with breast cancer, we have identified 154 women (from a screened population of 35,505) and 289 of their relatives between 50 and 64 years who have more than twice the age related risk of developing breast cancer. This constitutes 1.24% of the breast screening population attending the North East Scotland NHSBSP. For each woman identified to be at high risk, we have found 1.87 female relatives between 50 and 64 years and 1.85 relatives under 50 years also to be at high risk. Around 78% of the women identified with a significant family history of breast or other cancer have attended for counselling about their risks. The breast screening programme can be used to identify women at high risk of breast cancer in order to offer them (and their relatives) access to genetic counselling and appropriate screening.

Predominance of normal karyotype in colorectal tumors from hereditary non-polyposis colorectal cancer patients

We report the cytogenetic study of 9 colorectal tumors arising in patients with hereditary non-polyposis colorectal cancer (HNPCC). According to the cytogenetic classification of colorectal tumors previously proposed by us, 2 cases were of the trisomic type, 2 were of the monosomic type, and 5 had a normal karyotype. This represents a significant excess of tumors with normal karyotype in HNPCC tumors (56%) compared to sporadic cases (10/184 = 5%).

Molecular biology of colon polyps and colon cancer

From a histologic and endoscopic standpoint, colon and rectal cancer (CRC) begins as a small neoplastic polyp which progressively enlarges and transforms through a dysplasia stage into invasive cancer. Recently, molecular abnormalities underlying the adenomacarcinoma progression have been defined. The adenomatous polyposis coli (APC) gene and mismatch repair genes are found to be dysfunctional early in the neoplastic process; either as inherited or somatic mutations. Subsequently, polyps progress to cancer along one of two paths depending on which gene is abnormal. When the APC gene is the initial mutation tumor development follows the "loss of heterozygocity" (LOH) pathway. If mismatch repair genes are altered, the "replication error" (RER) pathway is followed. Somatic mutations of the K-ras oncogene and the MCC, DCC, and p53 tumor suppressor genes accumulate in the LOH pathway and mark the progression through polyp stages. Microsatellite instability is a characteristic of the RER pathway but the precise genes involved in this pathway currently are not known. Defining these pathways has led to a new classification scheme for CRC with resultant changes in our clinical approach to screening, surveillance, and treatment.

DNA mismatch repair and cancer

The genetic basis of cancer involves certain classes of genes, particularly oncogenes, tumor-suppressor genes, and DNA mismatch repair genes. Originally identified in bacteria and yeast, the human homologues of DNA mismatch repair genes have been implicated in the pathogenesis of the hereditary nonpolyposis colorectal cancer syndromes, as well as a variety of different sporadic cancers. An appreciation of their role in cancer is predicated on an understanding of their function in the processes of DNA repair. This article reviews the recent developments and advances in the biology of the human DNA mismatch repair genes and their involvement in the pathogenesis of cancer.

Founding mutations and Alu-mediated recombination in hereditary colon cancer

By screening members of Finnish families displaying hereditary nonpolyposis colorectal cancer (HNPCC) for predisposing germline mutations in MSH2 and MLH1, we show that two mutations in MLH1 together account for 63% (19/30) of kindreds meeting international diagnostic criteria. Mutation 1, originally detected as a 165-base pair deletion in MLH1 cDNA comprising exon 16, was shown to consist of a 3.5-kilobase genomic deletion most likely resulting from Alu-mediated recombination. Mutation 2 destroys the splice acceptor site of exon 6. A simple diagnostic test based on polymerase chain reaction was designed for both mutations. Our results show that these two ancestral founding mutations account for a majority of Finnish HNPCC kindreds and represent the first report of Alu-mediated recombination causing a prevalent, dominantly inherited predisposition to cancer.

Deletion of 1p loci and microsatellite instability in colorectal polyps

Previous cytogenetic studies have indicated that a subset of large bowel adenomas have distal 1p deletions. We addressed this question by examining 70 sporadic polyps (63 adenomas, 5 hyperplastic polyps, and 2 polyps of undetermined histology) from 55 patients for alterations at eight loci on the short arm of chromosome 1 and found allelic imbalance (AI) or loss of one allele (LOH) in 14 (20%). The locus most frequently changed was MSI, which maps to 1p33-35. Fluorescence in situ hybridisation with centromeric and telomeric probes for chromosome 1, performed for 11 polyps, did not yield an abnormal number of signals, in accordance with the interpretation that the observed AI and LOH were the result of interstitial deletions in 1p. Whereas allelic imbalance at five other loci (mapping to 5q, 8p, 10p, 11p and 17q) was found less frequently, and then mainly in large (> 2 cm) tumours, the 1p alterations were equally distributed among small (< 1 cm) and large polyps. They were preferentially found in left-side tumours. Instability at microsatellite loci--the mutator phenotype--is demonstrated by shifts in the electrophoretic mobility of normal alleles. The mutator phenotype was first associated with hereditary nonpolyposis colorectal cancer but is also occasionally found in sporadic colorectal carcinomas; however, it is still uncertain when in the adenoma-carcinoma sequence in this type of genomic instability arises. We therefore looked for it at 12 dinucleotide repeat loci and found that seven tumours (six adenomas and one hyperplastic polyp) from seven patients had acquired new alleles not seen in the patients' corresponding normal DNA. Our results suggest that inactivation of a putative suppressor gene distally in chromosome arm 1p is an early event in colorectal tumourigenesis. They also show that microsatellite instability can be detected in large bowel polyps, indicating that this phenomenon, too, probably plays a pathogenic role for some colorectal tumours early in the adenoma-carcinoma sequence.

Mutations in the MSH3 gene preferentially lead to deletions within tracts of simple repetitive DNA in Saccharomyces cerevisiae

Eukaryotic genomes contain tracts of DNA in which a single base or a small number of bases are repeated (microsatellites). Mutations in the yeast DNA mismatch repair genes MSH2, PMS1, and MLH1 increase the frequency of mutations for normal DNA sequences and destabilize microsatellites. Mutations of human homologs of MSH2, PMS1, and MLH1 also cause microsatellite instability and result in certain types of cancer. We find that a mutation in the yeast gene MSH3 that does not substantially affect the rate of spontaneous mutations at several loci increases microsatellite instability about 40-fold, preferentially causing deletions. We suggest that MSH3 has different substrate specificities than the other mismatch repair proteins and that the human MSH3 homolog (MRP1) may be mutated in some tumors with microsatellite instability.

Microsatellite instability in oral cancer

Generalized genomic instability, detected as somatic changes in allele sizes at microsatellite loci in tumors compared to peripheral lymphocyte DNA, is a recently recognized mechanism of mutation in cancer. Such instability results from the somatic loss of DNA mismatch repair capability. Germ-line mutations at DNA mismatch repair loci confer susceptibility to colon cancer in hereditary non-polyposis colorectal cancer. Somatic loss of DNA mismatch repair has been reported in a large variety of other tumor types. Our goal was to determine the frequency of microsatellite instability in a large series of oral tumors. Out of 91 tumors analyzed for microsatellite instability, 6 (7%) showed microsatellite instability. Instability was observed at multiple loci with a range of 50-74% of loci affected. Alterations include both increase (74%) and decrease (26%) in allele sizes. The proportion of alleles affected ranged from 30-58% of all alleles. Our data suggest that somatic genomic instability plays a role in the pathogenesis of a small subset of oral tumors.

In vitro transcription/translation assay for the screening of hMLH1 and hMSH2 mutations in familial colon cancer

BACKGROUND & AIMS

Hereditary nonpolyposis colorectal cancer (HNPCC) has been linked recently to a defect in repairing mismatched nucleotides in DNA. The aim of this study was to screen for germline mutations that result in prematurely truncated proteins in two of the mismatch repair genes identified at this time, hMLH1 and hMSH2, in a consecutive series of patients belonging to familial aggregations of colorectal cancer.

METHODS

Nineteen individuals with colorectal cancer from 19 families were consecutively referred because of a strong positive family history of colorectal cancer. Premature truncation mutations in hMLH1 and hMSH2 were sought from lymphocyte RNA by using an in vitro transcription/translation (IVTT) assay.

RESULTS

Protein truncating mutations in the hMLH1 or hMSH2 genes were found in 50% of families with HNPCC (6 of 12) but were not observed in any of the remaining familial aggregations that did not fulfill the standard criteria for HNPCC. In some of the IVTT-positive samples, the mutations were characterized by genomic sequencing.

CONCLUSIONS

IVTT may be a practical method to accomplish primary screening of germline mutations in DNA mismatch pair genes in HNPCC; however, a broader approach is necessary to obtain a more complete picture of the mutational spectrum in HNPCC and other familial aggregations of colorectal cancer.

Mismatch repair: mechanisms and relationship to cancer susceptibility

DNA mismatch-repair systems exist that repair mispaired bases formed during DNA replication, genetic recombination and as a result of damage to DNA. Some components of these systems are conserved in prokaryotes and eukaryotes. Genetic defects in mismatch-repair genes play an important role in common cancer-susceptibility syndromes and sporadic cancers.

Clinical impact of colonoscopic screening in first-degree relatives of patients with hereditary non-polyposis colorectal cancer

Sixty-one asymptomatic individuals with an affected first-degree relative from five large hereditary non-polyposis colorectal cancer (HNPCC) kindreds were screened by colonoscopy. Neoplasms were found in nine (15 per cent) of 61 individuals on the first screen. Five subjects had a single adenoma while two had two adenomas each. There were two patients (3 per cent) with malignant neoplasms: one with a Dukes B adenocarcinoma and one with synchronous Dukes C adenocarcinomas in the caecum and ascending colon. These findings support the hypothesis that adenomas do not occur in large numbers in HNPCC families but, because of the high malignant conversion rate, biennial colonoscopy with removal of polyps seen is recommended.

Microsatellite instability and loss of heterozygosity in melanoma

Alterations in the repeat length of microsatellites have been identified recently in tumors arising in patients with hereditary nonpolyposis colon cancer and in several human sporadic tumors. We examined 40 sporadic melanomas and their corresponding nontumorous skin for microsatellite instability (MSI) and loss of heterozygosity (LOH) at chromosomes 2q, 3p25-26, 5q11.2-13.3, 5q21, 6q27, 9p21, 9p22-pter, 17p12, 17p12-p11.1, and 18q23. Specific loci were amplified by polymerase chain reaction, electrophoresed on polyacrylamide gels, transferred onto nylon membranes, and hybridized with 33P-end-labeled oligonucleotides. MSI was observed in eight of 40 (20%) melanomas at one of 10 loci examined. LOH was found at chromosome region 9p21 in 40%, at 9p22 in 22%, and at 17p in 13% of the informative cases. Comparison between clinicopathologic features of patients with and without MSI revealed no obvious differences. LOH at 9p21 was observed only in lesions greater than 1.5 mm in depth, suggesting that it does not represent an early event in sporadic melanoma. Our results indicate that 1) MSI is a genetic alteration in a proportion of sporadic melanoma, which may reflect a defect in genes involved in DNA replication fidelity; and 2) LOH at chromosome region 9p21 is a significant event in sporadic melanoma. The latter finding further supports the hypothesis that the 9p21 region may contain one or more tumor suppressor genes (e.g., MTS1/CDNK2) involved in the pathogenesis of melanoma.

Microsatellite instability in gastric carcinoma with special references to histopathology and cancer stages

To study the molecular mechanism of gastric carcinogenesis, the frequencies of microsatellite instability were evaluated with seven dinucleotide repeat loci in 59 patients with gastric carcinoma. Microsatellite instability at two or more loci was found in 41.5% (17/41) of advanced gastric carcinoma, 21.4% (3/14) of early gastric carcinoma, but not in remnant gastric carcinoma (0/4), with an overall frequency of 33.9% (20/59). Diffuse gastric carcinoma had a similar prevalence (32.1%, 9/28) to intestinal gastric carcinoma (40.7%, 11/27). The frequency of microsatellite instability in gastric carcinoma was not significantly different with respect to age, sex and Helicobacter pylori infection. Microsatellite instability tended to occur more frequently in cancers of the cardia (62.5%, 5/8) compared with cancers of other stomach regions (31.9%, 15/47), but the difference was not statistically significant. These data suggest that microsatellite instability occurs in early gastric carcinoma and its occurrence increases during tumour progression. Furthermore, its frequency was independent of age, gender, histological types and Helicobacter pylori infection.

Germ line mutations of hMSH2 and hMLH1 genes in Japanese families with hereditary nonpolyposis colorectal cancer (HNPCC): usefulness of DNA analysis for screening and diagnosis of HNPCC patients

Mutations in hMSH2 and hMLH1 genes were analyzed in patients from 11 Japanese families that had been diagnosed as carrying hereditary nonpolyposis colorectal cancer (HNPCC) by clinical examination. Germ line mutations of hMSH2 gene were identified in 5 independent families in which colorectal (87% of patients), endometrial (30%), ovarian (17%), gastric (14%), and other cancers existed. Five mutations detected between codons 136 and 811 included single-base substitutions (C-->T and T-->G), a T deletion, and an A insertion, all of which produced stop codons resulting in truncated proteins, and an A-->T substitution at splice donor site of exon 5 which resulted in deletion of this exon. Moreover, one HNPCC family was presumed to have germ line mutation of hMSH2 gene because a somatic mutation of hMSH2 gene was detected in a cancer from a patient in this family. In addition to these 11 families already diagnosed with HNPCC, 3 new families with germ line mutations of hMSH2 gene and hMLH1 gene were found through analysis of DNA from patients who had multiple cancers with alteration in microsatellite DNA. These mutations included an AG deletion at codons 877-878 of hMSH2 gene, an AAG deletion at codons 616-618 of hMLH1 gene, and a C-->T single-base substitution at codon 217 of hMLH1 gene. Seven of eight germ line mutations found in this study are new mutations that have not been reported previously. In families in which germ line mutations were identified presymptomatic examination was then carried out using polymerase chain reaction single-strand conformation polymorphism analysis of DNA from peripheral blood, and the result was the detection of family members predisposed to HNPCC who did not yet show signs of cancer. These results indicate the value of DNA analysis in the screening and diagnosis of HNPCC patients and families.

Analysis of microsatellite repeats in pediatric brain tumors

Tumorigenesis has been shown to proceed through a series of genetic alterations involving protooncogenes and tumor suppressor genes. However, investigation of genomic instability of microsatellites has disclosed a new mechanism for human carcinogenesis, which is involved not only in hereditary nonpolyposis colon cancer (HNPCC) but also in a number of other malignancies. To determine whether microsatellite instability is involved in pediatric brain tumors, we screened 15 such tumors using seven microsatellite marker loci on six chromosomes 4, 5, 9p, 9q, 11, 14, and 17. Using the polymerase chain reaction method, DNA samples from the tumors and from normal peripheral blood leukocytes from each patient were compared for the allelic pattern produced at each locus. Our preliminary results indicate loss of heterozygosity at the fatty acid binding protein (FABP) locus, located on chromosomal arm 4q28-q31, the only trinucleotide repeat in the panel of markers used, for 3 of 15 cases, suggesting the presence of previously unidentified sequences relevant to brain tumorigenesis at or in the vicinity of this locus. We did not observe any microsatellite instability in these samples, indicating that the mechanisms operating in HNPCC are not active in this subset of pediatric brain tumors.

Molecular genetics of exocrine pancreatic neoplasms

Molecular genetic studies of pancreatic ductal adenocarcinoma have revealed the common co-existence of K-ras, p53, and MTS1 mutations. The finding of K-ras mutations in epithelial lesions of ducts suggests them as a precursor intraepithelial neoplasm. The clinical importance of this line of work can only be anticipated at present, and a fuller understanding of genetic alterations in these neoplasms is necessary.

The genetics of breast and ovarian cancer

A number of genes are known to be involved in inherited susceptibility to breast and/or ovarian cancer. In the context of high-risk families the most important genes are BRCA1 on chromosome 17q, which is associated with a high penetrance of both breast and ovarian cancer, and BRCA2 on chromosome 13q, which causes a high risk of breast cancer but a lower risk of ovarian cancer. Other high-risk cancer genes that confer increased risks of breast or ovarian cancer in addition to other cancers include the hereditary non-polyposis colorectal cancer genes and the TP53 gene, which causes breast cancer as part of the Li-Fraumeni syndrome. The predisposing mutations in these genes are relatively rare in the population. More common genes which are associated with an increased, but lower, risk of breast cancer are the ataxiatelangiectasia gene and the HRAS1 gene. This paper reviews recent progress in mapping and cloning of these susceptibility genes, and provides estimates of the cancer risks associated with each gene and the frequency of predisposing mutations.

Familial aggregation of tumors and detection of hereditary non-polyposis colorectal cancer in 3-year experience of 2 population-based colorectal-cancer registries

The clinical data of 2 population-based registries, located in areas with different incidence rates of colorectal cancer, were used in order to assess the role of familial factors in the pathogenesis of these tumors. The occurrence of tumors in family members was investigated in 389 subjects with colorectal cancer registered in Modena (Northern Italy, an area characterized by a high incidence of colorectal malignancies) between 1984 and 1986; similar information was obtained in 213 patients with tumors of the large bowel registered in Ragusa (Sicily, Southern Italy, an area of similar magnitude and with low incidence rates for these tumors) in the 3-year period 1988 to 1990. In both series, colorectal cancer occurred significantly more often among relatives of patients. Controls were patients of the same sex and age (+/- 5 years) hospitalized during the study periods, but not for gastrointestinal or neoplastic diseases. There were 89 cancer cases (3.1%) among 2,851 relatives of patients in Modena, vs. 17 cases among 1,744 relatives (1.0%) in Ragusa (p < 0.01). Apart from colorectal cancer, there was no excess of other types of tumors in patients' families (in both series). During the 3 years of registration, 17 cases of hereditary non-polyposis colorectal cancer (HNPCC, or Lynch syndrome) were diagnosed in Modena; in contrast, this syndrome was more rare in Ragusa (one case only during 3 years of observation). Similarly, many more families with clinical suspicion of HNPCC were recorded in Northern regions (44 vs. 10). Although incidence rates of colorectal cancer are appreciably higher in Northern than in Southern Italian regions, the excess of this cancer type among close relatives is similar. However, full-blown HNPCC or suspected Lynch syndrome were significantly more frequent in Northern Italy.

TEL1, an S. cerevisiae homolog of the human gene mutated in ataxia telangiectasia, is functionally related to the yeast checkpoint gene MEC1

Patients with the genetic disorder ataxia telangiectasia (AT) have mutations in the AT mutated (ATM) gene, which is homologous to TEL1 and the checkpoint gene MEC1. A tel1 deletion mutant, unlike a mec1 deletion, is viable and does not exhibit increased sensitivity to DNA-damaging agents. However, increased dosage of TEL1 rescues sensitivity of a mec1 mutant, mec1-1, to DNA-damaging agents and rescues viability of a mec1 disruption. mec1-1 tel1 delta 1 double mutants are synergistically sensitive to DNA-damaging agents, including radiomimetic drugs. These data indicate that TEL1 and MEC1 are functionally related and that functions of the ATM gene are apparently divided between at least two S. cerevisiae homologs.

Microallelotyping defines the sequence and tempo of allelic losses at tumour suppressor gene loci during colorectal cancer progression

Microallelotyping of many regions from individual colorectal tumours was used to determine the sequence and tempo of allelic loss on 5q, 17p and 18q during neoplastic progression. No allelic losses were found in normal tissues surrounding colorectal neoplasms, but losses occurred abruptly on 5q at the transition from normal colonic epithelium to the benign adenoma, and on 17p at the transition from adenoma to carcinoma, indicating an essential role for these losses in tumour progression. Allelic losses were uniform throughout extensively microdissected benign adenomas and carcinomas. However, substantial allelic heterogeneity was found in high-grade dysplasia, the transition lesion between adenoma and carcinoma. Thus, allelic losses on 5q and 17p are associated with abrupt waves of clonal neoplastic expansion, and high-grade dysplasia is characterized by a high degree of allelic heterogeneity.

Construction of a gene expression profile of a human fetal liver by single-pass cDNA sequencing

We have obtained an overall gene expression profile of a human fetal liver by sequencing the 5' ends of random cDNA clones from an unbiased cDNA library. As a result, many novel genes that might be related to liver growth and hemopoiesis have been identified. Poly (A)+ RNA was purified from the liver of a human fetus obtained at the 22nd week of gestation, and a directional library was constructed with oligo d(T)-primed cDNAs synthesized without any normalizing procedures. The 5' end of each randomly chosen clone was sequenced by the dideoxy-chain termination methods, and each sequence was used for homology search in the public databases such as GenBank, SWISS-PROT, and PIR. Of 1231 random cDNA clones analyzed, 697 clones representing 204 different transcripts (57%), were identical to previously known human genes. The spectrum of the genes in this category reflected well the physiological characteristics of the fetal liver, a combination of hepatic and hemopoietic functions. About 4% of the clones represented novel gene transcripts with significant homologies to known genes of human or other organisms. These included several genes that are known to be involved in cellular differentiation and/or proliferation. About 25% of the clones had no statistically significant match to any known genes. In summary, we have identified 546 different gene transcripts consisting of 204 known human genes, 42 homologous genes, and 300 unknown genes. Thus, this approach appears to be a highly efficient way to identify novel genes of biological interest.

Hypertension caused by a truncated epithelial sodium channel gamma subunit: genetic heterogeneity of Liddle syndrome

Sensitivity of blood pressure to dietary salt is a common feature in subjects with hypertension. These features are exemplified by the mendelian disorder, Liddle's syndrome, previously shown to arise from constitutive activation of the renal epithelial sodium channel due to mutation in the beta subunit of this channel. We now demonstrate that this disease can also result from a mutation truncating the carboxy terminus of the gamma subunit of this channel; this truncated subunit also activates channel activity. These findings demonstrate genetic heterogeneity of Liddle's syndrome, indicate independent roles of beta and gamma subunits in the negative regulation of channel activity, and identify a new gene in which mutation causes a salt-sensitive form of human hypertension.

MSH2 deficient mice are viable and susceptible to lymphoid tumours

Alterations of the human MSH2 gene, a homologue of the bacterial MutS mismatch repair gene, co-segregate with the majority of hereditary non-polyposis colon cancer (HNPCC) cases. We have generated homozygous MSH2-/- mice. Surprisingly, these mice were found to be viable, produced offspring in a mendelian ratio and bred through at least two generations. Starting at two months of age homozygous-/- mice began, with high frequency, to develop lymphoid tumours that contained microsatellite instabilities. These data establish a direct link between MSH2 deficiency and the pathogenesis of cancer. These mutant mice should be good models to study the progression of tumours and also to screen carcinogenic and anti-cancer agents.

Microsatellite instability and pathological aspects of breast cancer

Genomic instability plays a key role in hereditary nonpolyposis colorectal cancer and in a significant sub-set of non-hereditary colorectal tumors. Recent evidence suggests that microsatellite instability also occurs in various sub-sets of common, non-hereditary forms of extra-colorectal carcinoma. To investigate the role of microsatellite instability in breast cancer, and to correlate this type of alteration with clinico-pathological characteristics, including tumor proliferative activity, we analyzed the status of 8 different microsatellite loci in 28 cases of primary mammary carcinoma. For this purpose, microsatellite banding patterns were compared in paired breast-cancer/peripheral-blood DNA samples. Microsatellite instability was observed in 6/28 (21%) of the cases. Four of the 6 tumors had low proliferative activity, one had high proliferative activity, and in one case proliferative activity values were not available. All chromosomal loci investigated demonstrated microsatellite instability in one or more representative tumors of the series. Shifts in length larger than 2 bp were the most frequent change. Microsatellite instability significantly correlated with the lobular histotype, and with lymph-node involvement. A trend was also observed associating microsatellite instability and large tumor size.

Microsatellite instability in Japanese esophageal carcinoma

Recent studies have shown that microsatellite instability (MSI) may play an important role in the development of various types of cancer. However, there have been only 2 reports describing MSI in esophageal carcinoma and the clinicopathologic significance of MSI in this malignancy has not yet been clarified. To better elucidate the role of genetic instability in the development of esophageal carcinoma, we investigated the presence of MSI in 32 cases of esophageal cancer using paired samples of fresh frozen tumor and normal tissue by a method based on the polymerase chain reaction. MSI was defined as occurring in tumors which showed altered banding patterns at one or more microsatellite loci. The incidence of MSI in esophageal carcinoma was 6 out of 32 patients. MSI was observed more frequently in cases with small-cell carcinoma (2 out of 2) than in cases with squamous-cell carcinoma (4 out of 29). No cases with adenocarcinoma or Barrett's metaplasia were included in our series. No significant correlations between MSI and other clinicopathologic parameters were observed. The present study suggests that (1) some Japanese esophageal carcinomas certainly correlate with DNA replication error, and (2) MSI may be more frequent in small-cell carcinoma of the esophagus than in squamous-cell carcinoma of the esophagus.

A hPMS2 mutant cell line is defective in strand-specific mismatch repair

Human cells contain several homologs of the bacterial mutL gene required for mismatch repair, including a gene on chromosome 7 designated hPMS2. We have identified an endometrial carcinoma cell line, HEC-1-A, that has a C-->T mutation in hPMS2 that generates a nonsense codon and yields a protein truncated at the C terminus. No wild-type gene or gene product was detected. The missing amino acids in hPMS2 are highly conserved among PMS homologs, suggesting that they may be critical for function. In support of this, extracts of HEC-1-A cells are defective in repairing a variety of mismatched substrates. Moreover, di-, tri-, and tetranucleotide repeated sequences are highly unstable in single cell clones of HEC-1-A cells, and HEC-1-A cells are resistant to killing by N-methyl-N'-nitro-N-nitrosoguanidine. The results provide strong experimental support for the involvement of the hPMS2 gene product in mismatch repair in human cells and support the concept that a defective hPMS2 gene may lead to predisposition to certain forms of cancer.

Dose-response of a radiation induction of a germline mutation at a hypervariable mouse minisatellite locus

Dose-response of an induction of a germline mutation was studied at a hypervariable mouse minisatellite locus, Ms6hm, which consists of tandem repeats of a sequence motif GGGCA. Male C3H/HeN mice were exposed to various doses of 60Co gamma-ray and mated with unirradiated C57BL/6N female mice. Matings were done at various time after irradiation to assess the effects of radiation on spermatozoa, spermatids and spermatogonia. DNA samples of F1 offsprings were analysed by Southern blotting for the repeat length mutation at the Ms6hm locus. The mutation frequency per gamete of the paternal allele was 9.1% for the unirradiated control group. The spermatids stage was most sensitive to radiation and a statistically significant dose-response was observed. The mutation frequency of the paternal allele in F1 mice increased to 22% for 1 Gy, 28% for 2 Gy, and 28% for 3 Gy. The spermatogonia stage was less sensitive to radiation, and the mutation frequencies of the paternal allele were 14% for 2 Gy, and 16% for 3 Gy. The spermatozoa stage germ cells were also less sensitive and the frequency of mutation of the paternal allele increased to 14% for 3 Gy. However, these increases were statistically not significant. Possible mechanisms of radiation induction of germline mutation at the hypervariable minisatellite locus will be discussed.

Microsatellite instability in colorectal cancer: improved assessment using fluorescent polymerase chain reaction

BACKGROUND & AIMS

Microsatellite instability was first described in hereditary nonpolyposis colorectal cancers and sporadic colorectal cancers, in which it was associated with a good prognosis. The aim of this study was to assess the advantages of a novel fluorescent assay for detecting microsatellite instability.

METHODS

Eleven fluorescently tagged microsatellites and an automated DNA sequencer were used to investigate 54 sporadic colorectal adenocarcinomas.

RESULTS

This fluorescent assay combined accurate allele sizing with cross-sectional data display and allowed improved assessment of microsatellite instability. Twenty-two percent of cancers (12 of 54) showed microsatellite instability with at least one marker. For tumors showing microsatellite instability, results were obtained for a minimum of eight markers. Six tumors showed microsatellite instability at high frequency (at least 63% of markers affected), and 42% of the patients who had a tumor showing microsatellite instability had a synchronous and/or metachronous colorectal tumor (vs. 7% of patients whose tumor did not show microsatellite instability). Patients with a microsatellite instability-positive tumor had an improved prognosis (P = 0.03).

CONCLUSIONS

The use of this fluorescent assay improved the assessment of microsatellite instability with the automated analysis and cross-sectional data display. The assay identified a subgroup of patients who showed microsatellite instability and who also showed clinical features that differed from the microsatellite instability-negative cases.

Allelic imbalance in gastric cancer: an affected site on chromosome arm 3p

In order to detect regions of DNA containing tumor suppressor genes involved in the development of gastric cancer, we performed an allelotype study on 78 gastric adenocarcinomas from a population composed largely of Texan Hispanics and Anglos, two ethnic groups that have a ratio of incidence rates of gastric cancer of approximately 2:1. In total, 42 microsatellite markers were employed, which detected at least one site per arm of each autosome in the human genome. These included several markers linked to known tumor suppressor genes (TP53, APC, DCC, RB1, and BRCA1). Sites showing quantitative allelic imbalance (AI) greater than 30% were located on 3p (36%), 11q (31%), 12q (38%), 13q (33%), 17p near TP53 (74%), and 17q near BRCAI (32%). Among the 22% of cases showing microsatellite instability (MI), a subset (4 of 17) showed instability at 59% or more of sites tested. No ethnic bias was detected in cases showing MI or in cases with AI at sites with rates of AI above 30%. Tumors of the intestinal subtype were significantly more likely than diffuse tumors to show AI at DI3S170 (P = 0.01). A deletion map of chromosome arm 3p was prepared for tumors with AI at D3S1478. These data indicate that a tumor suppressor gene on chromosome arm 3p is involved in the development of a subset of gastric cancers.

Microsatellite instability in ovarian neoplasms

Microsatellite instability has been observed in a variety of sporadic malignancies, but its existence in sporadic ovarian cancer has been the subject of conflicting reports. We have performed a polymerase chain reaction-based microsatellite analysis of DNAs extracted from the neoplastic and non-neoplastic tissues of 41 ovarian cancer patients. Tumour-associated alterations were observed in seven (17%) of these cases. Clinicopathological correlations revealed that: (1) alterations among tumours classified as serous adenocarcinomas occurred with relatively low frequency (2/24 or 8%); (2) most of the tumours with microsatellite alterations (5/7 or 71%) were of less common histopathological types (epithelial subtypes such as endometrioid and mixed serous and mucinous, or non-epithelial types such as malignant mixed Müllerian or germ cell tumours); (3) tumour-associated alterations were observed in 3/4 (75%) of the patients with stage I tumours vs 4/37 (11%) of the patients with stage II, III and IV tumours (P = 0.01); (4) tumour-associated microsatellite instability was found to occur with similar frequencies among patients with and without clinical features suggestive of familial disease, including positive family history, early onset, or multiple primary tumours. In summary, we have observed microsatellite alterations in the neoplastic tissues of ovarian cancer patients with diverse genetic backgrounds and clinicopathological features. The pattern of alterations is consistent with the possibility that multiple mechanisms may be responsible for microsatellite instability in ovarian neoplasms.

Inactivation of the mouse Msh2 gene results in mismatch repair deficiency, methylation tolerance, hyperrecombination, and predisposition to cancer

To investigate the role of the presumed DNA mismatch repair (MMR) gene Msh2 in genome stability and tumorigenesis, we have generated cells and mice that are deficient for the gene. Msh2-deficient cells have lost mismatch binding and have acquired microsatellite instability, a mutator phenotype, and tolerance to methylating agents. Moreover, in these cells, homologous recombination has lost dependence on complete identity between interacting DNA sequences, suggesting that Msh2 is involved in safeguarding the genome from promiscuous recombination. Msh2-deficient mice display no major abnormalities, but a significant fraction develops lymphomas at an early age. Thus, Msh2 is involved in MMR, controlling several aspects of genome stability; loss of MMR-controlled genome stability predisposes to cancer.

Male mice defective in the DNA mismatch repair gene PMS2 exhibit abnormal chromosome synapsis in meiosis

Using gene targeting in embryonic stem cells, we have derived mice with a null mutation in a DNA mismatch repair gene homolog, PMS2. We observed microsatellite instability in the male germline, in tail, and in tumor DNA of PMS2-deficient animals. We therefore conclude that PMS2 is involved in DNA mismatch repair in a variety of tissues. PMS2-deficient animals appear prone to sarcomas and lymphomas. PMS2-deficient males are infertile, producing only abnormal spermatozoa. Analysis of axial element and synaptonemal complex formation during prophase of meiosis I indicates abnormalities in chromosome synapsis. These observations suggest links among mismatch repair, genetic recombination, and chromosome synapsis in meiosis.

Hereditary aspects of endometrial adenocarcinoma

The importance of heredity in the etiology of endometrial cancer (EC) was examined in a series of 326 patients with EC diagnosed at age 60 years or less. If one or both of the proband's parents had died of cancer, a thorough family history of malignancies was studied. Altogether 291 cases with complete parental data were found. Nine kindred (3.1%) showed features compatible with the dominantly inherited cancer trait known as hereditary non-polyposis colorectal cancer (HNPCC). In another 9 cases, clustering of malignancies in 2 or more successive generations was indicative of familial cancer. Aspecific cancer aggregates were found in 112 probands' families, and family history was negative in 161 cases. No families had gynecological cancer as the only malignancy. HNPCC, the genetic etiology of which was recently revealed, seems to be an important risk factor for EC, indicating the significance of family-history investigations of all patients with EC. Colorectal carcinoma (CRC) was here associated with EC also in families with clusterings of malignancies, but in these families no typical features of any known hereditary cancer syndrome could be found. On the basis of the results of the present study, proper surveillance for colorectal cancer should be recommended for patients with endometrial carcinoma if they belong to a family with features indicative of HNPCC. Furthermore, healthy gene carriers in an HNPCC family also need careful surveillance for CRC, EC and perhaps for other extra-colonic malignancies typical for HNPCC. Prophylactic surgery should even be considered in these cases.

Requirement of the yeast RTH1 5' to 3' exonuclease for the stability of simple repetitive DNA

Simple repetitive DNA sequences are unstable in human colorectal cancers and a variety of other cancers. Mutations in the DNA mismatch repair genes MSH2, MLH1, and PMS1 result in elevated rates of spontaneous mutation and cause a marked increase in the instability of simple repeats. Compared with the wild type, a null mutation in the yeast RTH1 gene, which encodes a 5' to 3' exonuclease, was shown to increase the rate of instability of simple repetitive DNA by as much as 280 times and to increase the spontaneous mutation rate by 30 times. Epistasis analyses were consistent with the hypothesis that this RTH1-encoded nuclease has a role in the MSH2-MLH-1-PMS1 mismatch repair pathway.

Increased mRNA expression of the receptor-like protein tyrosine phosphatase alpha in late stage colon carcinomas

The protein tyrosine phosphatase alpha (PTP alpha) mRNA level in paired samples of late stage (Dukes' D) colorectal tumors and adjacent normal colon mucosa was quantified by RNase protection assays. After normalization against 18S RNA or beta-actin mRNA level, a 2-10-fold increase in PTP alpha mRNA was detected in 10 of 14 tumors (approximately 70%) compared to mucosa. In situ hybridization of digoxigenin-labelled antisense PTP alpha RNA to tumor and mucosa sections produced a signal only in neoplastic cells of the tumor sample, consistent with the high increase in PTP alpha mRNA detected by RNase protection assays of some of the tumors. This is the first report suggesting an association of a protein tyrosine phosphatase with colorectal carcinoma. PTP alpha is a receptor-like PTP thought to be involved in regulating cell proliferation. Its oncogenic properties when overexpressed in cultured fibroblasts suggest that PTP alpha overexpression could contribute to the tumorigenic process in colon carcinoma.

Association of a chromosome deletion syndrome with a fragile site within the proto-oncogene CBL2

The fragile site FRA11B has been localized to the p(CCG)n repeat of the CBL2 proto-oncogene. A proportion of Jacobsen (11q-) syndrome patients inherited a chromosome carrying a CBL2 p(CCG)n expansion, which was truncated close to FRA11B. These results have broad implications for the role of p(CCG)n repeat expansion in the aetiology of genetic disease involving chromosome rearrangements.

Update on the differential diagnosis, surveillance and management of hereditary non-polyposis colorectal cancer

Hereditary non-polyposis colorectal cancer (HNPCC) is the most common hereditary form of colorectal cancer (CRC), accounting for approximately 10% of the total CRC burden. HNPCC lacks premonitory physical stigmata, thereby making the family history crucial for diagnosis. Advances in molecular genetics during the past 2 years have led to the cloning of four HNPCC genes (MHS2, MLH1, PMS1 and PMS2). It is now possible to provide presymptomatic DNA testing followed by genetic counselling for gene carriers. Some studies have shown that adenomas in HNPCC are larger, more villous, and have more high grade dysplasia than sporadic cases, suggesting an accelerated adenoma-carcinoma sequence. Given the early age of onset and proximal predominance of CRC, we initiate colonoscopy at age 20-25 years and we recommend that it be performed every 1-2 years. The wealth of clinical and molecular genetic knowledge currently available to physicians about HNPCC can be used effectively for cancer control.

Risk factors for colon neoplasia--epidemiology and biology

Epidemiological, physiological and molecular models of colon carcinogenesis have been proposed. Consistent epidemiological risk factors include reduced plant-food intake (increased risk); elevated meat intake (increased risk); higher physical activity (reduced risk); and increased alcohol intake (increased risk). At the physiological level, these lifestyle variables may trigger processes that provide explanations for the associations: higher meat, fat and alcohol means more heterocyclic amines and higher levels of bile acids; higher plant food means higher intake of several anticarcinogens and fibre fermentation that produces volatile fatty acids; exercise has a variety of beneficial effects. This complexity is elaborated further in the context of the colonic milieu where interactions among digesta, bacteria and epithelial cells occur. The long-term likelihood of cancer is the summation of moment-to-moment changes in the colonic milieu brought about by this interaction. Possible relationships between established epidemiological risk factors, genetic susceptibility and somatic genetic changes are outlined.

Hereditary nonpolyposis colorectal cancer: results of long-term surveillance in 50 families

A surveillance programme comprising either colonoscopy of sigmoidoscopy plus barium enema every 2-3 years was instituted in 50 hereditary nonpolyposis colorectal cancer (HNPCC) families. The families included 238 patients with colorectal cancer (CRC) (mean age at diagnosis: 43.7 years; range: 16-86 years). These patients had 597 first-degree relatives of whom 493 could be traced and 388 (79%) accepted the invitation for screening. The control group were relatives (index patients) with symptomatic CRC. The average follow-up duration was 5 years (1-20 years). Screening led to the detection of adenomas in 33 patients and CRC in 11 patients. Pathological examination revealed 1 Dukes' A, 7 Dukes' B and 3 Dukes' C cancers. In contrast, among the control group 47% had advanced CRC (Dukes' C or distant metastases). The 5-year survival of the screen-detected cases was 87% versus 63% in the control group. Of the 11 CRC cases in the screening group, 4 were detected within 1-4 years after a negative colonic examination. A large proportion of the polyps found in the screening and control groups showed a villous growth pattern and/or a high degree of dysplasia. We conclude that periodic examination of HNPCC families allows the detection of cancer at an earlier stage than in patients not under surveillance. Because of the possibly more aggressive nature of polyps associated with HNPCC, we recommend a screening interval of 1-2 years.

Genetic instability associated with adenoma to carcinoma progression in hereditary nonpolyposis colon cancer

BACKGROUND & AIMS

Genetic instability related to defective DNA mismatch repair genes may be involved in the pathogenesis of carcinoma in hereditary nonpolyposis colon cancer (HNPCC). However, nonneoplastic tissues from patients inheriting defects in human MSH2 or human MLH1 do not show significant genetic instability. The aim of this study was to determine whether acquisition of genetic instability at the adenoma stage promotes malignant transformation by studying adenoma-carcinoma progression in HNPCC.

METHODS

Dinucleotide repeat loci were analyzed by polymerase chain reaction from microdissected adenoma and/or carcinoma stages from formalin-fixed paraffin-embedded HNPCC tumors.

RESULTS

Although genetic instability was observed at some loci in almost all cases, the proportion of microsatellite loci altered was significantly less (P < 0.01) in completely benign adenomas (24%) than in benign areas of adenomas with malignancy (54%). Molecular fingerprints indicated intratumor heterogeneity, with evolution of related subclones of neoplastic cells. However, in all cases of tumor progression, at least one subclone from the adenoma stage was closely related to the carcinoma.

CONCLUSIONS

Some genetic instability develops at the benign adenoma stage in most HNPCC tumors. Adenomas with a greater rate of genetic instability are more likely to progress to carcinoma. Topographic genotyping data provides evidence supporting the hypothesis of adenoma-carcinoma progression in HNPCC.

Colorectal cancer: future population screening for early colorectal cancer

Colorectal cancer (CRC) is one of the most frequent cancers in Western countries. The identification of individuals at risk and the early diagnosis of CRC are of critical importance since a large proportion can be prevented or cured by surgical removal before metastasis has occurred. With increasing understanding of the genetic basis of hereditary and sporadic (non-hereditary) CRC, it becomes feasible to detect genetic alterations by molecular techniques. Familial adenomatous polyposis (FAP), hereditary nonpolyposis colorectal cancer (HNPCC), as well as early stages of spontaneous CRC, can be diagnosed by molecular characterisation of the adenomatous polyposis coli (APC) gene, the RAS oncogene and other genes in DNA from peripheral blood, stool or intestinal biopsies. With a better understanding of the genetic events leading to malignant transformation, molecular population screening should allow us to identify individuals at risk as well as patients with an early and potentially curable CRC. At present, careful patient and family history, physical examination and testing for occult blood as well as colonoscopy are still the key elements for clinical patient management. Molecular diagnosis will hopefully soon complement these analyses and should result in a reduction of morbidity and mortality from CRC.

p53 and its 14 kDa C-terminal domain recognize primary DNA damage in the form of insertion/deletion mismatches

Insertion/deletion (IDL) mismatches in DNA are lesions consisting of extra bases on one strand. Here, the binding of p53 and its 14 kDa C-terminal domain to DNAs containing one or three 3-cytosine IDL mismatches was examined. Electron microscopy showed that both p53 forms bound predominantly as tetramers at the lesions while single-stranded binding proteins did not bind. Gel retardation assays showed that p53 formed highly stable complexes when the DNA contained the IDL mismatches, but only unstable complexes when the DNA lacked lesions (but did contain free ends). The highly stable complexes had a half-life of > 2 hr, suggesting that upon encountering lesions, p53 may recruit other proteins to the site, providing a signal for DNA damage.