Mutation of a mutL homolog in hereditary colon cancer

The Saccharomyces cerevisiae Msh2 protein specifically binds to duplex oligonucleotides containing mismatched DNA base pairs and insertions

The yeast Saccharomyces cerevisiae encodes four proteins, Msh1, Msh2, Msh3, Msh4, that show strong amino acid sequence similarity to MutS, a central component of the bacterial mutHLS mismatch repair system. MutS has been shown to recognize base pair mismatches in DNA in vitro. Previous studies have suggested that Msh2 is the major mismatch recognition protein in yeast. In this study, the 109-kD Msh2 polypeptide was overexpressed and purified to analyze its DNA-binding properties. This analysis demonstrated that Msh2 can bind selectively to duplex oligonucleotide substrates containing a G/T mismatch, 1- to 14-nucleotide insertion mismatches, and palindromic (12- to 14-nucleotide) insertion mismatches. A general trend was that the affinity of Msh2 for substrate was proportional to the size of the insertion mispair present (+14 PAL, +12 PAL > +14 > +8 > GT, +6, +4, +2, +1). Kinetic studies indicated that the specificity of Msh2 to mismatch substrates was a function of its ability to form stable complexes with mispair-containing duplex DNAs. These complexes decayed more slowly than Msh2 complexes formed with homoduplex DNA.

Microsatellite instability in primary neoplasms from HIV + patients

AIDS is associated with a high risk of certain malignancies, notably Kaposi's sarcoma (KS) and B-cell non-Hodgkin's lymphoma (NHL). The pathogenesis of these malignancies is not fully understood. One mechanism of malignant transformation recently described in colon tumorigenesis results from defects in DNA mismatch repair, manifest as widespread microsatellite instability. We demonstrate a high rate of microsatellite instability in KS and aggressive lymphomas obtained from HIV-infected patients, whereas there is no evidence of instability in similar lesions from HIV-negative patients. Further elucidation of the underlying mechanisms responsible for HIV-associated instability in primary tumours may provide insight into the pathogenesis of these AIDS-related neoplasms.

Epidemiologic and genetic follow-up study of 544 Minnesota breast cancer families: design and methods

In 1944, a case-control family study was initiated at the Dight Institute for Human Genetics at the University of Minnesota to study the influences of childbearing breastfeeding, and hereditary susceptibility on the occurrence and age-of-onset of breast cancer. Index cases (probands) were women ascertained at the Tumor Clinic of the University of Minnesota Hospital. Medical history and life style information were obtained on probands and relatives, and all cancers were histologically verified. A total of 544 families were studied, with probands diagnosed between 1931 and 1952. All of the records and pathology slides have been maintained from the original study; for most probands this includes the original tissue blocks. We are conducting a historical cohort study of selected of selected first- and second-degree female relatives (sisters, daughters, nieces, granddaughters) of the probands and a group of control women identified as the spouses of all male first- and second-degree relatives (brothers, sons, grandsons, and nephews). The subsequent development of breast cancer is being determined to quantify the absolute risk associated with a positive family history. Current disease status is ascertained with mammography, and stromal density is measured using digital imaging. Segregation analysis will be applied to examine how non-genetic factors such as diet, exogenous hormone use, and body fat distribution influence risk in women at high risk because of family history. A subset of families are being selected for molecular analysis of the BRCA1 gene or for linkage analyses to identify putative susceptibility loci other than BRCA1. Documented cancer histories were known for at least three generations, and the current study extends the pedigrees up to four or five generations for every family, allowing a detailed description of familial risk. This cohort study of breast cancer families is likely to be important in both quantity and quality of data and will serve as a major genetic epidemiologic resource, being free of selection bias and having relevant non-genetic exposure determined in at least four generations.

Low frequency and late occurrence of p53 and dcc aberrations in colorectal tumours

Whilst p53 aberrations have been documented in numerous malignancies, reports of alterations to the deleted in colorectal cancer (dcc) gene are infrequent, and studies investigating the status of both genes in the same colon tumour are rare. In this study we have analysed a panel of 35 pairs of normal and neoplastic human colorectal tissues for abnormalities in these tumour-suppressor genes. In contrast to previous studies we have found only a low incidence of mutations and deletions. p53 point mutations were identified in 8/35 tumours (22%). All were G.C to A.T transitions, with 7/8 occurring at CpG dinucleotides. p53 allelic loss was detected in 4/11 informative cases (36%). Although not quite attaining statistical significance, p53 alteration correlated with the adenoma/carcinoma transition. Gross dcc alterations were identified by Southern blotting in 7/35 (20%) tumours. Microsatellite analysis using two markers, one within and one proximal to the dcc gene, detected a low frequency of deletion overall (41% informative cases). 18q/dcc aberrations were associated with the progression of early to late carcinoma, rather than with increasing adenoma size, as has been previously reported. Both p53 alterations and dcc deletions were detected at a higher frequency in distal tumours than in proximal malignancies. Two tumours exhibiting microsatellite instability in both markers were each of proximal origin.

Somatic mutations in VNTR-locus D1S7 in human colorectal carcinomas are associated with microsatellite instability

To elucidate mutation mechanisms in hypervariable VNTR loci, we have studied somatic mutation events with the minisatellite probe MS1 (VNTR locus D1S7) in 224 colorectal carcinomas (CRC). The D1S7 locus consists of a 9-basepair (bp) repeat unit. The copy number varies from about 100 to 2000, and the germline mutation rate is high. Here we demonstrate a high D1S7 somatic mutation rate in CRC (37/224), higher than indicated earlier by others. We also demonstrate that the most frequent mutational event by far (n = 34) involves small reductions in VNTR fragment size (median loss 22 repeat units, range 2-154), furthermore, in one-half of these cases, this event is biallelic. We wanted to test whether these somatic mutations mirror the same genetic instability as seen by RER (replication error), a phenomenon recently described in tumour DNA from both sporadic and familial cases of CRC. All blood/tumour DNA pairs displaying MS1 mutation (n = 37) as well as 37 randomly selected pairs without MS1 mutation were tested with four tetranucleotide short tandem repeats (STRs, microsatellites). There is a strong association between mutations at the D1S7 locus and the occurrence of new STR alleles (P < 0.001). This is the first report of the existence of a minisatellite as a marker for genetic instability/RER in colorectal carcinomas. The findings may also cast light upon the mechanism for somatic mutations in this minisatellite.

Regional assignment by hybrid mapping of 36 expressed sequence tags (ESTs) on human chromosome 6

We have determined the regional chromosome assignment of 36 cDNAs from infant brain libraries by assessing the concordant segregation of PCR products using a human-rodent hybrid mapping panel that subdivides chromosome 6 into 15 regions. These mapped sequences serve as markers for the physical and expression maps of chromosome 6, as well as candidate genes for various disease loci. Sequence analysis has identified putative functions and motifs for some of these genes.

Advances in molecular biology: implications for the future of clinical nutrition practice

Advances in molecular biology during the past decade have substantially contributed to our understanding of how genes influence physiologic processes and, ultimately, our health. Genes associated with many nutrition-related chronic diseases are being identified and characterized. Nutrients may directly or indirectly influence the transcription and/or translation of specific gene products. Identifying genetic markers for specific diseases and exploring gene therapy will provide new opportunities and challenges for clinical nutrition practice in the 21st century. Nutrition practitioners must be cognizant of developments in molecular biology to meet the challenges of providing nutrition care in the future.

Using information content and base frequencies to distinguish mutations from genetic polymorphisms in splice junction recognition sites

Predicting the effects of nucleotide substitutions in human splice sites has been based on analysis of consensus sequences. We used a graphic representation of sequence conservation and base frequency, the sequence logo, to demonstrate that a change in a splice acceptor of hMSH2 (a gene associated with familial nonpolyposis colon cancer) probably does not reduce splicing efficiency. This confirms a population genetic study that suggested that this substitution is a genetic polymorphism. The information theory-based sequence logo is quantitative and more sensitive than the corresponding splice acceptor consensus sequence for detection of true mutations. Information analysis may potentially be used to distinguish polymorphisms from mutations in other types of transcriptional, translational, or protein-coding motifs.

Mismatch repair gene defects in sporadic colorectal cancers with microsatellite instability

Microsatellite instability has been observed in both sporadic and hereditary forms of colorectal cancer. In the hereditary form, this instability is generally due to germline mutations in mismatch repair (MMR) genes. However, only one in ten patients with sporadic tumours exhibiting microsatellite instability had a detectable germline mutation. Moreover, only three of seven sporadic tumour cell lines with microsatellite instability had mutations in a MMR gene, and these mutations could occur somatically. These results demonstrate that tumours can acquire somatic mutations that presumably do not directly affect cell growth but result only in genetic instability. They also suggest that many sporadic tumours with microsatellite instability have alterations in genes other than the four now known to participate in MMR.

ARCAD: a method for estimating age-dependent disease risk associated with mutation carrier status from family data

We present ARCAD, a method to estimate the disease risk associated with mutation carrier status using data on families ascertained by affected individuals, in which a germline mutation has been detected. Because the event of interest, the age of onset, is a censored variable, the method uses the survival analysis approach to formulate the likelihood. Provided that selection criteria are clearly defined, the ascertainment bias is removed by including a correction term in the likelihood computation. We simulated family data and selected those with a proband affected before age 17, and at least one or at least two relatives affected before age 46. We show that including the correction for the ascertainment provides reliable estimates of the risk, even when many individuals are not tested for the mutation. An application to cancer risk and germline p53 mutations is presented. We routinely investigate the p53 status for all the children treated in the Department of Pediatric Oncology at the Institute Gustave Roussy, whose family displays at least one relative affected by cancer before age 46. We identified 5 families with an inherited germline p53 mutation. The risk for any cancer for a mutation carrier estimated by ARCAD was 42% within the age class 0-16 years, 38% within the age class 17-45 years, and 63% after 45 years, with a lifetime risk of 85%. These risks are almost entirely explained by the occurrence of the six most frequent cancers encountered in the Li-Fraumeni syndrome.

Cloning and expression of the Xenopus and mouse Msh2 DNA mismatch repair genes

Bacterial MutS protein and its yeast and human homologs MSH2 trigger the mismatch repair process by their initial binding to mispaired and unpaired bases in DNA. We describe the cloning and sequencing of genes from Xenopus laevis and Mus musculus encoding the homolog of the Saccharomyces cerevisiae MSH2 (the major DNA mismatch binding protein). Mutations in the human homolog of this gene have recently been implicated in microsatellite instability and DNA mismatch repair deficiency in tumour cells from patients with the most common hereditary predisposition to cancer (Lynch syndrome, or hereditary non-polyposis colorectal cancer, HNPCC), as well as in a significant percentage of sporadic tumours. Expression of the amphibian and murine Msh2 gene in different tissues appears to be ubiquitous. The Xenopus gene is highly expressed in eggs, a model system for the biochemistry of DNA mismatch repair. Expression of the murine gene is low in all tissues examined, and is relatively high in a rapidly dividing cell line. These data are suggestive of a role for MSH2 during DNA replication.

Direct mutational analysis in a family with hereditary non-polyposis colorectal cancer

BACKGROUND

It is now known that a proportion of cases of hereditary non-polyposis colorectal cancer (HNPCC) is caused by mutations in the human homologue of the yeast DNA mismatch repair gene MSH2. A proline to leucine change due to a C to T transition in codon 622 of hMSH2 has been identified in a large HNPCC family of over 240 individuals.

AIM

To develop an assay to detect the family-specific mutation and apply the findings to genetic screening.

METHODS

The C to T change in codon 622 creates a new Mae I site (CTAG) allowing a simple, non-radioactive assay to be developed in order to detect this mutation. The assay was applied to affected members of the family and their first degree relatives (siblings and offspring) between the ages of 17 and 77 years, a total of 75 subjects within two generations (IV and V).

RESULTS

13/13 (100%) subjects with cancer were mutation positive, 7/7 (100%) elderly subjects from generation IV and with no evidence of cancer were mutation negative, 23/57 (40%) subjects from generation V were mutation positive and 0/50 (100%) unrelated subjects were mutation negative. Following the demonstration of perfect segregation of the disease with the mutation, family members were invited to receive the results of the test. Sixty-three (84%) responded within six weeks of receiving the invitation. Genetic screening and counselling members of HNPCC families was perceived as beneficial overall, allowing non-carriers of the mutant gene (as well as their descendants) to be removed from a programme of colonoscopic surveillance.

Genetic alterations in non-melanoma skin cancer

Non-melanoma skin cancer is common and offers unrivaled opportunities to relate genetic changes to clinical and biologic behavior. Recent technical advances in molecular biology render genetic analysis of even the smallest skin cancers possible. In this review I will discuss the role of p53 gene in skin carcinogenesis, the relation between p53 immunostaining and p53 mutation, and recent evidence for the involvement of putative tumor suppressor genes both on chromosome 9 and other chromosomes in non-melanoma skin cancer.

Long DNA palindromes, cruciform structures, genetic instability and secondary structure repair

Long DNA palindromes pose a threat to genome stability. This instability is primarily mediated by slippage on the lagging strand of the replication fork between short directly repeated sequences close to the ends of the palindrome. The role of the palindrome is likely to be the juxtaposition of the directly repeated sequences by intra-strand base-pairing. This intra-strand base-pairing, if present on both strands, results in a cruciform structure. In bacteria, cruciform structures have proved difficult to detect in vivo, suggesting that if they form, they are either not replicated or are destroyed. SbcCD, a recently discovered exonuclease of Escherichia coli, is responsible for preventing the replication of long palindromes. These observations lead to the proposal that cells may have evolved a post-replicative mechanism for the elimination and/or repair of large DNA secondary structures.

Loss of the wild type MLH1 gene is a feature of hereditary nonpolyposis colorectal cancer

The mechanism by which germline mutations of DNA mismatch repair genes cause susceptibility to tumour formation is not yet understood. Studies in vitro indicate that heterozygosity for these mutations, unlike homozygosity, does not affect mismatch repair. Surprisingly, no loss of heterozygosity at the predisposing loci has so far been described in hereditary nonpolyposis colorectal cancers. Here, we show that loss of heterozygosity (LOH) of markers within or adjacent to the MLH1 gene on chromosome 3p occurs nonrandomly in tumours from members of families in which the disease phenotype cosegregates with MLH1. In every informative case, the loss affects the wild type allele. These results suggest that DNA mismatch repair genes resemble tumour suppressor genes in that two hits are required to cause a phenotypic effect.

Mismatch repair and cancer susceptibility

Mismatch-repair systems have been identified in organisms ranging from Escherichia coli to humans. They can repair almost all DNA base pair mismatches as well as small insertion/deletion mismatches. Molecular and biochemical analyses have shown that the core components of eukaryotic mismatch-repair systems are highly homologous to their bacterial counterparts. In humans, defects in four mismatch-repair genes have been linked both to hereditary non-polyposis colorectal cancer and to spontaneous cancers that exhibit rearrangements in DNA containing simple repeat sequences.

Diagnosis and the new genetics

The rapid pace of gene discovery has led to new opportunities for clinical diagnosis using molecular genetic technologies. Recent achievements include the culmination of the 10-year search for the Huntington's disease gene, the identification of predisposing genes for certain familial colon cancers, and the characterization of potential genetic risk indicators for Alzheimer's disease, hypertension, and coronary heart disease. These advances, coupled with the previous discoveries of important disease genes (e.g. those for cystic fibrosis, Duchenne muscular dystrophy, and fragile X syndrome) have quickly expanded the capacity of genetic analysis, allowing the design of enhanced and novel approaches for diagnostic testing. The transfer of molecular technology to the area of clinical genetic analysis, although associated with many potential benefits, has raised some concern regarding the possible misuse of genetic tests and information, particularly with regard to presymptomatic diagnosis of disease and population screening.

Binding of mismatched microsatellite DNA sequences by the human MSH2 protein

Alteration of the human mismatch repair gene hMSH2 has been linked to the microsatellite DNA instability found in hereditary nonpolyposis colon cancer and several sporadic cancers. This microsatellite DNA instability is thought to arise from defective repair of DNA replication errors that create insertion-deletion loop-type (IDL) mismatched nucleotides. Here, it is shown that purified hMSH2 protein efficiently and specifically binds DNA containing IDL mismatches of up to 14 nucleotides. These results support a direct role for hMSH2 in mutation avoidance and microsatellite stability in human cells.

DNA loop repair by human cell extracts

An activity in human cell extracts is described that repairs DNA with loops of five or more unpaired bases. Repair is strand-specific and is directed by a nick located 5' or 3' to the loop. This repair is observed in a colorectal cancer cell line that is devoid of a wild-type hMLH1 gene and is deficient in repair of mismatches. However, a cell line with deletions in both hMSH2 alleles is deficient in repair of both loops and mismatches. Defects in loop repair may be relevant to the repetitive-sequence instability observed in cancers and other hereditary diseases.

DNA damage tolerance, mismatch repair and genome instability

DNA mismatch repair is an important pathway of mutation avoidance. It also contributes to the cytotoxic effects of some kinds of DNA damage, and cells defective in mismatch repair are resistant, or tolerant, to the presence of some normally cytotoxic base analogues in their DNA. The absence of a particular mismatch binding function from some mammalian cells confers resistance to the base analogues O6-methylguanine and 6-thioguanine in DNA. Cells also acquire a spontaneous mutator phenotype as a consequence of this defect. Impaired mismatch binding can cause an instability in DNA microsatellite regions that comprise repeated dinucleotides. Microsatellite DNA instability is common in familial and sporadic colon carcinomas as well as in a number of other tumours. Several independent lines of investigation have identified defects in mismatch repair proteins that are causally related to these cancers.

Radiation-induced genomic instability

Quantitative assessment of the heritable somatic effects of ionizing radiation exposures has relied upon the assumption that radiation-induced lesions were 'fixed' in the DNA prior to the first postirradiation mitosis. Lesion conversion was thought to occur during the initial round of DNA replication or as a consequence of error-prone enzymatic processing of lesions. The standard experimental protocols for the assessment of a variety of radiation-induced endpoints (cell death, specific locus mutations, neoplastic transformation and chromosome aberrations) evaluate these various endpoints at a single snapshot in time. In contrast with the aforementioned approaches, some studies have specifically assessed radiation effects as a function of time following exposure. Evidence has accumulated in support of the hypothesis that radiation exposure induces a persistent destabilization of the genome. This instability has been observed as a delayed expression of lethal mutations, as an enhanced rate of accumulation of non-lethal heritable alterations, and as a progressive intraclonal chromosomal heterogeneity. The genetic controls and biochemical mechanisms underlying radiation-induced genomic instability have not yet been delineated. The aim is to integrate the accumulated evidence that suggests that radiation exposure has a persistent effect on the stability of the mammalian genome.

Southwestern internal medicine conference: hereditary predisposition to colorectal cancer: new insights

Hereditary predisposition to colorectal cancer assumes two well-defined forms: familial adenomatous polyposis and hereditary nonpolyposis colon cancer. These tumors segregate as autosomal dominant conditions whose penetrance increases with age; cancer is expected to develop ultimately in as much as 50% of the offspring of affected individuals. These traits account for less than 1% and approximately 5% of all colorectal cancer, respectively. In addition, first-degree relatives of patients with common (sporadic) colorectal neoplasia are at increased risk of colorectal cancer. This relative risk averages approximately twofold but is significantly higher for relatives of younger patients (age at diagnosis, < 55 years). Familial adenomatous polyposis and a major subset of hereditary nonpolyposis colon cancer are due to loss-of-function germline mutations of genes located on chromosomes 5q and 2p, respectively. Both of these genes have been cloned recently. The gene affected in familial polyposis, APC, encodes a protein of unknown function that normally is found on the surface of maturing cells in the upper colonic crypts. The relevant gene in many hereditary nonpolyposis colon cancer kindreds is hMSH2. This gene encodes the human homologue of a bacterial protein MutS, which is part of a system known to repair base mismatches in newly replicated DNA. Loss of hMSH2 function may explain the strikingly erroneous replication of short DNA repeats (microsatellites) in colon tumors from patients with hereditary nonpolyposis colon cancer. Because this error-prone replication is found in approximately 13% of nonfamilial colon cancers, defective mismatch repair may contribute to the development of both hereditary and sporadic colon neoplasia. Molecular genetic assays to detect mutated alleles of these genes will facilitate presymptomatic identification of carriers in families with familial polyposis and hereditary nonpolyposis colon cancer. Current recommendations for surveillance of family members are presented in the light of the new genetic understanding of these diseases.

A catalogue of genes in the cardiovascular system as identified by expressed sequence tags

The heart, which is composed of all the cellular components of the circulatory system, is a representative organ for obtaining genes expressed in the cardiovascular system in normal and disease states. We used partial sequences of cDNA clones, or expressed sequence tags, to identify and tag genes expressed in this organ. More than 3500 partial sequences representing > 3000 cDNA clones have been obtained from either the 5' or 3' end of inserts derived from human heart cDNA libraries. Of 3132 cDNA clones analyzed by sequence similarity searching against the GenBank/EMBL data bases, 1485 (47.4%) were found to represent additional, previously undiscovered genes, whereas 267 clones were matched to human brain expressed sequence tags. Clones matching to known genes were catalogued according to their putative structural and cellular functions. cDNA probes from reverse-transcribed mRNAs of fetal and adult hearts were used to study differential expression of selected clones in cardiac development. Cataloguing genes expressed in the heart may provide insight into the genes involved in health and cardiovascular disease.

Microsatellite alterations as clonal markers for the detection of human cancer

Microsatellite instability has been reported to be an important feature of tumors from hereditary nonpolyposis colorectal carcinoma (HNPCC) patients. The recent discovery of genetic instability in small cell lung carcinoma, a neoplasm not associated with HNPCC, led us to investigate the possible presence of microsatellite alterations in other tumor types. We examined 52 microsatellite repeat sequences in the DNA of normal and tumor pairs from 100 head and neck, bladder, and lung cancer patients by the polymerase chain reaction. Although alterations were rare in dinucleotide repeats, larger (tri- or tetranucleotide) repeats were found to be more prone to expansion or deletion. We screened 100 tumors with a panel of nine tri- and tetranucleotide repeat markers and identified 26 (26%) that displayed alterations in at least one locus. This observation prompted us to examine the possibility of using microsatellite alterations as markers to detect clonal tumor-derived cell populations in pathologic samples. The identical microsatellite alterations detected in the primary tumors were successfully identified in corresponding urine, sputum, and surgical margins from affected patients. This study demonstrates that appropriately selected microsatellite loci are commonly altered in many cancers and can serve as clonal markers for their detection.

Genetic susceptibility to colorectal cancer in patients under 45 years of age

A study was conducted to assess the genetic contribution to the development of colorectal cancer in young probands. Of 83 patients aged 45 years or under diagnosed with colorectal cancer in one health region over a 2-year period, 65 or their surviving next of kin were available for interview, from whom were obtained 60 detailed and five limited family histories. Five families fulfilled the Amsterdam criteria and a further eight satisfied less strict criteria for hereditary non-polyposis colorectal cancer, a total of 20 per cent of the cohort. Eleven of these families came from the subgroup of 13 probands who had one or more first-degree relatives with colorectal cancer. Overall the relative risk of colorectal cancer in close relatives was 5.2 (P < 0.0001). This risk was highest for female relatives at 9.7 (P < 0.0001) and relatives of female probands at 6.7 (P < 0.0001). This study highlights the importance of taking a family history in this group of patients. Screening by colonoscopy for all close relatives of young patients with colorectal cancer is recommended.

At least four genes and sex are associated with susceptibility to urethane-induced pulmonary adenomas in mice

Susceptibility to urethane-induced lung adenomas in mice has a polygenic mode of inheritance, with no obvious discontinuity in lung tumour counts among 37 AXB recombinant inbred strains. However, mean tumour counts were markedly higher in strains carrying the A/J allele at the Kras2 and H2 complex than in those carrying the C57BL/ allele. In 162 F2 hybrids and small numbers of both backcrosses between strain A/J (susceptible) and C57/BL/6 (resistant) mice, five factors influencing susceptibility were identified. Variation due to the 'major' Kras2 locus (chromosome 6) accounted for 60% of the total variation. 'Minor' loci linked to microsatellite markers Tnfb (in the H2 complex), D9Mit11 and D19Mit16 (on chromosomes 17, 9 and 19, respectively) accounted for a further 13% of the variation, and males had more tumours than females with sex differences accounting for 2% of the variation. No significant association with 32 other loci was detected. On a square-root transformed scale, heterozygotes at all marker loci were of intermediate susceptibility compared with homozygotes. The three minor loci and sex only affected lung tumour counts when at least one susceptible Kras2 allele was present.

Repair of heteroduplex DNA in Xenopus laevis oocytes

We have hypothesized that the inheritance of heteroallelic markers during recombination of homologous DNAs in Xenopus oocytes is determined by resolution of a heteroduplex intermediate containing multiple single-base mismatches. To test this idea, we prepared synthetic heteroduplexes carrying 8 separate mispairs in vitro and injected them into oocyte nuclei. DNA was recovered and analyzed directly, by Southern blot-hybridization, and indirectly, by cloning individual repair products in bacteria. Mismatch correction was quite efficient in the oocytes; markers on the same strand were commonly co-corrected, indicating a long-patch mechanism; and the distribution of markers was very similar to that obtained by recombination. This supports our interpretation of the recombination outcome in terms of a resection-annealing mechanism. The injected heteroduplexes carried strand breaks (nicks) as a result of their method of preparation. We tested the idea that mismatch correction might be nick-directed by ligating the strands of the heteroduplex substrate to form covalently closed circles. Repair in oocytes was still efficient, and long patches predominated; but the pattern of recovered markers was quite different than with the nicked substrate. This suggests that nicks, when present, do indeed direct repair, but that, in their absence, recognition of specific mismatches governs repair of the ligated heteroduplexes.

Genetic dissection of complex traits

Medical genetics was revolutionized during the 1980s by the application of genetic mapping to locate the genes responsible for simple Mendelian diseases. Most diseases and traits, however, do not follow simple inheritance patterns. Genetics have thus begun taking up the even greater challenge of the genetic dissection of complex traits. Four major approaches have been developed: linkage analysis, allele-sharing methods, association studies, and polygenic analysis of experimental crosses. This article synthesizes the current state of the genetic dissection of complex traits--describing the methods, limitations, and recent applications to biological problems.

Role of clinical criteria in the diagnosis of hereditary non-polyposis colorectal cancer (HNPCC): results of a multivariate analysis

Hereditary non-polyposis colorectal cancer (HNPCC) is an autosomal dominant disease characterized by early-onset colorectal tumors, primarily in the right colon, that are frequently associated with other cancers. In the absence of a reliable biomarker, clinical criteria for diagnosing HNPCC have been proposed by the International Collaborative Group on HNPCC (ICG-HNPCC). However, these criteria are often too restrictive for application in small families. We analyzed 6 clinical/pathological features suggestive of genetic colon-cancer risk in 970 colorectal-cancer patients defined according to the ICG criteria as HNPCC (96) or non-HNPCC (874). Logistic regression analysis was used to determine their relative potentials for predicting the diagnosis of HNPCC. The most predictive were then used to estimate HNPCC risk levels within the non-HNPCC group. Multivariate analysis showed the following associations with HNPCC diagnosis: vertically transmitted cancer ("highly predictive"), early-onset (< 50 yrs) colorectal cancer, aggregation of tumors in the nuclear pedigree and proximal-colon tumors (the last 3 considered "predictive"). Re-evaluation of all families revealed that 76 non-HNPCC families (8.9% of the whole series) satisfied our highly predictive vertical-transmission criterion plus at least one of the other "predictive" criteria. The presence of 2 consecutive generations affected by colorectal cancer or early primaries seems to be a major risk indicator of hereditary colorectal cancer. Using this approach we identified a large group of families that require further evaluation, although they do not currently meet the ICG-HNPCC criteria for HNPCC.

Detection of endogenous malondialdehyde-deoxyguanosine adducts in human liver

Endogenous DNA adducts may contribute to the etiology of human genetic disease and cancer. One potential source of endogenous DNA adducts is lipid peroxidation, which generates mutagenic carbonyl compounds such as malondialdehyde. A sensitive mass spectrometric method permitted detection and quantitation of the major malondialdehyde-DNA adduct, a pyrimidopurinone derived from deoxyguanosine. DNA from disease-free human liver was found to contain 5400 adducts per cell, a frequency comparable to that of adducts formed by exogenous carcinogens.

Mutagenesis of yeast MW104-1B strain has identified the uncharacterized PMS6 DNA mismatch repair gene locus and additional alleles of existing PMS1, PMS2 and MSH2 genes

The haploid yeast Saccharomyces cerevisiae MW104-1B strain was disomic for chromosome III (n + 1) and carried DNA mismatches at three different heteroallelic loci; leu2 (leu2-1/leu2-27), thr4 (thr4-1/thr4-16) and his4 (his4-4/his4-519) (Williamson, 1984). We mutagenized the MW104-1B strain and identified seven mutant isolates that display elevated mitotic/meiotic prototrophs due to mismatch repair failures at heteroallelic loci. Three mutants (pms1, pms2 and pms3) isolated earlier from MW104-1B were shown to correct in vitro constructed plasmids with defined DNA mismatches (G/T, A/C, G/G, etc.) poorly (Kramer et al., 1989a). Complementation tests were performed by crossing all seven new mutant isolates to pms1 and pms2 mutants and assaying for mutant phenotype in the diploids. Four mutant isolates failed to complement the two known pms alleles (pms1-1 and pms2-1). Two other mutant isolates complemented the pms1-1 and pms2-1 alleles, but failed to complement each other and were named as the pms5-1 allele of an uncharacterized gene (PMS5). One other mutant isolate complemented the pms1-1, pms2-1 and pms5-1 alleles and was named as the pms6-1 allele of another uncharacterized gene (PMS6). Subsequently, the pms5-1 mutant allele was shown to be complemented by a plasmid borne yeast MSH2 gene, implying that it is an allele of MSH2 (PMS5). The human homologs (hMSH2 and hMLH1) of two yeast DNA mismatch repair genes (MSH2 and MLH1) have been cloned recently and shown to be responsible for hereditary nonpolypnosis colon cancer (HNPCC) (Fishel et al., 1993; Leach et al., 1993; Bronner et al., 1994; Papadopoulos et al., 1994).

Mutations of two PMS homologues in hereditary nonpolyposis colon cancer

Hereditary nonpolyposis colorectal cancer (HNPCC) is one of man's commonest hereditary diseases. Several studies have implicated a defect in DNA mismatch repair in the pathogenesis of this disease. In particular, hMSH2 and hMLH1 homologues of the bacterial DNA mismatch repair genes mutS and mutL, respectively, were shown to be mutated in a subset of HNPCC cases. Here we report the nucleotide sequence, chromosome localization and mutational analysis of hPMS1 and hPMS2, two additional homologues of the prokaryotic mutL gene. Both hPMS1 and hPMS2 were found to be mutated in the germline of HNPCC patients. This doubles the number of genes implicated in HNPCC and may help explain the relatively high incidence of this disease.

MLH1, PMS1, and MSH2 interactions during the initiation of DNA mismatch repair in yeast

The discovery that mutations in DNA mismatch repair genes can cause hereditary nonpolyposis colorectal cancer has stimulated interest in understanding the mechanism of DNA mismatch repair in eukaryotes. In the yeast Saccharomyces cerevisiae, DNA mismatch repair requires the MSH2, MLH1, and PMS1 proteins. Experiments revealed that the yeast MLH1 and PMS1 proteins physically associate, possibly forming a heterodimer, and that MLH1 and PMS1 act in concert to bind a MSH2-heteroduplex complex containing a G-T mismatch. Thus, MSH2, MLH1, and PMS1 are likely to form a ternary complex during the initiation of eukaryotic DNA mismatch repair.

Colorectal tumourigenesis

Colorectal cancer is significant because of its lethality and high incidence in the Western world. Furthermore, it is a particularly suitable model for studying the events involved in the progression from normal tissue to invasive cancer. There is a great deal of epidemiological and histopathologic evidence to implicate the adenomatous polyp as the precursor to colorectal cancer. Moreover in recent years investigators have uncovered some of the molecular genetic events that underlie the progression from normal epithelium to polyp to cancer. It is hoped that an increased understanding of the molecular changes will afford opportunities for new diagnostic, prognostic and therapeutic strategies in the management of this disease.

Close linkage to chromosome 3p and conservation of ancestral founding haplotype in hereditary nonpolyposis colorectal cancer families

A susceptibility to hereditary nonpolyposis colorectal cancer (HNPCC) was recently shown to be due to mutations in the MSH2 gene on chromosome 2p. A second susceptibility locus has been mapped to chromosome 3p in two families. The present report describes the results of a genetic study of Finnish HNPCC kindreds. Of 18 apparently unrelated families living in different parts of the country, 11 could be genealogically traced to a common ancestry dating at least 13 generations back in a small geographic area. Linkage studies were possible in 9 families, revealing conclusive or probable linkage to markers on 3p in 8. Five of these were among those having shared ancestry. The location of the gene was refined by a linkage study comprising 12 marker loci. By analysis of recombinations in such families, the HNPCC locus could be assigned to the 1-centimorgan interval between marker loci D3S1561 and D3S1298. A haplotype encompassing 10 centimorgans around the HNPCC locus was conserved in five of the pedigrees with shared ancestry and present in 2 further families in which linkage analysis was not possible. Our results suggest the presence of a widespread single ancestral founding mutation. Moreover, the map position of the 3p gene for HNPCC susceptibility was greatly refined.