Mutation of a mutL homolog in hereditary colon cancer

Multiple mutations in human cancers

Increasing evidence indicates that most human cancers contain multiple mutations. The exact number of mutations, their origin, and types remain to be determined. An over-riding question is whether the multiple mutations that accumulate in cancers is rate-limiting for the carcinogenic process. In this review we consider the argument that the large numbers of mutations routinely reported in human cancers cannot be accounted for by the rate of spontaneous mutation observed in normal human cells. We will analyze different mechanisms that might account for the accumulation of mutations in cancer cells. We conclude that cancer cells are genetically unstable; i.e., they exhibit a mutator phenotype. The recent reports of microsatellite instability in a variety of human cancers have provided the first strong evidence for the presence of a mutator phenotype in human cancers. However, we still lack information about the relationship between microsatellite instability and mutations that allow cancer cells to proliferate, invade, and metastasize.

Recombination-dependent mutation in non-dividing cells

Over the past 6 years an unexpected way of making mutations in bacteria has challenged concepts of the genetic mechanisms behind evolution. Mechanistic studies of these so called 'adaptive' mutations are revealing a novel molecular mechanism involving DNA double-strand breaks, genetic recombination, probable DNA polymerase errors, and the possible suspension of mismatch repair during the reversion of a lac frameshift mutation in Escherichia coli. The molecular details of this process are altering our understanding of how mutations form in non-dividing cells.

Mismatch DNA recognition protein from an extremely thermophilic bacterium, Thermus thermophilus HB8

The mutS gene, implicated in DNA mismatch repair, was cloned from an extremely thermophilic bacterium, Thermus thermophilus HB8. Its nucleotide sequence encoded a 819-amino acid protein with a molecular mass of 91.4 kDa. Its predicted amino acid sequence showed 56 and 39% homology with Escherichia coli MutS and human hMsh2 proteins, respectively. The T.thermophilus mutS gene complemented the hypermutability of the E.coli mutS mutant, suggesting that T.thermophilus MutS protein was active in E.coli and could interact with E.coli MutL and/or MutH proteins. The T.thermophilus mutS gene product was overproduced in E.coli and then purified to homogeneity. Its molecular mass was estimated to be 91 kDa by SDS-PAGE but approx. 330 kDa by size-exclusion chromatography, suggesting that T.thermophilus MutS protein was a tetramer in its native state. Circular dichroic measurements indicated that this protein had an alpha-helical content of approx. 50%, and that it was stable between pH 1.5 and 12 at 25 degree C and was stable up to 80 degree C at neutral pH. Thermus thermophilus MutS protein hydrolyzed ATP to ADP and Pi, and its activity was maximal at 80 degrees C. The kinetic parameters of the ATPase activity at 65 degrees C were Km = 130 microM and Kcat = 0.11 s(-1). Thermus thermophilus MutS protein bound specifically with G-T mismatched DNA even at 60 degrees C.

Redundancy of Saccharomyces cerevisiae MSH3 and MSH6 in MSH2-dependent mismatch repair

Saccharomyces cerevisiae encodes six genes, MSH1-6, which encode proteins related to the bacterial MutS protein. In this study the role of MSH2, MSH3, and MSH6 in mismatch repair has been examined by measuring the rate of accumulating mutations and mutation spectrum in strains containing different combinations of msh2, msh3, and msh6 mutations and by studying the physical interaction between the MSH2 protein and the MSH3 and MSH6 proteins. The results indicate that S. cerevisiae has two pathways of MSH2-dependent mismatch repair: one that recognized single-base mispairs and requires MSH2 and MSH6, and a second that recognizes insertion/deletion mispairs and requires a combination of either MSH2 and MSH6 or MSH2 and MSH3. The redundancy of MSH3 and MSH6 explains the greater prevalence of hmsh2 mutations in HNPCC families and suggests how the role of hmsh3 and hmsh6 mutations in cancer susceptibility could be analyzed.

Recognition of DNA insertion/deletion mismatches by an activity in Saccharomyces cerevisiae

An activity in nuclear extracts of S.cerevisiae binds specifically to heteroduplexes containing four to nine extra bases in one strand. The specificity of this activity (IMR, for insertion mismatch recognition) in band shift assays was confirmed by competition experiments. IMR is biochemically and genetically distinct from the MSH2 dependent, single base mismatch binding activity. The two activities migrate differently during electrophoresis, they are differentially competable and their spectra of mispair binding are distinct. Furthermore, IMR activity is observed in extracts from an msh2- msh3- msh4- strain. IMR exhibits specificity for insertion mispairs in two different sequence contexts. Binding is influenced by the structure of the mismatch since an insertion with a hairpin configuration is not recognized by this activity. IMR does not result from single-strand binding because single-stranded probes to not yield IMR complex and single-stranded competitors are unable to displace insertion heteroduplexes from the complex. Similar results with intrinsically bent duplexes make it unlikely that recognition is conferred by a bend alone. Heteroduplexes bound by IMR do not contain any obvious damage. These findings are consistent with the idea that yeast contains a distinct recognition factor, IMR that is specific for insertion/deletion mismatches.

The risk of brain tumours in hereditary non-polyposis colorectal cancer (HNPCC)

Hereditary non-polyposis colorectal cancer (HNPCC) is known to be associated with several extracolonic cancers, e.g., cancers of the endometrium, stomach, urinary tract, small bowel and ovary. An association between HNPCC and brain tumours has also been reported, although previous risk analysis did not reveal an excess of this type of tumour. To determine whether HNPCC predisposes patients to brain tumours, we used risk analysis to compare families with HNPCC to those in the general population. Of the 1,321 subjects from 50 HNPCC families (with 60,237 person-years of follow-up) in the Dutch HNPCC Registry which satisfy the Amsterdam Criteria, 312 had colorectal cancer. The registry revealed 14 brain tumours in the HNPCC-patients and their first-degree relatives: 5 astrocytomas, 3 oligodendrogliomas, 1 ependymoma and 5 tumours for which a pathological report was not available. The relative risk of brain tumour in patients with HNPCC and their first-degree relatives was 6 times greater than in the general population (95% confidence interval, 3.5 to 10.1). After exclusion of the cases based only on family history, the relative risk was 4.3 (95% confidence interval, 2.3 to 8.0). Although the relative risk of brain tumour was increased, the lifetime risk was low (3.35%). Because it is not certain whether an improvement of the overall prognosis can be achieved by early diagnosis and intervention, and in view of the low lifetime risk, we do not recommend screening for brain tumours in HNPCC families.

Microsatellite instability in gastric cancer prone families

We examined for germ-line p53 mutations and microsatellite instability in three gastric cancer patients who had family histories of gastric cancer aggregation. Although no germ-line p53 mutation was detected in these three cases, the replication error (RER) phenotype was observed in two of them. One base deletion in the sequence of ten repeating adenines of the type II transforming growth factor-beta receptor gene was detected in one of these two cases. Furthermore, there were young patients of 50 years and downward in their families. Therefore, it is possible that inherited disorders in mismatch repair systems contribute to high susceptibility to gastric cancers in these families.

Products of DNA mismatch repair genes mutS and mutL are required for transcription-coupled nucleotide-excision repair of the lactose operon in Escherichia coli

To improve our understanding of the mechanism that couples nucleotide-excision repair to transcription in expressed genes, we have examined the effects of mutations in several different DNA repair genes on the removal of cyclobutane pyrimidine dimers from the individual strands of the induced lactose operon in UV-irradiated Escherichia coli. As expected, we found little repair in either strand of the lactose operon in strains with mutations in established nucleotide excision-repair genes (uvrA, uvrB, uvrC, or uvrD). In contrast, we found that mutations in either of two genes required for DNA-mismatch correction (mutS and mutL) selectively abolish rapid repair in the transcribed strand and render the cells moderately sensitive to UV irradiation. Similar results were found in a strain with a mutation in the mfd gene, the product of which has been previously shown to be required for transcription-coupled repair in vitro. Our results demonstrate an association between mismatch-correction and nucleotide-excision repair and implicate components of DNA-mismatch repair in transcription-coupled repair. In addition, they may have important consequences for human disease and may enhance our understanding of the etiology of certain cancers which have been associated with defects in mismatch correction.

Mutational analysis of mismatch repair genes, hMLH1 and hMSH2, in sporadic endometrial carcinomas with microsatellite instability

Microsatellite instability, monitored by replication error (RER), has been observed in both sporadic and hereditary types of endometrial carcinoma. In the hereditary tumors, this instability is considered to be caused by a germline defect in the DNA mismatch-repair system. We previously reported that nearly one-quarter of sporadic endometrial carcinomas examined revealed an RER-positive phenotype at multiple microsatellite loci. To investigate the role of genetic alterations of DNA mismatch-repair genes in sporadic endometrial carcinomas, we screened 18 RER(+) endometrial carcinomas for mutations of hMLH1 and hMSH2. Although we found no germline mutations, we detected two somatic mutations of hMLH1 in a single endometrial cancer; these two mutations had occurred on different alleles, suggesting that two separate mutational events had affected both copies of hMLH1 in this particular tumor. These data implied that mutations of hMLH1 or hMSH2 play limited roles in the development of sporadic endometrial carcinomas, and that the tumors with genetic instability might have alterations of other mismatch-repair genes, such as hPMS1 and hPMS2, or of unknown genes related to the mismatch-repair system.

Infrequent microsatellite instability during the evolution of myelodysplastic syndrome to acute myelocytic leukemia

Microsatellites are highly polymorphic, short-tandem repeat sequences dispersed throughout the genome. Instability of these repeat sequences at multiple gentic loci may result from mismatch repair errors and occur in hereditary nonpolyposis colorectal cancer and several other sporadic cancers, including chronic myelocytic leukemia as it progresses to blastic crisis. We investigated whether genetic instability occurred as myelodysplasia progressed to acute myelocytic leukemia. To this end, we studied microsatellite instability in 20 patients with myelodysplastic syndrome (MDS). These included five patients with refractory anemia (RA), three with refractory anemia with ringed sideroblast (RARS), nine with refractory anemia with excess blasts (RAEB) and three with chronic myelomonocytic leukemia (CMML). All of these patients transformed to acute myelocytic leukemia (AML) of various subtypes: three patients with M1, 11 with M2 and six patients with M4 (according to FAB classification). The DNA from both the MDS and AML phases of their disease was analyzed at 16 loci, and only four microsatellite instabilities were found in the 240 paired samples (1.6%) analyzed. These results indicate that mismatch repair errors such as microsatellite instability are not important in the evolution of MDS to AML.

The geneticist's approach to complex disease

Many studies are in progress worldwide to elucidate the genetics of complex diseases. Nevertheless, few articles are available that provide the scientific rationale and give guidelines for such ambitious endeavours. We describe the methodology and background necessary to study the genetics of complex disease and discuss how to analyze the data. We also provide a table of some ongoing studies. In particular, we wish to emphasize the analysis of intermediate, heritable, quantitative traits as a means of dissecting the genetic basis of a complex trait.

Psychogenic stress induces chromosomal and DNA damage

In this investigation, rats subjected to swim stress showed within 24 hours significant increases in both the level of chromosome aberrations and Sister Chromatid Exchanges (SCEs) in bone marrow cells. The generality of cytogenetic damage by behavioral stressors was demonstrated by exposing rats to both cold-and warm-water forced swims, to white noise, and to continuous or intermittent inescapable foot shock stress (IFS). The induction of chromosome aberrations and SCEs, to differing degrees, by stressors that differ both quantitatively and qualitatively, demonstrates that this is a general phenomenon of stress. The use of an additional measure, unscheduled DNA Synthesis (UDS) showed that stress-induced genotoxic damage can occur in a second cell type and on a molecular as well as chromosomal level. These results indicate that there may be a cellular genetic basis for some of the effects of stress.

CpG dinucleotides in the hMSH2 and hMLH1 genes are hotspots for HNPCC mutations

Hereditary nonpolyposis colon cancer (HN-PCC) is an autosomally inherited predisposition to cancer that has recently been linked to defects in the human mismatch repair genes hMSH2 and hMLH1. The identification of the causative mutations in HNPCC families is desirable, since it confirms the diagnosis and allows the carrier status of unaffected relatives at risk to be determined. We report six different new mutations identified in the hMSH2 and hMLH1 genes of Russian and Moldavian HNPCC families. Three of these mutations occur in CpG dinucleotides and lead to a premature stop codon, a splicing defect or an amino-acid substitution in an evolutionary conserved residue. Analysis of a compilation of published mutations including our new data suggests that CpG dinucleotides within the coding regions of the hMSH2 and hMLH1 genes are hotspots for single base-pair substitutions.

Mutator genes and mosaicism in colorectal cancer

Recent studies have shed light on the role of defective DNA mismatch repair in human cancer. An elevated mutation rate associated with mismatch repair deficiency has been demonstrated in the germline and normal tissue from patients with hereditary non-polyposis colorectal cancer and transgenic animals respectively. Thus mismatch repair deficiency may permit the accumulation of mutations in cancer genes that do not confer growth advantage. This represents one potential mechanism for the induction of mutational mosaicism in humans.

Molecular genetic analysis of clear cell adenocarcinomas of the vagina and cervix associated and unassociated with diethylstilbestrol exposure in utero

BACKGROUND

Prenatal exposure to the synthetic estrogen diethylstilbestrol (DES) is associated with the subsequent development of clear cell adenocarcinoma of the lower reproductive tract in young women, and data concerning the molecular genetic alterations involved in the etiology of this tumor type have not previously been reported. Such knowledge would be of potential value by providing insight into the molecular mechanisms of hormonal carcinogenesis in general, as well as by suggesting molecular markers for risk assessment in the estrogen-exposed population.

METHODS

A total of 24 samples of clear cell adenocarcinoma of the vagina or cervix, 16 associated with exposure in utero to DES and 8 with no history of DES exposure, were obtained as archival fixed and embedded tissue specimens. DNA was purified from these tissues and used to examine a number of biologically plausible molecular genetic endpoints for tumor specific alterations.

RESULTS

No evidence was found for mutations in the K-ras or H-ras protooncogenes, the Wilms' tumor (WT1) tumor suppressor gene, or the estrogen receptor gene. Sporadic overexpression of the p53 tumor suppressor gene was detected in some tumor cell nuclei by immunohistochemistry, but in the absence of detectable p53 gene mutation. Genetic instability as manifested by somatic mutation of microsatellite repeats was widespread in these tumors, with evidence of microsatellite instability in all DES-associated tumors examined, and in 50% of those tumors not associated with DES exposure.

CONCLUSIONS

These data are consistent with the hypothesis that the induction of genomic instability may be an important mechanism of DES-induced carcinogenesis.

Analysis of mismatch repair genes in hereditary non-polyposis colorectal cancer patients

Hereditary non-polyposis colorectal cancer (HNPCC) is an autosomal dominant disorder characterized by the early onset of colorectal cancer and linked to germline defects in at least four mismatch repair genes. Although much has been learned about the molecular pathogenesis of this disease, questions related to effective presymptomatic diagnosis are largely unanswered because of its genetic complexity. In this study, we evaluated tumors from 74 HNPCC kindreds for genomic instability characteristic of a mismatch repair deficiency and found such instability in 92% of the kindreds. The entire coding regions of the five known human mismatch repair genes were evaluated in 48 kindreds with instability, and mutations were identified in 70%. This study demonstrates that a combination of techniques can be used to genetically diagnose tumor susceptibility in the majority of HNPCC kindreds and lays the foundation for genetic testing of this relatively common disease.

Cancer mortality in relatives of women with ovarian cancer: the OPCS Study. Office of Population Censuses and Surveys

Mortality from cancer and other causes in first-degree relatives of women with ovarian cancer diagnosed before age 60 has been examined in a large population-based cohort study in England and Wales. Relatives of 1,188 ovarian-cancer cases diagnosed between 1954 and 1981 were identified through a register of households established in 1939. Some 4,111 first-degree relatives living in the same household and having the same surname as the index case were followed up through national records until the end of 1992. Over this period, 1,950 deaths (including 574 cancer deaths) occurred in the relatives. Mortality rates within the cohort were compared with age-, sex- and period-adjusted mortality rates for England and Wales. Mortality from ovarian cancer in first-degree relatives was significantly raised (SMR 223, 95% CI 155-310) although the excess was smaller than that found in case-control studies. The SMR increased with decreasing age of the relative, though not with decreasing age of the index case. After allowing for age, sisters of cases had higher ovarian-cancer mortality than mothers (sister:mother SMR ratio 1.89, p = 0.06). The SMR was greater in individuals having 2 first-degree relatives with ovarian cancer (4 deaths versus 0.17 expected, SMR 242). Relatives of ovarian cancer cases also had significantly increased mortality from cancers of the stomach (SMR 146, 69 deaths) and rectum (SMR 150, 33 deaths), and increased mortality from colon cancer, breast cancer and pancreatic cancer which failed to reach statistical significance. Individuals having a relative with colorectal cancer and a relative with ovarian cancer showed a high mortality from both cancers (11 colorectal-cancer deaths versus 1.23 expected, 4 ovarian-cancer deaths versus 0.66 expected.

hMSH2-independent DNA mismatch recognition by human proteins

Two distinct mismatch binding activities are detected using bandshift assays with human cell extracts and DNA with mispairs at defined positions. One requires hMSH2 protein and is absent from extracts of LoVo cells, which contain a partial deletion of the hMSH2 gene. The second activity is independent of hMSH2 and is present at normal levels in LoVo and three other cell lines, which are defective in in vitro hMSH2-dependent binding. The two mismatch recognition activities are distinguished by their sensitivity to polycations and can be resolved by chromatography on MonoQ. hMSH2-independent activity has been purified extensively from wild-type cells and from a cell line deficient in hMSH2-dependent binding. The purified material preferentially recognizes A-C, some pyrimidine-pyrimidine mismatches, and certain slipped mispaired structures. Binding exhibits some sequence preferences. The similar properties of the two mismatch binding activities suggest that they both contribute to mismatch repair.

Increase in hypoxanthine-guanine phosphoribosyl transferase gene mutations by exposure to high-density 50-Hz magnetic fields

Exposure to extremely low frequency magnetic field (ELFMF) of 50 Hz and 400 mT induced mutations in the hypoxanthine-guanine phosphoribosyl transferase gene of human melanoma MeWo cells. The mutant frequency was enhanced both by increasing the exposure period and the induced current intensity. Mutations induced by X-rays were enhanced by ELFMF exposure. No significant increase in mutant frequency occurred when DNA replication was inhibited during ELFMF exposure. DNA replication error is suspected of causing the mutations produced by ELFMF exposure.

Clinical implications of microsatellite instability in colorectal cancers

BACKGROUND

Microsatellite instability (MI) has been reported in some sporadic colon tumors and in cases of hereditary nonpolyposis colorectal cancer (HNPCC). The criteria for HNPCC have not been fully defined, and clinical criteria are used to identify as many HNPCC patients as possible. To clarify the conformity of these criteria with the identification of eligible HNPCC cases, we analyzed MI in HNPCC patients diagnosed using clinical criteria.

METHODS

Genomic DNA was extracted from surgical specimens of 56 colorectal cancers, including 36 from patients diagnosed with HNPCC using the clinical criteria. We analyzed four microsatellite loci using 32P-labeled primers.

RESULTS

Among HNPCC patients diagnosed using clinical criteria, patients who were positive for MI accounted for 62% of Group A (a confirmed group) and 35% of Group B (a high risk group); only 5% of randomly selected colorectal cancer patients (Group C), were positive for MI. Furthermore, MI-positive tumors were found in patients who had a tendency for tumors to involve the right side of the colon, an association with cancers in other organs, a lower incidence of p53 protein positivity, and a higher proportion of poorly differentiated cancers.

CONCLUSIONS

The presence of MI, in concert with modified clinical criteria, may identify legitimate cases of HNPCC in patients who might otherwise be excluded by the minimum criteria.

Recent developments in hereditary nonpolyposis colorectal cancer

BACKGROUND

Hereditary non-polyposis colorectal cancer (HNPCC) is characterized by early onset of colorectal carcinoma (CRC), usually located proximally to the splenic flexure and reportedly carrying a better survival as compared to sporadic-type CRC. Depending on the absence or presence of extracolonic tumours, particularly carcinomas of the endometrium, stomach and urinary tract, HNPCC can be divided into Lynch syndromes I and II, respectively. Although first described in 1913, the elucidation of the molecular basis of this disease has only recently started to unfold, and is reviewed in this article.

METHODS

Literature survey of published articles.

RESULTS

Comparison of HNPCC tumours with sporadic-type CRC had already revealed that no significant differences were found in APC, Ki-ras and p53 gene alterations. Instead, microsatellite instability was found to be the hallmark of HNPCC being present in 80% of cases compared to only 13% of sporadic type CRC. Since studies in yeast and bacteria had shown that microsatellite instability resulted from mutations in so-called postreplicative DNA mismatch repair genes, it was hypothesized that a similar mechanism might underlie the observed microsatellite instability in HNPCC and, hence, its hereditary character. In due course, four human homologues of yeast and bacterial postreplicative DNA mismatch repair genes were cloned and denoted hMSH2, hMLH1, hPMS1, and hPMS2. Current estimates suggest that mutations in hMSH2 account for 50%, in hMLH1 for 30%, in hPMS1 for 5% and in hPMS2 for 5% of HNPCC.

CONCLUSIONS

Despite many issues still to be resolved, accurate molecular screening tests will in all probability become the gold standard in the diagnosis of HNPCC. As a result, these developments will undoubtedly have profound implications for early detection and subsequent management of affected individuals.

Mutation of the hMSH2 gene in two families with hereditary nonpolyposis colorectal cancer

We examined 18 unrelated individuals who have colorectal cancer or cancers associated with the HNPCC syndrome and have a family history of cancer for mutations in exon 13 of the hMSH2 gene. Two of the 18 individuals had the same previously unreported single-base deletion in codon 705 of hMSH2, resulting in a frame-shift mutation. Two other individuals had a T-to-C base change in the intron sequence at -6 position of the splice acceptor site at the 5' end of exon 13 which has previously been reported to be a polymorphism found in normal individuals (Leach et al., 1993). Nucleotide sequence changes were not detected in the remaining 14 individuals. We examined DNA from additional family members of the two subjects with codon 705 mutations. Of these, two individuals were identified with the mutation who were older than 50 years and who are apparently cancer free. The occurrence of breast cancer in both families (including one individual with a confirmed codon 705 mutation) suggests that breast cancer may be a part of the HNPCC syndrome. This is the first study to describe mutations in the hMSH2 gene in families that do not fit the definition of HNPCC. Because both families with the mutations at codon 705 failed to meet the Amsterdam criteria for HNPCC (Vasen et al., Dis Colon Rectum 34:424-425, 1991), our findings suggest that these criteria should now be reconsidered for purposes of diagnosis of HNPCC.

Mammalian DNA repair responses and genomic instability

A cell responds to damage to its DNA in one of three ways: by tolerating the damage, by repairing the damage or by undergoing apoptosis. The latter two responses represent defenses against genomic instability and tumorigenesis resulting from unrepaired damage. There are multiple DNA repair pathways to cope with a variety of damage reflecting the importance of DNA repair in maintaining both cell viability and genomic stability. These include base excision repair, mismatch repair, double-strand break repair and nucleotide excision repair. Several signal transduction pathways are activated by DNA damage resulting in cell-cycle arrest. Cell-cycle arrest increases the time available for DNA repair before DNA replication and mutation fixation. Recently, there has been tremendous progress in our understanding of the molecular components repair processes and to examine recently observed interactions between DNA repair, signal transduction pathways and other cellular processes such as cell-cycle control, transcription, replication and recombination.

Contributions of molecular genetics to the clinical management of colorectal cancer

Molecular genetics is a tool that can be learned as a language to assist clinicians in the management of colorectal cancer patients. Following a brief review of the genetic controls of colorectal cancer, the author focuses on the models of the Registry for Familial Adenomatous Polyposis and the Registry for Hereditary Nonpolyposis Colon Cancer to demonstrate most vividly the impact molecular genetics is currently having on the practical management of colon cancer. Recent discoveries of K-ras oncogene mutations in stool cultures and the prognostic implications of mutations of the TP53 and DCC genes are discussed in the context of future applications to the management of patients.

The Lynch Syndrome: Melding Natural History and Molecular Genetics to Genetic Counseling and Cancer Control

Hereditary nonpolyposis colorectal cancer (HNPCC), also referred to as Lynch syndromes I and II, is an autosomal, dominantly inherited disorder that accounts for approximately 5% of all colorectal cancers. While colorectal cancer is the most frequently occurring malignancy in HNPCC, other types of cancer occur with increased statistical significance. A better understanding of its natural history, particularly early age of onset and the pattern of multiple primary cancer excess, is essential for the diagnosis and management of HNPCC.

Isolation and characterization of the human mismatch repair gene hMSH2 promoter region

Hereditary nonpolyposis colorectal cancer (HN-PCC) is one of man's commonest hereditary diseases. Several studies have identified four responsible genes that are involved in a process known as DNA mismatch repair; hMSH2 is the most important of these four genes. In addition to mutational analysis of these genes, investigations of transcriptional regulatory mechanisms are important. Therefore, our purpose has been to isolate the hMSH2 promoter region. Using direct sequencing of P1 recombinant DNA we have characterized 1100 bp of the hMSH2 promoter.

Loss of heterozygosity at chromosome regions 22q11-12 and 11p15.5 in renal rhabdoid tumors

Rhabdoid tumors of the kidney are highly malignant neoplasms that occur primarily within the first 3 years of life. Although they are regarded as distinct from Wilms' tumors, their pathogenesis remains unclear. Whereas most cytogenetic studies of these tumors have revealed normal karyotypes, a few reports have indicated abnormalities at chromosome regions 22q and 11p15.5. We analyzed 30 primary renal rhabdoid tumors for loss of heterozygosity (LOH) at both regions and found that 24 of 30 tumors (80%) had LOH at chromosome arm 22q and that 5 of 30 (17%) had LOH at chromosome band 11p15.5. All of the five tumors with LOH at chromosome arm 11p also had LOH at chromosome arm 22q. The data suggest that there is a gene in chromosome 22, probably a tumor suppressor, inactivation of which may be involved in the genesis of renal rhabdoid tumors. A second gene in chromosome segment 11p15.5, in the region of the putative WT2 gene, may also be involved, in at least a subset of rhabdoid tumors. In addition, five tumors were characterized by microsatellite instability at three or more of 21 loci examined, suggesting a possible role for a replicative error in tumorigenesis or progression in some cases of renal rhabdoid tumors. Genes Chromosom Cancer 15:10-17 (1996).

Multiple primary cancers with microsatellite instability: report of a case

We report here a patient who developed a variety of tumors both synchronously and metachronously over a 2-year period. The involved organs were the uterus, ureter, and small and large intestines. The patient underwent open surgery 3 times and polypectomies 6 times. Postoperative histopathologic analysis showed 2 adenomas and 8 carcinomas. Genetic analysis revealed microsatellite instabilities at the tested loci in all 10 tumors, indicating that replication errors played an essential role in the tumorigenesis. Early identification of microsatellite instability could be useful for predicting development of additional primary cancers.

Genetic counseling in a Navajo hereditary nonpolyposis colorectal cancer kindred

BACKGROUND

Cross-cultural genetic counseling was provided to an extended Navajo Indian family in which the MLH1 gene mutation for hereditary nonpolyposis colorectal cancer (HNPCC) had been identified. The family had been observed by the authors since 1983 and over the years had been provided with intensive education regarding the natural history of HNPCC as well as recommendations for cancer surveillance and management that was responsive to this natural history.

METHODS

Following identification of the MLH1 mutation, DNA from family members was evaluated by a reference laboratory (OncorMed, Gaithersburg, MD), where sequences were checked in both the forward and reverse directions against the published sequence for MLH1. The 4bp deletion beginning at the first nucleotide of codon 727 was easily visualized in the heterozygous condition in both affected and predispositional individuals. The family was reeducated as a group and then provided further education individually during genetic counseling sessions, at which time they were appraised of potential penalties, such as insurance and employer discrimination, and psychological sequelae that could result from knowledge of the MLH1 mutation. Strict confidentiality of this information was assured.

RESULTS

DNA testing was performed on 51 family members. Twenty-three individuals were counseled, seven of whom were positive for MHL1. Reactions ranged from full acceptance of the genetic implications to traditional Navajo reasoning such as the family had been cursed.

CONCLUSIONS

DNA-based genetic counseling requires comparison and empathy, coupled with intensive preeducation regarding potential penalties and advantages that might emanate from this knowledge. Special care must be given to patients' culture, beliefs, and traditions.

Nutrition and colorectal cancer

Epidemiologic evidence on the relation between nutrition and colorectal cancer is reviewed. Colon cancer varies approximately 20-fold internationally. Although there is clear evidence of genetic predisposition to colon cancer, much of this variation appears to be related to differences in dietary habits. At present, the data suggest that vegetables are associated with lower risk, and that fiber alone does not account for this association. Further, meat consumption is associated with increased risk but this, too, is not explained solely by its fat content. Several microconstituents of the diet may be associated with reduced risk--including folate and calcium--but phytochemicals of other sorts may be relevant. Mutagenic compounds, particularly heterocyclic amines, produced when protein is cooked, plausibly explain the meat association. The most consistent inverse association is with physical activity. Alcohol is associated, though inconsistently, with increased risk. Rectal cancer is less well studied but, at present, there are few data to suggest that the dietary risk factors are markedly different. Physical activity does not appear to be associated with a lower risk. Colorectal adenomatous polyps also appear to share the spectrum of risk factors seen with colon cancer, although, for adenomas, tobacco smoking is also a clear and consistent risk factor. There are a variety of links between the dietary epidemiology and physiology of colorectal neoplasia and the relevant pathologic and molecular changes. Other causal connections remain to be explicated.

Prophylaxis against colorectal cancer

Colorectal cancer is diagnosed in more than 3000 people every year in Denmark, with a population of 5 million, and 2000 die from this disease every year. The aetiology of the disease is complex, but an increasing number of cancers have been related to genetics and Denmark is contributing with a well-established register of familial adenomatous polyposis and a recently founded register for hereditary nonpolyposis colorectal cancer, both with major international relationships. The Danish tradition of epidemiology and clinical trials has also been demonstrated in population screening trials for colorectal cancer in average-risk persons as well as high-risk groups with precursors of the disease. The present review places Danish contributions within the prophylaxis of colorectal cancer during the last decade in an international context.

Induction of frameshift mutations in cultured mammalian cells within a transfected sequence containing a poly(dC-dA).poly(dT-dG) microsatellite

A cultured mouse cell line with an integrated copy of a plasmid that contains a short dinucleotide repeat sequence (microsatellite) has been used to determine the frequencies and types of mutation induced by two frameshift mutagens. The presence of the microsatellite, which consists of 17 repeats of a poly(dC-dA).poly(dT-dG) sequence, disrupts the reading frame of a gene coding for neomycin resistance. Revertants were selected in G418, and mutations were analyzed by PCR. ICR-170 was found to increase the reversion frequency by ten- to 15-fold at its LD50, although most of the frameshifts that it induced were single-base insertions outside the microsatellite sequence. NA-AAF brought about a more modest increase in mutation frequency, but nearly all of the revertants in the NA-AAF-treated cultures had insertions or deletions of multiples of two base pairs within the DNA segment that included the microsatellite. This system can be modified to include different short tandem repeat sequences as targets for testing of compounds that are suspected of having frameshift-inducing activities.

Validation of family history of breast cancer and identification of the BRCA1 and other syndromes using a population-based cancer registry

A major risk factor for breast cancer is family history of the disease in first-degree relatives. This study evaluates the validity of family history information on breast cancer in mothers and sisters of breast cancer probands from the cancer registry (CR) compared to personal interviews (PI) of 359 consecutive population-based cases of breast cancer. Breast cancer is seen in mothers of 14% of probands by CR compared to 12% by PI. Further, 13% of probands have a sister with breast cancer using CR compared to 12% by PI. Using PI as the standard, the sensitivity of the CR family history data in mothers is 92% and the specificity is 99%, while in sisters they are 88% and 99%, respectively. These estimates were calculated on cases where family history information is available in the CR. Sensitivity and specificity are recalculated, recording an "error" whenever family history information is not available, and they are 75% and 68%, respectively, for mothers and 72% and 70%, respectively, for sisters. Estimates of proband-mother and proband-sisters familial breast cancer from CR and PI are sufficiently similar to warrant the use of CR family history data in studies of genetic epidemiology. The family phenotype consistent with the BRCA1 syndrome was found in four (1.1%) probands, all below age 50 years, while for BRCA2 there were five (1.4%) probands, three below age 50 years and two 50 years or older. Site-specific familial breast cancer was found in 23 (6.4%) probands. Population-based multiple-case breast cancer families can rapidly be identified through CR. These families can make substantial contributions to the study of genetic and environmental etiology of the disease as well as benefit from preventive and therapeutic efforts. As new knowledge and tools in molecular genetics become available, there is an urgent need for large population-based registries of families at high risk for cancer.

Life-time risk of different cancers in hereditary non-polyposis colorectal cancer (HNPCC) syndrome

Identification of hereditary non-polyposis colorectal cancer (HNPCC) indicates theoretical life-time risks of 50% for the descendants of an affected family member and of 100% for the true gene carriers. However, besides colorectal cancer (CRC), many other cancer types and sites are also involved, which gives reason to evaluate the magnitude of risk for various other cancer types. A detailed pedigree analysis of 40 families with HNPCC identified 414 patients affected with cancer. A Kaplan-Meier life-table analysis for the cumulative risk of various cancers was performed on the basis of the 293 putative gene carriers who had adequate clinical and histological documentation of their tumors. Cumulative risks were highest for colorectal (78%) and endometrial cancers (43%, women only), followed by gastric, biliary tract, urinary tract and ovarian cancers (19-9%). For the other probably HNPCC-related cancer types, such as small bowel carcinoma and brain tumors, the life-time risk was only 1%. The risk of any metachronous cancer reached 90% after treatment of CRC and 75% after endometrial cancer; the second tumor was most often a new CRC or endometrial cancer. CRC remains the most important cancer type in the HNPCC syndrome but does not develop in all gene carriers. This makes the decision of possible prophylactic colectomy for test-detected gene carriers difficult. Of the many other cancer types involved, at least endometrial cancer is common enough to necessitate a specific surveillance program.

Microsatellite instability and alterations in the hMSH2 gene in human ovarian cancer

The role of the replication error-positive (RER+) phenotype in the development of specific subtypes of sporadic ovarian carcinomas was examined by screening for the presence of microsatellite instability (MI) in 47 tumors. The overall frequency of ovarian MI was 17% only. However, MI occurred in 50% of the ovarian endometrioid-type tumors, which was significantly more often than in all the other histological subtypes combined (8%). Five of the 8 RER+ tumors exhibited most marked type I instability, possibly representing a different mechanism than for the remaining type 2 tumors. The cDNA of the mutation suppression gene hMSH2, the gene most often associated with MI, was screened for alterations in 8 MI-positive and 5 MI-negative ovarian tumors. Only 3 changes were found. Complete loss of hMSH2 mRNA expression was detected in I tumor, while another expressed only an abnormal transcript containing a deletion of exon 3. One additional RER+ serous adenocarcinoma contained a rare polymorphism with a non-conservative amino acid change. One of 8 RER+ tumors showed loss of heterozygosity at the hMSH2 loci. Genetic instability, caused in part by alterations in the hMSH2 gene, may play an important role in the sporadic endometrioid subtype of ovarian tumors. Other mutator-phenotype genes may be responsible for the remaining cases of RER+ ovarian tumors.

Frequency and prognostic evaluation of 3p21-22 allelic losses in non-small-cell lung cancer

Previous loss of heterozygosity (LOH) studies of chromosome 3p loci have displayed a 60% deletion frequency in non-small-cell lung cancers (NSCLC), as opposed to small-cell lung cancers, in which the 3p deletion is consistently found. However, the high stromal-cell admixture found in NSCLC and the use of the Southern-blot method lead to under-evaluation of this frequency. In this study, we used a very precise microdissection technique followed by PCR amplification of 6 3p21-22 polymorphic genomic sequences to analyze LOH in 86 NSCLC and in normal adjacent tissue. We found the sensitivity of the microdissection-PCR-based LOH technique higher than the sensitivity of the Southern-blot technique: 87% of the squamous-cell carcinomas and 84% of the large-cell undifferentiated carcinomas showed a clear LOH for a 3p21-22 locus. All doubly informative cases but 4 showed concordant deletion at all 3p21-22 loci. The analysis of 3p microsatellite sequences displayed only 2 cases of genomic instability, one of them also displaying features of tumoral heterogeneity as regards the instability genotype. Four carcinomas in situ adjacent to these NSCLC showed the same allelic profile as the invasive tumors. The only prognostic factors in this study were the disease stage and histology. The 3p21-22 deletion was not related to the stage of the disease and did not appear to be a significant prognostic factor of survival. 3p21 loss appears, so far, to be the most frequent and the earliest genetic alteration described in NSCLC, but does not seem to carry significant prognostic information in invasive tumors.

Adaptive mutation sequences reproduced by mismatch repair deficiency

Adaptive reversions of a lac frameshift mutation in Escherichia coli are -1 deletions in small mononucleotide repeats, whereas growth-dependent reversions are heterogeneous. The adaptive mutations resemble instability of simple repeats, which, in hereditary colon cancer, in yeast, and in E. coli occurs in the absence of mismatch repair. The postulate that mismatch repair is disabled transiently during adaptive mutation in E. coli is supported here by the demonstration that the growth-dependent mutation spectrum can be made indistinguishable from adaptive mutations by disallowing mismatch repair during growth. Physiologically induced mismatch repair deficiency could be an important mutagenic mechanism in cancers and in evolution.

Microsatellite instability in primary and metastatic lung carcinomas

Fifty-seven primary lung carcinomas and 35 metastatic lung carcinomas were analyzed for microsatellite instability at II different chromosomal loci. Although no instability was detected in 37 small cell lung carcinomas (SCLC), it was frequently detected in non-small cell lung carcinomas (NSCLC) (16/55, 29%). In NSCLC, the incidence of replication errors (RERs) in metastatic tumors (12/22, 55%) was significantly higher than that in primary tumors (4/33, 12%) (P = 0.0021). Among 10 pairs of primary tumors and corresponding metastases, there were 4 cases which manifested the identical RER phenotypes in both primary and metastatic tumors. In two cases, RER phenotypes were detected in metastatic but not in primary tumors. Never was an RER phenotype found only in a primary tumor but not in the metastases. RERs were detected more frequently in stage III or IV tumors (3/8, 38%) than stage I or II tumors (1/25, 4%) (P = 0.0359). Tumor cells with allelic losses on chromosome arm 3p or 18q tended to have RER phenotypes (P = 0.0432 and P = 0.0187, respectively). The data suggest that microsatellite instability is common in NSCLC but not in SCLC, and that genomic instability appears late in tumor progression and plays an important role in the acquisition of more malignant phenotypes in NSCLC.

Molecular genetic analysis of the von Hippel-Lindau disease (VHL) tumour suppressor gene in gonadal tumours

Chromosome 3p allele loss is frequent in ovarian and testicular tumours. The von Hippel-Lindau (VHL) disease tumour suppressor gene maps to chromosome 3p25. Gonadal tumours may occur in patients with VHL disease, so somatic VHL gene mutations might be involved in the pathogenesis of sporadic gonadal tumours. To investigate this hypothesis, we screened 60 gonadal tumours (36 ovarian and 24 testicular) for VHL gene mutations and chromosome 3p allele loss. Although 38% (10/26) of informative ovarian and 54% (7/13) of testicular tumours demonstrated 3p allele loss, no somatic VHL gene mutations were detected in the 60 gonadal tumours analysed. This suggested that chromosome 3p tumour suppressor gene(s) other than VHL are involved in gonadal tumorigenesis.

Is mucinous carcinoma of the colorectum a distinct genetic entity?

Mucinous carcinomas are defined on the basis of the amount of the mucus component in the tumour mass. Apart from this quantitative criterion, a number of clinicopathological parameters (such as localisation, prevalence in different countries and age groups, association with HNPCC and inflammatory processes) and genetic alterations (e.g. frequency of mutation in Ki-ras and p53 genes, level of MUC2 expression) differentiate these tumours from the non-mucinous ones. Since a different set of genetic lesions implies different inducing agents, these observations suggest that there may be a 'mucinous pathway of carcinogenesis'. Further identification of genetic changes characteristic of the mucinous phenotype will help to understand the aetiology of these tumours and possibly establish markers for detection of the high-risk group.

Molecular genetic analysis of exons 1 to 6 of the APC gene in non-polyposis familial colorectal cancer

Familial adenomatous polyposis coli is caused by constitutional mutations in the APC gene. The hallmark of familial adenomatous polyposis coli is the presence of numerous (> 100) colorectal polyps, but mutations in the 5' end of the APC gene have been associated with familial colorectal cancer without florid polyposis. Although familial adenomatous polyposis coli accounts for only a minority of familial colorectal cancer cases, we hypothesised that APC mutations which were not associated with florid polyposis might make a significant contribution to nonpolyposis familial colorectal cancer. To investigate this possibility, we analysed 40 unrelated patients with familial colorectal cancer without classical familial adenomatous polyposis coli for mutations in exons 1 to 6 (codons 1 to 243) of the APC gene. No mutations were detected, but a C-->T polymorphism at nucleotide 333 (Arg-->Trp at codon 99) was identified. No 5' APC mutations were detected in two patients with desmoid tumours and a family history of colorectal cancer and polyps. We conclude that mutations in exons 1 to 6 of the APC gene are infrequent in patients with familial colorectal cancer who do not have many colorectal polyps.

Ovarian cancer screening. The role of ultrasound in early detection

BACKGROUND

Because of the urgent need for effective techniques with which to detect organ-confined curable ovarian cancers, efforts have been focused on early detection. Current ovarian cancer screening trials have been hampered by deficiencies in our knowledge of the molecular and biologic events leading to ovarian tumorigenesis. The lack of early ovarian cancer symptoms and the intraperitoneal location of the ovaries contribute to the dilemma of early diagnosis.

METHODS

Real-time ultrasound screening is aimed at detecting the earliest possible architectural alterations in the ovary indicative of neoplastic growth. Color Doppler imaging detects early alterations in ovarian blood flow that accompany tumorigenesis.

RESULTS

To be effective, these modalities must diagnose asymptomatic curable Stage I ovarian cancers and improve ovarian disease-specific survival. Because of the relatively low prevalence of ovarian cancer in the general population, investigators have targeted women at increased risk of ovarian cancer either based on their increasing age or their family histories of cancer. Some of these studies have been underway since the late 1980s and have already demonstrated the potential usefulness and limitations of current sonographic techniques used for screening.

CONCLUSIONS

Ultrasound screening for ovarian cancer has not demonstrated adequate sensitivity and specificity, thus it is not used widely outside of the clinical trial setting. Current clinical guidelines for ovarian cancer screening as well as exploratory methods for use in early ovarian cancer detection are presented.

Microsatellite instability in keratoacanthoma

BACKGROUND

Tumors from patients with hereditary nonpolyposis colorectal cancer (HNPCC) and from a subset of patients with the related Muir-Torre syndrome (MTS) exhibit a novel type of genomic instability known as microsatellite instability (MIN). In general, this form of genomic instability results from mutations that inactivate DNA mismatch repair genes. The detection of MIN in a keratoacanthoma (KA) from a patient with MTS suggested that defective mismatch repair may play a role in the pathogenesis of these neoplasms.

METHODS

Randomly selected paraffin embedded KA from 53 patients and paraffin embedded tumors from an additional 12 patients diagnosed with KA and colorectal carcinoma were examined for MIN at six loci. In addition, several KA were examined for mutations within the hMSH2 gene.

RESULTS

Six of the 53 randomly selected KAs had MIN at two or more loci. One of these six patients had HNPCC, whereas another had MTS. Two patients with KAs lacking MIN had colon tumors that exhibited widespread MIN, and one of these patients had MTS. Three of the 12 additional patients diagnosed with a KA and a colorectal carcinoma had at least one tumor that had MIN at two or more loci, and one of these patients had HNPCC: A 2-base pair somatic deletion in exon 3 of the hMSH2 gene was identified in one of the MIN+ KAs.

CONCLUSIONS

Defective mismatch repair appears to play a role in the process of tumorigenesis in some KAs. Microsatellite instability in a KA or the cooccurrence of a colorectal carcinoma and a KA in a patient suggests that the patient may have either HNPCC or its phenotypic variant MTS.

Detection of new mutations in six out of 10 Swiss HNPCC families by genomic sequencing of the hMSH2 and hMLH1 genes

The cancer predisposition in most HNPCC families is believed to be associated with mutations in the human mismatch repair gene homologues hMSH2 and hMLH1. We searched for mutations in our collection of 10 Swiss HNPCC families by sequencing the exons and exon/intron boundaries of the hMSH2 and hMLH1 genes. In four families we found different mutations which are expected to lead to protein truncations of either the hMSH2 or the hMLH1 proteins owing to premature in frame stop codons or splice defects. In two more families we detected mutations leading to an amino acid deletion and an amino acid substitution in an evolutionary conserved residues respectively. None of these mutations has been reported in other families, which is consistent with the notion that HNPCC associated hMSH2 and hMLH1 mutations are heterogeneous and there is no striking founder effect in the Swiss population. Whenever this could be investigated, the presence of the mutations was confirmed in other family members who showed manifestations of HNPCC. Interestingly, an obligate carrier in one of the families developed a brain tumour at the age of 29, histologically verified as a glioblastoma multiforme, which was recently linked to HNPCC in the context of Turcot's syndrome.