Identification of mismatch repair genes and their role in the development of cancer

Saccharomyces cerevisiae pms2 mutations are alleles of MLH1, and pms2-2 corresponds to a hereditary nonpolyposis colorectal carcinoma-causing missense mutation

A number of mutant Saccharomyces cerevisiae strains having phenotypes consistent with defects in DNA mismatch repair have been described, but not all have been extensively characterized. In this study we demonstrate that the pms2-1 and pms2-2 alleles arise from missense mutations in the MLH1 gene which inactivate MLH1. One of these alleles, pms2-2, causes the same amino acid substitution in a highly conserved region of the known MutL homologs as that caused by a proposed missense mutation observed in a Swedish hereditary nonpolyposis colorectal carcinoma kindred. This observation supports the functional significance of missense mutations found in hereditary nonpolyposis colorectal carcinoma kindreds and indicates that in some cases S. cerevisiae can serve as a useful model system for the analysis of such mutations.

DNA-replication fidelity, mismatch repair and genome instability in cancer cells

It has been suggested that an early event in the multistep progression of a normal cell to a tumor cell could be a defect that leads to an elevated mutation rate, thus providing a pool of mutants upon which selection could act to yield a tumor. Such a mutator phenotype could result from a defect in any of several DNA transactions, including those that determine the DNA replication error rate or the ability to correct replication errors. Recent evidence for the latter is the mutator phenotype observed in tumor cells of patients having a hereditary form of colon cancer. These patients have a germline mutation in genes required for post-replication DNA mismatch repair. A second mutation arises somatically, yielding a greatly elevated mutation rate due to an inability to correct DNA replication errors. This connection between cancer, DNA replication errors and defective mismatch repair is the subject of this review, wherein we consider the key steps and principles for high fidelity replication and how their perturbation results in genome instability.

Cancer genetics clinics: target population and consultees' expectations

The aim of this study was to determine in healthy consultees attending cancer genetics clinics their risk status, their pathways leading to the clinics, their expectations and perception of cancer risk. In 1994, the consultees at six French centres completed a questionnaire before their first oncogenetic consultation. The oncogeneticists subsequently filled in a standardised form giving their risk assessment. Among the 206 healthy consultees, 91.3% were women, 92.2% had at least one cancer-affected first-degree relative and 73% had a "cancer family risk" as assessed by the oncogeneticist. Sixty-nine per cent of the consultees were referred to the clinics by a physician, 10.4% by their family and 18.8% on their own initiative: 83.5% of the sample perceived their family risk of cancer as being high and this belief was confirmed in 74.3% of the cases studied by the oncogeneticist. The families of self-referred consultees were less often at risk than those of consultees referred by a physician or by their family (P = 0.012). The majority (78%) expected to be informed about cancer prevention and screening, and this expectation depended on the consultee's level of education (P = 0.001). This study shows that medical pathways are more effective than the media as a means of reaching the members of the general population who are genuinely at risk, and shows that fuller information about prevention needs to be provided at cancer genetic consultations.

Mutations, malformations and mortality

Although DNA replication is a very accurate process, a small number of new mutations are generated at every cell division. The generation of a new mutation during the formation of an ovum or sperm cell can cause an early miscarriage or birth defect. The generation of new mutations during embryogenesis can cause a variety of localized birth defects. The molecular delineation of these errors in somatic and gonadal cells has clarified the basis of some birth defects, and has both refined and complicated genetic counselling for a number of paediatric conditions. The processes responsible for these new mutations are present in all cells. For this reason new mutations accumulate in all cells throughout life and contribute to the ageing process. Thus the molecular events that cause many miscarriages and birth defects are the same as those that ultimately lead to death.

Mismatch repair: mechanisms and relationship to cancer susceptibility

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

DNA-mismatch repair. The intricacies of eukaryotic spell-checking

Recent work suggests that the eukaryotic system responsible for repairing DNA mismatches, and so correcting replication errors, is more complex than was thought; its multiple components have many cellular functions.

MSH2 deficient mice are viable and susceptible to lymphoid tumours

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