Saccharomyces cerevisiae RRM3, a 5' to 3' DNA helicase, physically interacts with proliferating cell nuclear antigen

Proliferating cell nuclear antigen (PCNA) plays an essential role in eukaryotic DNA replication, and numerous DNA replication proteins have been found to interact with PCNA through a conserved eight-amino acid motif called the PIP-box. We have searched the genome of the yeast Saccharomyces cerevisiae for open reading frames that encode proteins with putative PIP-boxes and initiated testing of 135 novel candidates for their ability to interact with PCNA-conjugated agarose beads. The first new PCNA-binding protein identified in this manner is the 5' to 3' DNA helicase RRM3. Yeast two-hybrid tests show that N-terminal deletions of RRM3, which remove the PIP-box but leave the helicase motifs intact, abolish the interaction with PCNA. In addition, mutating the two phenylalanine residues in the PIP-box to alanine or aspartic acid reduces binding to PCNA, confirming that the PIP-box in RRM3 is responsible for interaction with PCNA. The results presented here suggest that the RRM3 helicase functions at the replication fork.

Isolation and characterization of new proliferating cell nuclear antigen (POL30) mutator mutants that are defective in DNA mismatch repair

A number of studies have suggested a role for proliferating cell nuclear antigen (PCNA) in DNA mismatch repair (MMR). However, the majority of mutations in the POL30 gene encoding PCNA that cause MMR defects also cause replication and other repair defects that contribute to the increased mutation rate caused by these mutations. Here, 20 new pol30 mutants were identified and screened for MMR and other defects, resulting in the identification of two mutations, pol30-201 and pol30-204, that appear to cause MMR defects but little if any other defects. The pol30-204 mutation altered an amino acid (C81R) in the monomer-monomer interface region and resulted in a partial general MMR defect and a defect in MSH2-MSH6 binding in vitro. The pol30-201 mutation altered an amino acid (C22Y) located on the surface of the PCNA trimer that slides over the DNA but did not cause a defect in MSH2-MSH6 binding in vitro. The pol30-201 mutation caused an intermediate mutator phenotype. However, the pol30-201 mutation caused almost a complete defect in the repair of AC and GT mispairs and only a small defect in the repair of a "+T" insertion, an effect similar to that caused by an msh6Delta mutation, indicating that pol30-201 primarily effects MSH6-dependent MMR. The chromosomal double mutant msh3-FF>AA msh6-FF>AA eliminating the conserved FF residues of the PCNA interacting motif of these proteins caused a small (<10%) defect in MMR but showed synergistic interactions with mutations in POL30, indicating that the FF>AA substitution may not eliminate PCNA interactions in vivo. These results indicate that the interaction between PCNA and MMR proteins is more complex than was previously appreciated.

The MER3 DNA helicase catalyzes the unwinding of holliday junctions

The MER3 protein of Saccharomyces cerevisiae is required for crossover in meiosis and has been suggested to act at the initiation of homologous pairing and the resolution of Holliday junctions. The purified MER3 protein is a DNA helicase that translocates along single-stranded DNA in the 3' to 5' direction displacing annealed DNA fragments. Here, MER3 was found to be able to unwind various double-stranded DNA (dsDNA) substrates, including a 30-bp dsDNA with a 20-nucleotide 3'-overhang, a 30-bp dsDNA with a 20-nucleotide 5'-overhang, a 50-bp dsDNA with blunt ends, and a Holliday junction with 25-bp arms, each of which had a blunt end. Efficient unwinding of the 3'-overhang substrate appeared to initiate by the binding of MER3 to the 3' single-stranded tail in a reaction that required six or more unpaired bases. Unwinding of the blunt end and 5'-overhang substrates appeared to initiate at the blunt ends of these substrates. Unwinding of the Holliday junction was more efficient than the unwinding of the blunt and 5'-overhang substrates and was influenced by Mg(2+) concentrations that cause changes in the structure of the junction. Possible roles for Holliday junction unwinding in meiotic crossover are discussed.

Maintenance of genome stability in Saccharomyces cerevisiae

Most human cancer cells show signs of genome instability, ranging from elevated mutation rates to gross chromosomal rearrangements and alterations in chromosome number. Little is known about the molecular mechanisms that generate this instability or how it is suppressed in normal cells. Recent studies of the yeast Saccharomyces cerevisiae have begun to uncover the extensive and redundant pathways that keep the rate of genome rearrangements at very low levels. These studies, which we review here, have implicated more than 50 genes in the suppression of genome instability, including genes that function in S-phase checkpoints, recombination pathways, and telomere maintenance. Human homologs of several of these genes have well-established roles as tumor suppressors, consistent with the hypothesis that the mechanisms preserving genome stability in yeast are the same mechanisms that go awry in cancer.

Haploinsufficiency of Flap endonuclease (Fen1) leads to rapid tumor progression

Flap endonuclease (Fen1) is required for DNA replication and repair, and defects in the gene encoding Fen1 cause increased accumulation of mutations and genome rearrangements. Because mutations in some genes involved in these processes cause cancer predisposition, we investigated the possibility that Fen1 may function in tumorigenesis of the gastrointestinal tract. Using gene knockout approaches, we introduced a null mutation into murine Fen1. Mice homozygous for the Fen1 mutation were not obtained, suggesting absence of Fen1 expression leads to embryonic lethality. Most Fen1 heterozygous animals appear normal. However, when combined with a mutation in the adenomatous polyposis coli (Apc) gene, double heterozygous animals have increased numbers of adenocarcinomas and decreased survival. The tumors from these mice show microsatellite instability. Because one copy of the Fen1 gene remained intact in tumors, Fen1 haploinsufficiency appears to lead to rapid progression of cancer.

A hereditary nonpolyposis colorectal carcinoma case associated with hypermethylation of the MLH1 gene in normal tissue and loss of heterozygosity of the unmethylated allele in the resulting microsatellite instability-high tumor

Fourteen suspected hereditary nonpolyposis colorectal carcinoma cases with microsatellite unstable(microsatellite instability-high; MSI-H) tumors but no germ-line MSH2, MSH6, or MLH1 mutations were examined for hypermethylation of CpG sites in the critical promoter region of MLH1. The methylation patterns were determined using methylation-specific PCR and by sequence analysis of sodium bisulfite-treated genomic DNA. In one case, DNA hypermethylation of one allele was detected in DNA isolated from blood. In the MSI-H tumor from this case, the unmethylated MLH1 allele was eliminated by loss of heterozygosity, and the methylated allele was retained. This biallelic inactivation resulted in loss of expression of MLH1 in the tumor as confirmed by immunohistochemistry. These results suggest a novel mode of germ-line inactivation of a cancer susceptibility gene.

Dominant Saccharomyces cerevisiae msh6 mutations cause increased mispair binding and decreased dissociation from mispairs by Msh2-Msh6 in the presence of ATP

A previous study described four dominant msh6 mutations that interfere with both the Msh2-Msh6 and Msh2-Msh3 mismatch recognition complexes (Das Gupta, R., and Kolodner, R. D. (2000) Nat. Genet. 24, 53-56). Modeling predicted that two of the amino acid substitutions (G1067D and G1142D) interfere with protein-protein interactions at the ATP-binding site-associated dimer interface, one (S1036P) similarly interferes with protein-protein interactions and affects the Msh2 ATP-binding site, and one (H1096A) affects the Msh6 ATP-binding site. The ATPase activity of the Msh2-Msh6-G1067D and Msh2-Msh6-G1142D complexes was inhibited by GT, +A, and +AT mispairs, and these complexes showed increased binding to GT and +A mispairs in the presence of ATP. The ATPase activity of the Msh2-Msh6-S1036P complex was inhibited by a GT mispair, and it bound the GT mispair in the presence of ATP, whereas its interaction with insertion mispairs was unchanged compared with the wild-type complex. The ATPase activity of the Msh2-Msh6-H1096A complex was generally attenuated, and its mispair-binding behavior was unaffected. These results are in contrast to those obtained with the wild-type Msh2-Msh6 complex, which showed mispair-stimulated ATPase activity and ATP inhibition of mispair binding. These results indicate that the dominant msh6 mutations cause more stable binding to mispairs and suggest that there may be differences in how base base and insertion mispairs are recognized.

Evaluation of microsatellite instability and immunohistochemistry for the prediction of germ-line MSH2 and MLH1 mutations in hereditary nonpolyposis colon cancer families

Forty-eight hereditary nonpolyposis colorectal carcinoma (HNPCC) families for which a tumor sample was available were evaluated for the presence of germ-line mutations in MSH2 and MLH1, tumor microsatellite instability (MSI), and where possible, expression of MSH2 and MLH1 in tumors by immunohistochemistry. Fourteen of 48 of the families had a germ-line mutation in either MSH2 or MLH1 that could be detected by genomic DNA sequencing, and 28 of 48 of the families had MSI-H tumors. Four additional families showed loss of expression of MSH2, and one additional family showed loss of expression of MLH1 but did not have germ-line mutations in MSH2 or MLH1 that could be detected by DNA sequencing. MSI-H, as defined using the National Cancer Institute recommended five-microsatellite panel, had a 100% sensitivity for identifying samples having MSH2 or MLH1 mutations or loss of expression. In contrast, loss of MSH2 and MLH1 expression did not identify all samples having germ-line mutations in MSH2 or MLH1, because in five cases, a mutant protein product was expressed that could be detected by IHC. A combination of the Bethesda criteria for HNPCC and an MSI-H phenotype defined the smallest number of cases having all of the germ-line MSH2 and MLH1 mutations that could be detected by DNA sequencing.

Saccharomyces cerevisiae Mer3 is a DNA helicase involved in meiotic crossing over

Crossing over is regulated to occur at least once per each pair of homologous chromosomes during meiotic prophase to ensure proper segregation of chromosomes at the first meiotic division. In a mer3 deletion mutant of Saccharomyces cerevisiae, crossing over is decreased, and the distribution of the crossovers that occur is random. The predicted Mer3 protein contains seven motifs characteristic of the DExH box type of DNA/RNA helicases. The mer3G166D and the mer3K167A mutation, amino acid substitutions of conserved residues in a putative nucleotide-binding domain of the helicase motifs caused a defect in the transition of meiosis-specific double-strand breaks to later intermediates, decreased crossing over, and reduced crossover interference. The purified Mer3 protein was found to have DNA helicase activity. This helicase activity was reduced by the mer3GD mutation to <1% of the wild-type activity, even though binding of the mutant protein to single- and double-strand DNA was unaffected. The mer3KA mutation eliminated the ATPase activity of the wild-type protein. These results demonstrate that Mer3 is a DNA helicase that functions in meiotic crossing over.

Suppression of genome instability by redundant S-phase checkpoint pathways in Saccharomyces cerevisiae

Cancer cells show increased genome rearrangements, although it is unclear what defects cause these rearrangements. Previous studies have implicated the Saccharomyces cerevisiae replication checkpoint in the suppression of spontaneous genome rearrangements. In the present study, low doses of methyl methane sulfonate that activate the intra-S checkpoint but not the G1 or G2 DNA damage checkpoints were found to cause increased accumulation of genome rearrangements in both wild-type strains and to an even greater extent in strains containing mutations causing defects in the intra-S checkpoint. The rearrangements were primarily translocations or events resulting in deletion of a portion of a chromosome arm along with the addition of a new telomere. Combinations of mutations causing individual defects in the RAD24 or SGS1 branches of the intra-S checkpoint or the replication checkpoint showed synergistic interactions with regard to the spontaneous genome instability rate. PDS1 and the RAD50-MRE11-XRS2 complex were found to be important members of all the S-phase checkpoints in suppressing genome instability, whereas RAD53 only seemed to play a role in the intra-S checkpoints. Combinations of mutations that seem to result in inactivation of the S-phase checkpoints and critical effectors resulted in as much as 12,000-14,000-fold increases in the genome instability rate. These data support the view that spontaneous genome rearrangements result from DNA replication errors and indicate that there is a high degree of redundancy among the checkpoints that act in S phase to suppress such genome instability.

Mutation of a new gene encoding a putative pyrin-like protein causes familial cold autoinflammatory syndrome and Muckle-Wells syndrome

Familial cold autoinflammatory syndrome (FCAS, MIM 120100), commonly known as familial cold urticaria (FCU), is an autosomal-dominant systemic inflammatory disease characterized by intermittent episodes of rash, arthralgia, fever and conjunctivitis after generalized exposure to cold. FCAS was previously mapped to a 10-cM region on chromosome 1q44 (refs. 5,6). Muckle-Wells syndrome (MWS; MIM 191900), which also maps to chromosome 1q44, is an autosomal-dominant periodic fever syndrome with a similar phenotype except that symptoms are not precipitated by cold exposure and that sensorineural hearing loss is frequently also present. To identify the genes for FCAS and MWS, we screened exons in the 1q44 region for mutations by direct sequencing of genomic DNA from affected individuals and controls. This resulted in the identification of four distinct mutations in a gene that segregated with the disorder in three families with FCAS and one family with MWS. This gene, called CIAS1, is expressed in peripheral blood leukocytes and encodes a protein with a pyrin domain, a nucleotide-binding site (NBS, NACHT subfamily) domain and a leucine-rich repeat (LRR) motif region, suggesting a role in the regulation of inflammation and apoptosis.

Mutation of a new gene encoding a putative pyrin-like protein causes familial cold autoinflammatory syndrome and Muckle-Wells syndrome

Familial cold autoinflammatory syndrome (FCAS, MIM 120100), commonly known as familial cold urticaria (FCU), is an autosomal-dominant systemic inflammatory disease characterized by intermittent episodes of rash, arthralgia, fever and conjunctivitis after generalized exposure to cold. FCAS was previously mapped to a 10-cM region on chromosome 1q44 (refs. 5,6). Muckle-Wells syndrome (MWS; MIM 191900), which also maps to chromosome 1q44, is an autosomal-dominant periodic fever syndrome with a similar phenotype except that symptoms are not precipitated by cold exposure and that sensorineural hearing loss is frequently also present. To identify the genes for FCAS and MWS, we screened exons in the 1q44 region for mutations by direct sequencing of genomic DNA from affected individuals and controls. This resulted in the identification of four distinct mutations in a gene that segregated with the disorder in three families with FCAS and one family with MWS. This gene, called CIAS1, is expressed in peripheral blood leukocytes and encodes a protein with a pyrin domain, a nucleotide-binding site (NBS, NACHT subfamily) domain and a leucine-rich repeat (LRR) motif region, suggesting a role in the regulation of inflammation and apoptosis.

The distinct spectra of tumor-associated Apc mutations in mismatch repair-deficient Apc1638N mice define the roles of MSH3 and MSH6 in DNA repair and intestinal tumorigenesis

In mammalian cells, mismatch recognition has been attributed to two partially redundant heterodimeric protein complexes of MutS homologues, MSH2-MSH3 and MSH2-MSH6. We have conducted a comparative analysis of Msh3 and Msh6 deficiency in mouse intestinal tumorigenesis by generating Apc1638N mice deficient in Msh3, Msh6 or both. We have found that Apc1638N mice defective in Msh6 show reduced survival and a 6-7-fold increase in intestinal tumor multiplicity. In contrast, Msh3-deficient Apc1638N mice showed no difference in survival and intestinal tumor multiplicity as compared with Apc1638N mice. However, when Msh3 deficiency is combined with Msh6 deficiency (Msh3(-/-)Msh6(-/-)Apc1638N), the survival rate of the mice was further reduced compared to Msh6(-/-)Apc(1638N) mice because of a high multiplicity of intestinal tumors at a younger age. Almost 90% of the intestinal tumors from both Msh6(-/-)Apc1638N and Msh3(-/-)Msh6(-/-)Apc1638N mice contained truncation mutations in the wild-type Apc allele. Apc mutations in Msh6(-/-)Apc1638N mice consisted predominantly of base substitutions (93%) creating stop codons, consistent with a major role for Msh6 in the repair of base-base mismatches. However, in Msh3(-/-)Msh6(-/-)Apc1638N tumors, we observed a mixture of base substitutions (46%) and frameshifts (54%), indicating that in Msh6(-/-)Apc1638N mice frameshift mutations in the Apc gene were suppressed by Msh3. Interestingly, all except one of the Apc mutations detected in mismatch repair-deficient intestinal tumors were located upstream of the third 20-amino acid beta-catenin binding repeat and before all of the Ser-Ala-Met-Pro repeats, suggesting that there is selection for loss of multiple domains involved in beta-catenin regulation. Our analysis therefore has revealed distinct mutational spectra and clarified the roles of Msh3 and Msh6 in DNA repair and intestinal tumorigenesis.

exo1-Dependent mutator mutations: model system for studying functional interactions in mismatch repair

EXO1 interacts with MSH2 and MLH1 and has been proposed to be a redundant exonuclease that functions in mismatch repair (MMR). To better understand the role of EXO1 in mismatch repair, a genetic screen was performed to identify mutations that increase the mutation rates caused by weak mutator mutations such as exo1Delta and pms1-A130V mutations. In a screen starting with an exo1 mutation, exo1-dependent mutator mutations were obtained in MLH1, PMS1, MSH2, MSH3, POL30 (PCNA), POL32, and RNR1, whereas starting with the weak pms1 allele pms1-A130V, pms1-dependent mutator mutations were identified in MLH1, MSH2, MSH3, MSH6, and EXO1. These mutations only cause weak MMR defects as single mutants but cause strong MMR defects when combined with each other. Most of the mutations obtained caused amino acid substitutions in MLH1 or PMS1, and these clustered in either the ATP-binding region or the MLH1-PMS1 interaction regions of these proteins. The mutations showed two other types of interactions: specific pairs of mutations showed unlinked noncomplementation in diploid strains, and the defect caused by pairs of mutations could be suppressed by high-copy-number expression of a third gene, an effect that showed allele and overexpressed gene specificity. These results support a model in which EXO1 plays a structural role in MMR and stabilizes multiprotein complexes containing a number of MMR proteins. A similar role is proposed for PCNA based on the data presented.

Multiple pathways cooperate in the suppression of genome instability in Saccharomyces cerevisiae

Gross chromosome rearrangements (GCRs), such as translocations, deletion of a chromosome arm, interstitial deletions and inversions, are often observed in cancer cells. Spontaneous GCRs are rare in Saccharomyces cerevisiae; however, the existence of mutator mutants with increased genome instability suggests that GCRs are actively suppressed. Here we show by genetic analysis that these genome rearrangements probably result from DNA replication errors and are suppressed by at least three interacting pathways or groups of proteins: S-phase checkpoint functions, recombination proteins and proteins that prevent de novo addition of telomeres at double-strand breaks (DSBs). Mutations that inactivate these pathways cause high rates of GCRs and show synergistic interactions, indicating that the pathways that suppress GCRs all compete for the same DNA substrates.

The MER3 helicase involved in meiotic crossing over is stimulated by single-stranded DNA-binding proteins and unwinds DNA in the 3' to 5' direction

The meiosis-specific MER3 protein of Saccharomyces cerevisiae is required for crossing over, which ensures faithful segregation of homologous chromosomes at the first meiotic division. The predicted sequence of the MER3 protein contains the seven motifs characteristic of the DExH-box type of DNA/RNA helicases. The purified MER3 protein is a DNA helicase, which can displace a 50-nucleotide fragment annealed to a single-stranded circular DNA. MER3 was found to have ATPase activity, which was stimulated either by single- or double-stranded DNA. The turnover rate, k(cat), of ATP hydrolysis was approximately 500/min in the presence of either DNA. MER3 was able to efficiently displace relatively long 631-nucleotide fragments from single-stranded circular DNA only in the presence of the S. cerevisiae single-stranded DNA-binding protein, RPA (replication protein A). It appears that RPA inhibits re-annealing of the single-stranded products of the MER3 helicase. The MER3 helicase was found to unwind DNA in the 3' to 5' direction relative to single-stranded regions in the DNA substrates. Possible roles for the MER3 helicase in meiotic crossing over are discussed.

Suppression of spontaneous chromosomal rearrangements by S phase checkpoint functions in Saccharomyces cerevisiae

Cancer cells show increased genome rearrangements, although it is unclear what defects cause these rearrangements. Mutations in Saccharomyces cerevisiae RFC5, DPB11, MEC1, DDC2 MEC3, RAD53, CHK1, PDS1, and DUN1 increased the rate of genome rearrangements up to 200-fold whereas mutations in RAD9, RAD17, RAD24, BUB3, and MAD3 had little effect. The rearrangements were primarily deletion of a portion of a chromosome arm along with TEL1-dependent addition of a new telomere. tel1 mutations increased the proportion of translocations observed, and in some cases showed synergistic interactions when combined with mutations that increased the genome rearrangement rate. These data suggest that one role of S phase checkpoint functions in normal cells is to suppress spontaneous genome rearrangements resulting from DNA replication errors.

SGS1, the Saccharomyces cerevisiae homologue of BLM and WRN, suppresses genome instability and homeologous recombination

The Escherichia coli gene recQ was identified as a RecF recombination pathway gene. The gene SGS1, encoding the only RecQ-like DNA helicase in Saccharomyces cerevisiae, was identified by mutations that suppress the top3 slow-growth phenotype. Relatively little is known about the function of Sgs1p because single mutations in SGS1 do not generally cause strong phenotypes. Mutations in genes encoding RecQ-like DNA helicases such as the Bloom and Werner syndrome genes, BLM and WRN, have been suggested to cause increased genome instability. But the exact DNA metabolic defect that might underlie such genome instability has remained unclear. To better understand the cellular role of the RecQ-like DNA helicases, sgs1 mutations were analyzed for their effect on genome rearrangements. Mutations in SGS1 increased the rate of accumulating gross chromosomal rearrangements (GCRs), including translocations and deletions containing extended regions of imperfect homology at their breakpoints. sgs1 mutations also increased the rate of recombination between DNA sequences that had 91% sequence homology. Epistasis analysis showed that Sgs1p is redundant with DNA mismatch repair (MMR) for suppressing GCRs and for suppressing recombination between divergent DNA sequences. This suggests that defects in the suppression of rearrangements involving divergent, repeated sequences may underlie the genome instability seen in BLM and WRN patients and in cancer cases associated with defects in these genes.

Proliferating cell nuclear antigen and Msh2p-Msh6p interact to form an active mispair recognition complex

Proliferating cell nuclear antigen (PCNA) is required for mismatch repair (MMR) and has been shown to interact with complexes containing Msh2p or MLH1 (refs 1-4). PCNA has been implicated to act in MMR before and during the DNA synthesis step, although the biochemical basis for the role of PCNA early in MMR is unclear. Here we observe an interaction between PCNA and Msh2p-Msh6p mediated by a specific PCNA-binding site present in Msh6p. An msh6 mutation that eliminated the PCNA-binding site caused a mutator phenotype and a defect in the interaction with PCNA. The association of PCNA with Msh2p-Msh6p stimulated the preferential binding of Msh2p-Msh6p to DNA containing mispaired bases. Mutant PCNA proteins encoded by MMR-defective pol30 alleles were defective for interaction with Msh2p-Msh6p and for stimulation of mispair binding by Msh2p-Msh6p. Our results suggest that PCNA functions directly in mispair recognition and that mispair recognition requires a higher-order complex containing proteins in addition to Msh2p-Msh6p.

Characterization of hereditary nonpolyposis colorectal cancer families from a population-based series of cases

BACKGROUND

The incidence of hereditary nonpolyposis colon cancer (HNPCC) in the general population is not well defined because of the lack of large population-based studies. We characterized the incidence of HNPCC in a large, population-based cohort of colorectal cancer probands and analyzed the location of colorectal tumors.

METHODS

Of the participating 1134 probands from three counties in Southern California, 907 had a negative family history of colorectal cancer and 227 had a positive family history of colorectal cancer. In addition, 11 referral case subjects with HNPCC were used to study mutation frequencies in two mismatch repair genes (MSH2 and MLH1) and microsatellite instability. All statistical tests were two-sided.

RESULTS

Among the probands diagnosed in Orange County during 1994 (population-based sample, all ages), five were consistent with the Amsterdam criteria for HNPCC (0.9%; 95% confidence interval [CI] = 0. 3%-2.1%). Among probands diagnosed at less than 65 years of age-from the wider three-county area and a longer time span-16 (2.1%; 95% CI = 1.2%-3.4%) had a clinical history consistent with the Amsterdam criteria for HNPCC. Five (approximately 45%) of 11 of the referral HNPCC case subjects had a mutation in MSH2 or MLH1 and also showed microsatellite instability. The family members of case subjects with mutations tended to show an earlier age at diagnosis of HNPCC and more multiple primary cancers than those of case subjects without detectable mutations. Many of the known characteristics of HNPCC, including the presence of ureteral and endometrial cancers, were seen in both sets of families. The previously reported proximal location of colorectal tumors in HNPCC kindreds was not seen in the population-based dataset but was similar to the location reported in the referral cases.

CONCLUSIONS

On the basis of our data, we believe that the prevalence of HNPCC in the general population is likely to be closer to 1% than to 5%. Furthermore, our study suggests that some previously reported characteristics of HNPCC, such as the proximal location of tumors in the syndrome, may not always hold true in a population-based sample.

Checkpoint-dependent activation of mutagenic repair in Saccharomyces cerevisiae pol3-01 mutants

The Saccharomyces cerevisiae DNA polymerase delta proofreading exonuclease-defective mutation pol3-01 is known to cause high rates of accumulating mutations. The pol3-01 mutant was found to have abnormal cell cycle progression due to activation of the S phase checkpoint. Inactivation of the S phase checkpoint suppressed both the pol3-01 cell cycle progression defect and mutator phenotype, indicating that the pol3-01 mutator phenotype was dependent on the S phase damage checkpoint pathway. Epistasis analysis suggested that a portion of the pol3-01 mutator phenotype involves members of the RAD6 epistasis group that function in both error-free and error-prone repair. These results indicate that activation of a checkpoint in response to certain types of replicative defects can result in the accumulation of mutations.

Sensitivity and specificity of clinical criteria for hereditary non-polyposis colorectal cancer associated mutations in MSH2 and MLH1

BACKGROUND AND AIMS

There are multiple criteria for the clinical diagnosis of hereditary non-polyposis colorectal cancer (HNPCC). The value of several of the newer proposed diagnostic criteria in identifying subjects with mutations in HNPCC associated mismatch repair genes has not been evaluated, and the performance of the different criteria have not been formally compared with one another.

METHODS

We classified 70 families with suspected hereditary colorectal cancer (excluding familial adenomatous polyposis) by several existing clinical criteria for HNPCC, including the Amsterdam criteria, the Modified Amsterdam criteria, the Amsterdam II criteria, and the Bethesda criteria. The results of analysis of the mismatch repair genes MSH2 and MLH1 by full gene sequencing were available for a proband with colorectal neoplasia in each family. The sensitivity and specificity of each of the clinical criteria for the presence of MSH2 and MLH1 mutations were calculated.

RESULTS

Of the 70 families, 28 families fulfilled the Amsterdam criteria, 39 fulfilled the Modified Amsterdam Criteria, 34 fulfilled the Amsterdam II criteria, and 56 fulfilled at least one of the seven Bethesda Guidelines for the identification of HNPCC patients. The sensitivity and specificity of the Amsterdam criteria were 61% (95% CI 43-79) and 67% (95% CI 50-85). The sensitivity of the Modified Amsterdam and Amsterdam II criteria were 72% (95% CI 58-86) and 78% (95% CI 64-92), respectively. Overall, the most sensitive criteria for identifying families with pathogenic mutations were the Bethesda criteria, with a sensitivity of 94% (95% CI 88-100); the specificity of these criteria was 25% (95% CI 14-36). Use of the first three criteria of the Bethesda guidelines only was associated with a sensitivity of 94% and a specificity of 49% (95% CI 34-64).

CONCLUSIONS

The Amsterdam criteria for HNPCC are neither sufficiently sensitive nor specific for use as a sole criterion for determining which families should undergo testing for MSH2 and MLH1 mutations. The Modified Amsterdam and the Amsterdam II criteria increase sensitivity, but still miss many families with mutations. The most sensitive clinical criteria for identifying subjects with pathogenic MSH2 and MLH1 mutations were the Bethesda Guidelines; a streamlined version of the Bethesda Guidelines may be more specific and easier to use in clinical practice.

Identification of a locus on chromosome 1q44 for familial cold urticaria

Familial cold urticaria (FCU) is a rare autosomal dominant inflammatory disorder characterized by intermittent episodes of rash with fever, arthralgias, conjunctivitis, and leukocytosis. These symptoms develop after generalized exposure to cold. Some individuals with FCU also develop late-onset reactive renal amyloidosis, which is consistent with Muckle-Wells syndrome. By analyzing individuals with FCU from five families, we identified linkage to chromosome 1q44. Two-point linkage analysis revealed a maximum LOD score (Zmax) of 8.13 (recombination fraction 0) for marker D1S2836; multipoint linkage analysis identified a Zmax of 10. 92 in the same region; and haplotype analysis defined a 10.5-cM region between markers D1S423 and D1S2682. Muckle-Wells syndrome was recently linked to chromosome 1q44, which suggests that the two disorders may be linked to the same locus.

MutS homolog 4 localization to meiotic chromosomes is required for chromosome pairing during meiosis in male and female mice

Msh4 (MutS homolog 4) is a member of the mammalian mismatch repair gene family whose members are involved in postreplicative DNA mismatch repair as well as in the control of meiotic recombination. In this report we show that MSH4 has an essential role in the control of male and female meiosis. We demonstrate that MSH4 is present in the nuclei of spermatocytes early in prophase I and that it forms discrete foci along meiotic chromosomes during the zygotene and pachytene stages of meiosis. Disruption of the Msh4 gene in mice results in male and female sterility due to meiotic failure. Although meiosis is initiated in Msh4 mutant male and female mice, as indicated by the chromosomal localization of RAD51 and COR1 during leptonema/zygonema, the chromosomes fail to undergo normal pairing. Our results show that MSH4 localization on chromosomes during the early stages of meiosis is essential for normal chromosome synapsis in prophase I and that it acts in the same pathway as MSH5.