Abstract A14: Transforming growth factor beta signaling regulates the fate of intestinal stem cells

Transforming growth factor beta signaling has important roles in stem cell behavior, differentiation, cell motility, cell cycle and apoptosis. Here, we have utilized a novel mouse model that facilitates the sporadic inactivation of TgfβR2 via frameshift reversion of Cre in single, isolated cells and concomitant lineage tracing with a Cre-reporter. Short-term lineage tracing of TgfβR2-deficient crypts revealed reduced expansion suggesting a growth disadvantage among crypts. In contrast, long-term lineage tracing of TgfβR2-deficient crypts revealed an increased number suggesting a growth advantage within the crypt. To reconcile these apparently contradictory findings, we studied the dynamics of proliferative, Lgr5+ stem cells following TgfβR2 loss. We found that TgfβR2-deficient stem cell lineages required more time to reach monoclonality within the crypt. Using mathematical modeling of stem cell neutral drift, we found that increased “stemness” or a greater probability of a self-renewing asymmetrical outcome for TgfβR2-deficient stem cells was more consistent with our experimental results. Further analysis of TgfβR2-deficient stem cells in vivo revealed a decreased production of Paneth cells, a finding we corroborated in cultured intestinal organoids by treating with either a TgfβR1/2 inhibitor or low levels of the Tgfβ1 ligand. Recent studies by others suggested that quiescent stem cells are intermediaries between proliferative Lgr5+ stem cells and Paneth cells and that Tgfβ signaling is important in the formation of quiescent stem cells in other tissues. Therefore, we are currently asking whether Tgfβ signaling is important in the formation of quiescent stem cells in the intestine.

Citation Format: Jared M. Fischer, Ashleigh Miller, Peter P. Calabrese, Darryl Shibata, R Michael Liskay. Transforming growth factor beta signaling regulates the fate of intestinal stem cells. [abstract]. In: Proceedings of the Third AACR International Conference on Frontiers in Basic Cancer Research; Sep 18-22, 2013; National Harbor, MD. Philadelphia (PA): AACR; Cancer Res 2013;73(19 Suppl):Abstract nr A14.

Occult progression by Apc-deficient intestinal crypts as a target for chemoprevention

Although Apc mutation is widely considered an initiating event in colorectal cancer, little is known about the earliest stages of tumorigenesis following sporadic Apc loss. Therefore, we have utilized a novel mouse model that facilitates the sporadic inactivation of Apc via frameshift reversion of Cre in single, isolated cells and subsequently tracks the fates of Apc-deficient intestinal cells. Our results suggest that consistent with Apc being a 'gatekeeper', loss of Apc early in life during intestinal growth leads to adenomas or increased crypt fission, manifested by fields of mutant but otherwise normal-appearing crypts. In contrast, Apc loss occurring later in life has minimal consequences, with mutant crypts being less prone to either increased crypt fission or adenoma formation. Using the stem cell-specific Lgr5-CreER mouse, we generated different sized fields of Apc-deficient crypts via independent recombination events and found that field size correlates with progression to adenoma. To evaluate this early stage prior to adenoma formation as a therapeutic target, we examined the chemopreventive effects of sulindac on Apc-deficient occult crypt fission. We found that sulindac treatment started early in life inhibits the morphologically occult spread of Apc-deficient crypts and thus reduces adenoma numbers. Taken together these results suggest that: (i) earlier Apc loss promotes increased crypt fission, (ii) a field of Apc-deficient crypts, which can form via occult crypt fission or independent neighboring events, is an important intermediate between loss of Apc and adenoma formation and (iii) normal-appearing Apc-deficient crypts are potential unappreciated targets for cancer screening and chemoprevention.

Conservation of functional asymmetry in the mammalian MutLα ATPase

The DNA mismatch repair (MMR) protein dimer MutLα is comprised of the MutL homologues MLH1 and PMS2, which each belong to the family of GHL ATPases. These ATPases undergo functionally important conformational changes, including dimerization of the NH₂-termini associated with ATP binding and hydrolysis. Previous studies in yeast and biochemical studies with the mammalian proteins established the importance of the MutLα ATPase for overall MMR function. Additionally, the studies in yeast demonstrated a functional asymmetry between the contributions of the Mlh1 and Pms1 ATPase domains to MMR that was not reflected in the biochemical studies. We investigated the effect of mutating the highly conserved ATP hydrolysis and Mg²(+) binding residues of MLH1 and PMS2 in mammalian cell lines. Amino acid substitutions in MLH1 intended to impact either ATP binding or hydrolysis disabled MMR, as measured by instability at microsatellite sequences, to an extent similar to MLH1-null mutation. Furthermore, cells expressing these MLH1 mutations exhibited resistance to the MMR-dependent cytotoxic effect of 6-thioguanine (6-TG). In contrast, ATP hydrolysis and binding mutants of PMS2 displayed no measurable increase in microsatellite instability or resistance to 6-TG. Our findings suggest that, in vivo, the integrity of the MLH1 ATPase domain is more critical than the PMS2 ATPase domain for normal MMR functions. These in vivo results are in contrast to results obtained previously in vitro that showed no functional asymmetry within the MutLα ATPase, highlighting the differences between in vivo and in vitro systems.

Embryonic lethality after combined inactivation of Fancd2 and Mlh1 in mice

DNA repair defects are frequently encountered in human cancers. These defects are utilized by traditional therapeutics but also offer novel cancer treatment strategies based on synthetic lethality. To determine the consequences of combined Fanconi anemia (FA) and mismatch repair pathway inactivation, defects in Fancd2 and Mlh1 were combined in one mouse model. Fancd2/Mlh1 double-mutant embryos displayed growth retardation resulting in embryonic lethality and significant underrepresentation among progeny. Additional inactivation of Trp53 failed to improve the survival of Fancd2/Mlh1-deficient embryos. Mouse fibroblasts were obtained and challenged with cross-linking agents. Fancd2-deficient cells displayed the FA-characteristic growth inhibition after mitomycin C (MMC) exposure. In primary fibroblasts, the absence of Mlh1 did not greatly affect the MMC sensitivity of Fancd2-deficient and Fancd2-proficient cells. However, in Trp53 mutant immortalized fibroblasts, Mlh1 deficiency reduced the growth-inhibiting effect of MMC in Fancd2 mutant and complemented cells. Similar data were obtained using psoralen/UVA, signifying that MLH1 influences the cellular sensitivity to DNA interstrand cross-links. Next, the effect of MLH1 deficiency on the formation of chromosomal aberrations in response to cross-linking agents was determined. Surprisingly, Mlh1 mutant fibroblasts displayed a modest but noticeable decrease in induced chromosomal breakage and interchange frequencies, suggesting that MLH1 promotes interstrand cross-link repair catastrophe. In conclusion, the combined inactivation of Fancd2 and Mlh1 did not result in synthetic lethality at the cellular level. Although the absence of Fancd2 sensitized Mlh1/Trp53 mutant fibroblasts to MMC, the differential survival of primary and immortalized fibroblasts advocates against systemic inactivation of FANCD2 to enhance treatment of MLH1-deficient tumors.

Mutations affecting a putative MutLalpha endonuclease motif impact multiple mismatch repair functions

Mutations in DNA mismatch repair (MMR) lead to increased mutation rates and higher recombination between similar, but not identical sequences, as well as resistance to certain DNA methylating agents. Recently, a component of human MMR machinery, MutLalpha, has been shown to display a latent endonuclease activity. The endonuclease active site appears to include a conserved motif, DQHA(X)(2)E(X)(4)E, within the COOH-terminus of human PMS2. Substitution of the glutamic acid residue (E705) abolished the endonuclease activity and mismatch-dependent excision in vitro. Previously, we showed that the PMS2-E705K mutation and the corresponding mutation in Saccharomyces cerevisiae were both recessive loss of function alleles for mutation avoidance in vivo. Here, we show that mutations impacting this endonuclease motif also significantly affect MMR-dependent suppression of homeologous recombination in yeast and responses to S(n)1-type methylating agents in both yeast and mammalian cells. Thus, our in vivo results suggest that the endonuclease activity of MutLalpha is important not only in MMR-dependent mutation avoidance but also for recombination and damage response functions.

A mutation in EXO1 defines separable roles in DNA mismatch repair and post-replication repair

Replication forks stall at DNA lesions or as a result of an unfavorable replicative environment. These fork stalling events have been associated with recombination and gross chromosomal rearrangements. Recombination and fork bypass pathways are the mechanisms accountable for restart of stalled forks. An important lesion bypass mechanism is the highly conserved post-replication repair (PRR) pathway that is composed of error-prone translesion and error-free bypass branches. EXO1 codes for a Rad2p family member nuclease that has been implicated in a multitude of eukaryotic DNA metabolic pathways that include DNA repair, recombination, replication, and telomere integrity. In this report, we show EXO1 functions in the MMS2 error-free branch of the PRR pathway independent of the role of EXO1 in DNA mismatch repair (MMR). Consistent with the idea that EXO1 functions independently in two separate pathways, we defined a domain of Exo1p required for PRR distinct from those required for interaction with MMR proteins. We then generated a point mutant exo1 allele that was defective for the function of Exo1p in MMR due to disrupted interaction with Mlh1p, but still functional for PRR. Lastly, by using a compound exo1 mutant that was defective for interaction with Mlh1p and deficient for nuclease activity, we provide further evidence that Exo1p plays both structural and catalytic roles during MMR.

The E705K mutation in hPMS2 exerts recessive, not dominant, effects on mismatch repair

The hPMS2 mutation E705K is associated with Turcot syndrome. To elucidate the pathogenesis of hPMS2-E705K, we modeled this mutation in yeast and characterized its expression and effects on mutation avoidance in mammalian cells. We found that while hPMS2-E705K (pms1-E738K in yeast) did not significantly affect hPMS2 (Pms1p in yeast) stability or interaction with MLH1, it could not complement the mutator phenotype in MMR-deficient mouse or yeast cells. Furthermore, hPMS2-E705K/pms1-E738K inhibited MMR in wild-type (WT) mammalian cell extracts or yeast cells only when present in excess amounts relative to WT PMS2. Our results strongly suggest that hPMS2-E705K is a recessive loss-of-function allele.

Differing patterns of genetic instability in mice deficient in the mismatch repair genes Pms2, Mlh1, Msh2, Msh3 and Msh6

Defects in genes associated with DNA mismatch repair (MMR) have been linked to hereditary colon cancer. Because the MMR pathway includes multiple factors with both overlapping and divergent functions, we sought to compare the impact of deficiencies in each of several MMR genes on genetic instability using a collection of knock-out mouse models. We investigated mutation frequencies and patterns in MMR-deficient mice using two transgenic reporter genes, supFG1 and cII, in the context of mice deficient for Pms2, Mlh1, Msh2, Msh3 or Msh6 or both Msh2 and Msh3 or both Msh3 and Msh6. We found that the mean mutation frequencies of all of the MMR-deficient mice were significantly higher than the mean mutation frequencies of wild-type mice. Mlh1-deficient mice and Msh2-deficient mice had the highest mutation frequencies in a comparison of the single nullizygous mice. Of all the mice studied, mice nullizygous for both Msh2 and Msh3 and those nullizygous for both Msh3 and Msh6 displayed the greatest overall increases in mutation frequencies compared with wild-type mice. Sequence analysis of the mutated reporter genes revealed significant differences between the individual groups of MMR-deficient mice. Taken together, our results further characterize the functions of the MMR factors in mutation avoidance and provide in vivo correlation to biochemical models of the MMR pathway.

Overexpression of the DNA mismatch repair factor, PMS2, confers hypermutability and DNA damage tolerance

Inherited defects in genes associated with DNA mismatch repair (MMR) have been linked to familial colorectal cancer. Cells deficient in MMR are genetically unstable and demonstrate a tolerance phenotype in response to certain classes of DNA damage. Some sporadic human cancers also show abnormalities in MMR gene function, typically due to diminished expression of one of the MutL homologs, MLH1. Here, we report that overexpression of the MutL homolog, human PMS2, can also cause a disruption of the MMR pathway in mammalian cells, resulting in hypermutability and DNA damage tolerance. A mouse fibroblast cell line carrying a recoverable lambda phage shuttle vector for mutation detection was transfected with either a vector designed to express hPMS2 or with an empty vector control. Cells overexpressing hPMS2 were found to have elevated spontaneous mutation frequencies at the cII reporter gene locus. They also showed an increase in the level of mutations induced by the alkylating agent, methynitrosourea (MNU). Clonogenic survival assays demonstrated increased survival of the PMS2-overexpressing cells following exposure to MNU, consistent with the induction of a damage tolerance phenotype. Similar results were seen in cells expressing a mutant PMS2 gene, containing a premature stop codon at position 134 and representing a variant found in an individual with familial colon cancer. These results show that dysregulation of PMS2 gene expression can disrupt MMR function in mammalian cells and establish an additional carcinogenic mechanism by which cells can develop genetic instability and acquire resistance to cytotoxic cancer therapies.

Crossover and noncrossover pathways in mouse meiosis

During meiosis, recombination between homologous chromosomes generates crossover (CR) and noncrossover (NCR) products. CRs establish connections between homologs, whereas intermediates leading to NCRs have been proposed to participate in homologous pairing. How these events are differentiated and regulated remains to be determined. We have developed a strategy to detect, quantify, and map NCRs in parallel to CRs, at the Psmb9 meiotic recombination hot spot, in male and female mouse germ lines. Our results report direct molecular evidence for distinct CR and NCR pathways of DNA double-strand break (DSB) repair in mouse meiosis based on three observations: both CRs and NCRs require Spo11, NCR products have shorter conversion tracts than CRs, and only CRs require the MutL homolog Mlh1. We show that both products are formed from middle to late pachytene of meiotic prophase and provide evidence for an Mlh1-independent CR pathway, where mismatch repair does not require Mlh1.

Novel PMS1 alleles preferentially affect the repair of primer strand loops during DNA replication

Null mutations in DNA mismatch repair (MMR) genes elevate both base substitutions and insertions/deletions in simple sequence repeats. Data suggest that during replication of simple repeat sequences, polymerase slippage can generate single-strand loops on either the primer or template strand that are subsequently processed by the MMR machinery to prevent insertions and deletions, respectively. In the budding yeast Saccharomyces cerevisiae and mammalian cells, MMR appears to be more efficient at repairing mispairs comprised of loops on the template strand compared to loops on the primer strand. We identified two novel yeast pms1 alleles, pms1-G882E and pms1-H888R, which confer a strong defect in the repair of "primer strand" loops, while maintaining efficient repair of "template strand" loops. Furthermore, these alleles appear to affect equally the repair of 1-nucleotide primer strand loops during both leading- and lagging-strand replication. Interestingly, both pms1 mutants are proficient in the repair of 1-nucleotide loop mispairs in heteroduplex DNA generated during meiotic recombination. Our results suggest that the inherent inefficiency of primer strand loop repair is not simply a mismatch recognition problem but also involves Pms1 and other proteins that are presumed to function downstream of mismatch recognition, such as Mlh1. In addition, the findings reinforce the current view that during mutation avoidance, MMR is associated with the replication apparatus.

Contributions by MutL homologues Mlh3 and Pms2 to DNA mismatch repair and tumor suppression in the mouse

Germ line DNA mismatch repair mutations in MLH1 and MSH2 underlie the vast majority of hereditary non-polyposis colon cancer. Four mammalian homologues of Escherichia coli MutL heterodimerize to form three distinct complexes: MLH1/PMS2, MLH1/MLH3, and MLH1/PMS1. Although MLH1/PMS2 is generally thought to have the major MutL activity, the precise contributions of each MutL heterodimer to mismatch repair functions are poorly understood. Here, we show that Mlh3 contributes to mechanisms of tumor suppression in the mouse. Mlh3 deficiency alone causes microsatellite instability, impaired DNA-damage response, and increased gastrointestinal tumor susceptibility. Furthermore, Mlh3;Pms2 double-deficient mice have tumor susceptibility, shorter life span, microsatellite instability, and DNA-damage response phenotypes that are indistinguishable from Mlh1-deficient mice. Our data support previous results from budding yeast that show partial functional redundancy between MLH3 and PMS2 orthologues for mutation avoidance and show a role for Mlh3 in gastrointestinal and extragastrointestinal tumor suppression. The data also suggest a mechanistic basis for the more severe mismatch repair-related phenotypes and cancer susceptibility in Mlh1- versus Mlh3- or Pms2-deficient mice. Contributions by both MLH1/MLH3 and MLH1/PMS2 complexes to mechanisms of mismatch repair-mediated tumor suppression, therefore, provide an explanation why, among MutL homologues, only germ line mutations in MLH1 are common in hereditary non-polyposis colon cancer.

Localization of MMR proteins on meiotic chromosomes in mice indicates distinct functions during prophase I

Mammalian MutL homologues function in DNA mismatch repair (MMR) after replication errors and in meiotic recombination. Both functions are initiated by a heterodimer of MutS homologues specific to either MMR (MSH2-MSH3 or MSH2-MSH6) or crossing over (MSH4-MSH5). Mutations of three of the four MutL homologues (Mlh1, Mlh3, and Pms2) result in meiotic defects. We show herein that two distinct complexes involving MLH3 are formed during murine meiosis. The first is a stable association between MLH3 and MLH1 and is involved in promoting crossing over in conjunction with MSH4-MSH5. The second complex involves MLH3 together with MSH2-MSH3 and localizes to repetitive sequences at centromeres and the Y chromosome. This complex is up-regulated in Pms2-/- males, but not females, providing an explanation for the sexual dimorphism seen in Pms2-/- mice. The association of MLH3 with repetitive DNA sequences is coincident with MSH2-MSH3 and is decreased in Msh2-/- and Msh3-/- mice, suggesting a novel role for the MMR family in the maintenance of repeat unit integrity during mammalian meiosis.

EXO1-A multi-tasking eukaryotic nuclease

Exo1 was first isolated as a 5' --> 3' exonuclease activity induced during meiosis in fission yeast and since that time has been implicated in a multitude of eukaryotic DNA metabolic pathways that include DNA repair, recombination, replication, and telomere integrity. Involvement in multiple pathways affecting genomic stability makes EXO1 a logical target for mutation during oncogenesis. Here, we review studies in several experimental systems that shed light on the role of Exo1 in these DNA transaction pathways, particularly those that may relate to oncogenesis.

Apoptosis and mutation in the murine small intestine: loss of Mlh1- and Pms2-dependent apoptosis leads to increased mutation in vivo

The mismatch repair (MMR) protein Msh2 has been shown to function in the apoptotic response to alkylating agents in vivo. Here, we extend these studies to the MutL homologues (MLH) Mlh1 and Pms2 by analysing the apoptotic response within the small intestine of gene targeted strains. We demonstrate significant differences between Msh2, Mlh1 and Pms2 mutations in influencing apoptotic signalling following 50mg/kg N-methyl-nitrosourea (NMNU), with no obvious reliance upon either Mlh1 or Pms2. However, following exposure to 100mg/kg temozolomide or lower levels of NMNU (10mg/kg) both Mlh1- and Pms2-dependent apoptosis was observed, indicating that the apoptotic response at these levels of DNA damage is dependent on the MutL homologues. Given our ability to observe a MutLalpha dependence of the apoptotic response, we tested whether perturbations of this response directly translate into increases in mutation frequency in vivo. We show that treatment with temozolomide or 10mg/kg NMNU significantly increases mutation in both the Mlh1 and Pms2 mutant mice. At higher levels of NMNU, where the apoptotic response is independent of Mlh1 and Pms2, no gene dependent increase in mutation frequency was observed. These results argue that the MutSalpha and MutLalpha are not equally important in their ability to signal apoptosis. However, when MMR does mediate apoptosis, perturbation of this response leads to long-term persistence of mutant cells in vivo.

Mlh1 deficiency accelerates myeloid leukemogenesis in neurofibromatosis 1 (Nf1) heterozygous mice

Defects in DNA mismatch repair (MMR) have been implicated in the genesis of a diverse set of human cancers. Recent studies have suggested that one of the targets of MMR is the neurofibromatosis 1 (NF1) gene. To evaluate the contribution of Mlh1 MMR deficiency to Nf1 tumorigenesis, Mlh1-/-;Nf1+/- mice were generated. All Mlh1-/-;Nf1+/- mice (n=21) were dead by 260 days compared to none of the Nf1+/- mice. In all, 50% of the Mlh1-/-;Nf1+/- mice were dead at 150 days compared to 252 days for Mlh1-/- mice. Nine of the Mlh1-/-;Nf1+/- mice were found to harbor intrathoracic NOS2-immunoreactive myeloid leukemias similar to the hematopoietic malignancies observed in older Nf1+/- mice. As expected, significant microsatellite instability was observed in six of six tumors and neurofibromin expression was lost in all tumors analysed. These results suggest that MMR deficiency can accelerate myeloid leukemogenesis in Nf1+/- mice, presumably by inactivating Nf1 gene expression.

Neurofibromatosis type 1 gene as a mutational target in a mismatch repair-deficient cell type

DNA mismatch repair (MMR) is the process by which incorrectly paired DNA nucleotides are recognized and repaired. A germline mutation in one of the genes involved in the process may be responsible for a dominantly inherited cancer syndrome, hereditary nonpolyposis colon cancer. Cancer progression in predisposed individuals results from the somatic inactivation of the normal copy of the MMR gene, leading to a mutator phenotype affecting preferentially repeat sequences (microsatellite instability, MSI). Recently, we identified children with a constitutional deficiency of MMR activity attributable to a mutation in the h MLH1 gene. These children exhibited a constitutional genetic instability associated with clinical features of de novo neurofibromatosis type 1 (NF1) and early onset of extracolonic cancer. Based on these observations, we hypothesized that somatic NF1 gene mutation was a frequent and possibly early event in MMR-deficient cells. To test this hypothesis, we screened for NF1 mutations in cancer cells. Genetic alterations were identified in five out of ten tumor cell lines with MSI, whereas five MMR-proficient tumor cell lines expressed a wild-type NF1 gene. Somatic NF1 mutations were also detected in two primary tumors exhibiting an MSI phenotype. Finally, a 35-bp deletion in the murine Nf1 coding region was identified in mlh1-/- mouse embryonic fibroblasts. These observations demonstrate that the NF1 gene is a mutational target of MMR deficiency and suggest that its inactivation is an important step of the malignant progression of MMR-deficient cells.

Characterization of nuclease-dependent functions of Exo1p in Saccharomyces cerevisiae

Exo1p is a member of the Rad2p family of structure-specific nucleases that contain conserved N and I nuclease domains. Exo1p has been implicated in numerous DNA metabolic processes, such as recombination, double-strand break repair and DNA mismatch repair (MMR). In this report, we describe in vitro and in vivo characterization of full-length wild-type and mutant forms of Exo1p. Herein, we demonstrate that full-length yeast Exo1p possesses an intrinsic 5'-3' exonuclease activity as reported previously, but also possesses a flap-endonuclease activity. Our study indicates that Exo1p shares similar, but not identical structure-function relationships to other characterized members of the Rad2p family in the N and I nuclease domains. The two exo1p mutants we examined, showed deficiencies for both double-stranded DNA (dsDNA) 5'-3' exonuclease and flap-endonuclease activities. Examining the genetic interaction of these two exo1 mutations with rad27Delta suggest that the Exo1p flap-endonuclease activity and not the dsDNA 5'-3' exonuclease is redundant to Rad27p for viability. In addition, our in vivo results also indicate that many exo1Delta phenotypes are dependent on the complete catalytic activities of Exo1p. Finally, our findings plus those of other investigators suggest that Exo1p functions both in a catalytic and a structural capacity during DNA MMR.

Alleles of the yeast Pms1 mismatch-repair gene that differentially affect recombination- and replication-related processes

Mismatch-repair (MMR) systems promote eukaryotic genome stability by removing errors introduced during DNA replication and by inhibiting recombination between nonidentical sequences (spellchecker and antirecombination activities, respectively). Following a common mismatch-recognition step effected by MutS-homologous Msh proteins, homologs of the bacterial MutL ATPase (predominantly the Mlh1p-Pms1p heterodimer in yeast) couple mismatch recognition to the appropriate downstream processing steps. To examine whether the processing steps in the spellchecker and antirecombination pathways might differ, we mutagenized the yeast PMS1 gene and screened for mitotic separation-of-function alleles. Two alleles affecting only the antirecombination function of Pms1p were identified, one of which changed an amino acid within the highly conserved ATPase domain. To more specifically address the role of ATP binding/hydrolysis in MMR-related processes, we examined mutations known to compromise the ATPase activity of Pms1p or Mlh1p with respect to the mitotic spellchecker and antirecombination activities and with respect to the repair of mismatches present in meiotic recombination intermediates. The results of these analyses confirm a differential requirement for the Pms1p ATPase activity in replication vs. recombination processes, while demonstrating that the Mlh1p ATPase activity is important for all examined MMR-related functions.

Diet, cancer and aging in DNA mismatch repair deficient mice

Diet is an important risk factor for many cancers. High fat/low calcium (HFLC) diets are associated with increased tumorigenesis, whereas caloric restriction (CR) reproducibly increases lifespan and decreases tumors. Mutations are involved in aging and cancer, and different diets may alter mutagenesis. However, a number of repair pathways normally counteract mutations by correcting errors before they can be fixed in the genome. To further understand interactions between diet, aging and cancer, mice deficient in a major repair pathway called DNA mismatch repair (MMR) were fed HFLC, CR or control diets. Mlh1 deficient mice are prone to lymphomas and intestinal adenomas and carcinomas. No significant changes in adenocarcinoma or lymphoma incidence were observed with HFLC or CR diets. Significantly more (2.2-fold) adenomas occurred with HFLC diets although adenoma numbers were unchanged with CR. Only a small increase in lifespan (116% of control) was achieved with CR. In addition, levels of microsatellite mutations in the small and large intestines were unchanged with the different diets. Our studies indicate that MMR deficiency may be epistatic to certain otherwise strong environmental influences on carcinogenesis or aging.

Contribution of human mlh1 and pms2 ATPase activities to DNA mismatch repair

MutLalpha, a heterodimer composed of Mlh1 and Pms2, is the major MutL activity in mammalian DNA mismatch repair. Highly conserved motifs in the N termini of both subunits predict that the protein is an ATPase. To study the significance of these motifs to mismatch repair, we have expressed in insect cells wild type human MutLalpha and forms altered in conserved glutamic acid residues, predicted to catalyze ATP hydrolysis of Mlh1, Pms2, or both. Using an in vitro assay, we showed that MutLalpha proteins altered in either glutamic acid residue were each partially defective in mismatch repair, whereas the double mutant showed no detectable mismatch repair. Neither strand specificity nor directionality of repair was affected in the single mutant proteins. Limited proteolysis studies of MutLalpha demonstrated that both Mlh1 and Pms2 N-terminal domains undergo ATP-induced conformational changes, but the extent of the conformational change for Mlh1 was more apparent than for Pms2. Furthermore, Mlh1 was protected at lower ATP concentrations than Pms2, suggesting Mlh1 binds ATP with higher affinity. These findings imply that ATP hydrolysis is required for MutLalpha activity in mismatch repair and that this activity is associated with differential conformational changes in Mlh1 and Pms2.

Evidence for the lack of mismatch-repair directed antirecombination during mouse meiosis

Meiotic recombination was studied in DNA mismatch repair (MMR)-deficient mice using a strain carrying a Pms2 knockout mutation. Using single-sperm typing, recombination was analyzed over five intervals on four chromosomes in four Pms2 -/- animals. A total of 1936 meioses were studied and compared to 1848 meioses from three Pms2 +/+ controls. A smaller study was carried out on a single interval in each of two chromosomes in an MMR-deficient mouse homozygous for the Msh2 knockout mutation. A total of 792 meioses were examined in the Msh2 -/- and 880 meioses in the Msh2 +/+ animal. Recombination fractions were not significantly different in either of the MMR-deficient mouse strains when compared to MMR-proficient controls. Our results appear to conflict with mouse embryonic stem (ES) cell gene-targeting experiments where MMR plays a major role in determining the efficiency of homologous recombination between nonidentical sequences. A number of possibilities could explain the apparent lack of a significant effect on meiosis.

Maternal effect for DNA mismatch repair in the mouse

DNA mismatch repair (DMR) functions to maintain genome stability. Prokaryotic and eukaryotic cells deficient in DMR show a microsatellite instability (MSI) phenotype characterized by repeat length alterations at microsatellite sequences. Mice deficient in Pms2, a mammalian homolog of bacterial mutL, develop cancer and display MSI in all tissues examined, including the male germ line where a frequency of approximately 10% was observed. To determine the consequences of maternal DMR deficiency on genetic stability, we analyzed F(1) progeny from Pms2(-/-) female mice mated with wild-type males. Our analysis indicates that MSI in the female germ line was approximately 9%. MSI was also observed in paternal alleles, a surprising result since the alleles were obtained from wild-type males and the embryos were therefore DMR proficient. We propose that mosaicism for paternal alleles is a maternal effect that results from Pms2 deficiency during the early cleavage divisions. The absence of DMR in one-cell embryos leads to the formation of unrepaired replication errors in early cell divisions of the zygote. The occurrence of postzygotic mutation in the early mouse embryo suggests that Pms2 deficiency is a maternal effect, one of a limited number identified in the mouse and the first to involve a DNA repair gene.

Interactions of Exo1p with components of MutLalpha in Saccharomyces cerevisiae

Previously, we reported evidence suggesting that Saccharomyces cerevisiae MutLalpha, composed of Mlh1p and Pms1p, was a functional member of the gyrase b/Hsp90/MutL (GHL) dimeric ATPase superfamily characterized by highly conserved ATPase domains. Similar to other GHL ATPases, these putative ATPase domains of MutLalpha may be important for the recruitment and/or activation of downstream effectors. One downstream effector candidate is Exo1p, a 5'-3' double stranded DNA exonuclease that has previously been implicated in DNA mismatch repair (MMR). Here we report yeast two-hybrid results suggesting that Exo1p can interact physically with MutLalpha through the Mlh1p subunit. We also report epistasis analysis involving MutLalpha ATPase mutations combined with exo1Delta. One interpretation of our genetic results is that MutLalpha ATPase domains function to direct Exo1p and other functionally redundant exonucleases during MMR. Finally, our results show that much of the increase in spontaneous mutation observed in an exo1Delta strain is REV3-dependent, in turn suggesting that Exo1p is also involved in one or more MMR-independent mutation avoidance pathways.

Hypermutability to ionizing radiation in mismatch repair-deficient, Pms2 knockout mice

DNA mismatch repair (MMR) has been shown to play a role in the cytotoxicity of ionizing radiation (IR), as cell lines established from MMR-deficient mice exhibit higher clonogenic survival after IR than do cell lines from wild-type littermates. To test whether this tolerance phenotype would render MMR-deficient animals hypermutable to IR, we compared IR mutagenesis of Pms2-deficient versus wild-type transgenic mice carrying a lambda shuttle vector for mutation detection. In Pms2 nullizygous animals, the mutation frequency in the supFG1 reporter gene was increased from 210 x 10(-5) in untreated animals to 734 x 10(-5) after 6 Gy of IR (an increase of 524 mutants per 10(5)), whereas the frequency in wild-type mice increased from 1.9 x 10(-5) to 10.2 x 10(-5) (an increase of only 8.3 mutants per 10(5)). Similarly, when the lambda cII gene was used as a reporter, the mutation frequency in nullizygous mice was increased from 16.3 x 10(-5) to 42.3 x 10(-5) after IR (an increase of 26.0 x 10(-5)), whereas the frequency in wild-type mice increased from 2.4 x 10(-5) to 9.4 x 10(-5) (an increase of only 7.0 x 10(-5)). The pattern of IR-induced mutations in the MMR-deficient animals was notable for single bp deletions and insertions in mononucleotide repeat sequences, along with a slight increase in transversions. Overall, these results suggest that MMR-deficiency confers hypermutability to IR, and that much of this hypermutability can be attributed to induced instability of simple sequence repeats. Hence, MMR influences not only the survival but also the mutability of cells in response to IR.

MSH-MLH complexes formed at a DNA mismatch are disrupted by the PCNA sliding clamp

In the yeast Saccharomyces cerevisiae, mismatch repair (MMR) is initiated by the binding of heterodimeric MutS homolog (MSH) complexes to mismatches that include single nucleotide and loop insertion/deletion mispairs. In in vitro experiments, the mismatch binding specificity of the MSH2-MSH6 heterodimer is eliminated if ATP is present. However, addition of the MutL homolog complex MLH1-PMS1 to binding reactions containing MSH2-MSH6, ATP, and mismatched substrate results in the formation of a stable ternary complex. The stability of this complex suggests that it represents an intermediate in MMR that is subsequently acted upon by other MMR factors. In support of this idea, we found that the replication processivity factor proliferating cell nuclear antigen (PCNA), which plays a critical role in MMR at step(s) prior to DNA resynthesis, disrupted preformed ternary complexes. These observations, in conjunction with experiments performed with streptavidin end-blocked mismatch substrates, suggested that PCNA interacts with an MSH-MLH complex formed on DNA mispairs.

Elevated mutant frequencies and increased C : G-->T : A transitions in Mlh1-/- versus Pms2-/- murine small intestinal epithelial cells

Mutations in DNA mismatch repair (MMR) genes are associated with increased genomic instability and susceptibility to cancer. Mice rendered deficient in either Mlh1 or Pms2 as a result of gene targeting are prone to tumorigenesis, particularly, lymphomas. In addition, although Mlh1-/- mice also develop small intestinal adenomas and adenocarcinomas, Pms2-/- animals remain free of such tumors. To establish whether this phenotypic dichotomy might be associated with a quantitative and/or qualitative difference in genomic instability in these mice, we determined small intestinal epithelial cell DNA mutant frequency and mutation spectrum using a transgenic lambda-phage lacI reporter system. Mutant frequencies obtained from both Mlh1-/- and Pms2-/- mice revealed elevations of 18- and 13-fold, respectively, as compared to their wild-type littermates. Interestingly, we found that C : G-->T : A transitions were significantly elevated in Mlh1-/- mice, accounting in large measure for the 1.5-fold lacI mutant frequency increase seen in these animals. We hypothesize that the increased level of C : G-->T : A mutations may explain, in part, why Mlh1-/- mice, but not Pms2-/- mice, develop small intestinal tumors. Furthermore, the difference in the lacI mutational spectrum of Mlh1-/- and Pms2-/- mice suggests that other MutL-like heterodimers may play important roles in the repair of G : T mispairs arising within murine small intestinal epithelial cells.

Analysis of yeast MSH2-MSH6 suggests that the initiation of mismatch repair can be separated into discrete steps

The yeast MSH2-MSH6 complex is required to repair both base-pair and single base insertion/deletion mismatches. MSH2-MSH6 binds to mismatch substrates and displays an ATPase activity that is modulated by mispairs that are repaired in vivo. To understand early steps in mismatch repair, we analyzed mismatch repair (MMR) defective MSH2-msh6-F337A and MSH2-msh6-340 complexes that contained amino acid substitutions in the MSH6 mismatch recognition domain. While both heterodimers were defective in forming stable complexes with mismatch substrates, only MSH2-msh6-340 bound to homoduplex DNA with an affinity that was similar to that observed for MSH2-MSH6. Additional analyses suggested that stable binding to a mispair is not sufficient to initiate recruitment of downstream repair factors. Previously, we observed that MSH2-MSH6 forms a stable complex with a palindromic insertion mismatch that escapes correction by MMR in vivo. Here we show that this binding is not accompanied by either a modulation in MSH2-MSH6 ATPase activity or an ATP-dependent recruitment of the MLH1-PMS1 complex. Together, these observations suggest that early stages in MMR can be divided into distinct recognition, stable binding, and downstream factor recruitment steps.

Ionizing radiation-induced apoptosis via separate Pms2- and p53-dependent pathways

The cytotoxicity of ionizing radiation (IR) has been associated with both the p53 pathway and with DNA mismatch repair (MMR). p53 mediates cell cycle arrest and apoptosis in response to X-ray damage, whereas the MMR complex is thought to recognize damaged bases and initiate a signal transduction pathway that can include phosphorylation of p53. To determine whether p53 and MMR mediate X-ray cytotoxicity via the same pathway, mice with targeted disruptions in either the p53 gene or the MutL homologue MMR gene Pms2 were interbred and primary fibroblasts were established from the progeny with genotypes of either wild type, p53 null, Pms2 null, or double null. Cells with either p53 or Pms2 separately disrupted showed reduced levels of apoptosis after IR in comparison with wild type, but the double null cells showed even lower levels, consistent with nonoverlapping roles for p53 and PMS2 in the X-ray response. In transformed cell lines established from the primary cells at early passage, similar differences in the apoptotic response to IR were seen, and clonogenic survival assays following low dose rate IR further showed that nullizygosity for Pms2 confers increased survival on cells in both wild-type and p53 null backgrounds. These results indicate that both p53 and MMR contribute to X-ray-induced apoptosis and that the role of MMR in the cytotoxicity of IR does not depend on p53.

Functional studies on the candidate ATPase domains of Saccharomyces cerevisiae MutLalpha

Saccharomyces cerevisiae MutL homologues Mlh1p and Pms1p form a heterodimer, termed MutLalpha, that is required for DNA mismatch repair after mismatch binding by MutS homologues. Recent sequence and structural studies have placed the NH(2) termini of MutL homologues in a new family of ATPases. To address the functional significance of this putative ATPase activity in MutLalpha, we mutated conserved motifs for ATP hydrolysis and ATP binding in both Mlh1p and Pms1p and found that these changes disrupted DNA mismatch repair in vivo. Limited proteolysis with purified recombinant MutLalpha demonstrated that the NH(2) terminus of MutLalpha undergoes conformational changes in the presence of ATP and nonhydrolyzable ATP analogs. Furthermore, two-hybrid analysis suggested that these ATP-binding-induced conformational changes promote an interaction between the NH(2) termini of Mlh1p and Pms1p. Surprisingly, analysis of specific mutants suggested differential requirements for the ATPase motifs of Mlh1p and Pms1p during DNA mismatch repair. Taken together, these results suggest that MutLalpha undergoes ATP-dependent conformational changes that may serve to coordinate downstream events during yeast DNA mismatch repair.

Mutagenesis in PMS2- and MSH2-deficient mice indicates differential protection from transversions and frameshifts

DNA mismatch repair (MMR) deficiency leads to an increased mutation frequency and a predisposition to neoplasia. 'Knockout' mice deficient in the MMR proteins Msh2 and Pms2 crossed with mutation detection reporter (supF, lacI and cII) transgenic mice have been used to facilitate a comparison of the changes in mutation frequency and spectra. We find that the mutation frequency was consistently higher in Msh2-deficient mice than Pms2-deficient mice. The lacI target gene, which is highly sensitive to point mutations, demonstrated that both Msh2- and Pms2-deficient mice accumulate transition mutations as the predominant mutation. However, when compared with Msh2(-/-) mice, lacI and cII mutants from Pms2-deficient mice revealed an increased proportion of +/-1 bp frameshift mutations and a corresponding decrease in transversion mutations. The supF target gene, which is sensitive to frameshift mutations, and the cII target gene revealed a strong tendency for -1 bp deletions over +1 bp insertions in Msh2(-/-) compared with Pms2(-/-) mice. These data indicate that Msh2 and Pms2 deficiency have subtle but differing effects on mutation avoidance which may contribute to the differences in tumor spectra observed in the two 'knockout' mouse models. These variances in mutation accumulation may also play a role, in part, in the differences seen in prevalence of MSH2 and PMS2 germline mutations in hereditary non-polyposis colorectal cancer patients.

Mlh1 deficiency enhances several phenotypes of Apc(Min)/+ mice

Defects in APC and DNA mismatch repair genes are associated with a strong predisposition to colon cancer in humans, and numerous mouse strains with mutations in these genes have been generated. In this report we describe the phenotype of Min/+ Mlh1-/- mice. We find that these doubly mutant mice develop more than three times the number of intestinal adenomas compared to Min/+ Mlh1+/+ or +/- mice but that these tumors do not show advanced progression in terms of tumor size or histological appearance. Full length Apc protein was not detected in the tumor cells from Min/+ Mlh1-/- mice. Molecular analyses indicated that in many tumors from Min/+ Mlh1-/- mice, Apc was inactivated by intragenic mutation. Mlh1 deficiency in Min/+ mice also led to an increase in cystic intestinal crypt multiplicity as well as enhancing desmoid tumorigenesis and epidermoid cyst development. Thus, Mlh1 deficiency influences the somatic events involved in the development of most of the phenotypes associated with the Min mutation. Oncogene (2000).

Mammalian DNA mismatch repair

DNA mismatch repair (MMR) is one of multiple replication, repair, and recombination processes that are required to maintain genomic stability in prokaryotes and eukaryotes. In the wake of the discoveries that hereditary nonpolyposis colorectal cancer (HNPCC) and other human cancers are associated with mutations in MMR genes, intensive efforts are under way to elucidate the biochemical functions of mammalian MutS and MutL homologs, and the consequences of defects in these genes. Genetic studies in cultured mammalian cells and mice are proving to be instrumental in defining the relationship between the functions of MMR in mutation and tumor avoidance. Furthermore, these approaches have raised awareness that MMR homologs contribute to DNA damage surveillance, transcription-coupled repair, and recombinogenic and meiotic processes.

Different mutator phenotypes in Mlh1- versus Pms2-deficient mice

Deficiencies in DNA mismatch repair (MMR) result in increased mutation rates and cancer risk in both humans and mice. Mouse strains homozygous for knockouts of either the Pms2 or Mlh1 MMR gene develop cancer but exhibit very different tumor spectra; only Mlh1(-/-) animals develop intestinal tumors. We carried out a detailed study of the microsatellite mutation spectra in each knockout strain. Five mononucleotide repeat tracts at four different chromosomal locations were studied by using single-molecule PCR or an in vivo forward mutation assay. Three dinucleotide repeat loci also were examined. Surprisingly, the mononucleotide repeat mutation frequency in Mlh1(-/-) mice was 2- to 3-fold higher than in Pms2(-/-) animals. The higher mutation frequency in Mlh1(-/-) mice may be a consequence of some residual DNA repair capacity in the Pms2(-/-) animals. Relevant to this idea, we observed that Pms2(-/-) mice exhibit almost normal levels of Mlh1p, whereas Mlh1(-/-) animals lack both Mlh1p and Pms2p. Comparison between Mlh1(-/-) animals and Mlh1(-/-) and Pms2(-/-) double knockout mice revealed little difference in mutator phenotype, suggesting that Mlh1 nullizygosity is sufficient to inactivate MMR completely. The findings may provide a basis for understanding the greater predisposition to intestinal cancer of Mlh1(-/-) mice. Small differences (2- to 3-fold) in mononucleotide repeat mutation rates may have dramatic effects on tumor development, requiring multiple genetic alterations in coding regions. Alternatively, this strain difference in tumor spectra also may be related to the consequences of the absence of Pms2p compared with the absence of both Pms2p and Mlh1p on as yet little understood cellular processes.

A mutation of the yeast gene encoding PCNA destabilizes both microsatellite and minisatellite DNA sequences

The POL30 gene of the yeast Saccharomyces cerevisiae encodes the proliferating cell nuclear antigen (PCNA), a protein required for processive DNA synthesis by DNA polymerase delta and epsilon. We examined the effects of the pol30-52 mutation on the stability of microsatellite (1- to 8-bp repeat units) and minisatellite (20-bp repeat units) DNA sequences. It had previously been shown that this mutation destabilizes dinucleotide repeats 150-fold and that this effect is primarily due to defects in DNA mismatch repair. From our analysis of the effects of pol30-52 on classes of repetitive DNA with longer repeat unit lengths, we conclude that this mutation may also elevate the rate of DNA polymerase slippage. The effect of pol30-52 on tracts of repetitive DNA with large repeat unit lengths was similar, but not identical, to that observed previously for pol3-t, a temperature-sensitive mutation affecting DNA polymerase delta. Strains with both pol30-52 and pol3-t mutations grew extremely slowly and had minisatellite mutation rates considerably greater than those observed in either single mutant strain.

The human MLH1 cDNA complements DNA mismatch repair defects in Mlh1-deficient mouse embryonic fibroblasts

The DNA mismatch repair gene hMLH1 is reported to function in mutation avoidance, cell cycle checkpoint control, the cytotoxicity of various DNA-damaging agents, and transcription-coupled nucleotide excision repair. Formal proof of the involvement of hMLH1 in these processes requires single gene complementation. We have stably expressed hMLH1 from a transfected cDNA in Mlh1-deficient mouse embryonic fibroblasts. Expression of hMLH1 restored normal levels of mPMS2 protein, reduced spontaneous base substitution and microsatellite mutations, increased sensitivity to the toxic effects of 6-thioguanine (6-TG), and restored 6-TG-induced cell cycle arrest. Our studies confirm that hMLH1 has an essential role in the maintenance of genomic stability and the potentiation of 6-TG cytotoxicity and provide a system for detailed structure/function analysis of the hMLH1 protein.

Altered spectra of hypermutation in antibodies from mice deficient for the DNA mismatch repair protein PMS2

Mutations are introduced into rearranged Ig variable genes at a frequency of 10(-2) mutations per base pair by an unknown mechanism. Assuming that DNA repair pathways generate or remove mutations, the frequency and pattern of mutation will be different in variable genes from mice defective in repair. Therefore, hypermutation was studied in mice deficient for either the DNA nucleotide excision repair gene Xpa or the mismatch repair gene Pms2. High levels of mutation were found in variable genes from XPA-deficient and PMS2-deficient mice, indicating that neither nucleotide excision repair nor mismatch repair pathways generate hypermutation. However, variable genes from PMS2-deficient mice had significantly more adjacent base substitutions than genes from wild-type or XPA-deficient mice. By using a biochemical assay, we confirmed that tandem mispairs were repaired by wild-type cells but not by Pms2(-/-) human or murine cells. The data indicate that tandem substitutions are produced by the hypermutation mechanism and then processed by a PMS2-dependent pathway.

Enhanced intestinal adenomatous polyp formation in Pms2-/-;Min mice

Analysis of two human familial cancer syndromes, hereditary nonpolyposis colorectal cancer and familial adenomatous polyposis, indicates that mutations in either one of four DNA mismatch repair gene homologues or the adenomatous polyposis coli (APC) gene, respectively, are important for the development of colorectal cancer. To further investigate the role of DNA mismatch repair in intestinal tumorigenesis, we generated mice with mutations in both Apc and the DNA mismatch repair gene, Pms2. Whereas Pms2-deficient mice do not develop intestinal tumors, mice deficient in Pms2 and heterozygous for Min, an allele of Apc, develop approximately three times the number of small intestinal adenomas and four times the number of colon adenomas relative to Min and Pms2+/-;Min mice. Although Pms2 deficiency clearly increases adenoma formation in the Min background, histological analysis indicated no clear evidence for progression to carcinoma.

Tumour susceptibility and spontaneous mutation in mice deficient in Mlh1, Pms1 and Pms2 DNA mismatch repair

Germline mutations in the human MSH2, MLH1, PMS2 and PMS1 DNA mismatch repair (MMR) gene homologues appear to be responsible for most cases of hereditary non-polyposis colorectal cancer (HNPCC; refs 1-5). An important role for DNA replication errors in colorectal tumorigenesis has been suggested by the finding of frequent alterations in the length of specific mononucleotide tracts within genes controlling cell growth, including TGF-beta receptor type II (ref. 6), BAX (ref. 7) and APC (ref. 8). A broader role for MMR deficiency in human tumorigenesis is implicated by microsatellite instability in a fraction of sporadic tumours, including gastric, endometrial and colorectal malignancies. To better define the role of individual MMR genes in cancer susceptibility and MMR functions, we have generated mice deficient for the murine homologues of the human genes MLH1, PMS1 and PMS2. Surprisingly, we find that these mice show different tumour susceptibilities, most notably, to intestinal adenomas and adenocarcinomas, and different mutational spectra. Our results suggest that a general increase in replication errors may not be sufficient for intestinal tumour formation and that these genes share overlapping, but not identical functions.

Role of DNA mismatch repair in the cytotoxicity of ionizing radiation

The DNA mismatch repair (MMR) system in mammalian cells not only serves to correct base mispairs and other replication errors, but it also influences the cellular response to certain forms of DNA damage. Cells that are deficient in MMR are relatively resistant to alkylation damage because, in wild-type cells, the MMR system is thought to promote toxicity via futile repair of alkylated mispairs. Conversely, MMR-deficient cells are sensitive to UV light, possibly due to the requirement for MMR factors in transcription-coupled repair of active genes. MMR deficiency has been associated with familial and sporadic carcinomas of the colon and other sites, and so, we sought to determine the influence of MMR status on cellular response to ionizing radiation, an agent commonly used for cancer therapy. Fibroblast cell lines were established from transgenic mice carrying targeted disruptions of one of three MMR genes in mammalian cells: Pms2, Mlh1, or Msh2. In comparison to wild-type cell lines from related mice, the Pms2-, Mlh1-, or Msh2-nullizygous cell lines were found to exhibit higher levels of clonogenic survival following exposure to ionizing radiation. Because ionizing radiation generates a variety of lesions in DNA, the differences in survival may reflect a role for MMR in processing a subset of these lesions, such as damaged bases. These results both identify a new class of DNA-damaging agents whose effects are modulated by the MMR system and may help to elucidate pathways of radiation response in cancer cells.

Functional domains of the Saccharomyces cerevisiae Mlh1p and Pms1p DNA mismatch repair proteins and their relevance to human hereditary nonpolyposis colorectal cancer-associated mutations

The MutL protein is an essential component of the Escherichia coli methyl-directed mismatch repair system but has no known enzymatic function. In the yeast Saccharomyces cerevisiae, the MutL equivalent, an Mlh1p and Pms1p heterodimer, interacts with Msh2p bound to mismatch-containing DNA. Little is known of the functional domains of Mlh1p and Pms1p. In this report, we define the Mlh1p and Pms1p domains required for Mlh1p-Pms1p interaction. The Mlh1p-interactive domain of Pms1p is comprised of 260 amino acids near the carboxyl terminus while the Pms1p-interactive domain of Mlh1p resides in the final 212 residues. The two domains are sufficient for Mlh1p-Pms1p interaction, as determined by the two-hybrid assay and by in vitro protein affinity chromatography. Deletions within the domains completely eliminated Mlh1p-Pms1p interaction. Using site-directed mutagenesis, we altered a number of highly conserved residues in the Mlh1p and Pms1p proteins, including some alterations that mimic germline mutations observed for human hereditary nonpolyposis colorectal cancer. Alterations either in the consensus MutL box located in the amino-terminal portion of each protein or in the carboxyl-terminal homology motif of Mlh1p eliminated DNA mismatch repair function but had no effect on Mlh1p-Pms1p interaction. In addition, certain MLH1 and PMS1 mutant alleles caused a dominant negative mutator effect when overexpressed. We discuss the implications of these findings for the structural organization of the Mlh1p and Pms1p proteins and the importance of Mlh1p-Pms1p interaction.

Intestinal stem cell division and genetic diversity. A computer and experimental analysis

Somatic mutations are expected to arise with age. This process is accelerated in mice lacking the DNA mismatch repair gene Pms2. The distributions of microsatellite alleles present in small patches of normal Pms2 -/- intestines revealed a general increase in genetic diversity or the number of mutations with age. However, the patterns were complex with different distributions and variances present within a single mouse. Computer simulations indicate that the experimental data are consistent with mutation rates between 0.0020 and 0.0025 mutations per division, nonrandom cell death, and an effective population size of 20 or fewer cells. Small numbers of cells exacerbate the random accumulation of mutations expected of a stochastic mutation process. The computer simulations and experimental data are consistent with known patterns of intestinal development and renewal by small numbers of stem cells and demonstrate relatively high mutation rates in histologically normal epithelium. These findings provide background for the analysis of microsatellite mutations in normal and tumor tissue lacking mismatch repair and further support the hypothesis that microsatellite loci can function as molecular tumor clocks.

Elevated levels of mutation in multiple tissues of mice deficient in the DNA mismatch repair gene Pms2

The Pms2 gene has been implicated in hereditary colon cancer and is one of several mammalian homologs of the Escherichia coli mutL DNA mismatch repair gene. To determine the effect of Pms2 inactivation on genomic integrity in vivo, hybrid transgenic mice were constructed that carry targeted disruptions at the Pms2 loci along with a chromosomally integrated mutation reporter gene. In the absence of any mutagenic treatment, mice nullizygous for Pms2 showed a 100-fold elevation in mutation frequency in all tissues examined compared with both wild-type and heterozygous litter mates. The mutation pattern in the nullizygotes was notable for frequent 1-bp deletions and insertions within mononucleotide repeat sequences, consistent with an essential role for PMS2 in the repair of replication slippage errors. Further, the results demonstrate that high rates of mutagenesis in multiple tissues are compatible with normal development and life and are not necessarily associated with accelerated aging. Also, the finding of genetic instability in all tissues tested contrasts with the limited tissue distribution of cancers in the animals, raising important questions regarding the role of mutagenesis in carcinogenesis.

Altered expression of hMSH2 and hMLH1 in tumors with microsatellite instability and genetic alterations in mismatch repair genes

To date, at least four genes involved in DNA mismatch repair (MMR) have been demonstrated to be altered in the germline of patients with hereditary nonpolyposis colon cancer: hMSH2, hMLH1, hPMS1, and hPMS2. Additionally, loss of MMR function has been demonstrated to lead to the phenomenon of microsatellite instability (MIN) in tumors from these patients. In this study, we have examined the protein expression pattern of hMSH2 and hMLH1 by immunohistochemistry in paraffin-embedded tumors from 7 patients with MIN+ sporadic cancer, 13 patients with familial colorectal cancer, and 12 patients meeting the strict Amsterdam criteria for hereditary nonpolyposis colon cancer. The relationship between the expression of these two gene products, the presence of germline or somatic mutations, and the presence of tumor MIN was examined. Nineteen of the 28 tumors studied demonstrated MIN, whereas mutations in hMLH1 and hMSH2 were detected in 6 and 2 patients, respectively. Of the eight MIN+/mutation+ cases, the absence of protein expression was observed for the corresponding gene product in all but one case (missense mutation in hMLH1). However, seven MIN+/mutation- cases also showed no expression of either hMLH1 (n = 5), hMSH2 (n = 1), or both (n = 1), whereas four MIN+/mutation- cases demonstrated normal expression for both. None of the MIN-/mutation- cases (n = 9) demonstrated an altered expression pattern for either protein. These data suggest that examination of protein expression by immunohistochemistry may be a rapid method for prescreening tumors for mutations in the MMR genes.

Requirement for PCNA in DNA mismatch repair at a step preceding DNA resynthesis

A two-hybrid system was used to screen yeast and human expression libraries for proteins that interact with mismatch repair proteins. PCNA was recovered from both libraries and shown in the case of yeast to interact with both MLH1 and MSH2. A yeast strain containing a mutation in the PCNA gene had a strongly elevated mutation rate in a dinucleotide repeat, and the rate was not further elevated in a strain also containing a mutation in MLH1. Mismatch repair activity was examined in human cell extracts using an assay that does not require DNA repair synthesis. Activity was inhibited by p21WAF1 or a p21 peptide, both of which bind to PCNA, and activity was restored to inhibited reactions by addition of PCNA. The data suggest a PCNA requirement in mismatch repair at a step preceding DNA resynthesis. The ability of PCNA to bind to MLH1 and MSH2 may reflect linkage between mismatch repair and replication and may be relevant to the roles of mismatch repair proteins in other DNA transactions.

Involvement of mouse Mlh1 in DNA mismatch repair and meiotic crossing over

Mice that are deficient in either the Pms2 or Msh2 DNA mismatch repair genes have microsatellite instability and a predisposition to tumours. Interestingly, Pms2-deficient males display sterility associated with abnormal chromosome pairing in meiosis. Here mice deficient in another mismatch repair gene, Mlh1, possess not only microsatellite instability but are also infertile (both males and females). Mlh1-deficient spermatocytes exhibit high levels of prematurely separated chromosomes and arrest in first division meiosis. We also show that Mlh1 appears to localize to sites of crossing over on meiotic chromosomes. Together these findings suggest that Mlh1 is involved in DNA mismatch repair and meiotic crossing over.

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

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

Spontaneous and restriction enzyme-induced chromosomal recombination in mammalian cells

We have derived Chinese hamster ovary (CHO) cell hybrids containing herpes simplex virus thymidine kinase (tk) heteroalleles for the study of spontaneous and restriction enzyme-induced interchromosomal recombination. These lines allowed us to make a direct comparison between spontaneous intrachromosomal and interchromosomal recombination using the same tk heteroalleles at the same genomic insertion site. We find that the frequency of interchromosomal recombination is less by a factor of at least 5000 than that of intrachromosomal recombination. Our results with mammalian cells differ markedly from results with Saccharomyces cerevisiae, with which similar studies typically give only a 10-to 30-fold difference. Next, to inquire into the fate of double-strand breaks at either of the two different Xho I linker insertion mutations, we electroporated PaeR7I enzyme, an isoschizomer of Xho I, into these hybrids. A priori, these breaks can be repaired either by recombination from the homology or by end-joining. Despite a predicted bias against recovering end-joining products in our system, all cells characterized by enzyme-induced resistance to hypoxanthine/aminopterin/thymidine were, in fact, due to nonhomologous recombination or end-joining. These results are in agreement with other studies that used extrachromosomal sequences to examine the relative efficiencies of end-joining and homologous recombination in mammalian cells, but are in sharp contrast to results of analogous studies in S. cerevisiae, wherein only products of homologous events are detected.

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

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