Sean M. Baker

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.

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.

Interaction of DNA polymerase β with GRIP1 during meiosis

Abstract DNA polymerase β (polβ) is an essential enzyme that has been shown to localize as discrete foci to the synaptonemal complex during meiosis in the mouse. To identify proteins that associate with polβ during meiosis, we employed the yeast two-hybrid screen. Here we show that a multiple PDZ domain-containing protein, the glutamate receptor interacting protein 1 (GRIP1), interacts specifically with polβ. The PDZ domain-containing proteins, including GRIP1, act as scaffolds to promote rapid and localized biochemical events that require the interaction of multiple proteins. GRIP1 localizes to discrete foci on meiotic bivalents of both spermatocyte and oocyte nuclei, and colocalizes with polβ. Together, these findings provide evidence that GRIP1 interacts with polβ during meiosis. Our findings are consistent with the possibility that GRIP1 acts as a scaffold to promote interaction between proteins that function during meiosis.

Genetics of DNA Mismatch Repair, Microsatellite Instability, and Cancer

In both prokaryotes and eukaryotes mismatched or unpaired bases in a DNA duplex can arise spontaneously through errors in DNA replication, genetic recombination, and deamination of 5-methylcytosine to thymine (T). Errors generated during the process of DNA replication can be thought of as a highly “primitive” type of DNA damage. Systems designed to minimize the production of errors, or to correct mismatches, are likely to have evolved very early in order to assure the fidelity of genetic transmission. Paradoxically, such repair systems cannot result in error-free DNA synthesis, since the constant generation of mutant alleles is a requirement for variation and evolution. In a changing environment, increased spontaneous mutation rates can confer benefits to a bacterial population (38). In fact, the discovery of bacterial mutants that have reduced spontaneous mutation rates (antimutators) suggests that optimum mutation rates have evolved (31,32). Two biological processes, DNA polymerase proofreading and DNA mismatch repair (DMR), are known to affect spontaneous mutation rates dramatically in both prokaryotes and eukaryotes (83,98).