Determining Phenotypes of Pathogenic Mismatch Repair Mutants Brett Palama Lab of Andrew Buermeyer, Ph.D. Dept. of Environmental/Molecular Toxicology

Research Goals  Determine functional phenotypes of cells containing certain mutations in a key DNA mismatch repair protein which lead to human cancer  Aid the development of relevant treatment and prevention methods

DNA Mismatch Repair (MMR)  Highly conserved pathway which helps ensure preservation of genomic integrity  May also play role in recognition of excessive DNA damage and subsequent signaling of apoptosis  Well-defined in prokaryotes; still lacking mechanistic information in eukaryotes

Mechanism for Mismatch Repair T G T T G T A ! Recognition Selection Excision Resynthesis

Eukaryotic MMR Proteins  MMR carried out by two MutS homologs and three MutL homologs, each with varying heteroduplex specificity  Also dependent upon several replicative proteins (e.g. PCNA); excision mechanism still unknown MLH1MLH3MLH1 PMS2PMS1 MSH2 MSH6MSH3 SubstitutionsShort LoopsLong Loops

Loss of MMR in Human Cancer  Germline mutations in one copy of a MMR protein are evident in a large majority of patients with Lynch Syndrome (HNPCC)  Accounts for 2-5% of annual colorectal cancer cases  70% of cases involve mutations of mlh1 or msh2 gene  Greatly increases likelihood of developing other cancers (endometrial, ovarian, etc.)  Characterized by microsatellite instability (MSI)  Pathogenic germline mutations in both copies of a MMR protein result in severe early-onset leukemia or lymphoma (lifespan < 5 yrs)  Inactivation of MMR (evident from MSI) also apparent in 13% of sporadic colorectal cancers

Pathogenic Mutations in hMLH1 Specific interest of study is the hMLH1 protein (the non- redundant member in human MutL complexes) Map of hMLH1 protein, tentative domains, and pathogenic mutations

Questions to Address  Are there pathogenic mlh1 mutants in which only one MMR function is inactivated (“separation of function” mutant)?  What is the biochemical mechanism for the deficiencies introduced by mutations in mlh1?  Which cellular phenotypes (error correction and DNA damage surveillance) are present/deficient in each mutant? Mutator phenotype = higher mutation frequency = increased chance of mutation in tumor suppressor genes/oncogenes = cancer DNA damage tolerance = higher DNA damage threshold = increased likelihood of propagation of damaged cells = cancer

L749ter and the CTH Domain  Mutation which inserts a premature stop codon in the hMLH1 sequence, deleting the final eight amino acids of the polypeptide  Previous data shows that L749ter has a mutator phenotype, high DNA damage tolerance, and inability to stabilize PMS2  Two mutations identified in human cancer are in the CTH domain (K751R and R755W)  Will these two mutations show phenotypes similar to those of L749ter?

Project Outline Site-directed mutagenesis of hMlh1 cDNA Subcloning into mammalian expression vector Transfection into mlh1 - (and pms2 - in transient transfections) mouse embryo fibroblasts Functional Assays

Mutagenesis/Subcloning hMLH1 pBS SK- Site-directed mutagenesis hMLH1 pBS SK- Sequencing of cDNA Removal of cDNA by restriction digestion; blunting of cDNA; gel isolation of cDNA Ligation into linear, blunted pCMVBamNeo vector; Transformation into E.Coli hMLH1 pCMV BamNeo Sequencing of cDNA Large-scale plasmid preparation DNA for use in functional assays!

Functional Assays After subcloning into pCMVBamNeo and transfection into MEF cells, assays are done to determine functional characteristics of mutant MLH1 protein PMS2 Binding Functions To Be Studied: DNA Damage Recognition PMS2 Stabilization PMS2 Subcellular Localization Error Correction

PMS2 Stabilization Question: Which pathogenic mutants are incapable of stabilizing PMS2? Evidence suggests that MLH1 has a stabilizing effect on PMS2. Approach: Transiently transfect double-knockout cells with mutant hMLH1 and PMS2; use SDS-PAGE and Western Blot to determine protein levels. MSH6 PMS2 MLH1 PMS2 alone PMS2 wt MLH1 PMS2 L749t PMS2 K751R PMS2 R755W

Concluding Questions  Why do the mutations K751R and R755W show stabilization of hPMS2 while L749ter does not?  Are the mutations at 751 and 755 pathogenic by some other means? Or are they neutral polymorphisms?  Will other mutations in the CTH behave similar to 751 and 755?

Acknowledgements  Buermeyer Lab  URISC  HHMI  Hays Lab