The Role of Mismatch-Repair The Role of Mismatch-Repair Protein MLH3 in Genomic Stability Protein MLH3 in Genomic Stability

in the Plant in the Plant ArabidopsisArabidopsis

Howard Hughes Medical Institute Summer Research Program

Laurel WheelerMentor: Dr. John Hays

DNA Repair SystemsDNA Repair Systems

•One major concern for an organism’s survival and fecundity is genomic stability

•Errors in DNA can occur during synthesis or post replication from environmental factors (i.e. UV radiation)

•This is a major problem because errors lead to mutations, which can lead to a variety of problems including cancer

•Organisms have multiple repair systems•Direct Reversal•Base Excision•Recombination•Nucleotide Excision•Mismatch Repair

Mismatch Repair (MMR)Mismatch Repair (MMR)

MMR is responsible for locating and MMR is responsible for locating and removing mismatched base pairsremoving mismatched base pairs

Highly conserved throughout evolutionHighly conserved throughout evolution

Increases genomic stability 100 to 1,000 Increases genomic stability 100 to 1,000 foldfold

Lack of MMR has been linked to several Lack of MMR has been linked to several forms of cancerforms of cancer

Recognition of Daughter Strand Recognition of Daughter Strand

MMR telling the difference MMR telling the difference between daughter and parent between daughter and parent stands is critical in repairstands is critical in repair

In prokaryotes MMR recognizes In prokaryotes MMR recognizes the difference between the two the difference between the two strands by looking for the strands by looking for the unmethylated strandunmethylated strand

A stand is methylated shortly A stand is methylated shortly following replication, thus the following replication, thus the unmethylated strand is the unmethylated strand is the daughter standdaughter stand

Prokaryotic MMRProkaryotic MMR

Mismatch is recognizedMismatch is recognizedMutS binds to DNA at MutS binds to DNA at mismatch sitemismatch siteMutL recruited by MutSMutL recruited by MutSMutL acts as matchmaker MutL acts as matchmaker between MutS and MutHbetween MutS and MutHRecognizes correct strand Recognizes correct strand because it is unmethylated because it is unmethylated MutH creates a nick 5’ to the MutH creates a nick 5’ to the unmethylated GATC siteunmethylated GATC siteExonuclease degrades Exonuclease degrades section of the strand section of the strand containing mismatchcontaining mismatchGap in DNA is resynthesizedGap in DNA is resynthesized

http://http://cmgm.stanford.edu/biochem201/Slides/DNA%20Repair/24%20mismatch%20rcmgm.stanford.edu/biochem201/Slides/DNA%20Repair/24%20mismatch%20r

Eukaryotic MMREukaryotic MMR

Instead of MutS, MutL, and MutH, there Instead of MutS, MutL, and MutH, there are several MutS and MutL homologuesare several MutS and MutL homologues

MutS and MutL homologues exist in MutS and MutL homologues exist in heterodimersheterodimers

Arabidopsis thaliana Arabidopsis thaliana

Investigation Investigation of the MLH3 of the MLH3 homologue homologue found in found in Arabidopsis Arabidopsis plants plants

MLH3 ???MLH3 ???

Unknown function of Unknown function of MLH3MLH3Use MLH3 knockout Use MLH3 knockout mutants to look for mutants to look for irregularities in plants irregularities in plants lacking this part of lacking this part of MMRMMRPossible problems Possible problems with meiosis? with meiosis? Sterility? Phenotypic Sterility? Phenotypic changes?changes?

Current Experiments UnderwayCurrent Experiments Underway

Look for MLH3 knockout homozygous Look for MLH3 knockout homozygous lines lines

Identify homozygotes through genotyping Identify homozygotes through genotyping

Confirm through MSI assaysConfirm through MSI assays

PCR using primers LBa1 PCR using primers LBa1 and MLH3-Rand MLH3-R

PCR using primers MLH3-F PCR using primers MLH3-F and MLH3-Rand MLH3-R

Genotyping the PlantsGenotyping the Plants

MMR Correction of Slip-MispairingMMR Correction of Slip-Mispairing

ATNNNATATAT ATATATNNNTATATA TATATATATATANNN

NNNATATATATATATNNNTATATATATATATATATANNN

NNNATATAT ATATATNNNTATATA TATATATATATANNN TA

+2 base insertion

no insertion or deletion

-2 base deletion

Replication

MMRMMR

MMRMMR

Microsatellite Instability (MSI)Microsatellite Instability (MSI)

• PCR was run using primers specific for microsatellite sites

• PCR products were further examined by the ABI 3100 capillary electrophoresis analyzer

• ABI prism software was used to interpret data

Relative MSI Frequency in Various MMR Defective Lines

0

5

10

15

20

25

30

Unique

Relative frequency of shifts

wt avg. MSH2-#1 RNAi MLH1- MLH3- PMS2-

This work was supported by a grant from This work was supported by a grant from the Howard Hughes Medical Institutethe Howard Hughes Medical Institute

As well as in part by the Dr. John Hays’ As well as in part by the Dr. John Hays’ lablab

Funding of the Project

AcknowledgementsAcknowledgements

Dr. John Hays for acting as a mentor Dr. John Hays for acting as a mentor throughout the research processthroughout the research process

Adela Torres, Peter Hoffman, Stephanie Adela Torres, Peter Hoffman, Stephanie Bollmann, and Dr. Huixian Wang for Bollmann, and Dr. Huixian Wang for training and guidancetraining and guidance