1. Bonny Blackard Biology 169 April 4, 2006
MSH2
Bonny Blackard
Biology 169
April 4, 2006

2. Outline Introduction to MSH2 Normal cell biological role
Normal biochemical role
Normal biological role
HNPCC
MSH2’s connection to cancer
Current implications

3. What is MSH2? mutS homolog 2 Gene: Protein: Kolodner (1993) 80 kb
Chromosome 2
Category: caretaker
Protein:
Nuclear localization
Part of the MMR process
Called mutS homolog 2 because it is the human homolog of the E. coli mismatch repair gene mutS; Kolodner cloned it and also linked it to HNPCC
Diagram of MSH2 gene – 16 exons; 80 kilobases;
MMR = mismatch repair process

4. Normal cell biological role
What is DNA MMR?
Why is it important?
Consequences: good and bad
What are its responsibilities?
IDLs and MSI
Checkpoints and apoptosis
MSH2’s role in MMR
DNA MisMatch Repair. Process that corrects mismatches generated during DNA replication
Consequences of not having it are either beneficial or detrimental. Increased mutation rate provides genetic variation in microbes; Initiation and promotion of multistage carcinogenesis in mammals
correcting base substitution mismatches and insertion-deletion mismatches (IDLs) generated during DNA replication.
Guard against microsatellite instability; Microsatellites = long repetitive DNA sequences prone to inaccurate replication because of strand slippage and inefficient proofreading; biomarker for lost of MMR activity in tumor cells.
Recognize certain DNA lesions thereby activating cell cycle checkpoints and apoptosis; loss of these functions decreases apoptosis, increases cell survival and results in resistance to chemotherapy.
4. Involved in the recognition of mismatched bases and the initiation of MMR

5. Normal biochemical role - Overview
There are 6 MutS homologs in yeast (MSH1-MSH6)
Initiation of MMR in eukaryotes requires MSH2, MSH3 and MSH6
These MutS homologs form heterodimeric complexes: MSH2-MSH6 (MutS-alpha) and MSH2-MSH3 (MutS-beta); MutS-alpha complex recognizes base-base mispairs and single-base insertion/deletion loops (IDLs). MutS-beta complex recognizes larger IDLs of 2-4 bases.
recruitment of MLH1-PMS2 activates downstream repair events facilitating excision, resynthesis and ligation
PCNA (proliferating cellular nuclear antigen) functions in recruiting to mismatch and along with EXO1, functions in excision.

6. Normal biochemical role – more detailed
How mismatch recognition occurs is best seen by examining crystal structures of E. coli and Taq (Thermus aquaticus) MutS, which are similar.
Structure of Taq MutS homodimer – has 5 domains
Taq MutS domain interaction with DNA. Can see how there is one base IDL mismatch and is unbound so it is flipped out and can be recognized as a mistake
Eukaryotic MutS proteins have five homologous domains plus an N-terminal region with a motif for interaction with PCNA

7. Normal biological role – mouse models
MSH2-/-
Normal and fertile
Decreased survival
MSH2+/-
normal
MSH2-deficient
2. Heterozygous mutant mice were indistinguishable from the wild-type mice. Small proportion developed tumors in their 2nd year.
3. 50% of the homozygous mutant mice died before 6 months of age and all were dead by 12 months; Early lethality because predisposed to tumor phenotype – developed T-cell lymphomas, adenomas and adenomacarcinomas of the small intestine, sebaceous gland tumors; tissues displayed high mutation frequencies.
4. Chimeric mouse - generated a cell line to identify components of the MMR system using genetic screens in culture.

8. HNPCC What is HNPCC? Most common cause of hereditary colon cancer
Autosomal dominant
inheritance
Still develop
polyps
1. Hereditary nonpolyposis colorectal cancer also known as Lynch Syndrome
2. (5-8%); it is not a form of cancer, it is only a syndrome that puts people at high risk for colon cancer
3. characterized by: childhood haematological malignancies – malignant transformation of bone marrow derived cells, brain tumors, café au lait patches, other cancers
4. still develop colon polyps as a precursor to colon cancer. Scientists use this term to distinguish HNPCC, which causes a small number of polyps to develop, from familial adenomatous polyposis syndrome, in which thousands of polyps develop.
5. Lynch Syndrome Cancer Risks Compared to the General Population

9. MSH2 and Cancer HNPCC – heterozygous germline mutation in MMR gene
Loss of w.t. MMR
allele in HNPCC
tumors
RERs
MSI
Involved in other
cancers
The wt MMR allele is lost by a somatic event in HNPCC tumors – rapidly acquire genomic mutations
this molecular mutator phenotype is called Replication Errors in Repeats (RERs);
microsatellite instability in many HNPCC cases because of mutations in MSH2
15 to 25% of sporadic colorectal cancers and other types of cancers are mismatch repair defective.

10. Detection and Treatment
Genetic testing
The Amsterdam II Criteria
Specific diet
Early exams
Colonoscopies
Screening for other
cancers
researchers have developed an experimental genetic test for defects in MSH2, the test predicts only the likelihood of developing colon cancer fostered by the MSH2 gene, and not from other causes
People found to carry an altered gene can be counseled to adopt a high-fiber, low-fat diet in the hope of preventing cancer.
start colonoscopies at about age 20/every 2 years; annually after age 40. Such examinations should help physicians to detect any precancerous growths on the colon so that they can remove them before the growths turn malignant; women should have regular screening for endometrial cancer

11. To Review What is DNA MMR? Why is it important?
What is MSH2’s role in MMR?
What happens when you make a MSH2 deficient mouse? What does this tell us about MSH2’s role in an organism?
What is the genotype of a person with HNPCC? What about a tumor that person has?
How is MSH2 connected to cancer?
Process that corrects mismatches generated during DNA replication; there are many reasons why its important but primarly because it maintains the integrity of the genome.
MSH2 forms a complex with either MSH3 or MSH6 to initiate MMR
They are viable, however they die within a year because of their predisposition to cancer; MSH2 is not required for development but is required for survival
heterozygous for a loss-of-function mutation in the patients and homozygous for a loss-of-function mutation in the tumors
Loss of both functional copies results in an accumulation of mutations and can eventually lead to cancer

12. …among my image searches for MSH2

13. References
(Giaever G, et al. (2002) Functional profiling of the S. cerevisiae genome. Nature 418 (6896): )
(Wei, et al (2002) Mouse models for human DNA mismatch-repair gene defects. Trends in Molecular Medicine Vol. 8 No.7)
(Kunkel TA, et al. (2004) DNA Mismatch Repair. Annual Review Biochemistry)
(Peltomaki. (2001) Deficient DNA mismatch repair: a common etiologic factor for colon cancer. Human Molecular Genetics. Vol. 10, No. 7)
(de Vos M, et al. (2005) Phenotype associated with recessively inherited mutations in DNA mismatch repair MMR genes. Biochem Soc Trans (Pt 4):718-20)
(Abdel-Rahman WM, et al (2006) The genetics of HNPCC: Application to diagnosis and screening. Crit Rev Oncol Hematol.