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MutS/MSH2

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MutS/MSH2 & HNPCC MMR & Hereditary Non-polyposis Colorectal Cancer MMR (Mismatch Repair) Originally identified in bacteria: inactivation ncrease in the rate of ... – PowerPoint PPT presentation

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Title: MutS/MSH2


1
MutS/MSH2 HNPCC
  • MMR Hereditary Non-polyposis Colorectal Cancer

2
MMR (Mismatch Repair)
  • Originally identified in bacteria
  • inactivation?? ?ncrease in the rate of
    spontaneous mutations due to inability to repair
    replication errors.
  • Importance realized in early 1990s w/ discovery
    inactivation of primary cause of
  • corresponding human ? Hereditary
    non-polyposis
  • pathway colorectal
    cancers

Sandrine Jacob, Francoise Praz. Biochemie 83
(2002) 27-47
3
HNPCC
  • Hereditary
  • Non-Polyposis
  • Colorectal
  • Cancer

www.endoskopischer-atlas.de/ k67e.htm
4
Discovery of HNPCC
Sandrine Jacob, Francoise Praz. Biochemie 83
(2002) 27-47
  • Study kindreds using linkage analysis
  • etiology of CRC (colorectal cancer)
  • map loci on chromosome
    2p21-22 and 3p21.

  • MutS MutL

  • hMSH2 hMLH1
  • Identified regions of Loss-of-Heterozygosity
    (LOH) in tumors by
  • ? MicroSatellite Instability
    (MSI)?
  • cause of MSI- mutator phenotype exhibited by
    the MMR-deficient bacteria


  • yeast.
  • Led to identification hMSH2 and hMLH1

5
Key Point
Sandrine Jacob, Francoise Praz. Biochemie 83
(2002) 27-47 The Cleveland Clinic
http//www.clevelandclinic.org/registries/inherite
d/hnpcc.htm
  • MSI ? in ? 90 of the HNPCC tumors
  • ? also in 15-20 of the sporadic colon
    cancers.
  • MSI is not only confined to the proximal colon.
  • Exhibited in 10-45 of
  • Pancreatic
  • Gastric
  • Breast
  • Ovarian
  • small-cell lung cancers

6
MutS/MSH2s NORMAL role in Cancer (HNPCC) is to
prevent MSI from occurring
  • Recognize damaged bases by MMR
  • initiate signal transduction pathway
  • trigger
    apoptosis.
  • (cell cycle checkpoint)
  • - p53
    dependent
  • - p53 independent

Sandrine Jacob, Francoise Praz. Biochemie 83
(2002) 27-47
7
How does this (MMR) work?
8
DNA MMR in E. Coli
  • MMR is mediated by 3 specialized proteins MutS,
    MutL, and MutH
  • MutS - recognition of the base/base mismatches
    and small insertion or deletion loops (IDL)
  • MutL - stimulates the endonuclease activity of
    MutH when engaged in a complex with MutS
  • MutH - plays the final role in MMR, as we talked
    about previously in this class, by acting as an
    endonuclease to nick the nascent strand

Sandrine Jacob, Francoise Praz. Biochemie 83
(2002) 27-47
9
  • Molecular cell biology / Harvey Lodish ... et
    al.. 4th ed. New York W.H. Freeman, c2000.

10
Important Difference b/w E. Coli and Yeast/ Humans
  • Signal that allows discrimination b/w the
    template and newly synthesized strand is still
    doubtful, but it DOES NOT involve DNA
    methylation.

Sandrine Jacob, Francoise Praz. Biochemie 83
(2002) 27-47
  • Molecular cell biology / Harvey Lodish ... et
    al.. 4th ed. New York W.H. Freeman, c2000.

11
C. Richard Boland http//www.hosppract.com/geneti
cs/9711gen.htm
12
Role of MutS in higher organisms
  • Mammals
  • 5 homologs of MutS (MSH2-MSH6)
  • 4 homologs of MutL (MLH1-MLH3 and PMS1)
  • Each of these play specialized or partly
    redundant fxns in yeast and mammals.
  • One to remember MSH2, the MutS homolog
  • However, MSH2, MSH3, and MSH6 all
    function in
  • stability of nuclear
    DNA.
  • ltHow does this work?gt

Sandrine Jacob, Francoise Praz. Biochemie 83
(2002) 27-47
13
MSH2 has ability to form heteroduplexes w/ both
MSH6 MSH3, forming MutS? and MutS?,
respectively.
?
MSH2 MSH6
MutS
MSH2 MSH3
MutS
?
  • MSH2 plays different roles depending on which
    protein it is associated with.

Sandrine Jacob, Francoise Praz. Biochemie 83
(2002) 27-47
14
  • MutS? - binds most base-base mismatches (
    except CC)
  • - loops of one or a few nucleotides.
  • MutS? - repairs heteroduplexes w/ ? 2
    extrahelical bases.
  • - Acts w/ MHL1 MHL3 in yeast to
    repair IDLs,
  • ? freq. Of frameshift mutations
  • HOWEVER, MutS? is the main instigator of mismatch
    recognition, and is also expressed at higher
    levels than MutS?.
  •  
  • Both
  • Repair insertion/deletions (?1 or 2 bp,
    respectively)
  • Repair mismatches in recombination intermediates.
  • Inhibit recombination b/w divergent sequences.

Sandrine Jacob, Francoise Praz. Biochemie 83
(2002) 27-47
15
As a result, the DNA-synthesizing enzymes copy a
region of the template strand a second time,
leading to an increase in length of nine
nucleotides (yellow) in this example.
16
MSH2 Mutant Phenotypes
  • Replication ERror" (RER), or MicroSatellite
    Instability (MSI)
  • already
    discussed !
  • 2) Tolerance to events that would normally
    cause a cell to commit to apoptosis.
  • ex. alkylating agent exposure
  • low ionizing radiation/ chronic oxidative
    stress
  • temozolomide (methylating agent)

17
MSH2 Knockout Mice Tolerance Phenotype
18
Tolerance reluctance to apoptosis
Mouse ES Cells
Mouse ES Cells
Theodore L. DeWeese. PNAS Vol. 95, No. 20 Sep.
29, 1998
19
HNPCC Characteristics (NEXT FEW SLIDES)
  • Inherited in autosomal dominant fashion

20
Cont
  • 1)      MSH2 tumor suppressor. 2 hit model
    to induce phenotypic effect
  • 2)  Inherited predisposition to early-onset CRC
    and extracolonic epithelial-derived tumors
  • gastrointestinal and
    urogenital tracts.
  • 3)  5 of all CRC assoc. w/ germline mutations
    of one of the MMR genes hMSH2 or hMLH1.
  • 4) VERY HIGH gene penetrance lifetime risk ?
    80
  • 5) HNPCC patients
  • - diagnosed before age 50 (45)
  • - individuals dev. tumors 2-3
    decades earlier than
  • individuals w/ nonfamilial
    sporadic colon cancer.

Sandrine Jacob, Francoise Praz. Biochemie 83
(2002) 27-47
21
And.a few more!
  • 6) hMSH2 mutations 40 of the mutations
    detected in HNPCC hMLH1 account for about 55
  • 7) Large genomic deletions in hMSH2 cause
    truncating mutations ?? a frequent cause of
    HNPCC.
  • 9) In HNPCC tumors w/ hMSH2 mutation,
  • SOMATIC MUTATIONS ? LOH at the MSH2 locus,
    which occur in 10 of tumors.
  • 10) Sporadic MSI tumors are caused by
    inactivation of
  • hMLH1 in 90 of cases.

Sandrine Jacob, Francoise Praz. Biochemie 83
(2002) 27-47
22
Detection
  • Blood test to test patients for the identified
    genes, hMSH2 or hMLH1.
  • However, any HNPCC family member who has
    previously had CRC first must be used to identify
    the affected gene, and some people with HNPCC
    will not have abnormalities of these genes.

Thomas Jefferson Univ Kimmel Cancer Center
www.kcc.tju.edu/hereditarycancer/ HNPCC/hnpcc.htm
The Cleveland Clinic http//www.clevelandclinic.or
g/registries/inherited/hnpcc.htm
23
CRC is nearly 100 curable if diagnosed within
the early stages.
  • Men Women
  • Full colonoscopy every year beginning
  • at age 25
  • Or 5 - 10 years before earliest cancer diagnosed
    in the family.
  • Women
  • Annual screening for endometrial cancer
  • beginning at age 25 - 35.

Thomas Jefferson Univ Kimmel Cancer Center
www.kcc.tju.edu/hereditarycancer/ HNPCC/hnpcc.htm
24
Treatment
  • Surgery removal of portion of the intestine
    that contains the cancer.
  • Individuals with HNPCC have VERY HIGH risk of
    developing a 2nd colon cancer
  • entire colon must be removed, and the healthy
    small intestine be connected to the rectum.

Thomas Jefferson Univ Kimmel Cancer Center
www.kcc.tju.edu/hereditarycancer/ HNPCC/hnpcc.htm

The Cleveland Clinic http//www.clevelandclinic.or
g/registries/inherited/hnpcc.htm
25
Questions?
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