Transcript MSH2 and Human Nonpolyposis Colon Cancer

MSH2 and Human
Nonpolyposis Colon
Cancer
Yael Aschner
Presentation Outline
What is MMR?
 What is MSH2?
 What is HNPCC?

DNA Mismatch Repair (MMR)
Identified in bacteria
 Mechanism for fixing errors in DNA
replication
 The frequency of mutations introduced by
DNA polymerase during DNA replication is
1/1012 base pairs; of these, 99.9% are
repaired by MMR.

MMR in E. coli
Three proteins
 MutS
 MutL
 MutH
MMR in E. coli
MutS performs first step of MMR:
recognition of base/base mismatches and
small insertion or deletion loops (IDL)
 ATPase acting as an asymmetric
homodimer
 Translocates along DNA and promotes
DNA loop formation

MMR in E. coli
MutS undergoes conformation change and
interacts with MutL in the presence of ATP
 MutL stimulates MutH endonuclease
activity, which removes newly replicated
DNA fragment with mismatch
 DNA polymerase fills in the gap

MMR in E. coli
MutS Homolog 2 (MSH2)





Human homolog of E. coli MutS
Chromosme 2p
15 exons
Encodes a 934 amino acid protein
Involved in the recognition of
mismatched bases and the initiation
of the MMR reaction
E. coli MutS Crystal Structure
The MutS monomer
(green) recognizes
the G:T mismatch
in the DNA (red)
and has ADP bound
(at the bottom of
the figure in red).
The monomer in
blue provides the
non-specific DNA
binding domains
and an empty
ATPase domain
Eukaryotic MMR

Highly conserved
 No

MutH homolog
Six homologs of MutS (MSH1-MSH6)
 Three
are needed for initiation of the repair
process: MSH2, MSH3, MSH6

MSH2 forms a heteroduplex with MSH6 or
MSH3, forming MutSa and MutSb,
respectively
MutSa

MutSa can bind to most base-base
mismatches and loops of one or a few
nucleotides
 Present
at higher concentrations
 Primary mismatch recognition factor
 Responsible for most mismatch recognition
MutSb

MutSb can bind to IDLs with up to ~8
unpaired nucleotides
 Not
involved in base-base mismatches
 Backup system
Some overlap between the complexes
 MSH2 acts as a scaffold while MSH3 and
MSH6 provide specific binding

Mismatch Recognition
MSH2 Inactivation

Inability to repair base-base mismatches
and small nucleotide insertions and
deletions that occur as the result of
misalignment or erroneous DNA
replication

Acts as a TS gene  CANCER
Human Nonpolyposis Colon
Cancer (HNPCC)


Associated with deficiency in MMR
Syndrome characterized by familial predisposition to
colorectal carcinoma and extracolonic cancers


Most common hereditary colorectal cancer syndrome

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2%-5% of all colon cancers in Western population
Very high penetrance (~85%)
Early onset


Gastrointestinal, urological, female reproductive tracts
42 years
Autosomal dominant inheritance (cellular level recessive)
Treatment: removal of all or most of the colon
Colon Adenoma of the Large
Intestine


Precursor polyps (or
adenomas) appears to
be no greater than the
number found in the
general population
These polyps are
thought to progress more
rapidly to cancer and
form at earlier ages
How is MSH2 Mutated in HNPCC?

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40% of HNPCC mutations are in MSH2 gene
Predominantly small deletions, insertions, splice
site alterations, nonsense or missense mutations
MSH2 is the most common gene to have
deletions
 Range


from one exon to whole gene
MSH2 or MLH1 mutations responsible for 50%80% of HNPCC families
20% of sporadic colorectal cancers due to MMR
mutations
HNPCC Inheritance
Individuals heterozygous for mutations
 Two Hit Hypothesis
 Functional wild type allele lost by somatic
mutation event

 MMR
inactivated
 Accumulation of mutations
Loss of Heterozygosity
Mouse Models for HNPCC
(Wei, et al)
Msh2+/- mutants indistinguishable from
wild type
 Homozygous Msh2-/- mice all died by 12
months (50% by 6 months)

 Strong
predisposition for tumor phenotype
 Cell extracts lost ability to repair single-base
mispairs and IDLs of up to four nucleotides
 Tissues displayed high mutation frequencies

Small intestinal tumors, lymphoid tumors