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Types of DNA Mutations

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Substitution mutations: one base pair for another, e.g. T for G. the most common form of mutation ... benzo[a]pyrene-dG adducts, cisplatin-DNA cross-links) ... – PowerPoint PPT presentation

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Title: Types of DNA Mutations


1
DNA Damage, Mutations, and Repair
See Stryer p. 768-773
2
DNA Mutations
1. Substitution mutations one base pair for
another, e.g. T for G the most common form
of mutation transitions purine to purine and
pyrimidine to pyrimidine
  • transversions purine to pyrimidine or
    pyrimidine to purine
  • Frameshift mutations
  • Deletion of one or more base pairs
  • Insertion of one or more base pairs

3
Spontaneous mutations due to DNA polymerase
errors
Very low rate of misincorporation (1 per 108 -
1 per 1010) Errors can occur due to the
presence of minor tautomers of nucleobases.
NH
N
N
N
HN
O
N
N
C
Rare imino tautomer of A
amino
10-4
Normal base pairing
Mispairing
4
Consider misincorporation due to a rare tautomer
of A
2nd replication
A T
1st replication
A(imino) C
5
A T
A(imino) T
G C
3
A T
Normal replication
Final result A ? G transition (same as T ? C in
the other strand)
5
Induced mutations result from DNA damage
  • Sources of DNA damage endogenous
  • Deamination
  • 2. Depurination 2,000 - 10,000 lesions/cell/day
  • 3. Oxidative stress 10,000 lesions/cell/day
  • Sources of DNA damage environmental
  • 1. Alkylating agents
  • 2. X-ray
  • 3. Dietary carcinogens
  • 4. UV light
  • 5. Smoking

6
Normal base pairing in DNA and an example of
mispairing via chemically modified nucleobase
G ?A
G
C
7
DNA oxidation
Reactive oxygen species HO, H2O2, 1O2, LOO
  • 10,000 oxidative lesions/cell/day in humans

8
Deamination
N
H
O
2
N
N
N
N
H
H
y
p
o
x
a
n
t
h
i
n
e
N
N
N
N
A
O
O
N
N
N
H
N
H
X
a
n
t
h
i
n
e
N
N
N
O
N
N
H
2
G
H
N
H
O
2
N
N
H
U
r
a
c
i
l
N
O
N
O
C
C
Rates increased by the presence of NO (nitric
oxide)
9
Depurination to abasic sites
2,000 10,000/cell/day
10
UV light-induced DNA Damage
CC
Pyrimidine dimer
Easily bypassed by Pol ? (eta) in an error-free
manner
11
Deletions and insertions can be caused by
intercalating agents
Stryer Fig. 27.44
12
Importance of DNA Repair DNA is the only
biological macromolecule that is repaired. All
others are replaced. More than 100 genes are
required for DNA repair, even in organisms with
very small genomes. Cancer is a consequence of
inadequate DNA repair.
13
DNA Repair Types
  • Direct repair
  • Alkylguanine transferase
  • Photolyase
  • Excision repair
  • Base excision repair
  • Nucleotide excision repair
  • Mismatch repair
  • Recombination repair

14
Direct repair O6-alkylguanine DNA
alkyltransferase (AGT)
  • Directly repaires O6-alkylguanines (e.g.
    O6-Me-dG, O6-Bz-dG)
  • In a stoichiometric reaction, the O6 alkyl group
    is transferred to a Cys residue in the active
    site. The protein is inactivated and degraded.

15
  • Excision Repair
  • Takes advantage of the double-stranded (double
    information) nature of the DNA molecule.
  • Four major steps
  • Recognize damage.
  • Remove damage by excising part of one DNA strand.
  • The resulting gap is filled using the intact
    strand as the template.
  • Ligate the nick.

16
Antiparallel DNA Strands contain the same genetic
information
5'
5'
5'
3'
3'
3'
A
T
A
T
A
T
G
C
G
C
G
T
A
T
A
T
A
5'
5'
5'
3'
3'
3'
Original DNA duplex
DNA duplex with one of the nucleotides removed
Repaired DNA duplex
17
Base excision repair (BER)
  • Used for repair of small damaged bases in DNA (AP
    sites, methylated bases, oxidized bases)
  • Human BER gene hogg1 is frequently deleted in
    lung cancer

18
Nucleotide Excision Repair
  • Corrects any damage that both distorts the DNA
    molecule and
  • alters the chemistry of the DNA molecule
    (pyrimidine dimers,
  • benzoapyrene-dG adducts, cisplatin-DNA
    cross-links).
  • Xeroderma pigmentosum is a genetic disorder
    resulting
  • in defective NER

19
Mismatch Repair Enzymes
  • Nucleotide mismatches can be corrected after DNA
    synthesis!
  • Repair of nucleotide mismatches
  • Recognize parental DNA strand (correct base) and
    daughter
  • strand (incorrect base)
  • Parental strand is methylated
  • 2. Replace a portion of the strand containing
    erroneous nucleotide
  • (between the mismatch and a nearby methylated
    site up to 1000 nt)

20
Genetic diseases associated with defective DNA
repair
Xeroderma Pigmentosum NER Hereditary
nonpolyposis MMR colorectal
cancer Cockraynes syndrome NER Falconis
anemia DNA ligase Blooms syndrome BER,
ligase Lung cancer (?) BER
21
DNA Repair Types
  • Direct repair
  • Alkylguanine transferase
  • Photolyase
  • Excision repair
  • Base excision repair
  • Nucleotide excision repair
  • Mismatch repair
  • Recombination repair

22
Direct repair O6-alkylguanine DNA
alkyltransferase (AGT)
  • Directly repaires O6-alkylguanines (e.g.
    O6-Me-dG, O6-Bz-dG)
  • In a stoichiometric reaction, the O6 alkyl group
    is transferred to a Cys residue in the active
    site. The protein is inactivated and degraded.

23
AGT inhibitor O6-benzylguanine is in clinical
trials to be used in conjunction with antitumor
alkylnitrosoureas
24
AGT overexpression in tumors makes them resistant
to alkylnitrosoureas
25
Combination therapy with O6-benzylguanine
overcomes tumor resistance to alkylnitrosoureas
26
  • Excision Repair
  • Takes advantage of the double-stranded (double
    information) nature of the DNA molecule.
  • Four major steps
  • Recognize damage.
  • Remove damage by excising part of one DNA strand.
  • The resulting gap is filled using the intact
    strand as the template.
  • Ligate the nick.

27
Antiparallel DNA Strands contain the same genetic
information
5'
5'
5'
3'
3'
3'
A
T
A
T
A
T
G
C
G
C
G
T
A
T
A
T
A
5'
5'
5'
3'
3'
3'
Original DNA duplex
DNA duplex with one of the nucleotides removed
Repaired DNA duplex
28
Base excision repair (BER)
  • Used for repair of small damaged bases in DNA (AP
    sites, methylated bases, deaminated bases,
    oxidized bases)
  • Human BER gene hogg1 is frequently deleted in
    lung cancer

O
N
N
H
N
N
O
H
29
Uracil DNA glycosylase removes deaminated C
No Me group
N
H
O
2
BER
C
N
N
H
N
O
N
O
Cytosine
Not normally present in DNA
U
r
a
c
i
l
Normal DNA base Not recognized by BER
30
Nucleotide Excision Repair
  • Corrects any damage that both distorts the DNA
    molecule and
  • alters the chemistry of the DNA molecule
    (pyrimidine dimers,
  • benzoapyrene-dG adducts, cisplatin-DNA
    cross-links).
  • Xeroderma pigmentosum is a genetic disorder
    resulting
  • in defective NER

31
Mismatch Repair Enzymes
  • Nucleotide mismatches can be corrected after DNA
    synthesis!
  • Repair of nucleotide mismatches
  • Recognize parental DNA strand (correct base) and
    daughter
  • strand (incorrect base)
  • Parental strand is methylated
  • 2. Replace a portion of the strand containing
    erroneous nucleotide
  • (between the mismatch and a nearby methylated
    site up to 1000 nt)

G T
32
Genetic diseases associated with defective DNA
repair
Xeroderma Pigmentosum NER Hereditary
nonpolyposis MMR colorectal
cancer Cockraynes syndrome NER Falconis
anemia DNA ligase Blooms syndrome BER,
ligase Lung cancer (?) BER
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