Molecular mechanisms of
mutagenesis, spontaneous and
induced mutations and reversions
1
Mutation - is a permanent change of the
nucleotide sequence in the genome of an
organism, virus, or extra chromosomal genetic
element.
- a process
Mutations = source of genetic variability
• heritable changes in the genetic material:
- changes at the level of individual
genes (substitutions, insertions, deletions)
- changes in the structure or number
of chromosomes
not to be mistaken with changes in genotype
(and hence the phenotype) that are the
result of new combinations of existing
genetic variants (recombination)
source of genetic variability - provide new
genetic variants that are a prerequisite for
the evolution of organisms
2
Occurrence and distribution of mutations
• in unicellular organisms, each mutation duplicates in
replication and passes to the next cell generation
• in multicellular organisms, mutations can be
transmitted to offspring only when they appear in
the genome of cells of the germ line
• mutations in the DNA of somatic cells occur only in
the progeny of these cells (population of cells that
are genetically different from the rest of the
body)
• Classification of mutations:
• gametic: in germ line cells, may cause hereditary
diseases
• somatic: in somatic cells, may cause cancer
3
4
Diploid state protects cells against the
harmful effects of mutations
• diploid organism: each gene has two copies
• when one becomes damaged, the other can still
provide the correct gene / protein - recessive mutation,
preventing the defect (that is if the mutation is not
dominant)
• estimate: each person carries so many harmful
mutations that would in haploid configuration cause
lethality about eight times
• people today are genetically different from their
predecessors, due to the accumulation of mutations
over the centuries
5
6
• Monoallelic expression can be best understood in comparison to biallelic
expression, the more prevalent form of gene expression (Figure
1). In most cases, both alleles of a gene are transcribed; this is known as
bi-allelic expression. However, a minority of genes show monoallelic
expression. In these cases, only one allele of a gene is expressed. Which
one of the two alleles is expressed may be determined by the parental
origin of the allele (such as in imprinting), or the choice may be random.
7
Mutations – from
genes to
chromosomes
• affect the structure of genes (and their products) or
regulatory regions of DNA
• change the structure of chromosomes (chromosome
aberrations: duplications, deletions, inversions, translocations)
• variations in chromosome numbers:
• aneuploidy – a particular chromosome undergoes a change in its
number- monosomy, trisomy - eg. Down syndrome
• euploidy – a variation in which the chromosomes are present in
a abnormal whole-number multiple of a haploid genome haploidy,
polyploidy)
8
9
10
Are mutations are useful or harmful?
• often harmful because they adversely affect the
function of the gene product and thereby damage
the cells
• altered product may be not only protein but also
RNA (tRNA, rRNA, etc.)
• mutations can damage also the non-coding, but
important signal sequences
• most mutations have no significant impact on the
survival of the organism - they are neutral
• rarely, mutation can have a positive effect on the
survival and reproduction of the organism
• accumulations of these beneficial mutations enable
the development of an organism in a changing
environment
11
Expression of Wild-type and Mutant Alleles
12
13
• Sickle-cell anemia - haemoglobin S
• Mutation in allele HBBA led to the emergence of allele HBSS, the substitution of one T: A
nucleotide pair to A: T
• Phenylketonuria
The best studied hereditary disease in phenylalanine-tyrosine
metabolism is phenylketonuria. An autosomal recessive disease caused
by the lack of phenylalanine hydroxylase which converts phenylalanine
to tyrosine. Newborns affected by phenylketonuria should have a strict
diet restricting phenylalanine intake, otherwise they may develop severe
mental retardation
• Albinism
Albinism, a disorder caused by a lack of pigmentation in the skin, hair
and eyes, is the result of mutations blocking the conversion of tyrosine
to dark pigment melanin
14
Types of mutations
• point mutation: substitution of a single base
• null mutation: complete loss of gene function
• tight mutation has a clear phenotype (eg. full
loss of ability to grow under certain conditions or prevent formation of
product of the biochemical pathway, if the mutation knocks out an enzyme)
• leaky mutation: partial activity of the gene product is maintained (eg. the
residual activity of the enzyme allows at least slow growth under certain
circumstances)
• direct/forward mutation - standard allele turns into mutant allele
• reverse mutation (reversion) - mutant allele changes back into standard
• suppressor mutation - a second mutation, which offsets the effects of the
first direct mutation
15
16
• Mutation: A Reversible
Process. Forward mutation—
mutation of a wild- type
allele to a mutant allele.
Reverse mutation
(reversion)—a second
mutation that restores the
original phenotype. –Back
mutation—a second mutation
at the same site. Suppressor
mutation—a second mutation
at a different location in the
genome.
mutations - classification
• substitution: substitution of bases
• inversion: a piece of DNA is inverted, remains in the same place
• duplication: a piece of DNA is duplicated, the second copy usually
remains just next to the original
• deletion: one or more bases are removed
• insertion: one or more bases are added
• translocation: stretch of DNA is transferred from its original location
to another location - either the same or different DNA molecule
17
Base substitution
• transition: pyrimidine
is replaced by another
pyrimidine (T for C
and vice versa) or the
purine is replaced by
another purine (A for
G and vice versa)
• transversion:
pyrimidine is replaced
by a purine or vice
versa
18
Substitution mutation outcomes
• missense mutation:
TGG (Trp) -> CGG (Arg)
• nonsense mutation – causes premature termination
by a stop codon:
TGG (Trp) -> TAG (Term.)
• neutral mutations - changes are not reflected in the
function:
AAA (Lys) -> AGA (Arg)
• silent mutation - different codon, but the same AA:
AAA (Lys) -> AAG (Lys)
19
Severity of mutation depends on their type and
location
• missense mutations are the most common - lead to the
replacement of one amino acid in a protein for another
• when the original amino acid is replaced by a chemically related usually
not serious consequences (conservative substitution)
• the consequences are serious when the protein folding or the
structure of the protein active site are affected (radical
replacement)
20
Nonsense mutations
• mutations, in which the codons encoding amino acids are
changed to meaningless codons - terminator (UAA, UAG,
UGA in RNA)
• premature termination of the polypeptide chain synthesis
• truncated polypeptide is not folded correctly
• usually undergoes degradation
21
• eg. temperature-sensitive
mutants will grow at one
temperature but not at
another.
• Suppressor-sensitive
mutants are viable only
when a second genetic
factor, a suppressor, is
present.
22
Some mutations are lethal only
under specific conditions
Example of a temperature
sensitive mutation
23
The albino mutation in Siamese cats results in a defective form of tyrosinase
which does not function at normal body temperature. Therefore, dark
coloration can only appear in parts of the body that are cooler than the core
body temperature. The extremities are always the coolest parts of the body.
The face is also cooler because of air passing through the sinuses. The back is
warmer than the extremities, being closer to the body core, but it is also
exposed. The result is a medium degree of tyrosinase function, resulting in a
medium degree of shading
Pointed cats are partial albinos. Albinism is a mutation in the
TYR gene, responsible for the enzyme tyrosinase. Tyrosinase,
in turn, is necessary for the production of melanin, or dark
pigmentation. Albinism is not a black or white trait; it has
shades of gray—literally. In cats, various mutated alleles in
TYR can result in phenotypes with different degrees of
pigmentation. Each of these phenotypes falls in a specific
place in an allelic series with a hierarchical inheritance
pattern from dominant to recessive—in other words, some of
these traits are dominant over others. The Siamese
phenotype falls in the middle of the series, recessive to
normal coloration but dominant over a completely white coat.
Transactivation Sequence-specific DNA binding
3´- 5´ Exonuclease
Oligomerization
50 150 200 250 300 350 393100
N CI II III IV V
NLS
C-terminal DNA binding
PuPuPuC(A/T)(T/A)GPyPyPy
TP53 point mutations will cause loss of p53 function as a tumor
suppressor
R175
G245
R249
R248
R273
R282
175
245
248
273
282
Cho et al. 1994
Reactions of normal and tumor cells to the oncogenic stress
wtp53 tumor suppressor
mutant p53
A.Merlo, 2004
Insertions and deletions
• Gain or loss of a DNA section
• random DNA segment incorporation into coding sequence
will usually inactivate the original sequence
• phenotypic response depends on the extent and location
(shorter insertion may allow for at least partial activity of
the original protein)
• mobile genetic elements (transposons) can be as long as
several thousand bp
26
Insertion can activate gene expression
• change to the
repressor
binding site
• change in
promoter - e.g.
conversion of
gene regulation
under the
control of the
transposon
promoter
27
Mutations altering the reading frame
• bases are read as codons (3 bases)
• the inclusion or removal of one or two bases inside the reading frame
fundamentally changes the genetic information – protein function is often lost
• three base insertion or deletion inside reading frame - reading frame does not
change drastically- the protein has 1 AA more or less, its function is usually not
significantly altered
28
Inversion
• DNA segment reversal
• coding sequence gets interrupted- loss of gene function
• when terminal sequences of the inverted portion are in
intergenic regions - gene can remain intact, if the inversion
includes promoter with the whole gene it can even be
transcribed in the reverse orientation - does not lose its
function
29
Translocation
• exclusion of a DNA segment from its original site and its
insertion to the same or another chromosome
• if the coding sequence is intact - protein function may not
be lost
• inclusion of one gene over another- loss or change in
function
30
Duplication
• stretch of DNA is duplicated and both copies remain on the
same chromosome
• the copy is usually adjacent to original (tandem duplication)
• can generate two copies of a gene - subsequent divergence
allows formation of homologous new genes during evolution
• multiple duplication (amplification) may significantly increase
the gene copy number and thus the product levels
31
Silent mutations
• do not alter phenotype
• in the noncoding intergenic regions
• introns (can not be in critical splicing sites)
• mutations not altering codon sense (e.g. glutamic acid
codons: GAA, GAG, or alanine 4 codons: GCU, GCC, GCA,
GCG = codon degeneracy)
32
Reversion = back mutation
reversion = original sequence is completely recovered
pseudoreversion = original function of the polypeptide is
restored, by a degenerate codon
intragenic suppressor mutations = restores the original
phenotype after suppressor-sensitive mutation in the same gene
occurs
intergenic suppressor mutations = arises in another gene.
Suppressor is a mutant allele of the gene - suppressing
phenotype of suppressor-sensitive mutation
33
Alleles and phenotype
Allele = gene variant
Standard allele = prevalent in the population
Mutant allele = changed by mutation
standard phenotype = standard allele is
expressed in the phenotype
mutant phenotype = expression of the mutant
allele in phenotype
34
Spontaneous and induced
mutations
Spontaneous mutations occur without apparent
external cause – resulting from metabolic disorders in
the body, mistakes in DNA replication or the presence
of unknown substances in the environment
induced mutations are caused by known physical,
chemical or biological factors capable of inducing
changes in DNA - mutagens
Mutagens can cause also neoplastic transformation -
carcinogens
1. mutagen - mutagenic directly
2. promutagen – require metabolic activation
35
How often do mutations occur?
• Spontaneous mutations occur sporadically, the actual
observed rates vary from gene to gene and between
different organisms.
• phages and bacteria: 10 -8 to 10 -10 mutations per base pair
and generation
• eukaryotes: 10 -7 - 10 -9 mutations per base pair and
generation
• The action of mutagenic substances may increase the
mutation rates by several orders.
36
KEY POINTS
• Mutations occur in both embryonic and somatic
cells but only mutations in the germ cells are
transmitted to progeny.
• Mutations can arise spontaneously or may be
induced by mutagenic substances in the
environment.
• Standard phenotype can be restored in the mutant
organism by reversions or suppressor mutations.
37
Spontaneous
mutations
tautomeric base changes
wobble base pairing
depurination and depyrimidination of bases
deamination of bases
uracil incorporation into DNA during replication
oxidative DNA damage
38
Incorrect base pairing arises in
these processes and from the
intrinsic base properties
Tautomeric base changes
• Bases in DNA are not
static. Hydrogen atoms
can move from one
position on purine or
pyrimidine to another for
example, from the
amino group to the ring.
• Such chemical
isomorphism is
referred to as
tautomerism. Although
tautomeric
rearrangements are
rare, they can have an
important role in DNA
metabolism, because
some of them are
changing the base
pairing
39
40
Tautomeric Shifts Affect Base-Pairing
Mutation caused by tautomeric shift
41
Wobble base pairing
CT,GA,TG – wobble base pairs possible
during replication
also between tRNA anticodon and the mRNA
codon
correction mechanism - DNA polymerase III
42
Depurination and depyrimidination
• glycosidic bond between the base and the sugar
can eventually be abolished and the base then
lost (phenomenon used also to repair mispaired
bases, as they can be excised and then
replaced)
• under physiological conditions in mammals daily
several thousand nucleotide bases are lost
• during replication any nucleotide can be
incorporated to the vacant slot, usually dAMP
(GC -> AT)
43
Uracil incorporation into DNA
 associated with spontaneous deamination of cytosine
 occasionally incorporated into the DNA - removed by uracil-DNA-
glycosylase
 a frequent phenomenon in human lymphocytes
 causes GC to AT transversion
44
Oxidative DNA damage
hydroxyl radical OH, acts on bases (mostly guanine)
formed from hydrogen peroxide in respiratory chain
causes transversion from GC to AT
45
Induced mutations: chemical mutagens
• influence DNA structure
• oxidizing agents (peroxides, oxygen radicals)
• deaminating substances (nitrites)
• alkylating agents (ethyl methanesulfonate, yperite)
• intercalating agents (acridines)
• aromatic amines (benzidine, naphthylamine)
• substances damaging cellular machinery for division of
genetic information during cell division – colchicine
• substances inducing mutation, regardless of the ongoing
replication of DNA (e.g., alkylating agents, nitrous acid)
• substances causing mutations only over DNA replication
(base analogues, acridine dye)
46
Mutagenic base analogues
• structurally similar to normal bases
• incorporated into the DNA during replication
• structural differences from normal bases, increase
mismatch rate, thereby promote mutations
• e.g. 5-bromouracil: thymine analogue - induces a
transition from AT to GC (different charge distribution
→ increased tautomerisation frequency to enol form,
which is then paired with guanine)
47
Acridine dyes
• e.g. proflavine, acridine orange, acridine blue
• intercalators – incorporation to DNA groove
• tighten and alter the conformation of the DNA double helix
• during replication deletions or insertions of one or more
base pairs occur- often frameshift mutations
48
Nitrous acid
• causes oxidative deamination of amino groups of adenine,
guanine and cytosine: amino groups thereby change to keto
groups
• adenine deamination - change to hypoxanthine, which is
paired with cytosine
• cytosine is deaminated to uracil, which pairs with adenine
49
Hydroxylamine
• NH2OH
• causes hydroxylation of amino group of cytosine
• resulting hydroxylaminecytosine pairs with adenine
(transition of GC to AT)
50
Alkylating agents
• Ethyl methanesulfonate, aziridine, yperite
• DNA bases methylation or ethylation, causes a change in
base-pairing
• induces all types of mutations (transitions, transversions,
frameshift mutations and chromosomal aberrations)
51
Physical mutagens
• ionizing radiation (X-ray, gamma, cosmic) induces breaks
in DNA
• non-ionizing radiation (UV) absorbed at a specific
wavelength of 260-280 nm, formation of thymine dimers
• the degree of DNA damage is equivalent to the
type and dose of radiation absorbed
52
Mutations induced by ionizing radiation
• shorter wavelength ~ higher energy than the
visible light
• penetrates deep into the tissue, strikes the
atoms, releases electrons to form positively
charged ions and radicals which give rise to other
ions (ionization process)
53
UV radiation
• lower energy than ionizing radiation
• only penetrates into the upper layers of cells, potent
mutagen in unicellular organisms
• does not cause ionization
• radiant energy is captured by atoms, electrons pass into
the excited state – increasing the reactivity of atoms and
molecules
• in DNA that leads to creation of mutations
• most mutagenic effects at 254 nm (absorption maximum
of the bases at this wavelength)
UV and pyrimidines
• After UV absorption pyrimidines that are
close react to form pyrimidine dimers and
hydrates
• thymine dimers disrupt the DNA structure
and disrupt the replication
54
Biological mutagens
• Viruses - incorporate into
the host DNA
• transposable elementstransposons:
DNA regions
which can move from one
place to another in the
genome
• Insertion of transposon
may inactivate a gene
(mutagenic)
55
DNA Repair mechanisms
56
Living organisms contain many
enzymes that scan their DNA and
initiate repair processes when
damage is detected.
DNA repair
• In cells, there are mechanisms by which cell recognizes and completely or
to some extent removes DNA damage. These repair mechanisms are
catalyzed by different sets of enzymes.
• Ability to repair damaged DNA is essential for maintaining the genome
integrity of the cell and for normal functioning of a multicellular organism.
• Tomas R. Lindahl, Paul L. Modrich, Aziz Sancar won the 2015 Nobel Prize in
Chemistry for their research on the molecular mechanisms of DNA repair.
57
Cell cycle arrest
senescence
Inhibition of transcription, replication,segregation of
chromosomes
apoptosis
Mutation, chromosome aberration
cancer, heridaty disease
Types of DNA repair:
• complete repair - repairs to the original state without
DNA synthesis
photoreactivation
direct repair of alkylated bases
• excision repair - excision of damaged sites, synthesis of
DNA
NER – nucleotide excision repair
BER – base excision repair
mismatch repair
• tolerant repair – function restoration without DNA
damage repair
SOS response
• double strand break repair
58
DNA repair mechanisms
• enzymes seek DNA damage and when it is found,
they activate any of the repair processes
existing from bacteria to humans, DIVERSITY
• mismatch repair controlled by methylation
• excision repair (base and nucleotide)
• photoreactivation-correction dependent on the
light - only in bacteria
• postreplication repair
• error-prone repair (SOS response)
• Mutation frequencies: 10-10 mutations / bp /
replication
59
E.coli: 5 mechanisms (photoreactivation, excision repair,
mismatch repair, postreplication, error-prone)
mammals: all except for photoreactivation
1A. Photoreactivation
• only in bacteria, eukaryotes, but
not in mammals
• total repair
• correction dependent on light
• removes thymine dimers, mediated
by the enzyme DNA photolyase,
which is activated by visible light
(especially blue 340-400nm)
• DNA photolyase recognizes dimers,
binds to them and cleaves covalent
cross-linking using light energy
• binding to dimers occurs in the
dark, cleavage only after activation
by light energy
• No endonuclease, no polymerase, no
DNA ligase
60
1) complete repair
1B. Direct repair of alkylated bases
• Another type of damage, methylation of guanine bases,
is directly reversed by the protein methyl guanine
methyl transferase (MGMT), the bacterial equivalent
of which is called ogt.
• This is an expensive process because each MGMT
molecule can be used only once; that is, the reaction is
stoichiometric rather than catalytic.
• A generalized response to methylating agents in
bacteria is known as the adaptive response and confers
a level of resistance to alkylating agents upon sustained
exposure by upregulation of alkylation repair enzymes.
• The third type of DNA damage reversed by cells is
certain methylation of the bases cytosine and adenine.
61
2. Excision repair
(Excision of the damaged sites, synthesis of new DNA)
• performed in three steps:
• DNA endonuclease recognizes damaged base in DNA,
it binds to it and cleaves it
• DNA polymerase fills the gap using the intact
complementary strand as a template
• DNA ligase connects strands retained by DNA
polymerase
• Base excision repair (BER) removes abnormal or
chemically modified bases from DNA
• Nucleotide excision repair (NER) removes larger
defects in DNA
62
2A Base excision repair
repair of damaged bases
(oxidation, alkylation, deamination,
removing U)
• creates apurinic or apyrimidinic sites (AP
sites)
• AP endonucleases recognize AP-space, which
they cleave - interrupt the sugar-phosphate
backbone, creating 3 OH
• sugar is cleaved off later with
phosphodiesterases (in humans APEX1,
APEX2)
• DNA polymerase replaces a missing
nucleotide by complementarity to the opposite
strand (has no proofreading activity, makes
mistakes → checked by APE1)
• Polb in eukaryotes, Pol1 in prokaryotes
• DNA ligase restores the sugar-phosphate
backbone
• Increased risk of colorectal tumors with
mutations in the Polb, DNA glycosylase 63
2B Nucleotide excision
repair (NER)
• removes DNA from
more damaged sites
that distort the double
helix, DNA adducts, UV
photoproducts
• specific endonucleases
(excision nuclease)
cleave on both sides of
the damaged section
• oligonucleotide
fragment containing
the damaged bases is
cleaved off
• gap is then filled by
DNA polymerase and
DNA ligase
64
• Nucleotide excision repair
(NER) repairs damaged DNA
which commonly consists of
bulky, helix-distorting damage,
such as pyrimidine dimerization
caused by UV light.
• Damaged regions are removed in
12-24 nucleotide-long strands in
a three-step process which
consists of recognition of
damage, excision of damaged
DNA both upstream and
downstream of damage by
endonucleases, and resynthesis
of removed DNA region.[19]
• NER is a highly evolutionarily
conserved repair mechanism
and is used in nearly all
eukaryotic and prokaryotic
cells.[19] In prokaryotes, NER is
mediated by Uvr proteins.[19] In
eukaryotes, many more proteins
are involved, although the
general strategy is the same.[19]
65
3 Correction of errors in DNA replication
mismatch repair
• repair mechanisms which repair 99% of mismatched bases
created during replication
• repair proteins find mismatches and bind to them, ensuring
removal of part of the newly synthesized strand and its
resynthesis
66
Mistake during:
repair
Mismatch repair systems are present in essentially
all cells to correct errors that are not corrected by
proofreading. These systems consist of at least two
proteins. One detects the mismatch, and the other
recruits an endonuclease that cleaves the newly
synthesized DNA strand close to the region of
damage. In E. coli , the proteins involved are the
Mut class proteins. This is followed by removal of
damaged region by an exonuclease, resynthesis by
DNA polymerase, and nick sealing by DNA ligase.[20]
Mismatch Repair in E. coli
• Mismatching or mispairing of G and T
(DNA polymerase/exonuclease
proofreading activity)
• The A in GATC sequences is methylated
subsequent to DNA replication.
• In newly replicated DNA, the parental
strand is methylated, but the new strand
is not. This difference allows the
mismatch repair system to distinguish
the new strand from the old strand.
• The mismatched nucleotide is excised
from the new strand and replaced with
the correct nucleotide, using the
methylated parental strand as a template.
67
distinguishment of a new strand with
an error from the original - E. coli
Dam methylase that methylates A
sequence GATC, just after the
replication hemimethylated status
Correction system recognizes
this fact based on the identification of
strand of the matrix that contains the
original nucleotide sequence, and a newly
synthesized strand which contains the
erroneously inserted bases (bug)
Mismatch
Repair in E. coli
• MutS recognizes mismatches and binds to them to initiate the repair process.
• MutH and MutL join the complex.
• MutH cleaves the unmethylated strand at hemimethylated GATC sequences on either
side of the mismatch.
• Excision requires MutS, MutL, MutU (DNA helicase II), and an exonuclease.
• DNA polymerase III fills in the gap, and DNA ligase seals the nick. 68
69
Tolerant repair- error prone bipass
70
Error-Prone Bypass
Induce the SOS
response
This causes DNA
to replicate even
though the
damaged region
cannot be read
correctly
Result is errors in
the newly made
DNA
Genetic stability depends on
DNA repair
• repair mechanisms are based on the existence of two
copies of the genetic information in the DNA double
helix
• corrupted strand is repaired according to the
undamaged strand
• the corrupted strand is identified by abnormal DNA
structures generated by errors (bulges, modified
bases, mispaired..)
• mutations in genes encoding repair proteins increase
the frequency of further mutations, creating a
predisposition to cancer
71
72
Inherited DNA repair gene mutations that increase cancer risk
DNA repair gene Protein Repair pathways affected Cancers with increased risk
breast cancer 1 & 2 BRCA1 BRCA2
HRR of double strand breaks and daughter strand
gaps
breast, ovarian
ataxia telangiectasia mutated ATM Different mutations in ATM reduce HRR, SSA or NHEJ leukemia, lymphoma, breast
Nijmegen breakage syndrome NBS (NBN) NHEJ lymphoid cancers
MRE11A MRE11 HRR and NHEJ breast
Bloom syndrome BLM (helicase) HRR
leukemia, lymphoma, colon, breast, skin, lung, auditory canal,
tongue, esophagus, stomach, tonsil, larynx, uterus
WRN WRN HRR, NHEJ, long patch BER soft tissue sarcoma, colorectal, skin, thyroid, pancreas
RECQL4 RECQ4 Helicase likely active in HRR
basal cell carcinoma, squamous cell carcinoma, intraepidermal
carcinoma
Fanconi anemia genes
FANCA,B,C,D1,D2,E,F,G,I,J,L,M,N
FANCA etc. HRR and TLS leukemia, liver tumors, solid tumors many areas
XPC, XPE (DDB2) XPC, XPE
Global genomic NER, repairs damage in both
transcribed and untranscribed DNA
skin cancer (melanoma and non-melanoma)
XPA, XPB, XPD, XPF, XPG
XPA XPB XPD
XPF XPG
Transcription coupled NER repairs the transcribed
strands of transcriptionally active genes
skin cancer (melanoma and non-melanoma)
XPV (also called polymerase H) XPV (POLH) Translesion synthesis (TLS) skin cancers (basal cell, squamous cell, melanoma)
mutS H2, mutS homolog 6, mutL
(E. coli) homolog 1, postmeiotic
segregation increased 2 (S.
cerevisiae)
MSH2 MSH6
MLH1 PMS2
MMR colorectal, endometrial
mutY homolog (E. coli) MUTYH BER of A paired with 8-oxo-dG colon
TP53 P53
Direct role in HRR, BER, NER and acts in DNA damage
responsefor those pathways and for NHEJ and MMR
sarcomas, breast cancers, brain tumors, and adrenocortical
carcinomas
NTHL1 NTHL1 BER for Tg, FapyG, 5-hC, 5-hU in dsDNA
Colon cancer, endometrial cancer, duodenal cancer, basal-cell
carcinoma
73
74
Hereditary diseases caused by disorders
in DNA repair
• xeroderma pigmentosum
• Ataxia-telangiectasia
• Fanconi anemia
• Bloom syndrome
• Werner syndrome
• Rothmund-Thomson syndrome
• Nijmegen breakage syndrome
• Errors in DNA repair (defective enzymes – helicases,
nucleases, regulatory proteins…)
• high frequency of chromosomal aberrations
• high risk of malignancy
75
Hereditary diseases in humans caused by
disorders in DNA repair
• xeroderma pigmentosum – extreme sensitivity
to sunlight, skin cancer predisposition
• affects about 1 in 250,000 newborns
• defective DNA repair after UV radiation (most
often aberration - thymine dimers)
• XP disease can arise as a result of defects in
any one of at least eight different genes.
Products of seven of these genes, XPA, XPB,
XPC, XPD, XPE, XPG and XPF are required for
nucleotide excision repair
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Hereditary diseases caused by DNA repair
disorders
• Cocayne syndrome - stunted growth, impaired
mental ability
• trichothiodystrophy - short limbs, brittle hair,
scaly skin, psychomotoric retardation
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Protein homology in between
different species
• changes in the DNA sequence accumulate
very slowly during evolution thanks to the
DNA repair mechanisms (and selection
pressure)
• there is about 5 million years of divergence
between man and chimpanzee, but the DNA
nucleotide sequences are 98% identical
• thanks to the precision in replication and
reparation processes, only minimal changes
in the genetic information occurred over
millions of years
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