Dna damage

DNA DAMAGE

Dr.Riddhi H Patel,

3rd year resident,

Biochemistry

An alteration in the chemical structure

of DNA, such as a break in a strand of DNA,

a base missing from the backbone of DNA

Difference between DNA damage

and Mutation

DNA Damage DNA Mutation

• Physical abnormalities in the DNA • is a change in the base

sequence of the DNA

• Can be recognized by enzymes

• cannot be recognized by

enzymes once the base change

is present in both DNA strands

• can be correctly repaired if redundant

information, such as the undamaged

sequence in the complementary DNA

strand or in a homologous chromosome,

is available for copying

• a mutation cannot be repaired

lTypes of Damage to DNA

Types of DNA Damage

• Single Base Alteration:a)Depurination

b)Deamination

c)Alkylation

d)Base analogue incorporation

e)Mismatch base

• Double base Alteration: a)Pymidine Dimer

b)Purine Dimer

• Chain break : a) Single Stranded Break

b) Double Stranded Break

• Cross linking : a)Between DNA to DNA

b)Between DNA to Protein

Single base alteration

A) Depurination

Causes: -Spontaneous

- Chemical induced like estrogen

- UV radiation

- Ionizing radiation

- Monofunctional alkylating agents

- Free radical

lDue to some chemical reaction of endogenous

metabolites

Hydrolytic Cleavage of beta-N glycosidic bond of

PURINE NUCLEOSIDES

Release of Nucleic base

Forms APURINIC SITE

Repaired effeciently by BER

If Not repaire causes Mutaion

•Depurination produced by chemical like estrogen

2)Deamination

• Hydrolytic removal of an amine group from a molecule

• Causes:- spontaneous

l -chemically induced(Nitric Oxide induced)

Spontaneous Deamination

deamination of 5-methylcytosine

forms thymine. This conversion of a DNA base

from cytosine (C) to thymine (T) can result in

a transition mutation

Nitric Oxide induced Deamination

• Auto-oxidation of nirtic acid or from Acidic nitrite

• Nitrosating agent like nitrous anhydrite (N2O3)

• react with amines, thiol and other necleophles

• Deamination of DNA

• Repair mainly done by Base excision reapir by

formation of AP site.

• If not repaired :

• Guanine and Adenine deamination leads to the

transition mutation

3)ALKYLATION

• Alkyl group:CnH2n+1

-examples: methyl, ethyl, propyl

• Transfer of an alkyl group from one molecule to

another

• Alkylating Agents:

l -Monofunctional agents

-Bifunctional agents

•electrophilic compounds

•electrophiles encounter negative centers in DNA

•attack by alkylating agents are the N7 of guanine and the

N3 of adenine and add alkyl group

•Making bond between sugar and base more labile

•Forms apurinic site

•Repair: -Base Excision Repair

•Not repaired: -Add incorrect base-- mutation

•Example of alkylating agent: ethylmethane sulfonate

(EMS)

Monofunctional Agents

Alkylation done by EMS

Bifunctional Agents

• Bifunctional agents have two Alkyl groups

• Causes cross linking in DNA

Base analogue incorporation

• Cell makes mistake for recognizing the bases due to

poor proof reading function sometimes

• Causes change in base

• Example: G could look more like an A

• Repair by Mismatch repair

• Mutation to the protein involve in the MMR is

associated with Hereditary NonPolyposis

Colorectal Cancer (HNPCC) also k\a Lynch

syndrome

• HNPCC- causes increase risk of developing

colon cancer

Single Base Alteration: a)Depurination

b)Deamination

c)Alkylation

d)Base analogue incorporation

e)Mismatch base

Double base Alteration: a)Pymidine Dimer

b)Purine Dimer

Chain break : a) Single Stranded Break

b) Double Stranded Break

Cross linking : a)Between DNA to DNA

b)Between DNA to Protein

Double Base Alteration

• Causes : UV radiation

• May be direct effect of UV radiation

UV Radiation

lUVR : 3 types • UVA:315nm-400nm :Poor efficiency for DNA Damage as

lnot absorbed by native DNA

l-able to generate singlet oxygen that can damage DNA via

indirect photosensitizing reactions

lUVB:280-315nm : Mainly absorbed by DNA : Main damage

occur by this

lUVC :<280nm : absorbed by oxygen and ozone in the Earth’s

atmosphere

1)Pyrimidine dimer

Pyrimidine dimer

Purine Dimer

UV induced DNA lesions

lPyrimidine dimer

lPurine dimer

lDepurination

lSingle and Double strand break

• Repair mainly done by Nucleotide Excixion repair

• If not reapir- during replication it may incorrectly insert

base

- may skip that dimer

• NER defect: Xeroderma Pigmentosa

-XP protein encoding gene defect

-Accumulation of these mutation induced by UV light

-Causes various skin cancer: Basal Cell Carcinoma

Squamous Cell Carcinoma

Melanoma

• Single Base Alteration:a)Depurination

b)Deamination

c)Alkylation

d)Base analogue incorporation

e)Mismatch base

• Double base Alteration: a)Pymidine Dimer

b)Purine Dimer

• Chain break : a) Single Stranded Break

b) Double Stranded Break

• Cross linking : a)Between DNA to DNA

b)Between DNA to Protein

Chain Break

Causes :- Mainly by irradiation

- Free radical damage

Repair : By Homologus end joining

Double strand break:

Causes : - Direct by irradiaction

- unrepaired UV-induced DNA lesions

- the repair of single strand breaks

passing through base excision repair

- Free radical injury

- Radiomimetic compound

Mechanism of DSB

Repair by Homologus End joining Or Non

Homologus End Joining

•Oxidative damage mainly occurs by formation of Free

redical or Radiomimetic compunds

•Free radical damage to the DNA:

-70% damage by OH-

-Radiolysis of H2O produce peroxides

-formation of OH. requires metal near to the DNA

-Fe+2 mainly forms OH. formation by fenton reaction

Fe2+ + H2O2 → Fe3+ + OH· + OH−

Free radical damage to DNA

• OH. Abstracts H from deoxyribose carbon

• Deoxyribosyl radical formation

• Radical react with molecular O2- forms peroxyl radical

• Break the DNA strand and release of Bases(AP site)

• Repair by BER

• When OH. Abstrcts 1-deoxyribose-- forms

2- deoxyribonolacton which is resistant to repair

enzyme --- Mutation

Depurination

Single strand break

Double strand break

Radiomimetic compounds

•radiomimetic compounds are enediyens

•Undergose cyclization

•Forms Para-benzene diredicals

•Highly reactive- Abstract hydrogen from any possible

hydrogen donor

•In DNA- it abstracts H from the deoxyribose sugar

backbone, predominantly at the C-1’,C-4’ and C-5’

positions

radical formation at the reacted carbon

• carbon radical reacts with molecular oxygen

• Para-benzyne is able to position itself in such a way

that it can abstract proximal hydrogens from both

strands of DNA

• produces a double-strand break in the DNA

• Mostly this will not be repaired

• Cell apoptosis

Direct Damage:

ionizing radiation directly interact with

target structure to cause ionization

initiating the chain of events to lead to

biological changes

direct action of radiation done by High

Linear energy transfer like alpha partical

and neutron

Indirect Damage : done by free radical

formation

Clustered DNA lesion : which represent two

or more lesions formed within one or two

helical turns of DNA

-less readily repaired than individual

lesions

-cytotoxic, mutagenic and carcinogenic

effects

• Single Base Alteration:a)Depurination

b)Deamination

c)Alkylation

d)Base analogue incorporation

e)Mismatch base

• Double base Alteration: a)Pymidine Dimer

b)Purine Dimer

• Chain break : a) Single Stranded Break

b) Double Stranded Break

• Cross linking : a)Between DNA to DNA

b)Between DNA to Protein

Cross linking

• Causes : - UV radiation

-ionizing radiation like X rays and gamma rays

- Free radical like H2O2 and OH.

• Mechanism: By irradiation – formation of H2O2

• Protein peroxidation occur

• Fornation of Protein hydroperoxide

• Causes cross linking with DNA