DNA repair systems help maintain the integrity of genetic material by correcting damage from mutagens. There are several types of DNA repair mechanisms, including direct damage reversal, mismatch repair, base excision repair, nucleotide excision repair, and recombination repair. Key DNA repair proteins like p53 play an important role in recognizing DNA damage and initiating cell cycle arrest to allow time for repair or inducing apoptosis if damage is irreparable. Double strand breaks are the most difficult to repair and can lead to chromosomal rearrangements if unrepaired.

1. DNA Repair
Mohammed Fathy Bayomy, MSc, MD
Lecturer
Clinical Oncology & Nuclear Medicine
Faculty of Medicine
Zagazig University

2. Mutation
 Definition: any permanent & heritable change in DNA base sequence
 Classification
* Germline mutation: mutation occurs in gamete cells, mutation
will be passed on to all cells of body
* Somatic mutation: mutation affects somatic cell it will be more
restricted & passes to only descendents of that cell lineage in
particular organ
1) According to cell type affected
2) According to structural changes
* Point mutation: involves single base that substitute one amino
acid for another
* Deletions or insertions: remove or add one or more bases
* Inversion mutation: inverts segment of DNA sequence

3. * Missense mutation: substitutes one amino acid for another thus
resulting in abnormal protein
* Nonsense mutation: substitutes a stop codon for amino acid
codon leading to incomplete or truncated protein
* Frameshift mutation: adds or deletes base with shift of base
sequence, hence, changing reading frame, resulting in
mistranslation of all amino acid sequence beyond mutation
* Splicing mutation: will act on splice junction usually causing
major changes in m-RNA & encoded protein
3) According to consequence of mutation on encoded protein

4.  Pathogenesis
* Mispairing of bases ( A-C pair instead of normal A-T pair) during
DNA synthesis ( replication error ) or recombination
* Slippage of newly synthesized DNA strand during/replication
with loop formation is another replication error resulting in
copying of bases twice
* Loss of purine bases ( depurination) occurs as a result of effect
of body heat and cellular metabolism
* De-amination of cytosine leading to C-T substitution
1) Spontaneous mutations

5. * Ultraviolet radiation produce pyrimidine dimers in which
adjacent thymine residues in same DNA strand become covalently
attached
* Exposure to ionizing radiation may produce various lesions such
as : base damage (e.g. conversion of thymine to thymine glycol or
change of guanine to hydroxy guanine), single or double strand
breaks in DNA phosphate-sugar backbone, and cross-links between
the two strands of DNA or between DNA & proteins
* Chemotherapeutic-agents may cause base damage (alkylating
agents), strand breaks (bleomycin) or various adducts or cross-
links (cisplatin)
2) Induced mutations

6. DNA Repair
 Cells have evolved an elaborate array of enzymatic systems to maintain
integrity of their genetic material in the face of numerous agents that alter
DNA structure or base sequence
 Cellular repair of macromolecule is known to occur only for DNA, vital
molecule for cell survival
 Tumor suppressor gene p5 3 plays key role in this regard, hence it is
called guardian of genome. Thus, with DNA injury, p53 causes arrest of
cell cycle to give time for DNA repair enzymes. If DNA lesions are
severe & irrepairable, p5 3 eliminates mutant cells by stimulating
apoptosis

7. DNA Repair Systems
 Damaged DNA is reverted to its original state without replacement
of constituents
 Example
1) Damage Reversal
* Repair of base damage, by alkyl transferase enzyme, removing
methyl & larger alkylating groups from DNA bases
* Rejoining single strand break by ligase
2) Mismatch repair (MMR)
 Four types of mismatch repair genes: hMSH-2 (2p), hMCH-1
(3p), hPMS-1, hPMS-2
 During replication, mismatch protein gene products act as "spell
checkers" and if there is replication error (erroneous pairing of G-T
instead of A-T), this system corrects defect

8.  Mismatched lesion is recognized by hMSH-2 protein, or hMSH-2:
GTBP/P160 protein heterodimer
 Removal of mismatch, followed by resynthesis & ligation completes
repair process
3) Excision repair
A) Base Excision Repair (BER)
 Limited to small lesions (de-aminated cytosine, single strand breaks
 Repair patches consist usually of only one or two nucleotides (short
patch repair)
 Base damage is recognized & excised by glycosylase, removal of
sugar residue by endonuclease & exonuclease, followed by
resynthesis & ligation

9. b) Nucleotide Excision Repair (NER)
 Involves replacement of long patches of DNA of order of thirty
nucleotides (long patch repair)
 There are 5 steps in NER process, namely: recognition of DNA
lesion, incision of damaged strand on each side of lesion, removal of
damaged nucleotides, synthesis of new nucleotides, & its ligation
4) Recombination repair
 Double strand breaks are most difficult to repair due to absence of
template strand
 Unrepaired or misrepaired double strand breaks may lead to serious
consequences: end-to-end joining of nonhomologous chromosomes, or
formation of new telomeres generating chromosomal rearrangements
deletions, use of nonhomologous DNA strands as template results in
formation of abnormal base sequence (mutation)

10.  Double strand breaks, in view of difficulty of repair, is most
common cause of cell lethality following genotoxic agents
 Repair requires recruitment of homologous or heterologous DNA
strands with formation of Holliday junction caused by crossing over
of single strands from two adjacent DNA duplexes
 This junction requires resolution, by enzymatic cutting of crossing
point, before repair & duplex separation are complete