Base excision repair (BER) is a cellular mechanism, studied in the fields of
biochemistry
Biochemistry or biological chemistry is the study of chemical processes within and relating to living organisms. A sub-discipline of both chemistry and biology, biochemistry may be divided into three fields: structural biology, enzymology and ...
and
genetics, that
repairs damaged DNA throughout the cell cycle. It is responsible primarily for removing small, non-helix-distorting base lesions from the genome. The related
nucleotide excision repair
Nucleotide excision repair is a DNA repair mechanism. DNA damage occurs constantly because of chemicals (e.g. intercalating agents), radiation and other mutagens. Three excision repair pathways exist to repair single stranded DNA damage: Nucleo ...
pathway repairs bulky helix-distorting lesions. BER is important for removing damaged bases that could otherwise cause
mutations by mispairing or lead to breaks in DNA during replication. BER is initiated by
DNA glycosylase
DNA glycosylases are a family of enzymes involved in base excision repair, classified under EC number EC 3.2.2. Base excision repair is the mechanism by which damaged bases in DNA are removed and replaced. DNA glycosylases catalyze the first st ...
s, which recognize and remove specific damaged or inappropriate bases, forming
AP site
In biochemistry and molecular genetics, an AP site (apurinic/apyrimidinic site), also known as an abasic site, is a location in DNA (also in RNA but much less likely) that has neither a purine nor a pyrimidine base, either spontaneously or due ...
s. These are then cleaved by an
AP endonuclease. The resulting single-strand break can then be processed by either short-patch (where a single nucleotide is replaced) or long-patch BER (where 2–10 new nucleotides are synthesized).
Lesions processed by BER
Single bases in DNA can be chemically damaged by a variety of mechanisms, the most common ones being deamination, oxidation, and alkylation. These modifications can affect the ability of the base to hydrogen-bond, resulting in incorrect base-pairing, and, as a consequence, mutations in the DNA. For example, incorporation of
adenine
Adenine () (symbol A or Ade) is a nucleobase (a purine derivative). It is one of the four nucleobases in the nucleic acid of DNA that are represented by the letters G–C–A–T. The three others are guanine, cytosine and thymine. Its derivativ ...
across from
8-oxoguanine
8-Oxoguanine (8-hydroxyguanine, 8-oxo-Gua, or OH8Gua) is one of the most common DNA lesions resulting from reactive oxygen species modifying guanine, and can result in a mismatched pairing with adenine resulting in G to T and C to A substitutions ...
(right) during
DNA replication causes a G:C base pair to be mutated to T:A. Other examples of base lesions repaired by BER include:
*Oxidized bases:
8-oxoguanine
8-Oxoguanine (8-hydroxyguanine, 8-oxo-Gua, or OH8Gua) is one of the most common DNA lesions resulting from reactive oxygen species modifying guanine, and can result in a mismatched pairing with adenine resulting in G to T and C to A substitutions ...
, 2,6-diamino-4-hydroxy-5-formamidopyrimidine (FapyG, FapyA)
*Alkylated bases:
3-methyladenine,
7-methylguanosine
7-Methylguanosine (m7G) is a modified purine nucleoside. It is a methylated version of guanosine and when found in human urine, it may be a biomarker of some types of cancer. In the RNAs, 7-methylguanosine have been used to study and examine the ...
*Deaminated bases:
hypoxanthine formed from deamination of
adenine
Adenine () (symbol A or Ade) is a nucleobase (a purine derivative). It is one of the four nucleobases in the nucleic acid of DNA that are represented by the letters G–C–A–T. The three others are guanine, cytosine and thymine. Its derivativ ...
.
Xanthine
Xanthine ( or ; archaically xanthic acid; systematic name 3,7-dihydropurine-2,6-dione) is a purine base found in most human body tissues and fluids, as well as in other organisms. Several stimulants are derived from xanthine, including caffein ...
formed from deamination of guanine. (
Thymidine
Thymidine (symbol dT or dThd), also known as deoxythymidine, deoxyribosylthymine, or thymine deoxyriboside, is a pyrimidine deoxynucleoside. Deoxythymidine is the DNA nucleoside T, which pairs with deoxyadenosine (A) in double-stranded DNA. ...
products following deamination of
5-methylcytosine are more difficult to recognize, but can be repaired by mismatch-specific glycosylases)
*
Uracil inappropriately incorporated in DNA or formed by
deamination of
cytosine
Cytosine () (symbol C or Cyt) is one of the four nucleobases found in DNA and RNA, along with adenine, guanine, and thymine (uracil in RNA). It is a pyrimidine derivative, with a heterocyclic aromatic ring and two substituents attached (an amin ...
In addition to base lesions, the downstream steps of BER are also utilized to repair single-strand breaks.
The choice between long-patch and short-patch repair
The choice between short- and long-patch repair is currently under investigation. Various factors are thought to influence this decision, including the type of lesion, the cell cycle stage, and whether the cell is terminally differentiated or actively dividing.
Some lesions, such as oxidized or reduced AP sites, are resistant to pol β lyase activity and, therefore, must be processed by long-patch BER.
Pathway preference may differ between organisms, as well. While human cells utilize both short- and long-patch BER, the yeast ''
Saccharomyces cerevisiae
''Saccharomyces cerevisiae'' () (brewer's yeast or baker's yeast) is a species of yeast (single-celled fungus microorganisms). The species has been instrumental in winemaking, baking, and brewing since ancient times. It is believed to have been ...
'' was long thought to lack a short-patch pathway because it does not have homologs of several mammalian short-patch proteins, including pol β, DNA ligase III, XRCC1, and the kinase domain of
PNKP
Bifunctional polynucleotide phosphatase/kinase is an enzyme that in humans is encoded by the ''PNKP'' gene. A detailed structural study of the crystallized mouse protein examined both the 5´-polynucleotide kinase and 3’-polynucleotide phosphata ...
. The recent discovery that the poly-A polymerase
Trf4 possesses 5' dRP lyase activity has challenged this view.
Proteins involved in base excision repair
DNA glycosylases
DNA glycosylases are responsible for initial recognition of the lesion. They
flip
Flip, FLIP, or flips may refer to:
People
* Flip (nickname), a list of people
* Lil' Flip (born 1981), American rapper
* Flip Simmons, Australian actor and musician
* Flip Wilson, American comedian
Arts and entertainment Fictional characters
* ...
the damaged base out of the double helix, as pictured, and cleave the N-glycosidic bond of the damaged base, leaving an
AP site
In biochemistry and molecular genetics, an AP site (apurinic/apyrimidinic site), also known as an abasic site, is a location in DNA (also in RNA but much less likely) that has neither a purine nor a pyrimidine base, either spontaneously or due ...
. There are two categories of glycosylases: monofunctional and bifunctional. Monofunctional glycosylases have only glycosylase activity, whereas bifunctional glycosylases also possess
AP lyase
The enzyme DNA-(apurinic or apyrimidinic site) lyase, also referred to as DNA-(apurinic or apyrimidinic site) 5'-phosphomonoester-lyase (systematic name) or DNA AP lyase (EC 4.2.99.18) catalyzes the cleavage of the C-O-P bond 3' from the apurinic o ...
activity. Therefore, bifunctional glycosylases can convert a base lesion into a single-strand break without the need for an
AP endonuclease. β-Elimination of an AP site by a glycosylase-lyase yields a 3' α,β-unsaturated aldehyde adjacent to a 5' phosphate, which differs from the AP endonuclease cleavage product.
Some glycosylase-lyases can further perform δ-elimination, which converts the 3' aldehyde to a 3' phosphate. A wide variety of glycosylases have evolved to recognize different damaged bases. Examples of DNA glycosylases include
Ogg1, which recognizes 8-oxoguanine,
MPG, which recognizes 3-methyladenine, and
UNG
Ung or UNG may refer to:
People
* Woong, a Korean given name also spelled Ung
* Ung (surname), a Cambodian and Norwegian surname
* Ung Thị (full name Nguyễn Phúc Ung Thị; 1913–2001), Vietnamese-born American businessman
* Franz Unger ...
, which removes
uracil from DNA.
AP endonucleases
The AP endonucleases cleave an
AP site
In biochemistry and molecular genetics, an AP site (apurinic/apyrimidinic site), also known as an abasic site, is a location in DNA (also in RNA but much less likely) that has neither a purine nor a pyrimidine base, either spontaneously or due ...
to yield a 3' hydroxyl adjacent to a 5' deoxyribosephosphate (dRP). AP endonucleases are divided into two families based on their homology to the ancestral bacterial AP endonucleaes
endonuclease IV and
exonuclease III
Exonuclease III (ExoIII) is an enzyme that belongs to the exonuclease family. ExoIII catalyzes the stepwise removal of mononucleotides from 3´-hydroxyl termini of double-stranded DNA. A limited number of nucleotides are removed during each bindi ...
. Many eukaryotes have members of both families, including the yeast
Saccharomyces cerevisiae
''Saccharomyces cerevisiae'' () (brewer's yeast or baker's yeast) is a species of yeast (single-celled fungus microorganisms). The species has been instrumental in winemaking, baking, and brewing since ancient times. It is believed to have been ...
, in which
Apn1 is the EndoIV homolog and
Apn2
APN may refer to:
Biology and chemistry
* 3-Arylpropiolonitriles, a class of chemical reagents
* Acute pyelonephritis, a urinary tract infection
* Acyl peroxy nitrates, respiratory and eye irritants in photochemical smog
Computing
* Access ...
is related to ExoIII. In humans, two AP endonucleases,
APE1 and
APE2, have been identified. It is a member of the ExoIII family.
End processing enzymes
In order for ligation to occur, a DNA strand break must have a hydroxyl on its
3' end
Directionality, in molecular biology and biochemistry, is the end-to-end chemical orientation of a single strand of nucleic acid. In a single strand of DNA or RNA, the chemical convention of naming carbon atoms in the nucleotide pentose-sugar-ri ...
and a phosphate on its
5' end
Directionality, in molecular biology and biochemistry, is the end-to-end chemical orientation of a single strand of nucleic acid. In a single strand of DNA or RNA, the chemical convention of naming carbon atoms in the nucleotide pentose-sugar-r ...
. In humans, polynucleotide kinase-phosphatase (
PNKP
Bifunctional polynucleotide phosphatase/kinase is an enzyme that in humans is encoded by the ''PNKP'' gene. A detailed structural study of the crystallized mouse protein examined both the 5´-polynucleotide kinase and 3’-polynucleotide phosphata ...
) promotes formation of these ends during BER. This protein has a kinase domain, which phosphorylates 5' hydroxyl ends, and a phosphatase domain, which removes phosphates from 3' ends. Together, these activities ready single-strand breaks with damaged termini for ligation. The AP endonucleases also participate in 3' end processing. Besides opening AP sites, they possess 3' phosphodiesterase activity and can remove a variety of 3' lesions including phosphates, phosphoglycolates, and aldehydes. 3'-Processing must occur before DNA synthesis can initiate because DNA polymerases require a 3' hydroxyl to extend from.
DNA polymerases
Pol β is the main human polymerase that catalyzes short-patch BER, with
pol λ able to compensate in its absence.
These polymerases are members of the
Pol X family and typically insert only a single nucleotide. In addition to polymerase activity, these enzymes have a lyase domain that removes the 5' dRP left behind by AP endonuclease cleavage. During long-patch BER, DNA synthesis is thought to be mediated by
pol δ and
pol ε along with the processivity factor
PCNA, the same polymerases that carry out
DNA replication. These polymerases perform displacing synthesis, meaning that the downstream 5' DNA end is "displaced" to form a flap (see diagram above). Pol β can also perform long-patch displacing synthesis and can, therefore, participate in either BER pathway.
Long-patch synthesis typically inserts 2-10 new nucleotides.
Flap endonuclease
FEN1
Flap endonuclease 1 is an enzyme that in humans is encoded by the ''FEN1'' gene.
Function
The protein encoded by this gene removes 5' overhanging "flaps" (or short sections of single stranded DNA that "hang off" because their nucleotide bases a ...
removes the 5' flap generated during long patch BER. This endonuclease shows a strong preference for a long 5' flap adjacent to a 1-nt 3' flap.
The yeast homolog of FEN1 is ''RAD27''. In addition to its role in long-patch BER, FEN1 cleaves flaps with a similar structure during
Okazaki fragment
Okazaki fragments are short sequences of DNA nucleotides (approximately 150 to 200 base pairs long in eukaryotes) which are synthesized discontinuously and later linked together by the enzyme DNA ligase to create the lagging strand during ...
processing, an important step in lagging strand
DNA replication.
DNA ligase
DNA ligase III along with its cofactor
XRCC1 catalyzes the nick-sealing step in short-patch BER in humans.
DNA ligase I
DNA ligase 1 is an enzyme that in humans is encoded by the ''LIG1'' gene. DNA ligase I is the only known eukaryotic DNA ligase involved in both DNA replication and repair, making it the most studied of the ligases.
Discovery
It was known that ...
ligates the break in long-patch BER.
Links with cancer
Defects in a variety of DNA repair pathways lead to cancer predisposition, and BER appears to follow this pattern.
Deletion mutations in BER genes have shown to result in a higher mutation rate in a variety of organisms, implying that loss of BER could contribute to the development of cancer. Indeed, somatic mutations in Pol β have been found in 30% of human cancers, and some of these mutations lead to transformation when expressed in mouse cells.
Mutations in the DNA glycosylase
MYH are also known to increase susceptibility to
colon cancer
Colorectal cancer (CRC), also known as bowel cancer, colon cancer, or rectal cancer, is the development of cancer from the colon or rectum (parts of the large intestine). Signs and symptoms may include blood in the stool, a change in bowel m ...
.
Epigenetic deficiencies in cancers
Epigenetic
In biology, epigenetics is the study of stable phenotypic changes (known as ''marks'') that do not involve alterations in the DNA sequence. The Greek prefix '' epi-'' ( "over, outside of, around") in ''epigenetics'' implies features that are "o ...
alterations (epimutations) in base excision repair genes have only recently begun to be evaluated in a few cancers, compared to the numerous previous studies of epimutations in genes acting in other DNA repair pathways (such as
MLH1
DNA mismatch repair protein Mlh1 or MutL protein homolog 1 is a protein that in humans is encoded by the MLH1 gene located on chromosome 3. It is a gene commonly associated with hereditary nonpolyposis colorectal cancer. Orthologs of human MLH1 ...
in mismatch repair and
MGMT in direct reversal). Some examples of epimutations in base excision repair genes that occur in cancers are summarized below.
MBD4
MBD4
Methyl-CpG-binding domain protein 4 is a protein that in humans is encoded by the ''MBD4'' gene.
Structure
Human MBD4 protein has 580 amino acids with a methyl-CpG-binding domain at amino acids 82–147 and a C-terminal DNA glycosylase domain a ...
(methyl-CpG-binding domain protein 4) is a glycosylase employed in an initial step of base excision repair. MBD4 protein binds preferentially to fully
methylated
In the chemical sciences, methylation denotes the addition of a methyl group on a substrate, or the substitution of an atom (or group) by a methyl group. Methylation is a form of alkylation, with a methyl group replacing a hydrogen atom. These t ...
CpG site
The CpG sites or CG sites are regions of DNA where a cytosine nucleotide is followed by a guanine nucleotide in the linear sequence of bases along its 5' → 3' direction. CpG sites occur with high frequency in genomic regions called CpG isl ...
s and to the altered DNA bases at those sites. These altered bases arise from the frequent hydrolysis of cytosine to uracil (see image) and hydrolysis of
5-methylcytosine to thymine, producing G:U and G:T base pairs.
If the improper uracils or thymines in these base pairs are not removed before DNA replication, they will cause
transition mutations. MBD4 specifically catalyzes the removal of T and U paired with guanine (G) within CpG sites.
This is an important repair function since about 1/3 of all
intragenic single base pair mutations in human cancers occur in CpG dinucleotides and are the result of G:C to A:T transitions.
These transitions comprise the most frequent mutations in human cancer. For example, nearly 50% of somatic mutations of the tumor suppressor gene
p53
p53, also known as Tumor protein P53, cellular tumor antigen p53 (UniProt name), or transformation-related protein 53 (TRP53) is a regulatory protein that is often mutated in human cancers. The p53 proteins (originally thought to be, and often ...
in
colorectal cancer
Colorectal cancer (CRC), also known as bowel cancer, colon cancer, or rectal cancer, is the development of cancer from the colon or rectum (parts of the large intestine). Signs and symptoms may include blood in the stool, a change in bowel m ...
are G:C to A:T transitions within CpG sites.
Thus, a decrease in expression of MBD4 could cause an increase in
carcinogenic
A carcinogen is any substance, radionuclide, or radiation that promotes carcinogenesis (the formation of cancer). This may be due to the ability to damage the genome or to the disruption of cellular metabolic processes. Several radioactive substan ...
mutations.
MBD4 expression is reduced in almost all colorectal
neoplasms
A neoplasm () is a type of abnormal and excessive growth of tissue. The process that occurs to form or produce a neoplasm is called neoplasia. The growth of a neoplasm is uncoordinated with that of the normal surrounding tissue, and persists ...
due to
methylation
In the chemical sciences, methylation denotes the addition of a methyl group on a substrate, or the substitution of an atom (or group) by a methyl group. Methylation is a form of alkylation, with a methyl group replacing a hydrogen atom. These t ...
of the
promoter region of MBD4.
Also MBD4 is deficient due to mutation in about 4% of colorectal cancers.
A majority of histologically normal fields surrounding neoplastic growths (adenomas and colon cancers) in the colon also show reduced MBD4 mRNA expression (a
field defect) compared to histologically normal tissue from individuals who never had a colonic neoplasm.
This finding suggests that epigenetic
silencing
Silencing is a visual illusion in which a set of objects that change iluminance
hue
carcinogenesis
Carcinogenesis, also called oncogenesis or tumorigenesis, is the formation of a cancer, whereby normal cells are transformed into cancer cells. The process is characterized by changes at the cellular, genetic, and epigenetic levels and abnor ...
.
In a Chinese population that was evaluated, the MBD4 Glu346Lys
polymorphism was associated with about a 50% reduced risk of cervical cancer, suggesting that alterations in MBD4 may be important in cancer.
NEIL1
NEIL1
Endonuclease VIII-like 1 is an enzyme that in humans is encoded by the ''NEIL1'' gene.
NEIL1 belongs to a class of DNA glycosylases homologous to the bacterial Fpg/Nei family. These glycosylases initiate the first step in base excision repair by ...
recognizes (targets) and removes certain
oxidatively-damaged bases and then incises the
abasic site via β,δ elimination, leaving 3′ and 5′ phosphate ends. NEIL1 recognizes oxidized
pyrimidine
Pyrimidine (; ) is an aromatic, heterocyclic, organic compound similar to pyridine (). One of the three diazines (six-membered heterocyclics with two nitrogen atoms in the ring), it has nitrogen atoms at positions 1 and 3 in the ring. The othe ...
s, formamidopyrimidines,
thymine
Thymine () (symbol T or Thy) is one of the four nucleobases in the nucleic acid of DNA that are represented by the letters G–C–A–T. The others are adenine, guanine, and cytosine. Thymine is also known as 5-methyluracil, a pyrimidine nuc ...
residues oxidized at the methyl group, and both stereoisomers of
thymine glycol.
The best substrates for human NEIL1 appear to be the
hydantoin lesions, guanidinohydantoin, and spiroiminodihydantoin that are further oxidation products of
8-oxoG. NEIL1 is also capable of removing lesions from single-stranded DNA as well as from bubble and forked DNA structures. A deficiency in NEIL1 causes increased mutagenesis at the site of an 8-oxo-Gua:C pair, with most mutations being G:C to T:A transversions.
A study in 2004 found that 46% of primary gastric cancers had reduced expression of NEIL1
mRNA, though the mechanism of reduction was not known.
This study also found that 4% of gastric cancers had mutations in NEIL1. The authors suggested that low NEIL1 activity arising from reduced expression and/or mutation in NEIL1 was often involved in gastric carcinogenesis.
A screen of 145 DNA repair genes for aberrant promoter methylation was performed on head and neck squamous cell carcinoma (HNSCC) tissues from 20 patients and from head and neck mucosa samples from 5 non-cancer patients.
This screen showed that NEIL1, with substantially increased hypermethylation, had the most significantly different frequency of methylation. Furthermore, the hypermethylation corresponded to a decrease in NEIL1 mRNA expression. Further work with 135 tumor and 38 normal tissues also showed that 71% of HNSCC tissue samples had elevated NEIL1 promoter methylation.
When 8 DNA repair genes were evaluated in
non-small cell lung cancer (NSCLC) tumors, 42% were hypermethylated in the NEIL1 promoter region.
This was the most frequent DNA repair abnormality found among the 8 DNA repair genes tested. NEIL1 was also one of six DNA repair genes found to be hypermethylated in their promoter regions in
colorectal cancer
Colorectal cancer (CRC), also known as bowel cancer, colon cancer, or rectal cancer, is the development of cancer from the colon or rectum (parts of the large intestine). Signs and symptoms may include blood in the stool, a change in bowel m ...
.
Links with cognition
Active
DNA methylation
DNA methylation is a biological process by which methyl groups are added to the DNA molecule. Methylation can change the activity of a DNA segment without changing the sequence. When located in a gene promoter, DNA methylation typically acts t ...
and
demethylation is required for the
cognition
Cognition refers to "the mental action or process of acquiring knowledge and understanding through thought, experience, and the senses". It encompasses all aspects of intellectual functions and processes such as: perception, attention, though ...
process of
memory formation and maintenance.
In rats, contextual
fear conditioning
Pavlovian fear conditioning is a behavioral paradigm in which organisms learn to predict aversive events. It is a form of learning in which an aversive stimulus (e.g. an electrical shock) is associated with a particular neutral context (e.g., a ...
can trigger life-long memory for the event with a single trial, and methylation changes appear to be correlated with triggering particularly long-lived memories.
With contextual
fear conditioning
Pavlovian fear conditioning is a behavioral paradigm in which organisms learn to predict aversive events. It is a form of learning in which an aversive stimulus (e.g. an electrical shock) is associated with a particular neutral context (e.g., a ...
, after 24 hours, DNA isolated from the rat brain
hippocampus region had 2097
differentially methylated genes, with a proportion being demethylated.
As reviewed by Bayraktar and Kreutz,
DNA demethylation is dependent on base excision repair (see figure).
Physical exercise has well established beneficial effects on learning and memory (see
Neurobiological effects of physical exercise). ''
BDNF
Brain-derived neurotrophic factor (BDNF), or abrineurin, is a protein found in the and the periphery. that, in humans, is encoded by the ''BDNF'' gene. BDNF is a member of the neurotrophin family of growth factors, which are related to the cano ...
'' is a particularly important regulator of learning and memory.
As reviewed by Fernandes et al.,
in rats, exercise enhances the
hippocampus expression of the gene ''
Bdnf
Brain-derived neurotrophic factor (BDNF), or abrineurin, is a protein found in the and the periphery. that, in humans, is encoded by the ''BDNF'' gene. BDNF is a member of the neurotrophin family of growth factors, which are related to the cano ...
'', which has an essential role in memory formation.
Enhanced expression of ''Bdnf'' occurs through demethylation of its
CpG island promoter at
exon IV and demethylation depends on base excision repair (see figure).
Decline in BER with age
The activity of the
DNA glycosylase
DNA glycosylases are a family of enzymes involved in base excision repair, classified under EC number EC 3.2.2. Base excision repair is the mechanism by which damaged bases in DNA are removed and replaced. DNA glycosylases catalyze the first st ...
that removes methylated bases in human
leukocytes declines with age.
The reduction in the excision of methylated bases from DNA suggests an age-dependent decline in
3-methyladenine DNA glycosylase, a BER enzyme responsible for removing alkylated bases.
Young rats (4- to 5 months old), but not old rats (24- to 28 months old), have the ability to induce
DNA polymerase beta and
AP endonuclease in response to oxidative damage.
See also
*
DNA mismatch repair
DNA mismatch repair (MMR) is a system for recognizing and repairing erroneous insertion, deletion, and mis-incorporation of bases that can arise during DNA replication and recombination, as well as repairing some forms of DNA damage.
Mismatch ...
*
DNA repair
*
Homologous recombination
Homologous recombination is a type of genetic recombination in which genetic information is exchanged between two similar or identical molecules of double-stranded or single-stranded nucleic acids (usually DNA as in cellular organisms but may be ...
*
Non-homologous end joining
Non-homologous end joining (NHEJ) is a pathway that repairs double-strand breaks in DNA. NHEJ is referred to as "non-homologous" because the break ends are directly ligated without the need for a homologous template, in contrast to homology direct ...
*
Nucleotide excision repair
Nucleotide excision repair is a DNA repair mechanism. DNA damage occurs constantly because of chemicals (e.g. intercalating agents), radiation and other mutagens. Three excision repair pathways exist to repair single stranded DNA damage: Nucleo ...
*
Host-cell reactivation assay
References
External links
*
{{DEFAULTSORT:Base Excision Repair
DNA repair