DNA repair protein XRCC4 also known as X-ray repair cross-complementing protein 4 or XRCC4 is a
protein
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, res ...
that in humans is encoded by the XRCC4
gene
In biology, the word gene (from , ; "... Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a b ...
. In addition to humans, the XRCC4 protein is also expressed in many other
metazoan
Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, and go through an ontogenetic stage in ...
s,
fungi
A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately from ...
and in
plants
Plants are predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all current definitions of Plantae exclude ...
.
The X-ray repair cross-complementing protein 4 is one of several core
protein
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, res ...
s involved in the
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 direc ...
(NHEJ) pathway to repair
DNA double strand breaks (DSBs).
NHEJ requires two main components to achieve successful completion. The first component is the cooperative binding and
phosphorylation of
artemis
In ancient Greek mythology and religion, Artemis (; grc-gre, Ἄρτεμις) is the goddess of the hunt, the wilderness, wild animals, nature, vegetation, childbirth, care of children, and chastity. She was heavily identified wit ...
by the catalytic subunit of the DNA-dependent protein kinase (
DNA-PKcs
DNA-dependent protein kinase, catalytic subunit, also known as DNA-PKcs, is an enzyme that in humans is encoded by the gene designated as ''PRKDC'' or ''XRCC7''. DNA-PKcs belongs to the phosphatidylinositol 3-kinase-related kinase protein family. ...
). Artemis cleaves the ends of damaged DNA to prepare it for
ligation
Ligation may refer to:
* Ligation (molecular biology), the covalent linking of two ends of DNA or RNA molecules
* In medicine, the making of a ligature (tie)
* Chemical ligation, the production of peptides from amino acids
* Tubal ligation, a meth ...
. The second component involves the bridging of DNA to DNA Ligase IV (
LigIV), by XRCC4, with the aid of
Cernunnos-XLF. DNA-PKcs and XRCC4 are anchored to
Ku70
Ku70 is a protein that, in humans, is encoded by the ''XRCC6'' gene.
Function
Together, Ku70 and Ku80 make up the Ku heterodimer, which binds to DNA double-strand break ends and is required for the non-homologous end joining (NHEJ) pathway o ...
/
Ku80
Ku80 is a protein that, in humans, is encoded by the ''XRCC5'' gene. Together, Ku70 and Ku80 make up the Ku heterodimer, which binds to DNA double-strand break ends and is required for the non-homologous end joining (NHEJ) pathway of DNA repair ...
heterodimer, which are bound to the DNA ends.
Since XRCC4 is the key protein that enables interaction of LigIV to damaged DNA and therefore ligation of the ends, mutations in the XRCC4 gene were found to cause embryonic lethality in mice and developmental inhibition and
immunodeficiency in humans.
Furthermore, certain mutations in the XRCC4 gene are associated with an increased risk of cancer.
Double strand breaks
DSBs are mainly caused by free radicals generated from ionizing radiation in the environment and from by-products released continually during cellular metabolism. DSBs that are not efficiently repaired may result in the loss of important protein coding genes and regulatory sequences required for gene expression necessary for the life of a cell.
DSBs that cannot rely on a newly copied sister chromosome generated by DNA replication to fill in the gap will go into the NHEJ pathway. This method of repair is essential as it is a last resort to prevent loss of long stretches of the chromosome.
NHEJ is also used to repair DSBs generated during
V(D)J recombination
V(D)J recombination is the mechanism of somatic recombination that occurs only in developing lymphocytes during the early stages of T and B cell maturation. It results in the highly diverse repertoire of antibodies/immunoglobulins and T cell re ...
when gene regions are rearranged to create the unique antigen binding sites of antibodies and T-cell receptors.
Sources of DNA damage
DNA damage occurs very frequently and is generated from exposure to a variety of both exogenous and endogenous genotoxic sources.
One of these include
ionizing radiation, such as
γ radiation and
X-rays
An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 picometers to 10 nanometers, corresponding to frequencies in the range 30&nbs ...
, which ionize the deoxyribose groups in the DNA backbone and can induce DSBs.
Reactive oxygen species,
ROS, such as
superoxide
In chemistry, a superoxide is a compound that contains the superoxide ion, which has the chemical formula . The systematic name of the anion is dioxide(1−). The reactive oxygen ion superoxide is particularly important as the product of t ...
(O
2–),
hydrogen peroxide
Hydrogen peroxide is a chemical compound with the formula . In its pure form, it is a very pale blue liquid that is slightly more viscous than water. It is used as an oxidizer, bleaching agent, and antiseptic, usually as a dilute solution (3% ...
(H
2O
2),
hydroxyl radical
The hydroxyl radical is the diatomic molecule . The hydroxyl radical is very stable as a dilute gas, but it decays very rapidly in the condensed phase. It is pervasive in some situations. Most notably the hydroxyl radicals are produced from the ...
s (HO
•), and
singlet oxygen (
1O
2), can also produce DSBs as a result of ionizing radiation as well as cellular metabolic processes that are naturally occurring.
DSBs can also be caused by the action of
DNA polymerase
A DNA polymerase is a member of a family of enzymes that catalyze the synthesis of DNA molecules from nucleoside triphosphates, the molecular precursors of DNA. These enzymes are essential for DNA replication and usually work in groups to create ...
while attempting to
replicate DNA over a
nick
Nick may refer to:
* Nick (given name)
* A cricket term for a slight deviation of the ball off the edge of the bat
* British slang for being arrested
* British slang for a police station
* British slang for stealing
* Short for nickname
Place ...
that was introduced as a result of DNA damage.
Consequences of DSBs
There are many types of
DNA damage
DNA repair is a collection of processes by which a cell identifies and corrects damage to the DNA molecules that encode its genome. In human cells, both normal metabolic activities and environmental factors such as radiation can cause DNA d ...
, but DSBs, in particular, are the most harmful as both strands are completely disjointed from the rest of the
chromosome
A chromosome is a long DNA molecule with part or all of the genetic material of an organism. In most chromosomes the very long thin DNA fibers are coated with packaging proteins; in eukaryotic cells the most important of these proteins are ...
. If an efficient repair mechanism does not exist, the ends of the DNA can eventually degrade, leading to a permanent loss of sequence.
A double-stranded gap in DNA will also prevent
replication from proceeding, resulting in an incomplete copy of that specific
chromosome
A chromosome is a long DNA molecule with part or all of the genetic material of an organism. In most chromosomes the very long thin DNA fibers are coated with packaging proteins; in eukaryotic cells the most important of these proteins are ...
, targeting the cell for
apoptosis. As with all DNA damage, DSBs can introduce new
mutation
In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA replication, DNA or viral repl ...
s that can ultimately lead to
cancer
Cancer is a group of diseases involving abnormal cell growth with the potential to invade or spread to other parts of the body. These contrast with benign tumors, which do not spread. Possible signs and symptoms include a lump, abnormal b ...
.
DSB repair methods
There are two methods for repairing DSBs depending on when the damage occurs during
mitosis.
If the DSB occurs after DNA replication has completed proceeding S phase of the
cell cycle
The cell cycle, or cell-division cycle, is the series of events that take place in a cell that cause it to divide into two daughter cells. These events include the duplication of its DNA (DNA replication) and some of its organelles, and sub ...
, the
DSB repair pathway will use
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 ...
by pairing with the newly synthesized daughter strand to repair the break. However, if the DSB is generated prior to synthesis of the sister chromosome, then the template sequence that is required will be absent.
For this circumstance, the
NHEJ pathway provides a solution for repairing the break and is the main system used to repair DSBs in humans and multicellular eukaryotes.
During NHEJ, very short stretches of complementary DNA, 1 bp or more at a time, are hybridized together, and the overhangs are removed. As a result, this specific region of the genome is permanently lost and the deletion can lead to cancer and premature aging.
Properties
Gene and protein
The human XRCC4
gene
In biology, the word gene (from , ; "... Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a b ...
is located on
chromosome 5
Chromosome 5 is one of the 23 pairs of chromosomes in humans. People normally have two copies of this chromosome. Chromosome 5 spans about 181 million base pairs (the building blocks of DNA) and represents almost 6% of the total DNA in cells. C ...
, specifically at 5q14.2. This gene contains eight
exons and three
mRNA
In molecular biology, messenger ribonucleic acid (mRNA) is a single-stranded molecule of RNA that corresponds to the genetic sequence of a gene, and is read by a ribosome in the process of synthesizing a protein.
mRNA is created during the ...
transcript variants, which encode two different
protein isoforms
A protein isoform, or "protein variant", is a member of a set of highly similar proteins that originate from a single gene or gene family and are the result of genetic differences. While many perform the same or similar biological roles, some iso ...
. Transcript variant 1, mRNA, RefSeq NM_003401.3, is 1688 bp long and is the shortest out of the three variants. It is missing a short sequence in the 3’
coding region as compared to variant 2. Isoform 1 contains 334
amino acid
Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha a ...
s. Transcript variant 2, mRNA, RefSeq NM_022406, is 1694 bp long and encodes the longest isoform 2, which contains 336
amino acids. Transcript variant 3, RefSeq NM_022550.2, is 1735 bp and is the longest, but it also encodes for the same isoform 1 as variant 1. It contains an additional sequence in the 5’UTR of the mRNA transcript and lacks a short sequence in the 3’
coding region as compared to variant 2.
Structure
XRCC4 protein is a
tetramer
A tetramer () ('' tetra-'', "four" + '' -mer'', "parts") is an oligomer formed from four monomers or subunits. The associated property is called ''tetramery''. An example from inorganic chemistry is titanium methoxide with the empirical formula ...
that resembles the shape of a dumbbell containing two globular ends separated by a long, thin stalk. The tetramer is composed of two
dimers, and each dimer is made up of two similar
subunits. The first subunit (L) contains amino acid residues 1 – 203 and has a longer stalk than the second subunit (S) which contains residues 1 – 178.
The globular
N-terminal domains of each subunit are identical. They are made up of two, antiparallel
beta sheets that face each other in a beta sandwich-like structure (i.e., a "flattened"
beta barrel
In protein structures, a beta barrel is a beta sheet composed of tandem repeats that twists and coils to form a closed toroidal structure in which the first strand is bonded to the last strand (hydrogen bond). Beta-strands in many beta-barrels are ...
) and are separated by two
alpha helices
The alpha helix (α-helix) is a common motif in the secondary structure of proteins and is a right hand-helix conformation in which every backbone N−H group hydrogen bonds to the backbone C=O group of the amino acid located four residues ear ...
on one side. The N-terminus begins with one beta sheet composed of strands 1, 2, 3, and 4, followed by a
helix-turn-helix
Helix-turn-helix is a DNA-binding protein (DBP). The helix-turn-helix (HTH) is a major structural motif capable of binding DNA. Each monomer incorporates two α helices, joined by a short strand of amino acids, that bind to the major groove of ...
motif of the two alpha helices, αA and αB, which continues into strands 5, 6, 7, and ending with one alpha-helical stalk at the
C-terminus. αA and αB are perpendicular to one another, and because one end of αB is partially inserted between the two beta sheets, it causes them to flare out away from each other. The beta sandwich structure is held together through three hydrogen bonds between antiparallel strands 4 and 7 and one hydrogen bond between strands 1 and 5.
The two helical stalks between subunits L and S intertwine with a single left-handed crossover into a
coiled-coil at the top, near the globular domains forming a palm tree configuration. This region interacts with the two alpha helices of the second dimer in an opposite orientation to form a
four-helix bundle A helix bundle is a small protein fold composed of several alpha helices that are usually nearly parallel or antiparallel to each other.
Three-helix bundles
Three-helix bundles are among the smallest and fastest known cooperatively folding struct ...
and the dumbbell-shaped tetramer.
Post-translational modifications
In order for XRCC4 to be sequestered from the
cytoplasm
In cell biology, the cytoplasm is all of the material within a eukaryotic cell, enclosed by the cell membrane, except for the cell nucleus. The material inside the nucleus and contained within the nuclear membrane is termed the nucleoplasm. ...
to the
nucleus
Nucleus ( : nuclei) is a Latin word for the seed inside a fruit. It most often refers to:
*Atomic nucleus, the very dense central region of an atom
* Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA
Nucl ...
to repair a DSB during NHEJ or to complete
V(D)J recombination
V(D)J recombination is the mechanism of somatic recombination that occurs only in developing lymphocytes during the early stages of T and B cell maturation. It results in the highly diverse repertoire of antibodies/immunoglobulins and T cell re ...
,
post-translational modification
Post-translational modification (PTM) is the covalent and generally enzymatic modification of proteins following protein biosynthesis. This process occurs in the endoplasmic reticulum and the golgi apparatus. Proteins are synthesized by ribos ...
at
lysine 210 with a small
ubiquitin
Ubiquitin is a small (8.6 kDa) regulatory protein found in most tissues of eukaryotic organisms, i.e., it is found ''ubiquitously''. It was discovered in 1975 by Gideon Goldstein and further characterized throughout the late 1970s and 1980s. Fo ...
-related modifier (SUMO), or
sumoylation, is required. SUMO modification of diverse types of DNA repair proteins can be found in
topoisomerases, base excision
glycosylase TDG, Ku70/80, and BLM
helicase
Helicases are a class of enzymes thought to be vital to all organisms. Their main function is to unpack an organism's genetic material. Helicases are motor proteins that move directionally along a nucleic acid phosphodiester backbone, separatin ...
. A common conserved motif is typically found to be a target of SUMO modification, ΨKXE (where Ψ is a bulky,
hydrophobic
In chemistry, hydrophobicity is the physical property of a molecule that is seemingly repelled from a mass of water (known as a hydrophobe). In contrast, hydrophiles are attracted to water.
Hydrophobic molecules tend to be nonpolar and, t ...
amino acid
Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha a ...
). In the case of the XRCC4 protein, the consensus sequence surrounding lysine 210 is IKQE.
Chinese hamster ovary cell
Chinese hamster ovary (CHO) cells are an epithelial cell line derived from the ovary of the Chinese hamster, often used in biological and medical research and commercially in the production of recombinant therapeutic proteins. They have found wide ...
s, CHO, that express the mutated form of XRCC4 at K210 cannot be modified with SUMO, fail recruitment to the nucleus and instead accumulate in the cytoplasm. Furthermore, these cells are
radiation sensitive and do not successfully complete V(D)J recombination.
Interactions
![Interaction of XRCC4 in NHEJ](https://upload.wikimedia.org/wikipedia/commons/6/6c/Interaction_of_XRCC4_in_NHEJ.jpg)
Upon generation of a DSB, Ku proteins will move through the cytoplasm until they find the site of the break and bind to it.
Ku recruits XRCC4 and
Cer-XLF and both of these proteins interact cooperatively with one another through specific residues to form a
nucleoprotein
Nucleoproteins are proteins conjugated with nucleic acids (either DNA or RNA). Typical nucleoproteins include ribosomes, nucleosomes and viral nucleocapsid proteins.
Structures
Nucleoproteins tend to be positively charged, facilitating int ...
pore complex that wraps around DNA. Cer-XLF is a homodimer that is very similar to XRCC4 in the structure and size of its
N-terminal and
C-terminal
The C-terminus (also known as the carboxyl-terminus, carboxy-terminus, C-terminal tail, C-terminal end, or COOH-terminus) is the end of an amino acid chain (protein or polypeptide), terminated by a free carboxyl group (-COOH). When the protein is ...
domains. Residues
arginine 64,
leucine
Leucine (symbol Leu or L) is an essential amino acid that is used in the biosynthesis of proteins. Leucine is an α-amino acid, meaning it contains an α- amino group (which is in the protonated −NH3+ form under biological conditions), an α- ...
65, and leucine 115 in Cer-XLF interact with lysines 65 and 99 in XRCC4 within their N-terminal domains. Together they form a filament bundle that wraps around DNA in an alternating pattern. Hyper-
phosphorylation of the C-terminal alpha helical domains of XRCC4 by
DNA-PKcs
DNA-dependent protein kinase, catalytic subunit, also known as DNA-PKcs, is an enzyme that in humans is encoded by the gene designated as ''PRKDC'' or ''XRCC7''. DNA-PKcs belongs to the phosphatidylinositol 3-kinase-related kinase protein family. ...
facilitates this interaction. XRCC4 dimer binds to a second dimer on an adjacent DNA strand to create a tetramer for DNA bridging early on in NHEJ. Prior to
ligation
Ligation may refer to:
* Ligation (molecular biology), the covalent linking of two ends of DNA or RNA molecules
* In medicine, the making of a ligature (tie)
* Chemical ligation, the production of peptides from amino acids
* Tubal ligation, a meth ...
, Lig IV binds to the C-terminal stalk of XRCC4 at the site of the break and displaces the second XRCC4 dimer.
The BRCT2 domain of Lig IV hydrogen bonds with XRCC4 at this domain through multiple residues and introduces a kink in the two alpha helical tails. The
helix-loop-helix clamp connected to the BRCT-linker also makes extensive contacts.
Mechanism
NHEJ
The process of NHEJ involves XRCC4 and a number of tightly coupled proteins acting in concert to repair the DSB. The system begins with the binding of one heterodimeric protein called Ku70/80 to each end of the DSB to maintain them close together in preparation for
ligation
Ligation may refer to:
* Ligation (molecular biology), the covalent linking of two ends of DNA or RNA molecules
* In medicine, the making of a ligature (tie)
* Chemical ligation, the production of peptides from amino acids
* Tubal ligation, a meth ...
and prevent their degradation.
Ku70/80 then sequesters one DNA-dependent protein kinase catalytic subunit (DNA-PKcs) to the DNA ends to enable the binding of Artemis protein to one end of each DNA-PKcs.
One end of the DNA-PKcs joins to stabilize the proximity of the DSB and allow very short regions of DNA complementarity to hybridize.
DNA-PKcs then
phosphorylates
In chemistry, phosphorylation is the attachment of a phosphate group to a molecule or an ion. This process and its inverse, dephosphorylation, are common in biology and could be driven by natural selection. Text was copied from this source, whi ...
Artemis at a
serine/
threonine to activate its
exonuclease activity and cleave
nucleotides
Nucleotides are organic molecules consisting of a nucleoside and a phosphate. They serve as monomeric units of the nucleic acid polymers – deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), both of which are essential biomolecules with ...
at the single strand tails that are not hybridized in a 5’ to 3’ direction.
Two XRCC4 proteins are post-
translationally modified for recognition and localization to Ku70/80 (5). The two XRCC4 proteins dimerize together and bind to Ku70/80 at the ends of the DNA strands to promote ligation. XRCC4 then forms a strong complex with DNA ligase IV, LigIV, which is enhanced by Cernunnos XRCC4-like factor, Cer-XLF.
Cer-XLF only binds to XRCC4 without direct interaction with LigIV. LigIV then joins the DNA ends by catalyzing a covalent
phosphodiester bond
In chemistry, a phosphodiester bond occurs when exactly two of the hydroxyl groups () in phosphoric acid react with hydroxyl groups on other molecules to form two ester bonds. The "bond" involves this linkage . Discussion of phosphodiesters is d ...
.
V(D)J recombination
V(D)J recombination is the rearrangement of multiple, distinct
gene
In biology, the word gene (from , ; "... Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a b ...
segments in germ-line DNA to produce the unique protein domains of
immune cells
White blood cells, also called leukocytes or leucocytes, are the cells of the immune system that are involved in protecting the body against both infectious disease and foreign invaders. All white blood cells are produced and derived from mult ...
,
B cells
B cells, also known as B lymphocytes, are a type of white blood cell of the lymphocyte subtype. They function in the humoral immunity component of the adaptive immune system. B cells produce antibody molecules which may be either secreted o ...
and
T cells
A T cell is a type of lymphocyte. T cells are one of the important white blood cells of the immune system and play a central role in the adaptive immune response. T cells can be distinguished from other lymphocytes by the presence of a T-cell re ...
, that will specifically recognize foreign
antigens such as
viruses
A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism. Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and archaea.
Since Dmitri Ivanovsky's ...
,
bacteria
Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typically a few micrometr ...
, and
pathogen
In biology, a pathogen ( el, πάθος, "suffering", "passion" and , "producer of") in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a germ ...
ic eukaryotes. B cells produce
antibodies that are secreted into the bloodstream and T cells produce receptors that once translated are transported to the outer
lipid bilayer
The lipid bilayer (or phospholipid bilayer) is a thin polar membrane made of two layers of lipid molecules. These membranes are flat sheets that form a continuous barrier around all cells. The cell membranes of almost all organisms and many vir ...
of the cell. Antibodies are composed of two light and two heavy chains. The antigen binding site consists of two variable regions, VL and VH. The remainder of the antibody structure is made up of constant regions, CL, CH, CH2 and CH3. The Kappa locus in the mouse encodes an antibody light chain and contains approximately 300 gene segments for the variable region, V, four J segments than encode a short protein region, and one constant, C, segment. To produce a light chain with one unique type of VL, when B cells are differentiating, DNA is rearranged to incorporate a unique combination of the V and J segments. RNA splicing joins the recombined region with the C segment. The heavy chain gene also contain numerous diversity segments, D, and multiple constant segments, Cμ, Cδ, Cγ, Cε, Cα.
Recombination occurs in a specific region of the gene that is located between two conserved sequence motifs called recombination signal sequences. Each motif is flanked by a 7 bp and 9 bp sequence that is separated by a 12 bp spacer, referred to as class 1, or a 23 bp spacer, referred to as class 2. A
recombinase made up of RAG1 and RAG2 subunits always cleave between these two sites. The cleavage results in two
hairpin
A hairpin or hair pin is a long device used to hold a person's hair in place. It may be used simply to secure long hair out of the way for convenience or as part of an elaborate hairstyle or coiffure. The earliest evidence for dressing the hai ...
structures for the V and J segments, respectively, and the non-coding region, are now separated from the V and J segments by a DSB. The hairpin coding region goes through the process of NHEJ where the closed end is cleaved and repaired. The non-coding region is circularized and degraded.
Thus, NHEJ is also important in the development of the immune system via its role in V(D)J recombination.
Pathology
Recent studies have shown an association between XRCC4 and potential susceptibility to a variety of pathologies. The most frequently observed linkage is between XRCC4 mutations and susceptibility to cancers such as bladder cancer, breast cancer, and lymphomas. Studies have also pointed to a potential linkage between XRCC4 mutation and endometriosis. Autoimmunity is also being studied in this regard. Linkage between XRCC4 mutations and certain pathologies may provide a basis for diagnostic biomarkers and, eventually, potential development of new therapeutics.
Cancer susceptibility
XRCC4
polymorphisms have been linked to a risk of susceptibility for cancers such as
bladder cancer
Bladder cancer is any of several types of cancer arising from the tissues of the urinary bladder. Symptoms include blood in the urine, pain with urination, and low back pain. It is caused when epithelial cells that line the bladder become ma ...
,
breast cancer
Breast cancer is cancer that develops from breast tissue. Signs of breast cancer may include a lump in the breast, a change in breast shape, dimpling of the skin, milk rejection, fluid coming from the nipple, a newly inverted nipple, or a r ...
,
prostate cancer,
hepatocellular carcinoma,
lymphoma
Lymphoma is a group of blood and lymph tumors that develop from lymphocytes (a type of white blood cell). In current usage the name usually refers to just the cancerous versions rather than all such tumours. Signs and symptoms may include enla ...
s, and
multiple myeloma.
With respect to bladder cancer, for example, the link between XRCC4 and risk of cancer susceptibility was based on hospital-based case-control histological studies of gene variants of both XRCC4 and XRCC3 and their possible association with risk for urothelial bladder cancer. The linkage with risk for urothelial bladder cancer susceptibility was shown for XRCC4, but not for XRCC3
With regard to breast cancer, the linkage with "increased risk of breast cancer" was based on an examination of functional polymorphisms of the XRCC4 gene carried out in connection with a meta-analysis of five case-control studies .
There is also at least one hospital-based case-control histological study indicating that polymorphisms in XRCC4 may have an "influence" on prostate cancer susceptibility.
Conditional (CD21-cre-mediated) deletion of the XRCC4 NHEJ gene in
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 s ...
-deficient peripheral mouse
B cells resulted in surface Ig-negative B-cell lymphomas, and these lymphomas often had a "reciprocal chromosomal translocation" fusing
IgH to
Myc
''Myc'' is a family of regulator genes and proto-oncogenes that code for transcription factors. The ''Myc'' family consists of three related human genes: ''c-myc'' (MYC), ''l-myc'' ( MYCL), and ''n-myc'' (MYCN). ''c-myc'' (also sometimes refe ...
(and also had "large chromosomal deletions or translocations" involving
IgK or
IgL, with IgL "fusing" to oncogenes or to
IgH).
XRCC4- and p53-deficient pro-B lymphomas "routinely activate c-myc by gene amplification"; and furthermore, XRCC4- and p53-deficient peripheral B-cell lymphomas "routinely ectopically activate" a single copy of c-myc.
Indeed, in view of the observation by some that “DNA repair enzymes are correctives for DNA damage induced by carcinogens and anticancer drugs”,
it should not be surprising that “SNPs in DNA repair genes may play an important part” in cancer susceptibility.
In addition to the cancers identified above, XRCC4 polymorphisms have been identified as having a potential link to various additional cancers such as
oral cancer
Oral cancer, also known as mouth cancer, is cancer of the lining of the lips, mouth, or upper throat. In the mouth, it most commonly starts as a painless white patch, that thickens, develops red patches, an ulcer, and continues to grow. When on ...
,
lung cancer
Lung cancer, also known as lung carcinoma (since about 98–99% of all lung cancers are carcinomas), is a malignant lung tumor characterized by uncontrolled cell growth in tissues of the lung. Lung carcinomas derive from transformed, malign ...
,
gastric cancer
Stomach cancer, also known as gastric cancer, is a cancer that develops from the lining of the stomach. Most cases of stomach cancers are gastric carcinomas, which can be divided into a number of subtypes, including gastric adenocarcinomas. Lymph ...
, and
glioma
A glioma is a type of tumor that starts in the glial cells of the brain or the spine. Gliomas comprise about 30 percent of all brain tumors and central nervous system tumours, and 80 percent of all malignant brain tumours.
Signs and symptoms
...
s.
Senescence
Declining ability to repair DNA double-strand breaks by
NHEJ
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 direc ...
may be a significant factor in the
aging process. Li et al.
found that, in humans, the efficiency of NHEJ repair declines from age 16 to 75 years. Their study indicated that decreased expression of XRCC4 and other NHEJ proteins drives an age-associated decline in NHEJ efficiency and fidelity. They suggested that the age related decline in expression of XRCC4 may contribute to cellular senescence.
Autoimmunity
Based on the findings that (1) several polypeptides in the NHEJ pathway are "potential targets of autoantibodies" and (2) "one of the autoimmune epitopes in XRCC4 coincides with a sequence that is a nexus for radiation-induced regulatory events", it has been suggested that exposure to DNA double-strand break-introducing agents "may be one of the factors" mediating autoimmune responses.
Endometriosis susceptibility
There has been speculation that "XRCC4 codon 247*A and XRCC4 promoter -1394*T related genotypes and alleles... might be associated with higher endometriosis susceptibilities and pathogenesis".
Potential use as a cancer biomarker
In view of the possible associations of XRCC4 polymorphisms with risk of cancer susceptibility (see discussion above), XRCC4 could be used as a
biomarker
In biomedical contexts, a biomarker, or biological marker, is a measurable indicator of some biological state or condition. Biomarkers are often measured and evaluated using blood, urine, or soft tissues to examine normal biological processes, p ...
for
cancer screening
Cancer screening aims to detect cancer before symptoms appear. This may involve blood tests, urine tests, DNA tests, other tests, or medical imaging. The benefits of screening in terms of cancer prevention, early detection and subsequent treatment ...
, particularly with respect to prostate cancer, breast cancer, and bladder cancer.
In fact, XRCC4 polymorphisms were specifically identified as having the potential to be novel useful markers for "primary prevention and anticancer intervention" in the case of urothelial bladder cancer.
Radiosensitization of tumor cells
In view of the role of XRCC4 in
DNA double-strand break repair, the relationship between impaired XRCC4 function and the
radiosensitization of tumor cells has been investigated. For instance, it has been reported that "
RNAi
RNA interference (RNAi) is a biological process in which RNA molecules are involved in sequence-specific suppression of gene expression by double-stranded RNA, through translational or transcriptional repression. Historically, RNAi was known by ...
-mediated targeting of noncoding and coding sequences in DNA repair gene messages efficiently radiosensitizes human tumor cells".
Potential role in therapeutics
There has been discussion in the literature concerning the potential role of XRCC4 in the development of novel therapeutics. For instance, Wu ''et al.'' have suggested that since the XRCC4 gene is "critical in NHEJ" and is "positively associated with cancer susceptibility", some XRCC4 SNPs such as G-1394T (rs6869366) "may serve as a common SNP for detecting and predict
ngvarious cancers (so far for breast, gastric and prostate cancers... )"; and, although further investigation is needed, "they may serve as candidate targets for personalized anticancer drugs".
The possibility of detecting endometriosis on this basis has also been mentioned, and this may also possibly lead to the eventual development of treatments.
In evaluating further possibilities for anticancer treatments, Wu ''et al''. also commented on the importance of “co-treatments of DNA-damaging agents and radiation”.
Specifically, Wu ''et al''. noted that the “balance between DNA damage and capacity of DNA repair mechanisms determines the final therapeutic outcome” and “the capacity of cancer cells to complete DNA repair mechanisms is important for therapeutic resistance and has a negative impact upon therapeutic efficacy”, and thus theorized that “
armacological inhibition of recently detected targets of DNA repair with several small-molecule compounds... has the potential to enhance the cytotoxicity of anticancer agents”.
Microcephalic primordial dwarfism
In humans, mutations in the XRCC4 gene cause microcephalic primordial dwarfism, a phenotype characterized by marked microcephaly, facial dysmorphism, developmental delay and short stature.
Although immunoglobulin junctional diversity is impaired, these individuals do not show a recognizable immunological phenotype.
In contrast to individuals with a LIG4 mutation, pancytopenia resulting in bone marrow failure is not observed in individuals with XRCC4 deficiency.
At the cellular level, disruption of XRCC4 induces hypersensitivity to agents that induce double-strand breaks, defective double-strand break repair and increased apoptosis after induction of DNA damage.
Anti-XRCC4 antibodies
Anti-XRCC4 antibodies including phosphospecific antibodies to pS260 and pS318 in XRCC4 have been developed.
[; ] Antibodies to XRCC4 can have a variety of uses, including use in immunoassays to conduct research in areas such as DNA damage and repair, non-homologous end joining, transcription factors, epigenetics and nuclear signaling.
History
Research carried out in the 1980s revealed that a Chinese hamster ovary (CHO) cell mutant called XR-1 was "extremely sensitive" with regard to being killed by gamma rays during the G1 portion of the cell cycle but, in the same research studies, showed "nearly normal resistance" to gamma-ray damage during the late S phase;
and in the course of this research, XR-1's cell-cycle sensitivity was correlated with its inability to repair DNA double-strand breaks produced by ionizing radiation and restriction enzymes.
In particular, in a study using somatic cell hybrids of XR-1 cells and human fibroblasts, Giaccia ''et al.'' (1989) showed that the XR-1 mutation was a recessive mutation;
and in follow-up to this work, Giaccia ''et al.'' (1990) carried out further studies examining the XR-1 mutation (again using somatic cell hybrids formed between XR-1 and human fibroblasts) and were able to map the human complementing gene to chromosome 5 using chromosome-segregation analysis.
Giaccia ''et al'', tentatively assigned this human gene the name “XRCC4” (an abbreviation of “X-ray-complementing Chinese hamster gene 4”) and determined that (a) the newly named XRCC4 gene biochemically restored the hamster defect to normal levels of resistance to gamma-ray radiation and bleomycin and (b) the XRCC4 gene restored the proficiency to repair DNA DSBs.
Based on these findings, Giaccia ''et al.'' proposed that XRCC4 ― as a single gene― was responsible for the XR-1 phenotype.
References
Further reading
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{{NLM content
Molecular biology
DNA repair