FANCA
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Fanconi anaemia, complementation group A, also known as FAA, FACA and FANCA, 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, respo ...
which in humans is encoded by the ''FANCA''
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 ba ...
. It belongs to the
Fanconi anaemia Fanconi anaemia (FA) is a rare genetic disease resulting in impaired response to DNA damage. Although it is a very rare disorder, study of this and other bone marrow failure syndromes has improved scientific understanding of the mechanisms of nor ...
complementation group (FANC) family of genes of which 12 complementation groups are currently recognized and is hypothesised to operate as a post-replication repair or a
cell cycle checkpoint Cell cycle checkpoints are control mechanisms in the eukaryotic cell cycle which ensure its proper progression. Each checkpoint serves as a potential termination point along the cell cycle, during which the conditions of the cell are assessed, wi ...
. FANCA proteins are involved in inter-strand DNA cross-link repair and in the maintenance of normal
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 ...
stability that regulates the differentiation of haematopoietic
stem cells In multicellular organisms, stem cells are undifferentiated or partially differentiated cells that can differentiate into various types of cells and proliferate indefinitely to produce more of the same stem cell. They are the earliest type o ...
into mature
blood cells A blood cell, also called a hematopoietic cell, hemocyte, or hematocyte, is a cell produced through hematopoiesis and found mainly in the blood. Major types of blood cells include red blood cells (erythrocytes), white blood cells (leukocytes), a ...
. Mutations involving the FANCA gene are associated with many somatic and congenital defects, primarily involving phenotypic variations of
Fanconi anaemia Fanconi anaemia (FA) is a rare genetic disease resulting in impaired response to DNA damage. Although it is a very rare disorder, study of this and other bone marrow failure syndromes has improved scientific understanding of the mechanisms of nor ...
,
aplastic anaemia Aplastic anemia is a cancer in which the body fails to make blood cells in sufficient numbers. Blood cells are produced in the bone marrow by stem cells that reside there. Aplastic anemia causes a deficiency of all blood cell types: red blood c ...
, and forms of
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 ...
such as
squamous cell carcinoma Squamous-cell carcinomas (SCCs), also known as epidermoid carcinomas, comprise a number of different types of cancer that begin in squamous cells. These cells form on the surface of the skin, on the lining of hollow organs in the body, and on the ...
and
acute myeloid leukaemia Acute myeloid leukemia (AML) is a cancer of the myeloid line of blood cells, characterized by the rapid growth of abnormal cells that build up in the bone marrow and blood and interfere with normal blood cell production. Symptoms may include ...
.


Function

The Fanconi anaemia complementation group (FANC) currently includes FANCA,
FANCB Fanconi anemia group B protein is a protein that in humans is encoded by the ''FANCB'' gene. Function The Fanconi anemia complementation group (FANC) currently includes FANCA, FANCB, FANCC, FANCD1 (also called BRCA2), FANCD2, FANCE, FANCF ...
,
FANCC Fanconi anemia group C protein is a protein that in humans is encoded by the ''FANCC'' gene. Structure Function The protein encoded by this gene delays the onset of apoptosis and promotes homologous recombination repair of damaged DNA. ...
, FANCD1 (also called
BRCA2 ''BRCA2'' and BRCA2 () are a human gene and its protein product, respectively. The official symbol (BRCA2, italic for the gene, nonitalic for the protein) and the official name (originally breast cancer 2; currently BRCA2, DNA repair associated) ...
),
FANCD2 Fanconi anemia group D2 protein is a protein that in humans is encoded by the ''FANCD2'' gene. The Fanconi anemia complementation group ( FANC) currently includes FANCA, FANCB, FANCC, FANCD1 (also called BRCA2), FANCD2 (this gene), FANCE, FANCF, F ...
,
FANCE Fanconi anemia, complementation group E protein is a protein that in humans is encoded by the ''FANCE'' gene. The Fanconi anemia complementation group (FANC) currently includes FANCA, FANCB, FANCC, FANCD1 (also called BRCA2), FANCD2, FANCE, FANC ...
,
FANCF Fanconi anemia group F protein is a protein that in humans is encoded by the ''FANCF'' gene. Interactions FANCF has been shown to interact with Fanconi anemia, complementation group C, FANCG, FANCA and FANCE. Function FANCF is an adaptor ...
,
FANCG Fanconi anemia group G protein is a protein that in humans is encoded by the ''FANCG'' gene. Function FANCG, involved in Fanconi anemia, confers resistance to both hygromycin B and mitomycin C. FANCG contains a 5-prime GC-rich untranslated ...
, and
FANCL E3 ubiquitin-protein ligase FANCL is an enzyme that in humans is encoded by the ''FANCL'' gene. Function The clinical phenotype of mutational defects in all Fanconi anemia (FA) complementation groups is similar. This phenotype is characteriz ...
. The previously defined group FANCH is the same as FANCA. The members of the Fanconi anaemia complementation group do not share sequence similarity; they are related by their assembly into a common nuclear protein complex. The FANCA gene encodes the protein for complementation group A. Alternative splicing results in multiple transcript variants encoding different isoforms.


Gene and protein

In humans, the gene FANCA is 79 kilobases (kb) in length, and is located on
chromosome 16 Chromosome 16 is one of the 23 pairs of chromosomes in humans. People normally have two copies of this chromosome. Chromosome 16 spans about 90 million base pairs (the building material of DNA) and represents just under 3% of the total DNA in cell ...
(16q24.3). The FANCA protein is composed of 1455
amino acids 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 am ...
. Within cells, the major purpose of FANCA belongs to its putative involvement in a multisubunit FA complex composed of FANCA,
FANCB Fanconi anemia group B protein is a protein that in humans is encoded by the ''FANCB'' gene. Function The Fanconi anemia complementation group (FANC) currently includes FANCA, FANCB, FANCC, FANCD1 (also called BRCA2), FANCD2, FANCE, FANCF ...
,
FANCC Fanconi anemia group C protein is a protein that in humans is encoded by the ''FANCC'' gene. Structure Function The protein encoded by this gene delays the onset of apoptosis and promotes homologous recombination repair of damaged DNA. ...
,
FANCE Fanconi anemia, complementation group E protein is a protein that in humans is encoded by the ''FANCE'' gene. The Fanconi anemia complementation group (FANC) currently includes FANCA, FANCB, FANCC, FANCD1 (also called BRCA2), FANCD2, FANCE, FANC ...
,
FANCF Fanconi anemia group F protein is a protein that in humans is encoded by the ''FANCF'' gene. Interactions FANCF has been shown to interact with Fanconi anemia, complementation group C, FANCG, FANCA and FANCE. Function FANCF is an adaptor ...
,
FANCG Fanconi anemia group G protein is a protein that in humans is encoded by the ''FANCG'' gene. Function FANCG, involved in Fanconi anemia, confers resistance to both hygromycin B and mitomycin C. FANCG contains a 5-prime GC-rich untranslated ...
, FANCL/PHF9 and FANCM. In complex with FANCF, FANCG and FANCL, FANCA interacts with HES1. This interaction has been proposed as essential for the stability and nuclear localization of FA core complex proteins. The complex with FANCC and FANCG may also include EIF2AK2 and HSP70. In cells, FANCA involvement in this ‘FA core complex’ is required for the activation of the
FANCD2 Fanconi anemia group D2 protein is a protein that in humans is encoded by the ''FANCD2'' gene. The Fanconi anemia complementation group ( FANC) currently includes FANCA, FANCB, FANCC, FANCD1 (also called BRCA2), FANCD2 (this gene), FANCE, FANCF, F ...
protein to a monoubiquitinated isoform (FANCD2-Ub) in response to
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 da ...
, catalysing activation of the FA/BRCA DNA damage-response pathway, leading to repair. FANCA binds to both single-stranded (ssDNA) and double-stranded (dsDNA) DNAs; however, when tested in an electrophoretic mobility shift
assay An assay is an investigative (analytic) procedure in laboratory medicine, mining, pharmacology, environmental biology and molecular biology for qualitatively assessing or quantitatively measuring the presence, amount, or functional activity of a ...
, its affinity for ssDNA is significantly higher than for dsDNA. FANCA also binds to RNA with a higher affinity than its DNA counterpart. FANCA requires a certain number of nucleotides for optimal binding, with the minimum for FANCA recognition being approximately 30 for both DNA and RNA. Yuan et al. (2012) found through affinity testing FANCA with a variety of DNA structures that a 5'-flap or 5'-tail on DNA facilitates its interaction with FANCA, while the complementing C-terminal fragment of Q772X, C772-1455, retains the differentiated nucleic acid-binding activity (i.e. preferencing RNA before ssDNA and dsDNA), indicating that the nucleic acid-binding domain of FANCA is located primarily at the C terminus, a location where many disease-causing mutations are found. FANCA is ubiquitously expressed at low levels in all cells with subcellular localisation in primarily
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 Nucle ...
but also
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. The ...
corresponding with its putative caretaker role in DNA damage-response pathways, and FA complex formation. The distribution of proteins in different tissues is not well understood currently. Immunochemical study of mouse tissue indicates that FANCA is present at a higher level in
lymphoid The lymphatic system, or lymphoid system, is an organ system in vertebrates that is part of the immune system, and complementary to the circulatory system. It consists of a large network of lymphatic vessels, lymph nodes, lymphatic or lymphoid o ...
tissues, the
testis A testicle or testis (plural testes) is the male reproductive gland or gonad in all bilaterians, including humans. It is homologous to the female ovary. The functions of the testes are to produce both sperm and androgens, primarily testostero ...
and the
ovary The ovary is an organ in the female reproductive system that produces an ovum. When released, this travels down the fallopian tube into the uterus, where it may become fertilized by a sperm. There is an ovary () found on each side of the body. ...
, and though the significance of this is unclear, it suggests that the presence of FA proteins might be related to cellular proliferation. For example, in human immortalized
lymphoblasts __NOTOC__ A lymphoblast is a modified naive lymphocyte with altered cell morphology. It occurs when the lymphocyte is activated by an antigen (from antigen-presenting cells) and increased in volume by nucleus and cytoplasm growth as well as new mRN ...
and
leukaemia Leukemia ( also spelled leukaemia and pronounced ) is a group of blood cancers that usually begin in the bone marrow and result in high numbers of abnormal blood cells. These blood cells are not fully developed and are called ''blasts'' or ' ...
cells, FA proteins are readily detectable by
immunoprecipitation Immunoprecipitation (IP) is the technique of precipitating a protein antigen out of solution using an antibody that specifically binds to that particular protein. This process can be used to isolate and concentrate a particular protein from a samp ...
.


Clinical significance

Mutations 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 or viral replication, mi ...
in this gene are the most common cause of Fanconi's anaemia. Fanconi anaemia is an inherited autosomal recessive disorder, the main features of which are
aplastic anaemia Aplastic anemia is a cancer in which the body fails to make blood cells in sufficient numbers. Blood cells are produced in the bone marrow by stem cells that reside there. Aplastic anemia causes a deficiency of all blood cell types: red blood c ...
in childhood, multiple congenital abnormalities, susceptibility to
leukemia Leukemia ( also spelled leukaemia and pronounced ) is a group of blood cancers that usually begin in the bone marrow and result in high numbers of abnormal blood cells. These blood cells are not fully developed and are called ''blasts'' or ' ...
and other cancers, and cellular hypersensitivity to interstrand DNA cross-linking agents. Generally cells from Fanconi anaemia patients show a markedly higher frequency of spontaneous chromosomal breakage and hypersensitivity to the clastogenic effect of DNA cross-linking agents such as
diepoxybutane Diepoxybutane (also known as butane diepoxide, butadiene diepoxide, or 1,2:3,4-diepoxybutane) is a chemical compound with two epoxide functional groups. It is used as a chemical intermediate, as a curing agent for polymers, as a cross-linking ag ...
(DEB) and mitomycin-C (MMC) when compared to normal cells. The primary diagnostic test for Fanconi anaemia is based on the increased chromosomal breakage seen in afflicted cells after exposure to these agents – the DEB/MMC stress test. Other features of the Fanconi anaemia cell phenotype also include abnormal cell cycle kinetics (prolonged G2 phase), hypersensitivity to
oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as wel ...
, increased
apoptosis Apoptosis (from grc, ἀπόπτωσις, apóptōsis, 'falling off') is a form of programmed cell death that occurs in multicellular organisms. Biochemical events lead to characteristic cell changes (morphology) and death. These changes incl ...
and accelerated
telomere A telomere (; ) is a region of repetitive nucleotide sequences associated with specialized proteins at the ends of linear chromosomes. Although there are different architectures, telomeres, in a broad sense, are a widespread genetic feature mos ...
shortening. FANCA mutations are by far the most common cause of Fanconi anaemia, accounting for between 60-70% of all cases. FANCA was cloned in 1996 and it is one of the largest FA genes. Hundreds of different
mutations 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 or viral replication, mi ...
have been recorded with 30% point mutations, 30% 1-5 base pair microdeletions or microinsertions, and 40% large deletions, removing up to 31
exons An exon is any part of a gene that will form a part of the final mature RNA produced by that gene after introns have been removed by RNA splicing. The term ''exon'' refers to both the DNA sequence within a gene and to the corresponding sequence ...
from the gene. These large deletions have a high correlation with specific breakpoints and arise as a result of Alu mediated recombination. A highly relevant observation is that different mutations produce Fanconi anaemia phenotypes of varying severity. Patients
homozygous Zygosity (the noun, zygote, is from the Greek "yoked," from "yoke") () is the degree to which both copies of a chromosome or gene have the same genetic sequence. In other words, it is the degree of similarity of the alleles in an organism. Mo ...
for null-mutations in this gene have an earlier onset of
anaemia Anemia or anaemia (British English) is a blood disorder in which the blood has a reduced ability to carry oxygen due to a lower than normal number of red blood cells, or a reduction in the amount of hemoglobin. When anemia comes on slowly, th ...
than those with mutations that produce an altered or incorrect protein. However, as most patients are
compound heterozygotes In medical genetics, compound heterozygosity is the condition of having two or more heterogeneous recessive alleles at a particular locus that can cause genetic disease in a heterozygous state; that is, an organism is a compound heterozygote when it ...
, diagnostic screening for mutations is difficult. Certain founder mutations can also occur in some populations, such as the deletion exon 12-31 mutation, which accounts for 60% of mutations in Afrikaners.


Involvement in FA/BRCA pathway

In cells from Fanconi anaemia patients, FA core complex induction of
FANCD2 Fanconi anemia group D2 protein is a protein that in humans is encoded by the ''FANCD2'' gene. The Fanconi anemia complementation group ( FANC) currently includes FANCA, FANCB, FANCC, FANCD1 (also called BRCA2), FANCD2 (this gene), FANCE, FANCF, F ...
ubiquitination 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 ...
is not observed, assumably a result from impaired complex formation due to the lack of a working FANCA protein. Ultimately, regardless of specific mutation, it is disruption of this FA/BRCA pathway that results in the adverse cellular and clinical
phenotypes In genetics, the phenotype () is the set of observable characteristics or traits of an organism. The term covers the organism's morphology or physical form and structure, its developmental processes, its biochemical and physiological proper ...
common to all FANCA-impaired Fanconi anaemia sufferers. Interactions between BRCA1 and many FANC proteins have been investigated. Amongst known FANC proteins, most evidence points for a direct interaction primarily between FANCA protein and BRCA1. Evidence from yeast two-hybrid analysis, coimmunoprecipitation from ''in vitro'' synthesis, and coimmunoprecipitation from cell extracts shows that the site of interaction is between the terminal amino group of FANCA and the central part of BRCA1, located within amino acids 740–1083. However, as FANCA and
BRCA1 Breast cancer type 1 susceptibility protein is a protein that in humans is encoded by the ''BRCA1'' () gene. Orthologs are common in other vertebrate species, whereas invertebrate genomes may encode a more distantly related gene. ''BRCA1'' is a h ...
undergo a constitutive interaction, this may not depend solely on detection of actual DNA damage. Instead BRCA1 protein may be more crucial in the detection of double stranded DNA breaks, or an intermediate in interstrand
crosslink In chemistry and biology a cross-link is a bond or a short sequence of bonds that links one polymer chain to another. These links may take the form of covalent bonds or ionic bonds and the polymers can be either synthetic polymers or natural ...
(ICL) repair, and rather serve to bring some of the many DNA repair proteins it interacts with to the site. One such protein would be FANCA, which in turn may serve as a docking site or anchor point at the site of ICL damage for the FA core complex. Other FANC proteins, such as
FANCC Fanconi anemia group C protein is a protein that in humans is encoded by the ''FANCC'' gene. Structure Function The protein encoded by this gene delays the onset of apoptosis and promotes homologous recombination repair of damaged DNA. ...
,
FANCE Fanconi anemia, complementation group E protein is a protein that in humans is encoded by the ''FANCE'' gene. The Fanconi anemia complementation group (FANC) currently includes FANCA, FANCB, FANCC, FANCD1 (also called BRCA2), FANCD2, FANCE, FANC ...
and
FANCG Fanconi anemia group G protein is a protein that in humans is encoded by the ''FANCG'' gene. Function FANCG, involved in Fanconi anemia, confers resistance to both hygromycin B and mitomycin C. FANCG contains a 5-prime GC-rich untranslated ...
are then assembled in this nuclear complex in the presence of FANCA as required for the action of
FANCD2 Fanconi anemia group D2 protein is a protein that in humans is encoded by the ''FANCD2'' gene. The Fanconi anemia complementation group ( FANC) currently includes FANCA, FANCB, FANCC, FANCD1 (also called BRCA2), FANCD2 (this gene), FANCE, FANCF, F ...
. This mechanic is also supported by the protein-protein interactions between BRG1 and both BRCA1 and FANCA, that serve to modulate cell-cycle kinetics alongside this. Alternatively, BRCA1 might localize FANCA to the site of DNA damage and then release it to initiate complex formation. The complex would allow
ubiquitination 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 ...
of FANCD2, a later functioning protein in the FA path, promoting ICL and DNA repair. FANCA’s emerging putative and clearly integral function within activation the FA core complex also provides an explanation for its particularly high correlation with mutations causing Fanconi anaemia. Whilst many FANC protein mutations account for only 1% of the total observed cases, they are also stabilized by FANCA within the complex. For example, FANCA stabilises
FANCG Fanconi anemia group G protein is a protein that in humans is encoded by the ''FANCG'' gene. Function FANCG, involved in Fanconi anemia, confers resistance to both hygromycin B and mitomycin C. FANCG contains a 5-prime GC-rich untranslated ...
within the core complex, and hence mutations in FANCG are compensated for as the complex can still catalyse FANCD2-
ubiquitination 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 ...
further downstream. FANCA
upregulation In the biological context of organisms' production of gene products, downregulation is the process by which a cell decreases the quantity of a cellular component, such as RNA or protein, in response to an external stimulus. The complementary proc ...
also increases expression of FANCG in cells, and the fact this transduction is not mutual – FANCG upregulation does not cause increased expression of FANCA – suggests that FANCA is not only the primary stabilizing protein in the core complex, but may act as a natural regulator in patients who would otherwise suffer from mutations in FANC genes other than FANCA or FANCD2.


Participation in haematopoiesis

FANCA is hypothesised to play a crucial role in adult (definitive)
haematopoiesis Haematopoiesis (, from Greek , 'blood' and 'to make'; also hematopoiesis in American English; sometimes also h(a)emopoiesis) is the formation of blood cellular components. All cellular blood components are derived from haematopoietic stem cells. ...
during embryonic development, and is thought to be expressed in all haematopoietic sites that contribute to the formation of haematopoietic stem cells and
progenitor cells In genealogy, the progenitor (rarer: primogenitor; german: Stammvater or ''Ahnherr'') is the – sometimes legendary – founder of a family, line of descent, clan or tribe, noble house, or ethnic group.. Ebenda''Ahnherr:''"Stammvater eines G ...
(HSPCs). Most patients with a mutation develop haematological abnormalities within the first decade of life, and continue to decline until developing its most prevalent adverse effect,
pancytopenia Pancytopenia is a medical condition in which there is significant reduction in the number of almost all blood cells (red blood cells, white blood cells, platelets, monocytes, lymphocytes, etc.). If only two parameters from the complete blood coun ...
, potentially leading to death. In particular many patients develop
megaloblastic anaemia Megaloblastic anemia is a type of macrocytic anemia. An anemia is a red blood cell defect that can lead to an undersupply of oxygen. Megaloblastic anemia results from inhibition of DNA synthesis during red blood cell production. When DNA synth ...
around the age of 7, with this macrocytosis being the first haematological marker. Defective in vitro
haematopoiesis Haematopoiesis (, from Greek , 'blood' and 'to make'; also hematopoiesis in American English; sometimes also h(a)emopoiesis) is the formation of blood cellular components. All cellular blood components are derived from haematopoietic stem cells. ...
has been recorded for over two decades resulting from mutated FANCA proteins, in particular developmental defects such as impaired granulomonocytopoiesis due to FANCA mutation. Studies using clonogenic myeloid progenitors (CFU-GM) have also shown that the frequency of CFU-GM in normal
bone marrow Bone marrow is a semi-solid tissue found within the spongy (also known as cancellous) portions of bones. In birds and mammals, bone marrow is the primary site of new blood cell production (or haematopoiesis). It is composed of hematopoietic ce ...
increased and their proliferative capacity decreased exponentially with age, with a particularly marked proliferative impairment in Fanconi anaemia afflicted children compared to age-matched healthy controls. As haematopoietic progenitor cell function begins at birth and continues throughout life, it is easily inferred that prolonged incapacitation of FANCA protein production results in total haematopoietic failure in patients.


Potential impact on erythroid development

The three distinct stages of
mammalian Mammals () are a group of vertebrate animals constituting the class (biology), class Mammalia (), characterized by the presence of mammary glands which in Female#Mammalian female, females produce milk for feeding (nursing) their young, a ...
erythroid development are primitive, foetal and adult definitive. Adult, or definitive
erythrocytes Red blood cells (RBCs), also referred to as red cells, red blood corpuscles (in humans or other animals not having nucleus in red blood cells), haematids, erythroid cells or erythrocytes (from Greek ''erythros'' for "red" and ''kytos'' for "holl ...
are the most common blood cell type and characteristically most similar across mammalian species. Primitive and foetal erythrocytes however, have markedly different characteristics. These include: they are larger in size (primitive even more so than foetal), circulate during early stages of development with a shorter lifespan, and, in particular, primitive cells are
nucleated The cell nucleus (pl. nuclei; from Latin or , meaning ''kernel'' or ''seed'') is a membrane-bound organelle found in eukaryotic cells. Eukaryotic cells usually have a single nucleus, but a few cell types, such as mammalian red blood cells, ...
. As the reasons for these disparities are not well understood, FANCA may be a gene responsible for instigating these morphological differences when considering its variations in erythrocyte expression. In primitive and foetal erythrocyte precursors, FANCA expression is low, and almost zero during
reticulocyte Reticulocytes are immature red blood cells (RBCs). In the process of erythropoiesis (red blood cell formation), reticulocytes develop and mature in the bone marrow and then circulatory system, circulate for about a day in the blood stream before ...
formation. The marginal overall increase in the foetal stage is dwarfed by its sudden increase in expression solely during adult definitive proerythroblast formation. Here, the mean expression increases by 400% compared to foetal and primitive erythrocytes, and covers a huge margin of deviation. As FANCA is heavily implicated in controlling cellular proliferation, and often results in patients developing
megaloblastic anaemia Megaloblastic anemia is a type of macrocytic anemia. An anemia is a red blood cell defect that can lead to an undersupply of oxygen. Megaloblastic anemia results from inhibition of DNA synthesis during red blood cell production. When DNA synth ...
around age 7, a haematological disorder marked physically by proliferation-impaired, oversized erythrocytes, it is possible that the size and proliferative discrepancies between primitive, foetal and adult erythroid lineages may be explained by FANCA expression. As FANCA is also linked to cell-cycling and its progression from G2 phase, the stage impaired in megaloblastic anaemia, its expression in definitive proerythroblast development may be an upstream determinant of erythroid size.


Implications in cancer

FANCA mutations have also been implicated in increased risks of
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 ...
and malignancies. For example, patients with homozygous null-mutations in FANCA have a markedly increased susceptibility to
acute myeloid leukaemia Acute myeloid leukemia (AML) is a cancer of the myeloid line of blood cells, characterized by the rapid growth of abnormal cells that build up in the bone marrow and blood and interfere with normal blood cell production. Symptoms may include ...
. Furthermore, as FANC mutations in general affect
DNA repair 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 dam ...
throughout the body and are predisposed to affect dynamic
cell division Cell division is the process by which a parent cell (biology), cell divides into two daughter cells. Cell division usually occurs as part of a larger cell cycle in which the cell grows and replicates its chromosome(s) before dividing. In eukar ...
particularly in
bone marrow Bone marrow is a semi-solid tissue found within the spongy (also known as cancellous) portions of bones. In birds and mammals, bone marrow is the primary site of new blood cell production (or haematopoiesis). It is composed of hematopoietic ce ...
, it is unsurprising that patients are more likely to develop
myelodysplastic syndromes A myelodysplastic syndrome (MDS) is one of a group of cancers in which immature blood cells in the bone marrow do not mature, and as a result, do not develop into healthy blood cells. Early on, no symptoms typically are seen. Later, symptoms may ...
(MDS) and
acute myeloid leukaemia Acute myeloid leukemia (AML) is a cancer of the myeloid line of blood cells, characterized by the rapid growth of abnormal cells that build up in the bone marrow and blood and interfere with normal blood cell production. Symptoms may include ...
.


Mouse knockout

Knockout mice A knockout mouse, or knock-out mouse, is a genetically modified mouse (''Mus musculus'') in which researchers have inactivated, or "knocked out", an existing gene by replacing it or disrupting it with an artificial piece of DNA. They are importan ...
have been generated for FANCA. However, both single and double knockout murine models are healthy, viable, and do not readily show the phenotypic abnormalities typical of human Fanconi anaemia sufferers, such as haematological failure and increased susceptibility to cancers. Other markers such as
infertility Infertility is the inability of a person, animal or plant to reproduce by natural means. It is usually not the natural state of a healthy adult, except notably among certain eusocial species (mostly haplodiploid insects). It is the normal state ...
however still do arise. This can be seen as evidence for a lack of functional redundancy in the FANCA gene-encoded proteins. Murine models instead require induction of typical anaemic phenotypes by elevated dosing with MMC that does not affect wild-type animals, before they can be used experimentally as preclinical models for bone marrow failure and potential
stem cell transplant Hematopoietic stem-cell transplantation (HSCT) is the transplantation of multipotent hematopoietic stem cells, usually derived from bone marrow, peripheral blood, or umbilical cord blood in order to replicate inside of a patient and to produc ...
or gene therapies. Both female and male mice homozygous for a FANCA mutation show
hypogonadism Hypogonadism means diminished functional activity of the gonads—the testes or the ovaries—that may result in diminished production of sex hormones. Low androgen (e.g., testosterone) levels are referred to as hypoandrogenism and low estrogen ...
and impaired
fertility Fertility is the capability to produce offspring through reproduction following the onset of sexual maturity. The fertility rate is the average number of children born by a female during her lifetime and is quantified demographically. Fertili ...
.
Homozygous Zygosity (the noun, zygote, is from the Greek "yoked," from "yoke") () is the degree to which both copies of a chromosome or gene have the same genetic sequence. In other words, it is the degree of similarity of the alleles in an organism. Mo ...
mutant females exhibit premature reproductive senescence and an increased frequency of
ovarian cyst An ovarian cyst is a fluid-filled sac within the ovary. Often they cause no symptoms. Occasionally they may produce bloating, lower abdominal pain, or lower back pain. The majority of cysts are harmless. If the cyst either breaks open or causes ...
s. In
spermatocyte Spermatocytes are a type of male gametocyte in animals. They derive from immature germ cells called spermatogonia. They are found in the testis, in a structure known as the seminiferous tubules. There are two types of spermatocytes, primary and s ...
s, the FANCA protein is ordinarily present at a high level during the
pachytene Meiosis (; , since it is a reductional division) is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, such as sperm or egg cells. It involves two rounds of division that ultimately resu ...
stage of
meiosis Meiosis (; , since it is a reductional division) is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, such as sperm or egg cells. It involves two rounds of division that ultimately resu ...
. This is the stage when chromosomes are fully synapsed, and
Holliday junction A Holliday junction is a branched nucleic acid structure that contains four double-stranded arms joined. These arms may adopt one of several conformations depending on buffer salt concentrations and the sequence of nucleobases closest to the ju ...
s are formed and then resolved into recombinants. FANCA mutant males exhibit an increased frequency of mispaired meiotic chromosomes, implying a role for FANCA in meiotic recombination. Also apoptosis is increased in the mutant
germ cell Germ or germs may refer to: Science * Germ (microorganism), an informal word for a pathogen * Germ cell, cell that gives rise to the gametes of an organism that reproduces sexually * Germ layer, a primary layer of cells that forms during embry ...
s. The Fanconi anemia DNA repair pathway appears to play a key role in meiotic recombination and the maintenance of reproductive germ cells. Loss of FANCA provokes neural progenitor
apoptosis Apoptosis (from grc, ἀπόπτωσις, apóptōsis, 'falling off') is a form of programmed cell death that occurs in multicellular organisms. Biochemical events lead to characteristic cell changes (morphology) and death. These changes incl ...
during forebrain development, likely related to defective DNA repair. This effect persists in adulthood leading to depletion of the neural stem cell pool with aging. The
Fanconi anemia Fanconi anaemia (FA) is a rare genetic disease resulting in impaired response to DNA damage. Although it is a very rare disorder, study of this and other bone marrow failure syndromes has improved scientific understanding of the mechanisms of no ...
phenotype can be interpreted as a premature aging of stem cells, DNA damages being the driving force of aging. (Also see
DNA damage theory of aging The DNA damage theory of aging proposes that aging is a consequence of unrepaired accumulation of naturally occurring DNA damage. Damage in this context is a DNA alteration that has an abnormal structure. Although both mitochondrial and nuclear ...
.)


Interactions

FANCA has been shown to
interact Advocates for Informed Choice, dba interACT or interACT Advocates for Intersex Youth, is a 501(c)(3) nonprofit organization using innovative strategies to advocate for the legal and human rights of children with intersex traits. The organizati ...
with: *
BRCA1 Breast cancer type 1 susceptibility protein is a protein that in humans is encoded by the ''BRCA1'' () gene. Orthologs are common in other vertebrate species, whereas invertebrate genomes may encode a more distantly related gene. ''BRCA1'' is a h ...
, *
CHUK Inhibitor of nuclear factor kappa-B kinase subunit alpha (IKK-α) also known as IKK1 or conserved helix-loop-helix ubiquitous kinase (CHUK) is a protein kinase that in humans is encoded by the ''CHUK'' gene. IKK-α is part of the IκB kinase ...
, *
ERCC4 ERCC4 is a protein designated as DNA repair endonuclease XPF that in humans is encoded by the ''ERCC4'' gene. Together with ERCC1, ERCC4 forms the ERCC1-XPF enzyme complex that participates in DNA repair and DNA recombination. The nuclease enzy ...
, *
FANCE Fanconi anemia, complementation group E protein is a protein that in humans is encoded by the ''FANCE'' gene. The Fanconi anemia complementation group (FANC) currently includes FANCA, FANCB, FANCC, FANCD1 (also called BRCA2), FANCD2, FANCE, FANC ...
, *
FANCF Fanconi anemia group F protein is a protein that in humans is encoded by the ''FANCF'' gene. Interactions FANCF has been shown to interact with Fanconi anemia, complementation group C, FANCG, FANCA and FANCE. Function FANCF is an adaptor ...
, *
FANCG Fanconi anemia group G protein is a protein that in humans is encoded by the ''FANCG'' gene. Function FANCG, involved in Fanconi anemia, confers resistance to both hygromycin B and mitomycin C. FANCG contains a 5-prime GC-rich untranslated ...
, *
FANCC Fanconi anemia group C protein is a protein that in humans is encoded by the ''FANCC'' gene. Structure Function The protein encoded by this gene delays the onset of apoptosis and promotes homologous recombination repair of damaged DNA. ...
, *
IKK2 IKK-β also known as inhibitor of nuclear factor kappa-B kinase subunit beta is a protein that in humans is encoded by the ''IKBKB'' (inhibitor of kappa light polypeptide gene enhancer in B-cells, kinase beta) gene. Function IKK-β is an enzy ...
, *
SMARCA4 Transcription activator BRG1 also known as ATP-dependent chromatin remodeler SMARCA4 is a protein that in humans is encoded by the ''SMARCA4'' gene. Function The protein encoded by this gene is a member of the SWI/SNF family of proteins and i ...
*
SNX5 Sorting nexin-5 is a protein that in humans is encoded by the ''SNX5'' gene. This gene encodes a member of the sorting nexin family. Members of this family contain a phox (PX) domain, which is a phosphoinositide binding domain, and are involved i ...
*
SPTAN1 Alpha II-spectrin, also known as Spectrin alpha chain, brain is a protein that in humans is encoded by the ''SPTAN1'' gene. Alpha II-spectrin is expressed in a variety of tissues, and is highly expressed in cardiac muscle at Z-disc structures, co ...
and *
HES1 Transcription factor HES1 (hairy and enhancer of split-1) is a protein that is encoded by the ''Hes1'' gene, and is the mammalian homolog of the hairy gene in ''Drosophila.'' HES1 is one of the seven members of the Hes gene family (HES1-7). Hes ge ...


References

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