OLIG2
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Oligodendrocyte transcription factor (OLIG2) is a basic helix-loop-helix ( bHLH) transcription factor encoded by the ''OLIG2'' gene. The protein is of 329 amino acids in length, 32 kDa in size and contains one basic helix-loop-helix DNA-binding domain. It is one of the three members of the bHLH family. The other two members are OLIG1 and OLIG3. The expression of OLIG2 is mostly restricted in
central nervous system The central nervous system (CNS) is the part of the nervous system consisting primarily of the brain, spinal cord and retina. The CNS is so named because the brain integrates the received information and coordinates and influences the activity o ...
, where it acts as both an anti-neurigenic and a neurigenic factor at different stages of development. OLIG2 is well known for determining
motor neuron A motor neuron (or motoneuron), also known as efferent neuron is a neuron whose cell body is located in the motor cortex, brainstem or the spinal cord, and whose axon (fiber) projects to the spinal cord or outside of the spinal cord to directly o ...
and
oligodendrocyte Oligodendrocytes (), also known as oligodendroglia, are a type of neuroglia whose main function is to provide the myelin sheath to neuronal axons in the central nervous system (CNS). Myelination gives metabolic support to, and insulates the axons ...
differentiation, as well as its role in sustaining replication in early development. It is mainly involved in diseases such as brain tumor and Down syndrome.


Function

OLIG2 is mostly expressed in restricted domains of the brain and
spinal cord The spinal cord is a long, thin, tubular structure made up of nervous tissue that extends from the medulla oblongata in the lower brainstem to the lumbar region of the vertebral column (backbone) of vertebrate animals. The center of the spinal c ...
ventricular zone which give rise to oligodendrocytes and specific types of neurons. In the spinal cord, the pMN region sequentially generates motor neurons and oligodendrocytes. During
embryogenesis An embryo ( ) is the initial stage of development for a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male ...
, OLIG2 first directs motor neuron fate by establishing a ventral domain of motor neuron progenitors and promoting neuronal differentiation. OLIG2 then switches to promoting the formation of oligodendrocyte precursors and oligodendrocyte differentiation at later stages of development. Apart from functioning as a neurogenic factor in specification and the differentiation of motor neurons and oligodendrocytes, OLIG2 also functions as an anti-neurogenic factor at early time points in pMN progenitors to sustain the cycling progenitor pool. This side of anti-neurogenicity of OLIG2 later plays a bigger role in malignancies like
glioma A glioma is a type of primary tumor that starts in the glial cells of the brain or spinal cord. They are malignant but some are extremely slow to develop. Gliomas comprise about 30% of all brain and central nervous system tumors and 80% of ...
. The role of
phosphorylation In biochemistry, phosphorylation is described as the "transfer of a phosphate group" from a donor to an acceptor. A common phosphorylating agent (phosphate donor) is ATP and a common family of acceptor are alcohols: : This equation can be writ ...
has been highlighted recently to account for the multifaceted functions of OLIG2 in differentiation and proliferation. Studies showed that the phosphorylation state of OLIG2 at Ser30 determines the fate of cortical progenitor cells, in which cortical progenitor cells will either differentiate into astrocytes or remain as neuronal progenitors. Phosphorylation at a triple serine motif (Ser10, Ser13 and Ser14) on the other hand was shown to regulate the proliferative function of OLIG2. Another phosphorylation site Ser147 predicted by bioinformatics was found to regulate motor neuron development by regulating the binding between OLIG2 and NGN2. Further, OLIG2 contains a ST box composed of a string of 12 contiguous serine and threonine residues at position Ser77-Ser88. It is believed that phosphorylation at ST box is biologically functional, yet the role of it still remains to be elucidated in vivo. OLIG2 has also been implicated in bovine horn ontogenesis. It was the only gene in the bovine ''polled'' locus to show differential expression between the putative horn bud and the frontal forehead skin.


Clinical Significance


OLIG2 in Cancer

OLIG2 is well recognized for its importance in cancer research, particularly in brain tumors and
leukemia Leukemia ( also spelled leukaemia; pronounced ) is a group of blood cancers that usually begin in the bone marrow and produce high numbers of abnormal blood cells. These blood cells are not fully developed and are called ''blasts'' or '' ...
. OLIG2 is universally expressed in
glioblastoma Glioblastoma, previously known as glioblastoma multiforme (GBM), is the most aggressive and most common type of cancer that originates in the brain, and has a very poor prognosis for survival. Initial signs and symptoms of glioblastoma are nons ...
and other diffuse gliomas ( astrocytomas, oligodendrogliomas and oligoastrocytomas), and is a useful positive diagnostic marker of these brain tumors. Although in normal brain tissue OLIG2 expresses mostly on oligodendrocytes but not on mature astrocytes, in adult
glioma A glioma is a type of primary tumor that starts in the glial cells of the brain or spinal cord. They are malignant but some are extremely slow to develop. Gliomas comprise about 30% of all brain and central nervous system tumors and 80% of ...
, OLIG2 expresses on both IDH1 or IDH2-mutant adult lower grade
astrocytoma Astrocytoma is a type of brain tumor. Astrocytomas (also astrocytomata) originate from a specific kind of star-shaped glial cell in the cerebrum called an astrocyte. This type of tumor does not usually spread outside the brain and spinal cord, an ...
and
oligodendroglioma Oligodendrogliomas are a type of glioma that are believed to originate from the oligodendrocytes of the brain or from a oligodendrocyte progenitor cell, glial precursor cell. They occur primarily in adults (9.4% of all primary brain and central ne ...
on similar levels, but it is expressed on a lower level on IDH-wildtype
glioblastoma Glioblastoma, previously known as glioblastoma multiforme (GBM), is the most aggressive and most common type of cancer that originates in the brain, and has a very poor prognosis for survival. Initial signs and symptoms of glioblastoma are nons ...
. OLIG2 overexpression is a good surrogate marker for IDH mutation with an AUC of 0.90, but predicts poorly (AUC = 0.55) for 1p/19q co-deletion, a class-defining chromosomal alteration for
oligodendroglioma Oligodendrogliomas are a type of glioma that are believed to originate from the oligodendrocytes of the brain or from a oligodendrocyte progenitor cell, glial precursor cell. They occur primarily in adults (9.4% of all primary brain and central ne ...
. In survival analysis, higher
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 Protein biosynthesis, synthesizing a protein. mRNA is ...
levels of OLIG2 were associated with better overall survival, but this association was completely dependent on IDH mutation status. In particular, OLIG2 is selectively expressed in a subgroup of glioma cells that are highly tumorigenic, and is shown to be required for proliferation of human glioma cells implanted in the brain of
severe combined immunodeficiency Severe combined immunodeficiency (SCID), also known as Swiss-type agammaglobulinemia, is a rare genetic disorder characterized by the disturbed development of functional T cells and B cells caused by numerous genetic mutations that result in diff ...
(SCID) mice. Though the molecular mechanism behind this tumorigenesis is not entirely clear, more studies have recently been published pinpointing diverse evidence and potential roles for OLIG2 in glioma progression. It is believed that OLIG2 promotes neural stem cell and progenitor cell proliferation by opposing
p53 p53, also known as tumor protein p53, cellular tumor antigen p53 (UniProt name), or transformation-related protein 53 (TRP53) is a regulatory transcription factor protein that is often mutated in human cancers. The p53 proteins (originally thou ...
pathway, which potentially contributes to glioma progression. OLIG2 has been shown to directly repress the p53 tumor-suppressor pathway effector p21WAF1/CIP1, suppress p53
acetylation : In chemistry, acetylation is an organic esterification reaction with acetic acid. It introduces an acetyl group into a chemical compound. Such compounds are termed ''acetate esters'' or simply ''acetates''. Deacetylation is the opposite react ...
and impede the binding of p53 to several enhancer sites. It is further found that the phosphorylation of triple-serine motif in OLIG2 is present in several glioma lines and is more tumorigenic than the unphosphorylated status. In a study using the U12-1 cell line for controlled expression of OLIG2, researchers showed that OLIG2 can suppress the proliferation of U12-1 by transactivating the ''p27Kip1'' gene and can inhibit the motility of the cell by activating
RhoA Transforming protein RhoA, also known as Ras homolog family member A (RhoA), is a small GTPase protein in the Rho family of GTPases that in humans is encoded by the ''RHOA'' gene. While the effects of RhoA activity are not all well known, it is ...
. Besides glioma, OLIG2 is also involved in leukemogenesis. The ''Olig2'' gene was actually first identified in a study in T-cell
acute lymphoblastic leukemia Acute lymphoblastic leukemia (ALL) is a cancer of the Lymphocyte, lymphoid line of blood cells characterized by the development of large numbers of lymphoblast, immature lymphocytes. Symptoms may include feeling tired, pale skin color, fever, ...
, in which the expression of OLIG2 was found elevated after t(14;21)(q11.2;q22)
chromosomal translocation In genetics, chromosome translocation is a phenomenon that results in unusual rearrangement of chromosomes. This includes "balanced" and "unbalanced" translocation, with three main types: "reciprocal", "nonreciprocal" and "Robertsonian" transloc ...
. The overexpression of OLIG2 was later shown present in malignancies beyond glioma and leukemia, such as breast cancer,
melanoma Melanoma is the most dangerous type of skin cancer; it develops from the melanin-producing cells known as melanocytes. It typically occurs in the skin, but may rarely occur in the mouth, intestines, or eye (uveal melanoma). In very rare case ...
and
non-small cell lung carcinoma Non-small-cell lung cancer (NSCLC), or non-small-cell lung carcinoma, is any type of epithelial lung cancer other than Small-cell carcinoma#Small-cell lung cancer, small-cell lung cancer (SCLC). NSCLC accounts for about 85% of all lung cancers. ...
cell lines. It also has been shown that up-regulation of OLIG2 together with
LMO1 Rhombotin-1 is a protein that in humans is encoded by the ''LMO1'' gene. LMO1 encodes a cysteine-rich, two LIM domain transcriptional regulator. It is mapped to an area of consistent chromosomal translocation in chromosome 11, disrupting it in T ...
and Notch1 helps to provide proliferation signals.


OLIG2 in Neural Diseases

OLIG2 is also associated with Down syndrome, as it locates at
chromosome 21 Chromosome 21 is one of the 23 pairs of chromosomes in humans. Chromosome 21 is both the smallest human autosome and chromosome, with 46.7 million base pairs (the building material of DNA) representing about 1.5 percent of the total DNA in cell ...
within or near the Down syndrome critical region on the long arm. This region is believed to contribute to the cognitive defects of Down syndrome. The substantial increase in the number of
forebrain In the anatomy of the brain of vertebrates, the forebrain or prosencephalon is the rostral (forward-most) portion of the brain. The forebrain controls body temperature, reproductive functions, eating, sleeping, and the display of emotions. Ve ...
inhibitory neurons often observed in Ts65dn mouse (a murine model of trisomy 21) could lead to imbalance between excitation and inhibition and behavioral abnormalities. However, genetic reduction of OLIG2 and OLIG1 from three copies to two rescued the overproduction of interneurons, indicating the pivotal role of OLIG2 expression level in Down syndrome. The association between OLIG2 and neural diseases (i.e.
schizophrenia Schizophrenia () is a mental disorder characterized variously by hallucinations (typically, Auditory hallucination#Schizophrenia, hearing voices), delusions, thought disorder, disorganized thinking and behavior, and Reduced affect display, f ...
and
Alzheimer's disease Alzheimer's disease (AD) is a neurodegenerative disease and the cause of 60–70% of cases of dementia. The most common early symptom is difficulty in remembering recent events. As the disease advances, symptoms can include problems wit ...
) are under scrutiny, as several
single nucleotide polymorphisms In genetics and bioinformatics, a single-nucleotide polymorphism (SNP ; plural SNPs ) is a germline substitution of a single nucleotide at a specific position in the genome. Although certain definitions require the substitution to be present in ...
(SNPs) associated with these diseases in OLIG2 were identified by genome-wide association work. OLIG2 also plays a functional role in neural repair. Studies showed that the number of OLIG2-expressing cells increased in the lesion after cortical stab-wound injury, supporting the role for OLIG2 in reactive
gliosis Gliosis is a nonspecific reactive change of glial cells in response to damage to the central nervous system (CNS). In most cases, gliosis involves the proliferation or hypertrophy of several different types of glial cells, including astrocytes ...
. OLIG2 was also implicated in generating reactive
astrocytes Astrocytes (from Ancient Greek , , "star" and , , "cavity", "cell"), also known collectively as astroglia, are characteristic star-shaped glial cells in the brain and spinal cord. They perform many functions, including biochemical control of end ...
possibly in a transient re-expression manner, but the mechanisms are unclear.


References


Further reading

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External links

* * * {{Transcription factors, g1 Transcription factors