Enhancer of zeste homolog 2 (EZH2) is a
histone-lysine N-methyltransferase
Histone methyltransferases (HMT) are histone-modifying enzymes (e.g., histone-lysine N-methyltransferases and histone-arginine N-methyltransferases), that catalyze the transfer of one, two, or three methyl groups to lysine and arginine residues ...
enzyme
Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products. A ...
(
EC 2.1.1.43) encoded by
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 ...
, that participates in
histone methylation
Histone methylation is a process by which methyl groups are transferred to amino acids of histone proteins that make up nucleosomes, which the DNA double helix wraps around to form chromosomes. Methylation of histones can either increase or decrea ...
and, ultimately,
transcriptional repression.
EZH2 catalyzes the addition of
methyl groups
In organic chemistry, a methyl group is an alkyl derived from methane, containing one carbon atom bonded to three hydrogen atoms, having chemical formula . In formulas, the group is often abbreviated as Me. This hydrocarbon group occurs in many ...
to
histone H3
Histone H3 is one of the five main histones involved in the structure of chromatin in eukaryotic cells. Featuring a main globular domain and a long N-terminal tail, H3 is involved with the structure of the nucleosomes of the 'beads on a stri ...
at
lysine
Lysine (symbol Lys or K) is an α-amino acid that is a precursor to many proteins. It contains an α-amino group (which is in the protonated form under biological conditions), an α-carboxylic acid group (which is in the deprotonated −C ...
27,
by using the
cofactor S-adenosyl-L-methionine
''S''-Adenosyl methionine (SAM), also known under the commercial names of SAMe, SAM-e, or AdoMet, is a common cosubstrate involved in methyl group transfers, transsulfuration, and aminopropylation. Although these anabolic reactions occur throug ...
. Methylation activity of EZH2 facilitates
heterochromatin
Heterochromatin is a tightly packed form of DNA or '' condensed DNA'', which comes in multiple varieties. These varieties lie on a continue between the two extremes of constitutive heterochromatin and facultative heterochromatin. Both play a role ...
formation thereby silences gene function.
Remodeling of chromosomal
heterochromatin
Heterochromatin is a tightly packed form of DNA or '' condensed DNA'', which comes in multiple varieties. These varieties lie on a continue between the two extremes of constitutive heterochromatin and facultative heterochromatin. Both play a role ...
by EZH2 is also required during cell mitosis.
EZH2 is the functional enzymatic component of the Polycomb Repressive Complex 2 (
PRC2
PRC2 (polycomb repressive complex 2) is one of the two classes of polycomb-group proteins or (PcG). The other component of this group of proteins is PRC1 (Polycomb Repressive Complex 1).
This complex has histone methyltransferase activity and pr ...
), which is responsible for healthy
embryonic development
An embryo is an initial stage of development of 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 sperm ...
through the
epigenetic
In biology, epigenetics is the study of stable phenotypic changes (known as ''marks'') that do not involve alterations in the DNA sequence. The Greek prefix '' epi-'' ( "over, outside of, around") in ''epigenetics'' implies features that are "o ...
maintenance of genes responsible for regulating development and
differentiation.
EZH2 is responsible for the methylation activity of PRC2, and the complex also contains proteins required for optimal function (
EED,
SUZ12
Polycomb protein SUZ12 is a protein that in humans is encoded by the ''SUZ12'' gene.
Function
This zinc finger gene has been identified at the breakpoints of a recurrent chromosomal translocation reported in endometrial stromal sarcoma. Recomb ...
,
JARID2
Protein Jumonji is a protein that in humans is encoded by the ''JARID2'' gene. JARID2 is a member of the alpha-ketoglutarate-dependent hydroxylase superfamily.
Jarid2 (jumonji, AT rich interactive domain 2) is a protein coding gene that function ...
,
AEBP2
Adipocyte Enhancer-Binding Protein is a zinc finger protein that in humans is encoded by the evolutionarily well-conserved gene AEBP2. It was initially identified due to its binding capability to the promoter of the adipocyte P2 gene, and was th ...
,
RbAp46/48, and
PCL).
Mutation or over-expression of EZH2 has been linked to many forms of cancer. EZH2 inhibits genes responsible for suppressing tumor development, and blocking EZH2 activity may slow tumor growth. EZH2 has been targeted for inhibition because it is upregulated in multiple cancers including, but not limited to, breast,
prostate
The prostate is both an Male accessory gland, accessory gland of the male reproductive system and a muscle-driven mechanical switch between urination and ejaculation. It is found only in some mammals. It differs between species anatomically, ...
,
[
*] melanoma
Melanoma, also redundantly known as malignant melanoma, is a type of skin cancer that develops from the pigment-producing cells known as melanocytes. Melanomas typically occur in the skin, but may rarely occur in the mouth, intestines, or eye ( ...
,
[
*] and bladder cancer.
Mutations in the EZH2 gene are also associated with
Weaver syndrome
Weaver syndrome is a rare autosomal dominant genetic disorder associated with rapid growth beginning in the prenatal period and continuing through the toddler and youth years. It is characterized by advanced osseous maturation and distinctive cra ...
, a rare
congenital
A birth defect, also known as a congenital disorder, is an abnormal condition that is present at birth regardless of its cause. Birth defects may result in disabilities that may be physical, intellectual, or developmental. The disabilities can ...
disorder,
and EZH2 is involved in causing
neurodegenerative
A neurodegenerative disease is caused by the progressive loss of structure or function of neurons, in the process known as neurodegeneration. Such neuronal damage may ultimately involve cell death. Neurodegenerative diseases include amyotrophic ...
symptoms in the nervous system disorder, ataxia
telangiectasia
Telangiectasias, also known as spider veins, are small dilated blood vessels that can occur near the surface of the skin or mucous membranes, measuring between 0.5 and 1 millimeter in diameter. These dilated blood vessels can develop anywhere on ...
.
Function
EZH2 is the catalytic
subunit
Subunit may refer to:
*Subunit HIV vaccine, a class of HIV vaccine
*Protein subunit, a protein molecule that assembles with other protein molecules
*Monomer, a molecule that may bind chemically to other molecules to form a polymer
*Sub-subunit, a ...
of the Polycomb Repressive Complex 2 (PRC2).
EZH2's catalytic activity relies on its formation of a complex with at least two other PRC2 components,
SUZ12
Polycomb protein SUZ12 is a protein that in humans is encoded by the ''SUZ12'' gene.
Function
This zinc finger gene has been identified at the breakpoints of a recurrent chromosomal translocation reported in endometrial stromal sarcoma. Recomb ...
and
EED.
As a
histone methyltransferase
Histone methyltransferases (HMT) are histone-modifying enzymes (e.g., histone-lysine N-methyltransferases and histone-arginine N-methyltransferases), that catalyze the transfer of one, two, or three methyl groups to lysine and arginine residues of ...
(HMTase), EZH2's primary function is to
methylate
In the chemical sciences, methylation denotes the addition of a methyl group on a substrate, or the substitution of an atom (or group) by a methyl group. Methylation is a form of alkylation, with a methyl group replacing a hydrogen atom. These t ...
Lys-27 on
histone 3
Histone H3 is one of the five main histones involved in the structure of chromatin in eukaryotic cells. Featuring a main globular domain and a long N-terminal tail, H3 is involved with the structure of the nucleosomes of the 'beads on a strin ...
(H3K27me) by transferring a methyl group from the cofactor S-adenosyl-L-methionine (SAM). EZH2 is capable of mono-, di-, and tri-
methylation
In the chemical sciences, methylation denotes the addition of a methyl group on a substrate, or the substitution of an atom (or group) by a methyl group. Methylation is a form of alkylation, with a methyl group replacing a hydrogen atom. These t ...
of H3K27 and has been associated with a variety of biological functions, including transcriptional regulation in
hematopoiesis
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. ...
, development, and
cell differentiation
Cellular differentiation is the process in which a stem cell alters from one type to a differentiated one. Usually, the cell changes to a more specialized type. Differentiation happens multiple times during the development of a multicellula ...
.
Recent studies have indicated that EZH2 is also capable of methylating non-
histone
In biology, histones are highly basic proteins abundant in lysine and arginine residues that are found in eukaryotic cell nuclei. They act as spools around which DNA winds to create structural units called nucleosomes. Nucleosomes in turn are wr ...
proteins.
Transcription repression
EZH2, as a part of PRC2, catalyzes trimethylation of H3K27 (
H3K27me3
H3K27me3 is an epigenetic modification to the DNA packaging protein Histone H3. It is a mark that indicates the tri-methylation of lysine 27 on histone H3 protein.
This tri-methylation is associated with the Downregulation and upregulation, dow ...
), which is a histone modification that has been characterized as part of the
histone code
The histone code is a hypothesis that the transcription of genetic information encoded in DNA is in part regulated by chemical modifications (known as ''histone marks'') to histone proteins, primarily on their unstructured ends. Together with sim ...
.
The histone code is the theory that chemical modifications, such as
methylation
In the chemical sciences, methylation denotes the addition of a methyl group on a substrate, or the substitution of an atom (or group) by a methyl group. Methylation is a form of alkylation, with a methyl group replacing a hydrogen atom. These t ...
,
acetylation
:
In organic 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 oppo ...
, and
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 histone proteins play distinctive roles in
epigenetic
In biology, epigenetics is the study of stable phenotypic changes (known as ''marks'') that do not involve alterations in the DNA sequence. The Greek prefix '' epi-'' ( "over, outside of, around") in ''epigenetics'' implies features that are "o ...
regulation of gene
transcription
Transcription refers to the process of converting sounds (voice, music etc.) into letters or musical notes, or producing a copy of something in another medium, including:
Genetics
* Transcription (biology), the copying of DNA into RNA, the fir ...
. EZH2-mediated catalysis of H3K27me3 is associated with long term transcription repression.
EZH2, as well as other Polycomb group proteins, are involved in establishing and maintaining gene repression through
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 ...
.
This transcriptionally repressive state is thought to be due to PRC2/EZH2-EED-mediated H3K27 methylation and subsequent recruitment of
PRC1
Protein Regulator of cytokinesis 1 (PRC1) is a protein that in humans is encoded by the ''PRC1'' gene and is involved in cytokinesis.
Function
PRC1 protein is expressed at relatively high levels during S and G2/M phases of the cell cycle before ...
which facilitates condensation of
chromatin
Chromatin is a complex of DNA and protein found in eukaryotic cells. The primary function is to package long DNA molecules into more compact, denser structures. This prevents the strands from becoming tangled and also plays important roles in r ...
and formation of
heterochromatin
Heterochromatin is a tightly packed form of DNA or '' condensed DNA'', which comes in multiple varieties. These varieties lie on a continue between the two extremes of constitutive heterochromatin and facultative heterochromatin. Both play a role ...
.
Heterochromatin is tightly packed chromatin which limits the accessibility of transcription machinery to the underlying DNA, thereby suppressing transcription.
During cell division, heterochromatin formation is required for proper
chromosome segregation
Chromosome segregation is the process in eukaryotes by which two sister chromatids formed as a consequence of DNA replication, or paired homologous chromosomes, separate from each other and migrate to opposite poles of the nucleus. This segregation ...
.
PRC2/EED-EZH2 complex may also be involved in the recruitment of DNA methyltransferases (
DNMT
In biochemistry, the DNA methyltransferase (DNA MTase, DNMT) family of enzymes catalysis, catalyze the transfer of a methyl group to DNA. DNA methylation serves a wide variety of biological functions. All the known DNA methyltransferases use S- ...
s), which results in increased
DNA methylation
DNA methylation is a biological process by which methyl groups are added to the DNA molecule. Methylation can change the activity of a DNA segment without changing the sequence. When located in a gene promoter, DNA methylation typically acts t ...
, another epigenetic layer of transcription repression.
Specific genes that have been identified as targets of EZH2-mediated transcriptional repression include
HOXA9
Homeobox protein Hox-A9 is a protein that in humans is encoded by the ''HOXA9'' gene.
In vertebrates, the genes encoding the class of transcription factors called homeobox genes are found in clusters named A, B, C, and D on four separate chromoso ...
,
HOXC8
Homeobox protein Hox-C8 is a protein that in humans is encoded by the ''HOXC8'' gene.
Function
This gene belongs to the homeobox family of genes. The homeobox genes encode a highly conserved family of transcription factors that play an import ...
,
MYT1, CDKN2A and
retinoic acid
Retinoic acid (used simplified here for all-''trans''-retinoic acid) is a metabolite of vitamin A1 (all-''trans''-retinol) that mediates the functions of vitamin A1 required for growth and development. All-''trans''-retinoic acid is required in ...
target genes.
Transcription activation
In cancer, EZH2 may play a role in activation of transcription, independently of
PRC2
PRC2 (polycomb repressive complex 2) is one of the two classes of polycomb-group proteins or (PcG). The other component of this group of proteins is PRC1 (Polycomb Repressive Complex 1).
This complex has histone methyltransferase activity and pr ...
.
In breast cancer cells, EZH2 has been demonstrated to activate
NF-κB
Nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) is a protein complex that controls transcription of DNA, cytokine production and cell survival. NF-κB is found in almost all animal cell types and is involved in cellular ...
target genes, which are involved in responses to stimuli.
The functional role of this activity and its mechanism are still unknown.
Development and cell differentiation
EZH2 plays an essential role in development. In particular, it helps control transcriptional repression of genes that regulate cell differentiation.
In embryonic stem cells, EZH2-mediated trimethylation of H3K27me3 in regions containing developmental genes appears to be important for maintenance of normal cell differentiation.
H3K27me3 is also important in driving
X-inactivation
X-inactivation (also called Lyonization, after English geneticist Mary Lyon) is a process by which one of the copies of the X chromosome is inactivated in therian female mammals. The inactive X chromosome is silenced by being packaged into ...
, the silencing of one
X-chromosome
The X chromosome is one of the two sex-determining chromosomes (allosomes) in many organisms, including mammals (the other is the Y chromosome), and is found in both males and females. It is a part of the XY sex-determination system and XO sex- ...
in females during development.
During X-inactivation, it is thought that EZH2 is involved in initiating heterochromatin formation by trimethylating H3K27 and that other
histone methyltransferase
Histone methyltransferases (HMT) are histone-modifying enzymes (e.g., histone-lysine N-methyltransferases and histone-arginine N-methyltransferases), that catalyze the transfer of one, two, or three methyl groups to lysine and arginine residues of ...
s and histone marks may be involved in maintaining the silenced state.
Further, EZH2 has been identified as an essential protein involved in development and differentiation of
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 or ...
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 rec ...
.
H3K27me3 is involved in suppressing genes that promote differentiation, thus maintaining an undifferentiated state of B- and T-cells and playing an important role in regulating
hematopoiesis
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. ...
.
Regulation of EZH2 activity
The activity of EZH2 is regulated by the post-translational
phosphorylation
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, wh ...
of
threonine
Threonine (symbol Thr or T) is an amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated −NH form under biological conditions), a carboxyl group (which is in the deprotonated −COO ...
and
serine
Serine (symbol Ser or S) is an α-amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated − form under biological conditions), a carboxyl group (which is in the deprotonated − form un ...
residues on EZH2.
Specifically,
phosphorylation
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, wh ...
of T350 has been linked to an increase in EZH2 activity while phosphorylation of T492 and S21 have been linked to a decrease in EZH2 activity.
Phosphorylation of T492 has been suggested to disrupt contacts between human EZH2 and its binding partners in the PRC2 complex, thus hindering its catalytic activity.
In addition to phosphorylation, it has also been shown that PRC2/EZH2-EED activity is antagonized by transcription-activating histone marks, such as
acetylation
:
In organic 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 oppo ...
of H3K27 (
H3K27ac
H3K27ac is an epigenetic modification to the DNA packaging protein histone H3. It is a mark that indicates acetylation of the lysine residue at N-terminal position 27 of the histone H3 protein.
H3K27ac is associated with the higher activation o ...
) and methylation of H3K36 (
H3K36me
H3K36me is an epigenetic modification to the DNA packaging protein Histone H3, specifically, the mono-methylation at the 36th lysine residue of the histone H3 protein.
There are diverse modifications at H3K36, such as phosphorylation, methylation, ...
).
EZH2 expression is regulated by estrogen signaling in human normal breast epithelium and human breast cancers.
Enzymatic activity
EZH2 function is highly dependent upon its recruitment by the PRC2 complex. In particular,
WD40-repeat protein embryonic ectoderm development (EED) and
zinc finger protein suppressor of zeste 12 (SUZ12) are needed to stabilize the interaction of EZH2 with its histone substrate
Recently, two isoforms of EZH2 generated from
alternative splicing
Alternative splicing, or alternative RNA splicing, or differential splicing, is an alternative splicing process during gene expression that allows a single gene to code for multiple proteins. In this process, particular exons of a gene may be ...
have been identified in humans: EZH2α and EZH2β.
Both
isoform
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 isof ...
s contain elements that have been identified as important for EZH2 function including the
nuclear localization signal A nuclear localization signal ''or'' sequence (NLS) is an amino acid sequence that 'tags' a protein for import into the cell nucleus by nuclear transport. Typically, this signal consists of one or more short sequences of positively charged lysines o ...
, the EED and SUZ12 binding sites as well as the conserved SET domain.
Most studies have thus far focused on the longer isoform EZH2α, but EZH2β, which lacks
exon
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 sequen ...
s 4 and 8, has been shown to be active.
Furthermore, PRC2/EZH2β complexes act on distinct genes from that of its PRC2/EZH2α counterpart suggesting that each isoform may act to regulate a specific subset of genes.
Additional evidence suggests that EZH2 may also be capable of lysine methylation independent of association with PRC2, when EZH2 is highly upregulated.
Lysine methylation
Methylation
In the chemical sciences, methylation denotes the addition of a methyl group on a substrate, or the substitution of an atom (or group) by a methyl group. Methylation is a form of alkylation, with a methyl group replacing a hydrogen atom. These t ...
is the addition of a -CH
3, or methyl group, to another molecule. In biology, methylation is typically catalyzed by enzymes, and methyl groups are commonly added to either proteins or nucleic acids. In EZH2-catalyzed methylation, the amino acid
lysine
Lysine (symbol Lys or K) is an α-amino acid that is a precursor to many proteins. It contains an α-amino group (which is in the protonated form under biological conditions), an α-carboxylic acid group (which is in the deprotonated −C ...
in the histone h3 is methylated. This amino acid residue can be methylated up to three times on its terminal
ammonium
The ammonium cation is a positively-charged polyatomic ion with the chemical formula or . It is formed by the protonation of ammonia (). Ammonium is also a general name for positively charged or protonated substituted amines and quaternary a ...
group. These methylated lysines are important in the control of mammalian gene expression and have a functional role in
heterochromatin
Heterochromatin is a tightly packed form of DNA or '' condensed DNA'', which comes in multiple varieties. These varieties lie on a continue between the two extremes of constitutive heterochromatin and facultative heterochromatin. Both play a role ...
formation,
X-chromosome inactivation
X-inactivation (also called Lyonization, after English geneticist Mary Lyon) is a process by which one of the copies of the X chromosome is inactivated in therian female mammals. The inactive X chromosome is silenced by being packaged into ...
and transcriptional regulation. In mammalian chromosomes, histone lysine methylation can either activate or repress genes depending the site of methylation. Recent work has shown that at least part of the silencing function of the EZH2 complex is the methylation of
histone H3
Histone H3 is one of the five main histones involved in the structure of chromatin in eukaryotic cells. Featuring a main globular domain and a long N-terminal tail, H3 is involved with the structure of the nucleosomes of the 'beads on a stri ...
on lysine 27. Methylation, and other modifications, take place on the histones. Methyl modifications can affect the binding of proteins to these histones and either activate or inhibit
transcription
Transcription refers to the process of converting sounds (voice, music etc.) into letters or musical notes, or producing a copy of something in another medium, including:
Genetics
* Transcription (biology), the copying of DNA into RNA, the fir ...
.
Mechanism of catalysis
EZH2 is a member of the
SET domain
Set, The Set, SET or SETS may refer to:
Science, technology, and mathematics Mathematics
*Set (mathematics), a collection of elements
* Category of sets, the category whose objects and morphisms are sets and total functions, respectively
Electr ...
family of lysine
methyltransferase
Methyltransferases are a large group of enzymes that all methylate their substrates but can be split into several subclasses based on their structural features. The most common class of methyltransferases is class I, all of which contain a Rossm ...
s which function to add methyl groups to lysine side chains of
substrate proteins.
SET methyltransferases depend on a S-Adenosyl methionine (SAM) cofactor to act as a methyl donor for their catalytic activity. SET domain proteins differ from other SAM-dependent
methyltransferases
Methyltransferases are a large group of enzymes that all methylate their substrates but can be split into several subclasses based on their structural features. The most common class of methyltransferases is class I, all of which contain a Rossm ...
in that they bind their substrate and SAM cofactor on opposite sides of the
active site
In biology and biochemistry, the active site is the region of an enzyme where substrate molecules bind and undergo a chemical reaction. The active site consists of amino acid residues that form temporary bonds with the substrate (binding site) a ...
of the enzyme. This orientation of substrate and cofactor allows SAM to dissociate without disrupting substrate binding and can lead to multiple rounds of lysine methylation without substrate dissociation.
Although neither a substrate-bound or SAM-bound crystal structure for EZH2 has been determined, STAMP structure alignment with the human SET7/9 methyltransferase shows conserved
tyrosine
-Tyrosine or tyrosine (symbol Tyr or Y) or 4-hydroxyphenylalanine is one of the 20 standard amino acids that are used by cells to synthesize proteins. It is a non-essential amino acid with a polar side group. The word "tyrosine" is from the Gr ...
residues in almost identical positions within the putative active site of EZH2.
It had been previously suggested that tyrosine 726 in the EZH2 active site was acting as a general base to de-protonate the substrate lysine but kinetic isotope effects have indicated that active site residues are not directly involved in the chemistry of the methyltransferase reaction.
Instead these experiments support a mechanism in which the residues lower the
pKa
PKA may refer to:
* Professionally known as:
** Pen name
** Stage persona
* p''K''a, the symbol for the acid dissociation constant at logarithmic scale
* Protein kinase A, a class of cAMP-dependent enzymes
* Pi Kappa Alpha, the North-American so ...
of the substrate lysine residue while simultaneously providing a channel for water to access the lysine side chain within the interior of the active site. Bulk solvent water can then easily
deprotonate
Deprotonation (or dehydronation) is the removal (transfer) of a proton (or hydron, or hydrogen cation), (H+) from a Brønsted–Lowry acid in an acid–base reaction.Henry Jakubowski, Biochemistry Online Chapter 2A3, https://employees.csbsju.edu ...
the lysine side chain, activating it for
nucleophilic attack
In chemistry, a nucleophile is a chemical species that forms bonds by donating an electron pair. All molecules and ions with a free pair of electrons or at least one pi bond can act as nucleophiles. Because nucleophiles donate electrons, they are ...
of the SAM cofactor in an
SN2-like reaction resulting in transfer of the methyl group from SAM to the lysine side chain.
EZH2 primarily catalyzes mono- and di-methylation of H3K27 but a clinically relevant mutation of residue tyrosine 641 to
phenylalanine
Phenylalanine (symbol Phe or F) is an essential α-amino acid with the formula . It can be viewed as a benzyl group substituted for the methyl group of alanine, or a phenyl group in place of a terminal hydrogen of alanine. This essential amino a ...
(Y641F) results in higher H3K27 tri-methylation activity.
It is proposed that the removal of the hydroxyl group on Y641 abrogates steric hindrance and allows for accommodation of a third methyl group on the substrate lysine.
Clinical significance
Cancer
EZH2 is an attractive target for anti-cancer therapy because it helps cancerous cells divide and proliferate. It is found in larger amounts than in healthy cells in a wide range of cancers including breast, prostate, bladder,
uterine
The uterus (from Latin ''uterus'', plural ''uteri'') or womb () is the hollow organ, organ in the female reproductive system, reproductive system of most female mammals, including humans that accommodates the embryonic development, embryonic an ...
, and
renal
The kidneys are two reddish-brown bean-shaped organs found in vertebrates. They are located on the left and right in the retroperitoneal space, and in adult humans are about in length. They receive blood from the paired renal arteries; blood ...
cancers, as well as
melanoma
Melanoma, also redundantly known as malignant melanoma, is a type of skin cancer that develops from the pigment-producing cells known as melanocytes. Melanomas typically occur in the skin, but may rarely occur in the mouth, intestines, or eye ( ...
and
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 enlar ...
. EZH2 is a
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 ...
suppressor, so when it becomes overexpressed, many tumor suppressor genes that are normally turned on, are turned off. Inhibition of EZH2 function shrinks
malignant
Malignancy () is the tendency of a medical condition to become progressively worse.
Malignancy is most familiar as a characterization of cancer. A ''malignant'' tumor contrasts with a non-cancerous ''benign'' tumor in that a malignancy is not s ...
tumor
A neoplasm () is a type of abnormal and excessive growth of tissue. The process that occurs to form or produce a neoplasm is called neoplasia. The growth of a neoplasm is uncoordinated with that of the normal surrounding tissue, and persists ...
s in some reported cases because those tumor suppressor genes are not silenced by EZH2.
EZH2 typically is not expressed in healthy adults; it is only found in actively dividing cells, like those active during fetal development.
Because of this characteristic, overexpression of EZH2 can be used as a diagnostic marker of cancer and some neurodegenerative disorders.
However, there are cases where it is difficult to tell whether overexpression of EZH2 is the cause of a disease, or simply a consequence. If it is only a consequence, targeting EZH2 for inhibition may not cure the disease. One example of a cancer pathway in which EZH2 plays a role is the pRB-E2F pathway. It is downstream from the
pRB-E2F pathway, and signals from this pathway lead to EZH2 overexpression.
Another important characteristic of EZH2 is that when EZH2 is overexpressed, it can activate genes without forming
PRC2
PRC2 (polycomb repressive complex 2) is one of the two classes of polycomb-group proteins or (PcG). The other component of this group of proteins is PRC1 (Polycomb Repressive Complex 1).
This complex has histone methyltransferase activity and pr ...
. This is an issue because it means the methylation activity of the enzyme is not mediated by complex formation. In breast cancer cells, EZH2 activates genes that promote cell proliferation and survival.
It can also activate regulatory genes like
c-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 referre ...
and
cyclin D1
Cyclin D1 is a protein that in humans is encoded by the ''CCND1'' gene.
Gene expression
The CCND1 gene encodes the cyclin D1 protein. The human CCND1 gene is located on the long arm of chromosome 11 (band 11q13). It is 13,388 base pairs lon ...
by interacting with
Wnt signaling
The Wnt signaling pathways are a group of signal transduction pathways which begin with proteins that pass signals into a cell through cell surface receptors. The name Wnt is a portmanteau created from the names Wingless and Int-1. Wnt signaling p ...
factors.
Importantly, the mutation of tyrosine 641 in the active SET domain to a number of different amino acids is a common feature of some B-cell lymphomas.
Inhibitors
Developing an inhibitor of EZH2 and preventing unwanted histone methylation of tumor suppressor genes is a viable area of cancer research. EZH2 inhibitor development has focused on targeting the
SET domain
Set, The Set, SET or SETS may refer to:
Science, technology, and mathematics Mathematics
*Set (mathematics), a collection of elements
* Category of sets, the category whose objects and morphisms are sets and total functions, respectively
Electr ...
active site of the protein. Several inhibitors of EZH2 have been developed as of 2015, including
3-deazaneplanocin A (DZNep), EPZ005687, EI1, GSK126, and UNC1999.
;DZNep: DZNep has potential antiviral and anti-cancer properties because it lowers EZH2 levels and induces
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 ...
in breast and colon cancer cells.
DZNep inhibits the hydrolysis of S-adenosyl-L-homocysteine (SAH), which is a product-based inhibitor of all protein methyltransferases, leading to increased cellular concentrations of SAH which in turn inhibits EZH2. However, DZNep is not specific to EZH2 and also inhibits other DNA methyltransferases.
;EPZ005687: In 2012, a company called Epizyme revealed EPZ005687, an S-adenosylmethionine (
SAM) competitive inhibitor that is more selective than DZNep; it has a 50-fold increase in selectivity for EZH2 compared to
EZH1 #REDIRECT EZH1 #REDIRECT EZH1
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{{R from other capitalization
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. The drug blocks EZH2 activity by binding to the SET domain active site of the enzyme. EPZ005687 can also inhibit the Y641 and A677 mutants of EZH2, which may be applicable for treating non-Hodgkin's lymphoma.
;Tazemetostat: In 2013, Epizyme began Phase I clinical trials with another EZH2 inhibitor,
tazemetostat
Tazemetostat, sold under the brand name Tazverik, is a medication used for the treatment of adults and adolescents aged 16 years and older with metastatic (when cancer cells spread to other parts of the body) or locally advanced (when cancer ha ...
(EPZ-6438), for patients with B-cell lymphoma. In 2020,
tazemetostat
Tazemetostat, sold under the brand name Tazverik, is a medication used for the treatment of adults and adolescents aged 16 years and older with metastatic (when cancer cells spread to other parts of the body) or locally advanced (when cancer ha ...
, with the tradename Tazverik, gained an FDA
accelerated approval The United States Food and Drug Administration (FDA) initiated the FDA Accelerated Approval Program in 1992 to allow faster approval of drugs for serious conditions that fill an unmet medical need. The faster approval relies on use of surrogate endp ...
for the treatment of metastatic or locally advanced epithelioid sarcoma and an
accelerated approval The United States Food and Drug Administration (FDA) initiated the FDA Accelerated Approval Program in 1992 to allow faster approval of drugs for serious conditions that fill an unmet medical need. The faster approval relies on use of surrogate endp ...
for the treatment of patients with relapsed follicular lymphoma later that year.
;Sinefungin: Sinefungin is another SAM-competitive inhibitor, however, like DZNep, it is not specific to EZH2.
It works by binding in the cofactor binding pocket of DNA methyltransferases to block methyl transfer. EI1 is another inhibitor, developed by
Novartis
Novartis AG is a Swiss-American multinational pharmaceutical corporation based in Basel, Switzerland and
Cambridge, Massachusetts, United States (global research).name="novartis.com">https://www.novartis.com/research-development/research-loc ...
, that showed EZH2 inhibitory activity in lymphoma tumor cells, including cells with the Y641 mutation.
The mechanism of this inhibitor also involves competing with the SAM cofactor for binding to EZH2.
;GSK126: GSK126 is a potent, SAM-competitive EZH2 inhibitor developed by
GlaxoSmithKline
GSK plc, formerly GlaxoSmithKline plc, is a British multinational pharmaceutical and biotechnology company with global headquarters in London, England. Established in 2000 by a merger of Glaxo Wellcome and SmithKline Beecham. GSK is the ten ...
, that has 150-fold selectivity over EZH1 and a K
i of 0.5-3 nM.
UNC1999 was developed as an analogue of GSK126, and was the first orally bioavailable EZH2 inhibitor to show activity. However, it is less selective than its counterpart GSK126, and it binds to EZH1 as well, increasing the potential for off-target effects.
Combination therapies are being studied as possible treatments when primary treatments begin to fail.
Etoposide
Etoposide, sold under the brand name Vepesid among others, is a chemotherapy medication used for the treatments of a number of types of cancer including testicular cancer, lung cancer, lymphoma, leukemia, neuroblastoma, and ovarian cancer. It is ...
, a
topoisomerase
DNA topoisomerases (or topoisomerases) are enzymes that catalyze changes in the topological state of DNA, interconverting relaxed and supercoiled forms, linked (catenated) and unlinked species, and knotted and unknotted DNA. Topological issues i ...
inhibitor, when combined with an EZH2 inhibitor, becomes more effective for non-small cell lung cancers with
BRG1
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 ...
and
EGFR mutations.
However, EZH2 and lysine methylation can have tumor suppressing activity, for example in
myelodysplastic syndrome
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 ...
,
indicating that EZH2 inhibition may not be beneficial in all cases.
Weaver Syndrome
Mutations in the EZH2 gene have been linked with
Weaver syndrome
Weaver syndrome is a rare autosomal dominant genetic disorder associated with rapid growth beginning in the prenatal period and continuing through the toddler and youth years. It is characterized by advanced osseous maturation and distinctive cra ...
, a rare disorder characterized by advanced bone age,
macrocephaly
Macrocephaly is a condition in which circumference of the human head is abnormally large. It may be pathological or harmless, and can be a familial genetic characteristic. People diagnosed with macrocephaly will receive further medical tests to ...
, and
hypertelorism
Hypertelorism is an abnormally increased distance between two organs or bodily parts, usually referring to an increased distance between the orbits (eyes), or orbital hypertelorism. In this condition the distance between the inner eye corners as ...
.
The
histidine
Histidine (symbol His or H) is an essential amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated –NH3+ form under biological conditions), a carboxylic acid group (which is in the de ...
residue in the active site of the
wild-type
The wild type (WT) is the phenotype of the typical form of a species as it occurs in nature. Originally, the wild type was conceptualized as a product of the standard "normal" allele at a locus, in contrast to that produced by a non-standard, "m ...
EZH2 was mutated to
tyrosine
-Tyrosine or tyrosine (symbol Tyr or Y) or 4-hydroxyphenylalanine is one of the 20 standard amino acids that are used by cells to synthesize proteins. It is a non-essential amino acid with a polar side group. The word "tyrosine" is from the Gr ...
in patients diagnosed with Weaver syndrome.
The mutation likely interferes with
cofactor binding and causes disruption of the natural function of the protein.
Taxonomic distribution
Enhancer of zeste (E(z)) was originally identified in
Drosophila melanogaster
''Drosophila melanogaster'' is a species of fly (the taxonomic order Diptera) in the family Drosophilidae. The species is often referred to as the fruit fly or lesser fruit fly, or less commonly the "vinegar fly" or "pomace fly". Starting with Ch ...
, and its mammalian homologs were subsequently identified and named
EZH1 #REDIRECT EZH1 #REDIRECT EZH1
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{{R from other capitalization
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(enhancer of zeste homolog 1) and EZH2 (enhancer of zeste homolog 2).
EZH2 is highly conserved through
evolution
Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes, which are passed on from parent to offspring during reproduction. Variation ...
. It and its homologs play essential roles in development, cell differentiation, and cell division in plants, insects, fish, and mammals.
The following taxonomic tree is a depiction of EZH2's distribution throughout a wide variety of species.
See also
*
Ezh2 gene
References
Further reading
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