Exostosin glycosyltransferase-2 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 ...

that in humans is encoded by the ''EXT2''

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 ...

. This gene encodes one of two glycosyltransferases involved in the chain elongation step of

heparan sulfate Heparan sulfate (HS) is a linear polysaccharide found in all animal tissues. It occurs as a proteoglycan (HSPG, i.e. Heparan Sulfate ProteoGlycan) in which two or three HS chains are attached in close proximity to cell surface or extracellular ma ...

biosynthesis. Mutations in this gene cause the type II form of Hereditary Multiple Exostoses (HME).

Gene location

The EXT2 gene is located on chromosome 11 in the human genome, its location is on the p arm of this chromosome. The p arm of a chromosome is the shorter arm of a chromosome.

Interactions

Included in the EXT family are EXT2,

EXT1 Exostosin-1 is a protein that in humans is encoded by the ''EXT1'' gene. This gene encodes one of the two endoplasmic reticulum-resident type II transmembrane glycosyltransferase – the other being EXT2 – which are involved in the chain elon ...

, EXTL1, EXTL2, and EXTL3. The proteins formed by these genes work together to form and extend heparan sulfate chains. Heparan sulfate chains are

proteoglycans Proteoglycans are proteins that are heavily glycosylated. The basic proteoglycan unit consists of a "core protein" with one or more covalently attached glycosaminoglycan (GAG) chain(s). The point of attachment is a serine (Ser) residue to whic ...

present in the extracellular matrix of most tissue types. There is a lot about its function that is not entirely understood, however it is known that they have an important role for bone and

cartilage Cartilage is a resilient and smooth type of connective tissue. In tetrapods, it covers and protects the ends of long bones at the joints as articular cartilage, and is a structural component of many body parts including the rib cage, the neck an ...

formation. Cartilage is located at the

growth plates The epiphyseal plate (or epiphysial plate, physis, or growth plate) is a hyaline cartilage plate in the metaphysis at each end of a long bone. It is the part of a long bone where new bone growth takes place; that is, the whole bone is alive, with ...

of long bones and is placed in a specific pattern before it is later ossified into bone when it grows further away from the growth plate. New cartilage in a growing bone is placed through signaling proteins which bind to the heparan sulfate chains. EXT2 (protein) has also 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

TRAP1 Heat shock protein 75 kDa, mitochondrial is a protein that in humans is encoded by the ''TRAP1'' gene. Interactions TRAP1 has been shown to interact with EXT2, EXT1 and Retinoblastoma protein The retinoblastoma protein (protein name abbrevi ...

, a heat shock protein. Heat shock proteins will bind to specific proteins to help them keep their shape when the cell is stressed. TRAP1 has been found to bind to a region (in the c-terminal end) of EXT1 and EXT2 proteins to help it keep its desired shape and function.

Species Distribution

This gene was found to be present in many species other than humans such as mice, chickens, dogs, cows and many more. Other

orthologs Sequence homology is the biological homology between DNA, RNA, or protein sequences, defined in terms of shared ancestry in the evolutionary history of life. Two segments of DNA can have shared ancestry because of three phenomena: either a spec ...

have been found including

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

Caenorhabditis elegans ''Caenorhabditis elegans'' () is a free-living transparent nematode about 1 mm in length that lives in temperate soil environments. It is the type species of its genus. The name is a blend of the Greek ''caeno-'' (recent), ''rhabditis'' (ro ...

Mutations

Mutations that change the amino acid sequence of the exostosin glycosyltransferase-2 protein can lead to it becoming unfunctional. When this protein is unfunctional it causes the heparan sulfate chains to become shorter. The chains are still formed and extended by the other proteins encoded by the EXT family genes, although not to the same extent. This increases the likelihood that a cartilage cell will be placed incorrectly, as heparan sulfate is a bone and cartilage tumor suppressor. Since bone has a very specific structure, misplacing a cartilage cell in early growth is comparable to misplacement of a brick early on in construction of a wall. Misplacement in cartilage will result in cartilage tumor or tumors at the growth plates of long bones. This condition is known as hereditary multiple exostoses (HME) or hereditary multiple osteochondromas (HMO). HME can also be the result of a mutation to the EXT1 gene or other EXT family genes. EXT1 mutations tend to be more severe with more exostoses and are the cause of 56-78% of human HME cases, except for in China where mutations of the EXT2 gene are more common. HME effects 1 in 50,000 people and is more commonly seen in males in a 1.5:1 ratio.

Heredity of the EXT2 Gene

EXT2 gene mutations are dominant

autosomal An autosome is any chromosome that is not a sex chromosome. The members of an autosome pair in a diploid cell have the same morphology, unlike those in allosome, allosomal (sex chromosome) pairs, which may have different structures. The DNA in au ...

(not sex-linked) and is lethal in the

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 ...

form. This means that if the mutated gene is inherited from both parents giving the offspring two copies of the mutated gene (the homozygous form of the mutant gene) this will result in early embryonic death in the

gastrula Gastrulation is the stage in the early embryonic development of most animals, during which the blastula (a single-layered hollow sphere of Cell (biology), cells), or in mammals the blastocyst is reorganized into a multilayered structure known as ...

stage of development. Reasons for why this happens is that heparan sulfate has more roles than just bone formation, it also plays a role in embryonic development. Heparan sulfate can bind signaling molecules used in development such as

transforming growth factor β Transforming growth factor beta (TGF-β) is a multifunctional cytokine belonging to the transforming growth factor superfamily that includes three different mammalian isoforms (TGF-β 1 to 3, HGNC symbols TGFB1, TGFB2, TGFB3) and many other sign ...

, Fgf proteins and

Wnt proteins 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 ...

. The only individuals with this mutation exist in the

heterozygous 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 ...

form, this means that they have one EXT2 gene that is normal and one that is mutated. For the inheritance of this gene mutation, for a mutated parent and a not mutated parent there is a 50% chance that the offspring will also have an EXT2 mutation. For two parents with the EXT2 mutation, of their living offspring 2 out of 3 will have the mutation.

References

External links

Multiple Hereditary Exostoses Research Foundation
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