vEDS is a disease of Type III Collagen, which plays an essential role in the structural support and stabilization of a wide variety of structures, including the walls of arteries and veins. Vascular Ehlers Danlos can be caused by a wide variety of mutations (250+), including insertions, deletions, and missense and nonsense mutations in the COL3A1 gene (Byers et al, 1979). The COL3A1 gene is extremely large, and encompasses more than 50 exons (Germain and Herrera-Guzman, 2004). Many patients have more than one mutation, and many of these mutations are unique to one family, showing that vEDS has a strong genetic component. The COL3A1 gene is essential because it encodes alpha-chains, the precursors to Type III Collagen; Col3 is an essential part of the extra cellular matrix, and must be synthesized, it cannot be obtained from the diet. Due to the wide variety of mutations, vEDS can actually be triggered via several different mechanisms.
To form one Col3 protein, three monomeric alpha-chains bind together and twist around each other in a corkscrew-like shape to form one, triple-helical protein; macroscopic collagen is made up of many of these homotrimer Col3 units bound together. These triple helices can be seen in the figure above. Each individual alpha-chain is composed of many glycine-proline-hydroxyproline repeats (Boudko et al, 2008). Although not solely composed of these three amino acids, the repeats of proline and hydroxyproline are useful because both are relatively inflexible cyclic amino acids that give each Col3 chain strength and stability. Repeating patterns of these three amino acids are generally located on the outside of the helices, and thus are important for binding between alpha-chains, which is essential for stability of the triple helix in Col3. Mutations of the glycine in the Gly-Pro-HydroxyPro motif account for the large majority of vEDS mutations, and this is highly disruptive to the overall structure of Col3; glycine is extremely small, so the substitution for a larger and/or charged amino acid can cause increased steric or electronic hindrance. This can greatly destabilize the protein and potentially preventing the formation of the triple helix, making the collagen nonfunctional (Jorgenson et al, 2014). Even if the triple helix is stable enough to form, a mutation may disrupt the shape of the helix, which is problematic because every Col3 chain is bonded to another collagen chain; the specific shape and diameter of the collagen chain is essential to normal function, as chains that are too thick or too thin are dysfunctional or nonfunctional (Wu et al, 2010). A single mutation in the Gly-Pro-HydroxyPro motif, then, could decrease the stability of the entire triple helix, which could correspondingly decrease the strength and stability of the entire collagen polymer chain (Boudko et al, 2008). The collagen polymer becomes messy and disorganize
Although the three alpha chains are stabilized by inter-chain bonding between the three helices of Col3, the protein is also stabilized at the C-terminal end via a cysteine knot. This knot holds the triple helix together using three disulfide bonds between each alpha-chain, which provides significant stabilization of the entire structure. The most severe forms of vEDS have a mutation in the coding region for the C-Terminal cysteine knot, implying that this region plays an essential role in the stability and functionality of Col3 (Boudko et al, 2008). The Gly-Pro-HydroxyPro motif repeats in this knot region as well, and mutating the glycine residue has been shown to be particularly detrimental to collagen stability. Interestingly, even though this region is termed a “knot,” it is not a static structure; instead, it is highly mobile and can take several different conformations. This is useful because it allows the triple helix of Col3 to change shape, which is probably important during the polymerization of collagen into long chains. Unfortunately, these different shapes of the knot distribute electron density, allowing only a partial crystal structure to be obtained, as shown below (Boudko et al, 2008).
Post Translational Modifications
In addition to structural and stability considerations, it has been shown that mutated monomeric alpha-chains take longer to form the requisite trimer of Col3 (Mizuno et al, 2013). In this study, the glycine of the Gly-Pro-HydroxyPro repeat was mutated, which caused an observable delay in the formation of a collagen trimer. This is problematic because each monomer is being post-translationally modified (PTM) during trimerization. Normally residues that are crucial for bonding or stability are moved into the middle of the helix during the trimerization process, where they are shielded from modification; however, since trimerization takes significantly longer in vEDS patients, the alpha chains are exposed to the cellular milieu for a longer period of time, and thus are more likely to be modified, usually via the addition of glycosyl units. Glycosylation of some, select residues is normal, but the arbitrary addition of glycosyl units to many residues can drastically affect the shape of each alpha-chain, and prevent the trimerization into Col3. Glycosyl units are also highly reactive, and can indiscriminately bind to other macromolecules in the cell, further disrupting the normal structure of collagen.
In addition to these more structural changes, some vEDS patients have nonsense mutations, in which the mRNA for Col3 becomes shortened during transcription due to an early stop codon. The alpha-chains that are translated from this mRNA are also shortened, which shortens, or prevents the formation of, the resulting triple helix; in both instances, the Col3 is dysfunctional. If the triple helix does form, it has vastly decreased thermal stability, and can spontaneously dissociate into individual alpha-chains, even at normal body temperatures (Superti-Furga et al, 1988).
Some of these shortened mRNA chains may also automatically activate nonsense-mediated decay pathways, an RNA degradation mechanism. This pathway decreases the number of transcripts that code for alpha-chains; this is problematic because some vEDS patients are surprisingly actually able to produce some normal alpha chains. The disease is autosomal dominant, and collagen has three separate chains, so the laws of probability state that 7/8 of collagen will contain one or more mutated chains; one mutant chain is usually enough to make the entire Col3 unit dysfunctional; about 1/8 of the collagen being produced should be normal. However, in mutations that cause mRNA degradation, even less normal collagen is being produced, which only serves to worsen symptoms.
A few vEDS mutations have been shown to cause haploinsufficiency, which also decreases the amount of collagen being synthesized (Schwarze et al, 2001). In this case, one allele codes for normal collagen, while the other allele produces mutated collagen. vEDS is usually inherited autosomally dominant, but in about 5% of all patients, it can also be haploinsufficient. These patients produce 50% of normal collagen and 50% of mutated collagen; frequently the mutated collagen is destroyed via nonsense mediated decay before translation. These patients produce much more normal collagen than the typical vEDS patient, and thus manifest the disease later in life, and generally with more mild symptoms.
Manifestations of the mutations
In all of the above mechanisms, the end result is the disruption of individual alpha chains, which is inherently problematic because it prevents the formation of normal triple helices. The triple helix is absolutely essential for normal Col3 function, as this is an extremely important part of the tertiary and quaternary structure of the protein. Without a norma triple helix, the collagen is simply nonfunctional, and cannot provide support for other tissues in the body. While collagen itself does not play a major metabolic role, it plays an absolutely essential part in providing structural support and tissue organization that allows the body to function as normal.
Since some normal triple helices are being produced by vEDS patients, symptoms often do not manifest for until the teenage years, or even beyond; however, functional Col3 is still essential for life, and it is only being incorporated into the walls of major blood vessels or organs at very small rates. Therefore, it is only a matter of time before the organs and vessels become so weak that they rupture, even when only exposed to a tiny amount of force. Thus, regardless of the specific mechanism of action, the symptoms of vEDS all stem from the disruption of the triple helix and general lack of bioavailability of functional Type III collagen.