Effect Of COL3A1 Gene Mutation On Collagen
Type IV of Ehlers-Danlos Syndrome affects the arteries, therefore is known as the vascular form. The COL3A1 gene mutations have a detrimental effect on the synthesis, structure and secretion of type III collagens. (Mao and Bristow, 2011).
Patients with vEDS are found to have abnormal procollagen. The effects include a decreased collagen fiber density, dilated rough endoplasmic reticulum and altered fibroblasts. There are multiple effects of the mutation in the COL3A1 gene depending on the nature and location of the sequence change. Though they commonly prevent normal processing and secretion of collagen III. COL3A1 mutations result in exon-skipping mutations or missense mutations, usually at the carboxyl terminus. (Pepin, et al 2000). Due to the homotrimeric formation of collagen III fibres, where the the normal type pro α1(III) and mutant alpha chains occur equally, nearly 90% of all α1(III) trimers will contain one or more mutant chains. (Leeming and Karsdal, 2016). Trimers that contain the mutated gene are consequently not secreted.
Missense Mutation
Missense Mutations involve single nucleotide change from one amino acid to another.
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Mutations regarding the addition of glycine accounts for a large majority of vEDS mutations. As Glycine is the amino acid with the smallest side chain of just one hydrogen (H), any mutation replacing the glycine results in a massive conformational change and has serious impact on the structure and formation of the collagen fibre.
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The substitution of a glycine at a residue site occurs in the carboxyl-terminal end of the triple-helical domain of pro alpha1(III) (Smith, et al, 1997). This results in an intracellular accumulation of fibrous material in the rough endoplasmic reticulum, along with the fibril diameters being considerably smaller than normal. A study conducted by the Departments of Medicine, at the University of Washington (Smith, et al, 1997), showed the collagen fibrils after affected by the COL3A1 gene mutation were (65-80 nm) compared to normal fibrils (95-110 nm). Figure 2 shows an image of both normal collagen fibres and mutated collagen fibres using microscopy. Mutations GI012R, GI018V, or GI021E result in extremely dilated rough endoplasmic reticulum (RER) and a thin dermis layer.
Effects of exon skipping
Figure 2
Microscopy image of collagen fibers.
Fig A shows normal collagen, with perfectly aligned fibres in long, thing fibrils.
Fig B shows the mutated collagen of a patient with vEDS. This collagen is highly irregular and disjointed, and therefore likely nonfunctional.
Cleaving of the N-terminus and C-terminus.
The forms of vEDS that have a mutation in the coding region for the C-Terminal are the most severe. This is due to the C-terminus and n-terminus acting as significant elements in the stability of the collagen filament. The trimeric protein of alpha helices are stabilised at the C-terminal end via a cysteine knot. As shown in figure 1, if a mutation occurs in this region there is possible disruption of fibril structure. Which can lead to a build up of fibrils. (Sharon, et al, 2017)
Figure 1 : A) Shows the N-terminus and C-terminus of the trimeric protein. Successful cleaving of both ends by specific protease enzymes (3). Followed by normal formation of cross-linked covalent bonds.
B) Mutation results in the interference with protease enzymes, so cleaving does not fully occur. This structural change results in the possible disruption of fibre cross-linkages and therefore the fibril structure.
A B
Exons are sections of the genetic code (DNA) that hold the instructions for the coding of proteins. Exon-skipping mutations occur during the splicing of the pre-mRNA and can have a dramatic effect on the formation of proteins. Patients with vEDS may have the exon-skipping mutations G769R, G373R, and G061E can loose exons 34 and 45. This results in the diameter of fibrils being 80-90 mm, which is smaller than the normal diameter being 95-110mm. The mutations G034R and G016C resulted in exponent's skipping mutations that deleted the sequences of exons 7, 8, 14, 18, 24, and 27. Consequently, the diameter of fibrils measured in the study by Smith, et al, were sized between 85 and 120 nm, which were more variable in size. Both mutations also resulted in a more dilated rough endoplasmic reticulum.
Haploinsufficiency
Haploinsufficiency is used to explain a pathological phenotype in which a diploid organism has lost one copy of a gene and is left with a single functional copy of that gene. (Schwarze et al, 2001). Therefore, in the case of vEDS one allele codes for the normal collagen III alpha1 gene and the other codes for the mutated collagen III alpha1 gene. Around 5% of patients are haploinsufficient, and produce 50% of normal collagen and 50% of mutated collagen. The low percentage of sufferers of this specific type is due to the mutated collagen frequently being destroyed via nonsense mediated decay before translation. These patients have much more mild symptoms and therefore produce much more normal collagen than the typical vEDS patient. The disease is commonly unnoticed or manifested in years.
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References:
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Pepin, M.G. Murray, M.L., Byers, P.H. (1999), ‘Vascular Ehlers-Danlos Syndrome’, Gene Reviews [Internet]
Mao, J.R. Bristow, J. (2001), ‘The Ehlers-Danlos syndrome: on beyond collagens’, J Clin Invest. 107(9): 1063–1069.
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Bowen, J., Tocher, J. (2016) Vascular Ehlers-Danlos syndrome. Available at: https://www.ehlers-danlos.org/information/vascular-ehlers-danlos-syndrome/ (Accessed: 03 November 2018).
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Smith. L, Schwarze. U, Goldstein. J, Byers. P, 1997, Mutations in the COL3A1 Gene Result in the Ehlers–Danlos Syndrome Type IV and Alterations in the Size and Distribution of the Major Collagen Fibrils of the Dermis, Journal of Investigative Dermatology
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