Collagen Flashcards
Describe collagen
Collagen is the most abundant protein in the human body with 29 different types. The most abundant type is type 1 which is found in bone and tendon and skin. Type 2 is found in cartilage, type 3 in skin, blood vessels, uterus and intestines and type 7 forms fibrils that anchor skin.
Collagen proteins are formed from a triplet of aa repeats whihc is usually glycine, proline, proline. Glycine is essential for the forming of the triple helix because it is small.
How is collagen synthesised?
Having been translated collagen has N and C terminal pro-peptides which are later cleaved. The triple helix self assembles from the C terminus to the N producing pro-collagen. Modification occurs during assembly involving glycosylation and hydroxylation but can only occur before the helix is assembled.
The collagen proteins are then moved extracellularly where they align and cross link to form fibrils.
How can mutations affect collagen proteins?
Missense mutaitons in the glycine codons can result in a possible 8 other aa. If these are bulkier than glycine (glycine is smallest aa so most are) then this interupts the triple helix formation and so extra modification occurs. The closer to the C terminus this occurs the more over modification occurs and so the greater effect.
This is the packing effect and results in and increased diameter of the alpha helix.
What is the outcome of over modified collagen?
Over modified collagen is degraded creating endoplasmic stress. if any over modified collagens do make it outside the cell then they are distorted and provide a poor scaffold for bone.
Which types of collagen causes whihc diseases?
Type 1 = OI and Ehthelo Danlos type VII
Type 2 = chondriplasias
Type 3 = vascular ehlers danlos and aortic aneurysms
Type 4 = dystrophic epidermolysis bullosa
Describe the 5 types of OI showing the number of genes identified and the inheritance pattern.
Order of increasing severity
Type 1 -classic non deforming OI with blue sclera. 2 genes involved and has autosomal dominant inheritance.
Type 4 - common variable, normal sclera. 7 genes involved and has both autosomal dominant and recessive inheritance.
Type 5 - calcification of interosseous membranes. 1 gene involved and autosomal dominant inheritance.
Type 3 - progressively deforming and normal sclera. 14 genes involved and shows both autosomal dominant and recessive inheritance.
Type 2 - perinatal lethal - 5 genes involved and shows both autosomal dominant and recessive inheritance.
Discuss type 1 OI
Reduced amounts of collage are formed due to a haploinsufficiency. Mostly cause by nonsense and frame-shoft mutations resulting in NMD. Few examples of substituitions, exon skipping or deletions.
Majority of cases due to COL1A1 gene mutations, some due to COL1A2. Variants are semi private and the blue sclera occurs because the defect results in very thin scerla making and the blue epithelium shine through.
How does exon skipping occur in type 1 OI?
Splice acceptor site shifts 1bp due to the a substitution of G>A. Introns always end AG whilst collagen sequences always start GGT due to glysine. If first G of exon changes to an A splice acceptor site moves one position down and the rest of the gene is frame shifted.
Can also cause a slightly worse phenotype is the new splice acceptor site is sub optimal meaning that some protein is produced but this protein is sub optimal and so interferes in fibril formation.
Describe OI types 2, 3 and 4.
Mostly due to aa substitutions but also some exon skipping or deletions as well.
Most examples of mutations are de novo so it appeared recessive at first and are semi private. However the same variant seem to reoccur i.e. G>A but is unclear why.
What other OI phenotypes can occur?
Scoliosis, rib malformation - popcorn appearance die to recurrent fractures during pregnancy.
How is collagen normally assembled and how does this explain the reason for changes in phenotype.
Requires 2 COL1A1 proteins and 1 COL1A2 proteins.
In a frame shoft or NMD of COL1A1 then there is half as much collagen produced = mild OI. Missense mutation = all collagen produced being affected.
Proximity to C terminus is also another variant involved.
Are there any ways to predict pattern of phenotypes?
Gradient model - should be a gradient of substitutions being non lethal at N terminus and lethal at C terminus. However the data doesn’t support this as there is alot of cross over. None the less there is a higher concentration of lethal variants at the C terminus and non lethal at the N terminus and there were no lethal variants at all at the N terminus.
Regional model - some areas of the gene sequence and so triple helix of the protein can tolerate substitutions better than others. I.e. some areas are not pivotal for extracellular interactions. Theory works best for COL1A2 where lethal variants cluster in 8 different loci including exon skips. These loci coincide with binding domains.
How does the aa that is substituted affect the phenotype?
For COL1A1 a 1/4 of all glycines have 2 or more indpendant occurrences of substitutions. One might be lethal the other mild. And one aa might cause one phenotype in one person in the other it might differ.
For COL1A2 a 1/5 of all glycines have 2 or more indpendant occurrences of substitutions. But phenotype are more concordant between different AAs and more concordant for the same AA between individuals.
Either way there within a family the phenotype for the same mutations may differ between individuals
What is the relative important of the two main collagen alleles?
COL1A1 homozygotes are embryonic lethal in mice and as its not been seen in humans this is probably true of us too.
COL1A2 homozygotes do exist and cause ehlers Danlos syndrome.
What role does mosaicism have to play in OI?
Type 2 3 and 4 all thought to be recessive originally. But recurrence was only 5-8% when it should have been 25%.
Turned out this was becasue most are dominant disorder than occur in a somatic mutation in development resulting in mosaicadults who are unaffected. If germline is mosaic for the mutant then this may be passed on or may not.
