Osteogenesis Imperfecta Flashcards
What is locus heterogeneity?
A particular phenotype that can be caused by a different gene
What is a ‘dominant negative’ mutation? And where is it common? (2 marks)
Mutant gene that loses its own function and prevents other genes from functioning correctly.
Common where mutation affects multimeric protein encoded by more than one gene
What is the BASIC structure of collagen? (1 mark)
3 alpha chains
Each alpha chain contains triplet repeat
Part of molecule exists with triple helix with glycine centre
Purely outline (so simple) the biosynthesis of collagen? (3 marks)
- Transcription
- Translation into alpha chain
- Proline and lysine hydroxylation
- Glycosylation of hydroxylysine
- Triple helix formation and secretion
Purely outline the extracellular process of collagen biosynthesis? (2 marks)
Removal of peptide
Fibrillogenesis: fibres line up in staggered pattern and form cross formation between fibres
What mutations causes the four types of OI?
COL1A1 or COL1A2
What is the most lethal type of OI and what are the main phenotypes associated?
Type II - get beaded ribs and can’t support breathing, reduced mineralisation and deformed bones
How is OI diagnosed?
Clinically and radiographically
What are the ranges of symptoms for OI? (6 marks)
- Fractures from mild trauma
- Bowing deformities of long bones
- Reduced height
- Hearing loss
- Blue sclera
- Scoliosis
What is osteopenia?
Reduced amount of bone in skeleton. From bone histomorphometry can see lowered bone density
What is bone histomorphometry? (1 mark)
A bone biopsy - invasive procedure that reveals low bone density and high bone turnover
What is dentinogenesis imperfecta? (2 marks)
Abnormality in dentin see tooth discolouration, weak teeth so ground down to gum level
What kind of mutation are ALL types of OI?
Autosomal mutation
Is the abnormal mutated gene product incorporated into the matrix or excluded?
It depends
What are the phenotypes of Type I OI and how severe is it? (5 marks)
MILD
- normal structure
- blue sclerae
- matrix contains 50% of usual amount of collagen BUT ITS ALL NORMAL
- little bone deformity
- mutation is EXCLUDED
What are the phenotypes of Type II OI and how severe is it? (4 marks)
LETHAL
- marked bone deformity
- minimal calvaria mineralisation
- stature is very short
- beaded ribs
What are the phenotypes of Type III OI and how severe is it? (4 marks)
SEVERE & PROGRESSION
- marked bone deformity
- minimal calvaria mineralisation
- stature is very short
- beaded ribs
In type II and III OI what kind of collagen is present in the ECM? (2 marks)
Mixture of abnormal type I collagen and so a reduced amount of collagen is secreted
What are the mutated gene products in the matrix in type II and III OI caused by and are they included in the matrix? How does the severity differ? (3 marks)
Point mutations. They are included. Included COL1A1 is the most severe and included COL1A2 are less severe.
What are the phenotypes for type IV OI, how severe is it and what mutation causes it? (4 marks)
MILD/ MODERATE
- normal sclerae
- mild/ moderate bone deformity
- included mutation of COL1A2 in the matrix causes deficient and defective matrix
What types of OI include the excluded mutation? (3 marks)
Type I only. Null allele mutation is the cause of this OI. Only 50% collagen laid down in the matrix includes the abnormal collagen molecules but the rest is ALL NORMAL
What types of OI include the included mutation? What does this cause? (3 marks)
Types II, III and IV.
- mutation causes reduced secretion of collagen
- what is laid down in matrix is the included abnormal collagen molecules due to the dominant negative affect
Why could collagen be affected by a dominant negative mutation? (3 marks)
The 3 polypeptide chains could be coded for by 2 different genes. So even though a certain mutation does not affect one of the alpha chains, it could affect the other two as they contain completely different forms of collagen
What are the main characteristics of type I collagen and what is it encoded by? (3 marks)
- major constituent of bone
- exists as triple helix (2 alpha1 chains and 1 alpha2 chains
- encoded by COL1A1 and COL1A2
How can the mutations of COL1A1 and COL1A2 be enhanced? (1 mark)
By the mutated chain interfering with function of normal alpha chain
What percentage of collagen molecules will be abnormal in a COL1A1 mutation and COL1A2 mutation? What chains do they encode? (4 marks)
COL1A2 allele mutation ill cause 50% of molecules to be abnormal (encodes alpha2 chain)
COL1A1 will cause 75% of molecules to be abnormal (encodes alpha1 chain)
What does a null allele at COL1A1 cause? (3 marks)
50% less of type 1 procollagen and causes a mild from of OI as some surplus alpha2 chains get degraded.
But mutations that replace glycine with any other AA have a dominant negative effect as it disrupts 3 chains in triple helix
What do 2 COL1A1 and 1 COL1A2 make up?
Type I collagen
Why do point mutations mainly cause type II OI?
- Because there is a change in glycine GLY-X-Y = major cause of OI
What does a substitution of glycine in triple helical domain result in? (4 marks)
- slower rate of triple helix helical formation and over modification
- lower triple-helix thermal stability
- slower rate of secretion of collagen molecules from cell
- poor molecule packing of molecules into fibril
What factors affect the severity of OI? (6 marks)
- Which gene - COL1A1 or COL1A2
- What amino acid replaces glycine
- Position of mutation
- Over modification
- Abnormal molecules
- Abnormal fibrils
How does the terminal end collagen forms from and the end the mutation arises in affect OI? (3 marks)
So, triple helix of collagen assembles from C terminal end and mutations that affect the 3’ end of mRNA will disrupt the triple helix. This will cause a higher degree of post-translational modifications
How does over modification affect the severity of OI?
2 marks
It will reduce the thermal stability of collagen and therefore increase intracellular degradation and slow secretion
How will abnormal molecules affect the severity of OI?
4 marks
Give rise to abnormal fibrils and over modified molecules.
This will cause inefficient cleavage of pro peptides. Therefore the fibrils become poor templates for mineralisation which is an essential process for bone formation
What mutation causes the most severe form of OI?
Mis-sense mutation at C-terminal end of COL1A1
What mutation is the second most severe form of OI?
Mis-sense mutation at C terminal end of COL1A2
What are the recessive/ less severe forms of OI due to?
Mutations in collagen modifications folding and crosslinking
How does the lethality of cases change along the alpha 1 chain?
Cases tend to be more lethal moving from the N terminal end to the C terminal end
What does osteoblast regulation do?
Make type I collagen in bone
What is antireabsorptive therapy? (3 marks)
Uses bisphosphonates to reduce osteoclast activity which increases bone volume. The efficiency in preventing fractures in children is moderate - not proven yet in adults
What is anabolic therapy?
Uses recombinant human growth hormone to elviate the short stature in children
What is anti-sclerostin?
Activates Wnt signalling to cause bone formation - not been tried in humans yet for OI
Explain the intracellular events of collagen biosynthesis.
10 marks, 8 steps
- Formation of collagen from specific RNA
- After transcription, gene spliced and makes functional mRNA
- Specific mRNA transported to rER
- Proline and lysine hydroxylation mediated by hydroxylases prolyl and lysl
- Galactosyltransferase and glycosyltransferase catalyse glycosylations
- At the same time have association of pro-alpha chains in correct chain registration and triple helix assembly occurs
- In alignment cysteine resides juxtaposed for formation of disulfide bridges which will link individual pro-alpha chains at C-terminal end
- Procollagen molecules then travel from rER toward Golgi apparatus through microsomal lumen. In Golgi, pro-molecules packed into secretory vesicles and translocated to surface of cell where secreted into extracellular environment by exocytosis
Explain the extracellular events of collagen biosynthesis.
6 marks, 4 steps
- In ECM synthesised pro-mol. interact with processing enzymes and undergo fibril formation and cross linking
- Enzymes involved: procollagen, N & C proteinases and lysyl oxidase
- Lysyl oxidase - initiates cross-linking of collagens via deamination of certain lysine hydroxylysine residues located in short N & C terminal non-helical regions - remain after removal of procollagen peptides
- Bifunctional cross links undergo further intra and intermolecular reactions to from variety of mature trifunctional crosslinks
