MSS25 Molecular Mechanism Of Bone Development And Basic Genetic Aspects Of Skeletal Disorders Flashcards
Cartilage end of growing long bone
最出 –> 最入:
- Epiphysis:
- Articular cartilage
- Secondary ossification centre
- Reserve / Resting cartilage
- Proliferating cells (–> either become cartilage / bone) - Metaphysis
- Hypertrophic cells
- Calcified cartilage - Diaphysis:
- Trabecular bone
- Cortical bone
Components in bone calcifications
- Chondrocytes:
- ***Collagen 2 –> Collagen X
(- COMP
- aggrecan
- CMP
- syndecan 3
- link protein) - Osteoblasts:
- ***Collagen 1
- osteocalcin
- osteopontin
- BSP
- AP - Chondroclasts / Osteoclasts:
- cathepsins
- MMPs - Bone calcification:
- OPG (osteoprotegerin) –> bind to RANK to prevent RANKL binding to RANK –> inhibit Osteoclast
- MGP (Matrix Gla-proteins)
Osteochondrodysplasia
- disorders that affect epiphysis / metaphysis / diaphysis
- large no. of murine and human inherited skeletal abnormalities
- genetic studies increasingly important in identifying new / key genes and regulatory pathways in bone development
- information –> more effective therapies
Molecular consequences of mutations
- **Point mutations most common
1. Missense: alter a.a. sequence
2. Nonsense: introduce premature stop codon
3. Alteration of promoter / enhancer (regulatory) sequences
4. Exon skips: mRNA splicing mutations that result in loss of exon
Collagen disorders
***Collagen Type I: Osteogenesis imperfecta (bone)
Collagen type II: Spondyloepiphyseal dysplasia (cartilage)
Collagen type III, V:
Ehlers-Danlos syndrome (skin, arteries)
Collagen type VI: Bethlem myopathy (muscle)
Collagen type X: Metaphyseal chondrodysplasia (growth plate cartilage)
Osteogenesis imperfecta signs and symptoms
- affects mainly bone, but tendon, ligament, sclera, dentin also affected
- fragile / brittle bones
- hypermobile joints
- tendon rupture
- hernias frequent
- thin skin
- thin sclera –> blue colour
- early onset deafness
Sillence classification of OI
Type I - XI
Type I - IV: affects ***genes of collagen type I (COL1A1, COL1A2)
Type I: mild
Type II: lethal
Type III: progressive (worse over time) –> severe bowing, scoliosis
Type IV: moderate to severe
Type VI - XI: affects ***synthesis of collagen type I
Type V: unknown: hypertrophic calluses at fracture sites
Gene mutation related to Osteogenesis imperfecta (type I - IV)
- Result from type I collagen mutation
- > 200 mutations in **COL1A1 and **COL1A2 genes –> code for α1, α2 chains respectively
- **Types of mutations:
1. Point mutations –> ***Glycine substitutions
2. Insertion, deletion
3. Exon skips
4. Null mutations –> ↓ level of expression
Type I Osteogenesis Imperfecta
- ***Not enough Collagen type I
- Blue sclera –> clue to diagnosis of type 1A
Causes:
-
**Loss-of-function gene mutation (mostly)
- -> ↓ amount of type I collagen in bone
- -> **Haploinsufficient (only 1 functional allele –> collagen reduction by half) - Nonsense mutations
- -> Premature stop codon
- -> ***Non-functional truncated chain
- -> Selective degradation of mutant mRNA (nonsense-mediated mRNA decay)
Type I procollagen
- Fibrillar collagen
- 2x α1 chains, 1x α2 chain
- Gly-X-Y sequence repeated (X, Y = Proline / Hydroxyproline)
Collagen biosynthesis
Intracellular events:
- mRNA
- -> signal peptide cleavage in ER
- -> protein synthesis in ER (translation)
- -> post-translational modification: - Hydroxylation in ER (Ascorbic acid as co-factor)
- Oligosaccharide processing (Glycosylation) in Golgi
Extracellular events:
- ***Procollagen processing
- proteolytic processing
- N- and C-proteinases remove propeptide domains -
**Fibril assembly + **Cross-linking
- collagen aggregates into ordered fibrils stabilized by covalent crosslinks
- catalysed by peptidyl **lysine oxidase
- requires O2, **Copper-II ion
Intracellular modifications
- Signal peptide
- hydrophobic: allow extrusion of chains into lumen of rER - Post-translational hydroxylation
- specific enzymes: **Prolyl + Lysyl hydroxylases –> **Hydroxylation of proline + lysine
- reaction requires ***Ascorbic acid, O2, α-ketoglutarate
- proline hydroxylation critical for helix formation + stability
Type II Osteogenesis imperfecta
- generally due to synthesis of **abnormal collagen I —> **Dominant negative
- perinatal lethal
- Mutation within triple helix
- ***minimal mineralization
- marked long bone deformity
- numerous interuterine fractures
Causes:
- ***Glycine mutation (more severe than X, Y mutation)
- -> more severe if mutation near C-terminal (∵ helix folding starts at C-terminal)
Consequences of helical mutations:
- ***Excessive post-translational modification (∵ it happens only when chain is unfolded)
***Dominant-negative effect
- Impaired secretion of molecules containing >=1 mutant chains
- -> 1 mutated / dysfunctional α1 chain can cause the whole collagen molecule to be defective (even though the collagen molecule has 1 normal α1 and 1 normal α2)
- -> 一條有問題搞到成條Collagen無用
N.B. in case of type II collagen (3 identical chains) –> problem much more severe
- Destabilized matrix
- -> altered collagen packing / cross-linking during fibril assembly
- -> 一條有問題搞到成個Matrix無用
Possible treatments for OI patients
- having inferior bone is better than not enough
- target bone remodeling process: balance between osteoblast / osteoclast
Treatment:
- Improve bone by reducing activity of osteoclast
- ***Bisphosphonate treatment
- ***Inhibitors of TGFβ signaling (more recent concept)
