Osteogenesis Imperfecta

Question 1

What is locus heterogeneity?

Answer 1

A particular phenotype that can be caused by a different gene

Question 2

What is a ‘dominant negative’ mutation? And where is it common? (2 marks)

Answer 2

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

Question 3

What is the BASIC structure of collagen? (1 mark)

Answer 3

3 alpha chains
Each alpha chain contains triplet repeat
Part of molecule exists with triple helix with glycine centre

Question 4

Purely outline (so simple) the biosynthesis of collagen? (3 marks)

Answer 4

1. Transcription
2. Translation into alpha chain
3. Proline and lysine hydroxylation
4. Glycosylation of hydroxylysine
5. Triple helix formation and secretion

Question 5

Purely outline the extracellular process of collagen biosynthesis? (2 marks)

Answer 5

Removal of peptide

Fibrillogenesis: fibres line up in staggered pattern and form cross formation between fibres

Question 6

What mutations causes the four types of OI?

Answer 6

COL1A1 or COL1A2

Question 7

What is the most lethal type of OI and what are the main phenotypes associated?

Answer 7

Type II - get beaded ribs and can’t support breathing, reduced mineralisation and deformed bones

Question 8

How is OI diagnosed?

Answer 8

Clinically and radiographically

Question 9

What are the ranges of symptoms for OI? (6 marks)

Answer 9

• Fractures from mild trauma
• Bowing deformities of long bones
• Reduced height
• Hearing loss
• Blue sclera
• Scoliosis

Question 10

What is osteopenia?

Answer 10

Reduced amount of bone in skeleton. From bone histomorphometry can see lowered bone density

Question 11

What is bone histomorphometry? (1 mark)

Answer 11

A bone biopsy - invasive procedure that reveals low bone density and high bone turnover

Question 12

What is dentinogenesis imperfecta? (2 marks)

Answer 12

Abnormality in dentin see tooth discolouration, weak teeth so ground down to gum level

Question 13

What kind of mutation are ALL types of OI?

Answer 13

Autosomal mutation

Question 14

Is the abnormal mutated gene product incorporated into the matrix or excluded?

Answer 14

It depends

Question 15

What are the phenotypes of Type I OI and how severe is it? (5 marks)

Answer 15

MILD

• normal structure
• blue sclerae
• matrix contains 50% of usual amount of collagen BUT ITS ALL NORMAL
• little bone deformity
• mutation is EXCLUDED

Question 16

What are the phenotypes of Type II OI and how severe is it? (4 marks)

Answer 16

LETHAL

• marked bone deformity
• minimal calvaria mineralisation
• stature is very short
• beaded ribs

Question 17

What are the phenotypes of Type III OI and how severe is it? (4 marks)

Answer 17

SEVERE & PROGRESSION

• marked bone deformity
• minimal calvaria mineralisation
• stature is very short
• beaded ribs

Question 18

In type II and III OI what kind of collagen is present in the ECM? (2 marks)

Answer 18

Mixture of abnormal type I collagen and so a reduced amount of collagen is secreted

Question 19

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)

Answer 19

Point mutations. They are included. Included COL1A1 is the most severe and included COL1A2 are less severe.

Question 20

What are the phenotypes for type IV OI, how severe is it and what mutation causes it? (4 marks)

Answer 20

MILD/ MODERATE

• normal sclerae
• mild/ moderate bone deformity
• included mutation of COL1A2 in the matrix causes deficient and defective matrix

Question 21

What types of OI include the excluded mutation? (3 marks)

Answer 21

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

Question 22

What types of OI include the included mutation? What does this cause? (3 marks)

Answer 22

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

Question 23

Why could collagen be affected by a dominant negative mutation? (3 marks)

Answer 23

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

Question 24

What are the main characteristics of type I collagen and what is it encoded by? (3 marks)

Answer 24

• major constituent of bone
• exists as triple helix (2 alpha1 chains and 1 alpha2 chains
• encoded by COL1A1 and COL1A2

Question 25

How can the mutations of COL1A1 and COL1A2 be enhanced? (1 mark)

Answer 25

By the mutated chain interfering with function of normal alpha chain

Question 26

What percentage of collagen molecules will be abnormal in a COL1A1 mutation and COL1A2 mutation? What chains do they encode? (4 marks)

Answer 26

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)

Question 27

What does a null allele at COL1A1 cause? (3 marks)

Answer 27

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

Question 28

What do 2 COL1A1 and 1 COL1A2 make up?

Answer 28

Type I collagen

Question 29

Why do point mutations mainly cause type II OI?

Answer 29

- Because there is a change in glycine GLY-X-Y = major cause of OI

Question 30

What does a substitution of glycine in triple helical domain result in? (4 marks)

Answer 30

- 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

Question 31

What factors affect the severity of OI? (6 marks)

Answer 31

1. Which gene - COL1A1 or COL1A2 2. What amino acid replaces glycine 3. Position of mutation 4. Over modification 5. Abnormal molecules 6. Abnormal fibrils

Question 32

How does the terminal end collagen forms from and the end the mutation arises in affect OI? (3 marks)

Answer 32

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

Question 33

How does over modification affect the severity of OI? | 2 marks

Answer 33

It will reduce the thermal stability of collagen and therefore increase intracellular degradation and slow secretion

Question 34

How will abnormal molecules affect the severity of OI? | 4 marks

Answer 34

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

Question 35

What mutation causes the most severe form of OI?

Answer 35

Mis-sense mutation at C-terminal end of COL1A1

Question 36

What mutation is the second most severe form of OI?

Answer 36

Mis-sense mutation at C terminal end of COL1A2

Question 37

What are the recessive/ less severe forms of OI due to?

Answer 37

Mutations in collagen modifications folding and crosslinking

Question 38

How does the lethality of cases change along the alpha 1 chain?

Answer 38

Cases tend to be more lethal moving from the N terminal end to the C terminal end

Question 39

What does osteoblast regulation do?

Answer 39

Make type I collagen in bone

Question 40

What is antireabsorptive therapy? (3 marks)

Answer 40

Uses bisphosphonates to reduce osteoclast activity which increases bone volume. The efficiency in preventing fractures in children is moderate - not proven yet in adults

Question 41

What is anabolic therapy?

Answer 41

Uses recombinant human growth hormone to elviate the short stature in children

Question 42

What is anti-sclerostin?

Answer 42

Activates Wnt signalling to cause bone formation - not been tried in humans yet for OI

Question 43

Explain the intracellular events of collagen biosynthesis. | 10 marks, 8 steps

Answer 43

1. Formation of collagen from specific RNA 2. After transcription, gene spliced and makes functional mRNA 3. Specific mRNA transported to rER 4. Proline and lysine hydroxylation mediated by hydroxylases prolyl and lysl 5. Galactosyltransferase and glycosyltransferase catalyse glycosylations 6. At the same time have association of pro-alpha chains in correct chain registration and triple helix assembly occurs 7. In alignment cysteine resides juxtaposed for formation of disulfide bridges which will link individual pro-alpha chains at C-terminal end 8. 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

Question 44

Explain the extracellular events of collagen biosynthesis. | 6 marks, 4 steps

Answer 44

1. In ECM synthesised pro-mol. interact with processing enzymes and undergo fibril formation and cross linking 2. Enzymes involved: procollagen, N & C proteinases and lysyl oxidase 3. 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 4. Bifunctional cross links undergo further intra and intermolecular reactions to from variety of mature trifunctional crosslinks