Brittle Bone Tutorial

Question 1

What is the medical name for brittle bones diseases?

Answer 1

Osteogenesis imperfecta

Question 2

What can osteogenesis imperfecta be misdiagnosed as?

Answer 2

Child abuse

Question 3

What is the main feature of osteogenesis imperfecta?

Answer 3

Repeated fracture of long bones

Question 4

What causes osteogenesis imperfecta?

Answer 4

Range of genetic disorders

Question 5

What is an example of a mutation leading to osteogenesis imperfecta?

Answer 5

Point mutation in COLA1 (collagen α1) gene
- G to T —> glycine to cystine

Question 6

Why does the mutation cause osteogenesis imperfecta? (2)

Answer 6

Glycine replaced by cysteine
- Cysteine too large to be inside triple helix —> kink in helix
- Cysteine contains reactive sulphydryl group —> disulphide bridge formation between α chains

Question 7

How can collagen I be analysed in the lab?

Answer 7

Gel electrophoresis (SDS-PAGE)

Question 8

What is the difference between the gel electrophoresis of cross-linked collagen in the presence of SDS vs 2-mercaptoethanol and why?

Answer 8

• SDS —> mutated collagen travels slower
• 2-mercaptoethanol —> cleaves disulfide bridges —> normal collagen pattern

Question 9

Why is the ColA1 mutation dominant?

Answer 9

Some α1 chains mutated —> affects packing of all chains —> all fibrils affected

Question 10

What does the ColA1 mutation affect the rate of in collagen production? (4)

Answer 10

1. Change transcription rate
2. Change translation rate
3. Change mRNA stability
4. Change protein stability

Question 11

How is bone formed?

Answer 11

Hydroxyapatite deposited on collagen I

Question 12

What is hydroxyapatite?

Answer 12

Form of calcium phosphate deposited in bones

Question 13

What are the 3 symptoms of osteogenesis imperfecta?

Answer 13

1. Skeletal abnormalities
2. Weaker bones —> many fractures
3. Issues in eyes, teeth, skin, ears

Question 14

How is prenatal osteogenesis imperfecta screening done? (2)

Answer 14

Genetic screening
1. Chronic villus sampling
2. Amniocentesis
—> PCR

Question 15

How is DNA with the ColA1 mutation identified in the lab?

Answer 15

1. PCR —> probe for mutation
2. PCR —> restriction enzyme testing

Question 16

What is the limitation of genetic screening for osteogenesis imperfecta?

Answer 16

Requires knowledge of what mutation could be
- Use family history

Question 17

What are the 2 biochemical consequences of ColA1 mutation?

Answer 17

1. Kinks in collagen fibrils
2. Disulfide bridges between α-chains

Question 18

What type of mutation is the ColA1 mutation?

Answer 18

Gain-of-function

Question 19

How often does whole skeleton turn over occur?

Answer 19

Every 5-10 years

Question 20

What can be done to an amniocentesis sample to test for osteogenesis imperfecta? (2)

Answer 20

RFLP
- PCR
- Gel electrophoresis
- Probe for mutation