Osteogenesis imperfecta

Question 1

What is the difference in the sizes of glycine and cysteine?

Answer 1

Cysteine is bigger than Glycine because it has a larger R group

Question 2

What is osteogenesis imperfecta?

Answer 2

Typical Feature - repeated fracture of long bones

Cause: point mutation in the gene for type 1 collagen where G —-> T

This results in substitution of glycine with cysteine

The larger amino acid causes a kink in the normally straight triple helix - this affects the assembly into fibres.

Question 3

What does the fact that cysteine is substituted mean for the helix?

Answer 3

Cysteine has a reactive sulphydryl group in it’s side chain so, not only is the formation of the triple helix disrupted, but there can be inappropriate disulphide bonds between the two a1 chains in the helix.

Question 4

What are the three chains that collagen has?

Answer 4

2* alpha 1

1* alpha 2

Question 5

What does the mutation to cysteine mean for electrophoresis?

Answer 5

The resulting cross-linked polypeptides will migrate more slowly than the individual chains when examined by electrophoresis in the presence of SDS (sodium dodecyl sulphate - the speed of migration of proteins on electrophoresis depends on their size and not their charge).

In the presence of 2-mercaptoethanol, the disulphide bonds will be cleaved allowing the chains to migrate across the gel according to their molecular mass which isn’t that different to the molecular mass of the normal chains.

Question 6

What are the major consequences of deformed collagen?

Answer 6

The major consequence is in the formation of bone.
Bone is formed by laying down hydroxyapatite (a form of calcium phosphate) on an ordered scaffold of collagen-I.
The abnormal collagen leads to defects in the mineralisation process, so the patient ends up with skeletal abnormalities and generally weak bones.

There are also problems with the eyes, skin and ears.

Question 7

Prenatal diagnosis of osteogenesis imperfecta

Answer 7

• Biopsy is impractical and risky on a foetus
• Chorionic Villus Sampling (CVS) - screen the foetal DNA obtained
• Amniocentesis which is amplified by PCR can also be used
• Specific probes must be designed which bind with the part of the DNA sequence where a mutation was known to occur.
• if a mutation altered a restriction enzyme recognition site it would allow identification as only mutated DNA wouldn’t be cleaved

Question 8

Subsequent investigations showed the disorder had a dominant pattern of mutation in the patient’s family. Explain this by reference to the structure of collagen.

Answer 8

Only need to affect one chain to cause the collagen to be altered because all the chains are wound around each other with the glycine on the interior side.