Osteogenesis Imperfecta

Question 1

What is the structure of collagen?

Answer 1

Triple helix of 3 alpha chains - 2 alpha 1 chains and 1 alpha 2 chain.
Glycine at every third position facing the interior of the helix.
Modified amino acids hydroxyproline and hydroxylysine.

Question 2

What mutations occur in Osteogenesis Imperfecta?

Answer 2

Mutations in glycine residues, producing defective structural abnormality.
Point mutation in the gene for type 1 collagen changing a G to a T.
Glycine AA is substituted for Cysteine AA.

Question 3

What effect does the mutation have?

Answer 3

Cysteine is larger than glycine. This causes steric hindrance and a kink in the normally straight triple helix.
Defect in assembly into fibres.
Cysteine contains a reactive sulphydryl group in its side chain which can form inappropraite disulphide bonds between the alpha 1 chains.

Question 4

What difference will occur between electrophoresis in presence of SDS for affected type 1 collagen and normal collagen?

Answer 4

The affected collagen will migrate more slowly than the individual chains as it forms cross-linked polypeptide chains.

Question 5

What will occur in electrophoresis in the presence of 2-mercaptoethanol?

Answer 5

The disulphide bonds will be cleaved allowing the chains to migrate to their Mr.

Question 6

Why will the majority of alpha 1 chains be affected rather than 50% if the patient is heterozygous?

Answer 6

Differences in the rate of of transcription and translation, stability of mRNA and stability of the protein.

Question 7

What type of mutation causes Osteogenesis Imperfecta?

Answer 7

Autosomal Dominant.

Question 8

How does osteogenesis imperfecta affect bone formation?

Answer 8

Bone is formed by laying hydroxyapatite on an ordered scaffold of collagen-I.
The abnormal collagen structure leads to defects in the mineralisation process.
Causes skeletal abnormalities and weak bones.

Question 9

What prenatal diagnostic tests can be used to detect if a foetus will have Osteogenesis Imperfecta?

Answer 9

Genetic screening of fetal DNA obtained by chorionic villus sampling or amniocentesis which is amplified by PCR. Use specific probes complementary to the DNA sequence where a mutation is known to occur.