Osteogenesis imperfecta Flashcards
What are the main features of osteogenesis imperfecta
The main feature of this disease is repeated fractures of long bones, and for this reason it can easily be misdiagnosed as child abuse. There are also malformed bones. There is a whole range of genetic disorders which can lead to the disease.
What is collagen
Collagen is an extracellular matrix protein synthesised by and secreted from a variety of cells, such as fibroblasts and osteoblasts, and organised into insoluble fibres.
What are the roles of collagen
These fibres are a major part of the extracellular matrix surrounding cells and five mechanical strength and rigidity to tissues and organs. In particular, they provide the tensile strength of skeletal tissues including bone, cartilage, tendons and ligaments.
How many types of collagen are there
There are at least 5 types of collagen which occur in different tissues.
What may be the defect in osteogenesis imperfecta
A point mutation (G–T) in the gene coding for collagen type 1. This results in substitution of the normal glycine at this residue for cysteine. The larger amino acid in the mutant molecule will cause steric hinderance which generates a kink in the normally straight triple helix, with a resulting defect in the assembly into fibres. As cysteine also contains a reactive sulfhydryl group in its side chain, there can also be inappropriate disulphide bonds between the two alpha 1 chains in the helix.
What do most cases of osteogenesis imperfecta result from
Most cases of osteogenesis imperfecta result from mutations in the glycine residues, producing defective structural assembly.
How can we detect this mutation
The resulting cross-linked polypeptide chains will migrate much more slowly than the individual chains when examined by gel electrophoresis in the presence of SDS. However, in the presence of 2-mercaptoethanol the disulphide bonds will be cleaved allowing the chains to migrate according to their Mr.
Describe the action of 2-mercaptoethanol
S-S becomes 2S-H in the presence of this reagent.
What is meant by dominant negative
A special class of loss of function mutations, where abnormal product interferes with the function of the normal product.
Describe the consequences of the patient being heterogenous for this allele
The patient in this case is heterozygous, so only some of her alpha 1 chains will be abnormal while the other allele is making the normal version. In principle 50% of the chains would be normal and 50% abnormal, although in practice this exact ratio rarely occurs in rare genetic diseases. There may be differences in the rates of transcription, translation, stability of mRNA or protein which lead to a different ratio. Because the collagen triple helix contains two alpha 1 chains and will be disrupted if only one is in the mutant form, the majority of collagen fibres will be affected, leading to a dominant phenotype.
What is the major consequence of osteogenesis imperfecta
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 1. The abnormal collagen leads to defects in the mineralisation process, so the patient ends up with skeletal abnormalities and generally weak bones. Other problems also occur, especially with the eyes, skin, teeth and ears.
How de we investigate osteogenesis imperfecta
Direct study of the collagen protein.
Describe the prenatal diagnosis of osteogenesis imperfecta.
Direct study of the collagen protein from a foetus would be impractical and risky. A better approach would be genetic screening of fetal DNA obtained by chorionic villus sampling or amniocentesis, and amplified by PCR. Specific probes could be used which were complementary to the part of the DNA sequence where the mutation was known to occur: under the right conditions of temperature and ionic strength the probe will only hybridise to the complementary sequence, allowing the normal and mutant genes to be distinguished. Alternatively, if the mutation altered a restriction recognition site, that would allow identification of normal and abnormal genes since only one would be cleaved by the enzyme to shorter fragments. Both methods rely on the mutation being known, but that could be checked on by other family members.
when is cFF DNA used
In X-linked conditions
What is the difference between the action of osteoblasts and fibroblasts
Osteoblasts- synthesise collagen in bone
Fibroblasts- synthesis collagen in connective tissue
