brittle bones Flashcards
what are fibroblasts?
most common cell type in connective tissue
produce collagen
what does type 1 collagen consist of?
2 alpha 1 polypeptide chains and 1 alpha 2 polypeptide chain
what is 2-mercaptoethanol?
used to reduce disulphide bridges present in proteins and can be a biological antioxidant
how might a mutation in the nucleotide sequences of normal and mutant COL1A1 genes caused an altered electrophoresis pattern?
sequence changes from glycine to cysteine
cysteines can form disulphide bridges linking 2 chains together
2-mercaptoethanol breaks these disulphide bridges
why are only some of the α1(I) collagen chains affected?
only one of the 2 copies of CoIA1 gene are mutated
so only some collagen molecules carry the mutation
to form complex, need 2 copies of mutated protein to combine
Predict possible biochemical consequences of the change on the assembly of type I collagen.
glycine is in the centre of triple helix
so other amino acids won’t fit
Gly, X, Y-
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.
gain of function mutation
the mutation disrupts the activity of the normal version of ColA1
only half of colα1 protein mutated
all fibrils will be affected due to packing
Why might the predicted change cause skeletal abnormalities and brittle bones?
initially skeleton laid down as collagen
later stage mineralisation
if collagen defective, then bone is defective
skeleton is an active tissue
whole skeleton turnover every 5-10 years
Suggest a suitable prenatal diagnostic test to identify foetus who will suffer from Osteogenesis imperfecta
need a sample derived from foetus
eg. amniocentesis chorionic villus sampling
then PCR
amplify region with mutation
gel electrophoresis
use a probe specific for mutation
sequence PCR product
what is osteogenesis imperfecta?
genetic bone disorder characterised by repeated fractures and malformation of long bones
what mutation causes osteogenesis imperfecta?
mutations in COL1A1 gene
results in single amino acid substitution of procollagen precursor of alpha 1 (I) collagen
describe the structure of collagen
3 chains wound together in tight triple helix
2x alpha 1 (I) polypeptide chain
1x alpha 2 (I) polypeptide chain
glycine in middle due to compact size
repeated Gly-X-Y sequence, where the glycine side chains occupy the middle of the triple helix due to its compact size
what does point mutation in glycine result in?
glycine substituted for cysteine
what can substituting glycine for cysteine cause?
steric hindrance: preventing proper formation of collagen triple helix –> introduces kinks and decreases tensile strength
formation of inappropriate disulphide bonds: (de to the sulfhydryl chain) between adjacent a1(I) chains
give a problem of chorionic villus sampling or amniocentesis
both invasive procedures w risk of miscarriage
