Gene therapy Flashcards
What is the aim of gene therapy?
To cure genetic disease by replacing the mutated gene with a non-mutated gene
How does the replacement gene get to the cell?
Via vector sequence or lyzosome
What is an example of a vector sequence?
AAV
A virus which is very good at penetrating into tissue
What are the types of genetic disorders?
Monogenic/Mendelian
Polygenic/Complex
What are the types of monogenic disorders?
Dominant
Recessive
X-linked
Mitochondrial
Give an example of a monogenic disorder
Marfan’s syndrome
Retinitis Pigmentosa
Leber’s Congenital Amaurosis
Give an example of a polygenic disorder
Cataracts
Glaucoma
Refractive error
Why is a polygenic disorder harder to ‘cure’ than a monogenic disorder?
Polygenic caused by multiple gene mutations, monogenic caused by one. Polygenic also affected by other environmental factors.
What are the difficulties of replacing non-functional genes?
Delivering gene to the correct tissue
Achieving the desired level of gene expression (generally only need a few %)
Preventing the vector from being degraded (needs to stay present, while not integrating into host cell DNA - so could be detected as virus)
Unwanted side effects (occur if vector integrates into host cell DNA)
What are the types of dominant disorders?
Haploinsufficiency
Dominant-negative effect
What is haploinsufficiency?
50% functional protein is present, which is not enough for normal function
How can gene therapy help haploinsufficiency? What does it need to achieve to work?
Replacement of dysfunctional genes
Expression needs to be brought up to 100% for normal function
What is the dominant-negative effect?
Abnormal protein has an adverse effect, preventing the function of the normal protein
What does gene therapy need to achieve to help the dominant-negative effect?
Block expression of abnormal protein but allow expression of normal protein
How is the abnormal protein blocked in treating the dominant negative effect?
Translation of dominant negative mRNA blocked by nucleic acid drugs which bind to the mutant mRNA and cleave it in two.
Delivered via ribozymes, antisense nucleic acids, siRNA or CRISPR-Cas
Give an example of a haploinsufficiency
Stickler syndrome - vitreous malfunction which causes strands to be seen on slit lamp
Give an example of the dominant-negative effect
Marfan’s syndrome - fibrillin gene mutation prevents triple helix formation so elastic fibres are not formed (e.g. if zonules of zinn dysfunctional, lens may dislocate)
What are the difficulties of blocking the expression of an abnormal protein?
Achieving specificity for the mutant mRNA without degrading non-mutant mRNA
Delivering the gene to the correct tissue
Achieving desired level of gene expression
Preventing the delivery agent from being degraded
How is CRISPR-Cas used for blood disorders?
Blood removed from px
CRISPR-Cas delivered to cell culture in lab (genes edited and cultured further)
Edited cells are returned to the px
Why is the ability to edit genes outside of the body good?
Risk can be assessed before returning cells to the px
Less side effects
More controlled environment
Less clinic time for px and practitioner?
What makes gene therapy difficult?
Mutation heterogeneity - mutation can occur anywhere on gene and usually varies between families
Non-mendelian diseases (polygenic) - difficult to correct multiple mutations
Ethics
