Treatment of genetic disorders Flashcards
Example of metabolic disorders treated by Cofactor administration (2)
Pyridoxine-responsive homocystinuria
Biotinidase deficiency
(50% of patients will respond)
Example of metabolic disorders treated by Replacing an extracellular protein
Factor VIII in hemophilia Well established, effective
Protein Replacement-Example alpha-1 antitrypsin deficiency
• Alpha-1 antitrypsin is a plasma protein that inhibits elastase (balances the tissue destructive
properties of elastase)
• ~100,000 homozygotes in US have an elevated risk of COPD and liver disease
• Conceptually, the disease is a deficiency –> recombinant protein replacement therapy
(PRT) can mitigate the effects of the disease
• Most effective therapy remains avoidance of smoking
Protein Replacement Therapy-Example Fabry disease
Fabry disease is an X-linked condition due to deficiency of alpha-galactosidase. Accumulation of glycosphingolipids causes widespread microvascular damage
Symptoms:
Neuron damage: neurologic pain crises in childhood
Sweat gland damage: reduced sweating, risk of heat stroke
Renal damage: progressive renal failure (cause of death prior to renal transplanation)
Vascular damage: risk of heart attacks and stroke
Cardiovascular: hypertrophy of cardiac tissue also seen
Recombinant enzyme replacement therapy appears to mitigate some aspects of the diseaseApproved in the United States à annual cost $150-200,000 / patient (lifelong)
Ex vivo vs In vivo gene therapy
Ex vivo (insertion of DNA/RNA occurs outside the patient in cells/tissues which are then given to the patient) or in vivo (DNA/RNA constructs injected/delivered directly to the patient).
Considerations for gene therapy
Targeting: the transgene must be delivered/targeted to the appropriate cells and not to inappropriate cells. You do not have to express the transgene in its ‘normal’ location. For
example, Factor VIII is typically made by the liver and secreted into the blood to help maintain normal clotting (Factor VIII deficiency à hemophilia A). Some limited success has been seen injecting transgene into muscle tissue and having muscle tissue secret transgenic Factor VIII.
Expression: the transgene must lead adequate expression (in terms of level or expression (how
much transgenic product is made) and duration (how long does the effect last). Often the goal is not to achieve ‘normal’ physiological levels of a transgenic protein.
Toxicity: the toxic side-effects of the transgene must be acceptable.
Gene Therapy Approaches (3)
Retroviral, Adnoviral, non-viral
Advantage of retroviral vector?
Integrate into cell genome, Minimal host immune reaction
Disadvantages/risks of retroviral vector?
Insert size of transgene limited to ~7-8kb
Infect only dividing cells (cannot reach quiescent tissues)
Risk of insertional mutagenesis / germline integration)
Efficiency of retroviral vector?
Retroviral titers relatively low
Efficient at infecting dividing cells
Duration of retroviral vector?
As these integrate into genomic DNA, the transgene can be
passed to daughter cells
Advantage of adenoviral (DNA viral) vector?
Wide variety of cell types can be infected Transgene insert size ~35-36kb
Stable and easy to get high titers
Disadvantages of adenoviral (DNA viral) vector?
Does NOT integrate into cell genome
Expression can be very transient
Risk of malignant transformation
Safety/risks of adenoviral vector?
Lower risk of insertional
mutatgenesis
Immune reactions can be severe (Jessie Gelsinger and OTC gene therapy attempt)
Efficacy/duration of adenoviral vector?
Can infect non-dividing cells, Higher titers are possible.
Typically short-lived effect; not passed to daughter cell lines
