12 - Gene Therapy for Ocular Diseases

Question 1

Main viruses used for gene therapy

Answer 1

• AAV
• Retrovirus
• Lentivirus
• Modified HSV
• Adenovirus

Question 2

Main conditions being treated with gene therapies

Answer 2

• Oncology
• Ophthalmology
• HAematology
• Inflammatory and immune system

Question 3

How is a gene therapy delivered?

Answer 3

• Gene augmentation therapy (introduce functioning gene)
• Gene specific targeted therapy (introduce therapeutic/suicide gene)
• Genome editing or correction therapy (introduce genome editing system)

Question 4

Adeno Associated Virus (AAV) advantages

Answer 4

• Non pathogenic (replication deficient)
• Transduces dividing and non divding cells
• High level and stable long term expression
• Does not integrate
• Low immunogenicity

Question 5

AAV disadvantages

Answer 5

• Small genome packaging capacity
• Cell/tissue specificity
• Pre existing immunity (endemic in human population, high levels of circulating antibodies)

Question 6

Why ocular gene therapy?

Answer 6

• Accessibility
• Small size
• Compartmentalization
• Immune privilege
• Existence of a contralateral control
• Highly permissive to AAV targeting

Question 7

Developing a gene therapy strategy

Answer 7

• Understand the biology of the disorder
• Develop the treatment approach
• Test effectiveness in biological models of the disease.
• Establish safety in humans.

Question 8

Developing the treatment approach

Answer 8

• AAV genome (choice of promotor. regulatory elements)
• AAV capsid (capsid engineering)
• Route of delivery

Question 9

Different routes of delivery

Answer 9

Subretinal or intravitreal

Question 10

Subretinal route of delivery

Answer 10

• Technically difficult surgery
• Smaller volume/dose of treatment
• Strong targeting of photoreceptor cells
• Regional treatment effect

Question 11

Intravitreal route of delivery

Answer 11

• Extremely easy injection
• Larger volume/dose of
  treatment
• Weak targeting of ONL and
  INL but good for ganglion cell
• Higher immune response

Question 12

Leber Congenital Amaurosis (LCA)

Answer 12

• Due to RPE65 mutations
• RPE65 protein essential in visual cycle
• Congenital inherited eye disease
• Severe blindness
• First ever FDA approved gene therapy treatment

Question 13

Age related macular degeneration (AMD)

Answer 13

• Age related loss of vision in the centre of the visual field
• Multifactorial (genetics and environment)
• Current treatment monthly eye injections

Question 14

Two sets of clinical presentation of AMD

Answer 14

• Dry AMD (associated with gradual loss of vision, hard to treat)
• Wet AMD (more aggressive, responsible for 90% of cases of severe vision loss)

Question 15

Wet AMD

Answer 15

• Complex, multifactorial disease
• New blood vessel that sprout into retinal tissue, killing photoreceptors
• Associated with high levels of VEGF

Question 16

Current treatment of wet AMD

Answer 16

• Delivery of soluble VEGF antagonists (e.g. sFLT-1) by antibody
• Bind VEGF or compete for VEGF receptor, thus blocking angiogenesis

Question 17

Limitations of AMD treatment

Answer 17

• Expensive
• Increased risk of infection
• Molecule degrades thus need injection 4-8 weeks (high treatment burden)
• Not everyone responds well

Question 18

Gene therapy for AMD

Answer 18

• No defective gene to replace
• Instead use of vector to transect any cell which can then use its machinery to make soluble VGEF and secrete it into vitreous of the eye, thus is the same as having an injection (sFLT-1 gets made constantly.)
• Reduces VEGF and angiogenesis levels

Question 19

sFLT-1

Answer 19

Soluble form of the VEGFR1 receptor and a highly potent inhibitor of VEGF

Question 20

Mouse studies for gene therapy in wet AMD

Answer 20

• The Kimba mouse is a model of chronic, progressive retinal neovascularisation and degeneration that was used to test the rAAV.sFlt-1 vector
• Flourescein angiography was used to examine the mice at several time points postinjection of the rAAV.sFlt-1 vector.