Glaucoma Gene Therapy

Glaucoma Gene Therapy – a Life-Changing Alternative for 70 Million Patients

5 Facts About Glaucoma Gene Therapy

  • Open angle glaucoma is not caused by a specific genetic mutation. There is no gene that could be corrected
  • Open angle glaucoma gene therapy is currently in the research stage
  • There are two areas of research. First is focused on reducing intraocular pressure by modifying cells of the trabecular meshwork. Second area of research is focused on protecting retinal ganglion cells from degeneration
  • Eye gene therapy is already a reality. Luxturna, a gene therapy for Leber’s congenital amaurosis, is already being offered to patients
  • There is a promising glaucoma cell therapy that is currently going through the Phase 2 of clinical trials

A Closer Look Inside Glaucoma Eye

There is a continuous flow of fluid inside the eye. 

The fluid, called aqueous humour, is produced by a structure called the ciliary body located behind the iris. The aqueous humour flows through the pupil into the anterior chamber, and then out of the eye through a drainage system called the trabecular meshwork.

Circulation of aqueous humour provides nutrients to the eye lens. Continuous creation and drainage of aqueous humour also maintains pressure inside the eye. 

When the trabecular meshwork gets obstructed and loses its drainage capacity, the pressure inside the eye starts building up. Over time elevated intraocular pressure starts damaging retinal ganglion cells located at the back of the eye. 

The optic nerve that connects the eye with the brain is formed by projections of retinal ganglion cells. Death of retinal ganglion cells leads to optic nerve degeneration and the resulting blindness.

Inside Glaucoma Eye

Why Gene Therapy is Better Than Eye Drops

Currently, the most effective treatment for open angle glaucoma is daily administration of eye drops that reduce intraocular pressure. 

Because glaucoma is progressive and irreversible, the patients have to endure using eye drops for the rest of their lives.

Even if a patient adheres to the application regiment, there is still a 1 in 8 chance that they will develop blindness at least in one eye (1).

It is annoyingly inconvenient to apply eye drops every day. 

Especially knowing that they might not work.

Gene therapies for glaucoma are designed so they can be applied once every few years, or even once a lifetime.

Animal experiments indicate that genetic modification of cells in the eye’s trabecular meshwork can last for two years or longer (2). If we were to produce a modification that increases humour drainage for such extended periods of time, it would be a great improvement for patients.

Lowering of Intraocular Pressure with Gene Therapy

Trabecular meshwork is an eye’s drainage system. 

The meshwork is made of cells embedded in an extracellular matrix made of collagen and other fibrous molecules. Cell shape and structure of the extracellular matrix determine the rate of outflow of aqueous humour, and ultimately the intraocular pressure.

A recent success with glaucoma gene therapy is reduction of intraocular pressure in monkeys achieved by modifying trabecular meshwork cells with enzyme C3 transferase (3).

C3 transferase is an enzyme isolated from a bacterium Clostridium botulinum. C3 interacts with cellular machinery responsible for maintaining cell cytoskeleton, and ultimately cell shape. 

In the above mentioned study a virus carrying C3 gene was injected into the anterior chamber of the eyes of rhesus monkeys. Following the injection animals showed a reduction of intraocular pressure. The intraocular pressure remained lowered for the duration of the experiment, which lasted 112 days.

Similar outcome was achieved with enzyme metallopeptidase 1, which breaks down collagen fibers and reworks extracellular matrix (4). A single injection of a virus carrying metallopeptidase 1 gene into anterior chamber of sheep eye led to reduction of intraocular pressure for one month.

The animal experiments described above are promising because single treatments lead to the lasting reduction of intraocular pressure.

The next step is to take studies with C3 and metallopeptidase 1 to preclinical studies. 

Reducing Intraocular Pressure by Modifying Cells in the Trabecular Meshwork

Using Gene Therapy for Protection of Retinal Ganglion Cells

Neuroprotection can prevent the development of glaucoma even in the presence of elevated intraocular pressure.

Although open angle glaucoma is usually accompanied by increased intraocular pressure, it is generally accepted among the experts that the main cause of this condition is the death of retinal ganglion cells and degeneration of the optic nerve.

So, how do you save the nerve cells from dying?

There is a whole class of well-characterized signalling molecules called neurotrophins. Neurotrophins promote growth and differentiation of neurons. They also prevent neurons from initiating programmed cell death.

Brain-derived neurotrophic factor, or BDNF, has been extensively tested in glaucoma gene therapy research. Experiments in rats have shown that injection in the eye of a virus carrying BDNF gene does provide a robust neuroprotective effect on retinal ganglion cells. However, the BDNF effect lasted for only 16 days (5).

A more promising approach involves simultaneous delivery of BDNF gene with a gene for TrkB, a cellular receptor that transmits BDNF function (6). 

Combination of BDNF and TrkB genes promoted survival of retinal ganglion cells in rats for over six months without noticeable adverse effects. 

There is a chance that BDNF-TrkB gene therapy would progress to preclinical trials (6).

Gene Delivery to the Eye

Gene delivery to the eye is “easy”.

Eye is easy to access. Viruses carrying genetic payload can be injected into the anterior chamber or the vitreous body using a syringe.

Eye is relatively isolated, so there is a low risk of a virus moving anywhere else in the body.

Eye is also “immune privileged”. There are no immune cells sneaking around trying to destroy viruses or cause inflammation.

Gene Delivery to the Eye

Finally, both trabecular meshwork cells and retinal ganglion cells are good at uptaking foreign genetic material. For example, a single intracameral injection of adeno-associated virus carrying GFP gene into the eyes of monkeys led to expression of green fluorescent protein in trabecular meshwork cells for two years (7).

Effective treatment of eye condition by gene therapy has been demonstrated with the launch of Luxturna.

Luxturna is an FDA approved gene therapy for a different eye condition called Leber’s congenital amaurosis. During the treatment adeno-associated virus carrying its genetic payload is injected with a needle under the retina to a layer of retinal pigment epithelial cells (8). A gene that is being delivered, RPE65, restores the mechanism of recycling of light sensing molecules, and helps to restore vision to patients.

Success of Luxturna made eye gene therapy a reality. 

Let’s hope that one day gene therapy for open angle glaucoma will be available as well.

Gene Therapy for Congenital Glaucoma and Early-Onset Glaucoma

In addition to open angle glaucoma, there are two other types of glaucoma that are caused by a known genetic defect.

First one is primary congenital glaucoma. It is a rare genetic disease that develops in infants between birth and three years of age. The incidence rate of primary congenital glaucoma is estimated between 1:1,250 and 1:22,000 (9).

Individuals with primary congenital glaucoma carry mutations in the cytochrome P4501B1 gene. Despite having a clear genetic target, there are no attempts to develop gene therapy for this condition at the present moment.

Second genetic condition is early-onset glaucoma. Its development has been linked to dominant gain-of-function mutations in myocilin gene (MYOC). MYOC mutations affect the cells in the trabecular meshwork, leading to increased intraocular pressure and eventually development of glaucoma.

Dominant gain-of-function mutations are tricky to fix. If we deliver a functional copy of the gene, the dominant mutation will simply override it.

Repairing MYOC mutations has to involve advanced genome editing tools like CRISPR/Cas9. A recent study at The University of Iowa has demonstrated that such editing indeed could be done (10).

It is still too early to tell if and when early-onset glaucoma will be treated with gene therapy. 

But in this case we know that it is possible.

Cell Therapy

NT-501 is a cell therapy for treatment of glaucoma that has already advanced to Phase 2 clinical trials (11).

NT-501 uses ciliary neurotrophic factor (CNTF), which, similarly to BDNF described above, activates cell signalling pathways that protect retinal neurons from degeneration.

A steady stream of CNTF is produced by modified human cells that are encapsulated in a semipermeable membrane and injected in the vitreous body of the eye. 

The technology for cell encapsulation has been developed by Neurotech Pharmaceuticals. 

The results of glaucoma Phase 2 clinical trials of NT-501 are expected in 2020.

References

  1. Ophthalmology. Long-term trends in glaucoma-related blindness in Olmsted County, Minnesota 
  2. Investigative Ophthalmology & Visual Science. Self-complementary AAV virus (scAAV) safe and long-term gene transfer in the trabecular meshwork of living rats and monkeys
  3. Molecular Therapy. Lentiviral Vector-Mediated Expression of Exoenzyme C3 Transferase Lowers Intraocular Pressure in Monkeys 
  4. Gene Therapy. Inducible scAAV2.GRE.MMP1 lowers IOP long-term in a large animal model for steroid-induced glaucoma gene therapy
  5. Proceedings of the National Academy of Sciences of the United States of America. Prolonged delivery of brain-derived neurotrophic factor by adenovirus-infected Muller cells temporarily rescues injured retinal ganglion cells 
  6. Cell Death & Disease. Neuroprotection of retinal ganglion cells by a novel gene therapy construct that achieves sustained enhancement of brain-derived neurotrophic factor/tropomyosin-related kinase receptor-B signaling
  7. Investigative Ophthalmology & Visual Science. Self-complementary AAV virus (scAAV) safe and long-term gene transfer in the trabecular meshwork of living rats and monkeys
  8. Luxturna. Mechanism of action
  9. Investigative Ophthalmology & Visual Science. Molecular genetics of primary congenital glaucoma in Brazil
  10. Proceedings of the National Academy of Sciences of the United States of America. CRISPR-Cas9-based treatment of myocilin-associated glaucoma
  11. ClinicalTrials.gov. Study of NT-501 encapsulated cell therapy for glaucoma neuroprotection and vision restoration