Aug 12, 2020

A Gene Therapy Primer for People with Inherited Retinal Diseases

Science Education

Everything patients and families want to know about gene therapy…and a little bit more

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Can Gene Therapy Address Your Inherited Retinal Disease?

When it comes to saving or restoring vision for people with inherited retinal diseases (IRDs) such as retinitis pigmentosa, Usher syndrome, or Stargardt disease, no approach has gained more attention than gene therapy. And rightfully so. The first gene therapy for the eye or any inherited disease was approved by the US Food & Drug Administration in late 2017 — that was LUXTURNATM for people with RPE65 mutations causing Leber congenital amaurosis or retinitis pigmentosa — and currently, there are about two dozen gene therapy clinical trials underway for IRDs. LUXTURNATM is bringing dramatic vision improvements to most who receive it, and many gene therapies in clinical trials are showing encouraging results.

One advantage of gene therapy is that it can be designed to address IRDs in different ways. Whom the gene therapy can help depends on which therapeutic gene is delivered to retinal cells. And the good news is that researchers are developing a range of gene therapies to address IRDs, even for those who don’t know the genetic profile of their disease. There are even gene therapies for those who have lost all of their photoreceptors, the retinal cells that make vision possible.

Here are summaries of four approaches that, together, can potentially address the needs of a majority of IRD patients:

  • Gene replacement or augmentation: This approach involves delivering new gene copies to replace the mutated copies. In some cases, the mutated copies may also need to be turned off. Gene replacement may be a good option if: you know what mutated gene is causing your disease, a gene therapy is in development that is targeting your gene, and you have some photoreceptors remaining. One of the many benefits of genetic testing is to identify your gene to see if there is a gene replacement therapy emerging for it.
  • Neuroprotective gene therapy: This approach is for slowing or halting disease progression regardless of what mutated gene is causing your vision loss. Neuroprotection is applicable to you if you would be satisfied with saving your remaining vision (i.e. you have photoreceptors left to preserve). The gene delivered in neuroprotection leads to the production of growth factors that keep your retinal cells healthy.
  • Optogenetics: If you have lost all of your photoreceptors, neither gene replacement nor neuroprotection will be helpful. However, optogenetics is a form of gene therapy for people who have lost all or most of their vision. It involves bestowing light sensitivity— delivering a gene that expresses a light-sensitive protein — to retinal ganglion cells which survive in many cases after photoreceptors are lost. In other words, optogenetics is designed to enable your ganglion cells to work something like photoreceptors.
  • For wet and dry age-related macular degeneration: Many AMD treatments (emerging and available) require regular injections of proteins into the eye for the life of the patient. For wet AMD, the protein inhibits the growth of vision-robbing leaky blood vessels. For dry AMD, the protein tempers an overactive and damaging immune system. However, gene therapies in clinical trials for wet and dry AMD are designed to provide sustained production of the desired proteins. If the approach is effective, a single injection will last for several years.

For most people on the journey to managing and potentially treating their IRD, genetic testing is an important step in the process. “With no-cost genetic testing available to IRD patients in the US, it is a great time to get tested. Genetic testing often gives a clearer diagnosis, informs the family about who else may be at risk, and it can help patients understand which clinical trials and emerging therapies, including gene therapies, may be relevant to them,” says Benjamin Yerxa, PhD, chief executive officer at the Foundation Fighting Blindness. “Also, signing up in a patient registry can get people on the radar screen of therapy developers recruiting for clinical trials.”

The Foundation Fighting Blindness provides no-cost genetic testing and a patient registry through its My Retina Tracker Program.


What Exactly is Gene Therapy?

Virtually every cell in our bodies carries a complete set of an estimated 20,000 genes. Nearly 300 of these genes, when defective, have been associated with IRDs. Genes are like our body’s instruction manual. They instruct our cells which proteins to make. These proteins are essential to the development, health, and functioning of all cells, including those of the retina.

Most inherited retinal degenerative diseases are caused by variations in a single gene. These variations are like misspellings in our instruction manuals. Even having just one incorrect letter can cause the wrong, or not enough, protein to be made. That can lead to serious consequences, like degeneration of the retinal cells that enable us to see. When a variation causes disease, it is referred to as a mutation.

Scientists are developing gene therapies to deliver copies of new, corrective genes, without the misspellings, to the cells in our retina, enabling them to make the right proteins, and stay healthy and function properly. Specially designed viruses are commonly used to deliver the corrective gene copies to the cells. The viruses are said to “transfect” or penetrate the cells with their therapeutic genetic cargo.

Gene therapy is administered by injecting a tiny drop of liquid, also known as a bleb, underneath or near the retina. The solution is absorbed into the retina over a period of hours.

In today’s world of retinal gene therapy development, adeno-associated viruses (AAVs) are most often used to deliver therapeutic genes to cells in the retina. That’s because AAVs are safe and able to penetrate cells with their genetic cargo. They naturally occur in humans and don’t cause any known illness. For regulators like the US Food & Drug Administration, that excellent safety profile is highly desirable.

You can think of an AAV as being like a very large container delivery system. The containers, which scientists call capsids, hold copies of the therapeutic gene. A retinal dose of AAV could contain 300-500 billion capsids. Not all capsids will make it into the nucleus of the retinal cell — where they need to be to work — and some capsids don’t have cargo. That’s why so many capsids need to be in the bleb for enough therapeutic gene to get into the retinal cells.

But once the genes are delivered, they work for many years, perhaps the lifetime of the patient.

“Gene therapy is a quickly evolving field. Researchers are enhancing AAV technology to be more effective and also address challenges such as delivering genes that are too big for current AAV capsids,” says Dr. Yerxa. “Dual vectors, delivering the therapeutic gene in two parts, and minigenes are two approaches that show promise for large gene delivery.”


How is a Gene Therapy Made?

Administering a gene therapy — injecting the bleb underneath the retina — may seem pretty simple. But what’s in that bleb is a highly complex human-engineered viral delivery system with genetic cargo. And, manufacturing the gene therapy on a large scale to be safe and effective in humans is no small feat.

“Gene therapy manufacturing differs from that of traditional small and large molecules because of the complexity of the systems,” says Dave Knop, PhD, executive director of process development at Applied Genetic Technologies Corporation (AGTC), which has gene therapy clinical trials underway for X-linked retinitis pigmentosa and achromatopsia.

AGTC produces AAV gene therapies in immortalized baby hamster kidney (BHK) cells — a cell line that has been safely and effectively used in the development of a variety of other therapies.

The process of producing AAV in the BHK cells is rather complex. There are a number of genetic components that need to be introduced into the BHK cells and they include:

  • two genes for making AAV,
  • the therapeutic gene (e.g., the healthy gene for replacing the mutated or missing gene),
  • and “helper” genes that enable the cell to replicate AAV (i.e., to build the many capsids).

All of the above genetic components are delivered into the BHK cells by two human-engineered (safe!) herpes simplex viruses (HSVs).

Then, the AAV production and expansion process occurs in a stirred single-use bioreactor, which holds 50 liters of material. The bioreactor has a plastic bag lining that is replaced after each use.

“You have this big mix of stuff in the cells that has all the right elements for replicating and producing the viral containers,” says Dr. Knop. “Once the AAV has matured, we break the cells open and pull out all of the AAV and put it into the purification operations.”

As part of the purification process, special plastic beads, called chromatography resin, are used to pull the AAV away from the rest of the unwanted material used in production.

The AAV is stored — for multiple years, if desired— in ultra-low temperature (-65 degrees Celsius) storage.

“That’s pretty cold. You don’t have that freezer hanging out in your kitchen,” says Knop. “We’re trying to come up with ways to store at higher temperatures, so it is a little less cumbersome.”

While the process to make AAV takes two to three weeks, the manufacturers need several months to make and test the biological reagents used in the process.

After the AAV is made, it goes through a long battery of tests for two to three months to ensure it is safe and has the desired profile.

Of course, it takes several years of lab studies to design and develop a vision-saving retinal disease gene therapy. Good manufacturing practices are an essential step in moving the treatment out of the lab and into human studies, where hopefully, it will deliver the magic of saving or restoring vision.