How Does Gene Therapy Work? Correcting Disease-Causing Genetic Variations

Summary

Not all genetic variations are associated with threats or harms to human health. Some even protect us, such as genetic variations that have been shown to make bones harder or the heart more impervious to disease. But while some genetic variations are positive, others can cause or contribute to disease. 

In this episode, we answer the question of how gene therapy works, and learn how gene therapy replaces and repairs certain gene variants, and is changing the trajectory of genetic diseases.

For several years, Dr. Jean Bennett at the University of Pennsylvania’s Department of Ophthalmology Center for Advanced Retinal and Ocular Therapeutics investigated the possibility of replacing gene variants in the retina – which cause blindness – with copies of healthy ones.

After successfully treating blind puppies, Dr. Bennett and her colleagues turned their attention to treating human eyesight.

The retina turned out to be a good target site for gene therapy. In most parts of the human body, our cells keep dividing after we’re born. But not the rods and cones in our retina – these photoreceptor cells don’t regenerate.

A major obstacle in the development of gene therapy is the tendency of cloned genes to get lost in the process of cell division, especially when the transgene is delivered in a generally non-integrating viral vector. Because retinal cells don’t divide after birth, the cloned gene may be expressed for a prolonged time.

In 2017, after decades of painstaking research, building on the efforts of countless scientists throughout history, Dr. Bennett and her colleagues had the evidence that gene therapy may be used to treat genetic conditions in humans.

Since then, gene therapy has undergone giant leaps in the treatment of specific diseases.

And record numbers of gene therapy trials are ongoing, including potential treatments for conditions such as sickle cell disease and Parkinson’s disease

It’s just one of many ways that the field of gene therapy is poised to change the world in the years to come.

Transcript

DDx SEASON 4, EPISODE 2

How Gene Therapy Works: Correcting Disease-Causing Genetic Variations

RAJ: This season of DDx is brought to you by Novartis Gene Therapies. 

Opening

KIM: Dr. Jean Bennett is heading to work at the University of Pennsylvania’s Department of Ophthalmology Center for Advanced Retinal and Ocular Therapeutics.1,2

Dr. Bennett is one of the world’s leading experts on genetic eye diseases.2 And for several years, she’s been investigating whether it’s possible to replace gene variants in the retina—which cause blindness—with copies of healthy ones.3,4

It’s the forefront of the exciting, emerging field of gene therapy. In the year 2000, she’s working with blind puppies.4

The dogs in Dr. Bennett’s lab all have a variant of a gene called RPE-65. This gene encodes instructions for certain retinal cells, causing them to produce a protein that helps convert light into electrical signals. Without a healthy version of RPE-65, the brain can’t transform light into vision, and either partial or total blindness occurs.4,5

In humans, RPE-65 variation causes a disease known as Leber congenital amaurosis, or LCA, which affects one in every 40,000 babies born in the U.S. each year.4,6 And Dr. Bennett and others like her hope to one day help them see again. In her lab, Dr. Bennett has introduced cloned genes into the retinas of three blind puppies, using adeno-associated virus—or AAV—vectors.4

And within weeks of receiving the reagent,4 something that feels miraculous happens: the dogs begin to look at her. They follow her as she moves around the room. In the ensuing weeks, lab technicians take the puppies outside to play. The dogs can see. It’s not perfect yet, but it’s vision.7,8

Now, Dr. Bennett and her colleagues wonder, if we can make blind puppies see again, can we do the same thing for children?4

Show intro 

RAJ: This is DDx, a podcast from Figure 1 about how doctors think.

I’m Dr. Raj Bhardwaj and that was Kim Handysides, my co-host for this season, as we take a deep dive into gene therapy.

Today we’re talking about how gene therapy works: How replacing and repairing certain gene variants is changing the trajectory of genetic diseases.

Kim explains.

Chapter 1 

KIM: That gene that Dr. Bennett cloned and replaced—RPE-65—is one of about 30,000 genes that comprise the human genome—the DNA blueprint for our entire species.9

Although each DNA sequence is standard, variations in these sequences do occur.

Not all genetic variations are associated with threats or harms to human health. Some even protect us, such as genetic variations that have been shown to make bones harder or the heart more impervious to disease.10,11

But some variations cause or contribute to diseases, including cystic fibrosis, sickle-cell disease, hemophilia,12 and of course Leber congenital amaurosis, the blinding disease Dr. Bennett is working on.4

Chapter 2 

KIM: Our ability to correct disease-causing genetic variation is largely dependent on our ability to identify and intimately understand every gene in the human body.13,14 And for this, we can thank one of the most incredible collaborative scientific efforts in history: the Human Genome Project. It’s considered among the greatest feats of scientific exploration ever undertaken—comparable to landing astronauts on the moon or sending spacecraft towards the edges of our galaxy. Except, instead of probing the vastness of outer space, the Human Genome Project set out to explore the infinite universe inside our DNA.15

On October 1, 1990, the Human Genome Project was officially launched15—publicly funded, involving scientists from across the world,12 all working to reveal the hidden map of our genes. It took 13 years and cost 2.7 billion dollars.9

When finally published, the Human Genome Project revealed a broad picture of all the genes in our DNA—every sequence, every nucleotide base pair, every set of instructions encoded for the development and function of our cells. And since then, genetic science has continued the work to identify each individual gene.14

It’s like every star in the universe suddenly coalescing into constellations, right before our eyes.

The human genome is a map of our genetic constellation. It’s also a user’s manual for our cells and a history book of our species—telling us about our genes, what they do, why they do it, and where and how they vary.

Finally, gene therapy had the blueprint it needed to emerge into the world of science.

Chapter 3 

KIM: When Dr. Jean Bennett was a post-doctoral student in the early 1980s, she followed the development of gene therapy with eager attention.16

In fact, she credits the “father of gene therapy,” Dr. W. French Anderson, with helping her choose her field.4 After earning a PhD in zoology,4 she was able to learn at the National Institutes of Health, where geneticists like Dr. Anderson16 were already working to identify genetic variations in mice and experiment with viral vectors as gene delivery systems. Inspired, she applied to Harvard Medical School to study diseases that are in dire need of treatments.4

In the 1990s, there was growing interest in retina as a gene therapy target. Thanks largely to the Human Genome Project,17 scientists have identified more than 260 genes whose variations can lead to retinal disease—including, of course, RPE-65.18

Following her success with the blind puppies, Dr. Bennett and her team at the University of Pennsylvania turned their attention to human eyesight.4

The retina turned out to be a good target site for gene therapy.4,6 In most parts of the human body, our cells keep dividing after we’re born. But not the rods and cones in our retina—these photoreceptor cells don’t regenerate.6 This is significant for the AAV vectors that Bennett used to deliver cloned RPE-65 genes to the retina.4,19 

A major obstacle in the development of gene therapy is the tendency of cloned genes to get lost in the process of cell division, especially when the transgene is delivered in a largely non-integrating viral vector. Because retinal cells don’t divide after birth, the RPE-65 may be expressed for a prolonged time.20

And what’s more, because the eye is what we call an immunoprivileged site, there is a much lower likelihood that the body’s immune system will attack the viral vector, which sometimes happens in other cells where gene therapy is applied.4,6,21

Dr. Bennett’s puppies were able to regain partial vision. And it’s why, after subsequent clinical trials in humans at the Children’s Hospital of Philadelphia, the results were equally stunning: children with visual impairments saw improvements in their retinal and visual function.22,23

In 2017, after decades of painstaking research, building on the efforts of countless scientists throughout history, including those who worked on the Human Genome Project, Dr. Bennett and her colleagues had the evidence that gene therapy may be used to treat genetic conditions in humans.23

That year, the FDA approved one of the first gene therapies for the treatment of a genetic blindness4 – specifically, for the treatment of patients with confirmed RPE-65 mutation-associated retinal dystrophy.24

It was a major breakthrough for medical science.24

Since then, gene therapy has undergone giant leaps in the treatment of specific diseases.

And record numbers of gene therapy trials are ongoing, including potential treatments for conditions such as sickle cell disease and Parkinson’s disease.25,26

It’s just one of many ways that the field of gene therapy is poised to change the world in the years to come.

Show Closing and Sponsorship Disclosure  

RAJ: Special thanks to Dr. Sabrina Yum – pediatric neurologist at Children’s Hospital of Philadelphia and specialist in Neuromuscular disease and Pediatric electromyography  – for sharing her expertise in the research of this episode.

This is DDx, a podcast by Figure 1.

Figure 1 is an app that lets doctors share clinical images and knowledge about difficult to diagnose cases.

I’m Dr. Raj Bhardwaj, co-host and story editor of DDx.

You can follow me on Twitter at Raj BhardwajMD.

Head over to “figure one dot com slash ddx”, where you can find full show notes, photos and speaker bios.

This episode was brought to you by Novartis Gene Therapies.

Thanks for listening.

References: 

  1. Penn inherited retinal disease program. Penn Medicine. Accessed October 13, 2021. https://www.pennmedicine.org/for-patients-and-visitors/find-a-program-or-service/ophthalmology/inherited-retinal-disease
  2. Jean Bennett, M.D., Ph.D. National Foundation for Cancer Research. Accessed October 13, 2021. https://www.nfcr.org/team/jean-bennett-m-d-ph-d/
  3. Jean Bennett, MD, PhD. Perelman School of Medicine, University of Pennsylvania. Accessed September 20, 2021. https://www.med.upenn.edu/apps/faculty/index.php/g275/p11214
  4. Bennett J. My career path for developing gene therapy for blinding diseases: the importance of mentors, collaborators, and opportunities. Hum Gene Ther. 2014;25(8):663-670.
  5. Bainbridge JW, Smith AJ, Barker SS, et al. Effect of gene therapy on visual function in Leber’s congenital amaurosis. N Engl J Med. 2008;358(21):2231-2239.
  6. Makin S. Four technologies that could transform the treatment of blindness [published online ahead of print, 2019 Apr 10]. Nature. 2019. Accessed September 18, 2021. https://www.nature.com/articles/d41586-019-01107-8
  7. Acland GM, Aguirre GD, Ray J, et al. Gene therapy restores vision in a canine model of childhood blindness. Nat Genet. 2001;28(1):92-95.
  8. Cornell Chronicle. Gene therapy restores vision to dogs blinded by inherited disease, bringing new hope to childhood sufferers of similar condition. Published April 27, 2001. Accessed September 23, 2021. https://news.cornell.edu/stories/2001/04/gene-therapy-restores-vision-dogs-blinded-inherited-disease-bringing-new-hope
  9. National Human Genome Research Institute (NHGRI). Human Genome Project FAQ. Accessed September 20, 2021. https://www.genome.gov/human-genome-project/Completion-FAQ
  10. Boyden LM, Mao J, Belsky J, et al. High bone density due to a mutation in LDL-receptor-related protein 5. N Engl J Med. 2002;346(20):1513-1521.
  11. Boston University School of Public Health. Rare Gene Mutations May Prevent Heart Disease. Published May 10, 2019. Accessed September 19, 2021. https://www.bu.edu/sph/news/articles/2019/rare-gene-mutations-may-prevent-heart-disease/
  12. Jackson M, Marks L, May GHW, et al. The genetic basis of disease [published correction appears in Essays Biochem. 2020 Oct 8;64(4):681]. Essays Biochem. 2018;62(5):643-723.
  13. What is the Human Genome Project? National Human Genome Research Institute. Accessed November 1, 2021. https://www.genome.gov/human-genome-project/What
  14. National Human Genome Research Institute (NHGRI). The NIH Almanac. Accessed September 17, 2021. https://www.nih.gov/about-nih/what-we-do/nih-almanac/national-human-genome-research-institute-nhgri
  15. The Human Genome Project. National Human Genome Research Institute. Accessed October 16, 2021. https://www.genome.gov/human-genome-project
  16. Wilson JM. Interview with Jean Bennett, MD, PhD. Hum Gene Ther Clin Dev. 2018;29(1):7-9.
  17. Daiger SP. Identifying retinal disease genes: how far have we come, how far do we have to go?. Novartis Found Symp. 2004;255:17-178.
  18. Duncan JL, Pierce EA, Laster AM, et al. Inherited retinal degenerations: current landscape and knowledge gaps. Transl Vis Sci Technol. 2018;7(4):6.
  19. Cavazzana-Calvo M, Fischer A. Gene therapy for severe combined immunodeficiency: are we there yet?. J Clin Invest. 2007;117(6):1456-1465.
  20. Brommel CM, Cooney AL, Sinn PL. Adeno-associated virus-based gene therapy for lifelong correction of genetic disease. Hum Gene Ther. 2020;31(17-18):985-995.
  21. Bennett J. Taking stock of retinal gene therapy: looking back and moving forward. Mol Ther. 2017;25(5):1076-1094.
  22. Chiu W, Lin TY, Chang YC, et al. An update on gene therapy for inherited retinal dystrophy: Experience in Leber congenital amaurosis clinical trials. Int J Mol Sci. 2021;22(9):4534.
  23. Bennett J, Wellman J, Marshall KA, et al. Safety and durability of effect of contralateral-eye administration of AAV2 gene therapy in patients with childhood-onset blindness caused by RPE65 mutations: a follow-on phase 1 trial. Lancet. 2016;388(10045):661-672.
  24. FDA approves novel gene therapy to treat patients with a rare form of inherited vision loss. FDA. Published December 18, 2017. Accessed October 16, 2021. https://www.fda.gov/news-events/press-announcements/fda-approves-novel-gene-therapy-treat-patients-rare-form-inherited-vision-loss
  25. Sickle Cell Disease. American Society of Gene + Cell Therapy. Published October 30, 2020. Accessed September 29, 2021. https://patienteducation.asgct.org/disease-treatments/sickle-cell-disease
  26. Parkinson’s Disease: Is Gene Therapy the Answer We Have Been Looking For? American Society of Gene + Cell Therapy. Published April 13, 2020. Accessed September 29, 2021. https://asgct.org/research/news/april-2020/parkinsons-disease-awareness-month