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Ophthalmic Professional

Article

Introduction to gene therapy

A review of the basic principles of genetic testing, types of gene therapy, and administration.

By: Thomas A. Ciulla, MD, MBA, Michael Gemayel, MD
Ophthalmic Professional January 1, 2020 Vol 9, Issue January 2020 Page(s): 26-28

The 2017 U.S. FDA approval of the first gene therapy for a genetic retinal disease punctuated a new cycle of innovation in ophthalmic therapies and heralded a bright future of possible treatment strategies.1 Gene therapy is a rapidly evolving technique that manipulates the expression of genes to treat disease. Typically, with gene therapy, genetic material is introduced into cells to restore production of a protein that may otherwise be faulty due to a patient’s DNA. This genetic material is typically introduced through a vector that is engineered to deliver the gene to a patients’ cells.

Given the potential for gene therapy in ophthalmology, it is important to be aware of genetic testing and therapeutic options. The retina is a prime target for gene therapy research, given the number of monogenic disorders, accessibility to target cell delivery, the noninvasive ability to monitor for disease progression or therapeutic response, and relative immune-privilege, which limits inflammatory response.2-4 This article will review the basic principles of gene therapy.

Gene therapy glossary

Biofactory approach: The use of gene therapy to introduce genes that produce proteins that are not native to the cell or organism.

Gene augmentation therapy: Used to treat diseases with mutations in a gene resulting in loss of function of gene product and absence of the functional protein. DNA containing a functional version of the gene is introduced into the cell to produce the missing protein.

Gene panels: Genetic testing of a select set of genes or gene regions that have known associations with the disease or phenotype under investigation.

Genetic testing: The sequencing of human DNA to discover human anomalies or mutations in chromosomes, genes, or proteins that may be pathologic.

Gene therapy: The therapeutic manipulation of genetic material to correct for the inability of missing or abnormal genes to produce functional protein.

Inherited retinal diseases: A group of rare eye disorders caused by an inherited gene mutation that lead to vision loss or blindness.

Variations of uncertain significance: An allele or gene variant identified via genetic testing, the significance of which to the health or function of the organism is unknown.

Viral vector: Genetically engineered carrier of genetic material to be introduced into cells.

Genetic testing

With more than 260 genes known to cause inherited retinal diseases (IRDs),5 such as retinitis pigmentosa or Stargardt macular dystrophy, a basic understanding of genetic testing is essential. Genetic testing is recommended by the American Academy of Ophthalmology (AAO) when clinical findings indicate retinal dystrophy that is possibly associated with genetic mutations.6 Genetic testing can confirm suspicion of an IRD, provide an accurate diagnosis as well as information about prognosis and management, and assist in counseling of families, including assessing risk in family members.7

Ordering and interpretation of genetic test results is complex, but facility with the basic underpinnings provides more clarity. While single-gene testing is inexpensive, it requires a working clinical diagnosis and consequently has limited utility given the nonspecific clinical findings in many IRDs.8 Gene panels are more commonly used and generally focus on diagnoses associated with multiple genes.8 Whole genome/exome sequencing is the most comprehensive but generates a high number of variants of uncertain significance (VUSs).8 Genetic testing results are not binary and involve a ranking system of each identified mutation based on standards released by American College of Medical Genetics and Genomics (pathogenic, likely pathogenic, VUS, likely benign, benign).9 VUSs can be especially difficult to address as they involve a mutation variant for which there is a lack of data of pathogenicity or non-pathogenicity.

Genetic counselors can assist patients in selecting genetic tests, interpreting results, and family planning. Fortunately, their services can be covered by insurance, and can be available via a telemedicine interaction.

Types of gene therapy

The major categories of gene therapy include gene augmentation (adding a gene to a cell), gene editing (revising the existing genetic code), gene inactivation (silencing a gene) and selective toxicity (as in chimeric antigen receptor or CAR T Cells to recognize cancer cells).

Ocular gene therapy has mostly involved gene augmentation. Recessive single-gene disorders are the most amenable to gene augmentation, because the mutations causing the disease generally lead to loss of function of the gene product and, therefore, total or near absence of functional protein. Gene augmentation can correct the lost function by delivery of the normal gene. The majority of gene therapies to treat inherited ocular diseases target autosomal recessive diseases. For example, RPE65 mutation-associated retinal dystrophy is a recessive monogenic disorder in which LUXTURNA (Spark Therapeutics), the only FDA-approved ocular gene therapy, corrects the lost function. In contrast, gain-of-function mutations produce dominant phenotypes. These diseases may be less amenable to treatment by gene therapy, as one gene copy expresses an abnormal product that has to be suppressed.10 Consequently, developing a gene therapy for autosomal dominant retinitis pigmentosa has been more challenging.

In addition to addressing genetic mutations in native genes, gene therapy can be utilized in a “biofactory approach” to produce non-native proteins. In neovascular AMD, gene therapy is already using this approach to chronically express anti-angiogenic proteins, such as aflibercept or ranibizumab. The complement system is another target for gene therapy. In AMD, deposition of membrane attack complex (MAC) in Bruch’s membrane and choriocapillaris increases significantly with aging,11 and gene therapies are being developed to inhibit components of the complement cascade.

Administration

Vector delivery to the target retinal tissue involves several potential methods. The most commonly investigated method involves pars plana vitrectomy followed by retinotomy and injection of the viral vector with genetic material into the subretinal space. This creates a temporary retinal detachment but allows for direct delivery to the cells of interest. The virus then “infects” or enters the RPE cells or photoreceptors, ultimately causing the host cell’s own translational machinery to express the protein.

Alternatively, injection of the vector into the vitreous cavity has been attempted. Although this method is less invasive and may have fewer procedure-related complications, the penetration of viral vector to the target tissue has been perceived as inferior to that of subretinal injections.12 However, techniques such “directed evolution” of viral vectors may enhance intravitreal delivery. Furthermore, clinic-based intravitreal or suprachoroidal delivery may circumvent some of the logistic issues of operating room-based subretinal delivery.

Conclusion

Gene therapy has evolved over the last decade and proven to be relatively safe in small clinical trials. With the potential for continued optimization and rapid therapeutic innovation in gene therapy, ophthalmology has an exciting future. To care for the patients of tomorrow, ophthalmic professionals must understand genetic testing and the basic aspects of gene therapy. OP

REFERENCES

  1. LUXTURNATM (voretigene neparvovec-rzyl) December 19, 2017 Approval Letter. 2017, US Food & Drug Administration: Silver Spring, MD.
  2. Streilein, J.W., Ocular immune privilege: therapeutic opportunities from an experiment of nature. Nat Rev Immunol, 2003. 3(11): p. 879-89.
  3. Takahashi, V.K.L., J.T. Takiuti, R. Jauregui, et al., Gene therapy in inherited retinal degenerative diseases, a review. Ophthalmic Genet, 2018: p. 1-9.
  4. Boye, S.E., S.L. Boye, A.S. Lewin, et al., A comprehensive review of retinal gene therapy. Mol Ther, 2013. 21(3): p. 509-19.
  5. RetNet: Summaries of Genes and Loci Causing Retinal Diseases. September 18, 2018 [cited 2018 January 16]; Available from: https://sph.uth.edu/retnet/sum-dis.htm .
  6. Duncan, J.L., P.S. Bernstein, D.G. Birch, et al. Recommendations on Clinical Assessment of Patients with Inherited Retinal Degenerations - 2016. 2016 [cited 2017 October 3]; Available from: https://www.aao.org/clinical-statement/recommendations-on-clinical-assessment-of-patients .
  7. Role of Genetic Tests in IRDs and RED. [cited 2018 October 3]; Available from: http://www.retina-international.org/role-genetic-tests-irds-and-red .
  8. Lee, K., S. Garg, Navigating the current landscape of clinical genetic testing for inherited retinal dystrophies. Genet Med, 2015. 17(4): p. 245-52.
  9. Richards, S., N. Aziz, S. Bale, et al., Standards and guidelines for the interpretation of sequence variants: a joint consensus recommendation of the American College of Medical Genetics and Genomics and the Association for Molecular Pathology. Genet Med, 2015. 17(5): p. 405-24.
  10. Sengillo, J.D., S. Justus, Y.T. Tsai, et al., Gene and cell-based therapies for inherited retinal disorders: An update. Am J Med Genet C Semin Med Genet, 2016. 172(4): p. 349-366.
  11. Mullins, R.F., D.P. Schoo, E.H. Sohn, et al., The membrane attack complex in aging human choriocapillaris: relationship to macular degeneration and choroidal thinning. Am J Pathol, 2014. 184(11): p. 3142-53. strategies to circumvent them. Rev Med Virol, 2013. 23(6): p. 399-413.
  12. Yu-Wai-Man, P., Genetic manipulation for inherited neurodegenerative diseases: myth or reality? Br J Ophthalmol, 2016. 100(10): p. 1322-31.

 

Dr. Ciulla is volunteer clinical professor of ophthalmology at Indiana University School of Medicine, a board member of Midwest Eye Institute, Indianapolis, and chief medical officer at Clearside Biomedical. He previously served a VP role as medical strategy lead-ophthalmology at Spark Therapeutics.
Dr. Ciulla is volunteer clinical professor of ophthalmology at Indiana University School of Medicine, a board member of Midwest Eye Institute, Indianapolis, and chief medical officer at Clearside Biomedical. He previously served a VP role as medical strategy lead-ophthalmology at Spark Therapeutics.

 

Dr. Gemayel is a PGY-3 Ophthalmology resident at Indiana University. He graduated from Wayne State University medical school in 2017.
Dr. Gemayel is a PGY-3 Ophthalmology resident at Indiana University. He graduated from Wayne State University medical school in 2017.

 

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