RPGR gene therapy presents challenges in cloning the coding sequence

ABSTRACT

Introduction: Currently, there are three Phase I/II clinical trials based on gene therapy ongoing to test different AAV.RPGR or deleted RPGR vectors on patients affected by X-linked retinitis pigmentosa. These three vectors differ in the adeno-associated viral (AAV) vector capsid used, and the coding sequences: two contain codon optimized versions of RPGR which give the full-length protein, whilst the third uses a wild-type sequence that contains a large deletion encoding part of the functional domain of the RPGR protein.

Areas covered: This review approaches the different studies that have led to the initiation of three different clinical trials for RPGR related X-linked retinitis pigmentosa.

Expert opinion: The development of a gene therapy vector to deliver a normal copy of the RPGR gene into the photoreceptors has presented a challenge for the scientific community. The instability of its sequence and the fact that its function is not well understood can lead to the production of a nonfunctional or deleterious protein for the human retina. Since the RPGR protein undergoes post-translational glutamylation in the protein domain that may be particularly affected by gene instability, a functional assay of glutamylation is essential to verify the correct coding sequence.

KEYWORDS:

• Adeno-associated virus
• clinical trials
• codon optimization
• gene therapy
• retinitis pigmentosa GTPase regulator (RPGR)

Article highlights

• X-linked retinitis pigmentosa has a high prevalence and a severe phenotype, and there is no approved treatment to date.

• Gene therapy strategies have been widely applied in preclinical and clinical studies to treat inherited retinal diseases due to its great potential to correct genetic defects, and to the advantages that the retina as a target organ presents.

• The development of a gene therapy vector that provides a correctly coded copy of the RPGR gene to retinal photoreceptors is not straightforward, since the repetitive nature of its sequence and natural splice donor site makes the introduction of mutations highly likely.

• Alternative approaches, such as codon optimization, need to be undertaken in order to produce a full-length stable protein without mutations that could affect post-translational modification, such as glutamylation which is required for its function.

• The safety and efficacy of two different AAV.RPGR vectors and one AAV vector containing a deleted RPGR are being investigated in three clinical trials.

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Declaration of interest

RE MacLaren is a consultant to Biogen, Spark Therapeutics and Novartis, which are developing AAV gene therapies for retinal disease. RE MacLaren is named inventor on a patent filed on behalf of the University of Oxford, relating to the expression cassette and codon optimization of RPGR coding sequence in general. RE MacLaren is the scientific advisor to The National Institute for Health and Care Excellence in the UK (NICE) retinal gene therapy committee. The authors have no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed.

Reviewer Disclosures

One of the reviewers on this paper is a on the Scientific Advisory Board of AGTC Inc. Other peer reviewers on this manuscript have no relevant financial relationships or otherwise to disclose.

Additional information

Funding

This paper was funded by the Medical Research Council (UK) MR/K003690/1 award to RE MacLaren, Oxford NIHR Biomedical Research Center and Royal College of Surgeons of Edinburgh.