rAAV5-hCNGB3 Gene Therapy for Achromatopsia: Safety and Efficacy in a Dog Model

DESCRIPTION provided by applicant Complete achromatopsia is an autosomal recessive inherited congenital disorder of retinal cone photoreceptors Patients with complete achromatopsia experience extreme light sensitivity and daytime blindness and best visual acuity under non bright light conditions is usually or worse and generally stable over time In addition to poor acuity hypersensitivity to light is an extremely troubling symptom No specific therapy is currently available Previous studies in a dog model of achromatopsia caused by mutations in the CNGB gene showed that subretinal injection of a recombinant adeno associated virus rAAV vector expressing human CNGB rescued cone photoreceptor function but at high doses was associated with findings consistent with immune mediated toxicity that may be due to the low amino acid identity between human and canine CNGB The objectives of the studies proposed in this Fast Track Phase I II STTR application are to confirm and extend these findings by comparing rAAV vectors expressing human or canine CNGB This will be accomplished by constructing an AAV proviral plasmid or rHSV helper virus containing canine CNGB cDNA driven by a cone specific promoter and using the construct to produce rAAV vectors expressing canine or human CNGB that will be tested for safety and efficacy in the dog model of CNGB related achromatopsia Previous attempts to clone a stable full length dog CNGB dCNGB coding region into a plasmid to generate an AAV expression cassette that could be used to packaging a rAAV CNGB vector have been uniformly unsuccessful To overcome this problem we will use two innovative approaches In one approach we will modify the codons of the dCNGB cDNA in a way that they favor gene expression in humans but are rarely utilized in E coli To overcome toxicity related to a presumed cryptic promoter we will also introduce a mutation at codon from methionine to leucine and additional silent mutations i e using synonymous codons upstream of codon in order to suppress possible internal promoter functions In a second approach we will directly introduce a synthesized dCNGB expression cassette into a recombinant herpes simplex virus HSV helper virus that can be used for rAAV production using AGTCandapos s HSV based rAAV production system thereby bypassing the potential of toxicity in E coli meditated by plasmids containing a dCNGB expression cassette These studies will also support development of several assays critical to support of human clinical studies of a product to treat Achromatopsia A better understanding of the effects of rAAV CNGB vectors in animals especially with respect to toxicity seen at higher doses will help to guide future development of rAAV CNGB gene therapy for human patients PUBLIC HEALTH RELEVANCE Complete achromatopsia is an inherited retinal disorder characterized by severely reduced visual acuity daytime blindness and complete loss of color discrimination It can be caused by mutations in any one of four different genes Gene therapy using a modified virus containing a normal copy of one of these genes can improve vision and correct daytime blindness in animal models of achromatopsia The current research project will provide information on how to evaluate the safety of gene therapy vectors in an animal model of the disease