CMT4A and CMT2K Gene Replacement Therapy with AAV9/GDAP1 in Rats

About This Grant

PROJECT SUMMARY/ABSTRACT Charcot–Marie–Tooth (CMT) neuropathies are a genetically and phenotypically heterogeneous group of disorders caused by pathogenic variants in over 100 different genes. Ganglioside-induced differentiation- associated protein 1 (GDAP1) gene mutations cause various forms of CMT including CMT4A and CMT2K. CMT4A is a severe, early onset neuropathy caused by autosomal recessive, loss of function mutations, while CMT2K is a milder, late onset neuropathy caused by autosomal dominant, gain of function mutations in the GDAP1 gene. Despite the clinical burden and the known GDAP1 cause in CMT4A and CMT2K, the potential of gene replacement therapy (GRT) for these patients remains unexplored. We propose to use an AAV9 vector to carry the GDAP1 gene to the spinal cord neurons and Schwann cells of newly characterized Gdap1 KO and KI rat lines and then to characterize the dose-responsive safety and efficacy of the AAV9/GDAP1 vector. Our central hypothesis is that AAV9-mediated GDAP1 GRT during early development will restore proper protein function and, therefore, ameliorate disease phenotypes in the newly characterized Gdap1 KO and KI rat models. We will test this hypothesis by pursuing two specific aims: Aim 1. Determine the safety and efficacy of AAV9/GDAP1 treatment in the Gdap1 KO rat model of CMT4A. Gdap1 KO rats will be dosed IT with the AAV9/GDAP1 vector at 2.5 or 12 months of age to mimic pre- and post-symptomatic intervention, respectively. A subset of rats treated at 2.5 months old will be euthanized at 1 month post injection to serve as the short-term study to evaluate transgene expression and safety of AAV9/GDAP1 (Aim1A). The remaining rats will be kept as the long-term study to evaluate the safety and efficacy of AAV9/GDAP1 (Aim1B). To evaluate safety, we will closely monitor all treated rats for signs of toxicity through comprehensive assessments. To evaluate efficacy, we will focus on longitudinal in-life behavioral and NCV tests, as well as postmortem histopathological changes on peripheral nerves. Aim 2: Determine the safety and efficacy of AAV9/GDAP1 treatment in the Gdap1 KI rat model of CMT2K. A key scientific question remains if our transgene expression can sufficiently outcompete the dominant- negative GDAP1 allele in the Gdap1R120W heterozygous KI CMT2K model to achieve a significant therapeutic rescue. Some studies have demonstrated the success of using GRT to rescue autosomal dominant mutations. Thus, Aim 2 is designed to evaluate this possibility. Our study will be carried out exactly as described in Aim 1 except in the Gdap1R120W KI rats. If these proof-of-concept studies in Aim 1 and 2 demonstrate favorable efficacy and an absence of toxicity, the data generated will serve as pivotal IND-enabling efficacy evidence, as well as supporting safety evidence. We will leverage our extensive experience to design and carry out the remaining IND-enabling studies. If successful, these efforts are designed to ultimately support the translation of this gene therapy approach into a first-in-human clinical trial. This will be of great significance not only to solve the unmet need for both CMT4A and CMT2K, but also providing a general proof-of-concept for other forms of CMT.