Gene therapy with AR isoform 2 rescues spinal and bulbar muscular atrophy phenotype by modulating AR transcriptional activity
Lim, W. et al. (2021) Gene therapy with AR isoform 2 rescues spinal and bulbar muscular atrophy phenotype by modulating AR transcriptional activity. Science Advances. 7, 34. DOI: 10.1126/sciadv.abi6896
Spinal and bulbar muscular atrophy (SBMA), also known as Kennedy’s disease, is an X-linked adult-onset neuromuscular disease. This condition is found in 1/50,000 men worldwide and is commonly misdiagnosed with other neuromuscular diseases. Patients with SBMA develop muscle weakness and atrophy following lower motor neuron and skeletal muscle degeneration. Fifteen to twenty years after onset patients usually become wheelchair bound. The disease is caused by a CAG expansion encoding a polyglutamine (polyQ) tract in androgen receptors. The disease mutation leads to partial loss of androgen receptor function that results in androgen insensitivity syndrome. The mutant protein also affects androgen receptor transcriptional activity.
Previous SBMA studies in Drosophila show that androgen receptors and their transcriptional coregulators must interact through the activation function 2 domain and androgen receptors must bind to target genes to trigger the disease pathogenesis. Currently, a therapeutic approach is needed to modulate transcriptional dysregulation without affecting critical androgen receptor functions. Further research is required to fully understand the mechanisms and potential off-target effects of emerging therapeutics. Notably, clinical trials over the past few years have indicated that skeletal muscles are the main drivers of SBMA pathogenesis. Skeletal muscle specific excision of polyQ AR in BAC mouse models, including a floxed AR first exon, has proved to prevent disease manifestation. Additionally, treatments that do not cross the blood-brain barrier are able to improve mouse neuromuscular phenotypes, indicating muscle is a target for SBMA therapy.
This study worked to establish a physiological role of androgen receptor isoform 2 (AR-2). One of the nine AR variants identified in iPS-derived motor neurons and human brains, AR-2 was found to be the most abundant and encodes a truncated AR variant, called AR45, which has an amino acid peptide in place of the N-terminal domain and includes a 45 kDa polyQ stretch, which is associated with the disease.
Researchers sought to provide evidence for a safe and efficient gene therapy strategy for delivering androgen receptor variants to treat SBMA. They used deep RNA-seq to generate quantitative expression profiles in human tissue and assess the transcriptome-wide effects of AR-2 silencing. Using ChIP assays at androgen response element sites they were able to measure the protein-DNA interactions.
Results of this proof-of-principle study provide evidence for a safe and efficient gene therapy strategy based on adeno-associated viruses (AAV9-mediated delivery) of the AR45 variant in SBMA mice. This strategy shows potential to be translated into human patient treatments. However, before moving forward to clinical testing, vector immunogenicity, duration of expression, therapeutic potency, and potential toxicity will have to be studied further.
Keywords: gene therapy, neuromuscular disease, AR-mediated transcription, preclinical study, mouse model, viral vector
