Systemic AAV8-mediated delivery of a functional copy of muscle glycogen phosphorylase (Pygm) ameliorates disease in a murine model of McArdle disease

Abstract

McArdle disease is a disorder of carbohydrate metabolism that causes painful skeletal muscle cramps and skeletal muscle damage leading to transient myoglobinuria and increased risk of kidney failure. McArdle disease is caused by recessive mutations in the muscle glycogen phosphorylase (PYGM) gene leading to absence of PYGM enzyme in skeletal muscle and preventing access to energy from muscle glycogen stores. There is currently no cure for McArdle disease. Using a preclinical animal model, we aimed to identify a clinically translatable and relevant therapy for McArdle disease. We evaluated the safety and efficacy of recombinant adeno-associated virus serotype 8 (rAAV8) to treat a murine model of McArdle disease via delivery of a functional copy of the disease-causing gene, Pygm. Intraperitoneal injection of rAAV8-Pygm at post-natal day 1–3 resulted in Pygm expression at 8 weeks of age, accompanied by improved skeletal muscle architecture, reduced accumulation of glycogen and restoration of voluntary running wheel activity to wild-type levels. We did not observe any adverse reaction to the treatment at 8 weeks post-injection. Thus, we have investigated a highly promising gene therapy for McArdle disease with a clear path to the ovine large animal model endemic to Western Australia and subsequently to patients.

Preclinical Research in McArdle Disease: A Review of Research Models and Therapeutic Strategies

“…Hence, systemic delivery of rAAV2/8 expressing murine Pygm under the control of a synthetic triple muscle-specific CK (MCK) gene promoter. MCK gene promoter was evaluated in McArdle mice at post-natal days 1–3 [ 75 ] ( Table 3 ). Detectable levels (0.5 to 21% of WT levels) of Pygm were observed in the quadriceps of treated mice 8 weeks post-injection.…”

“…Detectable levels (0.5 to 21% of WT levels) of Pygm were observed in the quadriceps of treated mice 8 weeks post-injection. In addition, a significant reduction was reported in muscle glycogen levels together with an improved performance in voluntary wheel running [ 75 ]. However, no improvement was found in treated mice for performance in the hang wire test, probably reflecting that the level of Pygm expression achieved was enough to meet the energy requirements of sub-maximal endurance exercise, but not to support short bursts of maximal effort [ 75 ].…”

Preclinical Research in Glycogen Storage Diseases: A Comprehensive Review of Current Animal Models

“…Despite such good results, expression of PYGM appeared to decrease with time, possibly due to an immune response against the human enzyme and also to the specific viral vectors [207]. Gene therapy was also tested in Pygm R50X/R50X mice via intraperitoneal injections of rAAV8-Pygm in the first postnatal days, resulting in Pygm expression at 8 weeks of age, as well as in improved skeletal muscle architecture, reduced accumulation of muscle glycogen, and restoration of voluntary running wheel activity to normal levels [214]. Introduction GSD-VII (OMIM #232800) was first described by Japanese physician Seiichiro Tarui in 1965 [94].…”

Absence of p.R50X Pygm read-through in McArdle disease cellular models

“…There is currently no curative therapy for McArdle disease. Gene therapy has been evaluated in the spontaneously occurring McArdle sheep model and recently in the knock-in (p.R50X homozygous) McArdle mouse model (McNamara et al, 2019;Howell et al, 2008). In the sheep model, intramuscular application of two different vectors (adenovirus 5 vector and an adeno-associated virus serotype 2) containing GP-M expression cassettes produced only local expression of functional GP-M, and the number of GP-Mexpressing fibers decreased with time (Howell et al, 2008).…”

“…In the sheep model, intramuscular application of two different vectors (adenovirus 5 vector and an adeno-associated virus serotype 2) containing GP-M expression cassettes produced only local expression of functional GP-M, and the number of GP-Mexpressing fibers decreased with time (Howell et al, 2008). Furthermore, intraperitoneal injection of recombinant adenoassociated virus serotype 8 containing a functional copy of Pygm in the McArdle mouse model resulted in Pygm expression, improved skeletal muscle architecture, reduced accumulation of glycogen and restoration of voluntary running wheel activity, but a lack of improvement in the hanging wire ability (McNamara et al, 2019). Thus, further studies are required in this field before this approach can be proposed in patients.…”