Thursday, December 5, 2019

Non-immunogenic utrophin gene therapy for the treatment of muscular dystrophy animal models

Song Y, Morales L, Malik AS, Mead AF, Greer CD, Mitchell MA, Petrov MT, Su LT, Choi ME, Rosenblum ST, Lu X, VanBelzen DJ, Krishnankutty RK, Balzer FJ, Loro E, French R, Propert KJ, Zhou S, Kozyak BW, Nghiem PP, Khurana TS, Kornegay JN, Stedman HH. Non-immunogenic utrophin gene therapy for the treatment of muscular dystrophy animal models. Nat Med. 2019 Oct;25(10):1505-1511.

The essential product of the Duchenne muscular dystrophy (DMD) gene is dystrophin, a rod-like protein that protects striated myocytes from contraction-induced injury. Dystrophin-related protein (or utrophin) retains most of the structural and protein binding elements of dystrophin. Importantly, normal thymic expression in DMD patients6 should protect utrophin by central immunologic tolerance. We designed a codon-optimized, synthetic transgene encoding a miniaturized utrophin (µUtro), deliverable by adeno-associated virus (AAV) vectors. Here, we show that µUtro is a highly functional, non-immunogenic substitute for dystrophin, preventing the most deleterious histological and physiological aspects of muscular dystrophy in small and large animal models. Following systemic administration of an AAV-µUtro to neonatal dystrophin-deficient mdx mice, histological and biochemical markers of myonecrosis and regeneration are completely suppressed throughout growth to adult weight. In the dystrophin-deficient golden retriever model, µUtro non-toxically prevented myonecrosis, even in the most powerful muscles. In a stringent test of immunogenicity, focal expression of µUtro in the deletional-null German shorthaired pointer model produced no evidence of cell-mediated immunity, in contrast to the robust T cell response against similarly constructed µDystrophin (µDystro). These findings support a model in which utrophin-derived therapies might be used to treat clinical dystrophin deficiency, with a favorable immunologic profile and preserved function in the face of extreme miniaturization.

Because of the limited number of successful therapies for DMD, “The ability of the animal models and ages used in this study to predict success in boys with Duchenne muscular dystrophy is not yet established,” George Dickson, PhD, emeritus professor of molecular cell biology at Royal Holloway University of London told Neurology Today.

Dr. Dickson, who was a co-discoverer of utrophin in 1991, said, “It is not yet clear what the clinical significance of the lack of nNOS scaffolding may be for this therapy, at least in part because the importance of nNOS signaling in DMD is poorly understood.”

“The main advantage of utrophin is that it is non-immunogenic,” commented Michio Hirano, MD, FAAN, professor of neurology and director of the H. Houston Merritt Neuromuscular Research Center at Columbia University Medical Center in New York City.

“I think the specific utrophin construct used here was well thought through,” he added, “and the indications are that it is rather successful. But it is very difficult to extrapolate from the dog to humans” without functional confirmation of the histologic benefit. “Nevertheless, the results are striking and very encouraging.”

In an editorial accompanying the Nature Medicine paper, Kay E. Davies, PhD, of the MDUK Oxford Neuromuscular Center at the University of Oxford in the UK, and Jeffrey S. Chamberlain, PhD, of the neurology, medicine and biochemistry department at the University of Washington School of Medicine, agreed that the findings were promising. They offered an analysis of the differences between using the dystrophin and utrophin genes. They noted that various studies using the truncated dystrophin gene had modified phenotypes into mild forms of muscular dystrophy such as Becker MD. But they also offered cautious optimism about the alternative utrophin.

“Although current gene therapy trials for DMD are showing encouraging results, a rigorous test of dystrophin immunity has not been performed, and the availability of μ-Utro vectors provides an important alternative approach to treating this devastating human genetic disorder,” they wrote.

Dr. Stedman agreed. A clinical trial is likely the only way to confirm whether utrophin can live up to its promise, and such a trial is likely to come soon, he said, perhaps as early as the coming year. “We believe we have the data we need to move to the clinic, and we are exploring a range of possible opportunities to get it there as fast as possible in a responsible way.”

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