Congenital muscular dystrophy type 1A (MDC1A) is caused by mutations in the Lama2 gene that lead to muscle wasting and the destruction of the protective myelin coating around peripheral nerves. Now a Canadian team has developed a CRISPR-based therapy that targets the disease regardless of the mutation type. The technique has shown promise in mouse models.
Rather than targeting the culprit Lama2 gene, a team at the Hospital for Sick Children (SickKids) in Toronto used the CRISPR gene-editing tool on a related gene, Lama1. By increasing expression of Lama1, the therapy not only prevented paralysis in pre-symptomatic mice but also reversed disease progression in animals that had already developed symptoms. The scientists published the results in Nature.
Ronald Cohn and colleagues at SickKids previously used CRISPR-Cas9 to correct a mutation in Lama2 in vivo. But there are over 350 identified MDC1A-causing Lama2 mutations, meaning multiple such therapies need to be designed and thoroughly evaluated for each mutation. By contrast, increasing the expression of Lama1 to produce a protein that’s structurally similar to the Lama2-coded protein may arguably benefit all MDC1A cases regardless of their mutation type, Cohn believes.
But targeting Lama1 has its own challenge: The gene is too large to be contained in the viral vectors that are traditionally used to deliver gene therapy. So the researchers again turned to CRISPR and developed a gene-activation system to increase the expression of Lama1 without making double-stranded breaks in DNA.
When pre-symptomatic neonatal mice with MDC1A were treated, the therapy led to a reduction in fibrosis and larger muscle fiber size, preventing the development of symptoms, the team reported.
More importantly, in mice that were already suffering paralysis, the therapy also improved the animals’ to stand up and move. And the researchers observed a significant increase in the speed of nerve conduction, which was believed to help restore the myelin sheath and improved neuromuscular function.
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CRISPR has previously yielded positive preclinical results in Duchenne muscular dystrophy (DMD), mainly by targeting mutations in the gene that produces dystrophin, a protein that’s key for muscle support. In a Duke University study supported by DMD drug maker Sarepta, scientists used CRISPR to remove dysfunctional dystrophin exons, leaving a shortened but still functional gene. The therapy was found to be effective for more than a year in mice. In another study at UT Southwestern, researchers applied CRISPR to target exon 51 and successfully restored dystrophin levels to 92% of normal in dog models of DMD.
Cohn’s team figures its strategy of using AAV-delivered CRISPR to upregulate Lama1 could also be applied in DMD. To achieve better efficacy, the system could be used in tandem with another technology that corrects the mutation, the researchers argue.
“An application as a combinatorial therapeutic approach, involving concurrent upregulation of protective disease-modifier genes and downregulation of detrimental genes would represent a new paradigm for lessening disease phenotypes,” they wrote in the study.