CRISPR-based therapeutic approach designed to treat fatal pediatric disease

Multisystemic smooth muscle dysfunction syndrome (MSMDS) is a rare condition associated with stroke, aortic dissection (tearing) and death in childhood. Currently, there is no effective treatment or cure for MSMDS. A single error in the genetic code of the ACTA2 gene, which encodes the smooth muscle actin protein, is the most common cause of MSMDS. To directly target this mutation, researchers from Mass General Brigham engineered a bespoke CRISPR-Cas9 gene-editing enzyme to develop a potential therapy for MSMDS, which substantially prolonged survival and reduced vascular disease and neurodegeneration in mouse models of MSMDS. Findings are published in Nature Biomedical Engineering.

The story of this research truly began at the bedside. An infant in critical condition first brought together our team, which includes experts on the clinical, genetic, biological and therapeutic aspects of this disease. Now, we have a clear roadmap toward bringing an experimental drug back to the bedside."

Patricia Musolino, MD, PhD, Department of Neurology, Massachusetts General Hospital (MGH)

The therapy developed by the researchers relies on a genome editing tool called a base editor, which is comprised of a CRISPR-Cas9 protein fused to DNA modifying enzyme. The Cas9 component is programmed by a guide RNA that helps direct the base editor to the proper site in the genome to make a precise DNA edit. The researchers realized that while a base-editor with a conventional Cas9 protein effectively corrected the ACTA2 mutation causing MSMDS, it also changed nearby DNA, nullifying the benefits of the correction.

In response, a team of researchers, led by corresponding author Benjamin Kleinstiver, PhD, designed and screened dozens of base-editors with custom-made Cas9 proteins to improve targeting of the ACTA2 mutation. This new base editor protein now achieved efficacious on-target correction with minimized unwanted editing. Ultimately, a single dose of the bespoke gene-editing therapy extended survival four-fold in a mouse model of MSMDS, which was also engineered by the researchers to help investigate new treatments in animals. Mice treated with a viral vector encoding the base editor showed improvement in both brain and aortic disease and other aspects of MSMDS such as exercise intolerance were improved.

"Our lab has made progress in engineering base editors to be safer, more effective, more precise, and therefore better suited to treating genetic disease," said Kleinstiver, an investigator in the Center for Genomic Medicine at MGH. Kleinstiver's team also recently designed a CRISPR-Cas9 enzyme that helped save the life of an infant born with a rare metabolic disease.

"MSMDS is a disease of tremendous unmet need, and our team was excited to leverage base editing to develop a treatment for it," said Kleinstiver.

To deliver the therapy to the vascular tissue effected in MSMDS, a team led by Casey Maguire, PhD, an investigator in the MGH Department of Neurology, designed a viral vector that specifically targets the smooth muscle that lines blood vessels. This is the first CRISPR-based therapeutic approach designed to specifically target the vasculature, which is diseased and rapidly progressing in infants with MSMDS.

The researchers have already engaged with the U.S. Food and Drug Administration, paving the way for a clinical trial. With guidance from Mass General Brigham's Innovation team as well as the Gene and Cell Therapy Institute, the program has advanced toward IND filing and secured FDA rare disease designations – milestones that will accelerate development.

Eventually, this research may also help advance cures for other conditions involving the vasculature, like moyamoya, Marfan syndrome and Loeys-Dietz syndrome, and even diseases like atherosclerosis, a leading cause of cardiovascular disease and the most common cause of death worldwide according to the World Health Organization.

"The impact of this work extends beyond just one disease," said Mark Lindsay, MD, PhD, a pediatric cardiologist within the Mass General Brigham Heart and Vascular Institute. "Our team has created tools that have accelerated the field of genome-editing and precision therapeutics to levels that were unthinkable just two years ago. Cures are possible, but only if we continue to support biomedical research."