Muscles need dystrophin in order to regenerate. Persons suffering from Duchenne muscular dystrophy lack this essential muscular protein due to mutations in the gene which is responsible for producing dystrophin. As a result, their existing muscle cells deteriorate over time and are gradually replaced by connective and fatty tissue; muscle strength weakens during the course of the disease. The first symptoms usually appear around the age of five. Children with the disease begin to have difficulties with movements they previously completed with ease, for example climbing stairs or getting up from the floor. At approximately the age of twelve, they are no longer able to walk, later losing movement in their arms and hands. Due to concomitant respiratory and cardiac failure, the majority of patients does not reach the age of 40. DMD affects mainly boys, since the responsible mutations are located in the dystrophin gene on the X chromosome.
An interdisciplinary Munich research team led by scientists from Technical University of Munich (TUM) has for the first time succeeded in correcting the mutated dystrophin gene in living pigs. In order to cut the defective gene sequence from the DNA of the animals' muscle and heart cells, the researchers modified the Crispr-Cas9 gene scissors. "These gene scissors are highly efficient and specifically corrected the dystrophin gene," says Professor Wolfgang Wurst, developmental geneticist at TUM and the German Research Center for Environmental Health. It became then again possible to viably read the gene which had been unreadable because of the genetic defect, thus allowing for a successful protein biosynthesis. Now the shorter but stably formed dystrophin protein was able to improve muscle function. The animals treated were less susceptible to cardiac arrhythmia and had an increased life expectancy compared to animals with the disease that did not receive the therapy.
Therapeutic success with clinically relevant model
"Muscle and heart cells are long-lived cell structures. One half of all myocardial cells remain functional from birth throughout the entire lifecycle of a human being," says Professor Christian Kupatt, cardiologist at university hospital TUM Klinikum rechts der Isar. "The genome of a cell is used for protein biosynthesis as long as the cell is alive, and once a cell has been affected by the therapy, it remains corrected. So if we change the genome of a myocardial cell, the correction is a long-term success, in contrast to the results of previous methods."
The gene sequence responsible for the dystrophin protein has already been successfully corrected in the past, however in mice and other animal models. "Our results are very promising, since for the first time, we have now been able to demonstrate therapeutic success in a clinically relevant large animal model," says Professor Maggie Walter, neurologist at the LMU university hospital. In terms of important biochemical, clinical and pathological changes, the pig model mirrors Duchenne muscular dystrophy in humans. "Since the disease proceeds faster in our pig model, we were able to verify the efficacy of the therapeutic approaches within a manageable period of time," says Professor Eckhard Wolf, Chair of Molecular Animal Breeding and Biotechnology at the Gene Center and the Department of Veterinary Sciences at LMU. (TUM/LMU)Nature Medicine 2020