Gene therapy restores key protein in models of rare muscular dystrophy
Study provides data supporting further development for X-linked EDMD patients
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A new gene therapy approach safely restored the production of emerin, the protein missing in people with X-linked Emery-Dreifuss muscular dystrophy (EDMD), in both a mouse model and human muscle cells, according to a study.
Although the preclinical study could not assess the gene therapy’s impact on muscle function, it provided early proof-of-concept data supporting further development for X-linked EDMD patients, researchers said.
The study, “Assessment of emerin gene transfer in models of X-linked Emery-Dreifuss muscular dystrophy,” was published in the journal Skeletal Muscle.
X-linked EDMD caused by inherited gene mutations
EDMD is a form of muscular dystrophy marked by slowly progressive muscle weakness and wasting of certain skeletal muscles (those attached to bones) and contractures (stiffening) of the elbows and ankles. It can also affect the heart muscles, leading to irregular heartbeats, which typically develop during or soon after the second decade of life.
X-linked EDMD is caused by inherited mutations in the EMD or FHL1 genes on the X chromosome that disrupt the production of emerin, a key protein found in the membrane of the nucleus, where most of the cell’s DNA is housed. As an X-linked disease, it affects men much more frequently than women.
Because almost all disease-causing EMD mutations result in a complete lack of emerin protein, the researchers thought that replacing the missing protein through gene therapy could be an effective treatment strategy.
“The goal of the current project was to generate a gene therapy vector to restore emerin expression and conduct a proof of expression and limited safety study in a small animal model,” the researchers wrote.
Robust emerin expression detected in hearts of treated mice
The team constructed a gene therapy vector called AAV-tMCK-hEMD, which used an adeno-associated virus (AAV) to deliver the human emerin gene to muscle tissue. They tested the therapy in human muscle cells (cardiomyocytes) and a strain of mice, both of which lacked the EMD gene.
Mice received a single intravenous (into-the-vein) dose of the gene therapy at six to eight weeks of age, and were assessed six months later.
Tests found no emerin in the hearts of untreated EDMD mice, but robust emerin expression in the hearts, calf muscle, and diaphragm of mice treated with the therapy. Further analysis confirmed that the emerin protein localized correctly around the rim of the nucleus, similar to its position in normal mice.
In heart tissue, 36% of cell nuclei were positive for emerin after treatment, compared with 59% in the calf muscle. Since roughly 30% of cells in the mouse heart are cardiomyocytes, the researchers noted that most cardiomyocytes successfully received the gene therapy and expressed the protein.
In a gene activity analysis, hearts from treated EDMD mice showed gene expression patterns similar to normal mice, while untreated hearts showed patterns separate from these groups. Moreover, the small number of gene activity changes after treatment demonstrated a lack of cardiac toxicity, the team noted.
In human muscle cells, the gene therapy generated detectable emerin expression, though at a lower level than in unmutated healthy cells. Similar to mice, microscopy confirmed emerin expression with proper localization at the nuclear rim in a portion of the treated cardiomyocytes.
“AAV-tMCKhEMD can drive expression of emerin in a human X-linked EDMD model cardiomyocyte system,” the researchers confirmed.
No treated mice died or showed signs of distress during study period
In a safety assessment, no mice treated with the vector died or showed signs of distress during the six-month study period. Body mass increased in both saline- and vector-treated groups, though vector-treated mice showed a trend toward reduced body mass gain during the first 90 days.
At six months, mean blood sugar (glucose) levels were significantly higher in treated mice than in saline-treated mice. Still, other blood tests for the health and function of major organ systems, including the liver, kidneys, muscles, and cardiovascular system, were within the normal range.
Given the robust protein expression and lack of overt toxicity, our initial preclinical results suggest that this approach warrants further preclinical development to determine its potential to possibly test in patients.
A pathologist’s assessment found no significant abnormalities in heart or upper leg muscle tissue. Emerin protein was also detected in the liver of treated mice, an organ known to readily take up AAV vectors. Still, this off-target expression was not associated with significant liver injury, apart from rare or mild inflammation observed in both saline- and vector-treated samples.
Of the study’s limitations, the researchers noted that these EDMD mice did not develop significant symptoms of muscular dystrophy or heart disease, which prevented them from assessing whether the gene therapy could improve muscle function.
Even so, “given the robust protein expression and lack of overt toxicity, our initial preclinical results suggest that this approach warrants further preclinical development to determine its potential to possibly test in patients,” the researchers concluded.
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