Metabolic processes altered in Pompe disease muscles: Study
Changes seen in cells of adults with late-onset disease
Several metabolic processes in Pompe disease muscles are altered, according to a detailed examination of gene activity in muscle cells from adults with late-onset Pompe disease (LOPD).
The changes, many of which occur in early disease progression, included a switch from energy production based on glucose to fat-like lipids and amino acids, the building blocks of proteins. There was also dysfunction in energy-producing mitochondria and autophagy, a process by which cells break down and eliminate unwanted proteins and other substances.
Enzyme replacement therapy (ERT) appeared to shift metabolic pathways toward normal and partially restore the activity of autophagy genes.
“These findings highlight significant cellular changes in Pompe disease and potential therapeutic avenues for intervention,” the researchers wrote in the study, “Decoding the muscle transcriptome of patients with late onset Pompe disease reveals markers of disease progression,” published in Brain.
Glycogen is a multi-branched form of glucose the body uses to store energy. When energy is needed, glycogen is broken down into glucose by an enzyme called acid alpha-glucosidase, or GAA.
Pompe disease muscles
People with Pompe carry mutations in the gene that provides instructions for the enzyme, disrupting GAA’s production or function. As a result, glycogen builds up to toxic levels inside cells, particularly in muscle cells with high energy needs.
Eventually, symptoms emerge, either in the first year of life, when the disease is referred to as infantile-onset Pompe disease, or later on, called late-onset Pompe.
Previous studies in mouse models have shown that glycogen build-up disrupts autophagy. Impaired autophagy leads to oxidative stress, a type of cell damage; the toxic buildup of protein clumps; and weakened mitochondria, which generate most of the cell’s energy.
An inability to break down glycogen into glucose may cause cells to use other metabolites, such as fat-like lipids and proteins, to obtain energy.
Still, “there is little evidence in humans coming from muscle samples of patients affected,” the researchers wrote.
The team of scientists from the U.K., Europe, and the U.S. examined the gene expression (activity) profiles of muscle biopsies from eight LOPD patients and four age- and sex-matched healthy individuals. Patients ranged in age from 33 to 60; five were women, and two received ERT.
A technique called single nuclei RNA sequencing was applied to muscle biopsy samples, measuring the gene expression in individual nuclei in muscle cells. Changes in gene expression were matched to those in muscle tissues, as examined under a microscope.
Pompe samples had a significantly higher percentage of nuclei expressing markers of slow-twitch muscle fibers, which are related to endurance, than controls. At the same time, there was a decrease in markers for fast-twitch muscle fibers, which are related to short, powerful movements and have high energy demands for glucose metabolism.
“It appears that the inability of fast fibers to efficiently convert glycogen into glucose leads to a switch of fast to slow type in order to find new resources to fill the energy demands,” the team wrote.
Gene expression
The most consistent feature in Pompe muscle fibers was a drop in the expression of genes encoding components of the mitochondria, affecting oxidative phosphorylation, a key process in energy generation that occurs inside mitochondria. Markers for glycolysis, a form of mitochondria-independent energy production, were also lower.
Pompe muscles had more nuclei expressing genes involved in the metabolism of fat-like lipids and amino acids. Accordingly, there was a decrease in the expression of genes associated with carbohydrate metabolism in fast-twitch fibers and an increase in the metabolism of lipids and amino acids in both fast- and slow-twitch muscle fibers.
The team said there was a build-up of fat-like lipids in Pompe muscle samples, “which could support a dysregulated lipid metabolism.”
Increases in gene expression related to inflammation and apoptosis, a type of programmed cell death, were observed in Pompe muscles. There was also a higher number of immune cells called macrophages. Consistent with mouse studies, autophagy markers were significantly higher in Pompe samples than in controls, a sign of dysregulated autophagy.
Pompe fibers without signs of advanced disease were compared to control samples to understand the earliest changes in gene expression. Results showed an early increase in the expression of genes involved in lipid metabolism and response to oxidative stress, and a decrease in genes related to glucose metabolism.
Finally, the team compared the gene expression in muscle samples from the two ERT patients before and after treatment. The therapy appeared to shift amino acid metabolism closer to control samples in fast- and slow-twitch muscle fibers. Slow-twitch fibers, in particular, showed a shift towards normal for all metabolic pathways. ERT also partially restored genes involved in autophagy.
“Our findings indicate that alternative metabolic pathways, specifically lipid and protein degradation, become essential to provide the energy for cellular functioning,” the authors wrote. “Notably, activation of these alternative metabolic pathways occurs early in the disease progression.”
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