Differences in Gene Activity May Explain Why Some Patients Have Difficult-to-treat Eye Disease, Study Shows
Researchers used laboratory models derived from patient cells to re-create MG muscle cells, and study their gene activity.
The findings were included in the study “Profiling of patient-specific myocytes identifies altered gene expression in the ophthalmoplegic subphenotype of myasthenia gravis,” which was published in the Orphanet Journal of Rare Diseases.
Myasthenia gravis is a chronic neuromuscular disorder triggered by an autoimmune response that cripples the normal communication between nerve cells and muscles, causing muscle weakness and fatigue.
Many patients first notice weakness in the muscles that control the movement of the eyes and eyelids, known as extraocular muscles. But normally these early symptoms respond to standard therapy.
However, in sub-Saharan African populations there is a high incidence of treatment-resistant ophthalmoplegia (OP-MG) characterized by persistent eye muscle weakness, which can range from severe to complete paralysis of eye muscles and significant sight problems.
It seems that this persistent ocular form of MG most commonly affects subjects with juvenile onset, however it is unknown which factors drive this condition.
Previously, a team of researchers from the University of Cape Town, in South Africa, found that some OP-MG patients harbor genetic variants that disturb the activity of the DAF (or CD55) and TGFB1 genes. These two genes generate proteins that, among other functions, are involved in keeping an immune pathway — called the complement — in check.
The complement system plays a critical role in inflammation and defense against some bacterial infections, but it also can be activated as part of autoimmune responses, contributing to the damage of the body’s own tissues.
Taken together, these observations suggest that an overactive complement can lessen the injury and difficult healing of eye muscles in people with OP-MG.
To provide new insights into the mechanisms underpinning this condition, researchers compared the activity of genes — if genes are slightly, highly, or not turned “on” — associated with OP-MG in patients with OP-MG versus other MG patients.
To do that, researchers developed cell models that reproduced muscle cells, or myocytes, from patients in the laboratory. For generating these models, skin cells from 10 OP-MG and six control MG patients were collected by punch biopsy, grown in petri dishes, and then genetically engineered to turn into muscle cells.
Part of these cultures were treated with serum from MG patients to replicate the conditions found in patients.
In each patients’ muscle cells, they measured gene activity of 93 genes, which included the OP-MG susceptibility genes previously identified by them, as well as other genes reported to change in MG or mouse models of the disease.
Compared to muscle cells from MG patients, cells of subjects with OP-MG showed altered activity (expression) of 14 genes, both at basal levels and following exposure to MG serums.
Among the altered genes, four were OP-MG susceptibility genes: PPP6R2, CANX, FAM1364 and FAM69A.
Genes with altered expression in OP-MG included genes with a known role in the production of gangliosphingolipids (molecules important for the integrity of the connection between nerve cells and muscles) mitochondrial metabolism (important for energy production within cells) and the IGF1-signaling pathway, which has been implicated in MG.
Pairing any of these genes — a type of analysis done to pinpoint correlations in expression between genes — revealed 15% of gene pairs that were strongly correlated with OP-MG muscle cells, but not with control MG samples.
The top three significantly correlated pairs consisted of OP-MG susceptibility and MG-associated genes “reflecting crosstalk between OP-MG and myasthenia pathways, which was not evident in control MG cells,” researchers wrote.
“Using a surrogate cell culture model our findings suggest that muscle gene expression and co-expression differ between OP-MG and control MG individuals in response to MG sera [serums]. These findings implicate pathways not previously considered in extraocular muscle involvement in myasthenia gravis and will inform future studies,” the researchers concluded.