In this FSHD University Webinar from April 15, 2021, Angela Lek, PhD, a research scientist at Yale University School of Medicine, speaks about how her husband’s diagnosis with limb-girdle muscular dystrophy led to the scientific couple’s interest in the genetics of muscle-wasting diseases. She gives an engaging and outstandingly lucid explanation of the biology of muscular dystrophies (MDs), and how FSHD is a “unique form” of MD. Dr. Lek describes how each type of MD affects a different constellation of muscles, and how various genetic alterations can lead to different types of MD.
Most treatment strategies for muscular dystrophy are based on replacing a missing or defective gene, she says. But in FSHD, there isn’t a missing gene. Instead, in FSHD, shortening of the D4Z4 region on chromosome 4 results in the expression of the DUX4 gene. Normally, DUX4 shows up at the 2-cell stage in the human embryo, shortly after an egg is fertilized, and vanishes by the 8-cell stage. But in FSHD, DUX4 is activated in adult muscle cells, something that is not seen in unaffected humans. “When DUX4 is activated, it causes premature muscle cell death,” Dr. Lek explains, showing dramatic videos of many nuclei “lighting up” with DUX4 inside a muscle fiber. Whether DUX4 is induced in mice, human skeletal muscle cells in a dish, or zebra fish, DUX4 has the common effect of damaging muscle. (DUX4 isn’t normally expressed in mice or fish, so these lab creatures have to be genetically engineered to mimic this process.)
This key insight about the toxic effects of DUX4 suggests several ways to treat FSHD:
- Prevent the D4Z4 region on chromosome from unwinding, thereby preventing DUX4 from getting expressed;
- Directly targeting the DUX4 messenger RNA or protein;
- Targeting the “downstream” biochemical events that are activated by DUX4, which cause the actual damage to muscles.
“The third approach is what we’re pursuing in our lab,” Dr. Lek says. This is because “there may be existing drugs [approved by the FDA for other diseases] that can target these downstream pathways.” She says this strategy can shorten the discovery phase and reduce the cost to develop treatments.
Can we “repurpose” drugs to treat FSHD?
To pursue this strategy, one needs to understand how DUX4 wreaks havoc in the cell. DUX4 activates hundreds of genes—a snowball effect. “Can we identify circuits in the cell that cause them to die?” Dr. Lek asks. “To answer this question, we performed a genetic screen, looking at each of 20,000 genes in the cell, turning them on and off one by one using CRISPR technology.” The results of her study strongly pointed to one pathway, called the “hypoxia signaling pathway,” that manages how cells function in low-oxygen circumstances.
Lek’s team honed in on 8 compounds that significantly reduced cell death in cells expressing DUX4 (in a lab dish). “There were a lot more than 8 candidates” found in their study, Lek noted, but “we selected drugs with minimal side effects because we knew an FSHD treatment would require long term dosing. In an initial screen, [the 8 drugs] showed strongest inhibition of cell death.” The first drug her team decided to test was everolimus, a drug used to prevent immune rejection of transplanted organs.
Using a mouse model developed in the lab of Peter and Takako Jones, Lek’s team found that the muscles in mice treated with everolimus looked healthier. However, they did not see improvement in the animals’ performance on a treadmill and a grip strength test. She said more work needed to be done to see if the drug dosing could be optimized to produce a measurable effect in the performance tests.
Dr. Lek’s talk helps us appreciate the multitude of factors that must be considered and the painstaking experimentation that must be done to determine whether a drug might be a good candidate to treat FSHD in actual patients. With her personal dedication to solving FSHD, backed by determination and a talented team, this is a lab to keep our eyes on.