Ginny Morriss, Ph.D., is exploring whether reducing levels of the CELF1 protein, which are abnormally high in DM1-affected skeletal muscles, has a positive effect on these muscles. She’ll be studying mice in which disease-causing repeat expansions can be induced at any age.

The expansion of CTG repeats in the DMPK gene on chromosome 19 has been understood to be the molecular cause of DM1 since the mid-1990s. More recently, studies from many laboratories around the world have shed light on the possible mechanisms by which these expanded repeats lead to skeletal muscle wasting and defects in multiple other body systems.

Two mechanisms that help explain DM1-related muscle wasting are the sequestration of MBNL1 protein by expanded CUG RNA repeats in skeletal muscle cells, with a resulting depletion of this critical splicing factor in these cells; and higher-than-normal levels of CELF1 protein in skeletal muscle cells, perhaps because its half-life is prolonged as yet another effect of expanded CUG RNA repeats.

The effects of MBNL1 depletion and the possible therapeutic effect of its replacement have been extensively explored in mouse studies. The effects of CELF1 overabundance and the possible therapeutic effect of its reduction are now receiving increasing scrutiny.

Exploring Muscle Development, Diseases

"Everything you do in life, whether you’re an elite athlete or just trying to walk up the stairs, requires muscles that function properly and grow in the right way," says Ginny Morriss, Ph.D., a postdoctoral associate in the Pathology Department at Baylor College of Medicine in Houston. "When I went for my postdoc, I was interested in muscle diseases, because there are defects in these diseases that involve some of the key developmental pathways in skeletal muscle. I got interested in the work that Dr. Tom Cooper was doing in his lab at Baylor. Not only were they studying myotonic dystrophy, but they were also delving pretty deeply into muscle development."

Dr. Morriss, now a postdoc in Dr. Cooper’s lab, has a 2016-2017 research fellowship from the UK-based Wyck Foundation in partnership with MDF to study the effects of abnormally high levels of CELF1 and of CELF1 reduction on skeletal muscles in an inducible mouse model of DM1.

"We can turn on the expanded repeats in these mice whenever we want," Morriss explains. "To mimic the congenital-onset type of DM1, we can turn them on very early, at the beginning of the postnatal period. Or we can wait until after the postnatal period of development and turn them on after that. We’re seeing clear signs of muscle wasting in these mice. What I’m proposing to do is start the expression of these repeats and then inject an adeno-associated virus with short hairpin RNA [shRNA] to CELF1 once the process of wasting has been established."

Normalizing MBNL1, CELF1 Levels

Morriss believes that reducing CELF1 levels and/or raising MBNL (muscleblind) protein levels have potential as DM1 therapies. "CELF1 overexpression and a decrease in muscleblind levels, primarily from muscleblind 1 and muscleblind 2 knockdown models, result in significant muscle wasting in mice," she says. "So restoring those proteins to the levels where they should be could be helpful." That’s something other investigators are working on, she notes.

She thinks it would probably be feasible to develop agents to increase MBNL1 and reduce CELF1 in patients. "Using adeno-associated virus [AAV] gene delivery is a relatively straightforward process that is being used extensively in a lot of clinical trials for other diseases," she says, noting that she counted 163 clinical trials that are now using AAV technology. "I think it is feasible to use AAV to put in muscleblind or to inject shRNAs to reduce CELF1. I think we may be able to deliver those to patients and target them pretty well. By the time we get there, a lot of the kinks will, I hope, have been worked out."

Morriss will use AAV9 to deliver shRNA to CELF1 to her mice. "It’s very highly specific for skeletal and cardiac muscle," she says, adding that reduced MBNL1 and CELF1 excess are relevant to heart as well as skeletal muscle. "For my studies, I’ll be injecting directly into the skeletal muscles, so there’s not a lot of chance of it getting up to the heart. But AAV9 also has the advantage of being good for systemic administration. If we wanted to use it for studying the heart phenotype in our models, we could."

Expecting to Stabilize Damage, Hoping to Reverse It

Asked whether targeting CELF1 with shRNA could actually reverse existing muscle damage, Morriss is cautious. "At this point, I’d probably say muscle damage would be closer to being stabilized than reversed. Until we know a little bit more about how well it can reverse damage in mice, we won’t know whether it might be possible to reverse it in patients. It might preserve muscle rather than reverse damage, but our goal is reversal."

Combining Therapies

Asked whether MBNL1 enhancement or CELF1 depletion or both could be useful in conjunction with therapies that target the CUG repeats, such as the antisense compound now being tested in patients, Morriss says she thinks they would be. "I think it would be helpful to have as many of these therapies as possible that are efficacious and safe, because there’s not going to be a one-size-fits-all treatment for this disease."