The field has made astonishingly fast progress
by Charis Himeda, PhD, University of Nevada, Reno
Nearly 150 researchers, clinicians, patients, and industry partners convened at the FSH Society’s 2018 International Research Congress, held in Las Vegas on June 8-9, 2018. The workshop reflected the major focus of the field: building on key discoveries in recent years in an effort to uncover targets for therapy.
Over the past decade, research has shown that FSHD is caused by dysregulation of the disease locus, either by a contraction of the D4Z4 repeat array on chromosome 4 (FSHD1) or by mutations in proteins required for normal silencing of the D4Z4 repeat region (FSHD2). Both cases lead to aberrant expression of the DUX4 gene, which is stably expressed from the distal-most repeat unit of the array. The DUX4 protein, in turn, activates a host of genes normally expressed in early development, which cause pathology when expressed in adult skeletal muscle. This model of FSHD pathogenesis has achieved widespread consensus in the field, stimulating the search for therapeutic targets both upstream and downstream of DUX4.
Indeed, several companies have been aggressively pursuing DUX4-based targets for several years. Following up on large-scale screens for drugs that inhibit DUX4 activity, Fulcrum Therapeutics, Novartis, and Genea Biocells are now engaged in preclinical validation of their small molecule targets.
While unbiased screens can be profitable, more specific targets might be revealed through a better understanding of the factors controlling DUX4 expression in both health and disease. To this end, Amy Campbell, PhD, and Stephen Tapscott, MD PhD, of the Fred Hutchinson Cancer Research Center in Seattle have analyzed the host of proteins binding D4Z4 repeats and identified the Nucleosome Remodeling Deacetylase and Chromatin Assembly Factor 1 complexes as normal repressors of DUX4 expression in healthy muscle cells.
Scott Harper, PhD, and his group at Nationwide Children’s Hospital in Columbus, Ohio, are identifying the proteins that interact with DUX4, and the ways in which DUX4 is modified inside cells. In an effort to understand the series of events leading to muscle pathology, Silvère van der Maarel, PhD, and colleagues at the Leiden University Medical Center in the Netherlands have examined the spectrum of mRNA transcripts in single muscle cells from patients. They have found a small population of cells with an FSHD-specific gene expression signature and developed a way to model changes in this signature over time.
Studying the biology of primary muscle cells is very informative, but these cells can only be grown in a culture dish, away from the normal physiological signals and connections that researchers would see inside a patient. Fortunately, there are now several animal models of FSHD in which levels of DUX4 expression can be controlled by the investigators (Jones, Harper, and Kyba labs), or in which patient cells have been xenografted into mouse limbs (Bloch and Emerson labs).
Importantly, every disease model comes with its own set of advantages and limitations, but each is capable of great utility when used to answer appropriate questions. For example, human xenografts in mice contain a patient’s cells with endogenous D4Z4 arrays, making them well suited to address the regulation of these arrays in the context of living tissue. However, xenografted mice are immunocompromised and thus not equipped to provide information relating to the effects of DUX4 on the immune system, or vice versa.
Genetics and improved diagnostics
In the realm of diagnostics, Frank Baas, MD PhD, and colleagues at Leiden University Medical Center, along with Frédérique Magdinier and colleagues at Marseille Medical Genetics, have validated and used an enhanced molecular combing technique for visualizing the FSHD locus. Giancarlo Deidda, PhD, and colleagues at the Institute of Cell Biology and Neurobiology, National Research Council of Italy (CNR), Rome, are integrating sequence information with DNA methylation—a repressive mark which is lost at the disease locus in FSHD—for more precise genotyping.
Sabrina Sacconi, MD PhD, and colleagues at Nice University Hospital in France are continuing their study comparing patients with FSHD1 and FSHD2, showing that the two forms of the disease are not distinct, but form a genetic continuum in which molecular defects correlate with disease severity. In a study of a subgroup of these patients, Richard Lemmers, PhD, of Leiden University Medical Center demonstrated that for FSHD2 patients with unusually long repeat lengths, small D4Z4 array duplications explain the increased susceptibility to disease.
FSHD2 is commonly caused by mutations in SMCHD1, a protein that silences repetitive DNA. Interestingly, mutations in this protein can also lead to arhinia, a rare developmental disorder characterized by the complete absence of an external nose. Several groups (Shaw, Talkowski, Van der Maarel, Van Engelen, and Blewitt labs) are now investigating how similar mutations in the same gene can lead to strikingly different diseases; their findings should aid the understanding and treatment of both conditions.
Progress toward clinical trials
Meanwhile, the search for robust and reliable biomarkers and outcome measures continues. In collaboration with Fulcrum Therapeutics, the FSHD Clinical Trial Research Network is now using a 3D camera to document “reachable workspace” (volume of space people can reach with their arms) as a measure of upper body function.
Baziel van Engelen, MD PhD, and colleagues at Radboud University Medical Centre in the Netherlands showed that muscle MRI is a promising biomarker of disease severity, with the ability to detect muscle dysfunction prior to the appearance of symptoms. Electrical impedance myography (Rabi Tawil, MD; Jeffrey Statland, MD PhD; and colleagues), expression of DUX4 target genes (Fulcrum Therapeutics), and the myostatin pathway (Julie Dumonceaux, PhD, and colleagues) are also potential biomarkers, once drug candidates are ready for clinical trials.
In the clinical trial arena, Statland presented the positive results of a Phase 2 trial on ACE-083, a myostatin inhibitor developed by Acceleron that increases muscle mass and may improve patient strength in multiple diseases of muscle weakness.
Understanding and curing a disease requires tremendous work on multiple fronts. Once the root cause is identified, many hurdles must be overcome before a treatment reaches the clinic. With this in mind, it is astonishing how far FSHD research has progressed in a relatively short amount of time. Ten years ago, the cause of FSHD was still unknown, and now we have an established model of pathogenesis, and many viable therapeutic candidates.
Although the life cycle of DUX4 in FSHD is being revealed—from the failure of its upstream regulatory mechanisms, through its expression and activation of an abnormal program of gene expression, to how that program causes muscle disease—many questions remain. Why is FSHD a late-onset disease? What causes the variability in disease manifestation, severity, and progression? Why aren’t all muscles affected equally? Answering these and other questions will surely uncover better and more specific targets for therapy.
Editor’s note: Charis Himeda, PhD, is a research assistant professor in the Department of Pharmacology at the University of Nevada, Reno, School of Medicine.