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Reduced Repeats, Unraveling the Genetic Complexities of Facioscapulohumeral muscular dystrophy (FSHD)

  • Writer: Perla Sandoval & Allison Brown
    Perla Sandoval & Allison Brown
  • Dec 19, 2025
  • 14 min read

Facioscapulohumeral muscular dystrophy (FSHD) is the third most common muscular dystrophy, affecting most skeletal muscles and originally described as impacting primarily the face (facio), shoulders (scapula), upper arms (humerus), legs, and core.


Introduction


Facioscapulohumeral muscular dystrophy (FSHD) is the third most common muscular dystrophy behind Duchenne and Becker muscular dystrophy.  There are two main forms of the disease, FSHD1 and FSHD2. FSHD1 is the most common form of FSHD, with about 95% of all FSHD cases diagnosed as FSHD Type 1 [1] [2].  Individuals with this disorder experience a range of symptoms, primarily muscle weakness, atrophy, chronic pain, and chronic fatigue.


FSHD1 and FSHD2 have identical clinical criteria, and are both caused by the presence of a toxic protein, DUX4, in skeletal muscle.  However, they have different genetic causes.  In this article, we will focus on FSHD1 and touch on FSHD2 as well.


Tell me about this rare disease.


Genetic basis: Both FSHD1 and 2 are caused by an aberrant expression of the DUX4 gene on chromosome 4, locus 4q35 [1].  This disorder is most frequently inherited in an autosomal dominant manner (70–90%), but can also be the result of a de novo mutation (10–30%) [3].  Many genes’ expressions are developmentally regulated, and this is the case for the DUX4 gene as well.  In healthy individuals, DUX4 is expressed only very early on in development and then is epigenetically silenced during the rest of development and aging [4] [5]. However, in FSHD, DUX4 expression is reactivated later in muscle development, and this reactivation causes the symptoms of the disorder.


More specifically, FSHD1 is caused by the pathogenic contraction of a macrosatellite repeat called D4Z4, which, when reduced in length, induces hypomethylation and chromatin relaxation [1]. The number of D4Z4 repeats in chromosome 4 varies between 11-110 repeats among healthy individuals. However, in FSHD1 patients this region has only 1 to 10 repeats (Figure 1) [1]. Researchers have found a correlation between the number of D4Z4 repeats and disease severity, where patients with fewer D4Z4 repeats display a more severe clinical phenotype [1] [6] [7].  However, this correlation is debated by the field. This chromatin relaxation is associated with inappropriate expression of DUX4, which induces apoptosis and inflammation in muscles. 

 

FSHD2 is the other form of FSHD and accounts for only 5% of all FSHD cases [1] [2].  This type of FSHD follows the same clinical presentation as FSHD1, however it has a digenic basis [1].  Patients with FSHD2 harbor heterozygous mutations in the SMCHD1, DNMT3B and LRIF genes, causing hypomethylation of the D4Z4 genomic region and relaxation of the chromatin, which in turn allows for the aberrant expression of DUX4 [1] [8] [9].


Figure 1: Genetic summary of FSHD1 & FSHD2 [55]
Figure 1: Genetic summary of FSHD1 & FSHD2 [55]

Clinical Presentation:  Individuals with FSHD (type 1 and 2) typically first experience symptoms of the disorder before early adulthood, with 90% of FSHD patients experiencing symptoms before age twenty and 10% of diagnosed individuals experiencing symptoms before age ten [10].


The primary symptoms of FSHD are muscle weakness and atrophy in the face (around the eyes & mouth), shoulder blades, and upper arms which then progresses to muscle degeneration in the torso (abs), lower legs, and pelvic girdle as well, (Figure 2) [10] [11].  This muscle weakness frequently occurs asymmetrically, meaning a patient may lose muscle tone in their left arm before their right, or in one side of their body more than the other [11].  The asymmetrical nature of muscle wasting in FSHD patients can be seen in Figure 2 illustrating fat infiltration in FSHD patients, where increased fat infiltration in the body leads to muscle degeneration [12].  FSHD is a progressive neuromuscular disorder.  Symptoms are expected to increase in severity as the individual ages, with around 20% of all people diagnosed with FSHD using a wheelchair by age fifty [11] [13].   


Figure 2: 3D representations generated from whole body magnetic resonance imaging (MRI) data illustrating the distribution of muscle fat infiltration in patients with FSHD. Muscles are colored from low (yellow) to high (orange/red) fat infiltration [12].
Figure 2: 3D representations generated from whole body magnetic resonance imaging (MRI) data illustrating the distribution of muscle fat infiltration in patients with FSHD. Muscles are colored from low (yellow) to high (orange/red) fat infiltration [12].

In addition to muscle degeneration, over 70% of people with FSHD also experience chronic pain and fatigue [11] [14] [15].  The most common areas of pain have been recorded in the shoulders and lower back, with 19% of individuals reporting their pain as severe [14].  This chronic pain can also begin before the onset of other muscle degeneration symptoms, and some individuals find this to be the most disabling aspect of FSHD [14]. Along with chronic pain, chronic fatigue is also something many FSHD patients experience [15].  Most of the time individuals do not know the cause of their chronic fatigue, but some individuals have identified their fatigue to be linked to muscle weakness, physical overachieving or underachieving, and stress [15].


Early-onset FSHD (symptoms starting before age five) occurs in about 5-10% of all FSHD patients [11].  Early-onset FSHD often comes with a more rapid progression of symptoms and an increase in possibility for vision or hearing issues compared to adult-onset, highlighting a significant need for early diagnosis and intervention [11].


Incidence:  FSHD affects an estimated 1 in 8,333 - 20,000 people.  It is the third most common muscular dystrophy after Duchenne and Becker muscular dystrophies [10] [11] [13].


Brief history:

  • 1885-1886: Landouzy and Dejerine coined the term “facioscapulohumeral” to differentiate the unnamed myopathy from Duchenne’s muscular dystrophy [16]

  • 1982: Padberg described 109 patients from 19 unrelated families with similar clinical presentations and named it “Facioscapulohumeral disease” [17] [18]

  • 1991: The FSH Society (now FSHD Society) was founded [17]

  • 1993: Research found 1-10 D4Z4 repeats associated with FSHD patients compared to 11-100 repeats in healthy patients [16]

  • 1999: The gene DUX4 was discovered and named [19]

  • 1999: Research found a link between a D4Z4 locus partial-deletion and FSHD [19]

  • 2001: MD-CARE Act was enacted into law, accelerating research for FSHD and other muscular dystrophies [20] [21]

  • 2007: Evidence of toxicity caused by DUX4 gene expression was published [22]

  • 2012: Mutations in the SMCHD1 gene were found to be associated with FSHD2 patients [23]

  • 2014: Study found the prevalence of FSHD to be 12/100,000 or about 1 in 8,333 people in the Netherlands [13]


The State of the Disorder Today:  Currently, there is no cure or targeted therapy for FSHD. Individuals with FSHD rely on medication and procedures to alleviate symptoms of the disease.  However, there are many drugs in the clinic and in development [24].


What is it like to be a patient with this disease?


Who are the patients?  People with FSHD come from all over the world and can be any sex, gender, or age.  While some FSHD patients may see an increase in severity of symptoms as they age, this disorder is not thought to affect lifespan.


What do current treatment options look like?  There is no cure or targeted therapy for FSHD. Available therapies help patients alleviate symptoms and improve quality of life and mobility [25].  Chronic pain and fatigue in FSHD patients are frequently overlooked and undertreated.  NSAIDs and physical therapy can be prescribed for patients with chronic pain [26].  Cognitive therapy and aerobic exercise training may improve some symptoms of chronic fatigue [15].  Patients may undergo laser photocoagulation to address retinal vascular and telangiectatic neovascular diseases that are common among FSHD patients [26].  Procedures such as Scapulothoracic fusion surgery can improve stability and mobility from weakness and pain in shoulder muscles and joints caused by FSHD [26] [27].


Are there advocacy groups?  Yes!  The FSHD Society and Friends of FSH Research are non-profit organizations dedicated to encouraging, promoting, and funding FSHD scientific and clinical research [28].  MyFSHD is dedicated to FSHD education, with the objective to inform and empower FSHD patients and their families [29].  FSHD Canada Foundation aims to spread awareness and fundraise for FSHD research [30].  Based in Australia but operating globally, the FSHD Global Research Foundation’s goal is to find a cure for FSHD by raising money to fund medical research grants [30]. 


Are there genetic tests?  Yes!  The main clinical ICD-10 code for FSHD is G71.02 and the ICD-11 code 8C70.3 may be used by clinicians to refer patients for genetic testing [31].  Formal genetic testing is required for patients to be included in therapeutic trials.  The FSHD Society offers TestFSHD, an accessible genetic testing program developed with Genome Medical that helps patients navigate insurance coverage or self-pay options, with expert genetic counselors available to guide individuals, including those who are asymptomatic but at risk, through the testing process [32] [33].  The FSHD Society also has resources on its website to help patients get their commercial test covered by insurance, especially those who may be asymptomatic but at risk of possessing or passing along FSHD [34].


How do scientists and clinicians study this disease?


Are there good/any model systems scientists can use to develop drugs?  Some common FSHD cell models used are human skeletal muscle myoblasts, some of which are derived from iPSCs or isolated from a muscle biopsy. These myoblasts are often then differentiated into myotubes [35] [36] [37].  The FLExDUX4 mouse is the most commonly used FSHD-like mouse model [38] [39] [40].  Since the D4Z4 macrosatellite region and DUX4 is only expressed in old world primates [4] [35], the DUX4 gene has been introduced to model systems such as pigs, flies [41], mice, and zebrafish (Figure 3) [42].  Recently, a large animal model for FSHD has been developed [43], and the Jones Lab is collaborating with industry partners to characterize the FSHD Göttingen minipig model and test novel therapeutics therein [44].


Figure 3: Zebrafish expressing DUX4 (on the right of each panel) exhibit muscle fiber replacement by collagen (A) and asymmetric muscle structure in the caudal fins (B) compared to wild-type zebrafish [45].
Figure 3: Zebrafish expressing DUX4 (on the right of each panel) exhibit muscle fiber replacement by collagen (A) and asymmetric muscle structure in the caudal fins (B) compared to wild-type zebrafish [45].

Have natural history studies been done?  Dr. Jeffrey Statland from the University of Kansas Medical Center and the Clinical Trial Research Network (CTRN), centers around the US, Canada, and Europe are currently recruiting patients for natural history studies entitled MOVE and MOVE+.  The overall goal of the studies is to understand how FSHD affects muscles, movement, and daily life [46] [47].  Although not exactly a natural history study, The BetterLife Research Gateway is a research portal for researchers, clinicians, and patients to access longitudinal data from FSHD patients [48].


Certain physicians, scientists or centers that are experts?  Jefferey Statland, MD leads the FSHD Clinical Trial Research Network (CTRN), which runs natural history studies and clinical trials focused on FSHD patients [49].  Johanna Hamel, MD, Associate Professor Department of Neurology, University of Rochester Medical Center runs the National Registry for FSHD Patients and Family Members, as well as the FSHD Biorepository [50].  The FSHD Research Center is a collaboration between the University of Rochester Medical Center, the Leiden University Medical Center’s Department of Human Genetics, and The Fred Hutchinson Cancer Research Institute; it aims to advance research for FSHD [51].  Additionally, Professors Peter L. Jones and Takako I. Jones co-lead an academic research laboratory at the University of Nevada, Reno School of Medicine focused on FSHD and helped create MyFSHD [52] [53].


What are the major challenges in studying and curing this disease?  DUX4 is very challenging to detect in patients with FSHD.  Novel circulating markers have been identified and must be validated, and historically, muscle biopsies have been required to detect DUX4 in patient muscle.  In addition, it can be challenging to detect consistent disease progression in FSHD patients due to the heterogeneity of muscle impairment between patients.  In preclinical models, DUX4 expression is low in control and non-patient cell models, therefore DUX4 needs to be artificially introduced into these systems [35].  Similarly, the in vivo animal models (mouse, flies, zebrafish) currently used for FSHD research do not express the D4Z4 macrosatellite region and DUX4 naturally [54].  Therefore, DUX4 and/or the D4Z4 region needs to be introduced artificially in vivo.  Meaning, these animal models do not exactly recapitulate the human FSHD disease phenotype [35].


The Cure Corner: What is needed for a cure?


What are current therapies and treatments lacking?  Currently, there is no cure, nor any targeted treatment option to stop or reverse muscle degeneration due to FSHD.  Available treatment options only manage symptoms of the disorder [35] [55].


Are there companies already developing drugs?  Many companies are currently developing drugs for FSHD.  Hoffmann-La Roche’s drug RO7204239 is in Phase 2 clinical trials. The drug inhibits production of myostatin protein to aid in muscle growth [56].  Fulcrum Therapeutics has been the closest company to achieve approval of a therapeutic for FSHD with its clinical trial for the small molecule losmapimod reaching Phase 3.  Unfortunately, it was discontinued in September 2024 due to failure to show significant difference in functional outcomes between placebo and treatment groups [57].  While losmapimod was theorized to work by reducing DUX4 through an indirect pathway [58], there are drugs in the clinic that attempt to directly target DUX4, such as Avidity Biosciences’ Del-brax siRNA drug, which we will discuss in the next section [59] [60].


Figure 4:  Avidity Biosciences’ Del-brax, is designed to reduce expression of DUX4 mRNA and DUX4 protein in FSHD patients [66].
Figure 4:  Avidity Biosciences’ Del-brax, is designed to reduce expression of DUX4 mRNA and DUX4 protein in FSHD patients [66].

Could an RNA therapeutic fit the need?  RNA therapeutics are a promising option for a targeted FSHD therapeutic, showing preclinical efficacy by reducing DUX4 expression in in vitro and in vivo studies (Figure 5) [35].  There are two siRNA therapeutics that are currently in early-stage clinical trials [35].  The first is Avidity Biosciences’ Del-brax, Figure 4, an siRNA conjugated to a monoclonal antibody designed to target transferrin receptor 1 [35] [59] [60].   In June 2025, results from a Phase 2 clinical trial announced that Del-brax was shown to reduce KHDC1L protein levels in FSHD patients, a potential biomarker of DUX4 expression [61].


The second siRNA in clinical trials is ARO-DUX4, an siRNA from Arrowhead Pharmaceuticals currently in Phase 1/2 clinical trials for FSHD type 1 [62].  Additionally, other siRNAs are being investigated for preclinical efficacy, such as Dyne Therapeutics’ DYNE-302, an siRNA therapeutic conjugated to an antibody targeting transferrin receptor 1 [63].  Adeno-associated viral (AAV) vector-delivered miRNA therapeutics have also demonstrated in vivo promise, correcting DUX4-associated myopathy in mouse muscle [64].  In addition, conjugated antisense oligonucleotides have demonstrated promise in vivo, inhibiting DUX4 expression, improving muscle strength, improving locomotor activity, and ameliorating muscle atrophy in FHSD-like mice (FLExDUX4) [65].



Figure 5: RNA therapeutics such as antisense oligonucleotides and siRNAs are promising therapeutics for FSHD through downregulation of DUX4 [35].
Figure 5: RNA therapeutics such as antisense oligonucleotides and siRNAs are promising therapeutics for FSHD through downregulation of DUX4 [35].

 

Conclusion


While FSHD is the third most common muscular dystrophy, there is still much to be discovered about this disorder and the lives of FSHD patients.  Currently, this indication lacks a targeted therapeutic. However, preclinical and clinical research have shown RNA therapeutics that downregulate DUX4 are possible future options for patients with FSHD.  With many drugs for FSHD in clinical development, and several academic labs conducting FSHD research, our hopes are high that soon there will be a targeted therapeutic for FSHD.

 

 

A very special thanks to the FSHD Society, Lucienne Ronco, Amanda Hill, and Ashley Ferreira for taking the time to review our article prior to publication.



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[64] Wallace LM, Liu J, Domire JS, et al. RNA interference inhibits DUX4-induced muscle toxicity in vivo: implications for a targeted FSHD therapy. Mol Ther. 2012;20(7):1417-1423. doi:10.1038/mt.2012.68


[65] Lu-Nguyen N, Malerba A, Herath S, Dickson G, Popplewell L. Systemic antisense therapeutics inhibiting DUX4 expression ameliorates FSHD-like pathology in an FSHD mouse model. Hum Mol Genet. 2021;30(15):1398-1412. doi:10.1093/hmg/ddab136


 
 
 

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