A multinational study on motor function in early-onset FSHD

Age at onset mediates genetic impact on disease severity in facioscapulohumeral muscular dystrophy

Facioscapulohumeral muscular dystrophy type 1 (FSHD1) patients exhibit marked variability in both age at onset (AAO) and disease severity. Early onset FSHD1 patients are at an increased risk of severe weakness, and early onset has been tentatively linked to the length of D4Z4 repeat units (RUs) and methylation levels. The present study explored potential relationships among genetic characteristics, AAO and disease severity in FSHD1. This retrospective and observational cohort study was conducted at the Fujian Neuromedical Centre (FNMC) in China. Genetically confirmed participants with FSHD1 recruited from 2001 to 2023 underwent distal D4Z4 methylation assessment. Disease severity was assessed by FSHD clinical score, age-corrected clinical severity score (ACSS) and onset age of lower extremity involvement. Mediation analyses were used to explore relationships among genetic characteristics, AAO and disease severity. Finally, machine learning was employed to explore AAO prediction in FSHD1. A total of 874 participants (including 804 symptomatic patients and 70 asymptomatic carriers) were included. Multivariate Cox regression analyses indicated that male gender, low DUZ4 RUs, low CpG6 methylation levels, non-mosaic mutation and de novo mutation were independently associated with early onset in FSHD1. Early onset patients (AAO < 10 years) had both a significantly higher proportion and an earlier median onset age of lower extremity involvement compared to the typical adolescent onset (10 ≤ AAO < 20 years), typical adult onset (20 ≤ AAO < 30 years) and late onset (AAO ≥ 30 years) subgroups. AAO was negatively correlated with both clinical score and ACSS. We found that AAO exerted mediation effects, accounting for 12.2% of the total effect of D4Z4 RUs and CpG6 methylation levels on ACSS and 38.6% of the total effect of D4Z4 RUs and CpG6 methylation levels on onset age of lower extremity involvement. A random forest model that incorporated variables including gender, age at examination, inheritance pattern, mosaic mutation, D4Z4 RUs and D4Z4 methylation levels (at CpG3, CpG6 and CpG10 loci) performed well for AAO prediction. The predicted AAO (pAAO) was negatively correlated with ACSS (Spearman's ρ = -0.692). Our study revealed independent contributions from D4Z4 RUs, D4Z4 methylation levels, mosaic mutation and inheritance pattern on AAO variation in FSHD1. AAO mediates effects of D4Z4 RUs and methylation levels on disease severity. The pAAO values from our random forest model informatively reflect disease severity, offering insights that can support efficacious patient management.

Long-read Nanopore sequencing identified D4Z4 contractions in patients with facioscapulohumeral muscular dystrophy

Facioscapulohumeral muscular dystrophy (FSHD) is a genetic muscle disorder caused by abnormal expression of the DUX4 protein, commonly resulting from a contraction of D4Z4 repeat units with the presence of a polyadenylation (polyA) signal. More than 10 units of the D4Z4 repeat, with a length of 3.3 kb per unit, are typically required to silence DUX4 expression. Consequently, molecular diagnosis of FSHD is challenging. We used Oxford Nanopore technology to perform whole-genome sequencing of seven unrelated patients with FSHD, their six unaffected parents, and 10 unaffected controls. All seven patients were successfully identified to harbor one to five D4Z4 repeat units and the polyA signal, whereas none of the 16 unaffected individuals met the molecular diagnostic criteria. Our newly developed method provides a straightforward and powerful molecular diagnostic tool for FSHD.

Systemic Manifestations and Symptom Burden of Facioscapulohumeral Muscular Dystrophy in a Referral Cohort

INTRODUCTION/AIMS

The full spectrum of the clinical phenotype of facioscapulohumeral muscular dystrophy (FSHD), beyond skeletal muscle weakness, remains poorly characterized. In this study, we describe systemic manifestations and symptom burden in a large series of FSHD patients.

METHODS

We performed a retrospective chart review of FSHD patients seen at our institution between 2000 and 2017. We reviewed patients' responses to a comprehensive review of symptoms and the results of diagnostic testing for sensorineural hearing loss, cardiac disease, dysphagia, ocular abnormalities, and respiratory insufficiency. We assessed the association between disease manifestations and age of onset, genetic profile, and disease duration.

RESULTS

We identified 87 patients with FSHD. The most common reported symptoms included pain (71%), difficulty sleeping (41%), headaches (27%), and altered mood (24%). When tested, 7/16 (44%) patients had sensorineural hearing loss, 20/60 (33%) had cardiac arrhythmias or conduction defects, 17/45 (38%) had echocardiogram abnormalities, 12/25 (48%) had reduced forced vital capacity, and 4/10 (40%) had oropharyngeal dysphagia. However, patients with these abnormalities represented 8%, 23%, 20%, 14%, and 5% of total number of patients respectively, as uniform screening was lacking. Ocular pathology attributable to FSHD was not detected.

DISCUSSION

FSHD demonstrates a broad clinical phenotype. Increased vigilance among neurologists to screen for systemic manifestations of the disease is warranted. More uniform screening and future population-based studies are needed to compare the findings in FSHD patients to the general population.

Prevalence and disease progression of genetically-confirmed facioscapulohumeral muscular dystrophy type 1 (FSHD1) in China between 2001 and 2020: a nationwide population-based study

BACKGROUND

Facioscapulohumeral muscular dystrophy type 1 (FSHD1) is a rare disease, which is often underdiagnosed due to its heterogeneous presentations and complex molecular genetic basis, leading to a lack of population-based epidemiology data, especially of prevalence and disease progression.

METHODS

Fujian Neuromedical Centre (FNMC) is a diagnosis centre for clinical-genetic FSHD in China, and the only one employing pulsed-field gel electrophoresis (PFGE)-based Southern blotting for all FSHD1 genetic tests. Three sources distributed across all six spatial zones in China, were used to obtain information regarding FSHD1 events, namely, FNMC, Genetic and Myopathy Group (branches of the Neurology Society of the Chinese Medical Association), and "FSHD-China" (an organization supported by FSHD patients). During 2001-2020, all genetically-confirmed FSHD1 from China were registered in FNMC. Follow-up was conducted in the 20-year period to obtain data on disease progression, which was mainly described in terms of independent ambulation loss.

FINDINGS

Of the 1,744 FSHD1 genetic tests (total test number 1,802) included in the analysis, 997 (57.2%) patients from 620 families were diagnosed with FSHD1. The estimated prevalence of genetically-confirmed FSHD1 in China is 0.75 per million (95% confidence interval [CI], 0.70-0.79) during 2001-2020, with 0.78 (95% CI, 0.72-0.85) in males and 0.71 (95% CI, 0.65-0.78) in females. The estimated prevalence increased from 0.22 (95% CI, 0.19-0.26) per million in 2001-2015 to 0.53 (95% CI, 0.49-0.57) per million in 2016-2020 (p < 0.001). The prevalence in Fujian province was 7.10 per million, 4.66 per million, and 2.44 per million, during 2001-2020, 2001-2015, and 2016-2020, respectively. Among the 861 symptomatic plus asymptomatic patients of the total 997 patients, the median onset age at first-ever muscle weakness was 16 years of age (range 1-81); the median number of contracted D4Z4 repeats was 5 units (range 1-9); the median 4qA-allele-specific methylation level was 41% (range 14%-69%). Of the 977 symptomatic patients followed-up during 2001-2020, 117 patients (12.0%) lost independent ambulation. The expected duration from onset of first-ever muscle weakness to onset of independent ambulation loss was 40 years. The group with loss of independent ambulation had a smaller number of contracted D4Z4 repeats (p < 0.001) and had an earlier onset age of first-ever muscle weakness (p < 0.001) compared to the group without loss of independent ambulation.

INTERPRETATION

Our research captures the largest genetically-confirmed FSHD1 population worldwide, to calculate its prevalence of 0.75 per million in China from 2001 to 2020. Approximately 12.0% of symptomatic plus asymptomatic patients of FSHD1 will lose independent ambulation in 40 years from onset of first-ever muscle weakness.

FUNDING

This work has been supported by the grants (U2005201, 81870902, N.W.) and (81974193, 81671237, Z.Q.W.) from the National Natural Science Foundation of China; Joint Funds for the Innovation of Science and Technology of Fujian Province (2018Y9082) (N.W.), and the Key Clinical Specialty Discipline Construction Program of Fujian (N.W.).

Natural History of Facioscapulohumeral Dystrophy in Children: A 2-Year Follow-up

Background and Objectives

Data on the natural history of facioscapulohumeral dystrophy (FSHD) in childhood are limited and critical for improved patient care and clinical trial readiness. Our objective was to describe the disease course of FSHD in children.

Methods

We performed a nationwide, single-center, prospective cohort study of FSHD in childhood assessing muscle functioning, imaging, and quality of life over 2 years of follow-up.

Results

We included 20 children with genetically confirmed FSHD who were 2 to 17 years of age. Overall, symptoms were slowly progressive, and the mean FSHD clinical score increased from 2.1 to 2.8 (p = 0.003). The rate of progression was highly variable. At baseline, 16 of 20 symptomatic children had facial weakness; after 2 years, facial weakness was observed in 19 of 20 children. Muscle strength did not change between baseline and follow-up. The most frequently and most severely affected muscles were the trapezius and deltoid. The functional exercise capacity, measured with the 6-minute walk test, improved. Systemic features were infrequent and nonprogressive. Weakness-associated complications such as lumbar hyperlordosis and dysarthria were common, and their prevalence increased during follow-up. Pain and fatigue were frequent complaints in children, and their prevalence also increased during follow-up. Muscle ultrasonography revealed a progressive increase in echogenicity.

Discussion

FSHD in childhood has a slowly progressive but variable course over 2 years of follow-up. The most promising outcome measures to detect progression were the FSHD clinical score and muscle ultrasonography. Despite this disease progression, an improvement on functional capacity may still occur as the child grows up. Pain, fatigue, and a decreased quality of life were common symptoms and need to be addressed in the management of childhood FSHD. Our data can be used to counsel patients and as baseline measures for treatment trials in childhood FSHD.

Pathomechanisms and biomarkers in facioscapulohumeral muscular dystrophy: roles of DUX4 and PAX7

Facioscapulohumeral muscular dystrophy (FSHD) is characterised by progressive skeletal muscle weakness and wasting. FSHD is linked to epigenetic derepression of the subtelomeric D4Z4 macrosatellite at chromosome 4q35. Epigenetic derepression permits the distal-most D4Z4 unit to transcribe DUX4, with transcripts stabilised by splicing to a poly(A) signal on permissive 4qA haplotypes. The pioneer transcription factor DUX4 activates target genes that are proposed to drive FSHD pathology. While this toxic gain-of-function model is a satisfying "bottom-up" genotype-to-phenotype link, DUX4 is rarely detectable in muscle and DUX4 target gene expression is inconsistent in patients. A reliable biomarker for FSHD is suppression of a target gene score of PAX7, a master regulator of myogenesis. However, it is unclear how this "top-down" finding links to genomic changes that characterise FSHD and to DUX4. Here, we explore the roles and interactions of DUX4 and PAX7 in FSHD pathology and how the relationship between these two transcription factors deepens understanding via the immune system and muscle regeneration. Considering how FSHD pathomechanisms are represented by "DUX4opathy" models has implications for developing therapies and current clinical trials.

Reliability and validity of the FSHD-composite outcome measure in childhood facioscapulohumeral dystrophy

This study aims to investigate intra-rater reliability and construct validity of the Facioscapulohumeral Dystrophy Composite Outcome Measure (FSHD-COM), in childhood FSHD. Participants included eighteen children with FSHD, and matched healthy controls. Reliability data were collected from 15 participants with FSHD over two testing sessions. Validity data were collected from all participants. Participants with FSHD completed; the FSHD-COM (and modified pediatric version), Motor Function Measure-32 (MFM-32), FSHD Severity Scales, Performance of the Upper Limb 2.0, Pediatric Quality of Life™ Neuromuscular Module and pediatric FSHD Health-Index Questionnaire. Both versions of the FSHD-COM showed excellent intra-rater reliability (ICC_1,2 > 0.99, lower 95%CI > 0.98) with a Minimal Detectable Change (MDC95%) of ≤14.5%. The FSHD-COM had robust and widespread correlations with other related outcome measures. The FSHD-COM versions and 6 min walk test effectively discriminated between children with and without FSHD; the MFM-32 and 10 m walk/run test did not. Ceiling effects were not observed on either version of the FSHD-COM. Reliability and validity findings in this childhood FSHD study concord with estimates in adults. Both versions of the FSHD-COM were effective in discriminating disease in children with mild FSHD symptoms. The FSHD-COM has the potential to be a useful measure of function across the life span.

A 5-year clinical follow-up study from the Italian National Registry for FSHD

BACKGROUND

The natural history of facioscapulohumeral muscular dystrophy (FSHD) is undefined.

METHODS

An observational cohort study was conducted in 246 FSHD1 patients. We split the analysis between index cases and carrier relatives and we classified all patients using the Comprehensive Clinical Evaluation Form (CCEF). The disease progression was measured as a variation of the FSHD score performed at baseline and at the end of 5-year follow-up (ΔFSHD score).

FINDINGS

Disease worsened in 79.4% (112/141) of index cases versus 38.1% (40/105) of carrier relatives and advanced more rapidly in index cases (ΔFSHD score 2.3 versus 1.2). The 79.1% (38/48) of asymptomatic carriers remained asymptomatic. The highest ΔFSHD score (1.7) was found in subject with facial and scapular weakness at baseline (category A), whereas in subjects with incomplete phenotype (facial or scapular weakness, category B) had lower ΔFSHD score (0.6) p < 0.0001.

CONCLUSIONS

The progression of disease is different between index cases and carrier relatives and the assessment of the CCEF categories has strong prognostic effect in FSHD1 patients.

Multi-Omics Identifies Circulating miRNA and Protein Biomarkers for Facioscapulohumeral Dystrophy

The development of therapeutics for muscle diseases such as facioscapulohumeral dystrophy (FSHD) is impeded by a lack of objective, minimally invasive biomarkers. Here we identify circulating miRNAs and proteins that are dysregulated in early-onset FSHD patients to develop blood-based molecular biomarkers. Plasma samples from clinically characterized individuals with early-onset FSHD provide a discovery group and are compared to healthy control volunteers. Low-density quantitative polymerase chain reaction (PCR)-based arrays identify 19 candidate miRNAs, while mass spectrometry proteomic analysis identifies 13 candidate proteins. Bioinformatic analysis of chromatin immunoprecipitation (ChIP)-seq data shows that the FSHD-dysregulated DUX4 transcription factor binds to regulatory regions of several candidate miRNAs. This panel of miRNAs also shows ChIP signatures consistent with regulation by additional transcription factors which are up-regulated in FSHD (FOS, EGR1, MYC, and YY1). Validation studies in a separate group of patients with FSHD show consistent up-regulation of miR-100, miR-103, miR-146b, miR-29b, miR-34a, miR-454, miR-505, and miR-576. An increase in the expression of S100A8 protein, an inflammatory regulatory factor and subunit of calprotectin, is validated by Enzyme-Linked Immunosorbent Assay (ELISA). Bioinformatic analyses of proteomics and miRNA data further support a model of calprotectin and toll-like receptor 4 (TLR4) pathway dysregulation in FSHD. Moving forward, this panel of miRNAs, along with S100A8 and calprotectin, merit further investigation as monitoring and pharmacodynamic biomarkers for FSHD.

Case Studies on the Genetic and Clinical Diagnosis of Facioscapulohumeral Muscular Dystrophy

Facioscapulohumeral muscular dystrophy is the second most common adult muscular dystrophy and is caused by DUX4 protein. DUX4 is expressed when the locus on chromosome 4q35 is hypomethylated. The clinical features can be nearly pathognomonic with facial weakness, scapular winging, and abdominal weakness with a positive Beevor sign. Diagnosis of late-onset or milder disease is often more challenging. Diseases mimicking the facioscapulohumeral muscular dystrophy phenotype should be recognized. We present 6 cases to illustrate both clinical and genetic diagnostic challenges in facioscapulohumeral muscular dystrophy and provide examples on how to navigate the different steps of genetic testing.

Early-Onset Infantile Facioscapulohumeral Muscular Dystrophy: A Timely Review

Facioscapulohumeral muscular dystrophy (FSHD)-the worldwide third most common inherited muscular dystrophy caused by the heterozygous contraction of a 3.3 kb tandem repeat (D4Z4) on a chromosome with a 4q35 haplotype-is a progressive genetic myopathy with variable onset of symptoms, distribution of muscle weakness, and clinical severity. While much is known about the clinical course of adult FSHD, data on the early-onset infantile phenotype, especially on the progression of the disease, are relatively scarce. Contrary to the classical form, patients with infantile FSHD more often have a rapid decline in muscle wasting and systemic features with multiple extramuscular involvements. A rough correlation between the phenotypic severity of FSHD and the D4Z4 repeat size has been reported, and the majority of patients with infantile FSHD obtain a very short D4Z4 repeat length (one to three copies, EcoRI size 10-14 kb), in contrast to the classical, slowly progressive, form of FSHD (15-38 kb). With the increasing identifications of case reports and the advance in genetic diagnostics, recent studies have suggested that the infantile variant of FSHD is not a genetically separate entity but a part of the FSHD spectrum. Nevertheless, many questions about the clinical phenotype and natural history of infantile FSHD remain unanswered, limiting evidence-based clinical management. In this review, we summarize the updated research to gain insight into the clinical spectrum of infantile FSHD and raise views to improve recognition and understanding of its underlying pathomechanism, and further, to advance novel treatments and standard care methods.

Phenotypic Variability Among Patients With D4Z4 Reduced Allele Facioscapulohumeral Muscular Dystrophy

IMPORTANCE

Facioscapulohumeral muscular dystrophy (FSHD) is considered an autosomal dominant disorder, associated with the deletion of tandemly arrayed D4Z4 repetitive elements. The extensive use of molecular analysis of the D4Z4 locus for FSHD diagnosis has revealed wide clinical variability, suggesting that subgroups of patients exist among carriers of the D4Z4 reduced allele (DRA).

OBJECTIVE

To investigate the clinical expression of FSHD in the genetic subgroup of carriers of a DRA with 7 to 8 repeat units (RUs).

DESIGN, SETTING, AND PARTICIPANTS

This multicenter cross-sectional study included 422 carriers of DRA with 7 to 8 RUs (187 unrelated probands and 235 relatives) from a consecutive sample of 280 probands and 306 relatives from the Italian National Registry for FSHD collected between 2008 and 2016. Participants were evaluated by the Italian Clinical Network for FSHD, and all clinical and molecular data were collected in the Italian National Registry for FSHD database. Data analysis was conducted from January 2017 to June 2018.

MAIN OUTCOMES AND MEASURES

The phenotypic classification of probands and relatives was obtained by applying the Comprehensive Clinical Evaluation Form which classifies patients in the 4 following categories: (1) participants presenting facial and scapular girdle muscle weakness typical of FSHD (category A, subcategories A1-A3), (2) participants with muscle weakness limited to scapular girdle or facial muscles (category B, subcategories B1 and B2), (3) asymptomatic or healthy participants (category C, subcategories C1 and C2), and (4) participants with myopathic phenotypes presenting clinical features not consistent with FSHD canonical phenotype (category D, subcategories D1 and D2).

RESULTS

A total of 187 probands (mean [SD] age at last neurological examination, 53.5 [15.2] years; 103 [55.1%] men) and 235 relatives (mean [SD] age at last neurologic examination, 45.1 [17.0] years; 104 [44.7%] men) with a DRA with 7 to 8 RUs and a molecular diagnosis of FSHD were evaluated. Of 187 probands, 99 (52.9%; 95% CI, 45.7%-60.1%) displayed the classic FSHD phenotype, whereas 86 (47.1%; 95% CI, 39.8%-54.3%) presented incomplete or atypical phenotypes. Of 235 carrier relatives from 106 unrelated families, 124 (52.8%; 95% CI, 46.4%-59.7%) had no motor impairment, whereas a small number (38 [16.2%; 95% CI, 9.8%-23.1%]) displayed the classic FSHD phenotype, and 73 (31.0%; 95% CI, 24.7%-38.0%) presented with incomplete or atypical phenotypes. In 37 of 106 families (34.9%; 95% CI, 25.9%-44.8%), the proband was the only participant presenting with a myopathic phenotype, while only 20 families (18.9%; 95% CI, 11.9%-27.6%) had a member with autosomal dominant FSHD.

CONCLUSIONS AND RELEVANCE

This study found large phenotypic variability associated with individuals carrying a DRA with 7 to 8 RUs, in contrast to the indication that a positive molecular test is the only determining aspect for FSHD diagnosis. These findings suggest that carriers of a DRA with 7 to 8 RUs constitute a genetic subgroup different from classic FSHD. Based on these results, it is recommended that clinicians use the Comprehensive Clinical Evaluation Form for clinical classification and, whenever possible, study the extended family to provide the most adequate clinical management and genetic counseling.

Clinical and genetic features of somatic mosaicism in facioscapulohumeral dystrophy

PURPOSE

To analyse the clinical spectrum, genetic features, specific D4Z4 hypomethylation status and genotype-phenotype correlations for somatic mosaicism in facioscapulohumeral dystrophy (FSHD).

METHODS

This was a prospective, hospital-based, case-control, observational study of 35 participants with FSHD with somatic mosaicism recruited over 10 years, with 17 penetrant patients and 18 non-penetrant mutation carriers. This study also included a univariate comparison of 17 paired mosaic and non-mosaic patients with FSHD.

RESULTS

Mosaic participants with FSHD varied in age of diagnosis (median 45; range 15-65 years), muscle strength (FSHD clinical score median 0; range 0-10 points), clinical severity (age-corrected clinical severity score (ACSS) median 0; range 0-467 points), D4Z4 repeats (median 3; range 2-5 units), mosaic proportion (median 55%; range 27%-72%) and D4Z4 methylation extent (median 49.82%; range 27.17%-64.51%). The genotypic severity scale and D4Z4 methylation extent were significantly associated with ACSS (p₁=0.003; p₂=0.002). Among the matched pairs, the 17 mosaic patients had shorter D4Z4 repeats, lower FSHD clinical scores and lower ACSS than non-mosaic patients. Additionally, 34 of 35 (97%) participants carried two mosaic arrays, while a single patient had three mosaic arrays (3%). Two cases also carried four-type non-mosaic arrays on chromosome 10 (translocation configuration).

CONCLUSIONS

Broadly, this large mosaic FSHD cohort exhibited significant clinical heterogeneity and relatively slight disease severity. Both genotypic severity scale and D4Z4 hypomethylation status served as modifiers of clinical phenotypes. Consistent with previous reports, mitotic interchromosomal/intrachromosomal gene conversion without crossover was here identified as a major genetic mechanism underlying mosaic FSHD.

Beatmung bei neuromuskulären Erkrankungen

Neuromuskuläre Erkrankungen betreffen das erste und zweite Motoneuron, die peripheren Nerven, die neuromuskulären Übertragung und die Muskelzelle. Es handelt sich um eine heterogene Gruppe von erblichen, degenerativen und autoimmunen Erkrankungen. Eine korrekte diagnostische Einordnung ist erforderlich, da zentralnervöse, kardiale, endokrine und weitere Begleitsymptome vorliegen können und für einige Erkrankungen bereits medikamentöse Therapien zur Verfügung stehen. Neuromuskuläre Erkrankungen haben eine große Bedeutung in der neuromuskulären Beatmungsmedizin. Die respiratorische Symptomatik resultiert in der Regel aus Paresen der am Atmen, Schlucken oder Husten beteiligten Muskulatur mit konsekutiver ventilatorischer Insuffienz, Dysphagie bis hin zur Speichelaspiration und Sekretretention. Mittels eines strukturierte Sekretmanagements und einer effektive nichtinvasive oder invasive Beatmungstherapie können neuromuskuläre Patienten viele Jahre mit guter Lebensqualität überleben. Themen dieses Kapitels sind ein Überblick über die neuromuskulären Erkrankungen, die Indikationen und Strategien der nichtinvasiven und der invasiven Beatmung und eine ausführliche Darstellung beatmungsmedizinisch besonders relevanter neuromuskulärer Erkrankungen wie der amyotrophe Lateralsklerose, des Guillain-Barré-Syndroms, der Myasthenia gravis und der Critical-Illness-Polyneuropathie/-Myopathie.

Facioscapulohumeral Muscular Dystrophy: Update on Pathogenesis and Future Treatments

A reliable model of a disease pathomechanism is the first step to develop targeted treatment. In facioscapulohumeral muscular dystrophy (FSHD), the third most common muscular dystrophy, recent advances in understanding the complex genetics and epigenetics have led to the identification of a disease mechanism, moving the field towards targeted therapy development. FSHD is caused by expression of DUX4, a retrogene located on the D4Z4 macrosatellite repeat array on chromosome 4q35, a gene expressed in the germline but typically repressed in somatic tissue. DUX4 derepression results from opening of the chromatin structure either by contraction of the number of repeats (FSHD1) or by chromatin hypomethylation of the D4Z4 repeats resulting from mutations in SMCHD1, a gene involved in chromatin methylation (FSHD2). The resulting expression of DUX4, a transcriptional regulator, and its target genes is toxic to skeletal muscle. Efforts for targeted treatment currently focus on disrupting DUX4 expression or blocking 1 or more of several downstream effects of DUX4. This review article focuses on the underlying FSHD genetics, current understanding of the pathomechanism, and potential treatment strategies in FSHD. In addition, recent advances in the development of new clinical outcome measures as well as biomarkers, critical for the success of future clinical trials, are reviewed.