Facioscapulohumeral muscular dystrophy (FSHD) is the third most common hereditary myopathy, affecting 1:15,000-20,000 people . Age of onset and disease severity is variable, although in general presentation starts in the second decade of life . FSHD derives its name from the characteristic early involvement of the muscles of the face, shoulder girdle and upper arm, which, in contrast to other muscular dystrophies, is often asymmetrical. Further in the disease course weakness progresses to the foot dorsiflexors, abdominal and proximal leg muscles . About 20% of patients become wheelchair dependent .
FSHD1 (OMIM #158900), the most common type of FSHD, is an autosomal dominant disorder caused by a contraction of the D4Z4 repeat array, a macrosatellite repeat array consisting of 3.3 kb large D4Z4 units located on chromosome 4q. FSHD patients have 1 – 10 repeat units, in contrast to unaffected individuals who have 11 – 100 units [5, 6]. Residual repeat length is roughly and inversely correlated to disease severity and onset. Patients having 1 – 3 repeats usually show infantile onset and rapid disease progression . The repeat array contraction leads to a more open D4Z4 chromatin configuration, which is hypothesized to permit transcription of otherwise epigenetically silenced genes located on chromosome 4 [5, 6]. For FSHD to develop, there also needs to be a permissive genetic background of chromosome 4, as described below. In a minority of patients with a FSHD phenotype there is no D4Z4 contraction. These patients are categorized as FSHD2 and the majority of them have a mutation in the chromatin modifier SMCHD1 [8, 9]. Both FSHD1 and FSHD2 share the above-mentioned changes in D4Z4 chromatin configuration in somatic tissue on a FSHD-permissive chromosome 4 background.
FSHD candidate genes include Double Homeobox Protein 4 (DUX4) and FSHD Region Gene 1 (FRG1). Permissive chromosome 4 backgrounds contain a polymorphic DUX4 polyadenylation signal, which facilitates ectopic expression of DUX4 in skeletal muscle upon D4Z4 chromatin opening. DUX4 is currently considered the primary mediator of FSHD pathology, and activates the p53 and caspase-3 pathways when overexpressed in mice. Both of these pathways are involved in skeletal muscle differentiation, sarcomeric protein degradation and apoptosis [10, 11]. DUX4 may also activate ubiquitin-mediated protein degradation pathways including E3 ubiquitin ligases such as Atrogin-1 and MuRF1 [12, 13]. These muscle specific ligase enzymes specifically target sarcomeric proteins and have been implicated in the atrophic phenotype in FSHD myotubes [13, 14]. FRG1 is localized to the Z-disc of the sarcomere and functions as an actin-binding and bundling protein [15, 16]. Overexpression of FRG1 in mice causes muscular dystrophy and vascular abnormalities, both features of FSHD [17, 18]. Taken together, these findings suggest that expression of FSHD candidate genes such as DUX4 or FRG1 have downstream effects on sarcomere development and sarcomere turnover.
The sarcomere – the smallest contractile unit in muscle – is composed of several proteins that work together to enable muscle contraction. Upon activation with calcium, cross-bridges are formed between actin and myosin filaments. This induces filament sliding and enables muscle contraction. Other important constituents of the sarcomere include titin, which stabilizes the sarcomere and contributes to passive force, and nebulin, which plays an important role in the regulation of actin filament length.
We hypothesize that dysfunction of sarcomeric proteins contributes to the pathogenesis of muscle weakness in FSHD. A pilot study in 4 FSHD muscle biopsies performed in our laboratory supports this hypothesis [19
]. The present study aims to confirm these data in a larger cohort and elucidate the causes of sarcomeric dysfunction in FSHD. Secondary objectives are:
To determine whether sarcomeric dysfunction is specific for FSHD, or part of a generalized pathology common to muscular dystrophy and/or inflammatory myopathy. To investigate this, patients with oculopharyngeal muscular dystrophy (OPMD) and sporadic inclusion body myositis (sIBM) will be included in the study as disease controls.
To explore other contractile properties of FSHD muscle fibers such as cross-bridge cycling kinetics, calcium sensitivity and passive force generation. With this, we aim to clarify the cause of sarcomeric dysfunction in FSHD.
Compare sarcomeric function in muscles affected early (tibialis anterior) and late (quadriceps femoris) in the disease course of FSHD to shed light on the remarkable distribution of weakness in FSHD. Also, by assessing these muscles within the same patient we will be able to determine when sarcomeric dysfunction occurs in the pathological process.
Correlate in vitro sarcomeric function to in vivo clinical parameters such as physical activity, muscle strength, disease severity, residual D4Z4 fragment length, muscle/fat fraction and quantitative muscle studies.
This study will also provide extensive new data on sarcomeric function in OPMD and sIBM. This will result in a better understanding of these disorders and generate new hypotheses for research and treatment.