Facioscapulohumeral Muscular Dystrophy: Genetics

Abstract

Facioscapulohumeral muscular dystrophy (FSHD) is the third most common inherited muscular dystrophy that presents clinically with progressive weakness of the facial, scapular and humeral muscles, with later involvement of the trunk and lower extremities. FSHD is a unique disease with complex genetic and epigenetic aetiology and the underlying biological mechanisms are still not fully deciphered. FSHD1 is more frequent (95%) and is associated with the contraction of the D4Z4 macrosatellite repeat array located on a permissive 4qA chromosome combined with decreased methylation of cytosines at the 4q35‐linked D4Z4 units. D4Z4 contraction allows epigenetic derepression of the D4Z4 array allowing the expression of the toxic DUX4 transcription factor encoded within the terminal D4Z4 repeat in skeletal muscles. FSHD2 associated with approximately 2–3% of the FSHD patients results from haploinsufficiency of the SMCHD1 gene in individuals carrying a permissive 4qA allele and marked hypomethylation of 4q and 10q D4Z4, which leads to the derepression of DUX4 hence DUX4 appears to be a key player in both types of FSHD. Currently, there is no reliable treatment for this condition, therefore for the successful development of new treatments, an integration of clinical and pathogenetic information is vital.

Key Concepts

  • FSHD is the third most common inherited muscular dystrophy characterised by a typical phenotype and involvement of specific groups of muscles.
  • FSHD is characterised by a highly variable penetrance and clinical severity.
  • In most cases, FSHD involves shortening of a repetitive macrosatellite array.
  • The D4Z4 macrosatellite linked to FSHD encodes the DUX4 transcription factor.
  • A small proportion of FSHD patients carry mutation in the SMCHD1 gene.
  • Epigenetic changes in FSHD are closely associated with molecular features.
  • FSHD is a unique disease with complex genetic and epigenetic aetiology and the underlying biological mechanisms are still not fully deciphered.
  • There is no reliable treatment for the disease.

Keywords: facioscapulohumeral dystrophy; muscular dystrophy; D4Z4 macrosatellite; DUX4; SMCHD1; epigenetics; DNA methylation

Figure 1. Restriction map of the 4q35 region: relative positions of FRG1, TUB4Q, D4F104S1 (p13E‐11) and D4Z4. (a) EcoRI fragment detected by probe p13E‐11 (b) predominantly comprises an array of 3.3 kb tandem repeats that have a copy number of 11–100 in normal controls and usually 10 or less in FSHD patients. CEN: centromere; TEL: telomere. (c) Each 3.3 kb repeat comprises two homeodomains (shaded boxes) encompassing an ORF with an in‐frame start codon (ATG) and a stop codon. It encodes the DUX4 gene. The position of the GC and TACAA boxes in the promoter‐like sequence of DUX4 gene is indicated.
Figure 2. Location of D4Z4 repeats at 4q35 and 10q26. The 10q‐derived D4Z4 repeats differ from the 4q‐derived repeats in having an internal restriction site for BlnI. Deletion of D4Z4 repeats at 4q35 is associated with FSHD, whereas reduction in the copy number of 10q‐derived D4Z4 repeats does not result in any specific phenotype.
Figure 3. Length of the smallest EcoRI fragment associated with the disease in unrelated FSHD individuals. Ten microgram DNA was digested with EcoRI, fractionated on a 0.5% agarose gel and Southern blotted. DNA on the membrane was hybridised with radio‐labelled DNA probe p13E‐11. Lane 1 contains high molecular weight markers. Lanes 2–12 contain DNA samples from unrelated FSHD patients. Lane 5 contains DNA from an FSHD patient in whom the smallest EcoRI fragment is >48 kb. The corresponding smallest EcoRI fragment size is written at the top of each lane. The 8.9 kb fragment represents a Y‐specific sequence.
Figure 4. Subtelomeric sequence exchange between 4q35 and 10q26 in 20% of the normal population. In the control population, 80% of individuals carry a standard configuration, with 4‐type repeats on chromosome 4, and 10‐type repeats on chromosome 10. In 10% of individuals, 4‐derived repeats are also present on one of their chromosomes 10. Likewise, 10% of the control population carry 10‐derived repeats on one of their chromosomes 4. (a) 5 µg high molecular weight DNA digested sequentially with EcoRI and EcoRI/BlnI, and hybridised with probe p13E‐11. In an informative situation, four different‐sized EcoRI fragments are produced following a single digest, two derived from chromosome 4 and two from 10q. Digestion with enzyme BlnI will cleave two chromosome 10‐specific fragments. Chromosome 4‐specific fragments will be reduced by 3 kb owing to the presence of a BlnI site proximal to the first repeat but distal to the EcoRI site (d). However, in 10% of individuals (b), 4‐type repeats (BlnI resistant) are translocated to chromosome 10 and therefore with EcoRI/BlnI digestion, three alleles are seen instead of the expected two fragments. Similarly, 10% of control individuals (c) carry BlnI‐sensitive repeats on one of their chromosomes 4; therefore, with EcoRI/BlnI double digest, one allele (monosomy) is observed. Adapted from van Deutekom et al. .
Figure 5. Schematic representation of the 4q/10q loci and respective bar code used for DNA combing. (a) Representation of chromosome 4 and position of the 4q35 locus with colour schemes of the A‐ (red rectangle) and B‐ (blue rectangle) type alleles from the telomere (red arrows) to the proximal 4q35‐specific sequences. D4Z4 elements are represented by green arrows. (b) Barcode used to distinguish 4qA and 4qB alleles on combed DNA molecules. The size of signals is proportional to the size for the DNA fragment hybridised by the probe, allowing precise measurement of the D4Z4 array. (c) Representation of chromosome 10 and position of the 10q26 locus with colour scheme of the A‐ (red rectangle) and B‐ (blue rectangle) type alleles. (d) Barcode used to distinguish 10qA from 10qB alleles.
Figure 6. Schematic representation the most proximal and most distal D4Z4 elements and position of sites for which DNA methylation was analysed. In initial analyses, D4Z4 methylation was analysed after digestion with methylation‐sensitive restriction enzymes and hybridisation with the p13E11 probe located upstream of the first D4Z4 unit allowing the analysis of individual CG sites in the first repeat (upper panel). Different restriction sites have been tested (SmaI, FspI, BsaAI, FseI, CpoI). Position of the sites are indicated. The site showing the most significant hypomethylation in FSHD1 and FSHD2 patient is FseI (GGCCGGCC) showing an average decrease of 15–20% in FSHD1 patients, 35% in FSHD2 patients compared to controls. Hypomethylation is more moderate at the BsaAI site (YACGTR, 15% in FSHD1, 20–25% in FSHD2). More recently, DNA methylation has been assessed by sodium bisulfite sequencing with different primers encompassing all D4Z4 elements (DR1, 5P, BSS 5' DUX4, Mid, 3P) or specific to the end of the last repeat and adjacent 4qA sequence (4qA BS). Hypomethylated regions are indicated by light grey circles.
Figure 7. Model proposed for FSHD pathogenesis. In FSHD1 (left), patients carry a D4Z4 array of reduced size (between 1 and 10 repeated units) leading to hypomethylation, reduced SMCHD1 binding, chromatin decompaction and transcriptional activation of the DUX4 retrogene (Dux4‐fl transcript) encoded by the last D4Z4 repeat and adjacent A‐type allele containing a polyadenylation site (pLAM element). A large proportion of FSHD2 patients (right) carry mutation in SMCHD1 leading either to haploinsufficiency or loss of function. As in FSHD1, decreased SMCHD1 binding is associated with a marked hypomethylation and DUX4‐fl production. The DUX4 retrogene encodes a transcription factor containing two homeodomains able to activate expression of a number of genes leading to muscle defect.
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Magdinier, Frédérique, and Upadhyaya, Meena(Sep 2018) Facioscapulohumeral Muscular Dystrophy: Genetics. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0005915.pub3]