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Review
. 2021 Aug 9;13(8):e13695.
doi: 10.15252/emmm.202013695. Epub 2021 Jun 21.

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

Christopher R S Banerji  1 , Peter S Zammit  1
Affiliations
Review

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

Christopher R S Banerji et al. EMBO Mol Med. .

Abstract

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.

Keywords: DUX4; PAX7; biomarker; facioscapulohumeral muscular dystrophy (FSHD); pathology.

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Conflict of interest statement

The authors declare that they have no conflict of interest.

Figures

Figure 1
Figure 1. Early and late skeletal muscle involvement in FSHD
Muscles/muscle groups typically affected in FSHD are colour‐coded so those that exhibit early involvement are illustrated in mauve, while those with later involvement are highlighted in blue.
Figure 2
Figure 2. Microscopic muscle pathology in FSHD
(A) Transverse section of skeletal muscle from a healthy adult stained with haematoxylin and eosin (H&E). (B) Transverse section of an FSHD skeletal muscle biopsy stained with haematoxylin and eosin showing hallmarks of FSHD, including rounded rather than polygonal shaped myofibres, increased endomysial fibrosis (yellow arrow), atrophic myofibres (asterisk), perivascular inflammation (grey arrow) and a small regenerating muscle fibre with a basophilic sarcoplasm due to increased RNA levels (black arrow). (C) FSHD muscle biopsy with a necrotic muscle fibre undergoing phagocytosis (blue arrow) and two basophilic regenerating muscle fibres (black arrows). (D) An adjacent section to that in (C) immunolabelled for developmental myosin heavy chain isoforms (Dev MyHC) using Novocastra NCL‐MHCd (Clone RNMy2/9D2). The two small myofibres with basophilic sarcoplasm identified in (C) contain developmental myosin heavy chains (black arrows), confirming that they are regenerating. Published with permission of Rabi N. Tawil.
Figure 3
Figure 3. Macroscopic muscle pathology in FSHD
MRI images from the thighs of a healthy 24‐year‐old male (A and B) and a 31‐year‐old female with FSHD1 (C and D), where white areas denote high signal intensity of the corresponding sequence. STIR images reveal a uniform signal across the thigh muscles of the healthy male (A). STIR images of the FSHD1 patient (C) display bright signals revealing muscle oedema/inflammation in parts of the vastus medialis and vastus lateralis (C – yellow arrows). The T1‐weighted image of the healthy muscle is uniformly low (B), contrasting with the high (white) signal from the subcutaneous fat (B – red arrows). Muscles of the posterior thigh of the FSHD1 patient (D) have a bright appearance using T1, indicative of fat replacement, including the semitendinosus, semimembranosus, gracilis adductor magnus and biceps femoris (D – blue arrows). The T1 signal remains dark in those portions of the vastus medialis and vastus lateralis of the FSHD1 patient that were bright with STIR imaging (compare C and D – yellow arrows), signifying that fat replacement has not occurred. Published with permission of Giorgio Tasca.
Figure 4
Figure 4. Summary of DUX4 and PAX7 target gene expression in FSHD
Graphic illustration of how DUX4 and PAX7 target genes associate with FSHD muscle. Schematic render of Fig 2B that demonstrates characteristic FSHD pathology, including endomysial and perivascular inflammation. (A and C) DUX4 target genes are only reliably detected in actively inflamed FSHD muscle biopsies using DUX4 target gene signatures. (B) DUX4 and DUX4 target genes are expressed in a small proportion of primary and immortalised myoblast cell lines derived from FSHD patients. (D) In contrast, DUX4 and DUX4 target genes are robustly expressed in FSHD patient blood‐derived immortalised B‐lymphoblastoid clones. (A and C) The PAX7 target gene signature shows that PAX7 target genes are suppressed in all FSHD muscle biopsies regardless of inflammatory status, suggesting a more muscle specific effect. (B) Suppression of PAX7 target genes marks the majority of primary and immortalised FSHD myoblast cell lines. (D) PAX7 target genes are not suppressed in the FSHD B‐lymphoblastoid clones. (E) The lymphoblast score, composed of 237 genes upregulated in immortalised FSHD B‐lymphoblastoid cell lines, distinguishes FSHD from control muscle biopsies and is strongest when MRI is used to guide biopsy selection.
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