Loss of epigenetic silencing of the DUX4 transcription factor gene in facioscapulohumeral muscular dystrophy

Herpesviruses mimic zygotic genome activation to promote viral replication

Zygotic genome activation (ZGA) is crucial for maternal to zygotic transition at the 2-8-cell stage in order to overcome silencing of genes and enable transcription from the zygotic genome. In humans, ZGA is induced by DUX4, a pioneer factor that drives expression of downstream germline-specific genes and retroelements. Here we show that herpesviruses from all subfamilies, papillomaviruses and Merkel cell polyomavirus actively induce DUX4 expression to promote viral transcription and replication. Analysis of single-cell sequencing data sets from patients shows that viral DUX4 activation is of relevance in vivo. Herpes-simplex virus 1 (HSV-1) immediate early proteins directly induce expression of DUX4 and its target genes, which mimics zygotic genome activation. Upon HSV-1 infection, DUX4 directly binds to the viral genome and promotes viral transcription. DUX4 is functionally required for infection, since genetic depletion by CRISPR/Cas9 as well as degradation of DUX4 by nanobody constructs abrogates HSV-1 replication. Our results show that DNA viruses including herpesviruses mimic an embryonic-like transcriptional program that prevents epigenetic silencing of the viral genome and facilitates herpesviral gene expression.

Pioneer Transcription Factors: The First Domino in Zygotic Genome Activation

Zygotic genome activation (ZGA) is a pivotal event in mammalian embryogenesis, marking the transition from maternal to zygotic control of development. During the ZGA process that is characterized by the intricate cascade of gene expression, who tipped the first domino in a meticulously arranged sequence is a subject of paramount interest. Recently, Dux, Obox and Nr5a2 were identified as pioneer transcription factors that reside at the top of transcriptional hierarchy. Through co-option of retrotransposon elements as hubs for transcriptional activation, these pioneer transcription factors rewire the gene regulatory network, thus initiating ZGA. In this review, we provide a snapshot of the mechanisms underlying the functions of these pioneer transcription factors. We propose that ZGA is the starting point where the embryo's own genome begins to influence development trajectory, therefore in-depth dissecting the functions of pioneer transcription factors during ZGA will form a cornerstone of our understanding for early embryonic development, which will pave the way for advancing our grasp of mammalian developmental biology and optimizing in vitro production (IVP) techniques.

Voluntary wheel running improves molecular and functional deficits in a murine model of facioscapulohumeral muscular dystrophy

Endurance exercise training is beneficial for skeletal muscle health, but it is unclear if this type of exercise can target or correct the molecular mechanisms of facioscapulohumeral muscular dystrophy (FSHD). Using the FLExDUX4 murine model of FSHD characterized by chronic, low levels of pathological double homeobox protein 4 (DUX4) gene expression, we show that 6 weeks of voluntary, free wheel running improves running performance, strength, mitochondrial function, and sarcolemmal repair capacity, while slowing/reversing skeletal muscle fibrosis. These improvements are associated with restored transcriptional activity of gene networks/pathways regulating actin cytoskeletal signaling, vascular remodeling, inflammation, fibrosis, and muscle mass toward wild-type (WT) levels. However, FLExDUX4 mice exhibit blunted increases in mitochondrial content with training and persistent transcriptional overactivation of hypoxia, inflammatory, angiogenic, and cytoskeletal pathways. These results identify exercise-responsive and non-responsive molecular pathways in FSHD, while providing support for the use of endurance-type exercise as a non-invasive treatment option.

Pharmacotherapeutic Approaches to Treatment of Muscular Dystrophies

Muscular dystrophies are a heterogeneous group of genetic muscle-wasting disorders that are subdivided based on the region of the body impacted by muscle weakness as well as the functional activity of the underlying genetic mutations. A common feature of the pathophysiology of muscular dystrophies is chronic inflammation associated with the replacement of muscle mass with fibrotic scarring. With the progression of these disorders, many patients suffer cardiomyopathies with fibrosis of the cardiac tissue. Anti-inflammatory glucocorticoids represent the standard of care for Duchenne muscular dystrophy, the most common muscular dystrophy worldwide; however, long-term exposure to glucocorticoids results in highly adverse side effects, limiting their use. Thus, it is important to develop new pharmacotherapeutic approaches to limit inflammation and fibrosis to reduce muscle damage and promote repair. Here, we examine the pathophysiology, genetic background, and emerging therapeutic strategies for muscular dystrophies.

Knocking Down DUX4 in Immortalized Facioscapulohumeral Muscular Dystrophy Patient-Derived Muscle Cells

The third most common muscular dystrophy in the world, facioscapulohumeral muscular dystrophy (FSHD), is an inherited disorder characterized by distinct asymmetric, progressive skeletal muscle weakness that begins in the upper body and spreads to other regions with age. It is caused by mutations that induce aberrant expression of the DUX4 gene in skeletal muscle. DUX4 is highly cytotoxic in skeletal muscle, dysregulating numerous signaling pathways as a result of its transcription factor activity. A promising set of approaches being developed to treat FSHD uses antisense oligonucleotides (AOs) to inhibit DUX4 transcript expression. Both steric-blocking and gapmer AOs have been shown to induce efficient DUX4 transcript knockdown in vitro and in vivo. Here, we describe a protocol that allows reliable screening of DUX4-targeting AOs through the evaluation of DUX4 transcript expression by quantitative real-time polymerase chain reaction. We also describe methods to assess the efficacy of these AOs by looking at their effect on the expression of DUX4 downstream target and potential off-target genes, as well as on the amelioration of in vitro muscle cell phenotypes.

Oncogenic lesions and molecular subtypes in adults with B-cell acute lymphoblastic leukemia

B-cell acute lymphoblastic leukemia (B-ALL), a genetically heterogeneous disease, is classified into different molecular subtypes that are defined by recurrent gene rearrangements, gross chromosomal abnormalities, or specific gene mutations. Cells with these genetic alterations acquire a leukemia-initiating ability and show unique expression profiles. The distribution of B-ALL molecular subtypes is greatly dependent on age, which also affects treatment responsiveness and long-term survival, partly accounting for the inferior outcome in adolescents and young adults (AYA) and (older) adults with B-ALL. Recent advances in sequencing technology, especially RNA sequencing and the application of these technologies in large B-ALL cohorts have uncovered B-ALL molecular subtypes prevalent in AYA and adults. These new insights supply more precise estimations of prognoses and targeted therapies informed by sequencing results, as well as a deeper understanding of the genetic basis of AYA/adult B-ALL. This article provides an account of these technological advances and an overview of the recent major findings of B-ALL molecular subtypes in adults.

The evolution of DUX4 gene regulation and its implication for facioscapulohumeral muscular dystrophy

Double homeobox 4 (DUX4) is an early embryonic transcription factor whose expression in the skeletal muscle causes facioscapulohumeral muscular dystrophy (FSHD). Despite decades of research, our knowledge of FSHD and DUX4 biology is incomplete, and the disease has currently no cures or targeted therapies. The unusual evolutionary origin of DUX4, its extensive epigenetic and post-transcriptional gene regulation, and various feedback regulatory loops that control its expression and function all contribute to the highly complex nature of FSHD pathogenesis. In this minireview, I synthesize the current state of knowledge in DUX4 and FSHD biology to highlight key areas where further research is needed to better understand DUX4 regulation. I also emphasize post-transcriptional regulation of and by DUX4 via changes in RNA and protein stability that might underlie key features of FSHD pathophysiology. Finally, I discuss the various feedback loops involved in DUX4 regulation and the context-specific consequences of its expression, which could be key to developing novel therapeutic approaches to combat FSHD.

Functions and Regulation of Endogenous Retrovirus Elements during Zygotic Genome Activation: Implications for Improving Somatic Cell Nuclear Transfer Efficiency

Endogenous retroviruses (ERVs), previously viewed as deleterious relics of ancestral retrovirus infections, are silenced in the vast majority of cells to minimize the risk of retrotransposition. Counterintuitively, bursts of ERV transcription usually occur during maternal-to-zygotic transition (MZT) in preimplantation embryos; this is regarded as a major landmark event in the zygotic genome activation (ZGA) process, indicating that ERVs play an active part in ZGA. Evolutionarily, the interaction between ERVs and hosts is mutually beneficial. The endogenization of retrovirus sequences rewires the gene regulatory network during ZGA, and ERV repression may lower germline fitness. Unfortunately, owing to various limitations of somatic cell nuclear transfer (SCNT) technology, both developmental arrest and ZGA abnormalities occur in a high percentage of cloned embryos, accompanied by ERV silencing, which may be caused by the activation failure of upstream ERV inducers. In this review, we discuss the functions and regulation of ERVs during the ZGA process and the feasibility of temporal control over ERVs in cloned embryos via exogenous double homeobox (DUX). We hypothesize that further accurate characterization of the ERV-rewired gene regulatory network during ZGA may provide a novel perspective on the development of preimplantation embryos.

Facioscapulohumeral muscular dystrophy: genetics, gene activation and downstream signalling with regard to recent therapeutic approaches: an update

Whilst a disease-modifying treatment for Facioscapulohumeral muscular dystrophy (FSHD) does not exist currently, recent advances in complex molecular pathophysiology studies of FSHD have led to possible therapeutic approaches for its targeted treatment. Although the underlying genetics of FSHD have been researched extensively, there remains an incomplete understanding of the pathophysiology of FSHD in relation to the molecules leading to DUX4 gene activation and the downstream gene targets of DUX4 that cause its toxic effects. In the context of the local proximity of chromosome 4q to the nuclear envelope, a contraction of the D4Z4 macrosatellite induces lower methylation levels, enabling the ectopic expression of DUX4. This disrupts numerous signalling pathways that mostly result in cell death, detrimentally affecting skeletal muscle in affected individuals. In this regard different options are currently explored either to suppress the transcription of DUX4 gene, inhibiting DUX4 protein from its toxic effects, or to alleviate the symptoms triggered by its numerous targets.

Genetic Approaches for the Treatment of Facioscapulohumeral Muscular Dystrophy

Facioscapulohumeral muscular dystrophy (FSHD) is an autosomal dominant disorder characterized by progressive, asymmetric muscle weakness at the face, shoulders, and upper limbs, which spreads to the lower body with age. It is the third most common inherited muscular disorder worldwide. Around 20% of patients are wheelchair-bound, and some present with extramuscular manifestations. FSHD is caused by aberrant expression of the double homeobox protein 4 (DUX4) gene in muscle. DUX4 codes for a transcription factor which, in skeletal muscle, dysregulates numerous signaling activities that culminate in cytotoxicity. Potential treatments for FSHD therefore aim to reduce the expression of DUX4 or the activity of its toxic protein product. In this article, we review how genetic approaches such as those based on oligonucleotide and genome editing technologies have been developed to achieve these goals. We also outline the challenges these therapies are facing on the road to translation, and discuss possible solutions and future directions.

DUX4 Transcript Knockdown with Antisense 2'-O-Methoxyethyl Gapmers for the Treatment of Facioscapulohumeral Muscular Dystrophy

Facioscapulohumeral muscular dystrophy (FSHD) is an autosomal dominant disorder characterized by a progressive, asymmetric weakening of muscles, starting with those in the upper body. It is caused by aberrant expression of the double homeobox protein 4 gene (DUX4) in skeletal muscle. FSHD is currently incurable. We propose to develop a therapy for FSHD using antisense 2'-O-methoxyethyl (2'-MOE) gapmers, to knock down DUX4 mRNA expression. Using immortalized patient-derived muscle cells and local intramuscular injections in the FLExDUX4 FSHD mouse model, we showed that our designed 2'-MOE gapmers significantly reduced DUX4 transcript levels in vitro and in vivo, respectively. Furthermore, in vitro, we observed significantly reduced expression of DUX4-activated downstream targets, restoration of FSHD signature genes by RNA sequencing, significant improvements in myotube morphology, and minimal off-target activity. This work facilitates the development of a promising candidate therapy for FSHD and lays down the foundation for in vivo systemic treatment studies.

Expression of Dux family genes in early preimplantation embryos

After fertilization, the zygotic genome is activated through two phases, minor zygotic activation (ZGA) and major ZGA. Recently, it was suggested that DUX is expressed during minor ZGA and activates some genes during major ZGA. However, it has not been proven that Dux is expressed during minor ZGA and functions to activate major ZGA genes, because there are several Dux paralogs that may be expressed in zygotes instead of Dux. In this study, we found that more than a dozen Dux paralogs, as well as Dux, are expressed during minor ZGA. Overexpression of some of these genes induced increased expression of major ZGA genes. These results suggest that multiple Dux paralogs are expressed to ensure a sufficient amount of functional Dux and its paralogs which are generated during a short period of minor ZGA with a low transcriptional activity. The mechanism by which multiple Dux paralogs are expressed is discussed.

Inhibition of DUX4 expression with antisense LNA gapmers as a therapy for facioscapulohumeral muscular dystrophy

Facioscapulohumeral muscular dystrophy (FSHD), characterized by progressive muscle weakness and deterioration, is genetically linked to aberrant expression of DUX4 in muscle. DUX4, in its full-length form, is cytotoxic in nongermline tissues. Here, we designed locked nucleic acid (LNA) gapmer antisense oligonucleotides (AOs) to knock down DUX4 in immortalized FSHD myoblasts and the FLExDUX4 FSHD mouse model. Using a screening method capable of reliably evaluating the knockdown efficiency of LNA gapmers against endogenous DUX4 messenger RNA in vitro, we demonstrate that several designed LNA gapmers selectively and effectively reduced DUX4 expression with nearly complete knockdown. We also found potential functional benefits of AOs on muscle fusion and structure in vitro. Finally, we show that one of the LNA gapmers was taken up and induced effective silencing of DUX4 upon local treatment in vivo. The LNA gapmers developed here will help facilitate the development of FSHD therapies.

DUX4 Signalling in the Pathogenesis of Facioscapulohumeral Muscular Dystrophy

Facioscapulohumeral muscular dystrophy (FSHD) is a disabling inherited muscular disorder characterized by asymmetric, progressive muscle weakness and degeneration. Patients display widely variable disease onset and severity, and sometimes present with extra-muscular symptoms. There is a consensus that FSHD is caused by the aberrant production of the double homeobox protein 4 (DUX4) transcription factor in skeletal muscle. DUX4 is normally expressed during early embryonic development, and is then effectively silenced in all tissues except the testis and thymus. Its reactivation in skeletal muscle disrupts numerous signalling pathways that mostly converge on cell death. Here, we review studies on DUX4-affected pathways in skeletal muscle and provide insights into how understanding these could help explain the unique pathogenesis of FSHD.

Facioscapulohumeral dystrophy: activating an early embryonic transcriptional program in human skeletal muscle

Facioscapulohumeral dystrophy (FSHD) is the third most prevalent muscular dystrophy. A progressive disease, it presents clinically as weakness and wasting of the face, shoulder and upper arm muscles, with later involvement of the trunk and lower extremities. FSHD develops through complex genetic and epigenetic events that converge on a common mechanism of toxicity with mis-expression of the transcription factor double homeobox 4 (DUX4). There is currently no treatment available for FSHD. However, the consensus that ectopic DUX4 expression in skeletal muscle is the root cause of FSHD pathophysiology has allowed research efforts to turn toward cultivating a deeper understanding of DUX4 biology and the pathways that underlie FSHD muscle pathology, and to translational studies aimed at developing targeted therapeutics using ever more sophisticated cell and animal-based models of FSHD. This review summarizes recent advances in our understanding of FSHD, including the regulation and activity of DUX4 in its normal developmental roles as well as its pathological contexts. We highlight how these advances raise new questions and challenges for the field as it moves into the next decade of FSHD research.

Antisense Oligonucleotide Targeting of 3'-UTR of mRNA for Expression Knockdown

With the recent conditional approval of an antisense oligonucleotide (AON) that restores the reading frame of DMD transcript in a subset of Duchenne muscular dystrophy patients, it has been established that AONs sharing similar chemistry have clear clinical potential. Genetic diseases, such as facioscapulohumeral dystrophy (FSHD), can be the result of gain-of-function mutations. Since mRNA processing in terms of termination of transcription, its transport from the nucleus to the cytoplasm, its stability and translation efficiency are dependent on key 3'UTR elements, it follows that targeting these elements with AONs have the potential to induce gene silencing. Aberrant expression of the Double homeobox 4 (DUX4) transcription factor and the downstream consequences of such expression is the hallmark of FSHD. Here we describe the bioinformatic strategies behind the design of AONs targeting polyadenylation signals and the methodologies relevant to their in vitro screening for efficacy and safety, including analysis of expression at the transcript and protein level of the specific target and downstream genes, and measurement of the effect on the fusion index of myotubes. The targeting of permissive DUX4 and MSTN are used as examples. MSTN encodes for myostatin, a negative regulator of myogenesis; the downregulation of MSTN expression has the potential to address the muscular atrophy associated with muscular dystrophies, sarcopenia, cancer and acquired immunodeficiency syndrome.

Oncogenic Amplification of Zygotic Dux Factors in Regenerating p53-Deficient Muscle Stem Cells Defines a Molecular Cancer Subtype

The identity of tumor-initiating cells in many cancer types is unknown. Tumors often express genes associated with embryonic development, although the contributions of zygotic programs to tumor initiation and formation are poorly understood. Here, we show that regeneration-induced loss of quiescence in p53-deficient muscle stem cells (MuSCs) results in rhabdomyosarcoma formation with 100% penetrance. Genomic analyses of purified tumor cells revealed spontaneous and discrete oncogenic amplifications in MuSCs that drive tumorigenesis, including, but not limited to, the amplification of the cleavage-stage Dux transcription factor (TF) Duxbl. We further found that Dux factors drive an early embryonic gene signature that defines a molecular subtype across a broad range of human cancers. Duxbl initiates tumorigenesis by enforcing a mesenchymal-to-epithelial transition, and targeted inactivation of Duxbl specifically in Duxbl-expressing tumor cells abolishes their expansion. These findings reveal how regeneration and genomic instability can interact to activate zygotic genes that drive tumor initiation and growth.

Functional domains of the FSHD-associated DUX4 protein

Aberrant expression of the full-length isoform of DUX4 (DUX4-FL) appears to underlie pathogenesis in facioscapulohumeral muscular dystrophy (FSHD). DUX4-FL is a transcription factor and ectopic expression of DUX4-FL is toxic to most cells. Previous studies showed that DUX4-FL-induced pathology requires intact homeodomains and that transcriptional activation required the C-terminal region. In this study, we further examined the functional domains of DUX4 by generating mutant, deletion, and fusion variants of DUX4. We compared each construct to DUX4-FL for (i) activation of a DUX4 promoter reporter, (ii) expression of the DUX4-FL target gene ZSCAN4, (iii) effect on cell viability, (iv) activation of endogenous caspases, and (v) level of protein ubiquitination. Each construct produced a similarly sized effect (or lack of effect) in each assay. Thus, the ability to activate transcription determined the extent of change in multiple molecular and cellular properties that may be relevant to FSHD pathology. Transcriptional activity was mediated by the C-terminal 80 amino acids of DUX4-FL, with most activity located in the C-terminal 20 amino acids. We also found that non-toxic constructs with both homeodomains intact could act as inhibitors of DUX4-FL transcriptional activation, likely due to competition for promoter sites.

This article has an associated First Person interview with the first author of the paper.

Summary: Aberrant expression of DUX4 underlies facioscapulohumeral muscular dystrophy. This study identified functional domains of DUX4 and demonstrated that multiple pathological changes are related to DUX4-mediated transcriptional activation.

Dux4 controls migration of mesenchymal stem cells through the Cxcr4-Sdf1 axis

We performed transcriptome profiling of human immortalized myoblasts (MB) transiently expressing double homeobox transcription factor 4 (DUX4) and double homeobox transcription factor 4 centromeric (DUX4c) and identified 114 and 70 genes differentially expressed in DUX4- and DUX4c-transfected myoblasts, respectively. A significant number of differentially expressed genes were involved in inflammation, cellular migration and chemotaxis suggesting a role for DUX4 and DUX4c in these processes. DUX4 but not DUX4c overexpression resulted in upregulation of the CXCR4 (C-X-C motif Receptor 4) and CXCL12 (C-X-C motif ligand 12 also known as SDF1) expression in human immortalized myoblasts. In a Transwell cell migration assay, human bone marrow-derived mesenchymal stem cells (BMSCs) were migrating more efficiently towards human immortalized myoblasts overexpressing DUX4 as compared to controls; the migration efficiency of DUX4-transfected BMSCs was also increased. DUX4c overexpression in myoblasts or in BMSCs had no impact on the rate of BMSC migration. Antibodies against SDF1 and CXCR4 blocked the positive effect of DUX4 overexpression on BMSC migration. We propose that DUX4 controls the cellular migration of mesenchymal stem cells through the CXCR4 receptor.

Antisense Oligonucleotides Used to Target the DUX4 mRNA as Therapeutic Approaches in FaciosScapuloHumeral Muscular Dystrophy (FSHD)

FacioScapuloHumeral muscular Dystrophy (FSHD) is one of the most prevalent hereditary myopathies and is generally characterized by progressive muscle atrophy affecting the face, scapular fixators; upper arms and distal lower legs. The FSHD locus maps to a macrosatellite D4Z4 repeat array on chromosome 4q35. Each D4Z4 unit contains a DUX4 gene; the most distal of which is flanked by a polyadenylation site on FSHD-permissive alleles, which allows for production of stable DUX4 mRNAs. In addition, an open chromatin structure is required for DUX4 gene transcription. FSHD thus results from a gain of function of the toxic DUX4 protein that normally is only expressed in germ line and stem cells. Therapeutic strategies are emerging that aim to decrease DUX4 expression or toxicity in FSHD muscle cells. We review here the heterogeneity of DUX4 mRNAs observed in muscle and stem cells; and the use of antisense oligonucleotides (AOs) targeting the DUX4 mRNA to interfere either with transcript cleavage/polyadenylation or intron splicing. We show in primary cultures that DUX4-targeted AOs suppress the atrophic FSHD myotube phenotype; but do not improve the disorganized FSHD myotube phenotype which could be caused by DUX4c over-expression. Thus; DUX4c might constitute another therapeutic target in FSHD.

De novo mutations in SMCHD1 cause Bosma arhinia microphthalmia syndrome and abrogate nasal development

Bosma arhinia microphthalmia syndrome (BAMS) is an extremely rare and striking condition characterized by complete absence of the nose with or without ocular defects. We report here that missense mutations in the epigenetic regulator SMCHD1 mapping to the extended ATPase domain of the encoded protein cause BAMS in all 14 cases studied. All mutations were de novo where parental DNA was available. Biochemical tests and in vivo assays in Xenopus laevis embryos suggest that these mutations may behave as gain-of-function alleles. This finding is in contrast to the loss-of-function mutations in SMCHD1 that have been associated with facioscapulohumeral muscular dystrophy (FSHD) type 2. Our results establish SMCHD1 as a key player in nasal development and provide biochemical insight into its enzymatic function that may be exploited for development of therapeutics for FSHD.

Primary CIC-DUX4 round cell sarcoma of the kidney: A treatment-refractory tumor with poor outcome

The CIC-DUX4 sarcoma is a subset of the undifferentiated small round cell sarcoma family, presently recognized as a new clinicopathological entity. It is a rare and highly aggressive tumor usually arising in the soft parts of the limbs and the trunk. Only a very few cases of primitive visceral CIC-DUX4 have been hitherto described. We report the case of a 29 year-old male patient with a primary CIC-DUX4 sarcoma of the kidney with lung metastasis. The outcome of the disease was rapidly unfavorable. Despite radical nephrectomy, the patient experienced an early local retroperitoneal recurrence associated with lung and liver metastases. The tumor did not respond to four successive lines of chemotherapy nor to palliative radiotherapy. Due to partial morphologic and immunohistochemical overlap with Ewing sarcoma, CIC-DUX4 positive tumors have generally been considered as Ewing-like sarcomas and managed similarly. However, this tumor shows a high propensity to metastasize and is much less sensitive to chemotherapy than Ewing sarcomas. The management of this type of very aggressive sarcoma needs to be defined by comprehensive biological and clinical studies.

A complex interplay of genetic and epigenetic events leads to abnormal expression of the DUX4 gene in facioscapulohumeral muscular dystrophy

Facioscapulohumeral muscular dystrophy (FSHD), a prevalent inherited human myopathy, develops following a complex interplay of genetic and epigenetic events. FSHD1, the more frequent genetic form, is associated with: (1) deletion of an integral number of 3.3 Kb (D4Z4) repeated elements at the chromosomal region 4q35, (2) a specific 4q35 subtelomeric haplotype denominated 4qA, and (3) decreased methylation of cytosines at the 4q35-linked D4Z4 units. FSHD2 is most often caused by mutations at the SMCHD1 (Structural Maintenance of Chromosomes Hinge Domain 1) gene, on chromosome 18p11.32. FSHD2 individuals also carry the 4qA haplotype and decreased methylation of D4Z4 cytosines. Each D4Z4 unit contains a copy of the retrotransposed gene DUX4 (double homeobox containing protein 4). DUX4 gene functionality was questioned in the past because of its pseudogene-like structure, its location on repetitive telomeric DNA sequences (i.e. junk DNA), and the elusive nature of both the DUX4 transcript and the encoded protein, DUX4. It is now known that DUX4 is a nuclear-located transcription factor, which is normally expressed in germinal tissues. Aberrant DUX4 expression triggers a deregulation cascade inhibiting muscle differentiation, sensitizing cells to oxidative stress, and inducing muscle atrophy. A unifying pathogenic model for FSHD emerged with the recognition that the FSHD-permissive 4qA haplotype corresponds to a polyadenylation signal that stabilizes the DUX4 mRNA, allowing the toxic protein DUX4 to be expressed. This working hypothesis for FSHD pathogenesis highlights the intrinsic epigenetic nature of the molecular mechanism underlying FSHD as well as the pathogenic pathway connecting FSHD1 and FSHD2. Pharmacological control of either DUX4 gene expression or the activity of the DUX4 protein constitutes current potential rational therapeutic approaches to treat FSHD.

Segregation between SMCHD1 mutation, D4Z4 hypomethylation and Facio-Scapulo-Humeral Dystrophy: a case report

Background

The main form of Facio-Scapulo-Humeral muscular Dystrophy is linked to copy number reduction of the 4q D4Z4 macrosatellite (FSHD1). In 5 % of cases, FSHD phenotype appears in the absence of D4Z4 reduction (FSHD2). In 70-80 % of these patients, variants of the SMCHD1 gene segregate with 4qA haplotypes and D4Z4 hypomethylation.

Case presentation

We report a family presenting with neuromuscular symptoms reminiscent of FSHD but without D4Z4 copy reduction. We characterized the 4q35 region using molecular combing, searched for mutation in the SMCHD1 gene and determined D4Z4 methylation level by sodium bisulfite sequencing. We further investigated the impact of the SMCHD1 mutation at the protein level and on the NMD-dependent degradation of transcript.

In muscle, we observe moderate but significant reduction in D4Z4 methylation, not correlated with DUX4-fl expression. Exome sequencing revealed a heterozygous insertion of 7 bp in exon 37 of the SMCHD1 gene producing a loss of frame with premature stop codon 4 amino acids after the insertion (c.4614-4615insTATAATA). Both wild-type and mutated transcripts are detected.

Conclusion

The truncated protein is absent and the full-length protein level is similar in patients and controls indicating that in this family, FSHD is not associated with SMCHD1 haploinsufficiency.

Electronic supplementary material

The online version of this article (doi:10.1186/s12881-016-0328-9) contains supplementary material, which is available to authorized users.