FSHD Therapeutic Strategies: What Will It Take to Get to Clinic?

Facioscapulohumeral muscular dystrophy (FSHD) is arguably one of the most challenging genetic diseases to understand and treat. The disease is caused by epigenetic dysregulation of a macrosatellite repeat, either by contraction of the repeat or by mutations in silencing proteins. Both cases lead to chromatin relaxation and, in the context of a permissive allele, pathogenic misexpression of DUX4 in skeletal muscle. The complex nature of the locus and the fact that FSHD is a toxic, gain-of-function disease present unique challenges for the design of therapeutic strategies. There are three major DUX4-targeting avenues of therapy for FSHD: small molecules, oligonucleotide therapeutics, and CRISPR-based approaches. Here, we evaluate the preclinical progress of each avenue, and discuss efforts being made to overcome major hurdles to translation.

Targeted epigenetic repression by CRISPR/dSaCas9 suppresses pathogenic DUX4-fl expression in FSHD.

Facioscapulohumeral muscular dystrophy (FSHD) is caused by incomplete silencing of the disease locus, leading to pathogenic misexpression of DUX4 in skeletal muscle. Previously, we showed that CRISPR inhibition could successfully target and repress DUX4 in FSHD myocytes. However, an effective therapy will require both efficient delivery of therapeutic components to skeletal muscles and long-term repression of the disease locus. Thus, we re-engineered our platform to allow in vivo delivery of more potent epigenetic repressors. We designed an FSHD-optimized regulatory cassette to drive skeletal muscle-specific expression of dCas9 from Staphylococcus aureus fused to HP1α, HP1γ, the MeCP2 transcriptional repression domain, or the SUV39H1 SET domain. Targeting each regulator to the DUX4 promoter/exon 1 increased chromatin repression at the locus, specifically suppressing DUX4 and its target genes in FSHD myocytes and in a mouse model of the disease. Importantly, minimizing the regulatory cassette and using the smaller Cas9 ortholog allowed our therapeutic cassettes to be effectively packaged into adeno-associated virus (AAV) vectors for in vivo delivery. By engineering a muscle-specific epigenetic CRISPR platform compatible with AAV vectors for gene therapy, we have laid the groundwork for clinical use of dCas9-based chromatin effectors in skeletal muscle disorders.

The Good, The Bad, and The Unexpected: Roles of DUX4 in Health and Disease.

In this issue of Developmental Cell, Chew et al. (2019) show that the pioneer factor DUX4 is misexpressed in tumors, where it suppresses anti-tumor immune activity. Their findings provide a new mechanism for immune evasion in cancer and highlight the pathogenic effects of re-expressing an embryonic program in adult cells.

The Genetics and Epigenetics of Facioscapulohumeral Muscular Dystrophy.

Facioscapulohumeral muscular dystrophy (FSHD), a progressive myopathy that afflicts individuals of all ages, provides a powerful model of the complex interplay between genetic and epigenetic mechanisms of chromatin regulation. FSHD is caused by dysregulation of a macrosatellite repeat, either by contraction of the repeat or by mutations in silencing proteins. Both cases lead to chromatin relaxation and, in the context of a permissive allele, aberrant expression of the DUX4 gene in skeletal muscle. DUX4 is a pioneer transcription factor that activates a program of gene expression during early human development, after which its expression is silenced in most somatic cells. When misexpressed in FSHD skeletal muscle, the DUX4 program leads to accumulated muscle pathology. Epigenetic regulators of the disease locus represent particularly attractive therapeutic targets for FSHD, as many are not global modifiers of the genome, and altering their expression or activity should allow correction of the underlying defect.

Identification of Epigenetic Regulators of DUX4-fl for Targeted Therapy of Facioscapulohumeral Muscular Dystrophy.

Facioscapulohumeral muscular dystrophy (FSHD) is caused by epigenetic de-repression of the disease locus, leading to pathogenic misexpression of the DUX4 gene in skeletal muscle. While the factors and pathways involved in normal repression of the FSHD locus in healthy cells have been well characterized, very little is known about those responsible for the aberrant activation of DUX4-fl in FSHD myocytes. Reasoning that DUX4-fl activators might represent useful targets for small molecule inhibition, we performed a highly targeted, candidate-based screen of epigenetic regulators in primary FSHD myocytes. We confirmed several of the strongest and most specific candidates (ASH1L, BRD2, KDM4C, and SMARCA5) in skeletal myocytes from two other unrelated FSHD1 patients, and we showed that knockdown led to reduced levels of DUX4-fl and DUX4-FL target genes, as well as altered chromatin at the D4Z4 locus. As a second mode of validation, targeting the CRISPR/dCas9-KRAB transcriptional repressor to the promoters of several candidates also led to reduced levels of DUX4-fl. Furthermore, these candidates can be repressed by different methods in skeletal myocytes without major effects on certain critical muscle genes. Our results demonstrate that expression of DUX4-fl is regulated by multiple epigenetic pathways, and they indicate viable, druggable candidates for therapeutic target development.

Large family cohorts of lymphoblastoid cells provide a new cellular model for investigating facioscapulohumeral muscular dystrophy.

Facioscapulohumeral muscular dystrophy (FSHD) is associated with aberrant epigenetic regulation of the chromosome 4q35 D4Z4 macrosatellite repeat. The resulting DNA hypomethylation and relaxation of epigenetic repression leads to increased expression of the deleterious DUX4-fl mRNA encoded within the distal D4Z4 repeat. With the typical late onset of muscle weakness, prevalence of asymptomatic individuals, and an autosomal dominant mode of inheritance, FSHD is often passed on from one generation to the next and affects multiple individuals within a family. Here we have characterized unique collections of 114 lymphoblastoid cell lines (LCLs) generated from 12 multigenerational FSHD families, including 56 LCLs from large, genetically homogeneous families in Utah. We found robust expression of DUX4-fl in most FSHD LCLs and a good correlation between DNA hypomethylation and repeat length. In addition, DUX4-fl levels can be manipulated using epigenetic drugs as in myocytes, suggesting that some epigenetic pathways regulating DUX4-fl in myocytes are maintained in LCLs. Overall, these FSHD LCLs provide an alternative cellular model in which to study many aspects of D4Z4, DUX4, and FSHD gene regulation in a background of low genetic variation. Significantly, these non-adherent immortal LCLs are amenable for high-throughput screening of potential therapeutics targeting DUX4-fl mRNA or protein expression.

Scalpel or Straitjacket: CRISPR/Cas9 Approaches for Muscular Dystrophies.

Versatility of CRISPR/Cas9-based platforms makes them promising tools for the correction of diverse genetic/epigenetic disorders. Here we contrast the use of these genome editing tools in two myopathies with very different molecular origins: Duchenne muscular dystrophy, a monogenetic disease, and facioscapulohumeral muscular dystrophy, an epigenetic disorder with unique therapeutic challenges.

CRISPR/dCas9-mediated Transcriptional Inhibition Ameliorates the Epigenetic Dysregulation at D4Z4 and Represses DUX4-fl in FSH Muscular Dystrophy.

Facioscapulohumeral muscular dystrophy (FSHD) is one of the most prevalent myopathies, affecting males and females of all ages. Both forms of the disease are linked by epigenetic derepression of the D4Z4 macrosatellite repeat array at chromosome 4q35, leading to aberrant expression of D4Z4-encoded RNAs in skeletal muscle. Production of full-length DUX4 (DUX4-fl) mRNA from the derepressed D4Z4 array results in misexpression of DUX4-FL protein and its transcriptional targets, and apoptosis, ultimately leading to accumulated muscle pathology. Returning the chromatin at the FSHD locus to its nonpathogenic, epigenetically repressed state would simultaneously affect all D4Z4 RNAs, inhibiting downstream pathogenic pathways, and is thus an attractive therapeutic strategy. Advances in CRISPR/Cas9-based genome editing make it possible to target epigenetic modifiers to an endogenous disease locus, although reports to date have focused on more typical genomic regions. Here, we demonstrate that a CRISPR/dCas9 transcriptional inhibitor can be specifically targeted to the highly repetitive FSHD macrosatellite array and alter the chromatin to repress expression of DUX4-fl in primary FSHD myocytes. These results implicate the promoter and exon 1 of DUX4 as potential therapeutic targets and demonstrate the utility of CRISPR technology for correction of the epigenetic dysregulation in FSHD.

Individual epigenetic status of the pathogenic D4Z4 macrosatellite correlates with disease in facioscapulohumeral muscular dystrophy.

BACKGROUND: Both forms of facioscapulohumeral muscular dystrophy (FSHD) are associated with aberrant epigenetic regulation of the chromosome 4q35 D4Z4 macrosatellite. Chromatin changes due to large deletions of heterochromatin (FSHD1) or mutations in chromatin regulatory proteins (FSHD2) lead to relaxation of epigenetic repression and increased expression of the deleterious double homeobox 4 (DUX4) gene encoded within the distal D4Z4 repeat. However, many individuals with the genetic requirements for FSHD remain asymptomatic throughout their lives. Here we investigated family cohorts of FSHD1 individuals who were either affected (manifesting) or without any discernible weakness (nonmanifesting/asymptomatic) and their unaffected family members to determine if individual epigenetic status and stability of repression at the contracted 4q35 D4Z4 array in myocytes correlates with FSHD disease. RESULTS: Family cohorts were analyzed for DNA methylation on the distal pathogenic 4q35 D4Z4 repeat on permissive A-type subtelomeres. We found DNA hypomethylation in FSHD1-affected subjects, hypermethylation in healthy controls, and distinctly intermediate levels of methylation in nonmanifesting subjects. We next tested if these differences in DNA methylation had functional relevance by assaying DUX4-fl expression and the stability of epigenetic repression of DUX4-fl in myogenic cells. Treatment with drugs that alter epigenetic status revealed that healthy cells were refractory to treatment, maintaining stable repression of DUX4, while FSHD1-affected cells were highly responsive to treatment and thus epigenetically poised to express DUX4. Myocytes from nonmanifesting subjects had significantly higher levels of DNA methylation and were more resistant to DUX4 activation in response to epigenetic drug treatment than cells from FSHD1-affected first-degree relatives containing the same contraction, indicating that the epigenetic status of the contracted D4Z4 array is reflective of disease. CONCLUSIONS: The epigenetic status of the distal 4qA D4Z4 repeat correlates with FSHD disease; FSHD-affected subjects have hypomethylation, healthy unaffected subjects have hypermethylation, and nonmanifesting subjects have characteristically intermediate methylation. Thus, analysis of DNA methylation at the distal D4Z4 repeat could be used as a diagnostic indicator of developing clinical FSHD. In addition, the stability of epigenetic repression upstream of DUX4 expression is a key regulator of disease and a viable therapeutic target.

Facioscapulohumeral muscular dystrophy as a model for epigenetic regulation and disease.

SIGNIFICANCE: Aberrant epigenetic regulation is an integral aspect of many diseases and complex disorders. Facioscapulohumeral muscular dystrophy (FSHD), a progressive myopathy that afflicts individuals of all ages, is caused by disrupted genetic and epigenetic regulation of a macrosatellite repeat. FSHD provides a powerful model to investigate disease-relevant epigenetic modifiers and general mechanisms of epigenetic regulation that govern gene expression. RECENT ADVANCES: In the context of a genetically permissive allele, the one aspect of FSHD that is consistent across all known cases is the aberrant epigenetic state of the disease locus. In addition, certain mutations in the chromatin regulator SMCHD1 (structural maintenance of chromosomes hinge-domain protein 1) are sufficient to cause FSHD2 and enhance disease severity in FSHD1. Thus, there are multiple pathways to generate the epigenetic dysregulation required for FSHD. CRITICAL ISSUES: Why do some individuals with the genetic requirements for FSHD develop disease pathology, while others remain asymptomatic? Similarly, disease progression is highly variable among individuals. What are the relative contributions of genetic background and environmental factors in determining disease manifestation, progression, and severity in FSHD? What is the interplay between epigenetic factors regulating the disease locus and which, if any, are viable therapeutic targets? FUTURE DIRECTIONS: Epigenetic regulation represents a potentially powerful therapeutic target for FSHD. Determining the epigenetic signatures that are predictive of disease severity and identifying the spectrum of disease modifiers in FSHD are vital to the development of effective therapies.

Myogenic enhancers regulate expression of the facioscapulohumeral muscular dystrophy-associated DUX4 gene.

Facioscapulohumeral muscular dystrophy (FSHD) is linked to epigenetic dysregulation of the chromosome 4q35 D4Z4 macrosatellite. However, this does not account for the tissue specificity of FSHD pathology, which requires stable expression of an alternative full-length mRNA splice form of DUX4 (DUX4-fl) from the D4Z4 array in skeletal muscle. Here, we describe the identification of two enhancers, DUX4 myogenic enhancer 1 (DME1) and DME2 which activate DUX4-fl expression in skeletal myocytes but not fibroblasts. Analysis of the chromatin revealed histone modifications and RNA polymerase II occupancy consistent with DME1 and DME2 being functional enhancers. Chromosome conformation capture analysis confirmed association of DME1 and DME2 with the DUX4 promoter in vivo. The strong interaction between DME2 and the DUX4 promoter in both FSHD and unaffected primary myocytes was greatly reduced in fibroblasts, suggesting a muscle-specific interaction. Nucleosome occupancy and methylome sequencing analysis indicated that in most FSHD myocytes, both enhancers are associated with nucleosomes but have hypomethylated DNA, consistent with a permissive transcriptional state, sporadic occupancy, and the observed DUX4 expression in rare myonuclei. Our data support a model in which these myogenic enhancers associate with the DUX4 promoter in skeletal myocytes and activate transcription when epigenetically derepressed in FSHD, resulting in the pathological misexpression of DUX4-fl.

Pax3 synergizes with Gli2 and Zic1 in transactivating the Myf5 epaxial somite enhancer.

Both Glis, the downstream effectors of hedgehog signaling, and Zic transcription factors are required for Myf5 expression in the epaxial somite. Here we demonstrate a novel synergistic interaction between members of both families and Pax3, a paired-domain transcription factor that is essential for both myogenesis and neural crest development. We show that Pax3 synergizes with both Gli2 and Zic1 in transactivating the Myf5 epaxial somite (ES) enhancer in concert with the Myf5 promoter. This synergy is dependent on conserved functional domains of the proteins, as well as on a novel homeodomain motif in the Myf5 promoter and the essential Gli motif in the ES enhancer. Importantly, overexpression of Zic1 and Pax3 in the 10T1/2 mesodermal cell model results in enrichment of these factors at the endogenous Myf5 locus and induction of Myf5 expression. In our previous work, we showed that by enhancing nuclear translocation of Gli factors, Zics provide spatiotemporal patterning for Gli family members in the epaxial induction of Myf5 expression. Our current study indicates a complementary mechanism in which association with DNA-bound Pax3 strengthens the ability of both Zic1 and Gli2 to transactivate Myf5 in the epaxial somite.

Analysis of muscle gene transcription in cultured skeletal muscle cells.

The mechanisms by which muscle gene expression is initiated and maintained are not fully understood. Muscle genes are regulated by combinatorial interactions between numerous transcription factors bound to enhancers and promoters, and their associated protein complexes. Among the most important are the MyoD and MEF2 transcription factor families, but dozens of other factors play important regulatory roles, and many additional transcription factors are certain to be involved. Expression of muscle-specific genes varies among different anatomical muscles and in fast- vs. slow-twitch fiber types, suggesting different mechanisms of regulation in response to diverse physiological cues. Thus, identifying novel transcriptional regulators and interactions is key to understanding how different cells establish the muscle phenotype; it is also critical for developing methods to combat diseases such as muscular dystrophy. Using Muscle creatine kinase as a model, we outline the key steps involved in identifying muscle gene control elements, their binding factors, and mechanisms of transcriptional activation and repression. The basic principles described here can also be applied to the transcriptional analysis of other cell-type specific genes.

Differentiation and fiber type-specific activity of a muscle creatine kinase intronic enhancer.

BACKGROUND: Hundreds of genes, including muscle creatine kinase (MCK), are differentially expressed in fast- and slow-twitch muscle fibers, but the fiber type-specific regulatory mechanisms are not well understood. RESULTS: Modulatory region 1 (MR1) is a 1-kb regulatory region within MCK intron 1 that is highly active in terminally differentiating skeletal myocytes in vitro. A MCK small intronic enhancer (MCK-SIE) containing a paired E-box/myocyte enhancer factor 2 (MEF2) regulatory motif resides within MR1. The SIE's transcriptional activity equals that of the extensively characterized 206-bp MCK 5'-enhancer, but the MCK-SIE is flanked by regions that can repress its activity via the individual and combined effects of about 15 different but highly conserved 9- to 24-bp sequences. ChIP and ChIP-Seq analyses indicate that the SIE and the MCK 5'-enhancer are occupied by MyoD, myogenin and MEF2. Many other E-boxes located within or immediately adjacent to intron 1 are not occupied by MyoD or myogenin. Transgenic analysis of a 6.5-kb MCK genomic fragment containing the 5'-enhancer and proximal promoter plus the 3.2-kb intron 1, with and without MR1, indicates that MR1 is critical for MCK expression in slow- and intermediate-twitch muscle fibers (types I and IIa, respectively), but is not required for expression in fast-twitch muscle fibers (types IIb and IId). CONCLUSIONS: In this study, we discovered that MR1 is critical for MCK expression in slow- and intermediate-twitch muscle fibers and that MR1's positive transcriptional activity depends on a paired E-box MEF2 site motif within a SIE. This is the first study to delineate the DNA controls for MCK expression in different skeletal muscle fiber types.

Design and testing of regulatory cassettes for optimal activity in skeletal and cardiac muscles.

Gene therapy for muscular dystrophies requires efficient gene delivery to the striated musculature and specific, high-level expression of the therapeutic gene in a physiologically diverse array of muscles. This can be achieved by the use of recombinant adeno-associated virus vectors in conjunction with muscle-specific regulatory cassettes. We have constructed several generations of regulatory cassettes based on the enhancer and promoter of the muscle creatine kinase gene, some of which include heterologous enhancers and individual elements from other muscle genes. Since the relative importance of many control elements varies among different anatomical muscles, we are aiming to tailor these cassettes for high-level expression in cardiac muscle, and in fast and slow skeletal muscles. With the achievement of efficient intravascular gene delivery to isolated limbs, selected muscle groups, and heart in large animal models, the design of cassettes optimized for activity in different muscle types is now a practical goal. In this protocol, we outline the key steps involved in the design of regulatory cassettes for optimal activity in skeletal and cardiac muscle, and testing in mature muscle fiber cultures. The basic principles described here can also be applied to engineering tissue-specific regulatory cassettes for other cell types.

KLF3 regulates muscle-specific gene expression and synergizes with serum response factor on KLF binding sites.

This study identifies KLF3 as a transcriptional regulator of muscle genes and reveals a novel synergistic interaction between KLF3 and serum response factor (SRF). Using quantitative proteomics, KLF3 was identified as one of several candidate factors that recognize the MPEX control element in the Muscle creatine kinase (MCK) promoter. Chromatin immunoprecipitation analysis indicated that KLF3 is enriched at many muscle gene promoters (MCK, Myosin heavy chain IIa, Six4, Calcium channel receptor alpha-1, and Skeletal alpha-actin), and two KLF3 isoforms are upregulated during muscle differentiation. KLF3 and SRF physically associate and synergize in transactivating the MCK promoter independently of SRF binding to CArG motifs. The zinc finger and repression domains of KLF3 plus the MADS box and transcription activation domain of SRF are implicated in this synergy. Our results provide the first evidence of a role for KLF3 in muscle gene regulation and reveal an alternate mechanism for transcriptional regulation by SRF via its recruitment to KLF binding sites. Since both factors are expressed in all muscle lineages, SRF may regulate many striated- and smooth-muscle genes that lack known SRF control elements, thus further expanding the breadth of the emerging CArGome.

Quantitative proteomic identification of MAZ as a transcriptional regulator of muscle-specific genes in skeletal and cardiac myocytes.

We identified a conserved sequence within the Muscle creatine kinase (MCK) promoter that is critical for high-level activity in skeletal and cardiac myocytes (MCK Promoter Element X [MPEX]). After selectively enriching for MPEX-binding factor(s) (MPEX-BFs), ICAT-based quantitative proteomics was used to identify MPEX-BF candidates, one of which was MAZ (Myc-associated zinc finger protein). MAZ transactivates the MCK promoter and binds the MPEX site in vitro, and chromatin immunoprecipitation analysis demonstrates enrichment of MAZ at the endogenous MCK promoter and other muscle gene promoters (Skeletal alpha-actin, Desmin, and alpha-Myosin heavy chain) in skeletal and cardiac myocytes. Consistent with its role in muscle gene transcription, MAZ transcripts and DNA-binding activity are upregulated during skeletal myocyte differentiation. Furthermore, MAZ was shown to bind numerous sequences (e.g., CTCCTCCC and CTCCACCC) that diverge from the GA box binding motif. Alternate motifs were identified in many muscle promoters, including Myogenin and MEF2C, and one motif was shown to be critical for Six4 promoter activity in both skeletal and cardiac myocytes. Interestingly, MAZ occupies and is able to transactivate the Six4 promoter in skeletal but not cardiac myocytes. Taken together, these findings are consistent with a previously unrecognized role for MAZ in muscle gene regulation.

Design of tissue-specific regulatory cassettes for high-level rAAV-mediated expression in skeletal and cardiac muscle.

Systemic delivery of recombinant adeno-associated virus (rAAV) 6 vectors mediates efficient transduction of the entire striated musculature, making this an attractive strategy for muscle gene therapy. However, owing to widespread transduction of non-muscle tissues, optimization of this method would benefit from the use of muscle-specific promoters. Most such promoters either lack high-level expression in certain muscle types or are too large for inclusion in rAAV vectors encoding microdystrophin. Here, we describe novel regulatory cassettes based on enhancer/promoter regions of murine muscle creatine kinase (CK) and alpha-myosin heavy-chain genes. The strongest cassette, MHCK7 (770 bp), directs high-level expression comparable to cytomegalovirus and Rous sarcoma virus promoters in fast and slow skeletal and cardiac muscle, and low expression in the liver, lung, and spleen following systemic rAAV6 delivery in mice. Compared with CK6, our previous best cassette, MHCK7 activity is approximately 400-, approximately 50-, and approximately 10-fold higher in cardiac, diaphragm, and soleus muscles, respectively. MHCK7 also directs strong microdystrophin expression in mdx muscles. While further study of immune responses to MHCK7-regulated microdystrophin expression is needed, this cassette is not active in dendritic cell lines. MHCK7 is thus a highly improved regulatory cassette for experimental studies of rAAV-mediated transduction of striated muscle.

Quantitative proteomic identification of six4 as the trex-binding factor in the muscle creatine kinase enhancer.

Transcriptional regulatory element X (Trex) is a positive control site within the Muscle creatine kinase (MCK) enhancer. Cell culture and transgenic studies indicate that the Trex site is important for MCK expression in skeletal and cardiac muscle. After selectively enriching for the Trex-binding factor (TrexBF) using magnetic beads coupled to oligonucleotides containing either wild-type or mutant Trex sites, quantitative proteomics was used to identify TrexBF as Six4, a homeodomain transcription factor of the Six/sine oculis family, from a background of approximately 900 copurifying proteins. Using gel shift assays and Six-specific antisera, we demonstrated that Six4 is TrexBF in mouse skeletal myocytes and embryonic day 10 chick skeletal and cardiac muscle, while Six5 is the major TrexBF in adult mouse heart. In cotransfection studies, Six4 transactivates the MCK enhancer as well as muscle-specific regulatory regions of Aldolase A and Cardiac troponin C via Trex/MEF3 sites. Our results are consistent with Six4 being a key regulator of muscle gene expression in adult skeletal muscle and in developing striated muscle. The Trex/MEF3 composite sequence ([C/A]ACC[C/T]GA) allowed us to identify novel putative Six-binding sites in six other muscle genes. Our proteomics strategy will be useful for identifying transcription factors from complex mixtures using only defined DNA fragments for purification.

Differences in the function of three conserved E-boxes of the muscle creatine kinase gene in cultured myocytes and in transgenic mouse skeletal and cardiac muscle.

The 1256-base pair enhancer-promoter of the mouse muscle creatine kinase gene includes three CAnnTG E-boxes that are conserved among mammals and have flanking and middle sequences conforming to consensus muscle regulatory factor binding sites. This study seeks to determine whether these E-boxes are critical for muscle creatine kinase expression in physiologically distinct muscles. Mutations of the "right" and "left" E-boxes in the enhancer decreased expression in cultured skeletal myocytes approximately 10- and 2-fold, respectively, whereas a "promoter" E-box mutation had little effect. In neonatal myocardiocytes, the left E-box mutation decreased expression approximately 3-fold, whereas right or promoter E-box mutations had no effect. Very different effects were seen in transgenic mice, where the promoter E-box mutation decreased expression in quadriceps, extensor digitorum longus, and soleus approximately 10-fold, and approximately 100-fold in distal tongue, diaphragm, and ventricle. The right E-box mutation, tested in the presence of the other two mutations, caused a significant decrease in distal tongue, but not in quadriceps, extensor digitorum longus, soleus, or ventricle. Mutation of the left E-box actually raised expression in soleus, suggesting a possible repressor role for this control element. The discrepancies between mutation effects in differentiating skeletal muscle cultures, neonatal myocardiocytes, and adult mice suggested that the E-boxes might play different roles during muscle development and adult steady-state function. However, transgenic analysis of embryonic and early postnatal mice indicated no positive role for these three E-boxes in early development, implying that differences in E-box function between adult muscle and cultured cells are the result of physiological signals.