A novel IRES identified in DMD results in a functional N-truncated dystrophin, providing a potential route to therapy for patients with 5’ mutations.
ABSTRACT: Purpose: Ribosome profiling and RNA-Seq were used to map the location and abundance of translating ribosomes on human skeletal muscle transcripts from a patient with Becker muscular dystrophy. Methods: Tissue homogenates were prepared from frozen sections of a muscle biopsy obtained from a patient with an NM_004006:c.40_41delGA dystrophin mutation and a normal control. Ribosome-protected fragments and total RNA were prepared from a single homogenate, so starting RNA populations for both libraries were closely matched. Homogenates were not clarified before RNase digestion to avoid loss of ribosomes associated with large molecular weight complexes and RNA-Seq libraries were prepared after rRNA subtraction to avoid positional loss of 5’ reads. RPF-Seq libraries were built using the TruSeq Small RNA Sample Preparation Kit (Illumina) and RNA-Seq libraries were built using the TruSeq RNA Sample Preparation v2 Kit (Illumina). RPF-Seq and RNA-Seq libraries were subjected to 50 cycles of single-end sequencing on an Illumina HiSeq 2000 instrument. Trimmed and filtered RPF- and RNA-Seq reads were mapped to RefSeq fasta sequences downloaded from the UCSC genome browser (hg19 assembly). Results: Most mutations that truncate the reading frame of the DMD gene result in loss of dystrophin expression and lead to Duchenne muscular dystrophy. However, amelioration of disease severity can result from alternate translation initiation beginning in DMD exon 6 that results in the expression of a highly functional N-truncated dystrophin. This novel isoform results from usage of an internal ribosome entry site (IRES) within exon 5 that is glucocorticoid-inducible. IRES activity is confirmed in patient muscle by both peptide sequencing and ribosomal profiling. Conclusions: Our results provide a molecular explanation for the rescue of 5’ truncating mutations via a heretofore undescribed mechanism of post-transcriptional regulation of dystrophin expression. The presence of a glucocorticoid-inducible IRES within a highly conserved region of the DMD sequence strongly suggests a programmed role for alternate translation initiation, and ongoing efforts to understand the relevant cell lineage-specific and/or conditional activation signals will shed light on underlying mechanisms of IRES control and elucidate potentially novel functions of dystrophin. Skeletal muscle ribosome-protected fragment and RNA-Seq profiles from a patient with an NM_004006:c.40_41delGA dystrophin mutation and a normal control were generated by deep sequencing using the Illumina HiSeq 2000.
Project description:Most mutations that truncate the reading frame of the DMD gene cause loss of dystrophin expression and lead to Duchenne muscular dystrophy. However, amelioration of disease severity has been shown to result from alternative translation initiation beginning in DMD exon 6 that leads to expression of a highly functional N-truncated dystrophin. Here we demonstrate that this isoform results from usage of an internal ribosome entry site (IRES) within exon 5 that is glucocorticoid inducible. We confirmed IRES activity by both peptide sequencing and ribosome profiling in muscle from individuals with minimal symptoms despite the presence of truncating mutations. We generated a truncated reading frame upstream of the IRES by exon skipping, which led to synthesis of a functional N-truncated isoform in both human subject-derived cell lines and in a new DMD mouse model, where expression of the truncated isoform protected muscle from contraction-induced injury and corrected muscle force to the same level as that observed in control mice. These results support a potential therapeutic approach for patients with mutations within the 5' exons of DMD.
Project description:Utrophin, the autosomal homologue of dystrophin can functionally compensate for dystrophin deficiency. Utrophin upregulation could therefore be a therapeutic strategy in Duchenne Muscular Dystrophy (DMD) that arises from mutation in dystrophin gene. In contrast to its transcriptional regulation, mechanisms operating at post-transcriptional level of utrophin expression have not been well documented. Although utrophin-A 5'-UTR has been reported with internal ribosome entry site (IRES), its inhibitory effect on translation is also evident. In the present study we therefore aimed to compare relative contribution of cap-independent and cap-dependent translation with mouse utrophin-A 5'-UTR through m7G-capped and A-capped mRNA transfection based reporter assay. Our results demonstrate that cap-independent translation with utrophin-A 5'-UTR is not as strong as viral IRES. However, cap-independent mode has significant contribution as cap-dependent translation is severely repressed with utrophin-A 5'-UTR. We further identified two sequence elements and one upstream open reading frame in utrophin-A 5'-UTR responsible for repression. The repressor elements in utrophin-A 5'-UTR may be targeted for utrophin upregulation.
Project description:Purpose: Ribosome profiling and RNA-Seq were used to map the location and abundance of translating ribosomes on mouse heart and skeletal muscle transcripts. Methods: Tissue was rapidly harvested and snap-frozen to minimize bias to the pool of translating ribosomes. RNA was prepared from a single homogenate for each tissue so that starting RNA populations for both libraries were closely matched. Homogenates were not clarified before RNase digestion to avoid loss of ribosomes associated with large molecular weight complexes, and RNA-Seq libraries were prepared after rRNA subtraction to avoid positional loss of 5’ reads. Trimmed reads from 50 cycles of Illumina single-end sequencing were mapped onto a non-redundant set of 18,499 mouse protein-coding RefSeq transcripts from the nuclear genome. Results: Mapped sequence reads to myosin, actin and the giant protein titin together account for ~20% of the total mRNA-derived ribosome protected fragments (RPFs). We observed large-scale uniformity in the distribution of RPFs on the >30,000 codon titin open reading frame, from which we inferred an in vivo ribosome elongation error rate of ≤10-5. Ribosome footprints on Ttn mRNA also uncovered a novel 5’ UTR within a phylogenetically conserved intronic element that would produce ~2.35 mDa titin isoform that corresponds to the titin 'T2' band frequently described as a proteolytic artifact. Local translation efficiency across several >10 kb muscle mRNAs was also uniform, while their global translation efficiencies varied by ~20-fold suggesting initiation rate plays a major role in the translation efficiency of large mRNAs. Evidence for RPFs on 5’ UTRs was widespread with particular enrichment for ribosomes positioned at CUG codons. Comparison of global translation efficiency in cardiac and skeletal muscle revealed novel examples of tissue-specific translational control including synthesis of the myogenic factor Mef2c, and the titin-binding stress response protein Ankrd23. Conclusions: Our study represents the first detailed analysis of translation in an adult mammalian tissue generated by ribosome profiling technology. Current limitations to using ribosomal profiling in tissues include unknown perturbations to the dynamic state of translation despite rapidly harvested and snap-frozen samples. The uniform 5’ to 3’ coverage observed on individual large mRNAs and the ability to observe footprints on the extremely small phospholamban coding sequence, suggests that initiation and elongation were halted on similar time scales. More detailed examination of the positional information within CDS region requires further understanding of the bias introduced during the library preparation steps for both RPF-and RNA-Seq, as well as local biases induced as translation is arrested. Despite these qualifications, this initial view of active translation in muscle tissue highlights the potential for ribosome profiling to monitor the dynamic translation response to exercise, injury or disease pathology in animal models at a level of resolution not easily attainable with other quantitative approaches. Heart and skeletal muscle ribosome-protected fragment and RNA-Seq profiles of 10-week old C57BL/6J male mice were generated by deep sequencing using the Illumina HiSeq 2000.
Project description:Covalent nucleotide modifications in noncoding RNAs such as tRNAs affect a plethora of biological processes, with new functions continuing to be discovered for even well-known tRNA modifications. To systematically compare the functions of a large set of ncRNA modifications in gene regulation, we carried out ribosome profiling and RNA-Seq in budding yeast for 57 nonessential genes involved in tRNA modification. Overall design: Yeast strains were treated with cycloheximide and proceeded to ribosome profiling. For ribosome-protected footprints (RPF), 80S monosome fractions were isolated after RNase I treatment, and 27-34 nt RPF were isolated by denaturing PAGE. For RNA-Seq, total RNA was depleted of rRNA using Ribo-Zero, followed by zinc-based fragmentation. RNA fragments and RPF are constructed into Illumina deep-sequencing libraries by RNA 3’ adaptor ligation and cDNA circularization. Barcoded libraries were sequenced on an Illumina NextSeq 500. Raw fastq reads were de-multiplexed and removed of adaptor sequence. RPF reads were mapped to S. cerevisiae rDNA and the mapping reads were discarded. The remaining RPF reads and RNA-seq reads were mapped to sacCer3 genome. Uniquely mapping reads in length of 27-34 nt (RPF) or 27 nt (RNA-seq) were quantified as RPKM.
Project description:BACKGROUND:Duchenne muscular dystrophy (DMD) is a degenerative muscle disease caused by mutations in the dystrophin gene. Loss of dystrophin prevents the formation of a critical connection between the muscle cell membrane and the extracellular matrix. Overexpression of sarcospan (SSPN) in the mouse model of DMD restores the membrane connection and reduces disease severity, making SSPN a promising therapeutic target for pharmacological upregulation. METHODS:Using a previously described cell-based promoter reporter assay of SSPN gene expression (hSSPN-EGFP), we conducted high-throughput screening on libraries of over 200,000 curated small molecules to identify SSPN modulators. The hits were validated in both hSSPN-EGFP and hSSPN-luciferase reporter cells. Hit selection was conducted on dystrophin-deficient mouse and human myotubes with assessments of (1) SSPN gene expression using quantitative PCR and (2) SSPN protein expression using immunoblotting and an ELISA. A membrane stability assay using osmotic shock was used to validate the functional effects of treatment followed by cell surface biotinylation to label cell surface proteins. Dystrophin-deficient mdx mice were treated with compound, and muscle was subjected to quantitative PCR to assess SSPN gene expression. RESULTS:We identified and validated lead compounds that increased SSPN gene and protein expression in dystrophin-deficient mouse and human muscle cells. The lead compound OT-9 increased cell membrane localization of compensatory laminin-binding adhesion complexes and improved membrane stability in DMD myotubes. We demonstrated that the membrane stabilizing benefit is dependent on SSPN. Intramuscular injection of OT-9 in the mouse model of DMD increased SSPN gene expression. CONCLUSIONS:This study identifies a pharmacological approach to treat DMD and sets the path for the development of SSPN-based therapies.
Project description:Duchenne muscular dystrophy (DMD) is caused by a lack of the dystrophin protein and has no effective treatment at present. Zebrafish provide a powerful in vivo tool for high-throughput therapeutic drug screening for the improvement of muscle phenotypes caused by dystrophin deficiency. Using the dystrophin-deficient zebrafish, sapje, we have screened a total of 2640 compounds with known modes of action from three drug libraries to identify modulators of the disease progression. Six compounds that target heme oxygenase signaling were found to rescue the abnormal muscle phenotype in sapje and sapje-like, while upregulating the inducible heme oxygenase 1 (Hmox1) at the protein level. Direct Hmox1 overexpression by injection of zebrafish Hmox1 mRNA into fertilized eggs was found to be sufficient for a dystrophin-independent restoration of normal muscle via an upregulation of cGMP levels. In addition, treatment of mdx(5cv) mice with the PDE5 inhibitor, sildenafil, which was one of the six drugs impacting the Hmox1 pathway in zebrafish, significantly increased the expression of Hmox1 protein, thus making Hmox1 a novel target for the improvement of dystrophic symptoms. These results demonstrate the translational relevance of our zebrafish model to mammalian models and support the use of zebrafish to screen for new drugs to treat human DMD. The discovery of a small molecule and a specific therapeutic pathway that might mitigate DMD disease progression could lead to significant clinical implications.
Project description:PURPOSE: Mutations in the dystrophin (DMD) gene cause Duchenne or Becker muscular dystrophy (DMD/BMD). DMD contains a retina-specific promoter in intron 29. The short R-dystrophin transcript from this promoter has a retina-specific exon 1 (R1) joined to exon 30 of the DMD gene. It has been claimed that this is responsible for the ophthalmological problems observed in DMD/BMD. This research characterizes the structure of the 5'-untranslated region (5'-UTR) of human R-dystrophin. METHODS: The 5'-UTR of the human R-dystrophin transcript was amplified from human retina and 20 other human tissue RNAs by reverse transcription polymerase chain reaction (RT-PCR). Amplified products were identified by sequencing. The translational activities of transcripts bearing differing 5'-UTRs were measured using a dual luciferase assay system. RESULTS: RT-PCR amplification of the R-dystrophin transcript from the retina using a conventional primer set revealed one product comprising exon R1 and exons 30 to 32 (R-dys α). In contrast, three amplified products were obtained when a forward primer at the far 5'-end of exon R1 was employed for RT-PCR. R-dys α, and a shorter form in which 98 bp was deleted from exon R1 (R-dys β), were the two major products. A minor, short form was also identified, in which 143 bp was deleted from exon R1 (R-dys γ). The two primary retinal products (R-dys α and β) encoded an identical open reading frame. The 98 bp deleted in R-dys β was identified as a cryptic intron that was evolutionarily acquired in higher mammals. The shorter R-dys β was expressed in several tissues with a wide range in expression level, while R-dys α was retina specific. The 5'-UTRs of R-dys α and β were examined for translational activity using a dual luciferase assay system. Unexpectedly, the 5'-UTR of R-dys β showed lower translational activity than that of R-dys α. This lower activity was presumed to be due to the removal of internal ribosome entry sites by activation of cryptic intron splicing. CONCLUSIONS: An evolutionarily-acquired cryptic intron was identified in the 5'-UTR of the human R-dystrophin transcript. The two abundant R-dystrophin transcripts in the retina showed different translational activities in vitro owing to their differential splicing of the cryptic intron. This evolutionarily-acquired alternative splicing may act as a molecular switch that regulates translation of the R-dystrophin transcript.
Project description:The Duchenne muscular dystrophy (DMD) gene has a complex expression pattern regulated by multiple tissue-specific promoters and by alternative splicing (AS) of the resulting transcripts. Here, we used an RNAi-based approach coupled with DMD-targeted RNA-seq to identify RNA-binding proteins (RBPs) that regulate splicing of its skeletal muscle isoform (Dp427m) in a human muscular cell line. A total of 16 RBPs comprising the major regulators of muscle-specific splicing events were tested. We show that distinct combinations of RBPs maintain the correct inclusion in the Dp427m of exons that undergo spatio-temporal AS in other dystrophin isoforms. In particular, our findings revealed the complex networks of RBPs contributing to the splicing of the two short DMD exons 71 and 78, the inclusion of exon 78 in the adult Dp427m isoform being crucial for muscle function. Among the RBPs tested, QKI and DDX5/DDX17 proteins are important determinants of DMD exon inclusion. This is the first large-scale study to determine which RBP proteins act on the physiological splicing of the DMD gene. Our data shed light on molecular mechanisms contributing to the expression of the different dystrophin isoforms, which could be influenced by a change in the function or expression level of the identified RBPs.
Project description:Duchenne muscular dystrophy (DMD) is a fatal genetic disorder caused by mutations in the dystrophin gene. To enable the non-invasive analysis of DMD gene correction strategies in vivo, we introduced a luciferase reporter in-frame with the C-terminus of the dystrophin gene in mice. Expression of this reporter mimics endogenous dystrophin expression and DMD mutations that disrupt the dystrophin open reading frame extinguish luciferase expression. We evaluated the correction of the dystrophin reading frame coupled to luciferase in mice lacking exon 50, a common mutational hotspot, after delivery of CRISPR/Cas9 gene editing machinery with adeno-associated virus. Bioluminescence monitoring revealed efficient and rapid restoration of dystrophin protein expression in affected skeletal muscles and the heart. Our results provide a sensitive non-invasive means of monitoring dystrophin correction in mouse models of DMD and offer a platform for testing different strategies for amelioration of DMD pathogenesis.
Project description:Mutations in DMD disrupt the reading frame, prevent dystrophin translation, and cause Duchenne muscular dystrophy (DMD). Here we describe a CRISPR/Cas9 platform applicable to 60% of DMD patient mutations. We applied the platform to DMD-derived hiPSCs where successful deletion and non-homologous end joining of up to 725 kb reframed the DMD gene. This is the largest CRISPR/Cas9-mediated deletion shown to date in DMD. Use of hiPSCs allowed evaluation of dystrophin in disease-relevant cell types. Cardiomyocytes and skeletal muscle myotubes derived from reframed hiPSC clonal lines had restored dystrophin protein. The internally deleted dystrophin was functional as demonstrated by improved membrane integrity and restoration of the dystrophin glycoprotein complex in vitro and in vivo. Furthermore, miR31 was reduced upon reframing, similar to observations in Becker muscular dystrophy. This work demonstrates the feasibility of using a single CRISPR pair to correct the reading frame for the majority of DMD patients.