Monitoring duchenne muscular dystrophy gene therapy with epitope-specific monoclonal antibodies

Dystrophin deficiency impairs vascular structure and function in the canine model of Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is a muscle-wasting disease caused by dystrophin deficiency. Vascular dysfunction has been suggested as an underlying pathogenic mechanism in DMD. However, this has not been thoroughly studied in a large animal model. Here we investigated structural and functional changes in the vascular smooth muscle and endothelium of the canine DMD model. The expression of dystrophin and endothelial nitric oxide synthase (eNOS), neuronal NOS (nNOS), and the structure and function of the femoral artery from 15 normal and 16 affected adult dogs were evaluated. Full-length dystrophin was detected in the endothelium and smooth muscle in normal but not affected dog arteries. Normal arteries lacked nNOS but expressed eNOS in the endothelium. NOS activity and eNOS expression were reduced in the endothelium of dystrophic dogs. Dystrophin deficiency resulted in structural remodeling of the artery. In affected dogs, the maximum tension induced by vasoconstrictor phenylephrine and endothelin-1 was significantly reduced. In addition, acetylcholine-mediated vasorelaxation was significantly impaired, whereas exogenous nitric oxide-induced vasorelaxation was significantly enhanced. Our results suggest that dystrophin plays a crucial role in maintaining the structure and function of vascular endothelium and smooth muscle in large mammals. Vascular defects may contribute to DMD pathogenesis. © 2021 The Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

New monoclonal antibodies that recognize an unglycosylated, conserved, extracellular region of CD44 in vitro and in vivo, and can block tumorigenesis

CD44 is a transmembrane glycoprotein that binds to hyaluronic acid, plays roles in a number of cellular processes and is expressed in a variety of cell types. It is up-regulated in stem cells and cancer. Anti-CD44 monoclonal antibodies affect cell motility and aggregation, and repress tumorigenesis and metastasis. Here we describe four new anti-CD44 monoclonal antibodies originating from B cells of a mouse injected with a plasmid expressing CD44 isoform 12. The four monoclonal antibodies bind to the terminal, extracellular, conserved domain of CD44 isoforms. Based on differences in western blot patterns of cancer cell lysates, the four anti-CD44 mAbs separated into three distinct categories that include P4G9, P3D2, and P3A7, and P3G4. Spot assay analysis with peptides generated in Escherichia coli support the conclusion that the monoclonal antibodies recognize unglycosylated sequences in the N-terminal conserved region between amino acid 21-220, and analyses with a peptide generated in human embryonic kidney 293 cells, demonstrate that these monoclonal antibodies bind to these peptides only after deglycosylation. Western blots with lysates from three cancer cell lines demonstrate that several CD44 isoforms are unglycosylated in the anti-CD44 target regions. The potential utility of the monoclonal antibodies in blocking tumorigenesis was tested by co-injection of cells of the breast cancer-derived tumorigenic cell line MDA-MB-231 with the anti-CD44 monoclonal antibody P3D2 into the mammary fat pads of mice. All five control mice injected with MDA-MB-231 cells plus anti-IgG formed palpable tumors, while only one of the six test mice injected with MDA-MB-231 cells plus P3D2 formed a tiny tumor, while the remaining five were tumor-free, indicating that the four anti-CD44 mAbs may be useful therapeutically.

Characterization of 65 epitope-specific dystrophin monoclonal antibodies in canine and murine models of duchenne muscular dystrophy by immunostaining and western blot

Epitope-specific monoclonal antibodies can provide unique insights for studying cellular proteins. Dystrophin is one of the largest cytoskeleton proteins encoded by 79 exons. The absence of dystrophin results in Duchenne muscular dystrophy (DMD). Over the last two decades, dozens of exon-specific human dystrophin monoclonal antibodies have been developed and successfully used for DMD diagnosis. Unfortunately, the majority of these antibodies have not been thoroughly characterized in dystrophin-deficient dogs, an outstanding large animal model for translational research. To fill the gap, we performed a comprehensive study on 65 dystrophin monoclonal antibodies in normal and dystrophic dogs (heart and skeletal muscle) by immunofluorescence staining and western blot. For comparison, we also included striated muscles from normal BL10 and dystrophin-null mdx mice. Our analysis revealed distinctive species, tissue and assay-dependent recognition patterns of different antibodies. Importantly, we identified 15 antibodies that can consistently detect full-length canine dystrophin in both immunostaining and western blot. Our results will serve as an important reference for studying DMD in the canine model.

Monoclonal antibodies for clinical trials of Duchenne muscular dystrophy therapy

Most pathogenic mutations in Duchenne and Becker muscular dystrophies involve deletion of single or multiple exons from the dystrophin gene, so exon-specific monoclonal antibodies (mAbs) can be used to distinguish normal and mutant dystrophin proteins. In Duchenne therapy trials, mAbs can be used to identify or rule out dystrophin-positive "revertant" fibres, which have an internally-deleted dystrophin protein and which occur naturally in some Duchenne patients. Using phage-displayed peptide libraries, we now describe the new mapping of the binding sites of five dystrophin mAbs to a few amino-acids within single exons. The phage display method also confirmed previous mapping of MANEX1A (exon 1) and MANDRA1 (exon 77) by other methods. Of the 79 dystrophin exons, mAbs are now available against single exons 1, 6, 8, 12, 13, 14, 17, 21, 26, 28, 38, 41, 43, 44, 45, 46, 47, 50, 51, 58, 59, 62, 63, 75 and 77. Many have been used in clinical trials, as well as for diagnosis and studies of dystrophin isoforms.

Dystrophin rescue by trans-splicing: a strategy for DMD genotypes not eligible for exon skipping approaches

RNA-based therapeutic approaches using splice-switching oligonucleotides have been successfully applied to rescue dystrophin in Duchenne muscular dystrophy (DMD) preclinical models and are currently being evaluated in DMD patients. Although the modular structure of dystrophin protein tolerates internal deletions, many mutations that affect nondispensable domains of the protein require further strategies. Among these, trans-splicing technology is particularly attractive, as it allows the replacement of any mutated exon by its normal version as well as introducing missing exons or correcting duplication mutations. We have applied such a strategy in vitro by using cotransfection of pre–trans-splicing molecule (PTM) constructs along with a reporter minigene containing part of the dystrophin gene harboring the stop-codon mutation found in the mdx mouse model of DMD. Optimization of the different functional domains of the PTMs allowed achieving accurate and efficient trans-splicing of up to 30% of the transcript encoded by the cotransfected minigene. Optimized parameters included mRNA stabilization, choice of splice site sequence, inclusion of exon splice enhancers and artificial intronic sequence. Intramuscular delivery of adeno-associated virus vectors expressing PTMs allowed detectable levels of dystrophin in mdx and mdx4Cv, illustrating that a given PTM can be suitable for a variety of mutations.

AAV micro-dystrophin gene therapy alleviates stress-induced cardiac death but not myocardial fibrosis in >21-m-old mdx mice, an end-stage model of Duchenne muscular dystrophy cardiomyopathy

Duchenne muscular dystrophy (DMD) is a fatal genetic disease caused by the absence of the sarcolemmal protein dystrophin. Dilated cardiomyopathy leading to heart failure is a significant source of morbidity and mortality in DMD. We recently demonstrated amelioration of DMD heart disease in 16 to 20-m-old dystrophin-null mdx mice using adeno-associated virus (AAV) mediated micro-dystrophin gene therapy. DMD patients show severe heart disease near the end of their life expectancy. Similarly, mdx mice exhibit profoundly worsening heart disease when they reach beyond 21 months of age. To more rigorously test micro-dystrophin therapy, we treated mdx mice that were between 21.2 and 22.7-m-old (average, 22.1 ± 0.2 months; N=8). The ∆R4-23/∆C micro-dystrophin gene was packaged in the cardiotropic AAV-9 virus. 5×10(12) viral genome particles/mouse were delivered to mdx mice via the tail vein. AAV transduction, myocardial fibrosis and heart function were examined 1.7 ± 0.2 months after gene therapy. Efficient micro-dystrophin expression was observed in the myocardium of treated mice. Despite the robust dystrophin expression, myocardial fibrosis was not mitigated. Most hemodynamic parameters were not improved either. However, ECG abnormalities were partially corrected. Importantly, treated mice became more resistant to dobutamine-induced cardiac death. In summary, we have revealed for the first time the potential benefits and limitations of AAV micro-dystrophin therapy in end-stage Duchenne dilated cardiomyopathy. Our findings have important implications for the use of AAV gene therapy in dilated cardiomyopathy and heart failure.

A simplified immune suppression scheme leads to persistent micro-dystrophin expression in Duchenne muscular dystrophy dogs

Highly abbreviated micro-dystrophin genes have been intensively studied for Duchenne muscular dystrophy (DMD) gene therapy. Following adeno-associated virus (AAV) gene transfer, robust microgene expression is achieved in murine DMD models in the absence of immune suppression. Interestingly, a recent study suggests that AAV gene transfer in dystrophic dogs may require up to 18 weeks' immune suppression using a combination of three different immune-suppressive drugs (cyclosporine, mycophenolate mofetil, and anti-dog thymocyte globulin). Continued immune suppression is not only costly but also may cause untoward reactions. Further, some of the drugs (such as anti-dog thymocyte globulin) are not readily available. To overcome these limitations, we developed a novel 5-week immune suppression scheme using only cyclosporine and mycophenolate mofetil. AAV vectors (either AV.RSV.AP that expresses the heat-resistant human alkaline phosphatase gene, or AV.CMV.μDys that expresses the canine R16-17/H3/ΔC microgene) at 2.85×10(12) vg particles were injected into adult dystrophic dog limb muscles under the new immune suppression protocol. Sustained transduction was observed for nearly half year (the end of the study). The simplified immune suppression strategy described here may facilitate preclinical studies in the dog model.

Duchenne muscular dystrophy gene therapy: Lost in translation?

A milestone of molecular medicine is the identification of dystrophin gene mutation as the cause of Duchenne muscular dystrophy (DMD). Over the last 2 decades, major advances in dystrophin biology and gene delivery technology have created an opportunity to treat DMD with gene therapy. Remarkable success has been achieved in treating dystrophic mice. Several gene therapy strategies, including plasmid transfer, exon skipping, and adeno-associated virus-mediated microdystrophin therapy, have entered clinical trials. However, therapeutic benefit has not been realized in DMD patients. Bridging the gap between mice and humans is no doubt the most pressing issue facing DMD gene therapy now. In contrast to mice, dystrophin-deficient dogs are genetically and phenotypically similar to human patients. Preliminary gene therapy studies in the canine model may offer critical insights that cannot be obtained from murine studies. It is clear that the canine DMD model may represent an important link between mice and humans. Unfortunately, our current knowledge of dystrophic dogs is limited, and the full picture of disease progression remains to be clearly defined. We also lack rigorous outcome measures (such as in situ force measurement) to monitor therapeutic efficacy in dystrophic dogs. Undoubtedly, maintaining a dystrophic dog colony is technically demanding, and the cost of dog studies cannot be underestimated. A carefully coordinated effort from the entire DMD community is needed to make the best use of the precious dog resource. Successful DMD gene therapy may depend on valid translational studies in dystrophin-deficient dogs.