Qilong Lu

Efficacy of systemic morpholino exon-skipping in Duchenne dystrophy dogs.

Duchenne muscular dystrophy (DMD) is caused by the inability to produce dystrophin protein at the myofiber membrane. A method to rescue dystrophin production by antisense oligonucleotides, termed exon‐skipping, has been reported for the mdx mouse and in four DMD patients by local intramuscular injection. We sought to test efficacy and toxicity of intravenous oligonucleotide (morpholino)‐induced exon skipping in the DMD dog model.

Non-viral gene delivery in skeletal muscle: a protein factory

Ever since the publication of the first reports in 1990 using skeletal muscle as a direct target for expressing foreign transgenes, an avalanche of papers has identified a variety of proteins that can be synthesized and correctly processed by skeletal muscle. The impetus to the development of such applications is not only amelioration of muscle diseases, but also a range of therapeutic applications, from immunization to delivery of therapeutic proteins, such as clotting factors and hormones. Although the most efficient way of introducing transgenes into muscle fibres has been by a variety of recombinant viral vectors, there are potential benefits in the use of non-viral vectors. In this review we assess the recent advances in construction and delivery of naked plasmid DNA to skeletal muscle and highlight the options available for further improvements to raise efficiency to therapeutic levels.

The Status of Exon Skipping as a Therapeutic Approach to Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is associated with mutations in the dystrophin gene that disrupt the open reading frame whereas the milder Beckers form is associated with mutations which leave an in-frame mRNA transcript that can be translated into a protein that includes the N- and C- terminal functional domains. It has been shown that by excluding specific exons at, or adjacent to, frame-shifting mutations, open reading frame can be restored to an out-of-frame mRNA, leading to the production of a partially functional Becker-like dystrophin protein. Such targeted exclusion can be achieved by administration of oligonucleotides that are complementary to sequences that are crucial to normal splicing of the exon into the transcript. This principle has been validated in mouse and canine models of DMD with a number of variants of oligonucleotide analogue chemistries and by transduction with adeno-associated virus (AAV)-small nuclear RNA (snRNA) reagents encoding the antisense sequence. Two different oligonucleotide agents are now being investigated in human trials for splicing out of exon 51 with some early indications of success at the biochemical level.

Expansion of revertant fibers in dystrophic mdx muscles reflects activity of muscle precursor cells and serves as an index of muscle regeneration

Duchenne muscular dystrophy and the mdx mouse myopathies reflect a lack of dystrophin in muscles. However, both contain sporadic clusters of revertant fibers (RFs) that express dystrophin. RF clusters expand in size with age in mdx mice. To test the hypothesis that the expansion of clusters is achieved through the process of muscle degeneration and regeneration, we analyzed muscles of mdx mice in which degeneration and regeneration were inhibited by the expression of micro-dystrophins or utrophin transgenes. Postnatal RF expansion was diminished in direct correlation to the protective effect of the transgene expression. Similarly, expansion of RFs was inhibited when muscle regeneration was blocked by irradiation. However, in irradiated muscles, irradiation-tolerant quiescent muscle precursor cells reactivated by notexin effectively restored RF expansion. Our observations demonstrate that revertant events occur initially within a subset of muscle precursor cells. The proliferation of these cells, as part of the regeneration process, leads to the expansion of RF clusters within degenerating muscles. This expansion of revertant clusters depicts the cumulative history of regeneration, thus providing a useful index for functional evaluation of therapies that counteract muscle degeneration.

A Renaissance for Antisense Oligonucleotide Drugs in Neurology: Exon Skipping Breaks New Ground

Antisense oligonucleotides are short nucleic acid sequences designed for use as small-molecule drugs. They recognize and bind to specific messenger RNA (mRNA) or pre-mRNA sequences to create small double-stranded regions of the target mRNA that alter mRNA splicing patterns or inhibit protein translation. Antisense approaches have been actively pursued as a form of molecular medicine for more than 20 years, but only one has been translated to a marketed drug (intraocular human immunodeficiency virus treatment). Two recent advances foreshadow a change in clinical applications of antisense strategies. First is the development of synthetic DNA analogues that show outstanding stability and sequence specificity yet little or no binding to modulator proteins. Second is the publication of impressive preclinical and clinical data using antisense in an exon-skipping strategy to increase dystrophin production in Duchenne muscular dystrophy. As long-standing barriers are successfully circumvented, attention turns toward scale-up of production, long-term toxicity studies, and the challenges to traditional drug regulatory attitudes presented by tightly targeted sequence-specific drugs.

Exon skipping restores dystrophin expression, but fails to prevent disease progression in later stage dystrophic dko mice.

Antisense therapy with both chemistries of phosphorodiamidate morpholino oligomers (PMOs) and 2′-O-methyl phosphorothioate has demonstrated the capability to induce dystrophin expression in Duchenne muscular dystrophy (DMD) patients in phase II-III clinical trials with benefit in muscle functions. However, potential of the therapy for DMD at different stages of the disease progression is not understood. In this study, we examined the effect of peptide-conjugated PMO (PPMO)-mediated exon skipping on disease progression of utrophin-dystrophin-deficient mice (dko) of four age groups (21–29, 30–39, 40–49 and 50+ days), representing diseases from early stage to advanced stage with severe kyphosis. Biweekly intravenous (i.v.) administration of the PPMO restored the dystrophin expression in nearly 100% skeletal muscle fibers in all age groups. This was associated with the restoration of dystrophin-associated proteins including functional glycosylated dystroglycan and neuronal nitric synthase. However, therapeutic outcomes clearly depended on severity of the disease at the time the treatment started. The PPMO treatment alleviated the disease pathology and significantly prolonged the life span of the mice receiving treatment at younger age with mild phenotype. However, restoration of high levels of dystrophin expression failed to prevent disease progression to the mice receiving treatment when disease was already at advanced stage. The results could be critical for design of clinical trials with antisense therapy to DMD.

Muscle and Heart Function Restoration in a Limb Girdle Muscular Dystrophy 2I (LGMD2I) Mouse Model by Systemic FKRP Gene Delivery

Mutations in fukutin-related protein (FKRP) gene cause a wide spectrum of disease phenotypes including the mild limb-girdle muscular dystrophy 2I (LGMD2I), the severe Walker-Warburg syndrome, and muscle-eye-brain disease. FKRP deficiency results in α-dystroglycan (α-DG) hypoglycosylation in the muscle and heart, which is a biochemical hallmark of dystroglycanopathies. To study gene replacement therapy, we generated and characterized a new mouse model of LGMD2I harboring the human mutation leucine 276 to isoleucine (L276I) in the mouse alleles. The homozygous knock-in mice (L276I(KI)) mimic the classic late onset phenotype of LGMD2I in both skeletal and cardiac muscles. Systemic delivery of human FKRP gene by AAV9 vector in the L276I(KI) mice, at either neonatal age or at the age of 9 months, rendered body wide FKRP expression and restored glycosylation of α-DG in both skeletal and cardiac muscles. FKRP gene therapy ameliorated dystrophic pathology and cardiomyopathy such as muscle degeneration, fibrosis, and myofiber membrane leakage, resulting in restoration of muscle and heart contractile functions. Thus, these results demonstrated that the treatment based on FKRP gene replacement was effective.

Tris[2-(acryloyloxy)ethyl]isocyanurate cross-linked low-molecular-weight polyethylenimine as gene delivery carriers in cell culture and dystrophic mdx mice.

Hyperbranched poly(ester amine)s (PEAs) were successfully synthesized by Michael addition reaction between tris[2-(acryloyloxy)ethyl]isocyanurate (TAEI) and low-molecular-weight polyethylenimine (LPEI, M(w) 0.8k, 1.2k, and 2.0k) and evaluated in vitro and in vivo as gene carriers. PEAs effectively condensed plasmid DNA with particle sizes below 200 nm and surface charges between 11.5 and 33.5 mV under tested doses [at the ratios 2-10:1 of polymer/pDNA(w/w)]. The PEAs showed significantly lower cytotoxicities when compared with PEI 25k in two different cell lines. The PEAs (C series) composed of PEI 2k showed higher transgene expression compared to PEAs of PEI 0.8k (A series) or 1.2k (B series). Highest gene transfection efficiency in CHO, C2C12 myoblast, and human skeletal muscle (HSK) cell lines was obtained with TAEI/PEI-2K (C12) at a ratio of 1:2. Both C12, C14(TAEI/PEI-2K at a ratio of 1:4) demonstrated 5-8-fold higher gene expression as compared with PEI 25k in mdx mice in vivo through intramuscular administration. No obvious muscle damage was observed with these new polymers. Higher transfection efficiency and lower toxicity indicate the potential of the biodegradable PEAs as safe and efficient transgene delivery vectors.

Differential expression and secretion of alpha 1 anti-trypsin between direct DNA injection and implantation of transfected myoblast

Muscle can be used for systemic delivery of non-muscle proteins. In order to investigate the relative effectiveness of direct DNA plasmid injection versus implantation of genetically modified myogenic cell lines, we have used the human alpha 1 anti-trypsin (α1AT) cDNA driven by either cytomegalovirus (CMV) or the muscle creatine kinase 3.3 kb (MCK) promoter in immunodeficient mice. We dem- onstrate that the implantation of transfected myoblasts stably expressing the human α1AT cDNA generates a more persistent production of α1AT than does direct intramuscular injection of the same construct as plasmid DNA. Moreover, immunohistological labelling of muscle sections implanted with myoblasts show that the newly formed muscle fibres are those containing the human protein.

Polyethylenimine-modified Pluronics (PCMs) Improve Morpholino Oligomer Delivery in Cell Culture and Dystrophic mdx Mice

We investigated a series of small-sized polyethylenimine (PEI, 0.8/1.2 k)-conjugated pluronic copolymers (PCMs) for their potential to enhance delivery of an antisense phosphorodiamidate morpholino oligomer (PMO) in vitro and in dystrophic mdx mice. PCM polymers containing pluronics of molecular weight (Mw) ranging 2-6 k, with hydrophilic-lipophilic balance (HLB) 7-23, significantly enhanced PMO-induced exon-skipping in a green fluorescent protein (GFP) reporter-based myoblast culture system. Application of optimized formulations of PCMs with PMO targeted to dystrophin exon 23 demonstrated a significant increase in exon-skipping efficiency in dystrophic mdx mice. Consistent with our observations in vitro, optimization of molecular size and the HLB of pluronics are important factors for PCMs to achieve enhanced PMO delivery in vivo. Observed cytotoxicity of the PCMs was lower than Endo-porter and PEI 25 k. Tissue toxicity of PCMs in muscle was not clearly detected with the concentrations used, indicating the potential of the PCMs as effective and safe PMO carriers for treating diseases such as muscular dystrophy.