Update in Duchenne and Becker muscular dystrophy

Waldrop, Megan A., Flanigan, Kevin M.  Update in Duchenne and Becker muscular dystrophy, Current Opinion in Neurology: October 2019 - Volume 32 - Issue 5 - p 722-727 doi: 10.1097/WCO.0000000000000739

Abstract

Purpose of review

The purpose of this review is to highlight updates in the standard of care recommendations for DMD, and to describe approaches to and recent advances in genetic therapies for DMD.

Recent findings

Treatment of DMD patients with the corticosteroids prednisone or deflazacort remains the standard of care, and recent data shows that early treatment (as young as 5 months) with a weekend dosing regimen results in measurable improvement in motor outcomes. A mutation-specific therapy directed at restoring an open reading frame by skipping exon 51 is FDA-approved, and therapies directed at other exons are in trials. Gene replacement therapy shows significant promise in animal models, and trials are underway. Genome editing has received significant attention because of results in animal models, but challenges to implementation in humans remain.

Summary

The mainstay of treatment remains meeting well defined standards of care that have been shown to influence morbidity and mortality. These include use of systemic steroids, early nocturnal ventilatory support, appropriate cardiac care and prophylaxis, and wherever appropriate, scoliosis surgery. Early and accurate molecular diagnosis, along with appropriate and multidisciplinary care, provides the best opportunity for maximum benefit of both current standard and upcoming novel therapies for boys with DMD. Among the most promising of these is AAV-based gene replacement therapy, which is currently in clinical trials.

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From the article

The standard therapy for DMD is oral corticosteroids. Although patients were first treated with prednisone in 1974, it was not until shortly after discovery of the protein, that several studies were conducted evaluating daily prednisone for use in DMD and a benefit in terms of functional improvements were seen. However, because of concerns about side effects, prednisone was not widely used and management of the disease varied widely nationally and internationally. Over time, practice parameters were developed, but it was not until 2010 that the first international guidance for the care and management of DMD was published. This consensus guidance has recently been updated, and the interested reader is referred to these detailed recommendations. In this review, we will highlight some of these current standard of care recommendations for DMD, and promising upcoming therapies for the dystrophinopathies…

Current recommendations include initiation before substantial physical decline, typically by the age of 5, using daily dosing of either prednisone 0.75 mg/kg/day or deflazacort 0.9 mg/kg/day [22]. As earlier treatment may result in larger benefit in regards to loss of ambulation, many clinicians elect to initiate weekend dosing around age 3 years, using 5 or 10 mg/kg of prednisone per week dividing into two doses delivered on each Saturday and Sunday…

A recent article of interest addressed therapy with allogenic cardiosphere-derived cells (CDCs), to which have been attributed antifibrotic, anti-inflammatory, and regenerative properties via secretion of growth factors and mRNAs. A potential therapeutic benefit was suggested by a small (n = 25), open-label randomized trial of direct infusion of 75 million cells into the coronary arteries of patients, in which cardiac MRI suggested diminished fibrotic burden and improved regional myocardial function at 6 and 12 months; a subsequent randomized blinded trial is underway….

Corticosteroids, in addition to the interventions mentioned above, have greatly improved respiratory function via increased skeletal muscle strength and subsequent less severe scoliosis, which can directly impact ventilatory function. Although nocturnal ventilatory support has long been known to have an impact on morbidity and mortality, the coincident correction of spinal scoliosis in appropriate patients can increase survival by nearly a decade…

Muscle biopsy still has a place in the diagnosis of dystrophinopathies. Importantly, up to 7% of dystrophinopathy patients may have mutations that are undetectable by genomic DNA analysis, and many have deep intronic mutations that result in the inclusion of intronic sequence as pseudoexons that require analysis of muscle-derived mRNA to identify. Muscle biopsy also remains useful in assessing dystrophin protein expression in patients where the observed phenotype does not correlate with the phenotype predicted by the application of the ‘reading-frame rule.’ The most common category of these mutations consists of predicted nonsense, out-of-frame mutations (as identified from blood) that actually affect mRNA splicing and resulting in significant amounts of in-frame transcript and sufficient protein expression to alter the clinical course…

A major rationale for accurate genetic diagnosis is to make use of a growing number of mutation-specific therapies. In general, these are directed toward restoring an open-reading frame. The first of these is the antisense phosphorodiamidate morpholino oligomer (PMO) eteplirsen, directed toward altering splicing of the dystrophin mRNA to exclude exon 51. This therapy is applicable to multiple mutations; as examples, deletions of exons 45–50, 48–50, 50, or 52 (among others) are all predicted to have reading frames restored by exon 51 skipping, and are thus, amenable to eteplirsen treatment. Although treatment restores a relatively small amount of dystrophin, the antisense PMO showed improved ambulation and respiratory effects, and more favorable results than a trial of a competing 2’O-Me phosphorothioate antisense oligonucleotide. Following its Food and Drug Administration (FDA) approval, eteplirsen is in widespread clinical use; further treatment trials with morpholinos directed to other exons (casimersen for exon 45, and golidersen for exon 53) are underway. Cocktails of PMOs may ultimately be combined to generate a therapy that addresses skipping larger regions, such as exons 45–55, which would be therapeutic for up to 63% of patients. Another mutation-specific therapeutic approach is nonsense suppression, such as with the drug ataluren, although that drug failed to show a convincing benefit in a randomized placebo controlled trial and is not marketed in the United States…

Among the most promising experimental therapies is gene replacement using adenoassociated viral (AAV) vectors. AAV is not associated with human disease, and AAV genomes are essentially nonintegrating into chromosomes, factors that provide a margin of safety for considering therapeutic development. Different AAV serotypes have differential tropism to human tissues, and utilization of muscle tropic AAVs in combination with appropriate promoters has allowed the development of vectors designed for treatment of muscle diseases. The power of AAV gene replacement has recently been demonstrated by the remarkable results achieved in patients with spinal muscular atrophy type 1 (SMA1) treated with an AAV9 vector carrying the full-length SMN cDNA. The extraordinary survival benefit resulting from this treatment led to FDA approval of this viral therapy under the trade name Zolgensma in May 2019…

There is great excitement around the possibility that in the near future there will be novel therapies that may significantly alter the course of DMD, and the impact of these therapies will be maximized by the development of newborn screening strategies that result in presymptomatic diagnosis.  Nevertheless, the mainstay of treatment remains meeting well defined standards of care that have been shown to influence morbidity and mortality. These include use of systemic steroids, early nocturnal ventilatory support, appropriate cardiac care and prophylaxis, and wherever appropriate, scoliosis surgery. Nevertheless, the promise of novel therapies is significant. Importantly, the prospect of gene-directed therapies, including viral gene replacement, supports the need for early diagnosis and treatment, the rationale for which is increasingly well documented. Early and accurate molecular diagnosis, along with appropriate and multidisciplinary care, will provide the best opportunity for maximum benefit of novel therapies in each boy with DMD.

A challenge to viral gene replacement for DMD is that unlike the case with the SMN cDNA, the full-length DMD cDNA is too large, at approximately 11.5 kilobases (kb), to fit into an AAV genome, which has a maximum packaging capacity of around 5 kb. As a result, several groups have designed microdystrophin genes, essentially based upon the reading-frame rule; these encode miniaturized DMD constructs that encode protein products in which critical functional domains are included – generally, critical N-terminal and C-terminal-binding domains, with differing inclusion of other regions. A full discussion of the differences among these is beyond the scope of this article, but the interested reader is referred to a recent detailed review.  Three such competing microdystrophin trials are underway at present (sponsored by Pfizer, Sarepta Therapeutics, and Solid Therapeutics), with unpublished (to date) reports of early promising results. The ultimate durability of such therapy remains to be seen, as does the ultimate beneficial effect of the engineered microdystrophins, as no natural mutations as seen in BMD exist to inform expectations. Nevertheless, preclinical data for each construct is promising, and the results of each trial are eagerly awaited.

An alternate approach to viral gene therapy is the delivery of surrogate genes that can substitute for dystrophin function. One such example is the GALGT2 gene, which encodes an O-mannosyltransferase responsible for the terminal glycosylation of dystroglycan at the neuromuscular and myotendinous junctions, where utrophin replaces dystrophin in the dystroglycan-associated protein complex. Expression of exogenous GALGT2 via a viral vector results in expression of the glycosylated dystroglycan epitope and localization of utrophin across the entire myofiber, resulting in improvement in mouse models, and a pilot trial of gene delivery in humans is underway. Another promising approach delivers noncoding small nuclear RNAs (U7snRNA) with antisense sequences directed toward exon definition elements; these RNAs can induce highly efficient exon skipping, with promising results in animal models and planning for near-term trials underway…

Recent advances in genome editing at the DMD locus in model systems have raised interest in the prospect of somatic editing as a therapeutic approach. Most such approaches have utilized CRISPR/Cas9 systems, with guide RNAs (gRNAs) directing endonuclease activity to induce excision of one or more exons in order to restore an open reading frame. CRISPR/Cas9 editing has shown promising results in both mouse and canine models. Such an approach can theoretically extend to excision of the entire region of exon 45 to exon 55, with the previously stated goal of treating the largest number of DMD patients. However promising, multiple challenges remain prior to clinical trials. There are concerns regarding germline alteration, off-target effects, and immune responses to the bacterial Cas9 proteins used in many strategies. Depending upon the Cas9 used, packaging of the Cas9 gene along with the necessary gRNAs into a single AAV vector may not be possible, suggesting a need for dual vectors, and overall efficiency of editing remains a challenge. Despite enthusiasm of the patient community, genome editing therapy will likely require significant further preclinical studies…

There is great excitement around the possibility that in the near future there will be novel therapies that may significantly alter the course of DMD, and the impact of these therapies will be maximized by the development of newborn screening strategies that result in presymptomatic diagnosis . Nevertheless, the mainstay of treatment remains meeting well defined standards of care that have been shown to influence morbidity and mortality. These include use of systemic steroids, early nocturnal ventilatory support, appropriate cardiac care and prophylaxis, and wherever appropriate, scoliosis surgery. Nevertheless, the promise of novel therapies is significant. Importantly, the prospect of gene-directed therapies, including viral gene replacement, supports the need for early diagnosis and treatment, the rationale for which is increasingly well documented. Early and accurate molecular diagnosis, along with appropriate and multidisciplinary care, will provide the best opportunity for maximum benefit of novel therapies in each boy with DMD.