Targeted Treatments for Inherited Neuromuscular Diseases of Childhood

Alex J. Fay, MD, PhD; Renatta Knox, MD, PhD; Erin E. Neil, DO; Jonathan Strober, MD

Disclosures

Semin Neurol. 2020;40(3):335-341. 

In This Article

Duchenne Muscular Dystrophy

DMD is a progressive, fatal X-linked disorder due to mutation of the dystrophin gene, a protein that links the cytoskeleton to the muscle membrane. A significant role of dystrophin is to prevent injury to the muscle with each contraction. When dystrophin is abnormal, small tears in the membrane occur with contraction, and the cell's contents leak out, leading to inflammatory changes, loss of muscle fibers, and replacement with fat and connective tissue. These changes lead to the progressive reduction of skeletal muscle mass, weakness, and, eventually, respiratory failure and scoliosis. Cardiac muscle is affected in most boys, leading to a dilated cardiomyopathy. Cardiac and respiratory complications are the most frequent causes of death in this disease.

Until recently, the only therapy, outside of supportive care, was corticosteroids, which slow disease progression. The mechanism of action of steroids in slowing the disease course has not been clearly defined, though there is evidence for anti-inflammatory properties, stabilizing effects on muscle fiber membranes, inhibition of muscle proteolysis, and stimulation of myoblast proliferation through transcriptional regulation.[21] However, corticosteroids are fraught with many side effects including Cushingoid facies, obesity, growth impairment, pubertal delays, behavioral changes, cataracts, and osteoporosis.[22] Prednisone and prednisolone were the steroids initially used until the introduction of deflazacort, which is now approved by the FDA for use in DMD. Given the number of increasing studies for DMD patients, head-to-head analyses are now being published.[23] The posthoc analysis using the placebo arm from the trial studying another medication, Ataluren (discussed later), showed less mean decline from baseline to 48 weeks in the 6-minute walk test (6MWT), lower mean declines from baseline in the four-stair climb, and longer duration of ambulation in deflazacort-treated patients compared with prednisone/prednisolone-treated patients.[24] No significant differences were noticeable in the safety profiles; however, the deflazacort-treated group had a smaller mean increase in weight and BMI and a smaller mean increase in height.

Since the initial papers suggesting a benefit for prednisone in DMD,[23] evidence for the benefits of steroids has accumulated, including prolonged ambulation, slowed rate of respiratory decline, slowed loss of upper limb function, and reduced need for surgical intervention for scoliosis.[22] There are several regimens that are used in clinical practice today, including daily, intermittent, and high-dose weekend steroids.[25] Currently, a large multinational study is being conducted to compare several of these regimens, as well as to collect data for the first large-scale efficacy study of the use of steroids for DMD (3-year Finding Optimum Regimen for DMD [FOR-DMD]). A novel type of dissociative steroid, vamorolone, retains anti-inflammatory effects through the glucocorticoid receptor but has reduced transcriptional activation, which accounts for many of the side effects of other steroids. Vamorolone appears safe and reduced serum creatine kinase levels based on early phase II studies,[26] and further studies are underway to compare vamorolone with prednisone (NCT03439670).

Other strategies to target muscle pathology in DMD include antifibrotic medications and inhibition of myostatin, a negative regulator of muscle growth, whose deletion has been shown to increase skeletal muscle mass in animal models. Chronic activation of nuclear factor (NF) kB drives muscle degeneration and suppresses muscle regeneration in DMD.[27] Edasalonexent inhibits NF-kB-dependent inflammatory responses and downstream proinflammatory genes, and therefore it may play a role in replacing steroids for these patients. A phase 1/2 study showed it was well tolerated and inhibited the NF-kB pathways,[27] and a phase 3 study is currently underway (NCT03703882). In August 2018, the development of domagrozumab, a monoclonal antibody inhibitor of myostatin, was terminated after the drug did not meet the primary efficacy end point in a phase 2 clinical trial. RG6206 is a novel antimyostatin adnectin previously shown to inhibit myostatin activity in healthy adults. In a study of 48 ambulatory boys aged 5 to 10 years with DMD,[28] RG6206 was well tolerated and myostatin suppression was observed. Imaging data suggested an increased lean body mass and muscle volume. A phase 2/3 study is underway (NCT03039686).

Genetically Targeted Therapies for Duchenne Muscular Dystrophy

Exon Skipping. In DMD patients, deletions disrupt the reading frame of dystrophin, leading to the absence of dystrophin expression. Exon skipping alters pre-mRNA splicing by skipping key exons, which allows the reading frame to be restored. Skipping exon 51 of dystrophin would lead to the restoration of the reading frame in approximately 13% of DMD patients. Correction of the dystrophin reading frame should convert DMD to a milder, Becker muscular dystrophy-like phenotype.

Exon skipping in DMD is mediated by ASOs, which are short single-stranded chemically modified nucleic acids with homology to pre-mRNA sequences and which alter expression of the target RNA.[29] The two primary chemistries that are being tested in clinical trials for exon 51 skipping in DMD are 2'-O-methyl phosphorothioate (2OMeP) and phosphorodiamidate morpholino oligomers (PMOs).[30] While drisapersen, a 2OMeP compound, showed initial promise in phase 1 and phase 2 clinical trials, a larger phase 3 placebo-controlled trial failed to show statistically significant improvement in 6MWT in DMD patients compared with the control group.[31–33] Eteplirsen, a PMO compound, has shown functional improvement in the 6MWT in a cohort of 12 DMD patients treated for 4 years compared with a historical control cohort.[34] There is an ongoing phase 3 trial that has shown small increases in dystrophin restoration on muscle biopsy samples.[33] In September 2016, eteplirsen received accelerated approval in the United States by the FDA while awaiting results from an ongoing confirmatory clinical trial. Eteplirsen is not currently approved in Europe.[35]

Gene Replacement. Gene replacement with mini- and microdystrophin constructs with retained functional domains is also being actively investigated. This approach is based, in part, on the discovery of a family with a mild Becker phenotype found to have a deletion of 46% of the dystrophin gene.[36] One minidystrophin construct has been tested in a phase I clinical trial using a modified AAV2 vector (rAAV2.5–cytomegalovirus–minidystrophin); however, there was a robust T-cell mediated-immune response with no expression of dystrophin 90 days after injection.[37] In 2017, a new phase 1b clinical trial was launched using PF-06939926, a recombinant AAV9 minidystrophin under the control of a human muscle specific promoter.

Ribosomal Readthrough. Nonsense mutations resulting in premature stop codons and truncated proteins account for 10 to 15% of DMD cases, termed nmDMD.[38] Recent progress has been made using chemical compounds to promote ribosomal readthrough. PTC124, Ataluren (PTC Therapeutics Inc., South Plainfield, NJ), is an orally administered small molecule that promotes ribosomal readthrough for patients with nmDMD. In 2017, a phase 3 randomized placebo-controlled trial of Ataluren showed no significant difference in 6MWD between the treatment and placebo groups, though there was a statistically significant difference in a subgroup of patients with a baseline 6MWD of 300 to 400 m.[39] Based on these results and prior clinical trials, Ataluren was approved for nmDMD in several countries including Europe, South Korea, and Israel, but not in the United States.[40]

Genome Editing: CRISPR/Cas9. Discovered as components of a bacterial adaptive immune system in the past decade, CRISPR/Cas9 has rapidly become a promising technology for treating genetic diseases.[41] Cas9 is an endonuclease that induces double-strand breaks at specific genomic loci directed by a single-guide RNA (sgRNA) that is complementary to the targeted stretch of genomic DNA. Induction of one or more double-strand breaks in a target gene can, for example, inactivate a dominant allele responsible for a disease like CMT and leave behind the normal allele. Inactivation of the targeted allele occurs by insertions and deletions at the site of Cas9 cleavage induced by error-prone nonhomologous end joining. A less efficient repair process, with low activity in nondividing cells, is homology-directed repair (HDR), whereby a template DNA sequence introduced together with Cas9 and the sgRNA can replace the DNA sequence at the site targeted by Cas9. As HDR is still inefficient in tissues such as muscle and nerve, it will not be discussed further in this article.

DMD has attracted early attention as a target for CRISPR-based therapeutics based on the promise of exon skipping approaches discussed earlier. Directing Cas9 to delete an exon can function similarly to an ASO, allowing for the correction of out-of-frame dystrophin exon deletions or even point mutations. Targeting the region of exons 45 to 55 could allow for the correction of dystrophin mutations in up to 62% of patients.[42] Preclinical work in mouse and dog models of DMD has employed both dual-sgRNA and single-sgRNA delivery, along with Cas9 delivery, using AAV vectors. While the efficiency varies by muscle, this approach has been reported to achieve up to 80% rescue of dystrophin expression in some muscles in the canine model.[43] The mouse model,[44] using two sgRNAs, highlights some of the concerns regarding CRISPR/Cas9-based therapies: multiple types of insertions, deletions, and AAV vector integrations at the targeted site as well as in tissues other than skeletal muscle and heart; and induction of host immune response against the AAV vector, Cas9, and sgRNA, all of which are recognized as foreign by the adult (but not neonatal) mouse immune system. Thus, while the efficiency and versatility of CRISPR/Cas9-based therapeutics are indeed promising, there are safety considerations that still to be worked out regarding genomic and tissue specificity, and the host immune response.

Other Limb-girdle Muscular Dystrophies

Gene therapy trials are underway for several of the limb-girdle muscular dystrophies caused by mutations in subunits of the sarcoglycan complex, including α-sarcoglycan (LGMD-2D), β-sarcoglycan (LGMD-2E), and gamma-sarcoglycan (LGMD-2B). These dystrophies may manifest similar to both DMD and Becker muscular dystrophy, but due to the smaller size of the sarcoglycan genes compared with dystrophin, AAV delivery of an entire sarcoglycan gene is possible. After promising results from an initial trial of intramuscular delivery of scAAVrh74.tMCK.hSCGA, a more recent open-label trial of intrafemoral artery injection of the gene therapy resulted in the expression of α-sarcoglycan in lower extremity muscles and improvements in knee extensor strength and the 6MWT.[45] Future studies on systemic administration are in progress. Other targets under investigation based on promising preclinical data include gene therapy for dysferlin (LGMD-2B),[46] and ASOs for myotonic dystrophy type I[47] and Ullrich/Bethlem muscular dystrophies[48] caused by dominant collagen VI mutations.

Pompe's disease has been treatable with enzyme replacement for over a decade, and this therapy seems to improve skeletal muscle, cardiac muscle, and brain function in patients with infantile-onset disease.[49,50] However, these conclusions are drawn from mostly small, noncontrolled studies. Gene therapy with local delivery to the diaphragm has been reported, and further gene therapy clinical trials are in progress (see clinicaltrials.gov). Immunoreactivity against exogenous GAA is a concern for both enzyme replacement and gene therapy in patients without cross-reactive immunogenic material but appears to be manageable with immunosuppression.[51]

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