Article Text

Recent advances in the management of Duchenne muscular dystrophy
1. Eugen-Matthias Strehle,
2. Volker Straub
1. The John Walton Muscular Dystrophy Research Centre, Newcastle upon Tyne, UK
1. Correspondence to Professor Volker Straub, Institute of Genetic Medicine, Newcastle University, International Centre for Life, Central Parkway, Newcastle upon Tyne NE1 3BZ, UK; volker.straub{at}ncl.ac.uk

## Abstract

Duchenne muscular dystrophy (DMD) is the commonest inherited neuromuscular disorder of childhood and mainly affects males. Over the course of the last century, the average life expectancy of these patients has doubled and now stands at ∼25 years. This progress has been made possible through advances in the diagnosis, treatment and long-term care of patients with DMD. Basic and clinical research, national and international scientific networks, and parent and patient support groups have all contributed to achieving this goal. The advent of molecular genetic therapies and personalised medicine has opened up new avenues and raised hopes that one day a cure for this debilitating orphan disease will be found. The main purpose of this short review is to enable paediatricians to have informed discussions with parents of boys with DMD about recent scientific advances affecting their child's clinical care.

• Technology
• Statistics
• Pharmacology
• Outcomes research
• Neuromuscular

## Introduction

Duchenne muscular dystrophy is caused by out-of-frame mutations of the dystrophin (DMD) gene on chromosome Xp21 and transmitted in an X-linked recessive fashion but sporadic mutations are common. The mutations result in a deficiency of the protein dystrophin. DMD was first described by the English physician Edward Meryon in 1852.1 However, the condition is named after the French neurologist Guillaume-Benjamin-Amand Duchenne who in 1868 reported a case series of boys with ‘pseudo-hypertrophic muscular paralysis’.2 In 1954, Walton and Natrass published the first comprehensive classification of human myopathies based on 105 patients from the northeast of England. Forty-eight of them (46%) had DMD and were characterised in great detail.3 Other conditions mentioned in their pioneering paper were facioscapulohumeral muscular dystrophy, limb-girdle muscular dystrophy and myotonic dystrophy. Half a century later, Norwood et al4 presented epidemiological data including 1105 patients from a regional neuromuscular clinic in the north of England that showed a combined prevalence for the dystrophinopathies of 8.46/100 000 total population. The life expectancy of patients with DMD has risen steadily over the last decades thanks to the introduction of new medical and surgical treatments, and a multidisciplinary care approach.5 Passamano et al analysed 516 Italian Duchenne patients who were born in the 1960s, 1970s and 1980s. At 25 years of age, their respective survival rates were 13.5%, 31.6% and 49.2%.6 Here we will outline the improvements made in diagnosing and caring for patients with DMD during the last few years and highlight key aspects of evolving treatment strategies. Muscular dystrophy research takes place worldwide and is moving fast; therefore, not every aspect of it can be covered in this brief review.

## Diagnosis

Prompt diagnosis of DMD is important so that the available treatment options can be commenced as early as possible. It has been shown that often there is a lapse of 1–2 years between the first symptoms of the disease and the definitive diagnosis.7 ,8 Patients with DMD typically present in early childhood with delayed walking, difficulties in climbing stairs and hopping, a waddling gait or tip toeing. Physical signs include a reduced muscle bulk, pseudo-hypertrophy of the calf muscles and tightening of the Achilles tendons. These clinical features should prompt the clinician to check the child's plasma creatine kinase (CK), which is expressed in muscle and brain cells. The acceptable upper limit for a normal CK is ca. 300 IU/L, whereas CK levels in DMD usually exceed 1000 IU/L and can be as high as 30 000 IU/L. This increase is caused by leakage of the enzyme from the cytoplasm of the damaged muscle fibre into the blood circulation. CK levels are a poor indicator of disease progression and are not routinely measured once the diagnosis has been established. It has been suggested to check the CK level of every newborn so that DMD can be detected at an earlier age than the average of 4 years, but strictly speaking the disease does not fulfil the well-established criteria to justify the introduction of a neonatal screening test.9 In the past, the diagnosis was confirmed with a muscle biopsy and histological staining, but after the identification of the dystrophin gene by Louis Kunkel in 1985, molecular genetic testing became available and replaced the muscle biopsy in many diagnostic centres.10 ,11 The DMD gene is 2.4 Mb long and has at least 79 exons, making it the largest known human gene. The mutations leading to DMD tend to be clustered in certain ‘hot-spot’ regions of the gene. They can be divided into deletions (∼60%), duplications (∼10%) and point mutations (∼30%). Details about the latest developments in genetic testing such as multiplex ligation-dependent probe amplification can be gleaned from the textbook chapter on dystrophinopathies by Sinnreich.12

## Patient care

Table 1

Key standards of care for Duchenne muscular dystrophy

Table 2

Prescription drugs administered to boys with Duchenne muscular dystrophy (alphabetical order)

To conduct randomised controlled research trials with adequate patient numbers, academic medical centres have to collaborate nationally and globally. Examples of such networks are the UK-based North Star network,34 the global TREAT-NMD Alliance35 and the US-based network Cooperative International Neuromuscular Research Group.36 Parent support groups have been instrumental in promoting the cause of DMD, for instance, the Muscular Dystrophy Campaign (UK) and the Muscular Dystrophy Association (USA). Using standardised care guidelines not only benefits patients directly but also enables researchers to obtain better research data.37

## Clinical research

The last two decades have seen increasing efforts to find and test novel drug treatment for DMD by applying some of the methodologies listed in table 3. Currently, so-called ‘small molecule therapies’ that interfere with specific gene mutations carried by subgroups of boys with DMD appear to be the most promising candidates (personalised medicine).38 Ataluren (PTC124) is a polycyclic organic molecule that reduces the sensitivity of ribsosomes towards premature stop codons, resulting in a so-called ‘stop-codon read-through’.39 Questions have been raised in the scientific community as to whether ataluren's mechanism of action is real.40 In 2014, the drug was approved under the brand name Translarna by the European Medicines Agency and the European Commission for treatment of DMD caused by nonsense mutations, which account for ca. 13% of mutations.41 Bushby et al42 conducted a double-blind, randomised, placebo-controlled multicentre trial of oral ataluren involving 173 patients with DMD aged 5–20 years. After 48 weeks of treatment, study subject on 40 mg/kg/day ataluren but not those on 80 mg/kg/day showed a slower decline in their walking distance (–13 m) compared with the placebo group (–44 m). No serious adverse events were observed. Currently, a phase III trial of ataluren is underway, which should provide clarity regarding its efficacy or lack of it.

Table 3

Experimental treatments for Duchenne muscular dystrophy

Exon skipping with antisense oligonucleotides (AON, morpholinos) is another gene-modifying technique trialled in patients with DMD. AON are short nucleic acid analogues that attach themselves to messenger RNA or single-stranded DNA and prevent its translation into a protein, effectively silencing part of a gene. In the context of DMD, AON have been designed to target mutation-containing exons in one of the ‘hot-spots’ of the dystrophin gene. Normally, an out-of-frame mutation leads to a stop of dystrophin expression and the more severe DMD phenotype. However, by skipping the deleted or duplicated exon, the out-of-frame mutation is turned into an in-frame mutation. Dystrophin expression can therefore continue at a reduced level, which results in the milder Becker muscular dystrophy.43 ,44 Drisapersen is an AON that causes skipping of exon 51, which is a therapeutic strategy applicable to ∼13% of boys with DMD. It has been tested in three double-blind, placebo-controlled trials (DEMAND studies). In one 48-week study, drisapersen was administered subcutaneously at a dose of 6 mg/kg weekly to 18 patients (continuous regimen); another group of 17 patients received nine doses of drisapersen over 6 weeks followed by a 4-week break (intermittent regimen); and one group of 18 was given placebo. After 24 weeks but not at 48 weeks, the continuous regimen resulted in a statistically significant increase in walking distance (∼35 m) compared with placebo. Proteinuria was a common adverse effect.45 ,46 Eteplirsen is a morpholino drug that leads to removal of exon 51 during the RNA splicing process (excision of introns and joining of exons). The resulting dystrophin protein is functional but shorter than the normal one.47 ,48 Mendell et al enrolled 12 boys with DMD on GCs in a double-blind trial and divided them into three groups. One group received 30 mg/kg intravenous eteplirsen weekly, another 50 mg/kg weekly and a third group placebo. The patients had up to three muscle biopsies throughout the 48-week trial period. The boys in the eteplirsen groups showed increased dystrophin production of 40–50% on biopsy and were able to walk ∼67 m further than control after 48 weeks of treatment. The drug was well tolerated.49 It must be stressed that at the time of writing all types of gene-modifying therapies for DMD were at the experimental stage.

Survival rates are the ultimate outcome measure for many diseases including DMD. However, proving a reduction of mortality within time-limited clinical intervention studies is difficult, especially if the illness is progressing slowly. Researchers are continuously striving to develop more accurate and reliable clinical tests and laboratory parameters that can be used as study endpoints and prognostic indicators.50 Among others, the following assessments and investigations have been found useful for determining clinical outcomes and monitoring disease progress in patients with DMD: 6-min walk test,51 timed function test, North Star ambulatory assessment,52 muscle ultrasound,53 MRI and spectroscopy of pelvis and legs,54 ,55 and analysis of muscle biopsies with a variety of techniques.56 The 6-min walk test score and the North Star ambulatory assessment score show a gradual decline in patients with DMD from the age of 7 years. It has been suggested that a difference of 30 m in the 6-min walk test represents a clinically meaningful result even though it may not be statistically significant.57 Recently, microRNAs have become a focus of interest in DMD research. These short RNA molecules are present in mammalian blood and organ tissue, and have a regulatory function. Their expression levels in health and disease vary, which makes them likely candidates as valid future biomarkers.58 Nowadays the storage of human tissue (eg, blood, muscle, skin, DNA) is tightly controlled and requires written consent from the patient or legal guardian. Storage banks such as UK Biobank and EuroBioBank play a vital role for research into muscular dystrophy.

## Conclusion

Causative treatment of monogenic genetic diseases (Mendelian disorders) is one of the great challenges of modern medicine. Gene therapy has the potential to solve this problem, but it is still in its infancy despite several decades of intense research. Germline gene therapy is prohibited in the UK and many other countries due to ethical concerns, whereas somatic cell gene therapy is available for selected patients within research trials. The NICE (http://www.nice.org) does not recommend gene therapy for any medical condition. The USA does not have specific laws restricting research on gene therapy, but the Food and Drug Administration has yet to approve the first gene-modifying agent (Glybera for the treatment of lipoprotein lipase deficiency could be the first one). Drug costs are another important factor to take into consideration. According to the Kings Fund and the Nuffield Trust, the British government's annual budget for the National Health Service will remain static or decrease rather than increase during the coming years.59 ,60 Pharmaceutical companies invest large resources in the development of gene-modifying agents and naturally hope to make a profit from their new products. It is estimated that any type of drug acting on the dystrophin gene will cost in excess of £100 000 ($160 000) per patient per year. These costs have to be weighed against the economic burden of the disease.61 Landfeldt et al studied 770 patients with DMD in Germany, Italy, the UK and the USA. They found that the estimated annual costs per patient (direct, indirect and intangible costs) ranged from £53 000 to £79 000 ($80 000—\$121 000).62 One would like to see strong evidence that such expensive novel drugs are of definite overall benefit to patients with DMD, for instance, by reducing morbidity and mortality or by reaching a validated surrogate endpoint. Unfortunately as yet there is insufficient evidence that any of the newly developed drugs can achieve this desirable outcome. Furthermore, somatic gene-modifying therapy, should it eventually become available, is likely to remain an adjunct to standard treatment rather than a stand-alone therapy that will add to the overall costs of patient care. In medicine, scientific progress tends to occur in small steps rather than big leaps, and setbacks are far more common than breakthroughs. Approximately 9 out of 10 clinical pharmacological trials have negative outcomes. However, each study adds a new piece of knowledge and helps to solve the giant jigsaw of finding a lasting cure. Although recent clinical research trials in the field of DMD have shown some promising results, there is a long way to go before effective gene therapy will become available for children living in economically developed countries.63 ,64 Until such time standardised conventional therapy and regular monitoring by a dedicated team of healthcare specialists should remain the cornerstone of DMD management (figure 1).65

Figure 1

Diagram illustrating the multisystem character of Duchenne muscular dystrophy that is caused by a defect in the dystrophin–glycoprotein complex (DGC); the patient is integrated in a multidisciplinary support system.

## Acknowledgments

The authors would like to thank the paediatric muscle team at the Institute of Human Genetics in Newcastle for their support; and Angela Butler/7activestudio for preparing figure 1. Clinical research could not take place without close involvement of patients with DMD and their families who volunteer to participate in trials and give permission for invasive procedures.

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## Footnotes

• Contributors E-MS and VS have jointly written and agreed the manuscript.

• Competing interests VS is or has been a principal investigator for trials sponsored by Genzyme/Sanofi, GSK, Prosensa/Biomarin, ISIS Pharmaceuticals and Sarepta. He received speaker honoraria from Genzyme/Sanofi, is a member of the international Pompe advisory board of Genzyme/ Sanofi, and has been on advisory boards for Acceleron Pharma, Audentes, Italfarmaco S.p.A., Nicox, Pfizer, Prosensa, Santhera and TrophyNOD. He also has a research collaboration with Ultragenyx and Genzyme/Sanofi.

• Provenance and peer review Commissioned; externally peer reviewed.

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