Duchenne muscular dystrophy (DMD)

April 20, 2018

Associate Professor and Director of Medical School Curriculum at the University of Illinois, Dr Ahlke Heydemann underlines Duchenne muscular dystrophy (DMD) – a debilitating, progressive muscle weakening disease

Duchenne muscular dystrophy (DMD) is a debilitating, progressive muscle weakening disease. Currently, there are no cures and no effective treatments for muscular dystrophy (MD). However, there are multiple highly promising therapeutics in the pipeline that should give patients and their families’ significant hope. One exciting avenue of research is identifying novel methods of immune inhibition.

Immune inhibition is a promising therapeutic avenue because of the large body of data (from both lab animals and patients) that indicates reducing the chronic inflammation that always accompanies DMD is significantly beneficial (reviewed Evans 2009 and Tidball 2005). In MD mice, various scientists depleted CD8+ T-cells, or CD4+ T-cells, or macrophages, or neutrophils and have demonstrated significant improvement of pathology (reviewed in Evans). Now, researchers must achieve these immune-cell reductions in humans and without side effects.

MD pathobiology initiates with muscle cell membrane permeability, immune infiltrate, myofibre loss and fibrosis. Despite the complicated characteristics of the immune system, its role in acute muscle wounds and MD pathology can be described. In muscle wounds, the immune system is responsible to clean the wound, create scar tissue to halt bleeding, remodel the scar tissue and then terminate the immune system response. Neutrophils and type 1 macrophages initiate the immune infiltration followed by eosinophils and T-cells.

The type 1 macrophages then transition to the anti-inflammatory type 2 macrophages. The disease is so devastating because of the chronic, ongoing nature of the membrane damage and the resultant asynchronised healing response. One cell may be at the correct stage to repair its membrane and is secreting appropriate anti-inflammatory cytokines, while a neighbouring cell is secreting cytokines to attract macrophages and T-cells and thereby inhibit the first cell’s repair process. Interesting data even indicates that the immune cell infiltrate precedes disease histopathology (Spencer 2001), providing more impetus for inhibition of the immune system to treat MD.

In fact, the current standard of care – corticosteroids – is beneficial because it inhibits the immune response. Steroid therapies postpone the patient’s need for a wheelchair. However, the steroids cause significant side-effects and other therapeutics must be identified. Among the latest, clinically-relevant, immune inhibition therapeutics are:

1) Antibodies against NFκb;

2) All-trans retinoic acid;

3) TGFβ inhibition;

4) IL-10 injections;

5) Identifying the best dose and schedule for corticosteroid régime; and

6) Fingolimod.

Each of these compounds has a strong scientific rationale for its inclusion in trials against MD. However, my real optimism for these treatments to be effective against MD is that they can be combined with each other and therapeutics that target other MD molecular mechanisms. Thereby, clinicians can tailor-make therapies for each patient and minimise the doses to avoid patient-specific side effects, while still achieving maximum benefits.

MD research has been made possible and greatly accelerated due to the availability of many mouse models. The most commonly used mouse is the naturally occurring dystrophin deficient mdx mouse (muscular dystrophy on the X-chromosome). In addition, the individual sarcoglycans have been genetically mutated to make the range of sarcoglycanopathies, including the gamma-sarcoglycan mutations (Sgcg-/-). Both of these models, the mdx and the Sgcg-/-, have been breed onto the highly fibrotic DBA/2J mouse strain. Through this breeding, the mice closely resemble the pathology seen in patients.

My lab has focused on immune inhibition strategies because of the strong benefits are seen by this strategy. The mechanism my lab has focused upon is the use of the FDA approved sphingosine-1-phosphate receptor modulator Fingolimod. Fingolimod is FDA approved to treat relapsing multiple sclerosis (MS) and has proven very effective for these patients. MS is an auto-immune disease in which the bodies’ antibodies attack and often destroy the myelin sheaths surrounding the nerves. This causes profound muscle weakness and lesions in the brain. Many MS patients have been taking Fingolimod (Gilenya, Novartis) for six years, with few side effects. Fingolimod’s mechanism of action sequesters immune cells in the peripheral lymph tissue and thereby inhibits further damage.

This mechanism is very different from the immune inhibition provided by the steroids, thereby allowing the possibilities of co-therapy combining these two strategies. The most important, although rare side effects of Fingolimod are transient bradycardia after the first dose and lymphopenia after prolonged use. The lymphopenia is reversed with treatment discontinuation. Although MD is not a true auto-immune disease, the chronic immune response is pathogenic and must, therefore, be reduced. In addition to its immune inhibition, we have demonstrated that sphingosine-1-phosphate receptor modulation with Fingolimod provides additional benefits against MD.

In both the Sgcg-/- DBA/2J and the mdx DBA/2J mouse strains my lab has identified that Fingolimod has pleiotropic beneficial effects upon MD disease progression and even initiation. We have demonstrated that a three-week treatment course administered to young animals reduced the disease-proximal membrane permeability, reduced the immune infiltrate, reduced the resulting fibrosis and increased some of the respiratory functional parameters (Heydemann 2017).

Based on published research studies, our initial hypothesis was that Fingolimod would inhibit the pathogenic immune response and thereby reduce the necrosis-induced fibrosis. We had hoped that a slight improvement of membrane strength would also be achieved because of the reduction in cytokines. However, we were very surprised at the data demonstrating such a large reduction in membrane permeability. The treatment reduced the membrane permeability to not significantly different from wildtype levels.

As with most research studies, this study produced more questions than answers. Our current experiments are designed to answer the most important of these questions. Therefore, we are examining how Fingolimod strengthens the muscle membrane. In this pursuit, we have preliminary data that indicates that treatment with Fingolimod re-establishes the remaining – non-mutated – members of the dystrophin glycoprotein complex. We will now establish if this is through increases in transcription, translation, protein stability or complex stability. We are also conducting additional preclinical trials, such as dose-response curves, longer treatment times, prevention and reversal strategies and co-therapy trials.