Duchenne muscular dystrophy (DMD) is a genetic disorder characterized by progressive muscle degeneration. It usually starts in early childhood. In this article we review the background information and research insights of the dystrophin (DMD) protein-coding gene, a pathogenic gene of Duchenne Muscular Dystrophy.

Background Information – DMD Gene 

Overview of Duchenne Muscular Dystrophy (DMD) Research

Duchenne muscular dystrophy (DMD) was discovered in the 19th century and is one of the most common single-gene genetic diseases and one of the most-studied diseases in gene therapy research today. Duchenne muscular dystrophy (DMD) is caused by a defective dystrophin (DMD) gene, which is indispensable in ensuring the normal human muscle structure and function.

DMD is a huge gene with a length of more than 2.2M bp, such length also contributes to a significant increase in the number of potential mutation sites in this gene. This is exemplified by the weakened version of DMD called Becker Muscular Dystrophy (BMD). Although BMD is also due to a mutation of the DMD gene, it can still produce a small amount of functional protein due to the different mutation sites.

Duchenne muscular dystrophy (DMD) can cause skeletal muscle weakness and myocardial weakness, eventually leading to patient death. Some patients also have cognitive impairment and intestinal problems. Most patients lost their walking ability before 12 years old. Finally, dyspnea is aggravated due to respiratory muscle dysfunction, which requires ventilation support and cardiac dysfunction can lead to heart failure. With the assistance of ventilators and intensive care, DMD patients have an average life expectancy of 26, with many living into their 30s.

Figure 1. DMD protein structure and location. The figure shows the basic structure of the human DMD protein and its location near the cell membrane. NT represents the amino-terminal, CT represents the carboxy-terminal, CR represents the leucine repeat region, and the vertical olive region represents 24 spectrin repeat regions (spectrin-like repeats, SLR), in addition to 4 hinge structures marked H1-H4. Dystrophin has an Actin binding domain at the N-terminal and the middle region. SLRs 1-3 can bind to cell membranes; SLRs 16-17 can bind nNOS; SLRs 20-23 can interact with microtubules; H4 and CR It can bind to β-subunit of dystroglycan (βDG); the final CT can simultaneously bind to cotropin and dystroglycan. （Source: 10.1242/dmm.018424）

Although a complete, functional dystrophin protein has long been proven to be effective in treating DMD in animal models, its clinical application is very difficult because the full-length cDNA of DMD itself is close to 14kb and cannot be loaded into commonly used viral vectors. Clinical findings that truncated dystrophin in BMD restore most of the functions of full-length dystrophin suggest that adherence to full-length proteins may not be necessary to alleviate DMD. Therefore, Mini dystrophin with 6kb fragment and Micro dystrophin with 3.6kb fragment was successively developed to treat DMD. Although the function of Mini dystrophin is certainly not as good as that of the complete protein, the effect is still satisfactory.

Figure 2. The exploration of therapeutic DMD proteins.

(Source: 10.1016/j.ymthe.2017.02.019)

Duchenne Muscular Dystrophy (DMD) Research Models

The DMD protein has been described previously, and the mRNA structure of DMD is shown in Figure 3A, where ABD represents the coding region of the Actin binding domain. Deletions, duplications, insertions, and various point mutations may occur on this gene. If frameshift mutations or nonsense mutations occur, they will develop DMD; if no frameshift mutations occur, they are more likely to develop BMD with mild symptoms. One can make the protein expression bypass the partially mutated exons via Antisense Oligonucleotide (ASO) (rather lose part of the structure and eliminate the FRAM mutation) or directly load the truncated DMD with the virus for treatment.

Figure 3B shows various models related to DMD mutations, including MDX mice which were developed in the 1980s. There were examples of mice using the 23 C mutation, but the symptoms of the mice did not meet expectations, and life expectancy was only 1/4 that of wild type mice. This, people have tried to model many other sites, both on exons and introns, and have achieved certain phenotypes. However, overall, mice were less affected by the DMD mutation than humans.

Figure 3. Gene therapy of DMD and related models of dystrophin.

(Source: 10.1242/dmm.018424)

Cyagen’s Knockout Catalog Mouse Models

Cyagen Knockout Catalog Models has over 16,000 strains of KO and cKO/floxed mouse models, and is still growing. Researchers can quickly find the gene of interest they need knocked out, and obtain a group of live KO/cKO mouse models in as fast as 2 weeks.

Find more related mouse models in our selection of research-ready Cyagen Knockout Catalog Models.

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Related Resources

With the goal of helping the clinical transformation of gene therapy, Cyagen has been deeply involved in the field of gene therapy research. Below are a selection of our recent gene therapy resources:

>> On-Demand Webinar - The Use of Animal Models in Rare Disease Therapeutic Research

>> What is Gene Therapy? An Introduction to the Research

>> What are the Strategies for Gene Therapy?

References:

1. McGreevy JW, Hakim CH, McIntosh MA, Duan D. Animal models of Duchenne muscular dystrophy: from basic mechanisms to gene therapy. Dis Model Mech. 2015 Mar;8(3):195-213. doi: 10.1242/dmm.018424.

PMID: 25740330; PMCID: PMC4348559.

2. Chamberlain JR, Chamberlain JS. Progress toward Gene Therapy for Duchenne Muscular Dystrophy. Mol Ther. 2017 May 3;25(5):1125-1131. doi: 10.1016/j.ymthe.2017.02.019. Epub 2017 Apr 15.

PMID: 28416280; PMCID: PMC5417844.