The Effect of Age on Duchenne Muscular Dystrophy
The Effect of Age on Duchenne Muscular Dystrophy
Sindhu Ragunathan
Thomas Jefferson High School for Science and Technology
Sitting in a dark imaging room, I carefully inserted prepared slides under a microscope. Bright blue, green, and pink dots flared up on the screen in front of me, representing cell nuclei, membranes, and regenerated mice nuclei, respectively. At the Jaiswal Lab in the Children’s National Center for Genetic Medicine Research, I spent much of my time isolating, staining, and analyzing sections of mouse muscle to assess the regeneration capacity of muscle cells affected by Duchenne muscular dystrophy (DMD).
DMD is a heritable and chronic muscle-degenerative disease in which patients become wheelchair-dependent by age 12 and have fatal cardiorespiratory failure by age 30. DMD originates from genetic mutations in the dystrophin gene.3 Dystrophin is a protein in muscle cells that tethers the cytoskeleton, the “backbone” of the cell, to the surrounding cellular components, ensuring the cell’s integrity. The absence of dystrophin increases susceptibility to injury, leading to persistent muscle degeneration and chronic inflammation. Over time, fibrotic and/or adipose tissue replaces muscles from DMD patients with consequent loss of muscle function.6 However, while scientists know the genetic origin of DMD, the underlying mechanisms that result in the disease are still unknown.
Currently, researchers are using mdx mice, which have similar genetic mutations to humans with DMD, to study the progression of the disease. The two mouse models used to examine DMD are the B10-mdx and the D2-mdx models. The B10-mdx mouse model is the most widely used model in DMD research. However, these mice are poor models for a few reasons. Contrary to the short lifespan of humans diagnosed with DMD, they cope well with the lack of dystrophin and continue to have an almost normal lifespan. Recently, scientists introduced the same dystrophin mutation in the D2-mdx mouse model, which resulted in a more severe muscle pathology. These D2-mdx mice have similar disease progression to humans with DMD, unlike the B10-mdx mice, making D2-mdx a better model for DMD research.6 Despite these differences, both mouse models are useful when comparing disease progression and allow for a better understanding of DMD’s underlying mechanisms.3
Jaiswal Lab and van Putten et al. (2019) have been collecting data on the D2-mdx mice and have noticed that the disease is very severe in young D2-mdx mice compared to older D2-mdx mice. Other studies also found significantly reduced muscle function and increased calcification, or muscle hardening, in mice as young as seven weeks.1 Interestingly, van Putten et al. (2019) observed improved muscle pathology in 34-week-old D2-mdx mice compared to 10-week-old D2-mdx mice.
My project is a continuation of Jaiswal Lab’s ongoing research and aims to determine whether there is a significant difference in DMD severity between younger and older D2-mdx mice. I also hope to investigate the role of fibro-adipogenic progenitors (FAPs), which are cells that differentiate or transform into adipogenic, calcified, or fibrotic tissue.8 Annemieke Aartsma-Rus, a researcher at the Netherlands Leiden University Medical Center, strongly believes that FAPs underlie DMD in each species. She notes, “The mesenchymal stem cells [a subset of FAPs] can form all types of tissues, and probably local growth factors determine which pathway is dominant and which tissue is formed” (personal communication, 2020). FAPs’ primary purpose is to be regenerative, which is a crucial function in patients with a muscle degeneration disorder, but they appear to have negative consequences. In a paper under review, Jaiswal Lab has found that patients with DMD have high levels of transforming growth factor β (TGF-β), which is responsible for secreting FAPs and is linked to the promotion of muscle fibrosis and poor regeneration in younger mice. On the other hand, they found older mice have increased regeneration, indicating the effects of the disease are diminishing.4 Moratal, Arrighi, Dechesne, and Dani (2019) demonstrated that FAPs have an age-dependent relationship with DMD, which aligns with the improved muscle pathology shown by van Putten et al.5,6
Researchers are also evaluating potential treatments for DMD from a non-fundamental standpoint. They aim to fix the effects of the disorder without necessarily finding the cause of it. One such approach was approved by the Food and Drug Administration (FDA) as a treatment for DMD in December of 2019. A drug called Vyondys 53 increases the production of dystrophin, which DMD patients lack due to their genetic mutation. The FDA notes the drug is “reasonably likely to predict clinical benefit to patients”.2 A press release from the FDA (2019) indicates that the average percentage of dystrophin in trial DMD patients only increased from “0.10% of normal at baseline to 1.02% of normal after 48 weeks of treatment with the drug or longer”.2 However, due to the life-threatening and debilitating nature of the disease, the FDA has granted the drug an accelerated release as long as it passed the final clinical trial.
While treatments such as physical therapy and the Vyondys 53 drug offer some symptom alleviation, they cannot cure DMD. It is our role as researchers to try to understand the underlying pathways behind the disease to create targeted and effective treatments for those suffering. My project is fundamental to this understanding as it establishes the effectiveness of the D2-mdx mouse model and creates a link in the disease progression from a younger to older age. For now, I continue to parse through the bright blues, greens, and pinks. We hope that once our lab is able to display significant disease amelioration of DMD linked with increasing age, other researchers will discover pathways that explain this link and eventually create life-saving treatments for those suffering from DMD.
References
[1] Coley, W. D., Nagaraju, K., Lutz, C., Cox, G. A., Partridge, T. A., Austin, A., . . . Vila, M. C.
(2016). Effect of genetic background on the dystrophic phenotype in mdx mice. Human
Molecular Genetics, 25(1), 130-145. https://doi.org/10.1093/hmg/ddv460
[2] FDA grants accelerated approval to first targeted treatment for rare duchenne muscular
dystrophy mutation. (2019, December 12).
[3] Gordish-Dressman, H., Willmann, R., Pazze, L. D., Kreibich, A., van Putten, M.,
Heydemann, A., . . . Aartsma-Rus, A. (2018). "Of mice and measures": A project to improve
how we advance duchenne muscular dystrophy therapies to the clinic. Journal of Neuromuscular
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[6] Van Putten, M., & Aartsma-Rus, A. (2020). The use of genetically humanized animal models
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[7] van Putten, M., Putker, K., Overzier, M., Adamzek, W. A., Pasteuning-Vuhman, S., Plomp, J.
J., & Aartsma-Rus, A. (2019). Natural disease history of the D2-mdx mouse model for Duchenne
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[8] Wosczyna, M. N., & Rando, T. A. (2018). A muscle stem cell support group: Coordinated
cellular responses in muscle regeneration. Developmental Cell, 46(2), 135-143.
https://doi.org/10.1016/j.devcel.2018.06.018
