Pediatric cardiology researchers Jamie R. Johnston, PhD, and Elizabeth M. McNally, MD, have published a paper on the various genetic correction strategies currently available for Duchenne muscular dystrophy (DMD) and their impact on the heart. This is a unique paper in the sense that its focus is on cardiology, which features less commonly in DMD medical literature.

In their paper, they discussed 3 forms of genetic correction strategies: 

  • Antisense oligonucleotide-mediated exon skipping therapy 
  • Gene replacement therapy 
  • CRISPR (clustered regularly interspaced short palindromic repeat)-mediated genome editing  

Here, we will explore these strategies and how they affect the heart in patients with DMD. 


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Cardiac Benefits With Exon Skipping Unclear

There are estimates that up to 80% of patients with DMD possess genotypes that can be altered through exon-skipping therapies. “Exon-skipping relies on antisense oligonucleotides (AONs), roughly 20-30 nucleotides, to manipulate pre-messenger RNA (mRNA) alternative splicing to circumvent mutated exons, effectively converting an out-of-frame DMD mutation to an in-frame [Becker muscular dystrophy (BMD)] mutation,” Johnston and McNally explained. 

However, AON-mediated approaches are faced with a massive challenge: their ability to penetrate tissue is notably weak, especially when it comes to the heart. The reason for this is that AON-mediated therapies only offer limited restoration of dystrophin protein levels. To solve this problem, scientists chemically modify them in order to improve stability, reduce toxicity, and increase bioavailability. 

Read more about DMD etiology 

“It remains unknown at this time whether AON-exon skipping exerts benefit for cardiac function,” the researchers wrote. Although AON-mediated therapies have shown remarkable improvement in skeletal muscle fibers, improvement in cardiac muscle is less pronounced.

Johnston and McNally offer their own explanation as to why this might be the case: “Because of the mechanical role of dystrophin at the membrane, including [the] cardiomyocyte plasma membrane, increased cardiac contractile demand could exacerbate cardiomyocyte degeneration, thus creating a faster timeline for cardiac dysfunction in DMD.”

Research on Cardiac Impact of Gene Therapy Ongoing

The next therapy discussed is gene replacement therapy, which essentially is the replacement of faulty genes with normal ones. Abreu and Waldrop, in their paper on the uses of gene therapy in DMD and spinal muscular atrophy (SMA), provided a concise explanation of how gene therapy in DMD works: “Gene replacement therapy in DMD holds great promise among emerging therapies by directly providing a functional, albeit shortened, copy of the DMD gene, addressing the underlying genetic defect regardless of the underlying mutation.”

However, a significant problem remains: dystrophin genes tend to be larger in size than AAV vectors. How do scientists resolve this? Abreu and Waldrop explained, “Due to the large size of the dystrophin gene and small carrying capacity of AAV vectors, researchers have designed synthetic micro-dystrophin, removing many of the spectrin‐like repeats and the C‐terminal domain to produce a transgene that may fit within an AAV vector and produce a functional, internally truncated protein akin to “in‐frame” BMD‐like mutations.” 

Read more about DMD therapies 

Micro-dystrophins tend to be 40% smaller than truncated dystrophins in BMD patients. Johnston and McNally cautioned, “Complete replacement of the DMD gene is precluded due to the large size of the coding region required to produce full length dystrophin protein.” 

So what is the impact of gene therapy on cardiac function? Once again, it largely remains a mystery; the ability of micro-dystrophins to mitigate cardiomyopathy in DMD patients is still a subject of ongoing research.

CRISPR Strategy Shows Promise in Cardiac Muscle

The final category of genetic correction strategy mentioned in this study is CRISPR-mediated genome editing. “Somatic cell genome editing with CRISPR . . . -Cas9 technology is evolving at a rapid pace and has the potential to revolutionize the treatment or prevention of human disease, particularly monogenetic, X-linked recessive disorders like DMD,” Johnston and McNally wrote. 

It should be stressed here that this approach involves correcting somatic cells, not germline cells. CRISPR-Cas9 can modify the genome and the epigenome in a precise and targeted way; its function is to restore the production of dystrophin by producing internally truncated dystrophin protein that is partially functional. 

Studies investigating CRISPR-based gene editing demonstrated the robust expression of dystrophin in cardiac muscle, in addition to skeletal muscles.

Johnston and McNally were hopeful about the potential therapeutic impact of CRISPR-mediated genome editing on the heart: “Ultimately, the CRISPR-based genome editing toolkit may be used to therapeutically modulate the expression of genes in the heart that are expected to improve myocardial performance or delay adverse ventricular remodeling.”

A Work In Progress 

As we visit and revisit the medical literature around genetic correction strategies, we get a keen sense that they are incredibly innovative and brimming with promise. Granted, they are still at a relatively early stage of development and use, and more work needs to be done. In the specific case of DMD, we understand that genetic correction strategies have a positive impact on skeletal muscles; however, data on their impact on the heart is currently less satisfactory. 

Having appraised the available evidence for DMD genetic correction strategies, Johnston and McNally ended their study on an optimistic note: “The future looks to genetic correction strategies. In theory, genetic correction should restore expression of more of the internal components of dystrophin, which may have important functional consequences to cardiac function.”

References

Johnston JR, McNally EM. Genetic correction strategies for Duchenne muscular dystrophy and their impact on the heart. Published online November 2, 2021. Prog Pediatrc Cardiol. doi:10.1016/j.ppedcard.2021.101460

Abreu NJ, Waldrop MA. Overview of gene therapy in spinal muscular atrophy and Duchenne muscular dystrophy. Pediatr Pulmonol. 2021;56(4):710-720. doi:10.1002/ppul.25055