*1.3. Objective of This Review*

Understanding in greater detail the underlying mechanisms of DCM pathogenesis in DMD is critical to the development of targeted therapies for this disease. Current advancements in the treatment of skeletal muscle pathology may not benefit dystrophic cardiac muscle in the same way due to key differences in the function and calcium handling of these two striated muscle lineages [24]. Further, treatment of skeletal muscle without regard to the heart may exacerbate cardiac dysfunction [25], as can be seen in patients with X-linked dilated cardiomyopathy. These patients exhibit normal dystrophin expression in skeletal muscle and the absence of dystrophin in the heart [26,27]. Indeed, improved treatment options focused on ameliorating the skeletal muscle pathology of DMD have uncovered a previously underappreciated cardiac involvement, with an estimated 20–30% of deaths now resulting from cardiac failure in this population [28]. The purpose of this paper is to (1) discuss the role of calcium mishandling in DCM development and progression with a focus on DMD cardiomyopathy, (2) detail currently utilized therapies for DMD cardiomyopathy, and (3) evaluate experimental therapeutic strategies for correcting calcium mishandling and calcium overload in DCM and DMD cardiomyopathy.

## **2. Molecular Mechanisms of Dilated Cardiomyopathy**

#### *2.1. Genetic and Acquired Causes of DCM*

There are a multitude of genetic and acquired causes of DCM, making the pathology of the disease highly diverse and clinically vexing [23]. Many known genetic and novel mutations leading to DCM are contained within the sarcomere or cytoskeleton. Therefore, these structures will be the main focus here. A schematic outlining common causes of DCM is shown in Figure 1.

**Figure 1.** Common genetic and acquired causes of dilated cardiomyopathy in humans. ZASP: z-band alternatively spliced PDZ-motif.

#### 2.1.1. Sarcomere

The cardiac sarcomere consists of a repeating pattern of contractile and regulatory proteins organized into thick and thin myofilaments [29]. The sarcomere is the basic contractile unit of cardiac muscle. Therefore, mutations impacting sarcomeric protein function can have a severe effect on heart performance. Approximately 5–10% of DCM diagnoses are due to mutations in sarcomeric proteins [30]. Thin filament proteins, including actin, tropomyosin (Tm), and the three protein subunits that make up the troponin complex, are common targets of DCM-causing mutations [31]. Mutations in the troponin complex typically cause a reduction in myofilament calcium sensitivity and force production [32]. There is evidence however, that in end stage heart failure, there is an increase in myofilament calcium sensitivity [33,34]. This increase is thought to be due to changes in phosphorylation levels of myofilament proteins such as troponin I (TnI), possibly through protein kinase A (PKA) [35–37]. Another site of DCM mutations is within thick filament proteins, including the MYH7 gene (variant pAsn1918Lys), which encodes β-myosin heavy chain, the primary mechano-motor protein of the adult human heart [38]. These mutations are thought to disrupt the ability of myosin to interact with actin, leading to a contractility defect [30]. Finally, a sarcomeric protein that has recently

been identified as a target of DCM mutations is the giant protein titin [39]. Titin is a multifaceted protein that plays a key role in sarcomere assembly and helps maintain passive tension in muscles [40]. Mutations leading to the truncation of the titin protein account for 1–3% of DCM diagnoses, with disease characterized by significant diastolic dysfunction, highlighting the complex role of titin in regulating contraction of the sarcomere [23].
