**6. Differential contribution of mitochondria towards LVNC**

#### **6.1 Mitochondrial DNA (mtDNA) mutations leading to dysmorphic mitochondria**

mtDNA sequence variants are maternally inherited point mutations representing a predominate cause of inherited diseases within a tissue or organ. Studies performed on mitochondria isolated from patient's blood and *in vivo* cardiac tissues acknowledged the contribution of mtDNA mutations in the formation of cardiac noncompaction [115, 116]. In these studies, instability of cardiac-specific mtDNA is coupled to metabolic irregularities that impact cardiac functionality in response to a myriad of mitochondrial dysfunctions. Liu et al. [116] determined that hearts with noncompaction present with mitochondrial structural anomalies and have a reduced mtDNA copy number with the G4.5 gene on Xq28 that codes for acyltransferase tafazzin (TAZ); the first gene to be studied in detail in relation to the etiology of LVNC [117]. Tafazzin is a mitochondrial membrane component vital for proper functioning of the electron transport chain. This X-linked TAZ gene encodes for the mitochondrial membrane-associated phospholipid-modifying enzyme that alters cardiolipin through the addition of linoleic acid. As cardiolipin is a major player in the mitochondrial inner membrane and is requisite for the maintenance of mitochondrial shape, protein transport, and energy production within cells, it is essential for maintaining the organization of mitochondrial cristae [57]. TAZ gene mutations resulting in Bath Syndrome, perturbed mitochondrial cardiolipin metabolism and are associated with dilated cardiomyopathy. Inactivation of the TAZ gene produces dysfunctional tafazzin proteins. In this circumstance, linoleic acid is not present to alter cardiolipin. As a result, problems with normal mitochondrial shape and functions such as energy production and protein transport develop. High energy-demanding tissues including heart are more susceptible to cell death due to decreased mitochondrial energy

production [118]. Furthermore, TAZ deficiency impairs cardiolipin remodeling and alters the assembly and stability of mitochondrial super complexes of complex III and IV (intermittently impacting complex V as well) in the inner mitochondrial membrane [119].

These effects occasionally result in respiratory chain activity malfunctions. Mitochondrial transcription factor A (TFAM) is a nuclear-encoded protein that regulates the transcription, packaging, and stability of the mitochondrial genome. TFAM is crucial for the development and differentiation of the mitochondrial genome [120]. It is highly likely that TFAM regulates the mtDNA copy number by binding to the mtDNA promoter region to initiate transcription. Thus, the impact of TFAM on mtDNA copy number illustrates the direct influence of TFAM in mitochondrial function. Carnitine palmitoyl transferases (CPT) are mitochondrial enzymes involved in the transportation of Acyl-CoA to the mitochondrial Matrix for β-oxidation. Located in the outer mitochondrial membrane, CPT I convert fatty acyl CoA into fatty acylcarnitine. Located in the inner mitochondrial membrane, CPT II reconverts fatty acylcarnitine into free carnitine and fatty acyl CoA for β-oxidation for energy generation. Inherited defects in CPT I, II or CPT translocases can result in failure of longchain fatty acids transportation into the mitochondria for oxidation. This can result in a significant reduction in the ratio of free CoA to acyl CoA thus ultimately disrupting metabolic pathways and decreasing cardiac ATP production [24].

#### **6.2 Mitochondrial dynamics and dysfunction**

Adult cardiomyocytes are enriched with hypo-dynamic mitochondria that lack an interconnected reticular network. The fusion-fission cycle of cardiac mitochondria is common in cardiomyocyte mitochondrial dynamics [121]. Regulated by specialized proteins, enzymes, and adapter proteins that physically alter mitochondrial membranes, a delicate balance between mitochondrial fusion and mitochondrial fission is important in the regulation of cellular health. In adult cardiomyocytes progressive mitochondrial fission and fusion cycles occur frequently with the aid of proteins needed for the mitochondrial network remodeling. The key aspects of mitochondrial dynamics can be studied with cardiac-specific ablation of mitochondrial fission and fusion protein genes. As the cardiomyocyte mitochondria exhibit a certain degree of dynamism [122, 123], inability for the fusion or fission process could result in smaller or enlarged mitochondria, respectively. Hence, the imbalance between constant ongoing process of fusion and fission cycle are associated with increased caspases activity and subsequent cell death.

Mitochondrial dysfunction is frequently interrelated with LV noncompaction. Primary mitochondrial dysfunction arises from mutations in mitochondrial proteins involved with OXPHOS. Mitochondrial dysfunction refines mitochondrial dynamics with this phenomenon being associated with a variety of cardiac pathologies. For example, the MYH7 coding myosin protein is associated with the structure and function of cardiac sarcomere. Besides, the mutations in MYH7 linked with HCM and LVNC, makes myosin an important protein in cardiac function as defects in MYH7 expression are a common cause of cardiomyopathy [79]. Friedreich's ataxia (FRDA) is an example of a heart disease arising from mitochondrial dysfunction. FRDA is caused by the trinucleotide GAA expansion in the first intron of FXN gene (codes for mitochondrial matrix iron chaperone protein, frataxin) [123] with this triplet expansion resulting in transcription silencing and diminished frataxin expression. As a result, this mitochondrial localized protein influences iron homeostasis and

respiratory control. Therefore, impaired FXN gene function in FRDA patients is associated with dysfunctional ETC complexes, specifically the complex III with high ROS levels suggesting altered mitochondrial function [124].
