Barth syndrome (BTHS) is a rare, X-linked inborn error of mitochondrial phospholipid metabolism caused by pathogenic variants in the gene TAFAZZIN (TAZ), which leads to abnormal cardiolipin (CL) metabolism on the inner mitochondrial membrane. Although TAZ is ubiquitously expressed, BTHS involves a complex combination of tissue specific phenotypes including cardiomyopathy, neutropenia, skeletal myopathy, and growth delays, with a relatively minimal neurological burden. While the primary genetic and metabolic defects are well defined, there is limited mechanistic understanding of how pathogenic variants in TAZ, and therefore defects in CL metabolism, contribute to the mitochondrial pathogenicity of BTHS. Thus, there are limited targets for therapeutic monitoring and treatment.
To understand both the developmental and functional effects of TAZ-deficiency in different tissues, we generated isogenic TAZ knockout (TAZ-KO) and WT cardiomyocytes (CMs), skeletal muscle cell types (SKMs), and neural progenitor cells (NPCs) from CRISPR-edited induced pluripotent stem cells (iPSCs).
In TAZ-KO CMs we discovered evidence of dysregulated mitophagy including dysmorphic mitochondria and mitochondrial cristae, differential expression of key autophagy-associated genes, and an inability of TAZ-deficient CMs to properly initiate stress-induced mitophagy. Similarly, TAZ-deficient skeletal muscle cell types demonstrate failure to upregulate myogenic transcription factors and show preliminary evidence for dysregulated mitophagy. Further, in TAZ-deficient NPCs we identified novel phenotypes including a reduction in CIV abundance and CIV activity in the CIII2&CIV2 intermediate complex.
Using nutritional fatty acid supplementation strategies to manipulate CL acyl content, we discovered, while CL acyl chain manipulation was unable to alter mitophagy defects in TAZ-KO CMs, linoleic acid or oleic acid supplementation was able to partially restore CIV abundance in TAZ-deficient NPCs.
Taken together, our results have implications for understanding the tissue-specific pathology of BTHS and the potential for tissue-specific therapeutic targeting. Moreover, our results highlight an emerging role for mitophagy in the cardiac pathophysiology of BTHS, as well as in other skeletal muscle cell types, and we hypothesize that defective mitophagy could provide the missing link to unify the prenatal and postnatal cardiac complications in BTHS. Interestingly, we also reveal a potential neuron-specific bioenergetic phenotype and highlight the importance of evaluating downstream cellular defects in a tissue-targeted manner
