Elucidating Cardiolipin Immune Dysfunction in Barth Syndrome
Kate Schroder, PhD, The University of Queensland
Barth Syndrome (BTHS) is a potentially lethal genetic disease. It is caused by mutations in the protein tafazzin (TAZ), an enzyme that controls the maturation of the mitochondrial lipid cardiolipin (CL). Thanks to its four unsaturated chains, healthy CL preserves mitochondria function, whereas TAZ mutations deplete selected unsaturated chains, resulting in abnormal CL with saturated and oxidized chains. The consequent impairments in mitochondrial function and ATP production explain many clinical aspects of BTHS (e.g., cardiomyopathy, muscle weakness), while other immunological disease manifestations such as inflammation, neutropenia, and susceptibility to infection remain unexplained, despite being a leading cause of death in BTHS alongside heart failure. While standard heart-failure medications and broad-spectrum antibiotics have improved survival, these do not address the underlying immune dysfunction in BTHS. Using straightforward methodologies that are well established by the project team, comprising primary human cells and patient-derived cell lines to maximize relevance to human BTHS, this proposal will address simple, unexplored questions that are critical for treating immune dysfunction in BTHS. Our project team discovered new lipid-induced immunoregulatory pathways that may drive immune dysregulation in BTHS. PI Schroder elucidated a signaling pathway by which the bacterial lipid, lipopolysaccharide (LPS), triggers inflammatory responses and death of immune cells such as neutrophils. LPS activates two immune proteins, caspase-4(casp-4) and Toll-like Receptor 4 (TLR4), which generate responses critical for fighting bacteria; however, their uncontrolled activation is deleterious, causing a cytokine storm, multiorgan failure, and potentially death. Recent research by Dr Pizzuto, the researcher participating in this grant, showed that saturated CLs similar to those found in BTHS patients induce inflammatory reactions and cell death. By contrast, unsaturated CLs that are decreased in BTHS inhibit LPS-induced casp-4 and TLR4 activation. Modifications of CL chains thus dramatically affect immune system responses, leading us to hypothesize that immune dysfunction in BTHS patients is driven by aberrant TAZ-related CL modifications that generate pro-inflammatory CL species. We will define the immunomodulatory properties of CL species specific to BTHS by testing the ability of single aberrant CL species and the CL pool derived from BTHS patient-derived cells to modulatecasp-4 andTLR4inflammatory reactions in primary human monocytes, macrophages, and neutrophils, and human stem cell-derived cardiomyocytes. We will also characterize whether BTHS patient-derived cells exhibit basal or LPS-induced overactive immune responses compared to healthy controls, and determine whether compounds that modulate TLR4, casp-4 or CL function can restore immune homeostasis. Project outcomes include a mechanistic understanding of BTHS immune dysfunction, and new drug targets and drug candidates for treating BTHS patients affected by inflammatory disorders. Such new treatments would address the underlying cause of BTHS-associated immune dysfunction, rather than merely managing infections with antibiotics.
This project’s funding was made possible by a generous contribution from our affiliate Barth Syndrome Foundation of Canada