Modeling human cardiac genetic diseases in the zebrafish: Focus on cardiomyopathies and cohesinopathies

Cardiovascular disease is a leading cause of death worldwide. Currently, there is an increasing need to understand cardiac disease mechanisms, identify their pathological progression and find therapeutics to tackle triggers of the disease. Pathological conditions can occur due to genetic mutations leading to cardiac organ dysfunctions, caused by defective mechanical contraction and perturbed electrical signals. These dysfunctions result in structural remodeling and eventual heart failure. In order to understand human genetic diseases of the heart, we show case the zebrafish as an animal model to study these diseases.

In our work, sarcomeric and arrhythmogenic forms of cardiomyopathy are investigated. Troponin T (TNNT2), is an important sarcomeric protein that interacts with the contractile proteins upon the presence of calcium (Ca2+) to initiate heart contraction. Adult fish with RK94del heterozygous mutation in Troponin T result in cardiac structural remodeling, as well as slower contractility and changes in Ca2+ dynamics at the embryonic stages. Moreover, phospholamban (PLN) is a sarcoplasmic reticulum (SR) protein that regulate the activity of Sarco/endoplasmic reticulum Ca2+ ATPase (SERCA) to control the amount of Ca2+ entry into the SR. Mutants of cardiomyopathy-specific PLN R14del mutation display increased fibrofatty replacement and inflammation around the muscle region of the zebrafish ventricle, similarly to human situations of PLN R14del arrhythmogenic cardiomyopathy. Both mutant adults and embryos show a decrease in Ca2+ levels and dysregulation in the electrical signaling of the heart prior to the structural changes. Istaroxime, which is an enhancer of SERCA activity, causes PLN-SERCA interaction to be relieved. This in turn results in more Ca2+ entry to the mutant heart cells and rescue of the electrical signaling defects. Bulk RNA-sequencing and spatial transcriptomics show that genes involved in immunity and fat binding were increased in levels, while genes involved in contraction of the heart is decreased. Furthermore, immunity and fat binding genes were concentrated at the area with most structural changes. Mitochondria, the energy source of the cell, was also dysfunctional in PLN R14del, which may play a role in the fat buildup and the eventual structural changes.

In the last part of this thesis, we investigate cohesinopathies. These are diseases caused by genetic mutations in cohesins, the components of the cell that play a role in the process of cell division. Chronic atrial and intestinal dysrhythmia (CAID) is caused by mutation in Shugoshin-1 (SGO1), whereby patients have a genetic mutation that causes failure in the pace making ability of the cell in the heart and the gut. By targeting SGO1 in zebrafish, mutants show decrease in heart rate and reduction in function of several contraction parameters. Eye function in these mutants were disrupted due to structural damage in a layer of photoreceptors. These dysfunctions lead to early mortality in these fish as the heart does not function optimally and the fish is unable to visualize their food and environment. Therefore, the zebrafish is a promising model to use for understanding human heart genetic diseases and potentially identify therapies to treat the symptoms or alleviate them.