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Heart defects on a dish

About 1% of all newborns have structural cardiac malformations called congenital heart defects (CHD), which are the most common congenital malformations. CHD is an illustrative example of a complex disease, where both genes and adverse environmental stimuli can contribute to the pathogenesis. Around 10% of CHDs are estimated to be monogenic, however, in many cases the cause is thought to be oligogenic with two or more predisposing genetic variants. CHD has reduced penetrance, meaning that not all individuals with predisposing genetic variants develop disease. In addition, the genetic variants result in variable expressivity, where the same variant can cause different types of cardiac malformations in different individuals. Due to the complex inheritance pattern, identifying gene variants associated with CHD has been challenging. Further complicating the research field is the knowledge that certain environmental exposures, such as maternal diabetes, are known to be additional risks for CHD in the offspring.

Recent advances in stem cell biology and genome editing techniques have brought patient derived induced pluripotent stem cells (iPSC) as appealing disease modelling tools, which are especially attractive in complex disease. iPSCs can be reprogrammed from patients' skin or blood cells. The iPSCs can then further be differentiated into cells of interest, such as heart muscle cells (cardiomyocytes) or to cells lining blood vessels (endothelial cells). The properties of these patient derived cells can be studied to explore possible functional abnormalities.

Modern gene editing techniques allow us to do arts and crafts with genes. For example, the CRISPR/Cas9 technology is a useful tool in determining the pathogenicity of specific genetic variants. If patient derived cells are found to possess functional or phenotypic abnormalities, correcting the genetic variant suspected to cause disease should also correct these abnormalities. In practice this happens by 1) correcting the candidate variant in the patient’s iPSCs and 2) differentiating iPSCs containing the candidate variant and iPSCs in which the variant has been corrected to cells of interest and comparing the properties of these two cell lines.

The Finnish population is ideal for genetic studies and we have been able to collect a unique patient material from patients with congenital heart defects. We have identified potential disease causing variants in some of these individuals. During the last year, we have established iPS-cardiomyocyte (iPS-CM) and iPS-endothelial cell (iPS-EC) differentiation protocols in the Kivelä lab, and we are currently studying the pathogenicity and disease mechanisms of the candidate variants in patient derived iPSCs. Cardiac development is a complex interplay between multiple cell types, signaling pathways, and their response to mechanical forces of early blood flow. Co-cultures of iPS-CM and iPS-ECs and culturing the cells in flow are techniques we are using to mimic this complex system.

Our study will shed light on the origins of heart defects and model these defects on a dish. In addition to finding disease-associated genes, our study contributes to developing new methods to use patient derived iPSCs for disease modelling in complex cardiovascular disease. These methods can be applied to other forms of research involving the interplay between iPS-ECs and iPS-CMs, such as the cardiotoxicity of certain pharmaceuticals, and the role of endothelial cells in cardiomyocyte wellbeing in adult disease. In addition, our study results will shed light on the pathogenic mechanisms in diseases with a complex inheritance pattern in general.