Abstract
Background: In 2016 we discovered that novel mutations in the gene PPA2 caused the sudden cardiac death (SCD) of two siblings of a New Zealand family1. This led to the identification of multiple other families worldwide who were affected by SCD, typically following an intercurrent infection or exposure to alcohol. Since then, a large number of pathogenic variants in pyrophosphatase 2, (PPA2), a nuclear-encoded protein crucial for normal mitochondrial function, has been established as a cause of inherited SCD world-wide by us and subsequently others2-5. Despite the identification of the genetic basis of this condition, the mechanistic link between the identified triggers and sudden cardiac arrest remains unknown. In normal conditions, the mitochondrial matrix PPA2 enzyme hydrolyses diphosphate into individual orthophosphate (Pi) molecules for use in ATP production, among other things.
Purpose: To understand how affected individuals can avoid SCD in infancy and into adulthood.
Methods: Retrospective cohort analysis
Results: Key findings within this cohort of families are: 1) Patients present with cardiac arrest following particular physiological stressors, typically an intercurrent viral infection (for individuals presenting at or below the age of 3 years) or by the ingestion of small amounts of ethanol, typically in teenagers. 2) Midmyocardial fibrosis. 3. Characteristic neurological features.
We now aim to determine how PPA2 deficiency disrupts cellular function leading to arrythmia and heart failure. We will use CRISPR technology to generate a cardiomyocyte cell model of the disorder6-10. This approach allows us to more accurately assess the effect of PPA2 mutations on cellular, metabolic and electrical function of cells, in comparison to cell lines without the mutations and compare their response to mitochondrial stressors to that of normal cells11-12. We hypothesize that in affected individuals, without the presence of triggers, orthophosphate from the cytoplasm is imported into the mitochondrial matrix via mitochondrial membrane channels, compensating for an approximately 100 fold decrease of mitochondrial PPA2 enzyme activity. This cytoplasmic reservoir of Pi is produced by the cytoplasmic pyrophosphatase, PPA1. We aim to test whether viral infections drastically affect mitochondrial PIC channels’ ability to shunt Pi into the mitochondrial matrix. These channels are sensitive to the presence of cytokines, which are produced through immune responses to infection. Similarly, we will test whether ethanol blocks mitochondrial membrane VDAC channel function, as these also shunt Pi into the matrix and are very sensitive to ethanol. Ultimately, we believe that the two channels underlie the compensatory import of cytoplasmic Pi produced through the homologue enzyme PPA1 (a cytoplasmic pyrophosphatase), and that understanding this process will better define the mechanisms leading to cardiomyocyte crisis.
Conclusion: This study presents the curious condition of PPA2 deficiency, characterised by two distinct trigger mechanisms causing heart failure in two age groups – infancy and teenage years. Future work will inform therapeutic interventions aimed at preserving mitochondrial and cardiac function in affected individuals.