Arrhythmogenic right ventricular cardiomyopathy (ARVC) is a primary cardiomyopathy associated with sudden cardiac death (SCD) in young patients during high intensity activity1. The most common ARVC subtype 9 includes mutations in plakophilin2 (PKP2), affecting the desmosomal integrity2, 3. PKP2 directly and indirectly interacts with the cytoskeleton, the voltage gated sodium channel (NaV1.5), the inward rectifying potassium channel (Kir2.1), the gap junctions (Connexins) and modulators of excitation contraction coupling (ECC) such as CaMKII4. However, the detailed pro-arrhythmic mechanisms are not fully understood. Here we strive to elucidate the pro-arrhythmic changes in Ca handling and ion channel function in ARVC9 and the triggers for adverse development in some mutation carriers. We generated induced pluripotent stem cells (iPSC) from a patient who survived SCD and was diagnosed with ARVC9 (1bp-Deletion, c.1664delT; p.Phe555SerfsX8, Exon 7; indexpatient), an asymptomatic sibling carrying the same mutation and cells from a healthy proband (CTRL). After cardiac differentiation and a 3 month maturation, iPSC-derived cardiomyocytes (hiPSC-CM) were investigated. Ca transients (CaT; Fig. A) were measured by confocal microscopy using the Ca sensitive dye Fluo-4 AM. Compared to CTRL, CaT amplitude was significantly reduced in the indexpatient but not the sibling, and CaT decay slowed in iPSC-CM from the asymptomatic sibling and the indexpatient (CTRL n=43, sibling n=55, indexpatient n=80). Spontaneous CaT frequency (Fig. B) was significantly increased in cells from the sibling and the indexpatient. Importantly, we detected more frequency induced pro-arrhythmic events such as CaT alternans in cells from the sibling and the indexpatient (Fig. C). Action potentials (APs) and Na currents (INa and INaL) were acquired by whole-cell patch clamp. In accordance with increased CaT frequency, the resting membrane potential (RMP) was significantly more positive in cells from the indexpatient (-60 ±3.7 mV vs CTRL -70 ±2.4 mV; p=0.05). These changes cannot be explained by the reduced INa or INaL in cells from the indexpatient and the sibling (Fig. D and E). Thus, we measured protein expression of the inward rectifier Kir2.1 by Western blot technique. Kir2.1 expression was reduced from 0.45 in CTRL to 0.22 in the sibling and 0.23 in the indexpatient (Kir2.1/GAPDH), which might explain the shift in RMP. The intercellular communication via gap junctions, analyzed by fluorescence recovery after photobleach (FRAP) in cell clusters loaded with calcein, was also significantly impaired in cells from the indexpatient. The recovery rate of the bleached signal was reduced from 0.009 ±0.0008 1/s in CTRL cell clusters to 0.005.±0.0006 1/s in clusters from the indexpatient (p<0.001). We show for the first time that hiPSC-CM from an ARVC9 patient mutation have reduced intercellular communication and impaired ECC expressed in prolonged CaT and APD. Changes in Na currents and RMP were only detected in cells from the indexpatient while the impaired Ca cycling was also apparent in the asymptomatic sibling. We assume that early changes detected in Ca handling might set the basis for adverse ECC, but further changes are needed to cause pro-arrhythmic behavior.