Abstract Introduction SLC4A3 gene codes for an (Cl-/HCO3-) exchanger present on the surface of cardiomyocytes (AE3 channel) is involved in maintaining the physiological acid-base gradient of the cardiomyocyte at rest. Loss of function of the SLC4A3 gene leading to an increase in intracellular pH has. been reported to be responsible for short QT syndrome (SQT). Methods The aim of our study was to characterize the genotype-phenotype relationships of SLC4A3 mutations identified in the French databases (Paris, Nantes and Lyon) in patients with sudden death. Through systematic exome sequencing we identified 3 new SLC4A3 variants in 3 families with a history of sudden death in two universitary hospitals. Results Family 1. The c.3209G>C variant (p.Gly1070Ala) in the SLC4A3 gene was identified in a 7-yo boy with VF at sleep but admitted in encephalic death. EKG showed short QT (QTc 280ms). Clinical examination, blood tests, TTE and autopsy were normal. There was no family history of sudden death. His mother had EKG evidence of SQT and was carrier the SLC4A3 mutation. She was implanted with an ICD. His two older brothers and the rest of the family had no mutation or pathological ECG and were asymptomatic. Family 2. The c.1027C>T (p.Arg343Cys) mutation in the SLC4A3 gene was identified in a 29-year-old man who presented with sudden death at sleep (VF). Clinical examination, blood tests, TTE and autopsy were normal. A maternal uncle died suddenly while sleeping. His two brothers (30 and 27 yo) are both carriers of the SLC4A3 mutation and exhibit SQT (QTc 320 and 330 ms respectively). TEE, Holter monitoring and stress test were normal in both. Both brothers were treated with hydroquinidine. Over a 2-year follow-up they had no symptom nor AF. Family 3. The c.1022C>G (p.Thr341Arg) mutation was identified in the SLC4A3 gene in a 54-yoman who presented with VF during shower but admitted in encephalic death. There was no family history of sudden death. Clinical examination, blood tests and echocardiography as well as coronary angiogram were normal. The initial ECG showed a prolonged QT interval (QTc 520 ms). QTc decreased to 480 ms in the 48 following hours. His parents and his sister as well as his 18 yo daughter have normal QTc values. We are awaiting the genetic results. Conclusion Loss-of-activity-mutation in the cardiac chloride-bicarbonate exchanger AE3 has been recently reported to cause short QT syndrome but it seems that in rare cases it may also be responsible for LQTS. Further patients are needed to confirm these data. In order to understand the underlying electrophysiological mechanisms, we plan to characterize the cellular impact of these new mutations through IPS.