Atrial fibrillation (AF) is the most frequent sustained arrhythmia and can lead to structural cardiac changes, known as tachycardia-induced cardiomyopathy (TIC). HCN4 is implicated in spontaneous excitation of the sinoatrial node, while channel dysfunction has been associated with sinus bradycardia, AF and structural heart disease. We here asked whether HCN4 mutations may contribute to the development of TIC, as well. Mutation scanning of HCN4 in 60 independent patients with AF and suspected TIC followed by panel sequencing in carriers of HCN4 variants identified the HCN4 variant P883R [minor allele frequency (MAF): 0,88%], together with the KCNE1 variant S38G (MAF: 65%) in three unrelated patients. Family histories revealed additional cases of AF, sudden cardiac death and cardiomyopathy. Patch-clamp recordings of HCN4-P883R channels expressed in HEK293 cells showed remarkable alterations of channel properties shifting the half-maximal activation voltage to more depolarized potentials, while channel deactivation was faster compared to wild-type (WT). Co-transfection of WT and mutant subunits, resembling the heterozygous cellular situation of our patients, revealed significantly higher current densities compared to WT. In conclusion HCN4-P883R may increase ectopic trigger and maintenance of AF by shifting the activation voltage of I-f to more positive potentials and producing higher current density. Together with the common KCNE1 variant S38G, previously proposed as a genetic modifier of AF, HCN4-P883R may provide a substrate for the development of AF and TIC. (C) 2019 Elsevier Inc. All rights reserved.