we defined different models to group together various types of variants: ( 1 ) lof: only lof variants according to variant effect predictor ( vep ). ( 2 ) loftee: high - confidence lof variants according to loftee. ( 3 ) lofcadd: lof and missense ( mis ) when predicted deleterious with cadd phred score ≥ 25. ( 4 ) lof1misid: lof and mis when predicted deleterious by at least one of the following prediction methods: metasvm, metalr, or cadd phred score ≥ 25. lof: only lof variants according to variant effect predictor ( vep ). loftee: high - confidence lof variants according to loftee. lofcadd: lof and missense ( mis ) when predicted deleterious with cadd phred score ≥ 25. lof1misid: lof and mis when predicted deleterious by at least one of the following prediction methods: metasvm, metalr, or cadd phred score ≥ 25. in all models, we used minor allele frequency ( maf ) < 2 % to select variants for analyses. for case - control analyses, we used logistic regression under an additive model to test for association between carrying an lof variant in popdc2 ( lof, mis according to each model a – d ) and phenotypes, in which disease status was the dependent variable and genotype counts as the independent variable. for the analyses, we used software developed at decode genetics. for testing association with heart rate, measurements were inverse - normal transformed and analyzed using a linear mixed model implemented in bolt - lmm. meta - analysis was performed on the summary results from is, uk, dk, and us when available, using a fixed - effects inverse - variance - weighted method. to uncover the genetic cause of sinus node disease, av conduction defects and hcm in a child ( age at presentation 11 – 15 years; family a ), we performed whole - exome sequencing in the proband, both parents, and two of his unaffected siblings. his parents are first cousins ( figure 1, individual ii - 3 in the pedigree; tables 1 and s3; figure s1 ). table 1 clinical characteristics of affected individuals with bi - allelic popdc2 variants family a family b family c family d ii - 3 ii - 4 ii - 1 ii: 2 ii: 3 ii - 1 popdc2 variant p. gln172 _ tyr176delinshis p. arg263his p. arg263cys p. trp188ter; p. leu37serfs 20 effect on trek1 current lof lof not tested not tested consanguinity yes yes no no sex m m m m m m age at presentation ( years ) 11 – 15 21 – 25 21 – 25 21 – 25 16 – 20 11 – 15 symptoms at presentation palpitations palpitations none none cardiac arrest chest pain cardiac arrhythmia and ecg abnormalities cardiac arrest no no no arrest following bradycardia arrest following bradycardia minimum heart rate ( bpm ) 33 30 33 n / a n / a 67 sinus node disease sinus pauses ( 3 s ) sinus bradycardia no sa - block sinus pauses ( 5 s ) sa block sinus arrest no av conduction disease 2 avb ( type 1 ) pq time of 200 ms second avb first and second avb ( type 1 and 2: 1 ) first avb first and second avb ( type 1 ) first avb atrial arrhythmia atrial fibrillation and flutter no atrial fibrillation and flutter no episodes of high atrial rate no ventricular arrhythmia monomorphic nsvt, pvcs no monomorphic nsvt no monomorphic nsvt vt structural cardiac and extracardiac abnormalities hypertrophic cardiomyopathy yes yes no no no no cardiac mri findings ( mm ) septal hypertrophy ( 23 mm ) septal hypertrophy ( 16mm ) n / a n / a possible myoc arditis significant lv fibrosis and inflammation myoc arditis no no no no possible myoc arditis clinically ( not on cardiac histology ) treatment pacemaker implantation ( age, years ) yes ( 15 ) no yes ( 23 ) yes ( 21 ) yes ( 17 ) yes ( 15 ), temporary icd implantation ( age, years ) yes ( 15 ) no no no yes ( 33 ) no appropriate icd shocks no n / a n / a n / a no n / a avb, atrioventricular block, bpm, beats per minute; icd, implantable cardiac defibrillator; mri, magnetic resonance imaging; n / a, not available, nsvt, non - sustained ventricular tachy cardia; pvc, premature ventricular contractions; sa, sinoatrial; tte, transthoracic echo cardiogram; vt, ventricular tachy cardia. clinical characteristics of affected individuals with bi - allelic popdc2 variants source paper: pmc12256823 avb, atrioventricular block, bpm, beats per minute; icd, implantable cardiac defibrillator; mri, magnetic resonance imaging; n / a, not available, nsvt, non - sustained ventricular tachy cardia; pvc, premature ventricular contractions; sa, sinoatrial; tte, transthoracic echo cardiogram; vt, ventricular tachy cardia. no likely causal variant was found in genes previously associated with mendelian cardiomyopathies or arrhythmia syndromes ( either recessive or dominant; table s4 ). in line with the expected recessive inheritance pattern, we identified a rare segregating homozygous variant, nm _ 001308333. 2: c. 516 _ 527del; np _ 001295262. 1: p. ( gln172 _ tyr176delinshis ), in the popeye domain - containing protein 2 ( popdc2 ), as the most likely variant consistent with a recessive mode of inheritance and parental consanguinity ( see table s5 for overview of the two segregating coding - region variants identified in the homozygous state ). of note, the only other homozygous segregating variant has been found 15 times in gnomad and was therefore found unlikely to cause the disease in this individual. no de novo variants were found in the index affected individual. the cardiac arrhythmia phenotype in the affected individual is consistent with studies in mice that showed that loss of popdc2 resulted in sinus pauses and bradycardia and with findings made in zebrafish where morpholino knockdown of popdc2 resulted in av block. we therefore found the p. gln172 _ tyr176delinshis variant as an excellent candidate for this cardiac disor der. we then searched for additional individuals carrying bi - allelic variants in popdc2 by screening this gene in 78 individuals that presented with a similar phenotype to family a ( i. e., ccd with hcm ) and a search using genematcher and decipher. in total, we identified six affected individuals from four unrelated families of different ancestries ( affected individuals 3 – 5 are siblings, figures 1 a and 1b; tables 1 and s3 ) who harbor either homozygous or compound heterozygous rare popdc2 variants. all six affected individuals were diag nosed with ccds with or without hcm. in summary ( see tables 1 and s3; figure 1 b ), ( 1 ) av conduction disease was present in all six individuals, mainly consisting of first - degree av block or second - degree av block type 1 ( wenkebach ), ( 2 ) sinus node disease presenting as sinus bradycardia and sinus pauses was present in four out of six individuals, ( 3 ) cardiac arrest accompanied by sinus bradycardia or asystole was seen in two out of six individuals, ( 4 ) atrial arrhythmia ( i. e., atrial flutter or fibrillation ) was detected in three out of six individuals, ( 5 ) non - sustained ventricular tachy cardia occurred in four out of six individuals, and ( 6 ) hcm was diag nosed in two out of six individuals ( probands in families a and b with no other genetic variant causing hcm ). a pacemaker was implanted in five out of six affected individuals with the age at implantation ranging from 15 to 23 years. in two out of six affected individuals, an implantable cardiov erter - defibrillator ( icd ) was implanted to prevent lethal ventricular arrhythmia ( ranging from 15 to 33 years ), but no appropriate icd shocks occurred. unlike the recessive syndrome associated with the other popdc genes ( i. e., popdc1 and popdc3 - related limb - girdle muscular dystrophy ), none of the affected individuals showed signs of muscular dystrophy. of note, individual 6 ( ii - 1 in family d; figure 1 a ) presented with acute myoc arditis, for which he was admitted to the intensive care unit. detailed phenotypic descriptions of all affected individuals can be found in note s1.