High prevalence and distinctive clinical features of LMNA-associated atrioventricular block in young patients.

BACKGROUND AND AIMS: Atrioventricular block (AVB) is a degenerative disease and more commonly encountered in elderly patients, but unusual and often of unknown etiology in young patients. This study aimed to investigate the potential contributions of genetic variations to AVB of unknown reasons in young patients. METHODS: We enrolled 41 patients aged <55 years with high-degree AVB of unknown etiology whose clinical and genetic data were collected. RESULTS: Genetic variants were identified in 20 (20/41, 48.8%) patients, 11 (11/20, 55%) of whom had LMNA variants including 3 pathogenic (c.961C > T, c.936+1G > T and c.646C > T), 4 likely pathogenic (c.1489-1G > C, c.265C > A, c.1609-2A > G and c.1129C > T) and 3 of uncertain significance (c.1158-3C > G, c.776A > G and c.674G > T). Compared to those without LMNA variants, patients with LMNA variants demonstrated a later age at onset of AVB (41.45 ± 9.89 years vs 32.93 ± 12.07 years, P = .043), had more prevalent family history of cardiac events (81.8% vs 16.7%, P < .000), suffered more frequently atrial (81.8% vs 10.0%, P < .000) and ventricular (72.7% vs 10.0%, P < .000) arrhythmias, and were more significantly associated with enlargement of left atrium (39.91 ± 7.83 mm vs 34.30 ± 7.54 mm, P = .043) and left ventricle (53.27 ± 8.53 mm vs 47.77 ± 6.66 mm, P = .036). CONCLUSIONS: Our findings provide insights into the genetic etiology of AVB in young patients. LMNA variants are predominant in genotype positive patients and relevant to distinctive phenotypic properties.

Effects of body mass index and blood pressure on atrioventricular block: Two-sample mendelian randomization.

BACKGROUND: Observational studies have suggested associations between some atherogenic risk factors and atrioventricular (AV) block. OBJECTIVE: The purpose of this study was to investigate the causal effects of several cardiometabolic exposures on AV block and evaluate the role of coronary artery disease (CAD) as a mediator on the causal pathway by mendelian randomization analysis. METHODS: Two-sample bidirectional mendelian randomization was performed to assess the causal effects of cardiometabolic traits on AV block and examine causality inversely. The exposures of interest included body mass index (BMI), systolic blood pressure (SBP), diastolic blood pressure (DBP), fasting glucose, fasting insulin, low-density lipoprotein, high-density lipoprotein, and triglyceride. Multivariable mendelian randomization was then conducted to disentangle the effect of each significant exposure. Mediation effect of CAD on the causal pathways were estimated by two-step, two-sample mendelian randomization. RESULTS: Genetically predicted elevation of BMI (odds ratio [OR] 1.40; 95% confidence interval [CI] 1.10-1.78; P = .006), SBP (OR 1.02; 95% CI 1.00-1.03; P = .015), and DBP (OR 1.04; 95% CI 1.01-1.07; P = .005) were significantly associated with increased AV block risk. Effects of the other exposures were insignificant. There were no reverse causal effects. Multivariable mendelian randomization showed causal effects of increased BMI, SBP, and DBP on AV block after mutual adjustment. CAD mediated 14.20% (8.82%, 16.46%), 26.32% (25.00%, 26.47,%) and 12.20% (7.69%, 15.94%) of AV block risk from BMI, SBP and DBP, respectively. CONCLUSION: Elevated BMI, SBP, and DBP exhibited causal effects on AV block. The impacts were partly mediated by CAD.

A rare case with fetal autoimmune heart block and KNCH2 variant-induced long QT syndrome: a controversial opinion on prenatal management strategy.

BACKGROUND: Among all fetal heart block patients, > 50% cases are associated with maternal autoimmune diseases, and such patients should receive treatment. However, nearly half of fetal heart block cases involve a mother with negative results following autoimmune antibody screening. A few studies have reported long QT syndrome (LQTS) can also present as a severe fetal bradycardia, which does not respond to fetal treatment. Herein, we reported a rare case of an infant who presented with high-degree autoimmune-mediated fetal atrioventricular block (AVB) with LQTS induced by a novel KCNH2 variant. This case led us to review our prenatal therapeutic strategy. CASE PRESENTATION: A 1-year-old boy presented to our heart center having experienced syncope 5 times in the past year. He had previously presented with fetal bradycardia during the fetal stage from 27 + 3 gestational weeks. The fetal echocardiography demonstrated AVB (2:1 transmission). As the maternal autoimmune antibody results were positive, his mother had received dexamethasone treatment during pregnancy; subsequently, the fetal AVB had changed from 2:1 to 4:3 transmission with elevated ventricular beating rates. However, this patient was identified to have complete AVB after birth. The initial electrocardiogram and Holter measurements at hospital administration showed complete AVB, pleomorphic ventricular tachycardia, a prolonged QT interval (QT = 602 ms, corrected QT = 538 ms), and wide and deep inverted T-waves. Meanwhile, torsades de pointes could be observed in several transit ventricular tachycardias based on Holter monitoring review. Genetic testing revealed KCNH2 c.2483G > A variant-induced LQTS. An implantable cardioverter defibrillator device and permanent pacemaker were both considered as therapeutic alternations; his parents ultimately accepted the implantation of a permanent pacemaker. CONCLUSIONS: For fetuses with autoimmune-mediated AVB, intrauterine treatment should still be pursued immediately. However, once the treatment outcomes are deemed unacceptable or unexpected, other genetic variant-related channelopathies should be highly suspected. If the fetus lacks a positive family history, fetal genetic testing should be recommended to improve the prognosis of such patients by introducing integrative therapeutic strategies between the prenatal and postnatal phases.

Familial Associations of Complete Atrioventricular Block: A National Family Study in Sweden.

BACKGROUND: Complete atrioventricular block (CAVB) is a major reason for implantation of permanent pacemakers, but knowledge of CAVB inheritance is sparse. This nationwide study aimed to determine the occurrence of CAVB in first-, second-, and third-degree relatives (full siblings, half-siblings, and cousins). METHODS: The Swedish multigeneration register was linked to the Swedish nationwide patient register for the period 1997 to 2012. All Swedish full sibling, half-sibling, and cousin pairs born to Swedish parents between 1932 and 2012 were included. Competing risks and time-to-event, subdistributional hazard ratios (SHRs) according to Fine and Gray and hazard ratios using Cox proportional hazards model were estimated using robust SEs and considering the relatedness of relatives (full siblings, half-siblings, cousins). Additionally, odds ratios (ORs) for CAVB were calculated for traditional cardiovascular comorbidities. RESULTS: The study population (N=6 113 761) consisted of 5 382 928 full siblings, 1 266 391 half-siblings, and 3 750 913 cousins. In total, 6442 (0.11%) unique individuals were diagnosed with CAVB. Of these, 4200 (65.2%) were males. SHRs for CAVB were 2.91 for full siblings (95% CI, 2.43-3.49), 1.51 for half-siblings (0.56-4.10), and 3.54 for cousins (1.73-7.26) of affected individuals. Age-stratified analysis showed higher risk in young individuals born from 1947 to 1986: SHR, 5.30 (3.78-7.43) for full siblings, SHR, 3.30 (1.06-10.31) for half-siblings, and SHR, 3.15 (1.39-7.17) for cousins. Similar familial HRs according to Cox proportional hazard model and ORs were obtained without any major differences. Apart from familial relationship, CAVB was associated with hypertension (OR, 1.83), diabetes (OR, 1.41), coronary heart disease (OR, 2.08), heart failure (OR, 5.01), and structural heart disease (OR, 4.59). CONCLUSIONS: Risk of CAVB among relatives of affected individuals depends on relationship degree, being strongest in young siblings. The familial association extending to third-degree relatives indicates presence of genetic components in the cause of CAVB.

NKX2-5 genetic mutation in a young woman with an atrial septal defect presenting with complete heart block: ICD or bradycardia pacemaker?

A woman in her 40s was admitted following syncope. The 12-lead ECG showed atrial fibrillation with slow ventricular response and suspected complete atrioventricular (AV) block. Cardiac monitoring demonstrated non-sustained monomorphic ventricular tachycardia (VT). Her medical history included surgical repair of an atrial septal defect (ASD) aged 4 years. The patient's mother died suddenly in her early 50s and also had an ASD. Given the patient's syncope, background of familial sudden cardiac death (SCD), suspicion of complete AV block and non-sustained VT, she received an implantable cardiac defibrillator (ICD). She underwent genetic testing, revealing a heterozygous NKX2-5 genetic mutation. The signature phenotype in NKX2-5 mutations is ASD with AV conduction disturbance and an increased risk of SCD secondary to ventricular arrhythmias or severe bradycardia. SCD has been described in NKX2-5 mutation carriers despite functioning permanent pacemakers (PPMs). Therefore, we propose implantation of a preventive ICD, as opposed to a PPM.

Role of genetic testing in young patients with idiopathic atrioventricular conduction disease.

AIMS: To investigate the role of genetic testing in patients with idiopathic atrioventricular conduction disease requiring pacemaker (PM) implantation before the age of 50 years. METHODS AND RESULTS: All consecutive PM implantations in Southern Switzerland between 2010 and 2019 were evaluated. Inclusion criteria were: (i) age at the time of PM implantation: < 50 years; (ii) atrioventricular block (AVB) of unknown aetiology. Study population was investigated by ajmaline challenge and echocardiographic assessment over time. Genetic testing was performed using next-generation sequencing panel, containing 174 genes associated to inherited cardiac diseases, and Sanger sequencing confirmation of suspected variants with clinical implication. Of 2510 patients who underwent PM implantation, 15 (0.6%) were young adults (median age: 44 years, male predominance) presenting with advanced AVB of unknown origin. The average incidence of idiopathic AVB computed over the 2010-2019 time window was 0.7 per 100 000 persons per year (95% CI 0.4-1.2). Most of patients (67%) presented with specific genetic findings (pathogenic variant) or variants of uncertain significance (VUS). A pathogenic variant of PKP2 gene was found in one patient (6.7%) with no overt structural cardiac abnormalities. A VUS of TRPM4, MYBPC3, SCN5A, KCNE1, LMNA, GJA5 genes was found in other nine cases (60%). Of these, three unrelated patients (20%) presented the same heterozygous missense variant c.2531G > A p.(Gly844Asp) in TRPM4 gene. Diagnostic re-assessment over time led to a diagnosis of Brugada syndrome and long-QT syndrome in two patients (13%). No cardiac events occurred during a median follow-up of 72 months. CONCLUSION: Idiopathic AVB in adults younger than 50 years is a very rare condition with an incidence of 0.7 per 100 000 persons/year. Systematic investigations, including genetic testing and ajmaline challenge, can lead to the achievement of a specific diagnosis in up to 20% of patients. Heterozygous missense variant c.2531G > A p.(Gly844Asp) in TRPM4 gene was found in an additional 20% of unrelated patients, suggesting possible association of the variant with the disease.

Pathological and Comprehensive Genetic Investigation of Autopsy Cases of Idiopathic Bradyarrhythmia.

BACKGROUND: Idiopathic bradyarrhythmia is considered to be due to pathological degeneration of the cardiac conduction system (CCS) during aging. There appears to have been no comprehensive genetic investigations in patients with idiopathic bradyarrhythmia.Methods and Results: Ten autopsy cases with advanced bradyarrhythmia (6 men and 4 women; age: 70-94 years, 81.5±6.9 years; 5 cases each of sinus node dysfunction [SND] and complete atrioventricular block [CAVB]) were genetically investigated by using whole-exome sequencing. Morphometric analysis of the CCS was performed with sex-, age- and comorbidity-matched control cases. As a result, severe loss of nodal cells and distal atrioventricular conduction system were found in SND and CAVB, respectively. However, the conduction tissue loss was not significant in either the atrioventricular node or the proximal bundle of His in CAVB cases. A total of 13 heterozygous potential variants were found in 3 CAVB and 2 SND cases. Of these 13 variants, 4 were missense in the known progressive cardiac conduction disease-related genes: GATA4 and RYR2. In the remaining 9 variants, 5 were loss-of-function mutation with highly possible pathogenicity. CONCLUSIONS: In addition to degenerative changes of selectively vulnerable areas in the heart during advancing age, the vulnerability of the CCS, which may be associated with "rare variants of small effect," may also be a contributing factor to the degeneration of CCS, leading to "idiopathic" bradyarrhythmia.

Auxilin is a novel susceptibility gene for congenital heart block which directly impacts fetal heart function.

OBJECTIVE: Neonatal lupus erythematosus (NLE) may develop after transplacental transfer of maternal autoantibodies with cardiac manifestations (congenital heart block, CHB) including atrioventricular block, atrial and ventricular arrhythmias, and cardiomyopathies. The association with anti-Ro/SSA antibodies is well established, but a recurrence rate of only 12%-16% despite persisting maternal autoantibodies suggests that additional factors are required for CHB development. Here, we identify fetal genetic variants conferring risk of CHB and elucidate their effects on cardiac function. METHODS: A genome-wide association study was performed in families with at least one case of CHB. Gene expression was analysed by microarrays, RNA sequencing and PCR and protein expression by western blot, immunohistochemistry, immunofluorescence and flow cytometry. Calcium regulation and connectivity were analysed in primary cardiomyocytes and cells induced from pleuripotent stem cells. Fetal heart performance was analysed by Doppler/echocardiography. RESULTS: We identified DNAJC6 as a novel fetal susceptibility gene, with decreased cardiac expression of DNAJC6 associated with the disease risk genotype. We further demonstrate that fetal cardiomyocytes deficient in auxilin, the protein encoded by DNAJC6, have abnormal connectivity and Ca2+ homoeostasis in culture, as well as decreased cell surface expression of the Cav1.3 calcium channel. Doppler echocardiography of auxilin-deficient fetal mice revealed cardiac NLE abnormalities in utero, including abnormal heart rhythm with atrial and ventricular ectopias, as well as a prolonged atrioventricular time intervals. CONCLUSIONS: Our study identifies auxilin as the first genetic susceptibility factor in NLE modulating cardiac function, opening new avenues for the development of screening and therapeutic strategies in CHB.

Diagnostic Yield of Genetic Testing in Young Patients With Atrioventricular Block of Unknown Cause.

Background The cause of atrioventricular block (AVB) remains unknown in approximately half of young patients with the diagnosis. Although variants in several genes associated with cardiac conduction diseases have been identified, the contribution of genetic variants in younger patients with AVB is unknown. Methods and Results Using the Danish Pacemaker and Implantable Cardioverter Defibrillator (ICD) Registry, we identified all patients younger than 50 years receiving a pacemaker because of AVB in Denmark in the period from January 1, 1996 to December 31, 2015. From medical records, we identified patients with unknown cause of AVB at time of pacemaker implantation. These patients were invited to a genetic screening using a panel of 102 genes associated with inherited cardiac diseases. We identified 471 living patients with AVB of unknown cause, of whom 226 (48%) accepted participation. Median age at the time of pacemaker implantation was 39 years (interquartile range, 32-45 years), and 123 (54%) were men. We found pathogenic or likely pathogenic variants in genes associated with or possibly associated with AVB in 12 patients (5%). Most variants were found in the LMNA gene (n=5). LMNA variant carriers all had a family history of either AVB and/or sudden cardiac death. Conclusions In young patients with AVB of unknown cause, we found a possible genetic cause in 1 out of 20 participating patients. Variants in the LMNA gene were most common and associated with a family history of AVB and/or sudden cardiac death, suggesting that genetic testing should be a part of the diagnostic workup in these patients to stratify risk and screen family members.

Familial risk of atrioventricular block in first-degree relatives.

OBJECTIVE: Rare cases of genetically inherited atrioventricular block (AVB) have been reported; however, the heredity of AVB remains unknown. We aimed to assess the heredity of AVB. DESIGN, SETTING AND PARTICIPANTS: Using data from the Danish Civil Registration Registry, we established a nationwide cohort of individuals with parental links. Data were merged with information from the Danish Pacemaker and Implantable Cardioverter Defibrillator Registry, containing information on all pacemaker implantations performed in Denmark during the study period, to identify patients who received a first-time pacemaker because of AVB. RESULTS: A total of 4 648 204 individuals had parental links and a total of 26 880 consecutive patients received a first-time pacemaker due to AVB. Overall, the adjusted rate ratio (RR) of pacemaker implantation due to AVB was 2.1 (95% CI 1.8 to 2.5) if a father, mother or sibling had AVB compared with the risk in the general population. The adjusted RR was 2.2 (1.7-2.9) for offspring of mothers with AVB, 1.9 (1.5-2.4) for offspring of fathers with AVB and 3.5 (2.3-5.4) for siblings to a patient with AVB. The risk increased inversely proportionally with the age of the index case at the time of pacemaker implantation. The corresponding adjusted RRs were 15.8 (4.8-52.3) and 10.0 (3.3-30.4) if a mother or father, respectively, had a pacemaker implantation before 50 years. CONCLUSION AND RELEVANCE: First-degree relatives to a patient with AVB carry an increased risk of AVB with the risk being strongly inversely associated with the age of the index case at pacemaker implantation. These findings indicate a genetic component in the development of AVB in families with an early-onset disease.

The Role of TRPM4 Gene Mutations in Causing Familial Progressive Cardiac Conduction Disease: A Further Contribution.

Progressive cardiac conduction disease (PCCD) is a relatively common condition in young and elderly populations, related to rare mutations in several genes, including SCN5A, SCN1B, LMNA and GJA5, TRPM4. Familial cases have also been reported. We describe a family with a large number of individuals necessitating pacemaker implantation, likely due to varying degrees of PCCD. The proband is a 47-year-old-patient, whose younger brother died at 25 years of unexplained sudden cardiac death. Three paternal uncles needed a pacemaker (PM) implantation between 40 and 65 years for unspecified causes. At the age of 42, he was implanted with a PM for two episodes of syncope and the presence of complete atrioventricular block (AVB). NGS analysis revealed the missense variation c. 2351G>A, p.Gly844Asp in the exon 17 of the TRPM4 gene. This gene encodes the TRPM4 channel, a calcium-activated nonselective cation channel of the transient receptor potential melastatin (TRPM) ion channel family. Variations in TRPM4 have been shown to cause an increase in cell surface current density, which results in a gain of gene function. Our report broadens and supports the causative role of TRPM4 gene mutations in PCCD. Genetic screening and identification of the causal mutation are critical for risk stratification and family counselling.

Management of heart failure with concomitant complete atrioventricular block caused by a novel missense LMNA mutation.

A 30-year-old lady was admitted to the hospital with progressive exertional dyspnoea and bradycardia. A complete atrioventricular block was diagnosed using 12­lead electrocardiography and a transthoracic echocardiography revealed a severely impaired left ventricular systolic dysfunction with an ejection fraction of 20%. Following hospitalization, her coronary angiography was normal, so a whole exome sequencing was conducted. The novel Lamin A/C Gene missense mutation c.263C > A,p.Ala88Asp in exon 3 was identified. A CRT-D was implanted due to the high risk of life-threatening ventricular arrhythmias and low potential for left ventricular reverse remodelling. The patient is undergoing follow-ups at the outpatient clinic, showing a 25% improvement in left ventricular ejection fraction during the last visit.

Identification of a SCN5A founder mutation causing sudden death, Brugada syndrome, and conduction blocks in Southern Italy.

BACKGROUND: The genetic architecture of Brugada syndrome (BrS) is emerging as an increasingly complex area of investigation. The identification of genetically homogeneous populations can provide mechanistic insights and improve genotype-phenotype correlation. OBJECTIVE: To characterize and define the clinical implications of a novel BrS founder mutation. Using a haplotype-based approach we investigated whether 2 SCN5A genetic variants could derive from founder events. METHODS: Single nucleotide polymorphisms were genotyped in 201 subjects, haplotypes reconstructed, and mutational age estimated. Clinical phenotypes and historical records were collected. RESULTS: A SCN5A variant (c.3352C>T; p.Gln1118Ter) was identified in 3 probands with BrS originating from south Italy. The same mutation was identified in a proband from central Italy and in 1 U.S. resident subject with Italian ancestry. The 5 individuals carried a common core haplotype, whose frequency was extremely low in local noncarrier probands and in population controls (0%-6.06%). The clinical presentation included multigenerational dominant transmission of Brugada electrocardiographic pattern, high incidence of sudden cardiac death (SCD), and cardiac conduction defects (CCD). We reconstructed 7-generation pedigrees with common geographic origin. Variant's age estimates suggested that origin of the p.Gln1118Ter dates back 76 generations (95% confidence interval: 28-200). A second SCN5A variant (c.5350G>A; p.Glu1784Lys) identified in the region did not show similar founder signal. CONCLUSION: p.Gln1118Ter is a novel BrS/CCD/SCD founder mutation. We illustrate how these findings provide insights on the inheritance patterns and phenotypes associated with SCN5A mutation.

Risk of sudden cardiac death in EXOSC5-related disease.

The RNA exosome is a multi-subunit complex involved in the processing, degradation, and regulated turnover of RNA. Several subunits are linked to Mendelian disorders, including pontocerebellar hypoplasia (EXOSC3, MIM #614678; EXOSC8, MIM #616081: and EXOSC9, MIM #618065) and short stature, hearing loss, retinitis pigmentosa, and distinctive facies (EXOSC2, MIM #617763). More recently, EXOSC5 (MIM *606492) was found to underlie an autosomal recessive neurodevelopmental disorder characterized by developmental delay, hypotonia, cerebellar abnormalities, and dysmorphic facies. An unusual feature of EXOSC5-related disease is the occurrence of complete heart block requiring a pacemaker in a subset of affected individuals. Here, we provide a detailed clinical and molecular characterization of two siblings with microcephaly, developmental delay, cerebellar volume loss, hypomyelination, with cardiac conduction and rhythm abnormalities including sinus node dysfunction, intraventricular conduction delay, atrioventricular block, and ventricular tachycardia (VT) due to compound heterozygous variants in EXOSC5: (1) NM_020158.4:c.341C > T (p.Thr114Ile; pathogenic, previously reported) and (2) NM_020158.4:c.302C > A (p.Thr101Lys; novel variant). A review of the literature revealed an additional family with biallelic EXOSC5 variants and cardiac conduction abnormalities. These clinical and molecular data provide compelling evidence that cardiac conduction abnormalities and arrhythmias are part of the EXOSC5-related disease spectrum and argue for proactive screening due to potential risk of sudden cardiac death.

A novel LMNA indel mutation identified in a family with atrioventricular block and atrial fibrillation.

ABSTRACT: It is well known that many genetic factors are involved in the occurrence and progression of atrioventricular block (AV block) and atrial fibrillation (AF). However, the genetic variants discovered so far have only explained parts of these processes. More genes and variants remain to be identified. In the present study, a three-generation family with an autosomal dominant form of AV block and AF was enrolled. Whole exome sequencing was conducted in three affected and one unaffected family member. A total of 64 nonsynonymous variants was shared by three affected individuals and not present in the unaffected individual. By selection of variants absent in the known databases and were predicted to be deleterious, 4 novel variants were identified. Only one novel frameshift insertion in the LMNA gene (c.825_826insCAGG) was identified in another affected family member and not detected in other non-affected family members and the 100 controls. Our finding expanded the spectrum of variants associated with AV block and AF, and was valuable in the genetic diagnosis of AV block and AF.

Whole-Exome Sequencing Identifies a Novel TRPM4 Mutation in a Chinese Family with Atrioventricular Block.

Atrioventricular block (AVB) is a leading cause of sudden cardiac death, and most of AVB cases are presented as autosomal dominant. The electrocardiogram of AVB patients presents an abnormal progressive cardiac conduction disorder between atria and ventricles. Transient receptor potential melastatin 4 (TRPM4) is a nonselective Ca2+-activated cation channel gene defined as a novel disease-causing gene of AVB. So far, 47 mutations of TRPM4 have been recorded in Human Gene Mutation Database. The aim of this study was to explore the relationship between TRPM4 mutation and pathogenesis of AVB. We investigated a Chinese family with AVB by whole-exome sequencing. An arrhythmia-related gene filtering strategy was used to analyze the disease-causing mutations. Three different bioinformatics programs were used to predict the effects of the mutation result. A novel mutation of TRPM4 was identified (c.2455C>T/p.R819C) and cosegregated in the affected family members. The three bioinformatics programs predicted that the novel mutation may lead to damage. Our study will contribute to expand the spectrum of TRPM4 mutations and supply accurate genetic testing information for further research and the clinical therapy of AVB.

A New TBX5 Loss-of-Function Mutation Contributes to Congenital Heart Defect and Atrioventricular Block.

Congenital heart defect (CHD) represents the most common birth deformity, afflicting 1% of all births worldwide, and accounts for substantial morbidity and mortality. Increasing evidence highlights the pivotal roles of genetic etiologies in the pathogenesis of CHD, and pathogenic mutations in multiple genes, including TBX5 encoding a cardiac core transcription factor key to cardiovascular morphogenesis, have been involved in CHD. However, due to pronounced genetic heterogeneity of CHD, the genetic determinants underlying CHD in most cases remain obscure. In this investigation, by sequencing analysis of the coding exons and flanking introns of the TBX5 gene in 198 unrelated patients affected with CHD, a novel heterozygous mutation, NM_000192.3: c.692C>T; p. (Pro231Leu), was identified in an index patient with familial double outlet right ventricle (DORV), ventricular septal defect (VSD), and atrioventricular block (AVB). Genetic analysis of the proband's pedigree showed that the mutation co-segregated with the diseases. The missense mutation, which altered the amino acid conserved evolutionarily, was absent from 266 unrelated healthy subjects. Functional analyses with a dual-luciferase reporter assay system unveiled that the Pro231Leu-mutant TBX5 was associated with significantly reduced transcriptional activity on its target genes MYH6 and NPPA. Furthermore, the mutation disrupted the synergistic transactivation between TBX5 and NKX2-5 as well as GATA4, two other transcription factors causally linked to CHD. This study firstly links TBX5 loss-of-function mutation to familial DORV, VSD, and AVB, which provides novel insight into the mechanism underpinning CHD and AVB, suggesting potential implications for genetic evaluation and individualized treatment of patients affected by CHD and AVB.