A number sign (#) is used with this entry because of evidence that familial cardiomyopathy of the hypertrophic (CMH26), restrictive (RCM5), dilated (CMD1PP), or arrhythmogenic right ventricular (ARVD15) type is caused by heterozygous mutation in the FLNC gene (102565) on chromosome 7q32.

For a general phenotypic description and a discussion of genetic heterogeneity of familial hypertrophic cardiomyopathy, see CMH1 (192600); for familial restrictive cardiomyopathy, see RCM1 (115210); for familial dilated cardiomyopathy, see CMD1A (115200); and for arrhythmogenic right ventricular dysplasia, see ARVD1 (107970).

Familial cardiomyopathy caused by mutation in the FLNC gene has been described as hypertrophic, restrictive, dilated, or arrhythmogenic right ventricular cardiomyopathy. Affected individuals, especially those with dilated cardiomyopathy, are at risk for arrhythmias and sudden death. Arrhythmias without cardiomyopathy, and left ventricular noncompaction, have also been reported (Ortiz-Genga et al., 2016; Verdonschot et al., 2020).

Hypertrophic Cardiomyopathy

Valdes-Mas et al. (2014) studied a 4-generation Spanish family segregating autosomal dominant hypertrophic cardiomyopathy and mutation in the FLNC gene. The proband was diagnosed at age 20 years, based on electrocardiography (ECG) showing left ventricular hypertrophy due to volume overload and echocardiography showing a 27-mm hypertrophic septum. At age 32, she developed atrial fibrillation; MRI showed extensive areas of fibrosis involving the septum, apex, and anterior wall, and she received an implantable cardioverter defibrillator. At age 53, she was functionally grade I in the New York Heart Association classification, with a left ventricular outflow obstruction of 30 mm Hg. She had 2 affected maternal cousins, 1 who was diagnosed at age 45 with heart failure, left ventricular volume overload (LVVO), and a 21-mm hypertrophic septum, and the other, who was diagnosed at age 82, was asymptomatic but showed LVVO with a 15-mm septum. In addition, the proband's mother died suddenly at age 34, and her maternal grandmother and great-aunt died at ages 62 and 55, respectively, from cardiac causes.

Restrictive Cardiomyopathy

Brodehl et al. (2016) studied 2 Canadian families segregating autosomal dominant restrictive cardiomyopathy and mutation in the FLNC gene. Almost all affected individuals had normal contractility, with normal left ventricular wall thickness and systolic function, but showed various degrees of diastolic dysfunction and pulmonary hypertension, as well as left or biatrial enlargement. Atrial fibrillation was common, and significant conduction disease requiring pacemakers was common in 1 of the families. There was no clinically detectable skeletal involvement, and CK levels were in the normal or slightly elevated range. Tissue analysis of several biopsies and heart explants consistently demonstrated mild to moderate myocyte hypertrophy, interstitial fibrosis, and only mild myocyte disarray. In addition, explanted heart tissue showed eosinophilic cytoplasmic inclusions in the free left ventricular wall, and electron microscopy revealed disorganization of Z-bands and abnormal accumulations of filamentous material in occasional fibers of the left ventricle.

Dilated Cardiomyopathy

Begay et al. (2016) reported 3 families, 2 Italian and 1 from the US, segregating isolated autosomal dominant dilated cardiomyopathy (CMD) and mutation in the FLNC gene. No evidence of skeletal muscle disease or creatine kinase elevation was present in any of the affected individuals. Family TSFDC029 was a 4-generation Italian family in which the proband's mother and brother died sudden unexpected cardiac deaths, at ages 54 and 34 years, and the brother was found to have cardiomegaly at autopsy. The proband died of noncardiac causes at age 64 years, but had frequent and repetitive premature ventricular contractions (PVCs) in the absence of heart failure. The proband's son and a maternal aunt had mild CMD. Family TSFDC031 was a 3-generation Italian family in which a father with a history of well-compensated CMD died suddenly at age 48; his son exhibited atrial tachycardia and fibrillation, and later developed CMD with mild left ventricular dysfunction. In family DNFDC057, a sister and brother were affected. The sister presented at age 46 with sustained polymorphic ventricular tachycardia and paroxysmal atrial fibrillation, and was found to have CMD with severe left ventricular dysfunction. She underwent cardiac transplantation at age 60. Her brother presented at age 53 with severe left ventricular dysfunction and frequent PVCs. His cardiac function deteriorated, and he was awaiting placement of a left ventricular assist device.

Ortiz-Genga et al. (2016) studied 28 probands with inherited cardiovascular disease and truncating mutations in the FLNC gene, including 20 previously diagnosed with dilated cardiomyopathy, 7 with arrhythmogenic cardiomyopathy with predominantly left ventricular (LV) involvement, and 1 with restrictive cardiomyopathy. Exertional dyspnea and palpitations were the most frequent presenting symptoms in the 28 probands and 54 carrier relatives. Although 23 (43%) of the 54 relatives with the mutation were asymptomatic, cardiac alterations were found in 40 (74%). Most of the probands exhibited LV dilation and systolic dysfunction, with a mean ejection fraction of 34%; these findings were also frequent among carrier relatives. Abnormalities of the right ventricle (RV) including dilation, akinesia, dyskinesia, or systolic dysfunction were present in 10 probands, all of whom had LV involvement as well. Most patients were in sinus rhythm and cardiac conduction defects were infrequent and mild; however, a majority (82%) of mutation carriers experienced ventricular arrhythmias, primarily frequent PVCs and nonsustained ventricular tachycardia. Myocardial fibrosis predominantly affecting the LV wall was present in 75% of investigated probands and in 52% of evaluated relatives. Endomyocardial biopsy of the RV revealed extensive fibrosis in the proband (family 31277) diagnosed with restrictive cardiomyopathy. Proband 31277 was the only patient who exhibited lower extremity muscle weakness during follow-up, and electromyography showed moderate myopathic changes; however, those changes were attributed to her posttransplant treatment with simvastatin and corticosteroids. Palmoplantar keratoderma (PPK) cosegregated with the cardiac phenotype in 1 family (29876), but PPK was not observed in any of the other families. Cardiac arrest occurred in 12 mutation carriers, which was the first manifestation of disease in 4 of them. Cardiac defibrillators were implanted in 24 mutation carriers; 1 patient declined, and 1 died suddenly while awaiting implantation. Heart transplantation was performed in 5 mutation carriers. There were 40 sudden deaths in 21 of the 28 families, including 12 known mutation carriers and 28 without genetic study. Although some symptomatic individuals were designated as having a phenotype of dilated cardiomyopathy and others as having left-dominant arrhythmogenic cardiomyopathy (LDACM), patients with either designation exhibited frequent ventricular ectopy as well as ventricular tachycardia, sustained or nonsustained.

Arrhythmogenic Right Ventricular Cardiomyopathy

Brun et al. (2020) reported 2 unrelated patients with arrhythmogenic right ventricular cardiomyopathy/dysplasia (ARVD15) and mutation in the FLNC gene. In family TSRVD029, the proband was a 66-year-old woman with a 20-year history of syncope and palpitations. She had complete right bundle branch block on ECG, and echocardiography showed mild RV dilation, RV fractional area change of 18%, and RV apical inferolateral wall akinesia. The LV showed normal structure and function. A 24-hour Holter ECG revealed nonsustained ventricular tachycardia (VT) with left bundle branch block (LBBB) morphology and frequent PVCs (more than 500 per hour). The proband's sister had refractory supraventricular arrhythmias, and the proband's nephew, a 45-year-old asymptomatic athlete, was found to have frequent PVCs and nonsustained VT with LBBB morphology. Cardiac magnetic resonance (CMR) showed a dilated RV with impaired systolic function and diffuse hypokinesia, as well as a dilated LV with a low-normal ejection fraction. He was diagnosed as having possible ARVC, since his RV dysfunction was global rather than regional. The proband also had a male cousin who died of progressive heart failure while awaiting heart transplant. In family JHRVD001, the proband was a 37-year-old woman who experienced cardiac arrest while playing tennis. CMR showed major RV structural abnormalities, with regional dyskinesia in the RV outflow tract, RV dysfunction (ejection fraction, 32%), and normal LV function, fulfilling definite ARVC criteria. She underwent placement of an implantable cardioverter-defibrillator (ICD), and had recurrent sustained ventricular tachycardia and frequent PVCs during follow-up. No other family members were symptomatic.

The transmission pattern of cardiomyopathy in the families reported by Valdes-Mas et al. (2014), Brodehl et al. (2016), and Begay et al. (2016) was consistent with autosomal dominant inheritance.

Hypertrophic Cardiomyopathy

In the proband from a 4-generation Spanish family with hypertrophic cardiomyopathy, who was negative for mutation in 9 CMH-associated sarcomeric genes, Valdes-Mas et al. (2014) performed whole-exome sequencing and identified heterozygosity for a missense mutation in the FLNC gene (A1539T; 102565.0005). The mutation segregated with disease in the family and was not found in more than 400 Spanish controls or in the 1000 Genomes Project or Exome Sequencing Project databases. Sequencing of FLNC in 92 probands with CMH identified 7 more heterozygous mutations in 8 patients (see, e.g., 102565.0006 and 102565.0007). The FLNC variants showed strong cosegregation with disease in the families, with 14 of 16 mutation carriers over 40 years of age exhibiting symptoms of CMH, for a penetrance of more than 87%.

Restrictive Cardiomyopathy

In a Canadian family with restrictive cardiomyopathy, Brodehl et al. (2016) analyzed 90 cardiomyopathy-associated genes and identified heterozygosity for a missense mutation in the FLNC gene (S1624L; 102565.0008). Whole-exome sequencing in an unrelated Canadian family with RCM revealed heterozygosity for a different missense mutation in FLNC (I2160F; 102565.0009). Each mutation segregated fully with disease in the family, and neither was found in controls or public variant databases.

Dilated Cardiomyopathy

In affected members of 2 Italian families (TSFDC029 and TSFDC031) segregating autosomal dominant CMD, Begay et al. (2016) performed whole-exome sequencing and identified heterozygosity for the same splice site mutation in the FLNC gene (102565.0013). The mutation segregated with disease in both families, but was also present in the 34-year-old sister of the proband from family TSFDC029. The sister had a history of palpitations and an unremarkable echocardiogram; she declined further clinical follow-up. Analysis of nearby genotypes in the 2 Italian families was consistent with a shared ancestral haplotype. The authors noted that the rare splicing variant was 83% penetrant, and that sudden death had occurred before the age of 55 years in 3 (38%) of 8 known or suspected carriers. In a sister and brother from the US with CMD (family DNFDC057), the authors analyzed a panel of 4,813 phenotype-associated genes and identified a different heterozygous splice site mutation in the FLNC gene (102565.0014). Neither mutation was found in public variant databases.

In 2,877 patients with various inherited cardiovascular diseases, Ortiz-Genga et al. (2016) analyzed 213 genes associated with inherited cardiovascular disease and sudden death, and identified 28 probands with truncating mutations in the FLNC gene (see, e.g., 102565.0015 and 102565.0016), including 20 patients previously diagnosed with dilated cardiomyopathy, 7 with arrhythmogenic cardiomyopathy with predominantly LV involvement, and 1 with restrictive cardiomyopathy. Complete cosegregation of the mutation with the cardiac phenotype was shown in the 23 families that agreed to be investigated. Fifty-four carrier relatives were identified, of which 23 (43%) were asymptomatic, although cardiac changes were found in 40 (74%) of them. The authors noted that there was a high prevalence of ventricular arrhythmias and sudden cardiac death, even in the absence of severe LV dilation and dysfunction, and no evidence of skeletal muscle involvement. Immunohistochemical staining of myocardial tissue showed the presence of filamin C only in intercalated discs, in both patients and controls, suggesting that the mechanism involved is different than that previously observed with myofibrillar myopathy.

Arrhythmogenic Right Ventricular Cardiomyopathy

From a cohort of 156 patients with ARVC, who were negative for mutation in 6 known ARVC-associated genes and 2 cardiomyopathy-associated genes, Brun et al. (2020) identified 2 unrelated probands with heterozygous truncating mutations in the FLNC gene. In family TSRVD029, the 66-year-old female proband and her 45-year-old nephew had a nonsense mutation (E2189X; 102565.0017), and in family JHRVD001, the 37-year-old female proband had a de novo 1-bp deletion (102565.0018). The authors considered ARVC and CMD, involving respectively the right and left ventricles, to be 2 extremes of the same disease, which demonstrate phenotypic overlap when the arrhythmic pattern is evident and predominant.

Associations Pending Confirmation

In 2 asymptomatic half brothers, aged 8 and 6 years, from a family with a strong history of sudden death, Mangum and Ferns (2019) screened a panel of 39 cardiac arrhythmia-associated genes and identified heterozygosity for a 13-bp deletion in the FLNC gene in both boys. The 5 relatives who died suddenly at young ages included their mother, who died at age 23, their maternal uncle (age 21), their maternal grandfather (age 36), a maternal great-aunt (in her 20s), and another maternal great-aunt (in infancy). The maternal great-aunt who died in her 20s had an established diagnosis of long QT (see 192500), and several other family members were suspected of having long QT, including a maternal aunt who experienced syncope and underwent placement of an implantable cardiac defibrillator (ICD) device. Both boys had frequent PVCs on serial monitoring and also underwent placement of an ICD; their QT intervals or cardiac structural assessments were not reported. The authors stated that determining the causative status of the deletion would require further study.

Ader et al. (2019) screened a panel of 51 cardiomyopathy-associated genes in 1,150 probands with isolated cardiomyopathy, including 700 with CMH, 300 with CMD, 50 with RCM, and 100 with left ventricular noncompaction (LVNC; see 604169). Heterozygosity for a 'probably' or 'certainly' pathogenic variant in the FLNC gene was identified in 28 patients (see, e.g., 102565.0008), including 13 with CMH, 10 with CMD, and 4 with RCM, as well as in 1 sporadic patient diagnosed with LVNC. All patients with CMD had truncating variants, whereas missense variants or in-frame insertions or deletions were found in other phenotypes. Implantable cardiac defibrillators (ICDs) were placed in 32% of patients, and a family history of sudden cardiac death was present in 25% of cases. The authors noted that there was a significantly higher rate of personal or family history of sudden cardiac death in probands carrying truncating versus missense variants, suggesting that null variants may predispose to severe arrhythmias.

Verdonschot et al. (2020) reviewed published reports of FLNC-mutated patients for possible genotype-phenotype correlations. Truncating variants were strongly associated with dilated cardiomyopathy and arrhythmogenic potential and showed no clear clustering in any specific region of the gene; most truncating variants were predicted to result in nonsense-mediated decay. In contrast, missense variants, particularly in the ROD2 domain, were mostly associated with hypertrophic cardiomyopathy. The authors noted that the clinical course of FLNC-associated CMD appeared to be more malignant than that of other forms of CMD, characterized by ventricular arrhythmias, myocardial fibrosis, and a high risk of sudden cardiac death. Because FLNC variants associated with restrictive and other cardiomyopathies were less prevalent, the authors were unable to draw any conclusions regarding the role of FLNC in those phenotypes.

Gigli et al. (2021) analyzed an international cohort of 85 patients from tertiary care centers with cardiomyopathy due to truncating mutations in the FLNC gene (FLNCtv). Phenotypes were heterogeneous at presentation, including 49% dilated cardiomyopathy, 25% arrhythmogenic left dominant cardiomyopathy, and 3% arrhythmogenic right ventricular cardiomyopathy, with frequently overlapping forms, such as biventricular and left-dominant arrhythmogenic cardiomyopathy. Left ventricular ejection fraction (LVEF) was less than 50% in 64% of carriers, and 34% had right ventricular fractional area changes less than 35%. LVEF was associated with all-cause risk of death, need for heart transplantation (HT) or for left ventricle assist device (LVAD) as well as nonarrhythmic death/HT/LVAD, but not with the risk of sudden cardiac death or major ventricular arrhythmias. Because FLNCtv-associated cardiomyopathy is associated with a high risk of ventricular arrhythmias that are not associated with the severity of left ventricular dysfunction, the authors suggested that alternative strategies of stratification of the arrhythmic risk in FLNCtv-related cardiomyopathy are needed.

Begay et al. (2016) generated zebrafish with knockdown of flncb, the FLNC ortholog with greatest sequence similarity to the human gene. The morphant zebrafish showed pericardial edema, dysmorphic or dilated cardiac chambers, and abnormal looping of the heart tube suggestive of systolic dysfunction at 48 hours postfertilization (hpf) and 72 hpf. At 48 hpf, 9% of the morphants lacked circulation entirely, indicating a functional cardiac defect; the remaining 91% showed varying degrees of reduced blood circulation, with an increase in retrograde flow and overall weaker contractility. In addition, heart rate was slower in the morphants than controls. Ultrastructural analysis of transverse ventricular sections showed myofibrils composed of fewer consecutive sarcomeres in the morphants than wildtype zebrafish, or myofibrils in abnormal arrangements with Z-discs. In most cases, Z-discs appeared irregular or absent; however, cross-sections revealed that the primary arrangement of thick and thin filaments into hexagonal lattices appeared normal, suggesting that initial sarcomerogenesis was normal, but that myofibril growth was impaired. In addition, morphant cardiomyocytes developed small vacuoles associated with or near the cell membrane, suggesting that intercellular attachments had ruptured. The authors suggested that flncb plays an important role in maintaining sarcomere stability and cardiomyocyte attachment as mechanical stress increases in the developing embryonic heart.