Molecular Diagnosis of Primary Cardiomyopathy in 231 Unrelated Pediatric Cases by Panel-Based Next-Generation Sequencing: A Major Focus on Five Carriers of Biallelic TNNI3 Pathogenic Variants

Structural and functional insights into α-actinin isoforms and their implications in cardiovascular disease

α-actinin (ACTN) is a pivotal member of the actin-binding protein family, crucial for the anchoring and organization of actin filaments within the cytoskeleton. Four isoforms of α-actinin exist: two non-muscle isoforms (ACTN1 and ACTN4) primarily associated with actin stress fibers and focal adhesions, and two muscle-specific isoforms (ACTN2 and ACTN3) localized to the Z-disk of the striated muscle. Although these isoforms share structural similarities, they exhibit distinct functional characteristics that reflect their specialized roles in various tissues. Genetic variants in α-actinin isoforms have been implicated in a range of pathologies, including cardiomyopathies, thrombocytopenia, and non-cardiovascular diseases, such as nephropathy. However, the precise impact of these genetic variants on the α-actinin structure and their contribution to disease pathogenesis remains poorly understood. This review provides a comprehensive overview of the structural and functional attributes of the four α-actinin isoforms, emphasizing their roles in actin crosslinking and sarcomere stabilization. Furthermore, we present detailed structural modeling of select ACTN1 and ACTN2 variants to elucidate mechanisms underlying disease pathogenesis, with a particular focus on macrothrombocytopenia and hypertrophic cardiomyopathy. By advancing our understanding of α-actinin's role in both normal cellular function and disease states, this review lays the groundwork for future research and the development of targeted therapeutic interventions.

Homozygous TNNI3 frameshift variant in a consanguineous family with lethal infantile dilated cardiomyopathy

BACKGROUND

Dilated cardiomyopathy (DCM) is characterized by dilatation of the left ventricle, systolic dysfunction, and normal or reduced thickness of the left ventricular wall. It is a leading cause of heart failure and cardiac death at a young age. Cases with neonatal onset DCM were correlated with severe clinical presentation and poor prognosis. A monogenic molecular etiology accounts for nearly half of cases.

FAMILY DESCRIPTION

Here, we report a family with three deceased offspring at the age of 1 year old. The autopsy of the first deceased infant revealed a DCM. The second infant presented a DCM phenotype with a severely reduced Left Ventricular Ejection Fraction (LVEF) of 10%. Similarly, the third infant showed a severe DCM phenotype with LVEF of 30% as well, in addition to eccentric mitral insufficiency.

RESULTS

Exome sequencing was performed for the trio (the second deceased infant and her parents). Data analysis following the autosomal dominant and recessive patterns of inheritance was carried out along with a mitochondrial pathways-based analysis. We identified a homozygous frameshift variant in the TNNI3 gene (c.204delG; p.(Arg69AlafsTer8)). This variant has been recently reported in the ClinVar database in association with cardiac phenotypes as pathogenic or likely pathogenic and classified as pathogenic according to ACMG.

CONCLUSION

Genetic counseling was provided for the family and a prenatal diagnosis of choronic villus was proposed in the absence of pre-implantation genetic diagnosis possibilities. Our study expands the case series of early-onset DCM patients with a protein-truncating variant in the TNNI3 gene by reporting three affected infant siblings.

Challenging of ECMO application in pediatric restrictive cardiomyopathy: case report of a novel TNNI3 variant

Background

Restrictive cardiomyopathy (RCM) represents a rare cardiovascular disorder stemming from filament-associated genes. Nonetheless, treating RCM presents considerable challenges, particularly concerning device implantation and mechanical support. Furthermore, elucidating the molecular function of specific variants holds promise in benefiting patients and enhancing prognosis, given the significant heterogeneity among RCM variants.

Case presentation

The proband, an eight-year-old female, was admitted to our hospital post cardiopulmonary resuscitation due to sudden cardiac arrest. Echocardiography revealed bilateral atrial enlargement. Whole-exome sequencing uncovered a novel heterozygous mutation (c.509G>A, p.R170Q) in TNNI3. Evaluation using the MutationTaster application deemed c.509G>A pathogenic (probability = 0.99). Following clinical manifestations, imaging assessments, and genetic screening, the proband received an RCM diagnosis. ECMO was recommended along with continuous renal replacement therapy. However, persistent atrial flutter ensued post-ECMO withdrawal. Attempts to restore cardiac rhythm with cardioversion, metoprolol, and amiodarone proved futile. Subsequent heart failure led to the patient's demise due to cardiac shock. Based on crystal protein structural analysis, we observed that cTnI-R170Q and R170W exerted similar impacts on protein structural stability and formation. However, both differed significantly from cTnI-R170G, primarily influencing amino acid regions 32-79 and 129-149, involved in TnC and actin binding. Therefore, cTnI-R170Q was revealed to induce RCM via the same molecular mechanism as cTnI-R170W.

Conclusion

Managing RCM remains a critical challenge. This study underscores the discouragement of device implantations for cardiac pump functional support in RCM, particularly for non-short-term scheduled HTx. Additionally, considering catheter ablation for atrial fibrosis-induced AFs is recommended. Mechanistically, cTnI-R170Q primarily diminishes troponin-actin interactions and destabilizes thin filaments.

Exploring the complex spectrum of dominance and recessiveness in genetic cardiomyopathies

Discrete categorization of Mendelian disease genes into dominant and recessive models often oversimplifies their underlying genetic architecture. Cardiomyopathies (CMs) are genetic diseases with complex etiologies for which an increasing number of recessive associations have recently been proposed. Here, we comprehensively analyze all published evidence pertaining to biallelic variation associated with CM phenotypes to identify high-confidence recessive genes and explore the spectrum of monoallelic and biallelic variant effects in established recessive and dominant disease genes. We classify 18 genes with robust recessive association with CMs, largely characterized by dilated phenotypes, early disease onset and severe outcomes. Several of these genes have monoallelic association with disease outcomes and cardiac traits in the UK Biobank, including LMOD2 and ALPK3 with dilated and hypertrophic CM, respectively. Our data provide insights into the complex spectrum of dominance and recessiveness in genetic heart disease and demonstrate how such approaches enable the discovery of unexplored genetic associations.

Homozygous TNNI3 Mutations and Severe Early Onset Dilated Cardiomyopathy: Patient Report and Review of the Literature

The TNNI3 gene encodes for the cardiac isoform of troponin I, a pivotal component of the sarcomeric structure of the myocardium. While heterozygous TNNI3 missense mutations have long been associated with autosomal dominant hypertrophic and restrictive cardiomyopathies, the role of TNNI3 null mutations has been more debated due to the paucity and weak characterization of reported cases and the low penetrance of heterozygous genotypes. In recent years, however, an increasing amount of evidence has validated the hypothesis that biallelic TNNI3 null mutations cause a severe form of neonatal dilated cardiomyopathy. Here, we expand the case series reporting two unrelated patients afflicted with early onset dilated cardiomyopathy, due to homozygosity for the p.Arg98* TNNI3 variant, which had thus far been documented only in heterozygous patients and apparently healthy carriers, and the recurrent p.Arg69Alafs*8 variant, respectively. A review of previously reported biallelic TNNI3 loss-of-function variants and their associated cardiac phenotypes was also performed.

Identification of a novel TNNI3 synonymous variant causing intron retention in autosomal recessive dilated cardiomyopathy

BACKGROUND

Dilated cardiomyopathy type 2A (DCM2A, MIM: #611880) is a rare autosomal recessive heart disease leading to heart failure and sudden cardiac death. However, the causative role of TNNI3 in DCM2A is still questioned due to few cases reported and the conflicting molecular biological evidence.

METHODS

Trio whole-exome sequencing (trio-WES) was performed in a Chinese family with dilated cardiomyopathy. Sanger sequencing and real-time quantitative PCR were used to confirm the variants identified. Expression outcome caused by the synonymous mutation was validated by minigene splicing analyses.

RESULTS

The one-year-old girl presented severe left ventricular enlargement and significantly reduced left ventricular systolic function and she died of respiratory and heart failure soon after her diagnosis. Trio-WES revealed a compound heterozygous variants of TNNI3, a novel c.24G＞A (p.Ala8Ala) (NM_000363.4) in exon 2 and a deletion of entire gene. Minigene splicing analyses showed it led to an intron retention (c.24 + 1_24 + 45ins) by intron 2 cryptic splicing.

CONCLUSIONS

Our study describes and characterizes a synonymous mutation in TNNI3 gene, supporting the clinical diagnosis of an autosomal recessive DCM. Our study emphasizes the importance of functional analysis to assess the potential pathogenicity of synonymous mutations, especially when the synonymous variants are not annotated as benign.