Skeletal Muscle Involvement in Patients With Truncations of Titin and Familial Dilated Cardiomyopathy

Analysis of TTN Truncating Variants in >74 000 Cases Reveals New Clinically Relevant Gene Regions

BACKGROUND

Truncating variants (TTNtvs) in the titin (TTN) gene have been associated with cardiomyopathies or arrhythmias (C/A) and autosomal recessive neuromuscular diseases (NM). However, the clinical significance of TTNtvs across the entire coding sequence of TTN has not been comprehensively assessed. The purpose of this study was to examine the burden of TTNtvs in C/A and NM cases compared with controls in the genome aggregation database.

METHODS

This was a retrospective study of probands who underwent multigene testing (49 740 C/A panel, 24 514 NM panel) that included TTN from November 2017 to October 2021. Burden testing was performed using controls in the genome aggregation database v3.1.2 database, and the analysis was stratified by exon/band location and exon usage in cardiac or skeletal muscle. Frequency and odds ratio of TTNtv alleles in C/A or NM cases and genome aggregation database controls were measured.

RESULTS

There were 2446 (4.9%) C/A and 482 (2.0%) NM cases with 2446 and 528 TTNtv alleles, respectively. TTNtvs in all bands were significantly enriched in both C/A and NM cases compared with controls. A significant enrichment of TTNtvs in C/A was observed for exon 358 of the M-band (odds ratio, 2.55 [95% CI, 1.85-3.54]) but not the other M-band exons.

CONCLUSIONS

In the largest single-site cohort of C/A and NM cases with TTNtvs, an enrichment of TTNtvs across TTN was observed. These findings expand the clinically relevant regions of TTN.

Genotype-phenotype insights of pediatric dilated cardiomyopathy

Dilated cardiomyopathy (DCM) in children is a severe myocardial disease characterized by enlargement of the left ventricle or both ventricles with impaired contractile function. DCM can cause adverse consequences such as heart failure, sudden death, thromboembolism, and arrhythmias. This article reviews the latest advances in genotype and phenotype research in pediatric DCM. With the development of gene sequencing technologies, considerable progress has been made in genetic research on DCM. Research has shown that DCM exhibits notable genetic heterogeneity, with over 100 DCM-related genes identified to date, primarily involving functions such as calcium handling, the cytoskeleton, and ion channels. As human genomic variations are linked to phenotypes, DCM phenotypes are influenced by numerous genetic variations across the entire genome. Children with DCM display high genetic heterogeneity and are characterized by early onset, rapid disease progression, and poor prognosis. The genetic architecture of pediatric DCM markedly differs from that of adult DCM, necessitating analyses through clinical phenotyping, familial cosegregation studies, and functional validation. Clarifying the genotype-phenotype relationship can improve diagnostic accuracy, enhance prognosis, and guide follow-up treatment for genotype-positive and phenotype-negative patients identified through genetic testing, providing new insights for precision medicine. Future research should further explore novel pathogenic genes and mutations and strengthen genotype-phenotype correlation analyses to facilitate precise diagnosis and treatment of DCM in children.

Exercise training improves cardiovascular fitness in dilated cardiomyopathy caused by truncating titin variants

BACKGROUND

Participation in regular exercise activities is recommended for patients with chronic heart failure. However, less is known about the effect of exercise in patients with genetic dilated cardiomyopathy (DCM). We sought to examine the effect of vigorousintensity training on physical capacity in patients with DCM caused by truncating titin variants (TTNtv).

TRIAL DESIGN

Non-randomised clinical pre-post trial of exercise training.

METHODS

Individuals with DCM-TTNtv were included from outpatient clinics for inherited cardiac diseases. The trial consisted of 8 weeks of usual care followed by 8 weeks of regular vigorous-intensity cycling exercise, enclosed by three test days. The primary outcome was change in peak oxygen uptake (VO2). Secondary outcomes included change in blood volume, total haemoglobin mass, measures of systolic function and cardiac output/stroke volume during exercise.

RESULTS

Thirteen out of 14 included participants (43% women, age 48±11 years, body mass index: 30±6 kg/m2) completed the trial. In the exercise training period, peak VO2 increased by +1.9 mL/kg/min (95% CI +0.9 to +2.9, p=0.002). Compared with usual care, exercise training improved peak VO2 by +2.9 mL/kg/min (95% CI +1.2 to +4.5, p=0.002), corresponding to a 10% increase. Adaptations to exercise training included an increase in resting cardiac output (+0.8 L/min, p=0.042), total blood volume (+713 mL, p<0.001), total haemoglobin mass (+73 g, p<0.001), and improved left ventricular (LV) systolic function (LV ejection fraction: +3.2% (p=0.053) and global longitudinal strain: -2.0% (p=0.044)). No exercise-related adverse events or change in plasma biomarkers of cardiac or skeletal muscle damage were observed.

CONCLUSIONS

Our study shows that vigorous intensity exercise training improved peak VO2 in patients with DCM-TTNtv. Exercise training was associated with improved LV systolic function and increased blood volume and oxygen carrying capacity. Future research should investigate the effect of long-term exercise in this group.

TRIAL REGISTRATION NUMBER

NCT05180188.

Pathomechanisms of Monoallelic variants in TTN causing skeletal muscle disease

Pathogenic variants in the titin gene (TTN) are known to cause a wide range of cardiac and musculoskeletal disorders, with skeletal myopathy mostly attributed to biallelic variants. We identified monoallelic truncating variants (TTNtv), splice site or internal deletions in TTN in probands with mild, progressive axial and proximal weakness, with dilated cardiomyopathy frequently developing with age. These variants segregated in an autosomal dominant pattern in 7 out of 8 studied families. We investigated the impact of these variants on mRNA, protein levels, and skeletal muscle structure and function. Results reveal that nonsense-mediated decay likely prevents accumulation of harmful truncated protein in skeletal muscle in patients with TTNtvs. Splice variants and an out-of-frame deletion induce aberrant exon skipping, while an in-frame deletion produces shortened titin with intact N- and C-termini, resulting in disrupted sarcomeric structure. All variant types were associated with genome-wide changes in splicing patterns, which represent a hallmark of disease progression. Lastly, RNA-seq studies revealed that GDF11, a member of the TGF-β superfamily, is upregulated in diseased tissue, indicating that it might be a useful therapeutic target in skeletal muscle titinopathies.

Dilated Cardiomyopathy: A Genetic Journey from Past to Future

Dilated cardiomyopathy (DCM) is characterized by reduced systolic function and cardiac dilation. Cases without an identified secondary cause are classified as idiopathic dilated cardiomyopathy (IDC). Over the last 35 years, many cases of IDC have increasingly been recognized to be genetic in etiology with a core set of definitively causal genes in up to 40% of cases. While over 200 genes have been associated with DCM, the evidence supporting pathogenicity for most remains limited. Further, rapid advances in sequencing and bioinformatics have recently revealed a complex genetic spectrum ranging from monogenic to polygenic in DCM. These advances have also led to the discovery of causal and modifier genetic variants in secondary forms of DCM (e.g., alcohol-induced cardiomyopathy). Current guidelines recommend genetic counseling and screening, as well as endorsing a handful of genotype-specific therapies (e.g., device placement in LMNA cardiomyopathy). The future of genetics in DCM will likely involve polygenic risk scores, direct-to-consumer testing, and pharmacogenetics, requiring providers to have a thorough understanding of this rapidly developing field. Herein we outline three decades of genetics in DCM, summarize recent advances, and project possible future avenues for the field.