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

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.