Hendrikus L Granzier

Nemaline myopathy (NM), the most common non-dystrophic congenital myopathy, is characterized by generalized skeletal muscle weakness, often from birth. To date, no therapy exists that enhances the contractile strength of muscles of NM patients. Mutations in NEB, encoding the giant protein nebulin, are the most common cause of NM. The pathophysiology of muscle weakness in NM patients with NEB mutations (NEB-NM) includes a lower calcium-sensitivity of force generation. We propose that the lower calcium-sensitivity of force generation in NEB-NM offers a therapeutic target. Levosimendan is a calcium sensitizer that is approved for use in humans and has been developed to target cardiac muscle fibers. It exerts its effect through binding to slow skeletal/cardiac troponin C. As slow skeletal/cardiac troponin C is also the dominant troponin C isoform in slow-twitch skeletal muscle fibers, we hypothesized that levosimendan improves slow-twitch muscle fiber strength at submaximal levels of activation in patients with NEB-NM.

The small heat shock protein alphaB-crystallin interacts with N2B-Us, a large unique sequence found in the N2B element of cardiac titin. Using single molecule force spectroscopy, we studied the effect of alphaB-crystallin on the N2B-Us and its flanking Ig-like domains. Ig domains from the proximal tandem Ig segment of titin were also studied. The effect of wild type alphaB-crystallin on the single molecule force-extension curve was determined as well as that of mutant alphaB-crystallins harboring the dilated cardiomyopathy missense mutation, R157H, or the desmin-related myopathy mutation, R120G. Results revealed that wild type alphaB-crystallin decreased the persistence length of the N2B-Us (from approximately 0.7 to approximately 0.2 nm) but did not alter its contour length. alphaB-crystallin also increased the unfolding force of the Ig domains that flank the N2B-Us (by 51 +/- 3 piconewtons); the rate constant of unfolding at zero force was estimated to be approximately 17-fold lower in the presence of alphaB-crystallin (1.4 x 10(-4) s(-1) versus 2.4 x 10(-3) s(-1)). We also found that alphaB-crystallin increased the unfolding force of Ig domains from the proximal tandem Ig segment by 28 +/- 6 piconewtons. The effects of alphaB-crystallin were attenuated by the R157H mutation (but were still significant) and were absent when using the R120G mutant. We conclude that alphaB-crystallin protects titin from damage by lowering the persistence length of the N2B-Us and reducing the Ig domain unfolding probability. Our finding that this effect is either attenuated (R157H) or lost (R120G) in disease causing alphaB-crystallin mutations suggests that the interaction between alphaB-crystallin and titin is important for normal heart function.

Titin (TTN) is known as the largest sarcomeric protein that resides within the heart muscle. Due to alternative splicing of TTN, the heart expresses two major isoforms (N2B and N2BA) that incorporate four distinct regions termed the Z-line, I-band, A-band, and M-line. Next-generation sequencing allows a large number of genes to be sequenced simultaneously and provides the opportunity to easily analyze giant genes such as TTN. Mutations in the TTN gene can cause cardiomyopathies, in particular dilated cardiomyopathy (DCM). DCM is the most common form of cardiomyopathy, and it is characterized by systolic dysfunction and dilation of the left ventricle. TTN truncating variants have been described as the most common cause of DCM, while the real impact of TTN missense variants in the pathogenesis of DCM is still unclear. In a recent population screening study, rare missense variants potentially pathogenic based on bioinformatic filtering represented only 12.6% of the several hundred rare TTN missense variants found, suggesting that missense variants are very common in TTN and are frequently benign. The aim of this review is to understand the clinical role of TTN mutations in DCM and in other cardiomyopathies. Whereas TTN truncations are common in DCM, there is evidence that TTN truncations are rare in the hypertrophic cardiomyopathy (HCM) phenotype. Furthermore, TTN mutations can also cause arrhythmogenic right ventricular cardiomyopathy (ARVC) with distinct clinical features and outcomes. Finally, the identification of a rare TTN missense variant cosegregating with the restrictive cardiomyopathy (RCM) phenotype suggests that TTN is a novel disease-causing gene in this disease. Clinical diagnostic testing is currently able to analyze over 100 cardiomyopathy genes, including TTN; however, the size and presence of extensive genetic variation in TTN presents clinical challenges in determining significant disease-causing mutations. This review discusses the current knowledge of TTN genetic variations in cardiomyopathies and the impact of the diagnosis of TTN pathogenic mutations in the clinical setting.

The purpose of this study was to determine whether patients with heart failure and a preserved ejection fraction (HFpEF) have an increase in passive myocardial stiffness and the extent to which discovered changes depend on changes in extracellular matrix fibrillar collagen and cardiomyocyte titin.

Mechanisms underlying diastolic dysfunction need to be better understood.