We confirmed that higher cTnI levels were generally found in the second decade of life in patients with DMD and BMD. Second, the median serum cTnI level by age was higher in DMD patients until the age of 18 years, and abnormal cTnI values were more common in the DMD group than in the BMD group. In addition, the median maximum cTnI levels were found one year before the median abnormal LVEF value in DMD patients and three years before in BMD patients. Finally, we found that the ACTN3 XX genotype showed higher cTnI elevation earlier than the other two genotypes in DMD patients.
Cardiac dysfunction due to the progression of cardiomyopathy is the main cause of death in patients with DMD and BMD . Therefore, international guidelines recommend early detection and therapy for cardiomyopathy in DMD and BMD patients [2, 30]. There are, however, some difficulties associated with diagnosing cardiomyopathy since the age of onset and severity can be variable , and symptoms due to deterioration of cardiac function, such as orthopnea and dyspnea on exertion or rest, are rare in DMD patients . Nigro et al. reported that 61.5% of patients with DMD had preclinical cardiomyopathy without any symptom . Perloff et al. reported that even asymptomatic DMD patients showed regional wall motion abnormalities after 10 years of age . Early detection of cardiomyopathy in patients with DMD may, therefore, be relevant because timely initiation of medications or therapies will delay cardiac remodeling and relieve cardiac dysfunction. Biomarkers that can detect the onset of cardiomyopathy are thus required, and our study suggests that cTnI may be such a biomarker.
cTnI is a member of the troponin complex and a major component of myofibrils, and it is uniquely expressed in cardiac muscles . It appears in the blood following cardiac injury and is a specific cardiac injury marker . Its levels are elevated not only in acute but also in chronic pathogenic conditions. Recently, increasing cTnI levels have been shown in patients with cardiomyopathy or chronic HF in the general population [33, 34]. In the present study, 55.9% and 31.9% of the patients with DMD and BMD, respectively, had elevated cTnI levels. Although Kan et al. reported that DMD patients who had acute cardiomyopathy with acute chest pain had elevated cTnI levels and diffuse ST changes on ECG , none of the patients in our study complained of symptoms related to acute myocardial injury such as chest pain or dyspnea. Moreover, echocardiography and ECG did not show any sign of AMI. These results, therefore, indicated that chronic myocardial injury caused elevation of cTnI levels in patients with DMD and BMD in our study.
In the present study, we found higher serum cTnI levels with increasing age in DMD patients compared with BMD patients, and the proportion of patients with abnormal serum cTnI levels was larger in the DMD group at all ages and in the age range of 10 < years ≤ 18 when compared with the BMD group. In one study, cardiomyopathy was ubiquitously observed in patients with DMD (in more than 90% of patients over 18 years of age) . On the other hand, the onset of cardiomyopathy was variable in patients with BMD. The onset of dilated cardiomyopathy, the typical end form of cardiomyopathy, occurs in the mid-teen years to 20s in DMD patients [7, 36] and 30s to 40s in BMD patients . These results indicate that DMD generally has an earlier and more severe cardiac phenotype than BMD. In fact, DMD patients exhibited more severe LVEF decline compared to BMD patients. This difference in severity can be explained by the difference in dystrophin levels in the myocardium. In comparison with BMD patients who have an in-frame mutation that produces shortened and less functional dystrophin, DMD patients have a complete absence of dystrophin, resulting in myocardial disruption by mechanical stress . The difference in the level of cTnI between the two groups of patients is considered to reflect the different degrees of myocardial damage in the two muscular disorders.
Previous studies have reported the utility of cTnI for evaluating the cardiac function, especially in patients with DMD. Matsumura et al. reported that most DMD patients showed higher levels of cTnI in the second decade of their lives; however, no obvious correlation between cTnI and LVEF or brain natriuretic peptide was observed . Hammere-Lercher et al. reported that all patients with DMD, with a mean age of 7.5 years, had cTnI levels below the upper reference limit (URL), and there was no relation of cTnI level to clinical evidence of cardiac failure . Castro-Cago et al. also reported no relationship between cTnI levels and cardiac function . These reports suggest that cTnI cannot be used to evaluate cardiac function. However, as shown in our study, the cTnI level was transiently elevated in the second decade before the decline of LVEF. This indicated that the cTnI level was not associated with cardiac function at the time of measurement, but later, it was.
Recently, myocardial fibrosis (MF) in DMD patients has been demonstrated using cMRI with late gadolinium enhancement (LGE), which revealed that subepicardial fibrosis was the main characteristic of DMD patients . When the myocardium is injured, damaged cardiomyocytes are repaired by recruitment, proliferation, and activation of cardiac fibroblasts, which produce extracellular matrix components, resulting in the formation of fibrotic scars . Remarkably, MF has been reported in DMD cardiomyopathy before the onset of myocardial dysfunction in young patients with DMD . We hypothesize that serum cTnI levels may increase with the progression of MF because the observed timings of fibrosis and cTnI rise are the same (early second decade of life). Recently, Sonia et al. reported that cTnI values correlated with cMRI findings in patients with DMD cardiomyopathy . They showed that cTnI levels in DMD patients with mild LGE were significantly increased compared to those in patients without LGE. These studies and the present study indicate that cTnI, a standard marker for AMI, may have the potential to become an alternative, cost-effective, and noninvasive biomarker for detecting early signs of cardiac injury. In fact, cTnI has gained popularity as a biomarker in the diagnosis of HF , and the cost of cTnI assay has been reported to be 10‒100 times less than that of cardiac imaging .
As measurable plasma cTnI is found in the healthy population [24, 47], the abnormal value of cTnI is recommended to exceed the 99th percentile URL . However, there is no internationally accepted standard for the 99th percentile URL of cTnI, although a wide range of variables has been used as the 99th percentile URL . Caselli et al. recently reported plasma cTnI levels in healthy neonates, children, and adolescents; 357 participants had a high sensitive immunoassay similar to that used in our study. In their study, the cTnI showed the highest value in the first weeks of life, and it decreased progressively up to adulthood. Therefore, the 99th percentile URL needs to be defined according to age. They reported that the 99th percentile URL was age dependent; it was 61.3 ng/L for the whole population minus neonates and infants (1 < years ≤ 18) and 41.3 ng/L for the group of adolescents (10 < years ≤ 18) . Unfortunately, they did not report the 99th percentile URL for toddlers (1 < years ≤ 10). Therefore, we decided to define the 99th percentile URL according to the patient’s age as follows: ≥0.07 ng/mL, 1 < years ≤ 10; ≥0.05 ng/mL, 10 < years ≤ 18; and ≥ 0.03 ng/mL, > 18 years (manufacturer’s recommendation).
In the present study, we also examined the relationship between the ACTN3 genotype and cTnI levels in patients with DMD and found that the maximum cTnI level in patients with ACTN3 XX genotype was observed a few years earlier compared with the other two genotypes. These results suggest that patients with the XX genotype may have a higher risk for myocardial injury. Interestingly, the existence of alpha-actinin-3 has been reported not only in skeletal muscles but also in human fetal and adult hearts . We recently reported that the XX genotype is related to a lower LV dilation-free survival rate in patients with DMD . The impact of alpha-actinin-3 deficiency on cardiomyopathy progression was not elucidated in this study. Our results, however, indicate that alpha-actinin-3-deficient myocardium can be sensitive to mechanical and/or hypoxic damage that induces elevation of cTnI levels.
This study has some limitations. First, it was a retrospective observational study and was subject to selection bias. Second, although we evaluated a relatively large number of patients with DMD and BMD compared to previous studies, the number of participants may not be enough to allow generalization of our results to larger cohorts. However, the rarity of these muscular disorders may make it difficult to conduct studies on larger samples. Third, no patient had cTnI level measured over a long follow-up period; therefore, we could not elucidate precise changes in serum cTnI level for each patient by age. Finally, we did not assess the effects of cardioprotective medications that might affect cTnI levels. In addition, since we measured cTnI level only once at assessment for each patient, we do not know whether the value is reproducible or not. Despite these limitations, our study is unique in that it is a longitudinal study with a large number of DMD or BMD patients. Our findings may positively impact cardiac care by supporting the use of cTnI as a biomarker for cardiomyopathy.
In conclusion, we evaluated and compared serum cTnI levels and cardiac function in patients with DMD or BMD in a large cohort. cTnI levels were higher in DMD patients compared with BMD patients of each age group until the second decade of life, suggesting myocardial injury indicated was more severe in DMD patients. The ACTN3 null genotype may be a risk factor for early myocardial injury.