Cardiac features and effects of enzyme replacement therapy in Taiwanese patients with Mucopolysaccharidosis IVA

Mucopolysaccharidoses (MPSs; OMIM 252700) are composed of a group of rare genetic disorders caused by deficiencies in specific lysosomal enzymes and involve the sequential degradation of glycosaminoglycans (GAGs) which cause substrate accumulation in various cells and tissues, and progressive multi-organ dysfunction. Seven distinct types of MPS disorders (I, II, III, IV, VI, VII, and IX) with 11 specific lysosomal enzyme deficiencies have been described [1, 2]. The onset and severity of cardiovascular defects differ in each type of MPS, with the most documented abnormalities being cardiac valve thickening, valvular regurgitation and stenosis, and cardiac hypertrophy [3‐10]. It has been reported that valve defects and cardiomyopathy result from GAG accumulation in the spongiosa of cardiac valves, myointima of coronary arteries, and myocardium [11]. Mitral or aortic leaflet thickening and calcification may lead to stenosis or regurgitation, and deformities in cardiac structures resulting in cardiac dysfunction can significantly increase morbidity and mortality in patients with MPS [12‐14].

Among the different types of MPS diseases, MPS IVA (Morquio A syndrome; OMIM 253000) is a relatively rare autosomal recessive inherited disorder caused by N-acetylgalactosamine-6-sulfatase (GALNS) deficiency. MPS IVA can lead to excessive lysosomal storage of GAGs, keratan sulfate and chondroitin-6-sulfate in various tissues and organs. Patients with MPS IVA generally appear unaffected at birth, but may develop multiple clinical manifestations including systemic skeletal chondrodysplasia, short stature, valvular heart disease, corneal clouding, hearing loss, malformation of the thorax that impairs respiratory function, joint abnormalities, odontoid hypoplasia and ligamentous laxity, cervical spinal instability and potentially spinal cord compression [1, 2, 15]. Several studies have described the prevalence of cardiac defects in the more common types of MPS disease [3‐10], however, few studies have focused on the cardiac features of MPS IVA [16‐21]. Elosulfase alfa (Vimizim®; BioMarin Pharmaceutical Inc., Novato, CA, USA) is a recombinant human GALNS associated with increased endurance, reduced urine keratan sulfate and acceptable safety profile, and it has been approved as enzyme replacement therapy (ERT) for MPS IVA [22, 23]. Nonetheless, information regarding the cardiac characteristics and the long-term cardiac outcomes of ERT for patients with MPS IVA is limited [10]. The purpose of this study was to investigate the cardiologic features of Taiwanese patients with MPS IVA, and evaluate the impact of ERT on cardiac structure and function.

Table 1 showed the baseline clinical, echocardiographic and electrocardiographic features of 32 patients with MPS IVA. Echocardiographic examinations (n = 32) revealed that the mean z scores of LVMI, IVSd, LVPWd, and aortic diameter were 0.94, 2.70, 0.39, and 3.26, respectively (Table 2). Z scores > 2 were identified in 25%, 50%, 29%, and 69% of the LVMI, IVSd, LVPWd, and aortic diameter values, respectively. Diastolic dysfunction (E/A ratio < 1) was identified in four patients (13%), however, the ejection fraction (reference ≧ 55%) and shortening fraction (reference ≧ 28%) values were all normal and revealed normal systolic function. Sixteen patients (50%) had valvular heart disease and most were of mild degree. Fourteen (44%) had valvular stenosis, and 10 (31%) had regurgitation. Valvular stenosis was mild in all but one case who was adult (No. 32) and that regurgitation was also mild in all but three cases. No one had stenosis under the age of 5 years. Fourteen (44%) and 12 (38%) patients had mitral valve disease or aortic valve disease, respectively. Two (6%) patients had tricuspid valve disease, however none of the patients had pulmonary valve disease. The most prevalent cardiac valve abnormalities were mitral stenosis (38%) and aortic stenosis (38%), followed by mitral regurgitation (22%), aortic regurgitation (9%), and tricuspid regurgitation (6%). Overall, 22% and 31% of the patients had mitral valve prolapse and a thickened interventricular septum, respectively (Table 3). The z scores of LVMI, IVSd, IVSs, LVPWd, LVPWs, and aortic diameter, the severity scores of aortic stenosis and regurgitation, and the existence of a thickened interventricular septum were all positively correlated with increasing age (p < 0.05) (Tables 2 and 3, Figs. 1 and 2). The mean ages of the patients with and without valve thickening were 14.4 and 8.7 years, respectively. The mean LVMI values of the patients with and without valve thickening were 80.9 and 53.9 g/m^2.7, respectively. For the 14 patients with valve thickening, the z scores of LVMI, IVSd, IVSs, LVPWs, and aortic diameter were all larger than those of the 18 patients without valve thickening (p < 0.05) (Table 4). Twenty-one patients (78%, n = 27) had abnormal ECG findings, the most common of which was the presence of sinus tachycardia (37%), followed by sinus arrhythmia (33%), right or left axis deviation (19%), and cardiac enlargement (11%). The ECG abnormalities were usually of minor clinical significance. For the seven patients with MPS IVA (age range, 1.4 to 25.8 years) treated with weekly intravenous infusions of elosulfase alfa (2.0 mg/kg) for 3–6 years, echocardiography showed a decrease in mean LVMI z score from 0.77 to 0.73, a decrease in mean IVSd z score from 2.42 to 2.17, and a decrease in mean LVPWd z score from 0.41 to − 0.50. For the two patients who started ERT at a younger age (1.4 years and 2.8 years of age, respectively), the z scores for LVMI, IVSd, and LVPWd all decreased after ERT (Table 5). However, ERT seemed to have little or no effect on valvular heart disease (Additional file 1: Table S1).

To the best of our knowledge, this is the first report to describe the cardiac characteristics of Asian patients with MPS IVA and compare them with normal values derived from a population that included young adults according to the study of Kampmann et al. [31], and to measure the effects of ERT on cardiac structure and function in these patients. Few studies have focused on the cardiac features of patients with MPS IVA [16‐21]. Compared with the other types of MPS diseases, due to the severe skeletal deformities and dysmorphic stature of patients with MPS IVA, non-skeletal manifestations, including cardiovascular and respiratory impairment, have received relatively little attention [33]. Our results demonstrated that most of the patients with MPS IVA had cardiac hypertrophy, aortic dilatation, increased thickness of the interventricular septum, normal systolic function, and mildly valvular heart disease. The cardiac abnormalities in these patients worsened with increasing age in accordance with the progressive nature of this disease. Our results are consistent with those of previous studies from Caucasian populations [16, 17]. In addition, ERT appeared to be effective in stabilizing cardiac hypertrophy, and better results may have been associated with starting ERT at a younger age.

Most of the patients with MPS IVA in this study (97%) had cardiac abnormalities, and only one 1.3-year-old male patient had normal cardiac features in both echocardiography and ECG. Echocardiographic assessments revealed mean z scores of LVMI, IVSd, LVPWd, and aortic diameter of 0.94, 2.70, 0.39, and 3.26, respectively. Z scores > 2 were identified in 25%, 50%, 29%, and 69% of the LVMI, IVSd, LVPWd, and aortic diameter values, respectively. Kampmann et al. [16] reported thickened left ventricles and aortic root extensions in patients with MPS IVA, and Bolourchi et al. [34] reported that patients with MPS IVA had the highest prevalence of aortic root dilatation (62.5%) among all MPS types. Our results were consistent with theirs. Although left ventricular systolic function was normal in all of our patients, diastolic dysfunction with a reversed E/A ratio (< 1) was identified in four patients (13%). The general consensus in previous studies has been that abnormal catabolism of dermatan sulfate in patients with MPS I, II and VI leads to the accumulation of dermatan-sulfated GAGs in cardiac valves, resulting in valvular thickening and other cardiac defects [6, 7]. The main storage products of MPS IVA are keratan sulfate and chondroitin sulfate. Therefore, cardiac lesions may be less prominent in MPS IVA than in MPS I, II, and VI. In our cohort, substantial cardiac hypertrophy, aortic dilatation, as well as valvular stenosis and regurgitation were still present in the patients with MPS IVA, and the severity also worsened with increasing age.

Deformed mitral or aortic valves were more commonly found than tricuspid or pulmonary valves in our cohort. Previous studies have reported that left-sided valves are much more commonly involved than right-sided valves in patients with MPS IVA [17, 18], which is consistent with our results. There were varying degrees of valvular deformities, although most of the patients had mild stenosis or regurgitation [3, 7]. However, mitral or aortic stenosis was more common than regurgitation in our cohort, which is different to the results of most reports in Caucasian populations [7]. Further studies are warranted to clarify whether this difference is caused by actual ethnic differences or an artifact of a limited sample size.

For the 14 patients with valve thickening, the z scores of LVMI, IVSd, IVSs, LVPWs, and aortic diameter were all larger than those of the 18 patients without valve thickening (p < 0.05). This suggests that the patients with valve thickening required a higher cardiac muscle mass and higher heart rate-normalized work index to keep cardiac output constant, which is consistent with a previous report [16].

Seventy-eight percent of our ECGs showed abnormal findings, although the clinical significance was minor. The most common finding was sinus tachycardia (37%), which has frequently been observed in patients with MPS IVA. Although the underlying mechanism remains unclear, it seems to be a physiological reflex to maintain cardiac output in a small-sized heart with impaired filling patterns [16]. Cardiac enlargement was found in 11% of our patients by ECG, which was not consistent with the cardiac hypertrophy identified by echocardiography, possibly owing to the low electric conductance of GAGs [8]. Although ECG has been reported to be an unreliable tool for detecting cardiologic defects in MPS [8], one patient was found to have an irregular bundle branch block by ECG in this cohort. Therefore, we suggest that ECG should remain part of the follow-up examinations of patients with MPS IVA, especially to identify changes in conduction or rhythm abnormalities.

Cardiac disease can occur insidiously and lead to early mortality in patients with MPS IVA [13]. Comprehensive physical examinations, 12-lead complete ECG, and echocardiography when MPS is diagnosed, followed by routine cardiac function follow-up are important [35, 36]. Without regular cardiac monitoring, cardiac lesions may remain undetected because of a lack of physical activity due to skeletal dysplasia and pulmonary function impairment. If ventricular arrhythmia is detected, 24-h Holter monitoring should also be considered.

In our cohort, the mean z scores of LVMI, IVSd and LVPWd all decreased after ERT. This suggests that ERT has some effect on GAG accumulation in the cardiac tissue of patients with MPS, and thus is effective in reducing cardiac hypertrophy. However, ERT seemed to have little or no effect on valvular heart disease, which is consistent with previous studies [8, 10, 11, 37‐40]. Braunlin et al. [39] and Kampmann et al. [40] reported that ERT may have better long-term results when started at an early age for patients with MPS VI, which is consistent with our results. Several sibling control studies have also reported that ERT may prevent or delay the development of valvular heart disease when started early in life [41‐45]. As ERT appears to arrest rather than improve valvular heart disease, starting treatment early may be indicated. The increasing clinical awareness of MPS disease and increased ability to make a confirmative diagnosis has made an earlier diagnosis possible. Due to the progressive nature of MPS, initiating ERT before the occurrence of irreversible cardiac damage may contribute to a better clinical outcome. Thus, making an early diagnosis through screening programs for newborns or high-risk populations is of major importance [46‐48].

Our results showed that a substantial proportion of the patients with MPS IVA had cardiac hypertrophy, aortic dilatation, and mildly valvular heart disease. The cardiac abnormalities in these patients worsened with increasing age in accordance with the progressive nature of this disease. ERT appeared to be effective in stabilizing or reducing cardiac hypertrophy, and better results may have been associated with starting ERT at a younger age. However, previous sibling studies have suggested that ERT appears to arrest rather than improve valvular heart disease. Therefore, it is very important to make an early diagnosis through screening programs for newborns or high-risk populations in order to initiate ERT before the occurrence of irreversible cardiac damage.