PCD is a rare but well-treatable, inborn metabolic disorder caused by mutations in the SLC22A5 gene. As carnitine plays a crucial role in transporting long-chain fatty acids to the mitochondria, systemic carnitine deficiency leads to impaired long-chain fatty acid oxidation with consecutive energy deficiency. The cardiac muscle obtains 50–70% of its energy from fatty acids, thus the heart is one of the main affected organs in PCD [1]. Cardiomyopathy usually develops between 1 and 4 years of age, as was the case in our patient. OCTN2-associated cardiomyopathy responds poorly to standard therapy, and if the condition is not diagnosed correctly and no carnitine is administered, progressive heart failure may lead to transplantation or death [1].
Cardiomyopathy in PCD can be either dilated or hypertrophic [9]. Histological data are only available on a few patients [10]. Matsuishi et al. report excessive lipid droplets in the cardiac muscle biopsy of a Japanese patient [10]. Cardiomyopathy in PCD has been well-characterised in the JVS mouse model. Kuwajima et al demonstrated that the total carnitine content in JVS mouse heart was about 10% of that of control mouse heart at 4 and 8 weeks of age [11]. The JVS mice developed hypertrophic cardiomyopathy, a higher cardiac weight/body weight ratio, and larger wall areas of both ventricles and septum in JVS mice. Histologically, the myocyte diameter was increased in JVS mice. Electron microscopy studies revealed a significant increase in mitochondrial mass as well as a six-fold higher lipid fraction compared to control mice. Hypertrophy was associated with lower levels of ATP and ADP, and adenylate energy charge [11]. We were unfortunately unable to conduct any biochemical investigations or electron microscopy in patient 1’s explanted heart as only paraffin samples had been conserved. However, his muscle histology displayed in Figure 1 showed anomalies that closely resembled those detected in the mouse model.
According to the evidence from children and adults, cardiomyopathy in PCD responds well to carnitine supplementation, and is rapidly reversible under treatment [12, 13]. Wang et al. reported a series of six children with PCD who all presented with severe left ventricular dysfunction. After one month of carnitine treatment, left ventricular systolic function normalised in all six patients, and after six months of therapy, left ventricular volume normalised also [12]. Similar observations have been made in a 24-year-old OCTN2-deficient woman who developed severe dilated left-ventricular cardiomyopathy 3 months after discontinuing carnitine therapy. She demonstrated a dramatic improvement in biventricular function entailing normalised left and right ventricular systolic function just 5 days after re-initiating carnitine supplementation. Carnitine therapy might well have succeeded even in our patient with life-threatening cardiomyopathy, and, had he not required bridging with an assist device to alleviate severe cardiomyopathy and cardiac failure, could have rendered cardiac transplant obsolete. As carnitine supplementation can both prevent clinical symptoms from developing in asymptomatic patients and reverse even severe cardiac symptoms, we believe that PCD should be ruled out immediately in every patient presenting cardiomyopathy, and diagnostic gene panels for cardiomyopathy should include the SLC22A5 gene.
It is difficult to speculate about what would have happened to patient 1’s transplanted heart in the PCD setting over the longterm had carnitine supplementation not been started. The transplanted heart expresses a functional OCTN2 protein, however, the carnitine concentration in serum was still extremely low. The first three enzymes in endogenous carnitine biosynthesis are expressed in all body tissues, while the γ−butyrobetaine hydroxylase that catalyses the last step in carnitine biosynthesis is only present in kidney, liver, and brain tissues [14]. Therefore, the heart and skeletal muscle cannot endogenously synthesise carnitine and thus rely entirely on carnitine import for long chain fatty acid oxidation [1]. There is ample evidence that carnitine depletion can cause the intracellular accumulation of fatty acids, decreased detoxification, the removal of toxic acyl groups from mitochondria, and an increase in myocardial reactive oxygen species that may be arrhythmogenic [15]. We can thus assume that the transplanted organ would still have been at risk of cardiac arrhythmia.
Untreated patients with PCD carry a potentially fatal risk for cardiac arrhythmias. Both long-QT and short QT syndromes have been observed [1]. De Biase et al reported a female patient who presented with long QT syndrome leading to a syncopal episode due to ventricular tachycardia in her early twenties [16]. She was diagnosed with PCD after her newborn daughter screened positive for low free carnitine. Under treatment with L-carnitine, no further syncopal episodes occurred and her QT interval returned to normal. The association between PCD and short-QT syndrome was recently reported in a very few patients [17, 18]. The relationship between PCD, a short QT syndrome and arrhythmias was studied by Roussel et al in a mouse model of carnitine deficiency induced by long-term subcutaneous perfusion of MET88 [17]. They showed that MET88-treated mice developed cardiac hypertrophy associated with a remodelled mitochondrial network. Continuous electrocardiogramme monitoring confirmed a shortened QT interval that correlated negatively with the plasma carnitine concentration, anomalies that coincided with the genesis of ventricular premature beats, ventricular tachycardia, and fibrillation. Data from patients on the Faroe Islands (where PCD has an incidence of about 1:300) reveal that even asymptomatic adult patients are at risk of sudden death from cardiac arrhythmia. In a study by Rasmussen et al, all medico-legal cases of sudden death between 1979-2012 among subjects below age 45 years were systematically investigated [15], and the authors demonstrated a strong association between sudden death and untreated PCD, especially in females.
Our patients were homozygous for the variant c.1319C>T in exon 8 of the SLC22A5 gene. This variant results in a threonine by methionine exchange in position 440 of the OCTN2 protein and was first described by Lamhonwah et al. in 2002 [9]. Frigeni et al functionally characterised this mutant protein in carnitine-uptake studies, and identified only 0.3% of normal transport activity in fibroblasts in a patient homozygous for this variant [19]. While the c.1319C>T is usually associated with a multisystemic disease, Papadopoulou-Legbelou et al [20] reported a homozygous patient with a pure cardiac phenotype similar to our patient’s. Their boy presented with dilated cardiomyopathy that was fully reversible under carnitine supplementation [20].
PCD is part of newborn screening programmes in many countries. Patients are identified through significantly reduced free carnitine concentrations of < 2.5–10 % of the normal value [3]. The fact that the calculated frequency of mutant alleles based on the frequency of pathogenic alleles as reported in the ExAC Browser Beta and in the gnomAD Browser in about 120,000 healthy (heterozygotous) individuals is significantly higher than that reported in newborn screening suggests that the current neonatal screening protocols might be failing to detect some affected individuals [19]. It has been hypothesised that a diagnosis can fail if screening is done too soon after birth and no second screening is obtained, since carnitine is transferred from the mother to her child via the placenta, and the levels of free carnitine shortly after birth tend to reflect maternal carnitine concentrations [21]. Therefore, free-carnitine levels are usually lower in the infants of mothers with PCD immediately after birth than in infants themselves affected by the disease. While the free-carnitine levels decrease in infants with PCD over time, they remain stable or rise slightly in the infants of affected mothers [21]. Data from the Region 4 Stork collaborative project also show that a large proportion of patients with PCD exhibit C0 levels in their first newborn screening sample above the standard cut-offs that apply for low C0 in newborn screening [22]. A study conducted in the Faroe Islands revealed that post-neonatal screening beyond 2 months of age successfully identified additional affected patients whose newborn screening results had been unremarkable [23]. The low sensitivity and specificity of newborn screening for PCD and the numerous asymptomatic mothers identified makes including PCD generally within newborn screening programmes controversial [8]. In our family, patient 2’s having undergone newborn screening for PCD ultimately enabled its diagnosis in two children. Also patient 1, born before the “Newborn Screening 2020” pilot study was initiated, was indeed identified thanks to newborn screening, although PCD is not a target disease incorporated within the German newborn screening programme, since carnitine is essential to interpreting acylcarnitine data with respect to the other fatty acid-oxidation defects included in the newborn screening panel. However, and unexpectedly in a PCD patient, although his carnitine concentration in serum was at the lower limit of the reference range at age 3 weeks, it was not significantly reduced. As carnitine excretion in urine and tubular reabsorption were not measured, this boy was unfortunately not diagnosed until after PCD was confirmed in his baby sister following abnormal newborn-screening findings. As PCD is not included in all newborn screening programmes, and a PCD diagnosis can also be missed during newborn screening, we maintain that OCTN2 deficiency should be ruled out in all patients presenting suggestive symptoms even in the presence of a normal newborn-screening result.