Clinical and genetic characteristics of patients with fatty acid oxidation disorders identified by newborn screening

Fatty acid oxidation (FAO) is a key metabolic pathway for maintaining energetic substrates used to maintain metabolic homeostasis. FAO is important for some high-energy-requiring organs and provides the main energy supply during prolonged fasting, febrile illness, cold exposure, or muscular exertion. The prime pathway for the degradation of fatty acids is mitochondrial fatty acid oxidation, which is composed of the uptake and activation of fatty acids, carnitine cycles, beta-oxidation cycle, and electron transfer [1].

More than 15 distinct disorders have been described as affecting FAO; These include glutaric aciduria type 2, primary carnitine deficiency and deficiencies of carnitine palmitoyltransferase 1A (CPT1A), carnitine acylcarnitine translocase (CACT), very long chain acyl-CoA dehydrogenase (VLCAD), long chain hydroxyacyl-CoA dehydrogenase or mitochondrial trifunctional protein (LCHAD/MTP), medium chain acyl-CoA dehydrogenase (MCAD), medium/short chain hydroxyacyl-CoA dehydrogenase (M/SCHAD), and short chain acyl-CoA dehydrogenase (SCAD) [2‐4].

After the introduction of newborn screening by tandem mass spectrometry analysis of acylcarnitines, the detection of FAO disorders (FAODs) has increased. The estimated combined incidence of all FAODs is 1 in 9000, which is calculated from reports out of Australia, Germany, and the USA, but it seems to be much lower in Asian countries [5]. The estimated prevalence of long chain fatty acid oxidation disorders in Korea is 1 in 15,800, which is more frequently diagnosed following the introduction of tandem mass spectrometry newborn screening [6].

FAODs present with a heterogeneous clinical phenotype at various ages of onset, from neonate to adulthood. The most severe form manifests with hypertrophic cardiomyopathy, hepatic encephalopathy, or severe hypoketotic hypoglycemia in the neonatal period or infancy. The less-severe, later-onset myopathic form is characterized by exercise-induced myopathy and rhabdomyolysis [7, 8].

The diagnosis of FAODs are based on the measurement of abnormal acylcarnitines and confirmed by enzyme assay or molecular analysis. Early identification of FAODs became possible using expanded newborn screening using tandem mass spectrometry, which measures acylcarnitine levels in dried blood spots. Identification of FAODs in newborn screening is very important because intervention in the presymptomatic period helps improve patient prognosis [9].

In this respect, here we describe fourteen patients with diverse FAODs identified by newborn screening, and compare their clinical outcome and genetic characteristics with those of 8 patients diagnosed by symptomatic presentation. Our experience indicates the effectiveness of newborn screening for the early diagnosis of FAOD, especially in the presymptomatic period. However, careful observation and appropriate management is warranted to improve the clinical outcome of the affected patients.

The current study described the clinical and genetic features of 14 patients with FAODs identified by newborn screening, compared to those of 8 patients diagnosed with FAODs based on their symptomatic presentations.

The results of our current study indicate some important findings. Newborn screening helped to identify the broad spectrum of FAODs compared to FAODs with symptomatic presentation. These findings were comparable to previous reports regarding the newborn screening program [8, 22]. In addition, the relative frequency of each FAOD was reflected; VLCAD, LCHAD/MTP, and MCAD deficiencies were common FAODs identified by newborn screening. Alternatively, other FAODs, including primary carnitine deficiency, CPT1A or SCAD deficiencies, were identified in a small number of patients. Of note, VLCAD or LCHAD/MTP deficiencies were the most common among the FAODs identified either by newborn screening or by symptomatic presentation, whereas all patients with another common FAOD, MCAD deficiency, were identified by newborn screening, as symptomatic presentation of MCAD deficiency is expected to be very rare.

The purpose of newborn screening is to identify patients with inborn metabolic disorders in their presymptomatic period and intervene to prevent a metabolic crisis and improve their clinical outcome. In our current study, a fair outcome was observed in the patients with MCAD, CPT1A or primary carnitine deficiency identified by newborn screening. However, clinical outcomes were not significantly different among patients with long-chain FAODs, including VLCAD deficiency and LCHAD/MTP deficiencies identified either by newborn screening or by symptomatic presentation; most long-chain FAOD patients developed recurrent rhabdomyolysis and hypertrophic cardiomyopathy regardless of presymptomatic management.

The difference in outcomes among patients with FAODs appears to be related to disease characteristics and pathogenic effect of the mutations in addition to the mode of identification. In VLCAD or LCHAD/MTP deficiencies, patients identified by newborn screening experience a broad spectrum of severity even though the majority of patients were asymptomatic at diagnosis; symptoms develop in some patients even before newborn screening results were available, or some patients may remain asymptomatic through the long-term follow-up period [23‐25]. Most of our patients with VLCAD or LCHAD/MTP deficiencies (4 out of 6) experienced recurrent rhabdomyolysis or severe cardiomyopathy. These severe phenotypes are related to complete inactivating or null alleles [25‐28]. This correlation became more meaningful when the phenotype-genotype correlation was evaluated in all patients with VLCAD or LCHAD/MTP deficiencies, irrespective of the mode of identification. Seven patients with severe types of mutations, such as frameshift, nonsense, or splicing mutations, experienced severe phenotypes. Particularly, early death from severe cardiomyopathy was noted in 3 patients with severe mutations in LCHAD/MTP deficiencies. On the other hand, patients with missense mutations only either remained asymptomatic or experienced milder phenotypes (Tables 1 and 2).

As a long-term complication, peripheral polyneuropathy developed in four patients with LCHAD/MTP deficiencies, which has been reported in up to 80% of cases. In addition, pigmentary retinopathy develops in up 15–30% of patients [29, 30]. The mechanism responsible for these complications is not fully understood, although accumulation of 3-hydroxy fatty acid intermediate may be responsible [8]. Because the follow-up period was short for the patients in our current report, observation for these long-term complications is necessary.

The benign clinical course of MCAD deficiency can also be explained in part by the mutation type; most of the mutations were missense and only a small proportion of severe mutations such as frame-shift were noted. However, a life-long follow-up evaluation is required for patients with MCAD deficiency, considering the development of late-onset metabolic episodes. Between 3 and 24 months of age, patients may experience hypoketotic hypoglycemia, vomiting, lethargy, encephalopathy during febrile illness, or prolonged fasting. Sudden unexplained death may be the first presentation of MCAD deficiency [31, 32]. Even some adult patients may suffer from rhabdomyolysis, hepatic failure, encephalopathy or cardiac arrest triggered by alcohol consumption, pregnancy, or prolonged fasting [9, 33].

SCAD deficiency has also been classified as a benign condition because most of the newborns with SCAD deficiency as identified by newborn screening do not develop a clinical phenotype without any medical intervention. There exist controversies whether SCAD deficiency is benign biochemical phenotype, a clinical disorder with incomplete penetrance, or a clinically relevant part of multi-factorial or a multi-genetic disorder [34, 35]. Previously, some patients reported as having severe developmental delay, dysmorphic features and epilepsy, which would have been attributed to unknown genetic defects rather than to SCAD deficiency.

The CPT1A deficiency patient identified by newborn screening harbor heterozygous of two novel mutations. The patient remained asymptomatic during follow-up period, which was relatively short considering that CPT1A deficiency patients usually present by the age of 2 years [36]. The major manifestation of CPT1A deficiency is hepatic encephalopathy followed by febrile illness or prolonged fasting [21]. Even for a patient with neonatal presymptomatic presentation, late-onset hepatic failure develops, requiring lifelong evaluation. There exists a possibility that the novel missense mutation may not be deleterious, yet longer follow-up is needed in our patient in case of developing the symptomatic manifestations.

Newborn screening for FAODs revealed the relative frequency of each disease subtype and their general clinical characteristics. This screening helped to reduce the mortality and morbidity of each patient with FAODs, but their broad spectrum of disease severity was also encountered regardless the mode of diagnosis, which was explained in part by their respective genotype. Careful, life-long observation of patients with FAODs is required to improve clinical outcome.