1 INTRODUCTION

Malonic aciduria (MA, OMIM#248360) is an extremely rare genetic disorder caused by the deficiency of malonyl-CoA decarboxylase (MCD). Since being first described in 1984 (Brown, Scholem, Bankier, & Danks, 1984), less than 40 cases of MCD deficiency have been reported in the literature (Liu et al., 2016). Following a report in the early 2000s, which showed that malonylcarnitine (C3DC) in the blood could be measured using tandem mass spectrometry (TMS) (Santer et al., 2003), there have been several case reports of MA being diagnosed worldwide using the technique during the asymptomatic neonatal period.

MCD, which catalyzes the conversion of malonyl-CoA to acetyl-CoA and carbon dioxide, is encoded by the MLYCD gene (OMIM*606761) located on chromosome 16q23. The enzyme is expressed in heart, skeletal muscle, liver, kidneys, pancreas, and brain. MCD deficiency results in accumulation of malonyl-CoA in the cytoplasm, which in turn inhibits carnitine palmitoyltransferase 1 (McGarry & Brown, 1997), the rate-limiting enzyme in fatty acid beta-oxidation in mitochondria and peroxisomes. The characteristic phenotype is highly variable, and includes developmental delay, seizures, diarrhea, vomiting, metabolic acidosis, hypoglycemia, ketosis, and cardiomyopathy (Celato et al., 2013). Cardiomyopathy is a leading cause of mortality and morbidity in MA, and approximately first/third of the patients with MA show cardiomyopathy during the clinical course of the disease (Celato et al., 2013). A high middle-chain triglyceride (MCT)/low long-chain triglyceride diet combined with a carnitine supplement could be an effective method for preserving cardiac function in patients with MA (Ficicioglu, Chrisant, Payan, & Chace, 2005; Footitt et al., 2010).

Here, we describe the first case of a Korean child with MA who presented with dilated cardiomyopathy (DCMP) at the age of 3 months. Diagnosis was confirmed using biochemical and molecular analyses.

2 MATERIALS AND METHODS

2.3 Results of molecular genetic analysis

To understand the genetic basis of the clinical manifestations described above, we performed direct sequencing of the MLYCD gene (NM_012213.3) and quantitative PCR of each coding exon. Direct sequencing analysis identified a novel pathogenic mutation in our patient, c.1A>G. A partial deletion encompassing exon 1, 2, and 3 was also detected using quantitative PCR to screen copy number variation in MLYCD. Family screening revealed that his mother is a heterozygous carrier of the c.1A>G mutation and his father has a heterozygous exon 1–3 deletion. Therefore, the observed MCD deficiency was confirmed to be caused by compound heterozygous pathogenic mutations in MLYCD, the c.1A>G mutation, and an exon 1–3 deletion (Table 2; Figure 1).

Table 2. Results of the MLYCD gene analysis of the patient and his parents

	Allele 1	Allele 2
Patient	c.1A > G (p.?), hemizygote	exons 1–3 deletion, heterozygote
Father	−	+
Mother	+ (heterozygote)	−

Note

• Reference Sequence: NM_012213.3.

3 DISCUSSION

Until the early 21st century, there were no adequate methods to screen MA in the preclinical stage (Pourfarzam, Zadhoush, & Sadeghi, 2016). However, since 2003, early cases of MA have been diagnosed in the neonatal period owing to the introduction of nationwide newborn screening programs in many countries (Santer et al., 2003).

MA is extremely rare with an estimated prevalence of less than 1 in 400,000 newborns occurring worldwide (Lee et al., 2017; Wang et al., 2019). In Korea, from 2006 to 2015, only six infants have been suspected of having MA as shown by the results of first tier newborn screening using TMS(Lee et al., 2017; Santer et al., 2003), resulting in a detection rate of 1/574,206. Interestingly, newborn screening using TMS performed during the neonatal period failed to detect biochemical abnormalities in our patient. When the test was repeated at 3 months of age, it showed an elevated C3DC level, leading to a clinical diagnosis of MA. Our case emphasizes that if inherited metabolic disease is clinically suspected, repeated TMS with corroborative biochemical studies, including urine organic acid analysis, may be beneficial for diagnosis.

Approximately 50 molecular variations related to the MLYCD gene have been classified as pathogenic or likely pathogenic, according to the ClinVar database (https://www.ncbi.nlm.nih.gov/clinvar). However, the correlation between these genetic variants and resulting phenotypes has not been elucidated. Our patient exhibited a novel pathogenic variant, c.A> G, in the start codon of MLYCD. This variant was detected using Sanger sequencing for all coding exons of the gene. Because the chromatogram showed a homozygous c.1A>G change, we also studied this variant in his parents to verify segregation. Interestingly, his mother harbored the c.1A>G variant heterozygously, whereas his father did not. Therefore, we continued our analysis using quantitative PCR of each coding exons and detected a previously reported heterozygous exon 1–3 deletion inherited from his father (Liu et al., 2016).The phenotypes exhibited by our patient were more severe than those previously reported for a pediatric patient in China who also had an exon 1–3 deletion. The Chinese patient, who mainly exhibited poor performance at school, without cardiomyopathy or metabolic acidosis, was not diagnosed until the age of 9. The C3DC level and urine malonate excretion at diagnosis were also much higher in our patient. On the other allele, the novel mutation found in our patient is located in the start codon (c.1A>G) and is classified as a null variant, while the mutation in the Chinese patient was a missense variant, c.911G>A (p.Gly304Glu) in exon 4. The nature of the mutation on the other allele may explain the differences observed in phenotypes and disease severity between the two patients with the exon 1–3 deletion.

Our patient initially presented with DCMP and a decreased ejection fraction of 20%, requiring digoxin therapy at 3 months of age. After 2 weeks of medical therapy, the ejection fraction did not improve significantly. Following biochemical diagnosis of MA, a low-fat and high-carbohydrate diet with MCT and L-carnitine supplement was administered. This dramatically improved his cardiac function which reached ejection fractions of 43% and 63% at 3-weeks and 7-months of dietary intervention, respectively. Improvement of cardiac function following dietary intervention was consistent with previous MA cases exhibiting cardiomyopathy (Ficicioglu et al., 2005; Footitt et al., 2010).

Our patient is now 5 years old and his cardiac function remains stable; he is currently being treated with dietary intervention with L-carnitine and beta-blocker. He shows good clinical performance with normal growth profiles in parameters of height (25–50th percentile), weight (50–75th percentile), and head circumference (50–75th percentile). However, his developmental age was estimated at only 2.8 years old, when he was assessed using the Psychoeducational Profile Revised and Receptive and Expressive Vocabulary Test. Developmental delay in MA patients may be explained by the neurotoxic effects of accumulating malonyl-CoA esters and neonatal hypoglycemic events, although the mechanism is not fully understood (Xue et al., 2012).

Early diagnosis of MA (3 months old) enabled timely dietary interventions and reversed the deterioration of cardiac function with improvement of biochemical abnormalities in our patient. MA can be detected by newborn screening before the onset of signs or symptoms; however, negative newborn screening results do not necessarily exclude the possibility of disease. As early diagnosis and treatment is able to prevent or minimize progressive and life-threatening complications in MA and other metabolic myopathies, active metabolic screening to identify manageable causes is highly recommended, especially for infantile cardiomyopathies.

CONFLICT OF INTEREST

The authors declare no conflict of interest.

AUTHOR CONTRIBUTIONS

S.H.L., data assembly and manuscript preparation; M.K.S., clinical analysis about cardiomyopathy; J.S., biochemical analysis; K.S.P., molecular genetic analysis; J.M.K., design of this study.