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A severe genotype with favourable outcome in very long chain acyl-CoA dehydrogenase deficiency

Abstract

A patient with very long chain acyl-CoA dehydrogenase (VLCAD) deficiency is reported. He had a severe neonatal presentation and cardiomyopathy. He was found to be homozygous for a severe mutation with no residual enzyme activity. Tandem mass spectrometry on dried blood spots revealed increased long chain acylcarnitines. VLCAD enzyme activity was severely decreased to 2% of control levels. Dietary management consisted of skimmed milk supplemented with medium chain triglycerides and l-carnitine. Outcome was good and there was no acute recurrence.

  • fatty acids
  • newborn screening
  • cardiomyopathy
  • mass spectrometry

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Very long chain acyl-CoA dehydrogenase (VLCAD) deficiency is a recently identified inborn error of a membrane bound mitochondrial enzyme.1-3 We describe a patient with VLCAD deficiency of severe neonatal onset and cardiomyopathy, resulting from a severe mutation type with no residual enzyme activity or “null mutation”. Outcome was good with prompt resolution of cardiomyopathy.

Case report

The patient is the fifth child born to first cousin parents. Two sisters died early in life. The patient developed tachypnoea at 32 hours of life. Laboratory analysis revealed notable acidosis and low blood glucose (2.8 mmol/l). Other metabolic investigation showed very high serum lactate (20.2 mmol/l). Organic acid chromatography showed high urinary excretion of some saturated dicarboxylic acids: adipic, suberic, and sebacic acids. The patient was transferred to a university hospital on his fifth day of life and was found to be tachypnoeic on admission with metabolic acidosis (pH 7.24). He received intravenous bicarbonate and was started on oral citrate solution and regular feeding. Computed tomography scan of the abdomen revealed an enlarged liver; echocardiography showed mild biventricular hypertrophy with good systolic function. Brain magnetic resonance imaging did not show any abnormal signal. Because of persisting metabolic acidosis and the need for bicarbonate administration, thiamine 150 mg/day, biotin 7.5 mg/day,l-carnitine 250 mg/day, and coenzyme Q 120 mg/day were added to the regimen. Electrospray tandem mass spectrometry (ESI-MSMS) of dried blood spots on filter paper taken three days afterl-carnitine treatment identified increased long chain acylcarnitines suggestive of VLCAD deficiency.

At 1.5 months of age the patient was started on low fat milk formula (1% fat in dry powder) supplemented with carbohydrate, medium chain triglyceride (MCT) oil (1.5 g/kg/day), and l-carnitine 100 mg/kg/day. At 1 year of age a repeated echocardiogram was normal. Although the patient had some intercurrent infections, his outcome at 3 years of age is still good with normal growth and development and without any recurrence of the metabolic acidosis.

Results

Acylcarnitine profiling by ESI-MSMS analysis of the patient's blood spot after initiation of treatment with l-carnitine was consistent with VLCAD deficiency (fig 1). Free carnitine was estimated at 62.9 μmol/l (n = 2; normal 25–45 μmol/l) onl-carnitine supplementation. The profile showed significantly increased myristoleylcarnitine (C14:1) at 5.6 μmol/l (upper cut off value 0.30 μmol/l) and increased palmitoylcarnitine (C16) at 20.2 μmol/l (cut off 9.30 μM) as well as several other long chain acylcarnitines.

Figure 1

Blood spot free carnitine and acylcarnitine profiles obtained by ESI-MSMS analysis with precursor ion scanning for m/z 85. (A) Control; (B) patient. The peaks in the profile are the molecular ions (M+) of the free carnitine and acylcarnitine butyl esters. Their masses are as follows: free carnitine (FC), 218 (d3 isotope labelled, 221); acetyl (C2), 260; propionyl (C3), 274 (d3 isotope labelled, 277); octanoyl (C8), 344 (d3 isotope labelled, 347); decanoyl (C10), 372; dodecanoyl (C12), 400; tetradecanoyl (C14), 428; hexadecanoyl (C16), 456 (d3 isotope labelled, 459); octadecanoyl (C18), 484. Their unsaturated analogues appear 2 Da lower in mass. *Internal standard.

On dietary control the determination of tetradecenoic acid (C14:1, ω9) in plasma total fatty acids was 17 μmol/l (control <2).4 The analysis of plasma acylcarnitines also revealed mild elevation of tetradecenoylcarnitine estimated at 0.7 μmol/l (control <0.5). Oxidation of palmitic acid by cultured fibroblasts was reduced to 22% of controls, while VLCAD activity, determined by the dehydrogenation rate of palmitoyl-CoA with electron transferring flavoprotein in cultured fibroblasts, was severely decreased to 0.03 nmol/min/mg protein (control: 1.39 (0.37); n = 28).5

Sequence analysis of the entire coding region of the VLCAD gene from a patient as recently published by Andresen and colleagues,6revealed that he was apparently homozygous for a deletion of the nucleotides TG (corresponding to cDNA position 709–710) or GT (corresponding to cDNA position 710–711). This was the only change observed. The mutant allele encodes a VLCAD protein where the reading frame is shifted from the codon encoding cysteine at position 197 to 210 ending with a premature stop codon at codon 211 of the mature protein. To verify that the patient was in fact homozygous for the 2 bp deletion the parents were also analysed. Amplification and sequence analysis of exon 8 from the parents, showed that they were both heterozygous for the mutation, thus confirming that the patient is homozygous. The encoded protein would be expected to have no enzyme activity, as vital parts, such as the active site, were lacking. Moreover, analysis of VLCAD mRNA from alleles with premature stop codons, such as the stop codon resulting from the 2 bp deletion described in the present study, have shown that no normal sized mutant VLCAD mRNA can be observed.6

Discussion

Three general forms of clinical presentation in VLCAD deficient patients are known.5 6 The severe childhood form of the disease consists of patients with early onset of symptoms, a very high mortality, or a high number of disease episodes, presence of cardiomyopathy, and siblings who have died. The second group is the mild childhood form and includes patients presenting later in infancy and childhood with a generally milder presentation (fasting induced hypoketotic hypoglycaemia) and fewer episodes of disease precipitation. Cardiomyopathy is rare in this group and mortality much lower. The third group of patients presents in adulthood with an isolated muscular form of the disease (myopathy, rhabdomyolysis, and myoglobinuria). It has recently been shown that patients with the severe childhood form of the disease preferentially have “null” mutations that lead to no residual enzyme activity.6

Our patient is considered to have a severe neonatal form with cardiomyopathy and a severe homozygous mutation for a deletion of 2 bp in exon 8 corresponding to cDNA position 710–711. This mutation leaves absolutely no residual enzyme activity and this could explain the fact that the two undiagnosed sisters died at very early age.

In our patient, who has two null mutations, it is striking that all clinical manifestations including cardiomyopathy were completely resolved by appropriate treatment. From this observation it is apparent that patients with two null mutations may avoid disease episodes when fasting is avoided; and that the energy from MCT oil is sufficient to feed the heart and muscle. In conclusion, this study shows that although the heart is nearly obligatory dependent on long chain fatty acid oxidation it is possible to resolve cardiomyopathy in a child with a complete metabolic block of long chain fatty acid oxidation when adequately treated.

References

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