Functional Effects of Different Medium-Chain Acyl-CoA Dehydrogenase Genotypes and Identification of Asymptomatic Variants

<div><p>Medium-chain acyl-CoA dehydrogenase (MCAD) deficiency (OMIM 201450) is the most common inherited disorder of fatty acid metabolism presenting with hypoglycaemia, hepatopathy and Reye-like symptoms during catabolism. In the past, the majority of patients carried the prevalent c.985A>G mutation in the <em>ACADM</em> gene. Since the introduction of newborn screening many other mutations with unknown clinical relevance have been identified in asymptomatic newborns. In order to identify functional effects of these mutant genotypes we correlated residual MCAD (OMIM 607008) activities as measured by octanoyl-CoA oxidation in lymphocytes with both genotype and relevant medical reports in 65 newborns harbouring mutant alleles. We identified true disease-causing mutations with residual activities of 0 to 20%. In individuals carrying the c.199T>C or c.127G>A mutation on one allele, residual activities were much higher and in the range of heterozygotes (31%–60%). Therefore, both mutations cannot clearly be associated with a clinical phenotype. This demonstrates a correlation between the octanoyl-CoA oxidation rate in lymphocytes and the clinical outcome. With newborn screening, the natural course of disease is difficult to assess. The octanoyl-CoA oxidation rate, therefore, allows a risk assessment at birth and the identification of new <em>ACADM</em> genotypes associated with asymptomatic disease variants.</p> </div

Correlation of octanoyl-CoA oxidation residual activities [%] detected by HPLC-UV and by HPLC-ESI-MS/MS.

<p>Relative residual octanoyl-CoA oxidation activities are presented as a percentage of the mean value of healthy controls. The resulting regression line is y = 1.16×+0.03. The correlation coefficient R² is 0.96.</p

Correlation of octanoyl-CoA oxidation with <i>ACADM</i> genotype.

<p>The patients are grouped into 1. Wild-types, 2. Individuals with only one mutation found and 3. Homozygous and compound heterozygous newborns. Patients with identical genotypes are grouped in one bar. Homozygotes and compound heterozygotes are shown in black, heterozygotes in shaded and wild-types in white bars. * denotes novel identified missense mutations. Relative residual octanoyl-CoA oxidation activities are presented as a percentage of the mean value of healthy controls ± standard error of the mean (SEM).</p

Genotype and octanoyl-CoA oxidation measured in lymphocytes of 65 subjects with suspected MCAD deficiency^a.

a<p>Activity was given as % of the mean value of healthy controls (1.89±0.58 mU/mg).</p>b<p>Newborn screening octanoyl-carnitine results were given in µmol/L. The cut-off values were given bracketed.</p>c<p>Data not available.</p>d<p>no HPLC-UV measurement performed.</p>e<p>novel mutation.</p

Plasma octanoyl-carnitine (C8:0) concentrations determined in dried blood spots during initial newborn screening and subsequent follow-up from patients carrying two confirmed <i>ACADM</i> mutations.

<p>Plasma octanoyl-carnitine (C8:0) concentrations determined in dried blood spots during initial newborn screening and subsequent follow-up from patients carrying two confirmed <i>ACADM</i> mutations.</p

Octanoyl-CoA oxidation, plasma medium-chain acylcarnitine levels and gene analysis of both <i>ACADM</i> alleles in subjects with suspected MCAD-deficiency.

<p>Case 1 to 18 were identified in NBS, with the exception of 4 and 8.</p>1<p>Relative residual MCAD enzyme activities are presented as a percentage of the mean of lymphocytes from healthy control (2.38 nmol · min⁻¹ · mg⁻¹, n = 6).</p>2<p>First octanoylcarnitine specimen (C8:0 I) obtained on day 2–5 of life and subsequent repeat specimen (C8:0 II) are shown in µmol · L⁻¹, cut-off was set at 0.30.</p>3<p>Parents of 12.</p>4<p>Parents of 16.</p>5<p>Intervening sequences: IVS2-32C>G, IVS3+10T>C, IVS5+32C>G, IVS7-22C>A.</p>6<p>Intervening sequences: IVS2-32C>G, IVS3+10T>C, IVS5+32C>G, IVS6-14A>G and IVS7-22C>A.</p>7<p>subjects with clinical suspicion of MCADD.</p

Multiple Reactant Monitoring (MRM) chromatograms of quenched VLCAD assay samples.

<p>C16:OH-CoA, m/z 1022.4 → 515.2 (A and B) and C16:1-CoA, m/z 1004.4 → 497.2 (C and D) after 0 (A and C) and 5 (B and D) min. of incubation.</p