VLCAD enzyme activity determinations in newborns identified by screening: a valuable tool for risk assessment

Abstract

Tandem mass spectrometry-based newborn screening correctly identifies individuals with very long-chain acyl-CoA dehydrogenase deficiency (VLCADD). However, a great number of healthy individuals present with identical acylcarnitine profiles during catabolism in the first three days of life. We routinely perform an enzyme activity assay as confirmation analysis in newborns identified by screening. Whereas VLCAD residual activities of less than 10% are clearly diagnostic and indicate patients at risk of clinical disease, the clinical relevance of higher residual activities is unclear. In this study we assess the molecular basis in 34 individuals with residual activities of 10-50%. We identify two pathogenic mutations in patients that result in residual activities as high as 22%, while individuals with residual activities of 25-50% either present with a heterozygous or no mutation in the VLCAD gene. In addition, confirmed heterozygous parents present with residual activities as low as 32%.In conclusion, we identify individuals with 2 pathogenic mutations and those with only one heterozygous mutation in the residual activity range of 20-30%. Whereas heterozygosity is generally regarded as clinically irrelevant, identification of 2 VLCAD mutations leads to precautions in the management of the children. Based on our data we anticipate that individuals with a residual enzyme activity >20% present with a biochemical phenotype but likely remain asymptomatic throughout life. Studies in greater patient numbers are needed to correlate residual activities >10% with the genotype and the outcome.

Fatty acid oxidation flux predicts the clinical severity of VLCAD deficiency

“…10,11,27 Our study aimed to develop a tool that predicts the clinical phenotype in individuals with VLCADD detected by NBS. In this article we show a strong correlation between LC-FAO flux in cultured skin fibroblasts and the clinical severity of the phenotype in patients with VLCADD diagnosed before introduction of VLCADD in the Dutch NBS program.…”

“…8 However, since the introduction of VLCADD in NBS panels, it has become clear that a significant number of newborns with VLCADD actually have a very low risk for metabolic decompensation and may even remain fully asymptomatic if left untreated. [9][10][11] Unfortunately, there is currently no reliable method to assess the expected phenotypic severity at the time of diagnosis through NBS. The available literature is biased by reports of symptomatic patients; consequently, genotype-phenotype correlation studies concern more severe presentations.…”

“…These studies show that nonsense mutations in the encoding gene (ACADVL) result in a severe and early presentation of cardiomyopathy, [12][13][14] but the more frequent missense mutations are associated with both severe or attenuated presentations. 10 Functional tests, including determination of the residual activity of the VLCAD enzyme, 10,15 have been used to test the effects of various mutations on LC-FAO activity. VLCAD enzyme activity measurement as the sole functional readout is disadvantageous because it is not well suited to estimating the influence of genetic variations in potential compensatory enzymes (e.g., MCAD 15 or ACAD9 (ref.…”

The Newborn Screening Paradox: Sensitivity vs. Overdiagnosis in VLCAD Deficiency

“…However, it is not possible to define which outcomes are clinically relevant (Wilcken 2010;Bonham 2013). Children with a VLCAD activity of >20% appear to have no symptoms (Hoffmann et al 2011). One could argue that individual 5 is therefore not a case.…”

Cell and Gene Therapy for Genetic Diseases: Inherited Disorders Affecting the Lung and Those Mimicking Sudden Infant Death Syndrome

“…Residual enzyme activities of specific mutations have also been useful in predicting clinical outcomes. Patients with less than 10% activity will develop clinical features in absence of treatment, and patients with less than 20% may also be at risk for serious clinical disease (Hoffmann et al, 2012). For example, one fetal case occurred in a patient with a genotype encoding a protein that retains residual activity.…”