Identification of Six Novel Variants of ACAD8 in Isobutyryl-CoA Dehydrogenase Deficiency With Increased C4 Carnitine Using Tandem Mass Spectrometry and NGS Sequencing

Analysis of genotypes and biochemical phenotypes of neonates with abnormal metabolism of butyrylcarnitine

OBJECTIVES

To investigate the genotypes and biochemical phenotypes of neonates with abnormal metabolism of butyrylcarnitine (C4).

METHODS

One hundred and twenty neonates with increased C4 levels detected by tandem mass spectrometry in the neonatal screening at Children's Hospital, Zhejiang University School of Medicine from January 2018 to June 2023 were included. The initial screening data and recalled data of C4 and C4/C3 were collected and converted into multiples of C4 reference range. Next generation sequencing was performed and the exons with adjacent 50 bp regions of ACAD8 and ACADS genes were captured by liquid phase capture technique. Variant information was obtained by bioinformatic analysis and the pathogenicity were classified according to the American College of Medical Genetics and Genomics criteria. The Wilcoxon rank sum test was used to analyze the differences in C4 levels among neonates with different variation types.

RESULTS

In total, 32 variants in ACAD8 gene were detected, of which 7 variants were reported for the first time; while 41 variants of ACADS gene were detected, of which 17 variants have not been previously reported. There were 39 cases with ACAD8 biallelic variations and 3 cases with ACAD8 monoallelic variations; 34 cases with ACADS biallelic variations and 36 cases with ACADS monoallelic variations. Furthermore, 5 cases were detected with both ACAD8 and ACADS gene variations. Inter group comparison showed that the multiples of C4 reference range in initial screening and re-examination of the ACAD8 biallelic variations and ACADS biallelic variations groups were significantly higher than those of the ACADS monoallelic variations group (all P<0.01), while the multiples in the ACAD8 biallelic variations group were significantly higher than those in the ACADS biallelic variations group (all P<0.01). The multiples of C4 reference range in the initial screening greater than 1.5 times were observed in all neonates carrying ACAD8 or ACADS biallelic variations, while only 25% (9/36) in neonates carrying ACADS monoallelic variations.

CONCLUSIONS

ACAD8 and/or ACADS gene variants are the main genetic causes for elevated C4 in newborns in Zhejiang region with high genotypic heterogeneity. The C4 levels of neonates with biallelic variations are significantly higher than those of neonates with monoallelic variations. The cut-off value for C4 level could be modestly elevated, which could reduce the false positive rate in tandem mass spectrometry neonatal screening.

Butyrylcarnitine Elevation in Newborn Screening: Reducing False Positives and Distinguishing between Two Rare Diseases through the Evaluation of New Ratios

One of the main challenges of newborn screening programs, which screen for inherited metabolic disorders, is cutting down on false positives (FPs) in order to avoid family stresses, additional analyses, and unnecessary costs. False positives are partly caused by an insubstantial number of robust biomarkers in evaluations. Another challenge is how to distinguish between diseases which share the same primary marker and for which secondary biomarkers are just as highly desirable. Focusing on pathologies that involve butyrylcarnitine (C4) elevation, such as short-chain acylCoA dehydrogenase deficiency (SCADD) and isobutyrylCoA dehydrogenase deficiency (IBDD), we investigated the acylcarnitine profile of 121 newborns with a C4 increase to discover secondary markers to achieve two goals: reduce the FP rate and discriminate between the two rare diseases. Analyses were carried out using tandem mass spectrometry with whole blood samples spotted on filter paper. Seven new biomarkers (C4/C0, C4/C5, C4/C5DC\C6OH, C4/C6, C4/C8, C4/C14:1, C4/C16:1) were identified using a non-parametric ANOVA analysis. Then, the corresponding cut-off values were found and applied to the screening program. The seven new ratios were shown to be robust (p < 0.001 and p < 0.01, 0.0937 < ε2 < 0.231) in discriminating between FP and IBDD patients, FP and SCADD patients, or SCADD and IBDD patients. Our results suggest that the new ratios are optimal indicators for identifying true positives, distinguishing between two rare diseases that share the same primary biomarker, improving the predictive positive value (PPV) and reducing the false positive rate (FPR).