Biochemical and molecular genetic characterisation of MADD in a Southern African cohort

Abstract

Multiple acyl-coenzyme A dehydrogenase deficiency (MADD) is a heterogeneous group of inborn errors of metabolism caused by pathogenic autosomal recessive variants in ETFA, ETFB, and ETFDH. The latter is mainly affected and encodes for electron transfer flavoprotein-ubiquinone oxidoreductase (ETFQO). MADD presents as three phenotypes with neuromuscular involvement, including two mostly fatal neonatal subtypes (types I and II) and a riboflavin-responsive mild and/or late-onset subtype (type III). While the disorder has been well-studied in other countries, it remains under-recognised and under-diagnosed in ethnically diverse and understudied regions like Southern Africa, due to the paucity of population-specific data. To address these shortcomings, this study was initiated in collaboration with the International Centre for Genomic Medicine in Neuromuscular Diseases (ICGNMD), and aspired to realise two aims: Firstly, to characterise the clinical, metabolic, and genetic properties of a Southern African cohort diagnosed with MADD; and secondly, to evaluate the functional properties of selected pathogenic variants frequently identified in the cohort in vitro. Accordingly, a cohort of 12 unrelated families (12 White and two mixed ethnicity patients) could be recruited via academic medical centres across Southern Africa. For each, the genotype–phenotype association was assessed by extensive clinical evaluations, whole exome sequencing, and Sanger sequencing confirmation. The urinary diagnostic metabolome was profiled before and after treatment, where possible, via targeted gas chromatography–mass spectrometry, and liquid chromatography–triple quadrupole mass spectrometry. This study revealed one novel (c.[287dupA*]) and four known causal variants in ETFDH. Based on disease severity and treatment response, three groups emerged: (i) The most severe and fatal phenotypes accompanied the c.[1067G>A];c.[1067G>A] and c.[976G>C];c.[1067G>A] genotypes, which presented at birth as MADD types I and I/II. (ii) The c.[287dupA*];c.[1448C>T], c.[740G>T];c.[1448C>T], and c.[1448C>T];[1067G>A] genotypes displayed moderate early childhood-onset phenotypes of MADD types II/III and III, whereas (iii) the c.[1448C>T];c.[1448C>T] genotype was associated with mild, late-onset MADD type III. All groups exhibited the characteristic organic acids, acylcarnitines and amino acids of MADD (i>ii>iii), which normalised (ii<iii), together with the clinical symptoms, in only MADD type III following riboflavin and/or L-carnitine treatment. For the two main variants identified (c.[1067G>A] and c.[1448C>T]), the allele frequency was estimated as <0.00034–0.00084% in the four largest South African populations and both revealed a shared haplotype in the region of ETFDH, suggesting a founder effect for c.[1067G>A] in the White South African population. The pathogenicity of selected genotypes was further confirmed by investigating the relative quantity of ETFQO and mitochondrial complex I–V, together with respiration in patient fibroblasts. ETFQO was found to be most decreased in c.[287dupA*];c.[1448C>T] (-83%), followed by c.[1067G>A];c.[1448C>T] (-73%) and c.[1448C>T];c.[1448C>T] (-55%). Moreover, two adaptive responses to mild and severe genotypes of MADD, respectively, are hypothesised: The upregulation of mitochondrial biogenesis, and a shift from oxidative phosphorylation to alternative metabolic pathways. In conclusion, this study provides the first extensive genotype–phenotype, metabolic, and functional evaluation of MADD in the understudied populations of Southern Africa. The data and methods developed will enable timeous screening, genetic counselling, and patient-specific treatment, and support a policy of including MADD in a newborn screening program for Southern Africa.