Bi-allelic GOT2 Mutations Cause a Treatable Malate-Aspartate Shuttle-Related Encephalopathy.

Early-infantile encephalopathies with epilepsy are devastating conditions mandating an accurate diagnosis to guide proper management. Whole-exome sequencing was used to investigate the disease etiology in four children from independent families with intellectual disability and epilepsy, revealing bi-allelic GOT2 mutations. In-depth metabolic studies in individual 1 showed low plasma serine, hypercitrullinemia, hyperlactatemia, and hyperammonemia. The epilepsy was serine and pyridoxine responsive. Functional consequences of observed mutations were tested by measuring enzyme activity and by cell and animal models. Zebrafish and mouse models were used to validate brain developmental and functional defects and to test therapeutic strategies. GOT2 encodes the mitochondrial glutamate oxaloacetate transaminase. GOT2 enzyme activity was deficient in fibroblasts with bi-allelic mutations. GOT2, a member of the malate-aspartate shuttle, plays an essential role in the intracellular NAD(H) redox balance. De novo serine biosynthesis was impaired in fibroblasts with GOT2 mutations and GOT2-knockout HEK293 cells. Correcting the highly oxidized cytosolic NAD-redox state by pyruvate supplementation restored serine biosynthesis in GOT2-deficient cells. Knockdown of got2a in zebrafish resulted in a brain developmental defect associated with seizure-like electroencephalography spikes, which could be rescued by supplying pyridoxine in embryo water. Both pyridoxine and serine synergistically rescued embryonic developmental defects in zebrafish got2a morphants. The two treated individuals reacted favorably to their treatment. Our data provide a mechanistic basis for the biochemical abnormalities in GOT2 deficiency that may also hold for other MAS defects.

ACBD5 deficiency causes a defect in peroxisomal very long-chain fatty acid metabolism.

BACKGROUND: Acyl-CoA binding domain containing protein 5 (ACBD5) is a peroxisomal membrane protein with a cytosolic acyl-CoA binding domain. Because of its acyl-CoA binding domain, ACBD5 has been assumed to function as an intracellular carrier of acyl-CoA esters. In addition, a role for ACBD5 in pexophagy has been suggested. However, the precise role of ACBD5 in peroxisomal metabolism and/or functioning has not yet been established. Previously, a genetic ACBD5 deficiency was identified in three siblings with retinal dystrophy and white matter disease. We identified a pathogenic mutation in ACBD5 in another patient and studied the consequences of the ACBD5 defect in patient material and in ACBD5-deficient HeLa cells to uncover this role. METHODS: We studied a girl who presented with progressive leukodystrophy, syndromic cleft palate, ataxia and retinal dystrophy. We performed biochemical, cell biological and molecular studies in patient material and in ACBD5-deficient HeLa cells generated by CRISPR-Cas9 genome editing. RESULTS: We identified a homozygous deleterious indel mutation in ACBD5, leading to complete loss of ACBD5 protein in the patient. Our studies showed that ACBD5 deficiency leads to accumulation of very long-chain fatty acids (VLCFAs) due to impaired peroxisomal ß-oxidation. No effect on pexophagy was found. CONCLUSIONS: Our investigations strongly suggest that ACBD5 plays an important role in sequestering C26-CoA in the cytosol and thereby facilitates transport into the peroxisome and subsequent ß-oxidation. Accordingly, ACBD5 deficiency is a novel single peroxisomal enzyme deficiency caused by impaired VLCFA metabolism, leading to retinal dystrophy and white matter disease.