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

Abstract

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.

Brain Branched-Chain Amino Acids in Maple Syrup Urine Disease: Implications for Neurological Disorders

“…Hence, the decreased aspartate in MSUD may affect oxidative phosphorylation and adenosine triphosphate production in neurons (reviewed elsewhere [ 15 ]). Furthermore, studies reported that MAS deficits are associated with infantile-onset epileptic encephalopathies [ 65 , 66 , 67 , 68 , 77 , 78 ] and autism [ 79 , 80 , 81 , 82 ]. In short, reduced aspartate levels in the CNS of MSUD patients may decrease MAS flux and contribute to neurologic symptoms in MSUD patients.…”

Cysteine Aminotransferase (CAT): A Pivotal Sponsor in Metabolic Remodeling and an Ally of 3-Mercaptopyruvate Sulfurtransferase (MST) in Cancer

“…The physiological relevance of mCAT is pointed by the fact that homozygous GOT2 -knock-out mice and zebrafish are not viable, as showed by van Karnebeek and colleagues [ 59 ]. In the same study it was reported that GOT2 (mCAT) bi-allelic mutations promote a mitochondriopathy through deficiencies in mCAT functioning.…”

“…In the same study it was reported that GOT2 (mCAT) bi-allelic mutations promote a mitochondriopathy through deficiencies in mCAT functioning. Despite showing residual mCAT enzymatic activity, affected patients present clinical evidence of this deficiency, including metabolic encephalopathy with epilepsy, progressive microcephaly, and several biochemical abnormalities [ 59 ]. Importantly, mutated mCAT promotes a dysfunctional malate-aspartate shuttle (MAS), which leads to a deregulation in the cellular NADH/NAD + equilibrium, affecting NAD-dependent enzymes and pathways [ 59 ].…”

“…Despite showing residual mCAT enzymatic activity, affected patients present clinical evidence of this deficiency, including metabolic encephalopathy with epilepsy, progressive microcephaly, and several biochemical abnormalities [ 59 ]. Importantly, mutated mCAT promotes a dysfunctional malate-aspartate shuttle (MAS), which leads to a deregulation in the cellular NADH/NAD + equilibrium, affecting NAD-dependent enzymes and pathways [ 59 ]. MAS relies on the functionality of two main operators, glutamate oxaloacetate transaminases (GOT, also known as CAT) and malate dehydrogenases (MDH), both present in the mitochondria and in the cytoplasm [ 55 ].…”

“…Accordingly, the first step of de novo serine biosynthesis relies on the NAD + -dependent enzyme 3-phosphoglycerate dehydrogenase. Moreover, glycine production was also affected, which is consequently also related to the deficient MAS [ 59 ], since glycine biosynthesis depends on serine bioavailability. Additionally, deficient MAS also induced increased lactate serum levels [ 60 ].…”