Variant non ketotic hyperglycinemia is caused by mutations in LIAS, BOLA3 and the novel gene GLRX5

Abstract

Patients with nonketotic hyperglycinemia and deficient glycine cleavage enzyme activity, but without mutations in AMT, GLDC or GCSH, the genes encoding its constituent proteins, constitute a clinical group which we call 'variant nonketotic hyperglycinemia'. We hypothesize that in some patients the aetiology involves genetic mutations that result in a deficiency of the cofactor lipoate, and sequenced genes involved in lipoate synthesis and iron-sulphur cluster biogenesis. Of 11 individuals identified with variant nonketotic hyperglycinemia, we were able to determine the genetic aetiology in eight patients and delineate the clinical and biochemical phenotypes. Mutations were identified in the genes for lipoate synthase (LIAS), BolA type 3 (BOLA3), and a novel gene glutaredoxin 5 (GLRX5). Patients with GLRX5-associated variant nonketotic hyperglycinemia had normal development with childhood-onset spastic paraplegia, spinal lesion, and optic atrophy. Clinical features of BOLA3-associated variant nonketotic hyperglycinemia include severe neurodegeneration after a period of normal development. Additional features include leukodystrophy, cardiomyopathy and optic atrophy. Patients with lipoate synthase-deficient variant nonketotic hyperglycinemia varied in severity from mild static encephalopathy to Leigh disease and cortical involvement. All patients had high serum and borderline elevated cerebrospinal fluid glycine and cerebrospinal fluid:plasma glycine ratio, and deficient glycine cleavage enzyme activity. They had low pyruvate dehydrogenase enzyme activity but most did not have lactic acidosis. Patients were deficient in lipoylation of mitochondrial proteins. There were minimal and inconsistent changes in cellular iron handling, and respiratory chain activity was unaffected. Identified mutations were phylogenetically conserved, and transfection with native genes corrected the biochemical deficiency proving pathogenicity. Treatments of cells with lipoate and with mitochondrially-targeted lipoate were unsuccessful at correcting the deficiency. The recognition of variant nonketotic hyperglycinemia is important for physicians evaluating patients with abnormalities in glycine as this will affect the genetic causation and genetic counselling, and provide prognostic information on the expected phenotypic course.

Lipoic acid metabolism and mitochondrial redox regulation

“…Recently, the E. coli iron-sulfur cluster proteins NfuA and IscU were shown to reinstall the [4Fe-4S] cluster of LipA in order to facilitate additional turns of the enzyme (21). This mechanism is consistent with deficiencies in NFU1 resulting in phenotypes associated with lipoic acid deficiency and suggests that this mechanism may be conserved in mammals (22).…”

“…This enzyme can utilize both the (R)-and (S)-enantiomers of LA and primarily uses GTP to activate the natural (R)-lipoic acid, but so far there has been no substantial evidence to support that this enzyme functions in LA metabolism in vivo (36). This is consistent with the inability for exogenous LA to rescue defects in cells derived from LIAS deficient patients, embryonic lethality in LIAS deficient mice, or to ameliorate symptoms in patients with this disease (22,30,32). Taken together, this suggests that mammalian LA metabolism is similar to S. cerevisiae where LIPT2 transfers octanoate from ACP to the H-protein of GCS, LIAS inserts sulfur atoms into the octanoyl group on H-protein, and LIPT1 transfers the lipoyl group from the H-protein to E2 subunits.…”

Assembly of Lipoic Acid on Its Cognate Enzymes: an Extraordinary and Essential Biosynthetic Pathway

“…Mutations are found in two genes, that encoding lipoyl synthase (LIAS) and that encoding the defective lipoate ligase activity (LIPT1). As first seen in dihydrolipoamide dehydrogenase deficiency, the four known LIAS patients are clinically heterogeneous (144,(150)(151)(152). Two died early in life and two survived, albeit with severe neurological problems.…”

“…All four patients showed elevated glycine levels in plasma and cerebrospinal fluid plus low or absent glycine cleavage system activity. The administration of lipoic acid failed to reverse the metabolic defects (150). Fibroblasts from the two most severely affected patients had extremely reduced or undetectable levels of lipoylated proteins.…”

“…Fibroblasts from the two most severely affected patients had extremely reduced or undetectable levels of lipoylated proteins. The LIAS genes of all four patients contained mutations in highly conserved residues (150), and one of the LIAS alleles was shown to be deficient in restoring the growth of an E. coli lipA mutant strain (144).…”