Biallelic Mutations in LIPT2 Cause a Mitochondrial Lipoylation Defect Associated with Severe Neonatal Encephalopathy.

Lipoate serves as a cofactor for the glycine cleavage system (GCS) and four 2-oxoacid dehydrogenases functioning in energy metabolism (α-oxoglutarate dehydrogenase [α-KGDHc] and pyruvate dehydrogenase [PDHc]), or amino acid metabolism (branched-chain oxoacid dehydrogenase, 2-oxoadipate dehydrogenase). Mitochondrial lipoate synthesis involves three enzymatic steps catalyzed sequentially by lipoyl(octanoyl) transferase 2 (LIPT2), lipoic acid synthetase (LIAS), and lipoyltransferase 1 (LIPT1). Mutations in LIAS have been associated with nonketotic hyperglycinemia-like early-onset convulsions and encephalopathy combined with a defect in mitochondrial energy metabolism. LIPT1 deficiency spares GCS deficiency and has been associated with a biochemical signature of combined 2-oxoacid dehydrogenase deficiency leading to early death or Leigh-like encephalopathy. We report on the identification of biallelic LIPT2 mutations in three affected individuals from two families with severe neonatal encephalopathy. Brain MRI showed major cortical atrophy with white matter abnormalities and cysts. Plasma glycine was mildly increased. Affected individuals' fibroblasts showed reduced oxygen consumption rates, PDHc, α-KGDHc activities, leucine catabolic flux, and decreased protein lipoylation. A normalization of lipoylation was observed after expression of wild-type LIPT2, arguing for LIPT2 requirement in intramitochondrial lipoate synthesis. Lipoic acid supplementation did not improve clinical condition nor activities of PDHc, α-KGDHc, or leucine metabolism in fibroblasts and was ineffective in yeast deleted for the orthologous LIP2.

Hyperprolinemia in Type 2 Glutaric Aciduria and MADD-Like Profiles.

Classical neonatal-onset glutaric aciduria type 2 (MAD deficiency) is a severe disorder of mitochondrial fatty acid oxidation associated with poor survival. Secondary dysfunction of acyl-CoA dehydrogenases may result from deficiency for riboflavin transporters, leading to severe disorders that, nevertheless, are treatable by riboflavin supplementation. In the last 10 years, we identified nine newborns with biochemical features consistent with MAD deficiency, only four of whom survived past the neonatal period. A likely iatrogenic cause of riboflavin deficiency was found in two premature newborns having parenteral nutrition, one of whom recovered upon multivitamin supplementation, whereas the other died before diagnosis. Four other patients had demonstrated mutations involving ETF or ETF-DH flavoproteins, whereas the remaining three patients presumably had secondary deficiencies of unknown mechanism. Interestingly, six newborns among the seven tested for plasma amino acids had pronounced hyperprolinemia. In one case, because the initial diagnostic workup did not include organic acids and acylcarnitine profiling, clinical presentation and hyperprolinemia suggested the diagnosis. Analysis of our full cohort of >50,000 samples from >30,000 patients suggests that the proline/alanine ratio may be a good marker of MAD deficiency and could contribute to a more effective management of the treatable forms.