Sequential magnetic resonance spectroscopic changes in a patient with nonketotic hyperglycinemia

Ketogenic diet as a glycine lowering therapy in nonketotic hyperglycinemia and impact on brain glycine levels

BACKGROUND

Nonketotic hyperglycinemia (NKH) is a severe neurometabolic disorder characterized by increased glycine levels. Current glycine reduction therapy uses high doses of sodium benzoate. The ketogenic diet (KD) may represent an alternative method of glycine reduction.

AIM

We aimed to assess clinical and biochemical effects of two glycine reduction strategies: high dose benzoate versus KD with low dose benzoate.

METHODS

Six infants with NKH were first treated with high dose benzoate therapy to achieve target plasma glycine levels, and then switched to KD with low dose benzoate. They were evaluated as clinically indicated by physical examination, electroencephalogram, plasma and cerebral spinal fluid amino acid levels. Brain glycine levels were monitored by magnetic resonance spectroscopy (MRS).

RESULTS

Average plasma glycine levels were significantly lower with KD compared to benzoate monotherapy by on average 28%. Two infants underwent comparative assessments of brain glycine levels via serial MRS. A 30% reduction of brain glycine levels was observed in the basal ganglia and a 50% reduction in the white matter, which remained elevated above normal, and was equivalent between the KD and high dose benzoate therapies. CSF analysis obtained while participants remained on the KD showed a decrease in glycine, serine and threonine levels, reflecting their gluconeogenetic usage. Clinically, half the patients had seizure reduction on KD, otherwise the clinical impact was variable.

CONCLUSION

KD is an effective glycine reduction method in NKH, and may provide a more consistent reduction in plasma glycine levels than high-dose benzoate therapy. Both high-dose benzoate therapy and KD equally reduced but did not normalize brain glycine levels even in the setting of low-normal plasma glycine.

Nonketotic Hyperglycinemia: Insight into Current Therapies

Nonketotic hyperglycinemia (NKH) is a rare inborn error of glycine metabolism that is characterized by the accumulation of glycine in all tissues, especially in the central nervous system (CNS). Based on clinical outcomes, NKH can be divided into two forms, i.e., severe and attenuated NKH. A poor prognosis, including no developmental progress and intractable epilepsy, is typical of severe NKH, whereas patients with the attenuated form present with varied symptoms and neurodevelopmental outcomes. So far, no causal treatment of NKH is known. Currently, the therapy is based on sodium benzoate and NMDA (The N-methyl-D-aspartate receptor) receptor site antagonists (dextromethorphan, ketamine). Different clinical outcomes of the therapy raise doubts about the effectiveness of the treatment. The purpose of this review is to summarize the therapeutic potential, challenges and effectiveness of different NKH therapies.

Genotypic and phenotypic features in Turkish patients with classic nonketotic hyperglycinemia

Nonketotic hyperglycinemia is an autosomal recessive inborn error of glycine metabolism, characterized by deficient activity of the glycine cleavage enzyme system. Classic nonketotic hyperglycinemia is caused by mutations or genomic changes in genes that encode the protein components of the glycine cleavage enzyme system. We aimed to investigate clinical, biochemical, radiological findings and molecular genetic data in ten Turkish patients with classic nonketotic hyperglycinemia. Ten Turkish patients who were diagnosed with classic nonketotic hyperglycinemia in a single center from 2013 to 2019 were included in this study. Their clinical, radiological, electrophysiological and laboratory data were collected retrospectively. Sixty percent of the patients were in neonatal group, while 40 % of the patients were infantile. There were no late-onset patients. 90 % of the patients had the severe form. All patients had developmental delay and seizures. Mortality ratio was 30 % in all groups and 50 % in the neonatal group, while no mortality was seen in infantile group. Median (range) values of cerebrospinal fluid (CSF) glycine levels, plasma glycine levels and CSF/plasma glycine ratios were 148 (15-320) µmol/L, 896 (87-1910) µmol/L, 0.17 (0.09-0.21) respectively. Diffuse hypomyelination and corpus callosum anomaly were the most common cranial MRI findings and multifocal epileptic activity and burst supression pattern were the most common electroencephalographic findings. Six patients had variants in GLDC gene and four in AMT gene; five novel variants including AMT gene deletion were detected. Prognosis was poor and treatment was not effective, especially in the severe form. Classic nonketotic hyperglycinemia causes high morbidity and mortality. Neonatal-onset disease was more common and severe than infantile-onset disease. The ratio of AMT gene variants might be higher in Turkey than other countries. AMT gene deletion also plays a role in the etiology of classic nonketotic hyperglycinemia.

Brain imaging in classic nonketotic hyperglycinemia: Quantitative analysis and relation to phenotype

Patients with severe nonketotic hyperglycinemia (NKH) have absent psychomotor development and intractable epilepsy, whereas attenuated patients have variable psychomotor development and absent or treatable epilepsy; differences in brain magnetic resonance imaging (MRI) between phenotypes have not been reported. In a retrospective cross-sectional study, we reviewed 38 MRI studies from 24 molecularly proven NKH patients, and 2 transient NKH patients. Quantitative analyses included corpus callosum size, apparent diffusion coefficient, automated brain volumetric analysis, and glycine/creatine ratio by spectroscopy. All patients age <3 months had restricted diffusion in the posterior limb of the internal capsule, anterior brainstem, posterior tegmental tracts, and cerebellum, not present in transient NKH. In older infants, the pattern evolved and included generalized diffusion restriction in the supratentorial white matter, which quantitatively peaked between 3 and 12 months. No patient had absent corpus callosum or gyral malformation. The corpus callosum was relatively short in severe compared to attenuated phenotypes, and thin in severe cases only. The corpus callosum growth rate differed by severity; age-matched Z-scores of thickness worsened in severe cases only. Cerebral volume was decreased in the hippocampus, globus pallidus, cerebral cortex, thalamus, and cerebellum. Severe patients had greatest glycine/creatine ratios. In this study, no brain malformations were identified. The growth failure of the corpus callosum is worse in severe NKH, whereas the diffusion restriction pattern, reflecting microspongiosis, does not discriminate by phenotypic severity. NKH is therefore a disorder of brain growth best recognized in the corpus callosum, whereas spongiosis is not prognostic.

Evidence that glycine induces lipid peroxidation and decreases glutathione concentrations in rat cerebellum

Patients with non-ketotic hyperglycinemia (NKH) present severe neurological symptoms and brain abnormalities involving cerebellum. Although the pathomechanisms underlying the cerebellum damage have not been studied, high tissue levels of glycine (GLY), the biochemical hallmark of this disorder have been suggested to contribute to the neuropathology of this disease. We investigated the in vitro effects of GLY on important parameters of oxidative stress and energy metabolism in cerebellum of 30-day-old rats. Our results show that GLY increased 2',7'-dichlorofluorescin oxidation, suggesting that reactive species production are augmented by GLY in the cerebellum. However, hydrogen peroxide generation was not altered by GLY. GLY also increased thiobarbituric acid-reactive substances (TBA-RS) levels and reduced the glutathione (GSH) content, indicating that this amino acid provokes lipid oxidative damage and compromises the non-enzymatic antioxidant defenses, respectively, in cerebellum. The antioxidants melatonin and trolox (the hydrosoluble analog of vitamin E) prevented the GLY-induced increase of TBA-RS and decrease of GSH in cerebellum, indicating the involvement of hydroxyl and peroxyl radicals in these effects. The NMDA receptor antagonist MK-801 also attenuated GLY-induced decrease of GSH, suggesting that this effect is mediated through NMDA receptor. In contrast, GLY did not alter the protein carbonyl formation and total and protein-bound sulfhydryl group content, as well as catalase and superoxide dismutase activities. Furthermore, GLY did not alter the activities of the respiratory chain complexes and creatine kinase. Our present data indicate that oxidative stress elicited by GLY in vitro may be a potential pathomechanism involved in the cerebellar dysfunction observed in NKH.