Valine-sensitive nonketotic hyperglycinemia. Case report

[Clinical and molecular genetic study of nonketotic hyperglycinemia in a Chinese family]

Nonketotic hyperglycinemia (NKH) is a rare, inborn error of metabolism. In this case report, a Chinese male infant was diagnosed with NKH caused by GLDC gene mutation. The clinical characteristics and genetic diagnosis were reported. The infant presented with an onset of early metabolic encephalopathy and Ohtahara syndrome. Both blood and urinary levels of metabolites were in the normal range. Brain MRI images indicated a poor development of corpus callosum, and a burst suppression pattern was found in the EEG. Results of target gene sequencing technology combined with multiplex ligation-dependent probe amplification (MLPA) indicated a heterozygous missense mutation of c.1786 C>T (p.R596X) in maternal exon 15 and a loss of heterozygosity of 4-15 exon gross deletions in paternal GLDC gene. These definite pathogenic mutations confirmed the diagnosis of NKH. The infant's clinical condition was not improved after treatment with adreno-cortico-tropic-hormone, topiramate and dextromethorphan, and he finally died at 4 months of age. Patients with NKH often exhibit complicated clinical phenotypes and are lack of specific symptoms. NKH could be diagnosed by metabolic screening and molecular genetic analysis.

A rare case of glycine encephalopathy unveiled by valproate therapy

Glycine encephalopathy (GE) or nonketotic hyperglycinemia is an autosomal recessive disorder due to a primary defect in glycine cleavage enzyme system. It is characterized by elevated levels of glycine in plasma and cerebrospinal fluid usually presenting with seizures, hypotonia, and developmental delay. In our case, paradoxical increase in seizure frequency on starting sodium valproate led us to diagnose GE.

Nonketotic hyperglycinemia: A life-threatening disorder in Saudi newborns

Nonketotic hyperglycinemia (NKH) is an autosomal recessive disorder due to a fundamental defect in the glycine cleavage system, which leads to neuronal dysfunction caused by two receptor-mediated mechanisms. It is a life-threatening condition in the neonate. Until now, the disease has not been described from Saudi Arabia. We report on three Saudi newborns (two males and one female) who had NKH. Two of these were siblings (male and female). Following uneventful deliveries, they presented between the first and third day of life with progressive lethargy, poor feeding, recurrent apnea and severe hypotonia. Two newborns had myoclonic seizures, whereas electroencephalogram showed burst-suppression pattern in all of them. The diagnosis was confirmed by high cerebrospinal fluid/plasma glycine ratio (0.2 and 1.08) in two patients (normal < 0.030, whereas a sibling of one of the neonates had a high glycine level. Both siblings died during the second month of life despite therapy with dextromethorphan (an N-methyl-D-asparate [NMDA] receptor antagonist) in one of them. The third day had ketamine (noncompetitive NMDA receptor antagonist) and sodium benzoate (that conjugates with glycine, forming nontoxic hippuric acid). Although his seizures were controlled, he survived with severe neurological sequelae.

Response to sodium benzoate treatment in non-ketotic hyperglycinaemia

Therapy with benzoic acid in a case of classic neonatal non-ketotic hyperglycinaemia (NKH) was successful in stopping seizures but not in promoting mental development. Serum glycine levels were normalizable even by administering low doses of 53 mg sodium benzoate/kg body mass (BM) per day. Despite giving a higher dosage (240 mg/kg BM per day) normalization of glycine concentration in cerebrospinal fluid (CSF) was not achieved. However, seizures ceased. Restriction of protein intake (< or = 2 g/kg BM per day) seemed to be profitable. CSF glycine concentrations below 100 mumol/L may be sufficient to prevent seizures in older infants who have adapted to neuronal glycine exposure. No toxicity of sodium benzoate treatment was detected when administering doses of up to 470 mg/kg BM per day but side effects such as itching and hyperactivity were obvious.

Prenatal diagnosis of non-ketotic hyperglycinemia

We describe successful prenatal diagnosis in four pregnancies at risk for non-ketotic hyperglycinemia, two affected and two unaffected, using the glycine level and the glycine/serine ratio in amniotic fluid obtained at 16 weeks gestational age. Although this method of prenatal diagnosis for non-ketotic hyperglycinemia has been effective in our hands the narrow differences between affected and unaffected pregnancies indicate the need for caution concerning its reliability.

Nonketotic hyperglycinemia: clinical, biochemical, and therapeutic considerations

The salient features of nonketotic hyperglycinemia include apnea, feeding difficulties, lethargy, seizures, abnormal muscle tone and reflex activity, significant developmental delay, and, in most instances, early death. The pathogenesis of the biochemical defect leading to increased glycine concentration in blood, urine, and CSF is likely to concern derangements of the glycine cleavage enzyme and/or transport mechanisms of glycine. Our current state of knowledge of this disorder is incomplete. Therapeutic attempts, as described in Table 2, have been largely unsuccessful. Further basic research on the underlying biochemical perturbation, including additional documentation of the glycine cleavage enzyme deficiency patterns, of substrate inhibition of key metabolic pathways, and of glycine transport aberrations, as well as investigations of new pharmacologic approaches, will be a challenge for investigators in this field. It is hoped that new knowledge in these areas will eventually lead to reduction of morbidity and mortality in children with nonketotic hyperglycinemia.

A strategy for glycine encephalopathy therapy

An inherited defect in the glycine cleavage enzyme results in the condition of neonatal glycine encephalopathy which has not responded to the current innovative methods of therapy. A re-examination of the enzyme structure and metabolic pathways, leads us to recommend future clinical evaluation of (1) vitamin-responsiveness, e.g., pyridoxine, folate and lipoic acid, (2) methionine, (3) N5, N10-methylene tetrahydrofolate and (4) alpha-methylserine therapy during the critical period of neonatal brain growth and development.

Chronic hyperphenylalaninemia produces cerebral hyperglycinemia in immature rats

The amino acid content of three tissues was measured in 10-day-old rats made hyperphenylalaninemic from age 3 to 10 days by daily injection of phenylalanine plus alpha-methylphenylalanine to inhibit phenylalanine hydroxylase (PAH). At 12 h after the last injection, the concentrations of alanine, valine, methionine, isoleucine, and leucine in the cerebral hemispheres were depressed by 25-50%, whereas that of glycine was elevated 2.3-fold. In the spinal cord, the levels of phosphoserine, methionine, and leucine were decreased by 40-50%, and those of serine and threonine increased by 50%. Tyrosine and phenylalanine concentrations were high in all tissues, 2-3 and 15-30 times normal, respectively; of the amino acids investigated, they were the only ones changed in the liver. Cerebral hyperglycinemia was also produced by chronic treatment with phenylalanine plus p-chlorophenylalanine to inhibit PAH, but not by acute (12 h) hyperphenylalaninemia. An increase in cerebral phosphoserine phosphatase activity was greater in rats treated with phenylalanine plus PAH inhibitor than with inhibitor alone. The content of brain glycine normally declines with age from birth to 15 days; this decrease was prevented by chronic hyperphenylalaninemia. Attempts to reduce the cerebral glycine content of the hyperphenylalaninemic rats were unsuccessful. However, one of the therapeutic protocols, methionine loading, may be useful because it increased the methionine and decreased the phenylalanine contents in the brain.

Cerebrospinal fluid glycine in nonketotic hyperglycinemic: effect of treatment with sodium benzoate and a ventricular shunt

In three infants with nonketotic hyperglycinemia, glycine was increased three-to fourfold in plasma, 13- to 28-fold in lumbar spinal fluid, and was higher yet in ventricular fluid. Oral sodium benzoate lowered cerebrospinal fluid (CSF) glycine by greater than 40%, but did not change the abnormal plasma: CSF ratio. An adult control, made hyperglycinemic with oral glycine, had a normal plasma: CSF ratio. Treatment of one patient with sodium benzoate from birth did not prevent mental retardation; the degree of brain stem depression was a function of CSF glycine in another patient. The persistance of glycine elevation in CSF, although therapy maintained normal concentration in plasma, appears to be caused by overproduction in brain and limitation of the high-capacity lumbar spinal reabsorptive mechanism. Treatment through lowering of CNS glycine by use of a ventricular shunt was explored.

Nonketotic hyperglycinemia: report of a case and review of the clinical, chemical, and pathological changes

A female infant with neonatal hypotonia and lethargy was found to have nonketotic hyperglycinemia. She died at the age of 5 days. Autopsy revealed slightly retarded myelination and severe spongy change in the well-myelinated areas of the brain. Analysis of this and the other 26 reported cases suggests that patients with nonketotic hyperglycinemia develop severe mental retardation, not seen in ketotic hyperglycinemia. Elevated glycine levels in the brain and cerebrospinal fluid appear to differentiate these two forms of hyperglycinemia better than the presence of ketosis or leukopenia, and high glycine levels apparently occur in the same areas as the spongy change. While both forms show defective glycine cleavage in the liver, defective glycine cleavage in the brain has been reported only in nonketotic hyperglycinemia.