Despite its simple chemical structure, glycine plays a complex role in the body. The glycine cleavage system regulates brain glycine levels and is a key one-carbon donor to folate. Its metabolism is tightly integrated with that of serine. In addition to its biochemical role, glycine functions as a neurotransmitter and neuromodulator. Primary defects in the glycine cleavage system have long been known to cause human disease with a primarily neurological phenotype, and this was labelled as 'nonketotic hyperglycinaemia' in 1968. With increasing availability of molecular testing, many additional genetic conditions became apparent, as well as non-genetic factors that cause hyperglycinaemia. There is now a much greater appreciation of the marked clinical impact of this heterogeneity. The previous terminology of 'classical' and 'atypical' nonketotic hyperglycinaemia does not adequately address these numerous genetic aetiologies, nor does it account for the phenotypic spectrum within individual genetic disorders. We provide here a clinically relevant classification of the glycine encephalopathies, based on the underlying genetic aetiology and its relation to the glycine cleavage system. Characteristic clinical and biochemical features of each condition, as well as non-genetic phenocopies that cause hyperglycinaemia, are discussed in detail. This provides a readily usable framework for clinicians when faced with a patient with elevated glycine levels.

Neurotransmitter disorders are a group of heterogeneous conditions that comprise defects in synthesis, transport, receptor binding, and degradation of neurochemical messengers. These rare disorders range from mild intermittent dystonia to lethal encephalopathies. The natural history and clinical presentation remain far from established. The study was conducted between October 2015 and September 2024. This study aims to describe the spectrum of clinical presentation, laboratory, imaging features, and genetic profiles of children diagnosed with neurotransmitter disorders and to assess the treatment modalities and clinical outcomes in these children. Among 29 patients, the median age was 12 months, with a male predominance. Positive family history was noted in 9 cases. The most frequent presentation was global developmental delay (GDD), dystonia, and seizures with autonomic disturbances, with diurnal variation. Various subcategories of neurotransmitter disorders are aromatic L amino acid decarboxylase deficiency-7 cases, tyrosine hydroxylase deficiency-3 cases, dopamine transporter deficiency syndrome-1 case, vesicular monoamine transporter 2 deficiency (VMAT2)-2 cases, GTP cyclohydrolase type deficiency-1 case, 6-pyruvoyl-tetrahydropterin synthase deficiency-1 case, dihydropteridine reductase deficiency-3, sepiapterin reductase deficiency-1 case, glycine encephalopathy-1 case, FOLR1-related cerebral folate transport deficiency-3 cases, and succinic semialdehyde dehydrogenase deficiency-5 cases. Metabolic workups were normal in all cases, with elevated phenylalanine levels in tandem mass spectrometry (TMS) in 5 children. Neuroimaging and electroencephalogram (EEG) were abnormal in 7 and 5 children, respectively. Multi-pronged and early treatment ensured better outcomes in these children. The most common type of neurotransmitter disorder in our series was aromatic L-amino acid decarboxylase deficiency, with the most common presentation being global developmental delay and dystonia.

Nonketotic hyperglycinemia (NKH) is a rare, autosomal recessive-inherited disorder of amino acid metabolism known as glycine encephalopathy. Clinical manifestations arise because of the enzyme deficiency involved in glycine degradation. Currently, no effective treatment exists to alter the prognosis of NKH; available therapies focus primarily on reducing glycine accumulation in the body. MicroRNAs (miRNAs) are small noncoding RNAs that function as transcriptional and post-transcriptional regulators of gene expression. Here, we report the comparative profiling of small RNA sequencing (RNA-seq) data generated from clinical samples diagnosed with a specific condition. We identified miRNAs using miRNA-seq with samples obtained from three NKH patients, five individuals with heterozygous variants in NKH genes, and seven control cases. Utilising pathways from the PubChem database, we identified NKH-related pathways and used bioinformatics tools for miRNA, pathway, and disease prediction. This was followed by Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analyses to enrich the predicted target genes of differentially expressed miRNAs based on miRNA-target interactions. In our study, 10 known miRNAs were identified to be associated with NKH using at least two different tools. Our study is the first to demonstrate altered miRNA profiles in cases where the expression of AMT and GLDC genes is reduced. NKH is an ultrarare and difficult-to-diagnose disease. This study determines the miRNA-based biomarkers for early detection of NKH and provides a robust framework for advancing future experimental research and diagnostic strategies.

Non-ketotic hyperglycinemia (NKH) is a rare autosomal recessive disorder caused by defects in the mitochondrial glycine cleavage system, most commonly involving variants in GLDC. Clinical presentation is heterogeneous, ranging from severe neonatal encephalopathy with intractable epilepsy to attenuated forms with variable neurodevelopmental outcomes. We conducted a comprehensive clinical, biochemical, molecular, and genealogical analysis of eight Colombian patients with NKH, all of whom shared ancestry from the Paisa population. All patients were born at term after uncomplicated pregnancies and presented in the neonatal period with hypotonia, lethargy, feeding difficulties, and treatment-resistant seizures. Elevated glycine levels were detected in plasma for all patients and cerebrospinal fluid in three cases. Molecular testing identified the same homozygous GLDC variant (NM_000170.3:c.2714 T>G; p.Val905Gly) in all individuals. Although no consanguinity was reported, shared surname and regional ancestry suggested intra- and interfamilial isonymy, raising the possibility of a founder effect. Clinical outcomes varied despite the implementation of early supportive interventions. This is the first report of a recurrent GLDC variant in Colombian patients, supporting a potential founder effect in the Paisa population. These findings highlight the importance of regional genetic studies in enhancing diagnostic precision and guiding population-specific strategies for rare disease management.

Neonatal glycine encephalopathy is a rare genetic neurometabolic disorder secondary to glycine cleavage system deficiency. Patients typically present with early-onset intractable seizures, status epilepticus and encephalopathy. Seizures control remains challenging in view of their refractoriness to standard anti-seizure medications. Sodium benzoate is commonly used to control the elevated glycine level. Oral anti-NMDA receptor antagonists, ketamine and dextromethorphan, have been used in various combinations in the treatment of this complex disorder. In this report, we present a neonatal case of classical glycine encephalopathy with hypotonia and refractory myoclonic seizures. The status epilepticus was successfully treated using a combination of intravenous ketamine, oral dextromethorphan and sodium benzoate. Seizures resolved and the patient's development showed improvement on follow-up. The intravenous form of ketamine in the neonatal period is rarely used, and it has been reported in only two glycine encephalopathy patients in the literature. This is the first report in the literature of the efficacy of intravenous ketamine using the above triple therapy. This intervention might have implications on the management of neonatal intractable seizures in glycine encephalopathy, which might improve the outcome of this devastating disorder.