Published NKH Research Papers

Reviews and Broad Summaries

• Nonketotic Hyperglycinemia – GeneReviews® – Authoritative, continuously updated review covering clinical presentation, genetics, diagnosis, and management in detail. Source
  
  
• Clinical and genetic analysis of nonketotic hyperglycinemia: A case report (2022) – This case report demonstrates that plasma glycine levels do not reliably predict NKH prognosis, highlights corpus callosum abnormalities as part of disease involvement, and suggests a ketogenic diet may help control seizures in some patients. Source
  
  
• Glycine encephalopathy review (2022) – Comprehensive overview of genetic, biochemical and clinical presentations. Source
  
  
• Idiopathic hyperglycinemia and hyperglycinuria: a new disorder of amino acid metabolism (1961) – First description of nonketotic hyperglycinemia as a distinct inherited metabolic encephalopathy with severe neurologic impairment. Source
  
  
• Non-ketotic hyperglycinemia: a metabolic disorder associated with mental retardation (1975) – Early clinical and biochemical characterszation distinguishing NKH from organic acidemias and defining its neurodevelopmental severity. Source
  
  

Clinical/Natural History

• Natural history of nonketotic hyperglycinemia in 65 patients (2004) – Patients with NKH commonly present with neonatal lethargy, hypotonia, seizures and show high mortality within early childhood. Source
  
  
• Prenatal Brain Damage in Nonketotic Hyperglycinemia (1981) – NKH is present in utero, leading to abnormal glycine metabolism that may disrupt brain development before birth. Source
  
  
• Natural history and outcome of nonketotic hyperglycinemia in China (2024) – Study of 20 Chinese NKH patients reporting neonatal lethargy, hypotonia, apnea and seizures as common early symptoms, diverse MRI/EEG abnormalities, and variable survival outcomes. Source.
  
  
• Nonketotic Hyperglycinemia: Clinical range and outcome of a rare neurometabolic disease in a single‑center (2018) – Three‑year Turkish cohort documenting clinical onset between birth and 45 days with hypotonia, poor feeding, intractable seizures, variability in EEG and MRI abnormalities, and heterogeneous outcomes. Source
  
  

Diagnosis and Laboratory / Biochemical Studies

• Nonketotic hyperglycinemia (glycine encephalopathy): laboratory diagnosis (2001) – Elevated CSF glycine and CSF/plasma ratio are hallmark diagnostic markers, and molecular analysis improves prenatal and family diagnosis. Source
  
  
• Atypical variants of nonketotic hyperglycinemia (2005) – NKH exhibits neonatal, infantile and late-onset atypical phenotypes with overlapping but distinct biochemical profiles. Source
  
  
• Nonketotic hyperglycinemia (glycine encephalopathy): laboratory diagnosis (2001) – Reviews biochemical diagnostic criteria for NKH, emphasizing markedly elevated CSF glycine and CSF/plasma glycine ratio in classic neonatal cases, the role of glycine cleavage system enzymology, and the challenges of interpreting results in mild or atypical forms. Source
  
• Two novel laboratory tests facilitating diagnosis of glycine encephalopathy (nonketotic hyperglycinemia) (2011) – Describes two new diagnostic approaches — a non-invasive glycine breath test to measure glycine cleavage activity and MLPA for detecting large GLDC deletions — to improve NKH diagnosis. Source

• Cerebrospinal fluid amino acids glycine, serine, and threonine in nonketotic hyperglycinemia (2022) – Cross-sectional study showing that NKH patients have elevated CSF glycine with decreased serine and increased threonine, and that CSF amino acid ratios (glycine/serine) can aid discrimination of severe vs attenuated phenotypes and support biochemical diagnosis. Source
  
• Atypical nonketotic hyperglycinemia with normal cerebrospinal fluid to plasma glycine ratio (2000) – Reports that NKH can occur even when the CSF/plasma glycine ratio is normal, thus highlighting that enzyme assay or genetic testing may be needed when clinical features and peripheral glycine levels suggest NKH.Source
  
• Nonketotic Hyperglycinemia (Glycine Encephalopathy) ( 2019) – Provides detailed diagnostic guidance: simultaneous measurement of elevated plasma and CSF glycine and the CSF:plasma glycine ratio is the cornerstone of biochemical diagnosis, but careful specimen handling and exclusion of blood contamination are essential. Source
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• Nonketotic Hyperglycinemia (glycine encephalopathy): molecular and biochemical aspects (1997) – Reviews the glycine cleavage system defect underlying NKH and discusses how biochemical assays of enzyme components and clinical metabolite patterns (elevated glycine) are used for diagnosis. Source
  
• Pitfalls in the diagnosis of glycine encephalopathy (2002) – Identifies pitfalls in biochemical diagnosis, such as fluctuating CSF glycine levels and the importance of recognizing the characteristic CSF/plasma glycine ratio threshold (>0.08) while excluding other conditions with elevated glycine. Source
  
• Source summary available at Cambridge Core
• Non-ketotic hyperglycinemia: classical biochemical diagnostic criteria (ResearchGate summary) – Emphasizes that the simultaneous elevation of CSF and plasma glycine and a CSF/plasma glycine ratio >0.08 are essential biochemical features for confirming the diagnosis of NKH. Source
  
  
• False-positive diagnosis of nonketotic hyperglycinemia due to blood contamination of cerebrospinal fluid (1999) Shows how blood contamination during lumbar puncture can falsely elevate CSF glycine, leading to misdiagnosis of NKH. Source
  
  
• Prenatal diagnosis of nonketotic hyperglycinemia by molecular analysis (1998) – Demonstrates the utility of GLDC/AMT mutation analysis for prenatal diagnosis and family planning in NKH. Source
  
  
• One-carbon metabolism in health and disease (2017) – Comprehensive review of one-carbon metabolic pathways, frequently cited to contextualize glycine cleavage defects in NKH.
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Prognosis

• Biochemical and molecular predictors for prognosis in NKH (2015) – Correlates residual glycine cleavage activity with developmental outcomes to refine prognostic stratification. Source
  
  
• Prediction of long-term outcome in glycine encephalopathy: a clinical survey (2011) – Surveying 45 NKH patients, this study proposed a clinical severity scale correlating presentation, symptoms, and outcomes to help predict long-term prognosis. PubMed Source, Full Study Source
  
  
• Prognostic clues and outcome of early treatment of nonketotic hyperglycinemia (1996) – In six NKH infants, initial clinical severity and glycine levels did not reliably predict prognosis, but early treatment with NMDA modulators appeared to reduce long‑term neurological complications.
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• NKH prognosis overview (2023) – Severe deficiency in glycine cleavage enzyme activity presenting in the neonatal period is associated with poor prognosis, including early mortality or profound neurological impairment, whereas later onset can be linked to attenuated outcomes and variable developmental progress. Source
  
  

Genetics / Molecular Pathogenesis

• Recurrent mutations in P- and T-proteins of the glycine cleavage complex (2001) – Describes strategies for molecular characterisation of GLDC and AMT mutations in NKH patients. Source
  
  
• Glycine cleavage system H protein is essential for embryonic viability, implying additional function beyond the glycine cleavage system (2021) – This study suggests that mutations in GCSH are likely embryonically lethal, explaining why most NKH patients instead have mutations in AMT or GLDC. Source
  
  
• Large scale analyses of genotype-phenotype relationships of glycine decarboxylase mutations and neurological disease severity (2019) – Analyzing 255 GLDC missense mutations, this study shows plasma/CSF glycine does not predict severity and proposes a multi-parametric model linking specific mutations to four major symptom domains—cognitive disorders, seizures, movement dysfunction, and brain malformations—to predict clinical outcomes. Source
  
  
• Mutation in SLC6A9 encoding a glycine transporter causes a novel form of non-ketotic hyperglycinemia in humans (2016) – This report identifies mutations in the glycine transporter SLC6A9 as a novel cause of variant NKH in humans. Source
  
  
• The genetic basis of classic nonketotic hyperglycinemia due to mutations in GLDC and AMT (2016) – A large-scale study of 578 families identifies 410 unique GLDC/AMT mutations (246 novel), with GLDC mutations accounting for 80% of cases and an estimated NKH incidence of 1:76,000. Source
  
  
• Biochemical and molecular predictors for prognosis in nonketotic hyperglycinemia (2015) – In 124 NKH children, the combination of age of onset, clinical features, biochemical profile, and mutation type was used to develop a prognostic severity scale. Source
  
  
• Mutations in genes encoding the glycine cleavage system predispose to neural tube defects in humans (2012) – Demonstrates that GLDC and AMT mutations increase risk of human neural tube defects, linking NKH genetics to prenatal neurodevelopment. Source

Neuroimaging

• Brain imaging in classic nonketotic hyperglycinemia (2019) – MRI analyses in genetically confirmed NKH patients show quantitative cerebral abnormalities related to phenotype severity. Source
  
  
• Sequential MR Imaging Changes in Nonketotic Hyperglycinemia (2006) – This case study tracks progressive brain structural changes in a boy with NKH via serial MRI until death at 17 months, illustrating disease-related neurodegeneration over time. Source
  
  
• Magnetic Resonance Findings in an Infant with Nonketotic Hyperglycinemia (2011) – MRI of a 70-day-old infant with NKH revealed thin corpus callosum and symmetric white-matter signal abnormalities reflecting widespread spongiosis and intramyelinic microvacuole formation in tracts undergoing active neonatal myelination. Source
  
  
• Nonketotic hyperglycinemia: spectrum of imaging findings with emphasis on diffusion-weighted imaging (2017) – In seven children (8 days–2 years) with NKH, MRI showed variable white-matter damage and nonspecific brain atrophy including corpus callosum thinning, not strictly aligned with normal myelination patterns, though genetic heterogeneity limited conclusions. Source
  
  
• Abnormalities of the Brain in Nonketotic Hyperglycinemia: MR Manifestations (1988) – MRI of seven NKH patients (4 days–38 months) demonstrated early and progressive brain volume loss, delayed supratentorial myelination, and universally thin corpus callosum, with no correlation between imaging severity and glycine levels. Source.
  
• Prenatal Brain Damage in Nonketotic Hyperglycinemia (1981) – Early report suggesting that glycine metabolism abnormalities in NKH exist in utero and may contribute to delayed myelination and prenatal CNS pathology. Source
  
• The glycine site of the NMDA receptor—five years on (1993) – Seminal review detailing NMDA receptor glycine-binding physiology, central to understanding seizure generation and neurotoxicity in NKH. Source
  
  
• Electroencephalographic features of nonketotic hyperglycinemia (1998) – Characterises classic EEG patterns in NKH, including burst suppression and hypsarrhythmia-like abnormalities in neonates.
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Therapy / Management Strategies

• Nonketotic Hyperglycinemia: Insight into Current Therapies (2022) – Reviews sodium benzoate, NMDA antagonists, ketogenic diet, and other management strategies and challenges. Source
  
  
• Ketogenic diet as a glycine lowering therapy in nonketotic hyperglycinemia and impact on brain glycine levels (2022) – Compared ketogenic diet with high-dose benzoate, showing plasma glycine reduction and variable clinical impact. Source
  
  
• Neurodevelopmental Outcome and Treatment Efficacy of Benzoate and Dextromethorphan in Siblings with Attenuated Nonketotic Hyperglycinemia (2016) – In four sibling pairs with attenuated NKH, earlier treatment with sodium benzoate and dextromethorphan improved developmental milestones, developmental quotient, seizure frequency, and adaptive behavior, especially in socialization and daily living skills. Source
  
  
• Vigabatrin Caused Rapidly Progressive Deterioration in Two Cases With Early Myoclonic Encephalopathy Associated With Nonketotic Hyperglycinemia (Jan 2006) – Two NKH patients given Vigabatrin for seizures experienced rapid deterioration with acute encephalopathy, sleepiness, and respiratory failure, which resolved in one patient after discontinuation. Source
  
  
• CBD + NKH (Nov 2019) – This study identifies a potential interaction between cannabidiol (CBD) and the glycine cleavage system via a “CBD-sensitivity gene” similar to GCS-H, but its therapeutic relevance in NKH remains unclear. Source
  
  
• Up-regulation of neurotrophic factors by cinnamon and its metabolite sodium benzoate: Therapeutic implications for neurodegenerative disorders – Oral cinnamon administration results in sodium benzoate in the brain, which may upregulate neurotrophic factors relevant for neurodegenerative conditions. Source
  
  
• Can cinnamon spice down autoimmune diseases? (2020) – Highlights that sodium benzoate is a metabolite of cinnamon and is FDA-approved for NKH and urea cycle disorders, but does not provide evidence that cinnamon itself benefits NKH, but suggests more research is needed. Source
  
  
• The Efficacy of Vagus Nerve Stimulation in Intractable Epilepsy Associated With Nonketotic Hyperglycinemia in Two Children (2010) – Two infants with NKH and refractory seizures underwent vagus nerve stimulation, resulting in 75% seizure reduction, fewer medications, and improved quality of life sustained for at least three years. Source
  
  
• Using the Ketogenic Diet to Treat Intractable Epilepsy in a Case of Glycine Encephalopathy (2019) – An 11-year-old boy with NKH experienced dramatic seizure reduction, improved alertness, and normalized plasma glycine after starting a ketogenic diet, with over 50% seizure reduction maintained long-term. Source
  
  
• The impact of Glycine Reduction Therapies on Brain Glycine Levels in Nonketotic Hyperglycinemia (2021) – High-dose sodium benzoate and ketogenic diet with low-dose sodium benzoate normalized plasma glycine but only partially reduced CSF and brain glycine levels, and the clinical benefits on seizures and outcomes remain uncertain. Source
  
  
• Failure of early dextromethorphan and sodium benzoate therapy in an infant with nonketotic hyperglycinemia (1994) – Early treatment with NMDA antagonist dextromethorphan and sodium benzoate initially improved clinical and EEG features, but the child later deteriorated and died, demonstrating that this approach may not prevent poor outcomes in severe NKH. Source
  
  
• Long-term use of high-dose benzoate and dextromethorphan for the treatment of nonketotic hyperglycinemia (1998) – In four infants treated with high-dose sodium benzoate and dextromethorphan, plasma glycine normalized and seizures were reduced in some patients, but developmental outcomes varied widely and not all benefited equally. Source
  
  
• Efficacy of low-dose dextromethorphan in the treatment of nonketotic hyperglycinemia (1996) – In a case report, adding low-dose NMDA antagonist dextromethorphan to sodium benzoate and conventional therapy improved EEG patterns, resolved nystagmus, and modestly advanced developmental milestones in an infant with NKH. Source
  
  
• Response to sodium benzoate treatment in non-ketotic hyperglycinemia (1993) – Sodium benzoate therapy normalized plasma glycine levels and stopped seizures in a classic NKH infant, although CSF glycine remained elevated and neurological development did not improve significantly. Source
  
  

NKH Models

• A Nonketotic Hyperglycinemia Mouse Shows Wide‐Ranging Biochemical Consequences of Elevated Glycine, Reduced Folate One‐Carbon Charging, and Serine Deficiency (2026) – It describes a mouse model of NKH that exhibits biochemical and neurological consequences of elevated glycine and altered folate one-carbon metabolism, providing a tool to study disease mechanisms and potential therapies Source
  
  
• Regulation of glycine metabolism by the glycine cleavage system and conjugation pathway in mouse models of non-ketotic hyperglycinemia (2020) – This study in mice shows that glycine toxicity in NKH extends beyond glycine itself, demonstrates how sodium benzoate and cinnamate modify metabolism, and confirms that restoring GLDC function rescues tissue metabolic profiles. Source
  
  
• Glycine decarboxylase deficiency causes neural tube defects and features of nonketotic hyperglycinemia in mice (2014) – GLDC deficiency in mice causes both neural tube defects and postnatal NKH-like features, linked to impaired folate-mediated one-carbon metabolism, which can be rescued by formate supplementation. Source
  
  
• Impaired folate 1-carbon metabolism causes formate-preventable hydrocephalus in glycine decarboxylase–deficient mice (2019) – In Gldc-deficient mice, hydrocephalus arises from insufficient glycine-derived one-carbon units rather than glycine toxicity, and maternal formate supplementation or Mthfr ablation prevents ventriculomegaly and postnatal hydrocephalus. Source
  
  
• AAV‑mediated expression of mouse or human GLDC normalises metabolic biomarkers in a GLDC‑deficient mouse model of Non‑Ketotic Hyperglycinemia (2024) – In a GLDC‑deficient NKH mouse model, administering AAV9 vectors expressing GLDC restored enzyme expression and lowered elevated glycine and disrupted folate metabolism, validating the model for testing gene therapy.
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• Deep postnatal phenotyping of a new mouse model of nonketotic hyperglycinemia (2024) – Describes a Gldc p.Ala394Val mutant mouse model with reduced glycine cleavage enzyme activity, elevated glycine in blood/brain, developmental abnormalities including hydrocephalus and encephalopathic EEG changes, useful for testing therapeutic strategies.
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• Glycine decarboxylase deficiency–induced motor dysfunction in zebrafish is rescued by counterbalancing glycine synaptic level (2018) – A CRISPR/Cas9 gldc knockout zebrafish model recapitulates molecular features of NKH with elevated glycine, broad metabolic disturbances and severe hypotonic motor deficits, and the motor phenotype can be rescued by pharmacologically or genetically reducing synaptic glycine levels. Source
  
  
• Glycine disrupts myelin, glutamatergic neurotransmission, and redox homeostasis in a neonatal model for nonketotic hyperglycinemia (2024) – In a neonatal rat model and MO3.13 oligodendroglial cell line, excess glycine impaired myelin markers, altered glutamate transporter expression, induced oxidative stress, and reduced cell viability, suggesting mechanistic pathways of glycine neurotoxicity relevant to NKH. Source
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• Studies of glycine metabolism and transport in fibroblasts from patients with nonketotic hyperglycinemia (1980) – Human patient fibroblast cell lines showed glycine transport occurs via a sodium-dependent system with no significant difference between control and NKH cells, and glycine cleavage activity was undetectable in NKH fibroblasts, indicating a cell-based model of GCS deficiency. Source
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• Metabolic Rewiring and Altered Glial Differentiation in an iPSC-Derived Astrocytes Model Derived From a Nonketotic Hyperglycinemia Patient (2024) – In iPSC-derived neural precursor and astrocyte lineage cells from an NKH patient with GLDC mutations, altered serine-glycine-one-carbon metabolism and shifts in differentiation markers were observed, modeling cellular metabolic effects of GCS deficiency. Source
  
  
• Glycine potentiates the NMDA response in cultured mouse brain neurons (1987)
  – Demonstrated that glycine acts as a required NMDA receptor co-agonist, providing the mechanistic basis for glycine-mediated excitotoxicity in NKH. Source
  
  

Attenuated NKH

• Episodic encephalopathy in attenuated nonketotic hyperglycinemia (2000) – Describes stress-triggered neurologic crises in attenuated NKH, expanding the phenotype beyond neonatal presentations. Source
  
  

Case and Clinical Reports

• Clinical and genetic analysis of nonketotic hyperglycinemia: A case report (2022) – A Chinese infant with NKH due to compound heterozygous GLDC variants demonstrating corpus callosum dysplasia and refractory seizures. Source
  
  
• Neonatal Nonketotic Hyperglycinemia: A Rare Case Report from Pakistan(2020) – Case report illustrating the severe neonatal presentation with defects in glycine cleavage enzyme. Source
  
  
• The Mutation Analysis of the AMT Gene in a Chinese Family With Nonketotic Hyperglycinemia (2022) – This study identifies a previously unreported compound heterozygous mutation in the AMT gene in a Chinese girl with NKH using whole-exome sequencing. Source
  
  
• Novel GLDC variants causing nonketotic hyperglycinemia in Chinese patients (2021) – This paper reports two Chinese neonates with NKH caused by a novel GLDC splice-site mutation (c.862-2A>G), expanding the known mutational spectrum of the disease. Source
  
  
• Novel GLDC Compound Heterozygous Variant Leading to Nonketotic Hyperglycinemia: Case Report and Literature Review (2021) – This case describes a Chinese infant with NKH carrying two novel GLDC variants (c.1261G>C and c.450C>G) associated with a previously unreported clinical phenotype. Source
  
  
• Genotypic and phenotypic features in Turkish patients with classic nonketotic hyperglycinemia (2021) – This single-center study reviews the genetic findings and clinical outcomes of ten Turkish patients diagnosed with classic NKH between 2013 and 2019. Source
  
  
• Nonketotic Hyperglycinemia in Tunisia: Report upon a Series of 69 Patients (2020) – This large national case series suggests NKH is unusually frequent in Tunisia, particularly in the Kairouan region, which may have one of the highest incidence rates worldwide. Source
  
  
• Outcome of Nonketotic Hyperglycinemia in Lebanon: 14-Year Retrospective Review (2019) – This retrospective review of 12 Lebanese NKH patients shows high mortality, treatment-resistant seizures, and limited benefit from standard therapy, with ketogenic diet improving seizure control in two cases. Source
  
  
• Nonketotic Hyperglycinemia: Two Case Reports and Review (2019) – This paper follows the clinical progression of two neonates with classic NKH, illustrating severe early-onset disease and poor neurological outcomes. Source
  
  
• Nonketotic Hyperglycinemia of Infants in Taiwan (2016) – This review summarizes clinical features, treatments, and outcomes of 12 NKH cases diagnosed in Taiwan between 2000 and 2013. Source
  
  
• Nonketotic hyperglycinemia case series (2015) – This case series describes three children with NKH, noting early withdrawal of care in two and modest developmental survival in one child treated with dextromethorphan and a ketogenic diet. Source
  
  
• Nonketotic hyperglycinemia case series (2015) – This study reports three neonatal NKH cases initially presenting with hiccups, emphasising early clinical clues that may aid diagnosis. Source
  
  
• Neonatal Nonketotic Hyperglycinemia: A Case Study and Review of Management for the Advanced Practice Nurse (2013) – This paper presents a neonate with classic NKH and provides a comprehensive clinical overview of disease pathophysiology, diagnosis, symptoms, and management strategies. Source
  
  
• Adult Nonketotic Hyperglycinemia (NKH) Crisis Presenting as Severe Chorea and Encephalopathy (2006) – This report describes an adult with previously undiagnosed attenuated NKH whose episodic encephalopathy and chorea were triggered by fever and protein intake and improved with sodium benzoate and dextromethorphan. Source
  
  
• Nonketotic hyperglycinemia in two siblings with neonatal seizures (2003) – This report describes two Thai siblings with fatal NKH, one exhibiting rare transient hyperammonemia and hypouricemia, possibly related to sodium benzoate treatment. Source
  
  
• Clinical and genetic analysis of nonketotic hyperglycinemia: a case report (2022) – Case series showing typical early NKH manifestations including hiccups, hypotonia, coma, and seizures within the first week after birth, emphasizing corpus callosum abnormalities and challenges in prognosis.
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