Genomic analyses of glycine decarboxylase neurogenic mutations yield a large-scale prediction model for prenatal disease.

Hundreds of mutations in a single gene result in rare diseases, but why mutations induce severe or attenuated states remains poorly understood. Defect in glycine decarboxylase (GLDC) causes Non-ketotic Hyperglycinemia (NKH), a neurological disease associated with elevation of plasma glycine. We unified a human multiparametric NKH mutation scale that separates severe from attenuated neurological disease with new in silico tools for murine and human genome level-analyses, gathered in vivo evidence from mice engineered with top-ranking attenuated and a highly pathogenic mutation, and integrated the data in a model of pre- and post-natal disease outcomes, relevant for over a hundred major and minor neurogenic mutations. Our findings suggest that highly severe neurogenic mutations predict fatal, prenatal disease that can be remedied by metabolic supplementation of dams, without amelioration of persistent plasma glycine. The work also provides a systems approach to identify functional consequences of mutations across hundreds of genetic diseases. Our studies provide a new framework for a large scale understanding of mutation functions and the prediction that severity of a neurogenic mutation is a direct measure of pre-natal disease in neurometabolic NKH mouse models. This framework can be extended to analyses of hundreds of monogenetic rare disorders where the underlying genes are known but understanding of the vast majority of mutations and why and how they cause disease, has yet to be realized.

Neonatal nonketotic hyperglycinemia: a case study and review of management for the advanced practice nurse.

Nonketotic hyperglycinemia (NKH) is an autosomal recessive inborn error of glycine metabolism. In this article, I will present the case of baby girl S. who presented to the emergency room on Day 4 of life with severe lethargy, seizures, and respiratory depression requiring mechanical ventilation. A diagnosis of NKH was made secondary to elevated plasma and cerebrospinal fluid glycine concentrations. I will review the pathophysiology of NKH, methods of diagnosis, and the differential diagnosis. There are a variety of different pharmacologic and alternative therapies for NKH. Despite these treatments, the prognosis for infants with NKH is poor, with severe neurologic impairment, intractable seizures, and death common before 5 years of age. I will address the role of the advanced practice nurse in caring for an infant with NKH including clinical, educational, and research implications.

Poor outcome for neonatal-type nonketotic hyperglycinemia treated with high-dose sodium benzoate and dextromethorphan.

Neonatal-type nonketotic hyperglycinemia treatment remains unsatisfactory, even if started early. A review of six patients who underwent treatment for neonatal-type nonketotic hyperglycinemia in our hospital is presented. All patients were treated with a standardized protocol. Medical histories were retrieved from case notes. All six patients had elevated cerebrospinal fluid plasma glycine levels initially. All but one had received sodium benzoate and dextromethorphan from 1 month of age. All suffered from intractable seizures and severe mental retardation, and only two patients remain alive. One patient died at 5 days of age. No resuscitation was attempted in accordance with the family's wish after genetic counseling. The prognosis of neonatal nonketotic hyperglycinemia remains poor with current treatment. Genetic counseling helps parents cope with this devastating genetic disease.

Structure and expression of the glycine cleavage system in rat central nervous system.

The glycine cleavage system (GCS) is a mitochondrial multienzyme system consisting of four individual proteins, three specific components (P-, T-, and H-proteins) and one house-keeping enzyme, dihydrolipoamide dehydrogenase. Inherited deficiency of the GCS causes nonketotic hyperglycinemia (NKH), an inborn error of glycine metabolism. NKH is characterized by massive accumulation of glycine in serum and cerebrospinal fluids and severe neuronal dysfunction in neonates. To elucidate the neuropathogenesis of NKH, we cloned cDNAs encoding three specific components of the GCS and studied the gene expression in rat central nervous system. P-, T-, and H-protein cDNAs encoded 1024, 403, and 170 amino acids, respectively. In situ hybridization analysis revealed that P-protein mRNA was expressed mainly in glial-like cells, including Bergmann glias in the cerebellum, while T- and H-protein mRNAs were detected in both glial-like cells and neurons. T- and H-protein mRNAs, but not P-protein mRNA, were expressed in the spinal cord. Primary astrocyte cultures established from cerebral cortex had higher GCS activities than hepatocytes whereas those from spinal cord expressed only H-protein mRNA and had no enzymatic activity. An important role of glycine as inhibitory neurotransmitter has been established in the brainstem and spinal cord and another role of glycine as an excitation modulator of N-methyl-D-aspartate receptor is suggested in the hippocampus, cerebral cortex, olfactory bulbus, and cerebellum. Our results suggest that the GCS plays a major role in the forebrain and cerebellum rather than in the spinal cord, and that N-methyl-D-aspartate receptor may participate in neuropathogenesis of NKH.

Biochemical and molecular investigations of patients with nonketotic hyperglycinemia.

The investigation of 14 unrelated patients with nonketotic hyperglycinemia led to the identification of mutations in 4 cases. Patients were initially categorized into probable P- or T-protein defects of the glycine cleavage enzyme complex, by the use of the glycine exchange assay without supplemental H-protein, then screened for mutations in the P-protein and T-protein genes, respectively.