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Family pedigree, b esotropia and microcephaly, broad forehead, esotropia, low set ears, retrognathia, deep prominent philtrum and sparse eyebrows, c selected MR image of the brain at the level of basal ganglia, coronal T2-WI shows bilateral subinsular white matter hyper intensity (arrows) and right head of caudate atrophy with a tiny cyst (star), d axial T2-WI shows bilateral scattered subcortical and periventricular white matter hyperintensities (arrows)
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Glycine cleavage system (GCS) catalyzes the degradation of glycine and disruption of its components encoded by GLDC, AMT and GCSH are the only known causes of glycine encephalopathy, also known as non-ketotic hyperglycinemia (NKH). In this report, we describe a consanguineous family with one child who presented with NKH, but harbored no pathogenic...
Citations
... GLYT1 is primarily expressed by astrocytes adjacent to glycinergic neurons to facilitate the rapid clearance of glycine from the synaptic cleft (Supplemental Figure 1) (21)(22)(23). Homozygous mutations in the SLC6A9 gene that encodes GLYT1 cause glycine encephalopathy, also known as nonketotic hyperglycinemia (NKH), which is a severe neurological disease caused by abnormally high levels of glycine in the cerebrospinal fluid (CSF) and characterized by respiratory failure, progressive hypotonia, and startle-like reflexes (24,25). Interestingly, although most glycine encephalopathy patients die before 7 months of age, those who survive show progressive earlyonset scoliosis as a result of apparent neurological defects (26)(27)(28). ...
... Patients with glycine encephalopathy harboring homozygous SLC6A9 mutations were reported to have increased glycine concentrations in the CSF or plasma (24,25). Because the AIS patients in our study did not show any discernible neurological defects, it was not ethically justified to obtain CSF for measuring glycine concentrations. ...
Adolescent idiopathic scoliosis (AIS) is the most common form of spinal deformity affecting millions of adolescents worldwide, but it lacks a defined theory of etiopathogenesis. As such, treatment of AIS is limited to bracing and/or invasive surgery post onset. Pre-onset diagnosis or preventive treatment remains unavailable. Here we performed a genetic analysis of a large multi-center AIS cohort and identified disease-causing and predisposing variants of SLC6A9 in multi-generation families, trios, and sporadic patients. Variants of SLC6A9, which encodes glycine transporter 1 (GLYT1), reduced glycine uptake activity in cells, leading to an increased extracellular glycine level and aberrant glycinergic neurotransmission. Slc6a9 mutant zebrafish exhibited discoordination of spinal neural activities and pronounced lateral spinal curvature, a phenotype resembling human patients. The penetrance and severity of curvature was sensitive to the dosage of functional glyt1. Administration of a glycine receptor antagonist or a clinically-used glycine neutralizer (sodium benzoate) partially rescued the phenotype. Our results indicate a neuropathic origin for "idiopathic" scoliosis, involving the dysfunction of synaptic neurotransmission and central pattern generators (CPGs), potentially a common cause of AIS. Our work further suggests avenues for early diagnosis and intervention of AIS in preadolescents.
... Phosphatidic acid is a lipid second messenger to activate protein kinase C isoforms, ras guanyl nucleotide-releasing proteins, and some transient receptor potential channels [44]. Most of the top-upregulated genes in Jersey have been found to have a role in adipogenesis (ETS2) [45], adipocyte differentiation (OLR1, PARM1) [46,47], glucose transport (SLC6A9) (log 2 FC = 4.49) [48], glucose uptake (SLC45A4) [49], thyroid hormone synthesis (TG) [50], and aldosterone secretion (KCNK9) [51]. The upregulation of these genes in Jersey indicates their involvement in lipid biosynthesis in the mammary gland during lactation. ...
The effect of breed on milk components—fat, protein, lactose, and water—has been observed to be significant. As fat is one of the major price-determining factors for milk, exploring the variations in fat QTLs across breeds would shed light on the variable fat content in their milk. Here, on whole-genome sequencing, 25 differentially expressed hub or bottleneck fat QTLs were explored for variations across indigenous breeds. Out of these, 20 genes were identified as having nonsynonymous substitutions. A fixed SNP pattern in high-milk-yielding breeds in comparison to low-milk-yielding breeds was identified in the genes GHR, TLR4, LPIN1, CACNA1C, ZBTB16, ITGA1, ANK1, and NTG5E and, vice versa, in the genes MFGE8, FGF2, TLR4, LPIN1, NUP98, PTK2, ZTB16, DDIT3, and NT5E. The identified SNPs were ratified by pyrosequencing to prove that key differences exist in fat QTLs between the high- and low-milk-yielding breeds.
... [4] The difference between glycine encephalopathy due to nonketotic hyperglycinemia and SLC6A9 gene mutation is shown in Table 1. [7,8] The role of glycine is that it activates inhibitory glycine receptors and is a co-agonist for excitatory NMDA receptors. Dysregulation in glycine has been associated with hyperekplexia. ...
... [9] To date, only six cases of such encephalopathy due to GLYT1 dysfunction were noted [ Table 2]. [7,10] Clinically, the patient presents with difficulty in swallowing, weak cry, joint laxity, apnea, hyperekplexia, and failure to thrive, along with axial hypotonia, startle-like clonus, facial features of trigonocephaly, long myopathic facies, retrognathia, and microcephaly. [7] Examination findings of our patient included hypertonia in all four limbs, exaggerated reflexes, and bilateral ankle clonus with bilateral irreducible inguinal swelling with normal facial features. ...
... [7,10] Clinically, the patient presents with difficulty in swallowing, weak cry, joint laxity, apnea, hyperekplexia, and failure to thrive, along with axial hypotonia, startle-like clonus, facial features of trigonocephaly, long myopathic facies, retrognathia, and microcephaly. [7] Examination findings of our patient included hypertonia in all four limbs, exaggerated reflexes, and bilateral ankle clonus with bilateral irreducible inguinal swelling with normal facial features. However, there was neither microcephaly nor facial dysmorphism. ...
... 1,2 Disorders caused by pathogenic variants in genes related to GCS cofactors (lipoate and pyridoxal-phosphate, known as variant NKH) or glycine transport and disorders causing abnormal regulation or inhibition of GSC (eg, organic acidurias, intracellular cobalamin metabolism disorders) should be considered differential diagnoses. [3][4][5][6] Although the global incidence of NKH is unknown, it was estimated at 1:76,000 live births. 1 L-glycine is a nonessential amino acid involved in numerous metabolic pathways and also in neurotransmission. It acts as coagonist on inhibitory glycine receptors, found at postsynaptic membranes on brain stem neurons and the spinal cord. ...
Objective:
Glycine encephalopathy, also known as nonketotic hyperglycinemia (NKH), is an inherited neurometabolic disorder with variable clinical course and severity, ranging from infantile epileptic encephalopathy to psychiatric disorders. A precise phenotypic characterization and an evaluation of predictive approaches are needed.
Methods:
Longitudinal clinical and biochemical data of 25 individuals with NKH from the patient registry of International Working Group on Neurotransmitter related Disorders were studied with in silico analyses, pathogenicity scores and molecular modeling of GLDC and AMT variants.
Results:
Symptom onset (p<0· 01) and diagnosis occur earlier in life in severe NKH (p<0· 01). Presenting symptoms affect the age at diagnosis. Psychiatric problems occur predominantly in attenuated NKH. Onset-age ≥3 months (66% specificity, 100% sensitivity, AUC = 0·87) and cerebrospinal fluid (CSF)/plasma glycine ratio ≤0· 09 (57% specificity, 100% sensitivity, AUC = 0·88) are sensitive indicators for attenuated NKH while CSF glycine concentration ≥116· 5 μmol/L (100% specificity, 93% sensitivity, AUC = 0·97) and CSF/plasma glycine ratio ≥0· 15 (100% specificity, 64% sensitivity, AUC = 0·88) are specific for severe forms. A ratio threshold of 0· 128 discriminates the overlapping range. We present ten new GLDC variants. Two mild variants resulted in attenuated, while two severe variants or one mild and one severe variant lead to severe phenotype. Based on clinical, biochemical and genetic parameter we propose a severity prediction model.
Interpretation:
This study widens the phenotypic spectrum of attenuated NKH and expands the number of pathogenic variants. The multiparametric approach provides a promising tool to predict disease severity, helping to improve clinical management strategies. This article is protected by copyright. All rights reserved.
... Human genetical data demonstrated that defects of GlyT but also of Asc-1 functionality lead to severe clinical symptoms. Homozygous mutations in the GlyT1 (SLC6A9) gene have been shown to cause a disease phenotype very similar but not identical to glycine encephalopathy or nonketotic hyperglycinemia [88][89][90], a disease previously associated with defects in the glycine cleavage system. Similar to patients suffering from glycine encephalopathy, affected individuals have severe neurological problems reminiscent of the phenotype seen in GlyT1-deficient mice, characterized by hypotonia and severe respiratory depression directly after birth. ...
In addition to being involved in protein biosynthesis and metabolism, the amino acid glycine is the most important inhibitory neurotransmitter in caudal regions of the brain. These functions require a tight regulation of glycine concentration not only in the synaptic cleft, but also in various intracellular and extracellular compartments. This is achieved not only by confining the synthesis and degradation of glycine predominantly to the mitochondria, but also by the action of high-affinity large-capacity glycine transporters that mediate the transport of glycine across the membranes of presynaptic terminals or glial cells surrounding the synapses. Although most cells at glycine-dependent synapses express more than one transporter with high affinity for glycine, their synergistic functional interaction is only poorly understood. In this review, we summarize our current knowledge of the two high-affinity transporters for glycine, the sodium-dependent glycine transporters 1 (GlyT1; SLC6A9) and 2 (GlyT2; SLC6A5) and the alanine–serine–cysteine-1 transporter (Asc-1; SLC7A10).
... Regulation of SERT occurs largely through phosphorylation in mechanisms similar to NET (72). SERT activity is also regulated by the presence of phosphatidylinositol 4,5-biphosphate [PIP (2)] lipids in the plasma membrane (43). ...
... Patho)physiology GlyT inhibitors have been explored as antipsychotic treatments, schizophrenia, and neuropathic pain, however, none are routinely used clinically. Mutations in GlyT2 are associated with hyperekplexia, an excessive startle response(74), and nonketotic hyperglycinemia(2). ...
Regulation of the ability of a neurotransmitter [our focus: serotonin, norepinephrine, dopamine, acetylcholine, glycine, and gamma-aminobutyric acid (GABA)] to reach its receptor targets is regulated in part by controlling the access the neurotransmitter has to receptors. Transporters, located at both the cellular plasma membrane and in subcellular vesicles, carry a myriad of responsibilities that include enabling neurotransmitter release and controlling uptake of neurotransmitter back into a cell or vesicle. Driven largely by electrochemical gradients, these transporters move neurotransmitters. The regulation of the transporters themselves through changes in expression and/or posttranslational modification allows for fine-tuning of this system. Transporters have been best recognized as targets for psychoactive stimulants and remain a mainstay target of primarily central nervous system (CNS) acting drugs for treatment of debilitating diseases such as depression and anxiety. Studies reveal, however, that transporters are found and functional in tissues outside the CNS (gastrointestinal and cardiovascular tissues, for example). The importance of neurotransmitter transporters is underscored with discoveries that dysfunction of transporters can cause life-changing disease. This article provides a high-level review of major classes of both plasma membrane transporters and vesicular transporters. © 2021 American Physiological Society. Compr Physiol 11:2279-2295, 2021.
... Lastly, human mutations in SLC6A20, encoding the SLC6A20 protein that can mediate proline transport, have been linked to complex digenic iminoglycinuria (a renal disorder with impaired reabsorption of glycine and imino acids [proline and hydroxyproline]) and hyperglycinuria . In addition, known glycine transporters such as GlyT1 and GlyT2 encoded by SLC6A9 and SLC6A5, respectively, are associated with glycine encephalopathy (Alfadhel et al, 2016;Kurolap et al, 2016). GlyT1and GlyT2-mutant mice also show early postnatal lethality associated with impaired glycinergic transmission and phenotypes mimicking human glycine encephalopathy and hyperekplexia (Gomeza et al, 2003a;Gomeza et al, 2003c;Eulenburg et al, 2010). ...
Glycine transporters (GlyT1 and GlyT2) that regulate levels of brain glycine, an inhibitory neurotransmitter with co-agonist activity for NMDA receptors (NMDARs), have been considered to be important targets for the treatment of brain disorders with suppressed NMDAR function such as schizophrenia. However, it remains unclear whether other amino acid transporters expressed in the brain can also regulate brain glycine levels and NMDAR function. Here, we report that SLC6A20A, an amino acid transporter known to transport proline based on in vitro data but is understudied in the brain, regulates proline and glycine levels and NMDAR function in the mouse brain. SLC6A20A transcript and protein levels were abnormally increased in mice carrying a mutant PTEN protein lacking the C terminus through enhanced β-catenin binding to the Slc6a20a gene. These mice displayed reduced extracellular levels of brain proline and glycine and decreased NMDAR currents. Elevating glycine levels back to normal ranges by antisense oligonucleotide-induced SLC6A20 knockdown, or the competitive GlyT1 antagonist sarcosine, normalized NMDAR currents and repetitive climbing behavior observed in these mice. Conversely, mice lacking SLC6A20A displayed increased extracellular glycine levels and NMDAR currents. Lastly, both mouse and human SLC6A20 proteins mediated proline and glycine transports, and SLC6A20 proteins could be detected in human neurons. These results suggest that SLC6A20 regulates proline and glycine homeostasis in the brain and that SLC6A20 inhibition has therapeutic potential for brain disorders involving NMDAR hypofunction.
... 1,2 Disorders caused by pathogenic variants in genes related to GCS cofactors (lipoate and pyridoxal-phosphate, known as variant NKH) or glycine transport and disorders causing abnormal regulation or inhibition of GSC (eg, organic acidurias, intracellular cobalamin metabolism disorders) should be considered differential diagnoses. [3][4][5][6] Although the global incidence of NKH is unknown, it was estimated at 1:76,000 live births. 1 L-glycine is a nonessential amino acid involved in numerous metabolic pathways and also in neurotransmission. It acts as coagonist on inhibitory glycine receptors, found at postsynaptic membranes on brain stem neurons and the spinal cord. ...
... Furthermore, familybased analysis and studies of next-generation sequencing provide considerable power to detect common and rare variants [7], and family-based designs can be helpful for cosegregation (e.g., for prioritizing variants and genes) [8]. Additionally, several genes and disorders have been discovered by testing extended consanguineous families [9][10][11], mainly by autozygome and exome analysis [12]. However, one of the major limitations of these technologies in addition to their cost is the data interpretation. ...
Background:
Testing strategies is crucial for genetics clinics and testing laboratories. In this study, we tried to compare the hit rate between solo and trio and trio plus testing and between trio and sibship testing. Finally, we studied the impact of extended family analysis, mainly in complex and unsolved cases.
Methods:
Three cohorts were used for this analysis: one cohort to assess the hit rate between solo, trio and trio plus testing, another cohort to examine the impact of the testing strategy of sibship genome vs trio-based analysis, and a third cohort to test the impact of an extended family analysis of up to eight family members to lower the number of candidate variants.
Results:
The hit rates in solo, trio and trio plus testing were 39, 40, and 41%, respectively. The total number of candidate variants in the sibship testing strategy was 117 variants compared to 59 variants in the trio-based analysis. We noticed that the average number of coding candidate variants in trio-based analysis was 1192 variants and 26,454 noncoding variants, and this number was lowered by 50-75% after adding additional family members, with up to two coding and 66 noncoding homozygous variants only, in families with eight family members.
Conclusion:
There was no difference in the hit rate between solo and extended family members. Trio-based analysis was a better approach than sibship testing, even in a consanguineous population. Finally, each additional family member helped to narrow down the number of variants by 50-75%. Our findings could help clinicians, researchers and testing laboratories select the most cost-effective and appropriate sequencing approach for their patients. Furthermore, using extended family analysis is a very useful tool for complex cases with novel genes.
... Previous work suggested that mutations within the coding regions of the SLC6A9 gene might be causal for glycine transporter 1 encephalopathy in humans (Alfadhel et al., 2016;Kurolap et al., 2016), a disease displaying many but not all facets of nonketotic hyperglycinemia, previously associated with loss of function mutations of the mitochondrial glycine cleavage system (Applegarth and Toone, 2001). ...
... Up to now three different families with different mutations and in total 6 individuals carrying GlyT1 mutations homozygously and displaying the disease phenotype have been identified (Alfadhel et al., 2016;Kurolap et al., 2016). Symptoms described in all patients include arthrogryposis and increased nuchal translucency in ultrasound scans during pregnancy. ...
... Three different mutations in the SLC6A9 gene have been reported in the context of GlyT1 encephalopathy in three different families: a homozygous missense mutation c.1219 A>G (p.Ser407Gly), a homozygous small deletion c.928_932 delAAGTC (p.Lys310Pheþfs*31), and a homozygous nonsense mutation c.1717 C>T (p.Gln573*) (Alfadhel et al., 2016;Kurolap et al., 2016) due to consanguinity an autosomal recessive inheritance pattern is suggested (Alfallaj and Alfadhel, 2019). ...
Glycine constitutes a major inhibitory neurotransmitter predominantly in caudal regions of the CNS. The extracellular glycine concentration is regulated synergistically by two high affinity, large capacity transporters GlyT1 and GlyT2. Both proteins are encoded by single genes SLC6A9 and SLC6A5, respectively. Mutations within the SLC6A5 gene encoding for GlyT2 have been demonstrated to be causative for hyperekplexia (OMIM #614618), a complex neuromuscular disease, in humans. In contrast, mutations within the SLC6A9 gene encoding for GlyT1 have been associated with GlyT1 encephalopathy (OMIM #601019), a disease causing severe postnatal respiratory deficiency, muscular hypotonia and arthrogryposis. The consequences of the respective GlyT1 mutations on the function of the transporter protein, however, have not yet been analysed. In this study we present the functional characterisation of three previously published GlyT1 mutations, two mutations predicted to cause truncation of the GlyT1 (GlyT1Q573* and GlyT1K310F+fs∗31) and one predicted to cause an amino acid exchange within transmembrane domain 7 of the transporter (GlyT1S407G), that are associated with GlyT1 encephalopathy. Additionally, the characterization of a novel mutation predicted to cause an amino acid exchange within transmembrane domain 1 (GlyT1V118M) identified in two fetuses showing increased nuchal translucency and arthrogryposis in routine ultrasound scans is demonstrated. We show that in recombinant systems the two presumably truncating mutations resulted in an intracellular retained GlyT1 protein lacking the intracellular C-terminal domain. In both cases this truncated protein did not show any residual transport activity. The point mutations, hGlyT1S407G and hGlyT1V118M, were processed correctly, but showed severely diminished activity, thus constituting a functional knock-out in vivo. Taken together our data demonstrate that all analysed mutations of GlyT1 that have been identified in GlyT1 encephalopathy patients cause severe impairment of transporter function. This is consistent with the idea that loss of GlyT1 function is indeed causal for the disease phenotype.
