Hypomorphic variants of lactase-phlorizin hydrolase in congenital lactase deficiency are trafficking incompetent and functionally inactive.

Patients with the rare autosomal recessive disorder congenital lactase deficiency (CLD) present with severe, potentially life-threatening symptoms shortly after birth. Several variants have been characterized within the gene for lactase-phlorizin hydrolase (LCT) that are associated with CLD. Here, we analyze at the biochemical and cellular levels LCT mutants harboring the genetic variants p.Y1390*, p.E1612*, p.S1150Pfs*19, p.S1121L, p.R1587H, and p.S688P. Our data unequivocally demonstrate that these mutants are absolutely transport incompetent, some of which are readily degraded, and are enzymatically inactive. The current study contributes to and expands our understanding on the pathogenesis of CLD at the molecular level.

Characterization of ETFDH and PHGDH Mutations in a Patient with Mild Glutaric Aciduria Type II and Serine Deficiency.

Glutaric aciduria type II (GA-II) is a rare autosomal recessive disease caused by defects in electron transfer flavoprotein (ETF), ultimately causing insufficiencies in multiple acyl-CoA dehydrogenase (MAD). 3-phosphoglycerate dehydrogenase (3-PHGDH) deficiency, is another rare autosomal disorder that appears due to a defect in the synthesis of L-serine amino acid. Several mutations of ETFDH and PHGDH genes have been associated with different forms of GA-II and serine deficiency, respectively. In this study, we report a unique case of GA-II with serine deficiency using biochemical, genetic, and in silico approaches. The proband of Syrian descent had positive newborn screening (NBS) for GA-II. At two years of age, the patient presented with developmental regression, ataxia, and intractable seizures. Results of amino acid profiling demonstrated extremely low levels of serine. Confirmatory tests for GA-II and whole exome sequencing (WES) were performed to determine the etiology of intractable seizure. Sequencing results indicated a previously reported homozygous missense mutation, c.679 C>A (p.Pro227Thr) in the ETFDH gene and a novel missense homozygous mutation c.1219 T>C (p.Ser407Pro) in the PHGDH gene. In silico tools predicted these mutations as deleterious. Here, the clinical and biochemical investigations indicate that ETFDH:p.Pro227Thr and PHGDH:p.Ser407Pro variants likely underlie the pathogenesis of GA-II and serine deficiency, respectively. This study indicates that two rare autosomal recessive disorders should be considered in consanguineous families, more specifically in those with atypical presentation.

Milestones in treatments for inborn errors of metabolism: Reflections on Where chemistry and medicine meet.

From Sir Archibald Garrod's initial description of the tetrad of albinism, alkaptonuria, cystinuria, and pentosuria to today, the field of medicine dedicated to inborn errors of metabolism has evolved from disease identification and mechanistic discovery to the development of therapies designed to subvert biochemical defects. In this review, we highlight major milestones in the treatment and diagnosis of inborn errors of metabolism, starting with dietary therapy for phenylketonuria in the 1950s and 1960s, and ending with current approaches in genetic manipulation.

Colonic mucosal eosinophilia in children without inflammatory bowel disease.

Children undergoing colonoscopy and mucosal biopsies may show increased colonic mucosal eosinophils, which may or may not be associated with inflammatory bowel disease. There is not much clinical data on American children who have isolated increased colonic mucosal eosinophils. We sought to study the clinical correlates of children without inflammatory bowel disease who show increased mucosal eosinophils to understand their clinical presentation, etiological associations, and outcome. A retrospective analysis of children seen at a tertiary-level Children's hospital was performed by reviewing their medical charts and extracting pertinent data. There were 110 children in the study who had increased colonic mucosal eosinophils. Most children presented with abdominal pain, but several of them also had constipation, blood in stools, and diarrhea. Food allergies, irritable bowel syndrome, asthma, and lactase deficiency were the top four conditions present in these patients. Pathology of the colonic distribution revealed involvement of more than two colonic regions in 86% of the subjects, and only two subjects showing epithelial or crypt involvement by eosinophils. All subjects had a good outcome. Children with colonic mucosal eosinophilia (CME) who do not have an inflammatory bowel disease most frequently present with abdominal pain and primarily an increase of lamina propria eosinophils in two or more colonic regions. Based on the etiological associations we noted in the study, a work-up of children with CME may encompass detailed history for functional gastrointestinal disorders and lactose intolerance, testing for food and environmental allergies, stool examination for parasites, and peripheral blood counts. Almost all children had resolution of symptoms in the studied period suggesting that CME in children has a good clinical outcome.

An early endothelial cell-specific requirement for Glut1 is revealed in Glut1 deficiency syndrome model mice.

Paucity of the glucose transporter-1 (Glut1) protein resulting from haploinsufficiency of the SLC2A1 gene arrests cerebral angiogenesis and disrupts brain function to cause Glut1 deficiency syndrome (Glut1 DS). Restoring Glut1 to Glut1 DS model mice prevents disease, but the precise cellular sites of action of the transporter, its temporal requirements, and the mechanisms linking scarcity of the protein to brain cell dysfunction remain poorly understood. Here, we show that Glut1 functions in a cell-autonomous manner in the cerebral microvasculature to affect endothelial tip cells and, thus, brain angiogenesis. Moreover, brain endothelial cell-specific Glut1 depletion not only triggers a severe neuroinflammatory response in the Glut1 DS brain, but also reduces levels of brain-derived neurotrophic factor (BDNF) and causes overt disease. Reduced BDNF correlated with fewer neurons in the Glut1 DS brain. Controlled depletion of the protein demonstrated that brain pathology and disease severity was greatest when Glut1 scarcity was induced neonatally, during brain angiogenesis. Reducing Glut1 at later stages had mild or little effect. Our results suggest that targeting brain endothelial cells during early development is important to ensure proper brain angiogenesis, prevent neuroinflammation, maintain BDNF levels, and preserve neuron numbers. This requirement will be essential for any disease-modifying therapeutic strategy for Glut1 DS.

Untargeted metabolomics as an unbiased approach to the diagnosis of inborn errors of metabolism of the non-oxidative branch of the pentose phosphate pathway.

Inborn errors of metabolism (IEM) involving the non-oxidative pentose phosphate pathway (PPP) include the two relatively rare conditions, transketolase deficiency and transaldolase deficiency, both of which can be difficult to diagnosis given their non-specific clinical presentations. Current biochemical testing approaches require an index of suspicion to consider targeted urine polyol testing. To determine whether a broad-spectrum biochemical test could accurately identify a specific metabolic pattern defining IEMs of the non-oxidative PPP, we employed the use of clinical metabolomic profiling as an unbiased novel approach to diagnosis. Subjects with molecularly confirmed IEMs of the PPP were included in this study. Targeted quantitative analysis of polyols in urine and plasma samples was accomplished with chromatography and mass spectrometry. Semi-quantitative unbiased metabolomic analysis of urine and plasma samples was achieved by assessing small molecules via liquid chromatography and high-resolution mass spectrometry. Results from untargeted and targeted analyses were then compared and analyzed for diagnostic acuity. Two siblings with transketolase (TKT) deficiency and three unrelated individuals with transaldolase (TALDO) deficiency were identified for inclusion in the study. For both IEMs, targeted polyol testing and untargeted metabolomic testing on urine and/or plasma samples identified typical perturbations of the respective disorder. Additionally, untargeted metabolomic testing revealed elevations in other PPP metabolites not typically measured with targeted polyol testing, including ribonate, ribose, and erythronate for TKT deficiency and ribonate, erythronate, and sedoheptulose 7-phosphate in TALDO deficiency. Non-PPP alternations were also noted involving tryptophan, purine, and pyrimidine metabolism for both TKT and TALDO deficient patients. Targeted polyol testing and untargeted metabolomic testing methods were both able to identify specific biochemical patterns indicative of TKT and TALDO deficiency in both plasma and urine samples. In addition, untargeted metabolomics was able to identify novel biomarkers, thereby expanding the current knowledge of both conditions and providing further insight into potential underlying pathophysiological mechanisms. Furthermore, untargeted metabolomic testing offers the advantage of having a single effective biochemical screening test for identification of rare IEMs, like TKT and TALDO deficiencies, that may otherwise go undiagnosed due to their generally non-specific clinical presentations.

Hypomorphic SI genetic variants are associated with childhood chronic loose stools.

OBJECTIVE: The SI gene encodes the sucrase-isomaltase enzyme, a disaccharidase expressed in the intestinal brush border. Hypomorphic SI variants cause recessive congenital sucrase-isomaltase deficiency (CSID) and related gastrointestinal (GI) symptoms. Among children presenting with chronic, idiopathic loose stools, we assessed the prevalence of CSID-associated SI variants relative to the general population and the relative GI symptom burden associated with SI genotype within the study population. METHODS: A prospective study conducted at 18 centers enrolled 308 non-Hispanic white children ≤18 years old who were experiencing chronic, idiopathic, loose stools at least once per week for >4 weeks. Data on demographics, GI symptoms, and genotyping for 37 SI hypomorphic variants were collected. Race/ethnicity-matched SI data from the Exome Aggregation Consortium (ExAC) database was used as the general population reference. RESULTS: Compared with the general population, the cumulative prevalence of hypomorphic SI variants was significantly higher in the study population (4.5% vs. 1.3%, P < .01; OR = 3.5 [95% CI: 6.1, 2.0]). Within the study population, children with a hypomorphic SI variant had a more severe GI symptom burden than those without, including: more frequent episodes of loose stools (P < .01), higher overall stool frequency (P < .01), looser stool form (P = .01) and increased flatulence (P = .02). CONCLUSION: Non-Hispanic white children with chronic idiopathic loose stools have a higher prevalence of CSID-associated hypomorphic SI variants than the general population. The GI symptom burden was greater among the study subjects with a hypomorphic SI variant than those without hypomorphic SI variants.

Colonic mucosal inflammatory cells in children and adolescents with lactase deficiency.

INTRODUCTION AND OBJECTIVES: Many of the symptoms of patients with lactose intolerance are due to fermentation of undigested lactose in the colonic lumen, which may also lead to inflammatory cell changes in the colonic mucosa. The objective of our project was to understand the histopathological changes involving infiltration of eosinophils and mast cells in the colonic mucosa of children with lactase deficiency (LD). METHODS: In this retrospective study we studied colonic mucosa of children and adolescents with LD to determine if any pathological changes or inflammatory cell changes were present. Pathology reports and Hematoxylin and eosin stained slides were reviewed. Tryptase immunohistochemistry was performed for mast cell assessment. RESULT: There were 30 subjects in the study who had a LD and 15 presented with diarrhea and 15 without diarrhea. The colonic mucosa of 35.5 % of the subjects revealed increased mucosal eosinophils. There was no increase of mast cells or lymphocytic colitis in any of the subjects. Excepting for the increased eosinophils in a subset of the subjects, all had a normal appearance of the colonic mucosa. CONCLUSION: Colonic mucosa of children and adolescents with LD has a normal histological appearance in majority of the patients. However, 35 % of the patients could demonstrate elevated eosinophils. In primary LD without any comorbidity there is no increase of mast cells and lymphocytic cells in the colonic mucosa.

Sucrase-Isomaltase Deficiency as a Potential Masquerader in Irritable Bowel Syndrome.

BACKGROUND: Patients with irritable bowel syndrome (IBS) frequently have meal-related symptoms and can recognize specific trigger foods. Lactose intolerance is a well-established carbohydrate malabsorption syndrome that causes symptoms similar to IBS such as bloating, abdominal pain, and diarrhea. However, the prevalence of sucrase-isomaltase deficiency (SID) in this population is poorly defined. SID is a condition in which sucrase-isomaltase, an enzyme produced by brush border of small intestine to metabolize sucrose, is deficient. Just like lactase deficiency, SID causes symptoms of maldigestion syndromes including abdominal pain, bloating, gas, and diarrhea. In this study, we aim to determine the prevalence of SID in patients with presumed IBS-D/M and characterize its clinical presentation. METHODS: Patients with a presumed diagnosis of IBS-D/M based on symptoms of abdominal pain, diarrhea, and/or bloating who underwent esophagogastroduodenoscopy with duodenal biopsies and testing for disaccharidase deficiency were included. Patients with a history of inflammatory bowel disease, gastrointestinal malignancy, or celiac disease were excluded. Odds ratio was calculated for abdominal pain, diarrhea, and bloating in patients with versus without SID. RESULTS: A total of 31 patients with clinical suspicion for IBS-D/M were included with a median age of 46 years (IQR 30.5-60) and with 61% females. SID was present in 35% of patients. Among patients with SID, 63.6% had diarrhea, 45.4% had abdominal pain, and 36.4% had bloating. Patients with SID were less likely than controls to have abdominal pain (OR 0.16, 95% CI 0.03-0.81, p = 0.04) although no difference in diarrhea or bloating was found. Only two patients with SID underwent sucrose breath testing of which only one had a positive result. However, this patient also had a positive glucose breath test and may have had small intestinal bacterial overgrowth as a confounder. CONCLUSION: SID was found in 35% of patients with presumed IBS-D/M and should be considered in the differential diagnosis of patients presenting with abdominal pain, diarrhea, or bloating. Further studies should better characterize the clinical features of SID and investigate the effects of dietary modification in this group of patients.

Diagnostic Challenges Associated with GLUT1 Deficiency: Phenotypic Variabilities and Evolving Clinical Features.

GLUT1 deficiency is a rare neurometabolic disorder that can be effectively treated with ketogenic diet. However, this condition is underdiagnosed due to its nonspecific, overlapping, and evolving symptoms with age. We retrospectively reviewed the clinical course of nine patients diagnosed with GLUT1 deficiency, based on SLC2A1 mutations and/or glucose concentration in cerebrospinal fluid. The patients included eight boys and one girl who initially presented with seizures (44%, 4/9) or delayed development (44%, 4/9) before 2 years of age, except for one patient who presented with apnea as a neonate. Over the clinical course, all of the children developed seizures of the mixed type, including absence seizures and generalized tonic-clonic seizures. About half (56%, 5/9) showed movement disorders such as ataxia, dystonia, or dyskinesia. We observed an evolution of phenotype over time, although this was not uniform across all patients. Only one child had microcephaly. In five patients, ketogenic diet was effective in reducing seizures and movement symptoms, and the patients exhibited subjective improvement in cognitive function. Diagnosing GLUT1 deficiency can be challenging due to the phenotypic variability and evolution. A high index of clinical suspicion in pediatric and even older patients with epilepsy or movement disorders is key to the early diagnosis and treatment, which can improve the patient's quality of life.

Relationship between Size of the Foveal Avascular Zone and Carbohydrate Metabolic Disorders during Pregnancy.

AIM: To determine whether the area of the foveal avascular zone (FAZ), as a morphological indicator of the microcirculation of the perifoveal capillary network, changes in the carbohydrate metabolism disorders during pregnancy (the gestational age of patients with gestational diabetes mellitus (GDM) and preexisting diabetes (PexD)). METHODS: Ten normal individuals and 41 eyes of 41 patients, 28 with GDM and 13 with PexD, were studied. A 3 × 3 mm area of the FAZ of the superficial capillary plexus layer (SCP) and the deep capillary plexus layer (DCP) was determined by optical coherence tomography angiography (OCTA; RS-3000 Advance, NIDEK). The significance of the correlation between the size of the FAZ and the weeks of pregnancy was determined. RESULTS: The area of the FAZ of the SCP was 0.38 ± 0.11 mm2 (normal eyes), 0.41 ± 0.16 mm2 (GDM), and 0.43 ± 0.10 mm2 (PexD). The area of the FAZ of the DCP was 0.78 ± 0.23 mm2 (normal eyes), 0.69 ± 0.16 mm2 (GDM), and 0.79 ± 0.25 mm2 (PexD). No significant difference in the FAZ sizes was observed between the groups. The average number of weeks of pregnancy was 24.1 ± 8.2 weeks in the eyes with GDM and 23.3 ± 11.4 weeks in the eyes with PexD (P > 0.05). Significant correlations were found between the size of the FAZ of the SCP and the number of weeks (r = 0.37, P=0.04 for GDM, and r = 0.49, P=0.04 for PexD, Spearman's rank-order correlation coefficient). CONCLUSIONS: For GDM and PexD under established glycemic control, the area of the FAZ is not affected, but vascular changes occurred at the early phase of pregnancy.

Confirmation of a Rare Genetic Leukoencephalopathy due to a Novel Bi-allelic Variant in RPIA.

Ribose 5-phosphate isomerase deficiency is a rare genetic leukoencephalopathy caused by pathogenic sequence variants in RPIA, that encodes ribose 5-phosphate isomerase, an enzyme in the pentose phosphate pathway. Till date, only three individuals with ribose 5-phosphate isomerase deficiency have been described in literature. We report on a subject with RPIA associated progressive leukoencephalopathy with elevated urine arabitol and ribitol levels and a novel missense variant c.770T > C p.(Ile257Thr) in exon 8 of RPIA. We also compare the phenotypes of all the four subjects. Our report confirms the phenotype and the genetic cause of this condition.

Missense variant in TPI1 (Arg189Gln) causes neurologic deficits through structural changes in the triosephosphate isomerase catalytic site and reduced enzyme levels in vivo.

Mutations in the gene triosephosphate isomerase (TPI) lead to a severe multisystem condition that is characterized by hemolytic anemia, a weakened immune system, and significant neurologic symptoms such as seizures, distal neuropathy, and intellectual disability. No effective therapy is available. Here we report a compound heterozygous patient with a novel TPI pathogenic variant (NM_000365.5:c.569G>A:p.(Arg189Gln)) in combination with the common (NM_000365.5:c.315G>C:p.(Glu104Asp)) allele. We characterized the novel variant by mutating the homologous Arg in Drosophila using a genomic engineering system, demonstrating that missense mutations at this position cause a strong loss of function. Compound heterozygote animals were generated and exhibit motor behavioural deficits and markedly reduced protein levels. Furthermore, examinations of the TPIArg189Gln/TPIGlu104Asp patient fibroblasts confirmed the reduction of TPI levels, suggesting that Arg189Gln may also affect the stability of the protein. The Arg189 residue participates in two salt bridges on the backside of the TPI enzyme dimer, and we reveal that a mutation at this position alters the coordination of the substrate-binding site and important catalytic residues. Collectively, these data reveal a new human pathogenic variant associated with TPI deficiency, identify the Arg189 salt bridge as critical for organizing the catalytic site of the TPI enzyme, and demonstrates that reduced TPI levels are associated with human TPI deficiency. These findings advance our understanding of the molecular pathogenesis of the disease, and suggest new therapeutic avenues for pre-clinical trials.

Low catalytic activity is insufficient to induce disease pathology in triosephosphate isomerase deficiency.

Triosephosphate isomerase (TPI) deficiency is a fatal genetic disorder characterized by hemolytic anemia and neurological dysfunction. Although the enzyme defect in TPI was discovered in the 1960s, the exact etiology of the disease is still debated. Some aspects indicate the disease could be caused by insufficient enzyme activity, whereas other observations indicate it could be a protein misfolding disease with tissue-specific differences in TPI activity. We generated a mouse model in which exchange of a conserved catalytic amino acid residue (isoleucine to valine, Ile170Val) reduces TPI specific activity without affecting the stability of the protein dimer. TPIIle170Val/Ile170Val mice exhibit an approximately 85% reduction in TPI activity consistently across all examined tissues, which is a stronger average, but more consistent, activity decline than observed in patients or symptomatic mouse models that carry structural defect mutant alleles. While monitoring protein expression levels revealed no evidence for protein instability, metabolite quantification indicated that glycolysis is affected by the active site mutation. TPIIle170Val/Ile170Val mice develop normally and show none of the disease symptoms associated with TPI deficiency. Therefore, without the stability defect that affects TPI activity in a tissue-specific manner, a strong decline in TPI catalytic activity is not sufficient to explain the pathological onset of TPI deficiency.

Congenital Lactase Deficiency: Mutations, Functional and Biochemical Implications, and Future Perspectives.

Congenital lactase deficiency (CLD) is a severe autosomal recessive genetic disorder that affects the functional capacity of the intestinal protein lactase-phlorizin hydrolase (LPH). This disorder is diagnosed already during the first few days of the newborn's life due to the inability to digest lactose, the main carbohydrate in mammalian milk. The symptoms are similar to those in other carbohydrate malabsorption disorders, such as congenital sucrase-isomaltase deficiency, and include severe osmotic watery diarrhea. CLD is associated with mutations in the translated region of the LPH gene that elicit loss-of-function of LPH. The mutations occur in a homozygote or compound heterozygote pattern of inheritance and comprise missense mutations as well as mutations that lead to complete or partial truncations of crucial domains in LPH, such as those linked to the folding and transport-competence of LPH and to the catalytic domains. Nevertheless, the identification of the mutations in CLD is not paralleled by detailed genotype/protein phenotype analyses that would help unravel potential pathomechanisms underlying this severe disease. Here, we review the current knowledge of CLD mutations and discuss their potential impact on the structural and biosynthetic features of LPH. We also address the question of whether heterozygote carriers can be symptomatic for CLD and whether genetic testing is needed in view of the severity of the disease.

[Transaldolase deficiency - clinical outcome, pathogenesis, diagnostic process].

Transaldolase deficiency is a rare inborn autosomal recessive error of the pentose phosphate pathway that, to date, has been diagnosed in 33 patients, including 4 from Poland. The aim of this manuscript was to present the clinical presentation, pathogenesis and diagnostic process of transaldolase deficiency. The authors also present a diagnostic algorithm of transaldolase deficiency.

A frame-shift deletion in the PURA gene associates with a new clinical finding: Hypoglycorrhachia. Is GLUT1 a new PURA target?

PURA is a DNA/RNA-binding protein known to have an important role as a transcriptional and translational regulator. Mutations in the PURA gene have been documented to cause mainly a neurologic phenotype including hypotonia, epilepsy, development delay and respiratory alterations. We report here a patient with a frame-shift deletion in the PURA gene that apart from the classical PURA deficiency phenotype had marked hypoglycorrhachia, overlapping the clinical findings with a GLUT1 deficiency syndrome. SLC2A1 (GLUT1) mutations were discarded, so we hypothesized that GLUT1 could be downregulated in this PURA deficient scenario. We confirmed reduced GLUT1 expression in the patient's peripheral blood cells compared to controls predicting that this could also be happening in the blood-brain barrier and in this way explain the hypoglycorrhachia. Based on PURA's known functions as a transcriptional and translational regulator, we propose GLUT1 as a new PURA target. Further in vitro and in vivo studies are needed to confirm this and to uncover the underlying molecular mechanisms.

The face of Glut1-DS patients: A 3D Craniofacial Morphometric Analysis.

Glut1 deficiency syndrome (Glut1-DS) is a neurological and metabolic disorder caused by impaired transport of glucose across the blood brain barrier (BBB). Mutations on the SCL2A1 gene encoding the glucose transporter protein in the BBB cause the syndrome, which encompasses epilepsy, movement disorders, and mental delay. Such variability of symptoms presents an obstacle to early diagnosis. The patients seem to share some craniofacial features, and identification and quantification of these could help in prompt diagnosis and clinical management. We performed a three-dimensional morphometric analysis of the faces of 11 female Glut1-DS patients using a stereophotogrammetric system. Data were analyzed using both inter-landmark distances and Principal Component Analysis. Compared with data collected from age-, sex-, and ethnicity-matched control subjects, common and homogenous facial features were identified among patients, which were mainly located in the mandible and the eyes. Glut1-DS patients had a more anterior chin; their mandibular body was longer but the rami were shorter, with a reduced gonial angle; they had smaller and down-slanted eyes with a reduced intercanthal distance. This study highlights the importance of morphometric analysis for defining the facial anatomical characteristics of the syndrome better, potentially helping clinicians to diagnose Glut1-DS. Improved knowledge of the facial anatomy of these patients can provide insights into their facial and cerebral embryological development, perhaps further clarifying the molecular basis of the syndrome. Clin. Anat. 30:644-652, 2017. © 2017 Wiley Periodicals, Inc.

Somatic mosaicism for a SLC2A1 mutation: implications for genetic counseling for GLUT1 deficiency syndrome.

A heterozygous SLC2A1 mutation in the severely affected child was inherited from his less severely affected mother who was mosaic for the mutation.

Structural and Genetic Studies Demonstrate Neurologic Dysfunction in Triosephosphate Isomerase Deficiency Is Associated with Impaired Synaptic Vesicle Dynamics.

Triosephosphate isomerase (TPI) deficiency is a poorly understood disease characterized by hemolytic anemia, cardiomyopathy, neurologic dysfunction, and early death. TPI deficiency is one of a group of diseases known as glycolytic enzymopathies, but is unique for its severe patient neuropathology and early mortality. The disease is caused by missense mutations and dysfunction in the glycolytic enzyme, TPI. Previous studies have detailed structural and catalytic changes elicited by disease-associated TPI substitutions, and samples of patient erythrocytes have yielded insight into patient hemolytic anemia; however, the neuropathophysiology of this disease remains a mystery. This study combines structural, biochemical, and genetic approaches to demonstrate that perturbations of the TPI dimer interface are sufficient to elicit TPI deficiency neuropathogenesis. The present study demonstrates that neurologic dysfunction resulting from TPI deficiency is characterized by synaptic vesicle dysfunction, and can be attenuated with catalytically inactive TPI. Collectively, our findings are the first to identify, to our knowledge, a functional synaptic defect in TPI deficiency derived from molecular changes in the TPI dimer interface.