The gene mutated in thiamine-responsive anaemia with diabetes and deafness (TRMA) encodes a functional thiamine transporter

Thiamine deficiency in pregnancy and lactation: implications and present perspectives

During pregnancy, many physiologic changes occur in order to accommodate fetal growth. These changes require an increase in many of the nutritional needs to prevent long-term consequences for both mother and the offspring. One of the main vitamins that are needed throughout the pregnancy is thiamine (vitamin B1) which is a water-soluble vitamin that plays an important role in many metabolic and physiologic processes in the human body. Thiamine deficiency during pregnancy can cause can have many cardiac, neurologic, and psychological effects on the mother. It can also dispose the fetus to gastrointestinal, pulmonological, cardiac, and neurologic conditions. This paper reviews the recently published literature about thiamine and its physiologic roles, thiamine deficiency in pregnancy, its prevalence, its impact on infants and subsequent consequences in them. This review also highlights the knowledge gaps within these topics.

New Avenues of Heme Synthesis Regulation

During erythropoiesis, there is an enormous demand for the synthesis of the essential cofactor of hemoglobin, heme. Heme is synthesized de novo via an eight enzyme-catalyzed pathway within each developing erythroid cell. A large body of data exists to explain the transcriptional regulation of the heme biosynthesis enzymes, but until recently much less was known about alternate forms of regulation that would allow the massive production of heme without depleting cellular metabolites. Herein, we review new studies focused on the regulation of heme synthesis via carbon flux for porphyrin synthesis to post-translations modifications (PTMs) that regulate individual enzymes. These PTMs include cofactor regulation, phosphorylation, succinylation, and glutathionylation. Additionally discussed is the role of the immunometabolite itaconate and its connection to heme synthesis and the anemia of chronic disease. These recent studies provide new avenues to regulate heme synthesis for the treatment of diseases including anemias and porphyrias.

Mitochondrial transport and metabolism of the vitamin B-derived cofactors thiamine pyrophosphate, coenzyme A, FAD and NAD+ , and related diseases: A review

Multiple mitochondrial matrix enzymes playing key roles in metabolism require cofactors for their action. Due to the high impermeability of the mitochondrial inner membrane, these cofactors need to be synthesized within the mitochondria or be imported, themselves or one of their precursors, into the organelles. Transporters belonging to the protein family of mitochondrial carriers have been identified to transport the coenzymes: thiamine pyrophosphate, coenzyme A, FAD and NAD+ , which are all structurally similar to nucleotides and derived from different B-vitamins. These mitochondrial cofactors bind more or less tightly to their enzymes and, after having been involved in a specific reaction step, are regenerated, spontaneously or by other enzymes, to return to their active form, ready for the next catalysis round. Disease-causing mutations in the mitochondrial cofactor carrier genes compromise not only the transport reaction but also the activity of all mitochondrial enzymes using that particular cofactor and the metabolic pathways in which the cofactor-dependent enzymes are involved. The mitochondrial transport, metabolism and diseases of the cofactors thiamine pyrophosphate, coenzyme A, FAD and NAD+ are the focus of this review.

Identification of the amino acid residues involved in the species-dependent differences in the pyridoxine transport function of SLC19A3

Recent studies have shown that human solute carrier SLC19A3 (hSLC19A3) can transport pyridoxine (vitamin B6) in addition to thiamine (vitamin B1), its originally identified substrate, whereas rat and mouse orthologs of hSLC19A3 can transport thiamine but not pyridoxine. This finding implies that some amino acid residues required for pyridoxine transport, but not for thiamine transport, are specific to hSLC19A3. Here, we sought to identify these residues to help clarify the unique operational mechanism of SLC19A3 through analyses comparing hSLC19A3 and mouse Slc19a3 (mSlc19a3). For our analyses, hSLC19A3 mutants were prepared by replacing selected amino acid residues with their counterparts in mSlc19a3, and mSlc19a3 mutants were prepared by substituting selected residues with their hSLC19A3 counterparts. We assessed pyridoxine and thiamine transport by these mutants in transiently transfected HEK293 cells. Our analyses indicated that the hSLC19A3-specific amino acid residues of Gln⁸⁶, Gly⁸⁷, Ile⁹¹, Thr⁹³, Trp⁹⁴, Ser¹⁶⁸, and Asn¹⁷³ are critical for pyridoxine transport. These seven amino acid residues were found to be mostly conserved in the SLC19A3 orthologs that can transport pyridoxine, but not in orthologs that are unable to transport pyridoxine. In addition, these residues were also found to be conserved in several SLC19A2 orthologs, including rat, mouse, and human orthologs, which were all found to effectively transport both pyridoxine and thiamine, exhibiting no species-dependent differences. Together, these findings provide a molecular basis for the unique functional characteristics of SLC19A3 and also of SLC19A2.

Effects of Thiamin Restriction on Exercise-Associated Glycogen Metabolism and AMPK Activation Level in Skeletal Muscle

This study aimed to investigate the direct influence of a decrease in the cellular thiamin level, before the onset of anorexia (one of the symptoms of thiamin deficiency) on glycogen metabolism and the AMP-activated protein kinase (AMPK) activation levels in skeletal muscle at rest and in response to exercise. Male Wistar rats were classified as the control diet (CON) group or the thiamin-deficient diet (TD) group and consumed the assigned diets for 1 week. Skeletal muscles were taken from the rats at rest, those that underwent low-intensity swimming (LIS), or high-intensity intermittent swimming (HIS) conducted immediately before dissection. There were no significant differences in food intake, locomotive activity, or body weight between groups, but thiamin pyrophosphate in the skeletal muscles of the TD group was significantly lower than that of the CON group. Muscle glycogen and lactate levels in the blood and muscle were equivalent between groups at rest and in response to exercise. The mitochondrial content was equal between groups, and AMPK in the skeletal muscles of TD rats was normally activated by LIS and HIS. In conclusion, with a lowered cellular thiamin level, the exercise-associated glycogen metabolism and AMPK activation level in skeletal muscle were normally regulated.

Leber congenital amaurosis as an initial manifestation in a Chinese patient with thiamine-responsive megaloblastic anemia syndrome

Thiamine-responsive megaloblastic anemia syndrome (TRMA) is an autosomal recessive disorder, inherited by the defective SLC19A2 gene that encodes a high-affinity thiamine transporter (THTR-1). TRMA is characterized by the occurrence of classical triad manifestations including megaloblastic anemia, diabetes mellitus, and sensorineural deafness. In addition to the systemic manifestations, ophthalmic features can be present and include retinitis pigmentosa, optic atrophy, cone-rod dystrophy, maculopathy, and Leber congenital amaurosis. Here we report a 6-year-old boy presenting severe early-onset retinal dystrophy with the initial diagnosis of Leber congenital amaurosis, which followed for 12 years. Diabetes mellitus occurred 3 years after vision problem. Eosinophilic granuloma of the left scapula was confirmed at 13 years old. Whole-exome sequencing was performed to identify two novel compound heterozygous variants c.725dupC (p.Ala243Serfs*3) and c.121G>A (p.Gly41Ser) in SLC19A2 gene (NM_006996.3). Oral thiamine supplementation treatment was initiated at 13 years. This case demonstrates Leber congenital amaurosis can present as the first clinical feature before systemic manifestations. Phenotypic variety should be aware and multidisciplinary teamwork and regular follow-up are important for TRMA patient care.

Identification of novel compound heterozygous variants in SLC19A2 and the genotype-phenotype associations in thiamine-responsive megaloblastic anemia

BACKGROUND AND AIMS

Thiamine-responsive megaloblastic anemia (TRMA), caused by SLC19A2 loss-of-function variants, is characterized by the triad of megaloblastic anemia, progressive sensorineural deafness, and non-type 1 diabetes mellitus. Here, we present the case of a Chinese infant with two novel variants segregating in compound heterozygous form in SLC19A2 and reviewed genotype-phenotype associations (GPAs) in patients with TRMA.

MATERIALS AND METHODS

Whole-exome sequencing was performed to establish a genetic diagnosis. The clinical manifestations and genetic variants were collected by performing a literature review. The bioinformatics software SIFT, PolyPhen2, and Mutation Taster was applied to predict variant effects and analyze GPAs.

RESULTS

Two novel variants segregating in compound heterozygous form in SLC19A2 (NM_006996.2: exon2:c.336_363del:p.W112fs; exon2:c.358G>T:p.G120X) was identified. Thiamine supplementation corrected anemia and diabetes mellitus but did not improve the hearing defect. In the literature, 183 patients with TRMA with 74 variants in SLC19A2 have been reported, with high incidence in the Middle East, South Asia, and the northern Mediterranean. Patients with biallelic premature termination codon variants presented with more severe phenotypes, and truncating sites on extracellular domains was a protective factor for the hemoglobin level at diagnosis.

CONCLUSION

Two novel compound heterozygous variants (NM_006996.2: exon2:c.336_363del:p.W112fs; exon2:c.358G>T:p.G120X) were identified, and GPAs in TRMA indicated the predictability of clinical manifestations.

Case Report: Genetic and Clinical Features of Maternal Uniparental Isodisomy-Induced Thiamine-Responsive Megaloblastic Anemia Syndrome

Background: Thiamine-responsive megaloblastic anemia syndrome (TRMA) is a rare autosomal recessive hereditary disease due to mutations in SLC19A2. Some cases show familial inheritance. Case report: A female patient (from a gravida 1, para 1 mother) of 3.5 years of age was admitted to the Pediatric Hematology Department of Xianyang Caihong Hospital in June 2019. The patient had severe anemia, acupoint-size bleeding spots, and a few ecchymoses all over her body, as well as astigmatism and hyperopia. Hearing was normal. The patient had diabetes. Bone marrow biopsy suggested a myelodysplastic syndrome. The patient had a c.515G>A (p.G172D) homozygous mutation of SLC19A2 (NM_006996), indicating TRMA. Genetic testing revealed that the two alleles were inherited from her mother alone due to maternal uniparental isodisomy (UPD). The patient was treated with thiamine and a subcutaneous injection of insulin. The patient recovered well and was discharged. She continued thiamine and insulin at the same dose and was followed once a month. The last follow-up on September 15, 2020, showed no anemia or bleeding. She had a sound hearing and normal blood routine and fasting glucose levels. Hyperopia and astigmatism did not improve. Conclusion: The patient had TRMA induced by the c.515G>A (p.G172D) homozygous mutation of SLC19A2 inherited through maternal UPD. The genetic diagnosis of TRMA is of significance for guiding clinical treatment. Early treatment with exogenous thiamine can improve some of the clinical features of TRMA.

Wernicke's encephalopathy - from basic science to clinical practice. Part 1: Understanding the role of thiamine

Wernicke's encephalopathy (WE) is an acute neuropsychiatric state. Untreated, WE can lead to coma or death, or progress to Korsakoff syndrome (KS) - a dementia characterized by irreversible loss of anterograde memory. Thiamine (vitamin B1) deficiency lies at the heart of this condition. Yet, our understanding of thiamine regarding prophylaxis and treatment of WE remains limited. This may contribute to the current undertreatment of WE in clinical practice. The overall aim of this review is to identify the best strategies for prophylaxis and treatment of WE in regard to (a) dose of thiamine, (b) mode of administration, (c) timing of switch from one mode of administration to another, (d) duration of administration, and (e) use of magnesium along thiamine as an essential cofactor. Evidence from randomized controlled trials and other intervention studies is virtually absent. Therefore, we have to resort to basic science for proof of principle instead. Here, we present the first part of our clinical review, in which we explore the physiology of thiamine and the pathophysiology of thiamine deficiency. We first explore both of these in their historical context. We then review the pharmacodynamics and pharmacokinetics of thiamine, exploring the roles of the six currently known thiamine compounds, their transporters, and target enzymes. We also explore the significance of magnesium as a cofactor in thiamine-facilitated enzymatic reactions and thiamine transport. In the second (forthcoming) part of this review, we will use the findings of the current review to make evidence-based inferences about strategies for prophylaxis and treatment of WE.

GATA factor-regulated solute carrier ensemble reveals a nucleoside transporter-dependent differentiation mechanism

Developmental-regulatory networks often include large gene families encoding mechanistically-related proteins like G-protein-coupled receptors, zinc finger transcription factors and solute carrier (SLC) transporters. In principle, a common mechanism may confer expression of multiple members integral to a developmental process, or diverse mechanisms may be deployed. Using genetic complementation and enhancer-mutant systems, we analyzed the 456 member SLC family that establishes the small molecule constitution of cells. This analysis identified SLC gene cohorts regulated by GATA1 and/or GATA2 during erythroid differentiation. As >50 SLC genes shared GATA factor regulation, a common mechanism established multiple members of this family. These genes included Slc29a1 encoding an equilibrative nucleoside transporter (Slc29a1/ENT1) that utilizes adenosine as a preferred substrate. Slc29a1 promoted erythroblast survival and differentiation ex vivo. Targeted ablation of murine Slc29a1 in erythroblasts attenuated erythropoiesis and erythrocyte regeneration in response to acute anemia. Our results reveal a GATA factor-regulated SLC ensemble, with a nucleoside transporter component that promotes erythropoiesis and prevents anemia, and establish a mechanistic link between GATA factor and adenosine mechanisms. We propose that integration of the GATA factor-adenosine circuit with other components of the GATA factor-regulated SLC ensemble establishes the small molecule repertoire required for progenitor cells to efficiently generate erythrocytes.

GATA transcription factors endow blood stem and progenitor cells with activities to produce progeny that transport oxygen to protect cells and tissues, evade pathogens and control physiological processes. GATA factors regulate hundreds of genes, and the actions of these genes mediate important biological functions. While the genes have been documented, many questions remain regarding how the “network” components mediate biological functions. The networks include members of large gene families, and the relationships between the regulation and function of individual family members is not well understood. Analyzing datasets from genetic complementation and enhancer mutant systems revealed that GATA factors regulate an ensemble of membrane transporters termed solute carrier proteins (SLCs), which dictate the small molecule composition of cells. Genetic analyses with Slc29a1, which transports adenosine, revealed its function to promote erythrocyte development, and Slc29a1 attenuated anemia in a mouse model. This study revealed the importance of SLC transporters in GATA factor networks. We propose that the GATA factor-adenosine circuit integrates with other SLCs to establish/maintain the small molecule constitution of progenitor cells as a new mechanism to control blood cell development.

Thiamine-Responsive Megaloblastic Anemia-Related Diabetes: Long-Term Clinical Outcomes in 23 Pediatric Patients From the DPV and SWEET Registries

OBJECTIVES

To describe clinical presentation and long-term outcomes in a large cohort of children diagnosed with thiamine-responsive megaloblastic anemia (TRMA)-related diabetes.

METHODS

Data from the Diabetes Patienten Verlaufsdokumentation (DPV) and Better control in Pediatric and Adolescent diabeteS: Working to crEate CEnTers of Reference (SWEET) registries were used to identify cases. Complementary information was collected through a chart review of each case. Descriptive analyses with medians and interquartile ranges and numbers (proportions) were tabulated.

RESULTS

We identified 23 cases (52% male) in the 2 registries. Eighteen (78%) had genetic confirmation of TRMA. Median age at diabetes onset was 1.4 (quartiles 0.8 to 3.6) years and median age at initiation of thiamine treatment was 5.9 (2.4 to 12.4) years. At their most recent visit, patients' median age was 14.3 (8.1 to 17.5) years, glycated hemoglobin level was 6.9% (6.1% to 7.9%), insulin dose was 0.9 (0.4 to 1.2) units/kg per day and thiamine dose was 200 (100 to 300) mg/day. Three patients were not treated with insulin or antidiabetic drugs. There was no difference in diabetes outcomes in patients with initiation of thiamine ≤1 year after diabetes onset compared to patients with initiation of thiamine >1 year after diabetes onset.

CONCLUSIONS

This is the longest case series of pediatric TRMA-related diabetes reported to date. Diabetes onset often occurs several years before initiation of thiamine supplementation. Early initiation of thiamine (within 1 year of diabetes onset) was not linked to improved diabetes outcome. However, the role of thiamine in pancreatic function needs further assessment. Patients with TRMA-related diabetes maintained good glycemic control even after 9 years (median) of follow up.

Incidence and characteristics of childhood- and youth-onset diabetes in the Qalandarabad area in northern Pakistan

OBJECTIVES

Determine the types, incidence, mortality rate, and clinical status of youth diabetes at Bach Christian Hospital (BCH), Qalandarabad, Pakistan.

METHODS

Analysis of incidence and mortality data of all patients (<25 year (y)) diagnosed from January 2014-June 2019, and also analysis of clinical status of patients < 25y seen in 2018/2019.

RESULTS

Ninety-three patients were seen over the study period. Eighty-eight were type 1 diabetes (T1D), 51.1% female. Age of diagnosis was 0.8-24.5 years (y) (mean = 11.4 y, SD = 6.2y). 15.1% were 0-4y, 31.4% 5-9 y, 24.4% 10-14y, 19.8% 15-19y, and 9.3% 20-24y. Minimum incidence for the Mansehra tehsil administrative district was calculated as 1.0 per 100,000 population <15y/y, 1.2 per 100,000 < 20y/y and 1.1 per 100,000 < 25y/y; the degree of ascertainment could not be assessed. A further four patients were diagnosed with thiamine-responsive megaloblastic anaemia (TRMA), all male, three from the same consanguineous family, and were treated with high-dose thiamine. One other patient was diagnosed with type 2 diabetes. Three T1D and one TRMA patient died during the study period. The standardised mortality rate for T1D was 9.4, but vital status was unknown for 13 patients. The mean/median HbA1c of T1D patients seen in 2018/2019 was 9.1%/9.2% (76/77 mmol/mol).

CONCLUSIONS

Minimum T1D incidence in Mansehra tehsil is double the previously reported value for Pakistan (from 1990 to 1999), although is still low compared to most other countries. Considering the limited resources available, patients attending BCH are achieving fair glycemic control. The TRMA cases show the importance of genetic testing in atypical cases.

Enterohemorrhagic Escherichia coli infection inhibits colonic thiamin pyrophosphate uptake via transcriptional mechanism

Colonocytes possess a specific carrier-mediated uptake process for the microbiota-generated thiamin (vitamin B1) pyrophosphate (TPP) that involves the TPP transporter (TPPT; product of the SLC44A4 gene). Little is known about the effect of exogenous factors (including enteric pathogens) on the colonic TPP uptake process. Our aim in this study was to investigate the effect of Enterohemorrhagic Escherichia coli (EHEC) infection on colonic uptake of TPP. We used human-derived colonic epithelial NCM460 cells and mice in our investigation. The results showed that infecting NCM460 cells with live EHEC (but not with heat-killed EHEC, EHEC culture supernatant, or with non-pathogenic E. Coli) to lead to a significant inhibition in carrier-mediated TPP uptake, as well as in level of expression of the TPPT protein and mRNA. Similarly, infecting mice with EHEC led to a significant inhibition in colonic TPP uptake and in level of expression of TPPT protein and mRNA. The inhibitory effect of EHEC on TPP uptake by NCM460 was found to be associated with reduction in the rate of transcription of the SLC44A4 gene as indicated by the significant reduction in the activity of the SLC44A4 promoter transfected into EHEC infected cells. The latter was also associated with a marked reduction in the level of expression of the transcription factors CREB-1 and ELF3, which are known to drive the activity of the SLC44A4 promoter. Finally, blocking the ERK1/2 and NF-kB signaling pathways in NCM460 cells significantly reversed the level of EHEC inhibition in TPP uptake and TPPT expression. Collectively, these findings show, for the first time, that EHEC infection significantly inhibit colonic uptake of TPP, and that this effect appears to be exerted at the level of SLC44A4 transcription and involves the ERK1/2 and NF-kB signaling pathways.

Genetic defects of thiamine transport and metabolism: A review of clinical phenotypes, genetics, and functional studies

Thiamine is a crucial cofactor involved in the maintenance of carbohydrate metabolism and participates in multiple cellular metabolic processes within the cytosol, mitochondria, and peroxisomes. Currently, four genetic defects have been described causing impairment of thiamine transport and metabolism: SLC19A2 dysfunction leads to diabetes mellitus, megaloblastic anemia and sensory-neural hearing loss, whereas SLC19A3, SLC25A19, and TPK1-related disorders result in recurrent encephalopathy, basal ganglia necrosis, generalized dystonia, severe disability, and early death. In order to achieve early diagnosis and treatment, biomarkers play an important role. SLC19A3 patients present a profound decrease of free-thiamine in cerebrospinal fluid (CSF) and fibroblasts. TPK1 patients show decreased concentrations of thiamine pyrophosphate in blood and muscle. Thiamine supplementation has been shown to improve diabetes and anemia control in Rogers' syndrome patients due to SLC19A2 deficiency. In a significant number of patients with SLC19A3, thiamine improves clinical outcome and survival, and prevents further metabolic crisis. In SLC25A19 and TPK1 defects, thiamine has also led to clinical stabilization in single cases. Moreover, thiamine supplementation leads to normal concentrations of free-thiamine in the CSF of SLC19A3 patients. Herein, we present a literature review of the current knowledge of the disease including related clinical phenotypes, treatment approaches, update of pathogenic variants, as well as in vitro and in vivo functional models that provide pathogenic evidence and propose mechanisms for thiamine deficiency in humans.

Whole exome sequencing identifies a new mutation in the SLC19A2 gene leading to thiamine-responsive megaloblastic anemia in an Egyptian family

BACKGROUND

The Solute Carrier Family 19 Member 2 (SLC19A2, OMIM *603941) encodes the thiamine transporter 1 (THTR-1) that brings thiamine (Vitamin B1) into cells. THTR-1 is the only thiamine transporter expressed in bone marrow, cochlear, and pancreatic beta cells. THTR-1 loss-of-function leads to the rare recessive genetic disease Thiamine-Responsive Megaloblastic Anemia (TRMA, OMIM #249270).

METHODS

In vitro stimulated blood lymphocytes were used for cytogenetics and the isolation of genomic DNA used to perform whole exome sequencing (WES). To validate identified mutations, direct Sanger sequencing was performed following PCR amplification.

RESULTS

A 6-year-old male born from a consanguineous couple presenting bone marrow failure and microcephaly was referred to our clinic for disease diagnosis. The patient presented a normal karyotype and no chromosomal fragility in response to DNA damage. WES analysis led to the identification of a new pathogenic variant in the SLC19A2 gene (c.596C>G, pSer199Ter) allowing to identify the young boy as a TRMA patient.

CONCLUSION

Our analysis extend the number of inactivating mutations in SLC19A2 leading to TRMA that could guide future prenatal diagnosis for the family and follow-up for patients.

Ion Transporters, Channelopathies, and Glucose Disorders

Ion channels and transporters play essential roles in excitable cells including cardiac, skeletal and smooth muscle cells, neurons, and endocrine cells. In pancreatic beta-cells, for example, potassium K_ATP channels link the metabolic signals generated inside the cell to changes in the beta-cell membrane potential, and ultimately regulate insulin secretion. Mutations in the genes encoding some ion transporter and channel proteins lead to disorders of glucose homeostasis (hyperinsulinaemic hypoglycaemia and different forms of diabetes mellitus). Pancreatic K_ATP, Non-K_ATP, and some calcium channelopathies and MCT1 transporter defects can lead to various forms of hyperinsulinaemic hypoglycaemia (HH). Mutations in the genes encoding the pancreatic K_ATP channels can also lead to different types of diabetes (including neonatal diabetes mellitus (NDM) and Maturity Onset Diabetes of the Young, MODY), and defects in the solute carrier family 2 member 2 (SLC2A2) leads to diabetes mellitus as part of the Fanconi–Bickel syndrome. Variants or polymorphisms in some ion channel genes and transporters have been reported in association with type 2 diabetes mellitus.

Neurological, Psychiatric, and Biochemical Aspects of Thiamine Deficiency in Children and Adults

Thiamine (vitamin B1) is an essential nutrient that serves as a cofactor for a number of enzymes, mostly with mitochondrial localization. Some thiamine-dependent enzymes are involved in energy metabolism and biosynthesis of nucleic acids whereas others are part of the antioxidant machinery. The brain is highly vulnerable to thiamine deficiency due to its heavy reliance on mitochondrial ATP production. This is more evident during rapid growth (i.e., perinatal periods and children) in which thiamine deficiency is commonly associated with either malnutrition or genetic defects. Thiamine deficiency contributes to a number of conditions spanning from mild neurological and psychiatric symptoms (confusion, reduced memory, and sleep disturbances) to severe encephalopathy, ataxia, congestive heart failure, muscle atrophy, and even death. This review discusses the current knowledge on thiamine deficiency and associated morbidity of neurological and psychiatric disorders, with special emphasis on the pediatric population, as well as the putative beneficial effect of thiamine supplementation in autism spectrum disorder (ASD) and other neurological conditions.

An analytic approach to identifying the sources of the low-frequency round window cochlear response

The cochlear microphonic, traditionally thought of as an indication of electrical current flow through hair cells, in conjunction with suppressing high-pass noise or tones, is a promising method of assessing the health of outer hair cells at specific locations along the cochlear partition. We propose that the electrical potential recorded from the round window in gerbils in response to low-frequency tones, which we call cochlear response (CR), contains significant responses from multiple cellular sources, which may expand its diagnostic purview. In this study, CR is measured in the gerbil and modeled to identify its contributing sources. CR was recorded via an electrode placed in the round window niche of sixteen Mongolian gerbils and elicited with a 45 Hz tone burst embedded in 18 high-pass filtered noise conditions to target responses from increasing regions along the cochlear partition. Possible sources were modeled using previously-published hair cell and auditory nerve response data, and then weighted and combined using linear regression to produce a model response that fits closely to the mean CR waveform. The significant contributing sources identified by the model are outer hair cells, inner hair cells, and the auditory nerve. We conclude that the low-frequency CR contains contributions from several cellular sources.

The molecular genetics of sideroblastic anemia

The sideroblastic anemias (SAs) are a group of inherited and acquired bone marrow disorders defined by pathological iron accumulation in the mitochondria of erythroid precursors. Like most hematological diseases, the molecular genetic basis of the SAs has ridden the wave of technology advancement. Within the last 30 years, with the advent of positional cloning, the human genome project, solid-state genotyping technologies, and next-generation sequencing have evolved to the point where more than two-thirds of congenital SA cases, and an even greater proportion of cases of acquired clonal disease, can be attributed to mutations in a specific gene or genes. This review focuses on an analysis of the genetics of these diseases and how understanding these defects may contribute to the design and implementation of rational therapies.

Gastrointestinal Handling of Water-Soluble Vitamins

Nine compounds are classified as water-soluble vitamins, eight B vitamins and one vitamin C. The vitamins are mandatory for the function of numerous enzymes and lack of one or more of the vitamins may lead to severe medical conditions. All the vitamins are supplied by food in microgram to milligram quantities and in addition some of the vitamins are synthesized by the intestinal microbiota. In the gastrointestinal tract, the vitamins are liberated from binding proteins and for some of the vitamins modified prior to absorption. Due to their solubility in water, they all require specific carriers to be absorbed. Our current knowledge concerning each of the vitamins differs in depth and focus and is influenced by the prevalence of conditions and diseases related to lack of the individual vitamin. Because of that we have chosen to cover slightly different aspects for the individual vitamins. For each of the vitamins, we summarize the physiological role, the steps involved in the absorption, and the factors influencing the absorption. In addition, for some of the vitamins, the molecular base for absorption is described in details, while for others new aspects of relevance for human deficiency are included. © 2018 American Physiological Society. Compr Physiol 8:1291-1311, 2018.

Glutamine via α-ketoglutarate dehydrogenase provides succinyl-CoA for heme synthesis during erythropoiesis

During erythroid differentiation, the erythron must remodel its protein constituents so that the mature red cell contains hemoglobin as the chief cytoplasmic protein component. For this, ∼10⁹ molecules of heme must be synthesized, consuming 10¹⁰ molecules of succinyl-CoA. It has long been assumed that the source of succinyl-coenzyme A (CoA) for heme synthesis in all cell types is the tricarboxylic acid (TCA) cycle. Based upon the observation that 1 subunit of succinyl-CoA synthetase (SCS) physically interacts with the first enzyme of heme synthesis (5-aminolevulinate synthase 2, ALAS2) in erythroid cells, it has been posited that succinyl-CoA for ALA synthesis is provided by the adenosine triphosphate-dependent reverse SCS reaction. We have now demonstrated that this is not the manner by which developing erythroid cells provide succinyl-CoA for ALA synthesis. Instead, during late stages of erythropoiesis, cellular metabolism is remodeled so that glutamine is the precursor for ALA following deamination to α-ketoglutarate and conversion to succinyl-CoA by α-ketoglutarate dehydrogenase (KDH) without equilibration or passage through the TCA cycle. This may be facilitated by a direct interaction between ALAS2 and KDH. Succinate is not an effective precursor for heme, indicating that the SCS reverse reaction does not play a role in providing succinyl-CoA for heme synthesis. Inhibition of succinate dehydrogenase by itaconate, which has been shown in macrophages to dramatically increase the concentration of intracellular succinate, does not stimulate heme synthesis as might be anticipated, but actually inhibits hemoglobinization during late erythropoiesis.

Pancytopenia in an adult patient with thiamine-responsive megaloblastic anaemia

Thiamine-responsive megaloblastic anaemia (TRMA) is a syndrome associated with megaloblastic anaemia, diabetes mellitus and sensorineural deafness, due to mutations in the SLC19A2 gene, which codes for a thiamine carrier protein. Oral thiamine supplementation is the main treatment. We report the case of a 25-year-old woman known for TRMA, who presented with pancytopenia (haemoglobin 7.6 g/dL, leucocytes 2.9×10⁹/L, thrombocytes 6×10⁹/L) revealed by dyspnoea. Investigations excluded coagulopathy, a recent viral infection, vitamin and iron deficiencies, and a malignant process. We later found out that thiamine treatment had been discontinued 5 weeks before, due to prescription error. Parenteral thiamine administration resulted in the recovery of haematopoiesis within 3 weeks. Pancytopenia is uncommon in patients with TRMA. Pre-existing medullary impairment caused by the patient's daily antipsychotic medications or the natural course of the syndrome may explain the severity of the laboratory findings in our patient.

Pharmacogenomics in diabetes: outcomes of thiamine therapy in TRMA syndrome

AIMS/HYPOTHESIS

Diabetes is one of the cardinal features of thiamine-responsive megaloblastic anaemia (TRMA) syndrome. Current knowledge of this rare monogenic diabetes subtype is limited. We investigated the genotype, phenotype and response to thiamine (vitamin B1) in a cohort of individuals with TRMA-related diabetes.

METHODS

We studied 32 individuals with biallelic SLC19A2 mutations identified by Sanger or next generation sequencing. Clinical details were collected through a follow-up questionnaire.

RESULTS

We identified 24 different mutations, of which nine are novel. The onset of the first TRMA symptom ranged from birth to 4 years (median 6 months [interquartile range, IQR 3-24]) and median age at diabetes onset was 10 months (IQR 5-27). At presentation, three individuals had isolated diabetes and 12 had asymptomatic hyperglycaemia. Follow-up data was available for 15 individuals treated with thiamine for a median 4.7 years (IQR 3-10). Four patients were able to stop insulin and seven achieved better glycaemic control on lower insulin doses. These 11 patients were significantly younger at diabetes diagnosis (p = 0.042), at genetic testing (p = 0.01) and when starting thiamine (p = 0.007) compared with the rest of the cohort. All patients treated with thiamine became transfusion-independent and adolescents achieved normal puberty. There were no additional benefits of thiamine doses >150 mg/day and no reported side effects up to 300 mg/day.

CONCLUSIONS/INTERPRETATION

In TRMA syndrome, diabetes can be asymptomatic and present before the appearance of other features. Prompt recognition is essential as early treatment with thiamine can result in improved glycaemic control, with some individuals becoming insulin-independent.

DATA AVAILABILITY

SLC19A2 mutation details have been deposited in the Decipher database ( https://decipher.sanger.ac.uk/ ).

Not quite type 1 or type 2, what now? Review of monogenic, mitochondrial, and syndromic diabetes

Diabetes mellitus is a heterogeneous group of conditions defined by resultant chronic hyperglycemia. Given the increasing prevalence of diabetes mellitus and the increasing understanding of genetic etiologies, we present a broad review of rare genetic forms of diabetes that have differing diagnostic and/or treatment implications from type 1 and type 2 diabetes. Advances in understanding the genotype-phenotype associations in these rare forms of diabetes offer clinically available examples of evolving precision medicine where defining the correct genetic etiology can radically alter treatment approaches. In this review, we focus on forms of monogenic diabetes, mitochondrial diabetes, and syndromic diabetes.

Recovered insulin production after thiamine administration in permanent neonatal diabetes mellitus with a novel solute carrier family 19 member 2 (SLC19A2) mutation

BACKGROUND

Solute carrier family 19 member 2 (SLC19A2) gene deficiency is one of the causes of permanent neonatal diabetes mellitus (PNDM) and can be effectively managed by thiamine supplementation. Herein we report on a male patient with a novel SLC19A2 mutation and summarize the clinical characteristics of patients with SLC19A2 deficiency.

METHODS

The genetic diagnosis of the patient with PNDM was made by sequencing and quantitative polymerase chain reaction. The clinical characteristics of PNDM were summarized on the basis of a systematic review of the literature.

RESULTS

The patient with PNDM had c.848G>A (p.W283X) homozygous mutation in SLC19A2. His father had a wild-type SLC19A2 (c.848G) and his mother was c.848G/A heterozygous. The patient and his father both had a diploid genotype (c.848A/A and c.848G/G). After oral thiamine administration, the patient's fasting C-peptide levels increased gradually, and there was a marked decrease in insulin requirements. A search of the literature revealed that thiamine treatment was effective and improved diabetes in 63% of patients with SLC19A2 deficiency.

CONCLUSIONS

A novel SLC19A2 mutation (c.848G>A; p.W283X) was identified, which was most likely inherited as segmental uniparental isodisomy. Insulin insufficiency in PNDM caused by SLC19A2 deficiency can be corrected by thiamine supplementation. The differential diagnosis of SLC19A2 deficiency should be considered in children with PNDM accompanied by anemia or hearing defects to allow for early treatment.

First 2 cases with thiamine-responsive megaloblastic anemia in the Czech Republic, a rare form of monogenic diabetes mellitus: a novel mutation in the thiamine transporter SLC19A2 gene-intron 1 mutation c.204+2T>G

Thiamine-responsive megaloblastic anemia (TRMA) is a rare autosomal recessive disorder caused by mutations in the SLC19A2 gene. To date at least 43 mutations have been reported for the gene encoding a plasma membrane thiamine transporter protein (THTR-1). TRMA has been reported in less than 80 cases worldwide. Here, we illustrate 2 female patients with TRMA first diagnosed in the Czech Republic and in central Europe being confirmed by sequencing of the THTR-1 gene SLC19A2. Both subjects are compound heterozygotes with 3 different mutations in the SLC19A2 gene. In case 2, the SLC19A2 intron 1 mutation c.204+2T>G has never been reported before. TRMA subjects are at risk of diabetic ketoacidosis during intercurrent disease and arrythmias. Thiamine supplementation has prevented hematological disorders over a few years in both pediatric subjects, and improved glycaemic control of diabetes mellitus. Patient 1 was suffering from hearing loss and rod-cone dystrophy at the time of diagnosis, however, she was unresponsive to thiamine substitution. Our patient 2 developed the hearing loss despite the early thiamine substitution, however no visual disorder had developed. The novel mutation described here extends the list of SLC19A2 mutations causing TRMA.

Molecular mechanisms involved in the adaptive regulation of the colonic thiamin pyrophosphate uptake process

A considerable amount of the thiamin generated by gut microbiota exists in the form of thiamin pyrophosphate (TPP). We have previously shown that human colonocytes possess an efficient carrier-mediated uptake process for TPP that involves the SLC44A4 system and this uptake process is adaptively regulated by prevailing extracellular TPP level. Little is known about the molecular mechanisms that mediate this adaptive regulation. We addressed this issue using human-derived colonic epithelial NCM460 cells and mouse colonoids as models. Maintaining NCM460 cells in the presence of a high level of TPP (1 mM) for short (2 days)- and long-term (9 days) periods was found to lead to a significant reduction in [³H] TPP uptake compared with cells maintained in its absence. Short-term exposure showed no changes in level of expression of SLC44A4 protein in total cell homogenate (although there was a decreased expression in the membrane fraction), mRNA, and promoter activity. However, a significant reduction in the level of expression of the SLC44A4 protein, mRNA, and promoter activity was observed upon long-term maintenance with the substrate. Similar changes in Slc44a4 mRNA expression were observed when mouse colonoids were maintained with TPP for short- and long-term periods. Expression of the transcription factors ELF3 and CREB-1 (which drive the SLC44A4 promoter) following long-term exposure was unchanged, but their binding affinity to the promoter was decreased and specific histone modifications were also observed. These studies demonstrate that, depending on the period of exposure, different mechanisms are involved in the adaptive regulation of colonic TPP uptake by extracellular substrate level.

Megaloblastic Anemias: Nutritional and Other Causes

Vitamin B₁₂ and folate deficiencies are major causes of megaloblastic anemia. Causes of B₁₂ deficiency include pernicious anemia, gastric surgery, intestinal disorders, dietary deficiency, and inherited disorders of B₁₂ transport or absorption. The prevalence of folate deficiency has decreased because of folate fortification, but deficiency still occurs from malabsorption and increased demand. Other causes include drugs and inborn metabolic errors. Clinical features of megaloblastic anemia include anemia, cytopenias, jaundice, and megaloblastic marrow morphology. Neurologic symptoms occur in B₁₂ deficiency, but not in folate deficiency. Management includes identifying any deficiency, establishing its cause, and replenishing B₁₂ or folate parenterally or orally.

Treatable Inborn Errors of Metabolism Due to Membrane Vitamin Transporters Deficiency

B vitamins act as cofactors for strategic metabolic processes. The SLC19 gene family of solute carriers has a significant structural similarity, transporting substrates with different structure and ionic charge. Three proteins of this family are expressed ubiquitously and mediate the transport of 2 important water-soluble vitamins, folate, and thiamine. SLC19A1 transports folate and SLC19A2 and SLC19A3 transport thiamine. PCFT and FOLR1 ensure intestinal absorption and transport of folate through the blood-brain barrier and SLC19A25 transports thiamine into the mitochondria. Several damaging genetic defects in vitamin B transport and metabolism have been reported. The most relevant feature of thiamine and folate transport defects is that both of them are treatable disorders. In this article, we discuss the biology and transport of thiamine and folate, as well as the clinical phenotype of the genetic defects.

Structure-function characterization of the human mitochondrial thiamin pyrophosphate transporter (hMTPPT; SLC25A19): Important roles for Ile(33), Ser(34), Asp(37), His(137) and Lys(291)

Thiamin plays a critical role in cellular energy metabolism. Mammalian cells obtain the vitamin from their surroundings, converted it to thiamin pyrophosphate (TPP) in the cytoplasm, followed by uptake of TPP by mitochondria via a carrier-mediated process that involves the MTPPT (product of the SLC25A19 gene). Previous studies have characterized different physiological/biological aspects of the human MTPPT (hMTPPT), but less is known about structural features that are important for its function. Here, we used a protein-docking model ("Phyre2" and "DockingServer") to predict residues that may be important for function (substrate recognition) of the hMTPPT; we also examined the role of conserved positively-charged residues predicted ("PRALINE") to be in the trans-membrane domains (TMDs) in uptake of the negatively-charged TPP. Among the six residues predicted by the docking model (i.e., Thr(29), Arg(30), Ile(33), Ser(34), Asp(37) and Phe(298)), only Ile(33), Ser(34) and Asp(37) were found to be critical for function. While no change in translational efficiency/protein stability of the Ser(34) mutant was observed, both the Ile(33) and Asp(37) mutants showed a decrease in this parameter(s); there was also a decrease in the expression of the latter two mutants in mitochondria. A need for a polar residue at position 34 of the hMTPPT was evident. Our findings with the positively-charged residues (i.e., His(82), His(137), Lys(231) and Lys(291)) predicted in the TMD showed that His(137) and Lys(291) are important for function (via a role in proper delivery of the protein to mitochondria). These investigations provide important information about the structure-function relationship of the hMTPPT.

Perinatal thiamine deficiency causes cochlear innervation abnormalities in mice

Neonatal thiamine deficiency can cause auditory neuropathy in humans. To probe the underlying cochlear pathology, mice were maintained on a thiamine-free or low-thiamine diet during fetal development or early postnatal life. At postnatal ages from 18 days to 22 wks, cochlear function was tested and cochlear histopathology analyzed by plastic sections and cochlear epithelial whole-mounts immunostained for neuronal and synaptic markers. Although none of the thiamine-deprivation protocols resulted in any loss of hair cells or any obvious abnormalities in the non-sensory structures of the cochlear duct, all the experimental groups showed significant anomalies in the afferent or efferent innervation. Afferent synaptic counts in the inner and outer hair cell areas were reduced, as was the efferent innervation density in both the outer and inner hair cell areas. As expected for primary neural degeneration, the thresholds for distortion product otoacoustic emissions were not affected, and as expected for subtotal hair cell de-afferentation, the suprathreshold amplitudes of auditory brainstem responses were more affected than the response thresholds. We conclude that the auditory neuropathy from thiamine deprivation could be produced by loss of inner hair cell synapses.

Novel nonsense mutation (p.Ile411Metfs*12) in the SLC19A2 gene causing Thiamine Responsive Megaloblastic Anemia in an Indian patient

Thiamine-responsive megaloblastic anemia (TRMA), an autosomal recessive disorder, is caused by mutations in SLC19A2 gene encodes a high affinity thiamine transporter (THTR-1). The occurrence of TRMA is diagnosed by megaloblastic anemia, diabetes mellitus, and sensorineural deafness. Here, we report a female TRMA patient of Indian descent born to 4th degree consanguineous parents presented with retinitis pigmentosa and vision impairment, who had a novel homozygous mutation (c.1232delT/ter422; p.Ile411Metfs*12) in 5th exon of SLC19A2 gene that causes premature termination of hTHTR-1. PROSITE analysis predicted to abrogate GPCRs family-1 signature motif in the variant by this mutation c.1232delT/ter422, suggesting uncharacteristic rhodopsin function leading to cause RP clinically. Thiamine transport activity by the clinical variant was severely inhibited than wild-type THTR-1. Confocal imaging had shown that the variant p.I411Mfs*12 is targeted to the cell membrane and showed no discrepancy in membrane expression than wild-type. Our findings are the first report, to the best of our knowledge, on this novel nonsense mutation of hTHTR-1 causing TRMA in an Indian patient through functionally impaired thiamine transporter activity.

A novel homozygous SLC19A2 mutation in a Portuguese patient with diabetes mellitus and thiamine-responsive megaloblastic anaemia

Thiamine-responsive megaloblastic anaemia (TRMA) is a rare syndrome where patients present with early onset diabetes mellitus, megaloblastic anaemia and sensorineural deafness. This report describes a new case of TRMA syndrome in a female patient of Portuguese descent, born to unrelated parents. The patient was found to have a novel homozygous change R397X in exon 4 of the SLC19A2 gene, leading to a premature stop codon. The patient’s diabetes and anaemia showed a good response to daily thiamine doses, reducing the daily insulin dose requirement. The report further indicates that TRMA is not only limited to consanguineous or ethnically isolated families, and should be considered as a differential diagnosis for patients presenting with suggestive clinical symptoms.

Fundus autofluorescence and optical coherence tomography findings in thiamine responsive megaloblastic anemia

BACKGROUND

To report ophthalmologic fundus autofluorescence and spectral domain optical coherence tomography findings in a patient with thiamine responsive megaloblastic anemia (TRMA).

METHODS

A 13-year-old girl with genetically proven TRMA was ophthalmologically (visual acuity, funduscopy, perimetry, electroretinogram) followed up over >5 years. Fundus imaging also included autofluorescence and spectral domain optical coherence tomography.

RESULTS

During a 5-year follow-up, visual acuity and visual field decreased, despite a special TRMA diet. Funduscopy revealed bull's eye appearance, whereas fundus autofluorescence showed central and peripheral hyperfluorescence and perifoveal hypofluorescence. Spectral domain optical coherence tomography revealed affected inner segment ellipsoid band and irregularities in the retinal pigment epithelium and choroidea.

CONCLUSION

Autofluorescence and spectral domain optical coherence tomography findings in a patient with TRMA show retinitis pigmentosa-like retina, retinal pigment epithelium, and choroid alterations. These findings might progress even under special TRMA diet, indispensable to life. Ophthalmologist should consider TRMA in patients with deafness and ophthalmologic disorders.

Comprehensive review on lactate metabolism in human health

Metabolic pathways involved in lactate metabolism are important to understand the physiological response to exercise and the pathogenesis of prevalent diseases such as diabetes and cancer. Monocarboxylate transporters are being investigated as potential targets for diagnosis and therapy of these and other disorders. Glucose and alanine produce pyruvate which is reduced to lactate by lactate dehydrogenase in the cytoplasm without oxygen consumption. Lactate removal takes place via its oxidation to pyruvate by lactate dehydrogenase. Pyruvate may be either oxidized to carbon dioxide producing energy or transformed into glucose. Pyruvate oxidation requires oxygen supply and the cooperation of pyruvate dehydrogenase, the tricarboxylic acid cycle, and the mitochondrial respiratory chain. Enzymes of the gluconeogenesis pathway sequentially convert pyruvate into glucose. Congenital or acquired deficiency on gluconeogenesis or pyruvate oxidation, including tissue hypoxia, may induce lactate accumulation. Both obese individuals and patients with diabetes show elevated plasma lactate concentration compared to healthy subjects, but there is no conclusive evidence of hyperlactatemia causing insulin resistance. Available evidence suggests an association between defective mitochondrial oxidative capacity in the pancreatic β-cells and diminished insulin secretion that may trigger the development of diabetes in patients already affected with insulin resistance. Several mutations in the mitochondrial DNA are associated with diabetes mellitus, although the pathogenesis remains unsettled. Mitochondrial DNA mutations have been detected in a number of human cancers. d-lactate is a lactate enantiomer normally formed during glycolysis. Excess d-lactate is generated in diabetes, particularly during diabetic ketoacidosis. d-lactic acidosis is typically associated with small bowel resection.

Many faces of monogenic diabetes

Monogenic diabetes represents a heterogeneous group of disorders resulting from defects in single genes. Defects are categorized primarily into two groups: disruption of β-cell function or a reduction in the number of β-cells. A complex network of transcription factors control pancreas formation, and a dysfunction of regulators high in the hierarchy leads to pancreatic agenesis. Dysfunction among factors further downstream might cause organ hypoplasia, absence of islets of Langerhans or a reduction in the number of β-cells. Many transcription factors have pleiotropic effects, explaining the association of diabetes with other congenital malformations, including cerebellar agenesis and pituitary agenesis. Monogenic diabetes variants are classified conventionally according to age of onset, with neonatal diabetes occurring before the age of 6 months and maturity onset diabetes of the young (MODY) manifesting before the age of 25 years. Recently, certain familial genetic defects were shown to manifest as neonatal diabetes, MODY or even adult onset diabetes. Patients with neonatal diabetes require a thorough genetic work-up in any case, and because extensive phenotypic overlap exists between monogenic, type 2, and type 1 diabetes, genetic analysis will also help improve diagnosis in these cases. Next generation sequencing will facilitate rapid screening, leading to the discovery of digenic and oligogenic diabetes variants, and helping to improve our understanding of the genetics underlying other types of diabetes. An accurate diagnosis remains important, because it might lead to a change in the treatment of affected subjects and influence long-term complications.

Association of TM4SF4 with the human thiamine transporter-2 in intestinal epithelial cells

BACKGROUND

The human thiamine transporter-2 (hTHTR-2) is involved in the intestinal absorption of thiamine. Recent studies with membrane transporters of other nutrients/substrates have shown that they have associated proteins that affect different aspects of their physiology and cell biology. Nothing is known about protein(s) that interact with hTHTR-2 in intestinal epithelial cells and influence its physiological function and/or its cell biology.

AIMS

The aim of this study was to identify protein partner(s) that interact with hTHTR-2 in human intestinal cells and determine the physiological/biological consequence of that interaction.

METHODS

The yeast split-ubiquitin two-hybrid approach was used to screen a human intestinal cDNA library. GST-pull-down and cellular co-localization approaches were used to confirm the interaction between hTHTR-2 and the associated protein(s). The effect of such an interaction on hTHTR-2 function was examined by (3)H-thiamine uptake assays.

RESULTS

Our screening results identified the human TransMembrane 4 SuperFamily 4 (TM4SF4) as a potential interactor with hTHTR-2. This interaction was confirmed by an in vitro GST-pull-down assay, and by live-cell confocal imaging of HuTu-80 cells co-expressing hTHTR-2-GFP and mCherry-TM4SF4 (the latter displayed a significant overlap of these two proteins in intracellular vesicles and at the cell membrane). Co-expression of hTHTR-2 with TM4SF4 in HuTu-80 cells led to a significant induction in thiamine uptake. In contrast, silencing TM4SF4 with gene-specific siRNA led to a significant decrease in thiamine uptake.

CONCLUSIONS

These results show for the first time that the accessory protein TM4SF4 interacts with hTHTR-2 and influences the physiological function of the thiamine transporter.

Thiamine responsive megaloblastic anemia: the puzzling phenotype

BACKGROUND

Thiamine responsive megaloblastic anemia (TRMA) is characterized by a triad of megaloblastic anemia, non-type 1 diabetes mellitus and sensorineural deafness. Other clinical findings have been described in few cases. The SLC19A2 gene on chromosome 1q 23.3 is implicated in all cases with TRMA. Our aim is to discuss the clinical manifestations of all Omani children diagnosed with TRMA and determine genotype-phenotype relationship.

PROCEDURE

Clinical and laboratory data of all patients diagnosed in Oman were retrospectively collected. Mutation analysis of affected families was conducted using two Microsatellite markers. Genotyping was performed with fluorescent-labeled PCR primers. To define the deletion breakpoint region, PCR reactions were carried out using different primer pairs located at the introns 3 and 3'-untranslated region with Expand Long Template PCR kit.

RESULTS

A total of six children have been diagnosed with this syndrome. They were five females and one male. They all presented with sensorineural deafness at birth while the age of anemia presentation ranged between 6 weeks to 19 months. They all belong to same family with complex interfamilial marriages and presented with the typical triad. Of interest is the very rare presentation of one patient with Uhl cardiac anomaly (total absence of right ventricular myocardium with apposition of endocardium and pericardium) that has never been described before in patients with TRMA. All patients have a novel large deletion of 5,224 bp involving exons 4, 5, and 6 of SLC19A2.

CONCLUSIONS

TRMA is a disease of expanding phenotypic spectrum with poor genotype-phenotype correlation.

Effect of the cigarette smoke component, 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), on physiological and molecular parameters of thiamin uptake by pancreatic acinar cells

Thiamin is indispensable for the normal function of pancreatic acinar cells. These cells take up thiamin via specific carrier-mediated process that involves thiamin transporter-1 and -2 (THTR-1 and THTR-2; products of SLC19A2 and SLC19A3 genes, respectively). In this study we examined the effect of chronic exposure of pancreatic acinar cells in vitro (pancreatic acinar 266-6 cells) and in vivo (wild-type and transgenic mice carrying the SLC19A2 and SLC19A3 promoters) to the cigarette smoke component 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) on physiological and molecular parameters of the thiamin uptake process. The results show that chronic exposure of 266-6 cells to NNK (3 µM, 24 h) leads to a significant inhibition in thiamin uptake. The inhibition was associated with a significant decrease in the level of expression of THTR-1 and -2 at the protein and mRNA levels as well as in the activity of SLC19A2 and SLC19A3 promoters. Similarly chronic exposure of mice to NNK (IP 10 mg/100 g body weight, three times/week for 2 weeks) leads to a significant inhibition in thiamin uptake by freshly isolated pancreatic acinar cells, as well as in the level of expression of THTR-1 and -2 protein and mRNA. Furthermore, activity of the SLC19A2 and SLC19A3 promoters expressed in transgenic mice were significantly suppressed by chronic exposure to NNK. The effect of NNK on the activity of the SLC19A2 and SLC19A3 promoters was not mediated via changes in their methylation profile, rather it appears to be exerted via an SP1/GG and SP1/GC cis-regulatory elements in these promoters, respectively. These results demonstrate, for the first time, that chronic exposure of pancreatic acinar cells to NNK negatively impacts the physiological and molecular parameters of thiamin uptake by pancreatic acinar cells and that this effect is exerted, at least in part, at the level of transcription of the SLC19A2 and SLC19A3 genes.

Recent advances in transport of water-soluble vitamins in organs of the digestive system: a focus on the colon and the pancreas

This review focuses on recent advances in our understanding of the mechanisms and regulation of water-soluble vitamin (WSV) transport in the large intestine and pancreas, two important organs of the digestive system that have only recently received their fair share of attention. WSV, a group of structurally unrelated compounds, are essential for normal cell function and development and, thus, for overall health and survival of the organism. Humans cannot synthesize WSV endogenously; rather, WSV are obtained from exogenous sources via intestinal absorption. The intestine is exposed to two sources of WSV: a dietary source and a bacterial source (i.e., WSV generated by the large intestinal microbiota). Contribution of the latter source to human nutrition/health has been a subject of debate and doubt, mostly based on the absence of specialized systems for efficient uptake of WSV in the large intestine. However, recent studies utilizing a variety of human and animal colon preparations clearly demonstrate that such systems do exist in the large intestine. This has provided strong support for the idea that the microbiota-generated WSV are of nutritional value to the host, and especially to the nutritional needs of the local colonocytes and their health. In the pancreas, WSV are essential for normal metabolic activities of all its cell types and for its exocrine and endocrine functions. Significant progress has also been made in understanding the mechanisms involved in the uptake of WSV and the effect of chronic alcohol exposure on the uptake processes.

Analysis of thiamine transporter genes in sporadic beriberi

OBJECTIVE

Thiamine or vitamin B1 deficiency diminishes thiamine-dependent enzymatic activity, alters mitochondrial function, impairs oxidative metabolism, and causes selective neuronal death. We analyzed for the first time, the role of all known mutations within three specific thiamine carrier genes, SLC19 A2, SLC19 A3, and SLC25 A19, in a patient with atrophic beriberi, a multiorgan nutritional disease caused by thiamine deficiency.

METHODS

A 44-year-old male alcoholic patient from Morocco developed massive bilateral leg edema, a subacute sensorimotor neuropathy, and incontinence. Despite normal vitamin B1 serum levels, his clinical picture was rapidly reverted by high-dose intramuscular thiamine treatment, suggesting a possible genetic resistance. We used polymerase chain reaction followed by amplicon sequencing to study all the known thiamine-related gene mutations identified within the Human Gene Mutation Database.

RESULTS

Thirty-seven mutations were tested: 29 in SLC19 A2, 6 in SLC19 A3, and 2 in SLC25 A19. Mutational analyses showed a wild-type genotype for all sequences investigated.

CONCLUSION

This is the first genetic study in beriberi disease. We did not detect any known mutation in any of the three genes in a sporadic dry beriberi patient. We cannot exclude a role for other known or unknown mutations, in the same genes or in other thiamine-associated genes, in the occurrence of this nutritional neuropathy.

Adaptive regulation of human intestinal thiamine uptake by extracellular substrate level: a role for THTR-2 transcriptional regulation

The intestinal thiamine uptake process is adaptively regulated by the level of vitamin in the diet, but the molecular mechanism involved is not fully understood. Here we used the human intestinal epithelial Caco-2 cells exposed to different levels of extracellular thiamine to delineate the molecular mechanism involved. Our results showed that maintaining Caco-2 cells in a thiamine-deficient medium resulted in a specific and significant increase of [3H]thiamine uptake compared with cell exposure to a high level of thiamine (1 mM). This adaptive regulation was also associated with a higher level of mRNA expression of thiamine transporter-2 (THTR-2), but not thiamine transporter-1 (THTR-1), in the deficient condition and a higher level of promoter activity of gene encoding THTR-2 (SLC19A3). Using 5'-truncated promoter-luciferase constructs, we identified the thiamine level-responsive region in the SLC19A3 promoter to be between -77 and -29 (using transcriptional start site as +1). By means of mutational analysis, a key role for a stimulating protein-1 (SP1)/guanosine cytidine box in mediating the effect of extracellular thiamine level on SLC19A3 promoter was established. Furthermore, extracellular level of thiamine was found to affect SP1 protein expression and binding pattern to the thiamine level-responsive region of SLC19A3 promoter in Caco-2 cells as shown by Western blotting and electrophoretic mobility shift assay analysis, respectively. These studies demonstrate that the human intestinal thiamine uptake is adaptively regulated by the extracellular substrate level via transcriptional regulation of the THTR-2 system, and report that SP1 transcriptional factor is involved in this regulation.