Thiamine-responsive megaloblastic anemia, sensorineural deafness, and diabetes mellitus: A new syndrome?

Metabolic Cardiomyopathies and Cardiac Defects in Inherited Disorders of Carbohydrate Metabolism: A Systematic Review

Heart failure (HF) is a progressive chronic disease that remains a primary cause of death worldwide, affecting over 64 million patients. HF can be caused by cardiomyopathies and congenital cardiac defects with monogenic etiology. The number of genes and monogenic disorders linked to development of cardiac defects is constantly growing and includes inherited metabolic disorders (IMDs). Several IMDs affecting various metabolic pathways have been reported presenting cardiomyopathies and cardiac defects. Considering the pivotal role of sugar metabolism in cardiac tissue, including energy production, nucleic acid synthesis and glycosylation, it is not surprising that an increasing number of IMDs linked to carbohydrate metabolism are described with cardiac manifestations. In this systematic review, we offer a comprehensive overview of IMDs linked to carbohydrate metabolism presenting that present with cardiomyopathies, arrhythmogenic disorders and/or structural cardiac defects. We identified 58 IMDs presenting with cardiac complications: 3 defects of sugar/sugar-linked transporters (GLUT3, GLUT10, THTR1); 2 disorders of the pentose phosphate pathway (G6PDH, TALDO); 9 diseases of glycogen metabolism (GAA, GBE1, GDE, GYG1, GYS1, LAMP2, RBCK1, PRKAG2, G6PT1); 29 congenital disorders of glycosylation (ALG3, ALG6, ALG9, ALG12, ATP6V1A, ATP6V1E1, B3GALTL, B3GAT3, COG1, COG7, DOLK, DPM3, FKRP, FKTN, GMPPB, MPDU1, NPL, PGM1, PIGA, PIGL, PIGN, PIGO, PIGT, PIGV, PMM2, POMT1, POMT2, SRD5A3, XYLT2); 15 carbohydrate-linked lysosomal storage diseases (CTSA, GBA1, GLA, GLB1, HEXB, IDUA, IDS, SGSH, NAGLU, HGSNAT, GNS, GALNS, ARSB, GUSB, ARSK). With this systematic review we aim to raise awareness about the cardiac presentations in carbohydrate-linked IMDs and draw attention to carbohydrate-linked pathogenic mechanisms that may underlie cardiac complications.

Thiamine and diabetes: back to the future?

The first reports of a link between thiamine and diabetes date back to the 1940s. Some years later, a role for thiamine deficiency in diabetic neuropathy became evident, and some pilot studies evaluated the putative effects of thiamine supplementation. However, the administration of thiamine and its lipophilic derivative benfotiamine for the treatment of this complication gained consensus only at the end of the '90 s. The first evidence of the beneficial effects of thiamine on microvascular cells involved in diabetic complications dates to 1996: from then on, several papers based on in vitro and animal models have addressed the potential use of this vitamin in counteracting diabetic microangiopathy. A few pilot studies in humans reported beneficial effects of thiamine administration on diabetic nephropathy, but, despite all promising proofs-of-concept, the possible role of thiamine in counteracting development or progression of retinopathy has not been addressed until now. Thiamine is a water-soluble vitamin, rapidly expelled from the body, with no issues of over-dosage or accumulation; unfortunately, it is non-patentable, and neither industry nor independent donors are interested in investing in large-scale randomized controlled clinical trials to investigate its potential in diabetes and its complications. Consequently, science will not be able to disprove a promising hypothesis and, more importantly, diabetic people remain deprived of a possible way to ameliorate their condition.

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.

Dyserythropoiesis and myelodysplasia in thiamine-responsive megaloblastic anemia syndrome

The case of thiamine-responsive megaloblastic anemia (TRMA) presented here speculates the need early diagnosis, continuous monitoring, follow-up, and regulated treatment plan for the patients. Complications and systemic manifestations are likely to enhance in otherwise circumstances.

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.

[Thiamine-responsive megaloblastic anemia or Rogers syndrome: A literature review]

Thiamine-responsive megaloblastic anemia (TRMA), also known as Rogers syndrome, is a rare autosomal recessive disease characterized by three main components: megaloblastic anemia, diabetes mellitus and sensorineural deafness. Those features occur in infancy but may arise during adolescence. Diagnosis relies on uncovering genetic variations (alleles) in the SLC19A2 gene, encoding for a high affinity thiamine transporter. This transporter is essentially present in hematopoietic stem cells, pancreatic beta cells and inner ear cells, explaining the clinical manifestations of the disease. Based on a multidisciplinary approach, treatment resides on lifelong thiamine oral supplementation at pharmacological doses, which reverses anemia and may delay development of diabetes. However, thiamine supplementation does not alleviate already existing hearing defects.

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.

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.

Beta cell function and clinical course in three siblings with thiamine-responsive megaloblastic anemia (TRMA) treated with thiamine supplementation

Three siblings with thiamine-responsive megaloblastic anemia (TRMA) with a homozygous c.454delGGCATinsAT mutation in SLC19A2 are described. The index case presented at 14 months' old with severe non-ketotic hyperglycemia, dehydration, seizures and sinovenous thrombosis. She was started on insulin and developed sensorineural hearing loss around 2 years old. Two siblings were found to have the same mutation and were started on thiamine. One sibling developed transient hyperglycemia after several years of thiamine supplementation of 12 mg/kg that resolved with an increased thiamine dose (23 mg/kg). A younger sibling continues to remain diabetes-free on thiamine (24 mg/kg). The clinical course in this family suggests that there is an effect of thiamine on pancreatic beta cell function in patients with TRMA given the resolution of impaired fasting glucose with increasing thiamine dose in one sibling and the lack of diabetes to date in the siblings that were treated early with thiamine.

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.

Novel mutation in the SLC19A2 gene in an Iranian family with thiamine-responsive megaloblastic anemia: a series of three cases

Thiamine-responsive megaloblastic anemia (TRMA) is a clinical triad characterized by megaloblastic anemia, non-autoimmune diabetes mellitus, and sensory-neural hearing loss. Mutations in the thiamine transporter gene, solute carrier family 19, member 2 (SLC19A2), have been associated with TRMA. Three pediatric patients from a large consanguineous Iranian family with hyperglycemia, anemia, and hearing loss were clinically diagnosed with TRMA. In all three patients, TRMA was confirmed by direct sequencing of the SLC19A2 gene that revealed a novel missense homozygous mutation c.382 G>A (p.E128K). This mutation results in the substitution of glutamic acid to lysine at position 128 in exon 2 and was not detected in 200 control chromosomes. Thiamine therapy reversed the anemia and alleviated the hyperglycemia in all three patients. We recommend sequence analysis of the SLC19A2 gene in individuals with a clinical triad of diabetes mellitus, hearing loss, and anemia. The administration of thiamine ameliorates the megaloblastic anemia and the hyperglycemia in patients with TRMA.

Thiamine responsive megaloblastic anemia: a novel SLC19A2 compound heterozygous mutation in two siblings

Thiamine responsive megaloblastic anemia (TRMA) is an autosomal recessive disease caused by loss of function mutations in the SLC19A2 gene. TRMA is characterized by anemia, deafness, and diabetes. In some cases, optic atrophy or more rarely retinitis pigmentosa is noted. We now report two sisters, the eldest of which presented to a different hospital during childhood with sensorineural deafness, which was treated with a hearing prosthesis, insulin requiring diabetes, retinitis pigmentosa, optic atrophy, and macrocytic anemia. These features initially suggested a clinical diagnosis of Wolfram syndrome (WS). Therapy with thiamine was initiated which resulted in the resolution of the anemia. The younger sister, who was affected with sensorineural deafness, was referred to our hospital for non-autoimmune diabetes. She was found to have macrocytosis and ocular abnormalities. Because a diagnosis of TRMA was suspected, therapy with insulin and thiamine was started. Sequencing analysis of the SLC19A2 gene identified a compound heterozygous mutation p.Y81X/p.L457X (c.242insA/c.1370delT) in both sisters. Non-autoimmune diabetes associated with deafness and macrocytosis, without anemia, suggests a diagnosis of TRMA. Patients clinically diagnosed with WS with anemia and/or macrocytosis should be reevaluated for TRMA.

Thiamine-responsive megaloblastic anemia: identification of novel compound heterozygotes and mutation update

OBJECTIVE

To determine causative mutations and clinical status of 7 previously unreported kindreds with TRMA syndrome, (thiamine-responsive megaloblastic anemia, online Mendelian inheritance in man, no. 249270), a recessive disorder of thiamine transporter Slc19A2.

STUDY DESIGN

Genomic DNA was purified from blood, and SLC19A2 mutations were characterized by sequencing polymerase chain reaction-amplified coding regions and intron-exon boundaries of all probands. Compound heterozygotes were further analyzed by sequencing parents, or cloning patient genomic DNA, to ascertain that mutations were in trans.

RESULTS

We detected 9 novel SLC19A2 mutations. Of these, 5 were missense, 3 were nonsense, and 1 was insertion. Five patients from 4 kindreds were compound heterozygotes, a finding not reported previously for this disorder, which has mostly been found in consanguineous kindreds.

CONCLUSION

SLC19A2 mutation sites in TRMA are heterogeneous; with no regional "hot spots." TRMA can be caused by heterozygous compound mutations; in these cases, the disorder is found in outbred populations. To the extent that heterozygous patients were ascertained at older ages, a plausible explanation is that if one or more allele(s) is not null, partial function might be preserved. Phenotypic variability may lead to underdiagnosis or diagnostic delay, as the average time between the onset of symptoms and diagnosis was 8 years in this cohort.

Thiamine transporter mutation: an example of monogenic diabetes mellitus

OBJECTIVE

Thiamine-responsive megaloblastic anemia (TRMA) is a rare syndrome characterized by diabetes mellitus (DM), anemia, and sensorineural deafness. We describe the clinical course and the molecular defect of a young woman who was diagnosed to have this syndrome.

CASE

The patient is an 18-year-old girl who was born to non-consanguous parents. She was noted to be deaf-mute in the first year of life. She was diagnosed with DM at the age of 9 months and with severe anemia at the age of 2 years. An extensive work up could not identify the cause. She was treated with blood transfusions every 3-4 weeks for the past 16 years. A diagnosis of TRMA was suspected and the patient was treated with thiamine hydrochloride. Hemoglobin and platelets increased to normal values after a few weeks of thiamine therapy. Diabetic control significantly improved but she had no noticeable changes in the deafness.

METHODS

Peripheral blood DNA was extracted from the patient, her mother, aunt, and a healthy sister. Exons and exon-intron boundaries of the thiamine transporter gene SLC19A2 were PCR amplified and directly sequenced.

RESULTS

A G515C homozygous mutation was identified in the SLC19A2 gene of the patient. This mutation changes Gly to Arg at codon 172 (G172R). The mother, an aunt, and a sister had a heterozygous G172R mutation.

CONCLUSIONS

Mutations in thiamine transporter gene, SLC19A2, causes a rare form of monogenic diabetes, anemia, and sensorineural deafness. Thiamine induces a remarkable hematological response and improvement in the diabetic control but has no effect on deafness.

Neurodegenerative disorders associated with diabetes mellitus

More than 20 syndromes among the significant and increasing number of degenerative diseases of neuronal tissues are known to be associated with diabetes mellitus, increased insulin resistance and obesity, disturbed insulin sensitivity, and excessive or impaired insulin secretion. This review briefly presents such syndromes, including Alzheimer disease, ataxia-telangiectasia, Down syndrome/trisomy 21, Friedreich ataxia, Huntington disease, several disorders of mitochondria, myotonic dystrophy, Parkinson disease, Prader-Willi syndrome, Werner syndrome, Wolfram syndrome, mitochondrial disorders affecting oxidative phosphorylation, and vitamin B(1) deficiency/inherited thiamine-responsive megaloblastic anemia syndrome as well as their respective relationship to malignancies, cancer, and aging and the nature of their inheritance (including triplet repeat expansions), genetic loci, and corresponding functional biochemistry. Discussed in further detail are disturbances of glucose metabolism including impaired glucose tolerance and both insulin-dependent and non-insulin-dependent diabetes caused by neurodegeneration in humans and mice, sometimes accompanied by degeneration of pancreatic beta-cells. Concordant mouse models obtained by targeted disruption (knock-out), knock-in, or transgenic overexpression of the respective transgene are also described. Preliminary conclusions suggest that many of the diabetogenic neurodegenerative disorders are related to alterations in oxidative phosphorylation (OXPHOS) and mitochondrial nutrient metabolism, which coincide with aberrant protein precipitation in the majority of affected individuals.

Male infertility due to germ cell apoptosis in mice lacking the thiamin carrier, Tht1. A new insight into the critical role of thiamin in spermatogenesis

A mouse model of thiamin-responsive megaloblastic anemia (diabetes mellitus, deafness, megaloblastic anemia) lacking functional Slc19a2 has been generated and unexpectedly found to have a male-specific sterility phenotype. We describe here the characterization of the testis-specific effects of absence of the high-affinity thiamin transporter, Tht1. Null males were found to have hypoplastic testes secondary to germ cell depletion. Morphologic and expression analysis revealed that under conditions of standard thiamin intake, tissues affected in the syndrome (pancreatic beta-cell, hematopoietic cells, auditory nerve) maintained normal function but pachytene stage spermatocytes underwent apoptosis. Under conditions of thiamin challenge, the apoptotic cell loss extended to earlier stages of germ cells but spared Sertoli cells and Leydig cells. Injection of high-dose thiamin was effective in reversing the spermatogenic failure, suggesting that the absence of the thiamin carrier could be overcome by diffusion-mediated transport at supranormal thiamin concentrations. These observations demonstrated that male germ cells, particularly those with high thiamin transporter expression beyond the blood-testis barrier, were more susceptible to apoptosis triggered by intracellular thiamin deficiency than any other tissue type. The findings described here highlight an unexpected and critical role for thiamin transport and metabolism in spermatogenesis.

Novel mutation in theSLC19A2 gene in an African-American female with thiamine-responsive megaloblastic anemia syndrome

Thiamine-responsive megaloblastic anemia (TRMA) syndrome is an autosomal recessive disorder characterized by diabetes mellitus (DM), progressive sensorineural deafness, and thiamine-responsive anemia. Mutations in the SLC19A2 gene encoding a high-affinity thiamine transporter protein THTR-1 are responsible for the clinical features associated with TRMA syndrome. We report an African-American female with TRMA-syndrome associated with thyroid disease and retinitis pigmentosa caused by a novel mutation in the SLC19A2 gene. The patient presented at 12 months of age with paroxysmal atrial tachycardia and hepatosplenomegaly. One month later, she developed DM requiring intermittent insulin therapy. At 2-1/2 years of age, profound sensorineural hearing loss was discovered. By 4 years of age, daily insulin therapy (0.5 U/kg/day) was instituted and her insulin requirement gradually increased to 1.0 U/kg/day by 9 years of age. She developed optic atrophy, retinitis pigmentosa, and visual impairment by 12 years of age with severe restriction of peripheral vision by 16 years. At age 19, a thiamine-responsive normocytic anemia was discovered. She was diagnosed with autoimmune thyroiditis at 20 years and she experienced a psychotic episode associated with a mood disorder at age 21. With oral thiamine therapy, her insulin requirement decreased by 30% over a 20 month period. Molecular analysis revealed that the patient is homozygous for a missense mutation (C152T) in exon 1 of the SLC19A2 gene.

Cardiac manifestations in thiamine-responsive megaloblastic anemia syndrome

Thiamine-responsive megaloblastic anemia (TRMA) syndrome is a rare autosomal recessive disorder defined by the occurrence of megaloblastic anemia, diabetes mellitus, and sensorineural deafness, responding in varying degrees to thiamine treatment. Other features of this syndrome gradually develop. We describe three TRMA patients with heart rhythm abnormalities and structural cardiac anomalies. Eight other reported TRMA patients also had cardiac anomalies. Recently, the TRMA gene, SLC19A2, was identified, encoding a functional thiamine transporter. Characterization of the metabolic defect of TRMA may shed light on the role of thiamine in common cardiac abnormalities.

TRMA syndrome (thiamine-responsive megaloblastic anemia): a case report and review of the literature

Thiamine-responsive megaloblastic anemia syndrome (TRMA) is an autosomal recessive disorder with features that include megaloblastic anemia, mild thrombocytopenia and leukopenia, sensorineural deafness and diabetes mellitus. In this disease, the active thiamine uptake into cells is disturbed. Treatment with pharmacological doses of thiamine ameliorates the megaloblastic anemia and diabetes mellitus. Previous studies have demonstrated that the disease is caused by mutations in the SLC19A2 gene encoding a high-affinity thiamine transporter. We present a 5-yr-old-boy with TRMA and, because of its rarity, we review the literature.

Identification of transcriptional start sites and splicing of mouse thiamine transporter gene THTR-1 (Slc19a2)

We have previously reported the cDNA cloning of the mouse thiamine transporter THTR-1 as a p53 transcriptional target gene (renamed THTR-1a hereinafter). The mouse THTR-1a is predicted to encode a protein of 12 hydrophobic stretches and a hydrophilic loop of 87 amino acids between transmembrane helices VI and VII. The mouse THTR-1 gene has been cloned, two major transcriptional start sites located at -175 and -183 relative to the translation start codon were identified. In addition, we have cloned a spliced variant, designated THTR-1b, from mouse liver cDNA library. This isoform is characterized by an inframe deletion of 114 nucleotides from the 3'-terminal region of exon 2, predicting the expression of a truncated protein lacking the central 38 amino acids of the loop region. THTR-1b coexpressed with THTR-1a in many of the mouse tissues and in day-7 to day-17 embryos, but in lower levels than the THTR-1a. When expressed in mammalian cells, both isoforms were able to mediate the transport of thiamine. Therefore, the transport function of the mouse THTR-1 is not determined by the central 38 amino acids of its loop region.

Mitochondrial diabetes, DIDMOAD and other inherited diabetes syndromes

Inherited diabetes syndromes are individually rare but collectively make up a significant proportion of patients attending diabetes clinics, some of whom have multiple handicaps. This chapter focuses on syndromes in which major advances have been made in our understanding of the underlying molecular genetics. These conditions demonstrate novel genetic mechanisms such as maternal inheritance and genetic imprinting. They are also fascinating as they aid our understanding of insulin metabolism, both normal and abnormal. As the causative genes are identified, future issues will be the availability of genetic testing, their contribution to the genetic heterogeneity of the more common types of diabetes, and functional studies of the relevant proteins. It is probable that other subtypes of diabetes will be identified as the relevant metabolic pathways are characterized. This is an exciting time to be a diabetes physician as diabetology returns to being a diagnostic rather than a mainly management-based speciality.

Slc19a2: cloning and characterization of the murine thiamin transporter cDNA and genomic sequence, the orthologue of the human TRMA gene

Recently, our group and others cloned the TRMA disease gene, SLC19A2, which encodes a thiamin transporter. Here, we report the cloning and characterization of the full-length cDNA and genomic sequences of mouse Slc19a2. The Slc19a2 cDNA contained a 1494-bp open-reading frame, and had 5'- and 3'-untranslated regions of 189 and 1857 bp, respectively. A putative GC-rich, TATA-less promoter was identified in genomic sequence directly upstream of the identified 5' end. The Slc19a2 gene spanned 16.3 kb and was organized into six exons, a gene structure conserved with the human orthologue. The predicted Slc19a2 protein, like SLC19A2, was predicted to have 12 transmembrane domains and shared a number of other conserved sequence motifs with the human orthologue, including one potential N-glycosylation site (N(63)) and several potential phosphorylation sites. Comparison of the Slc19a2 amino acid sequence with those of the other known SLC19A solute carriers highlighted interesting patterns of conservation and divergence in various domains, allowing insight into potential structure-function relationships. The identification of the mouse Slc19a2 cDNA and genomic sequences will facilitate the generation of an animal model of TRMA, permitting future studies of disease pathogenesis.

A novel mutation in the SLC19A2 gene in a Tunisian family with thiamine-responsive megaloblastic anaemia, diabetes and deafness syndrome

Thiamine-responsive megaloblastic anaemia (TRMA) syndrome with diabetes and deafness was found in two patients from a Tunisian kindred. The proband was homozygous for a novel mutation, 287delG, in the high-affinity thiamine transporter gene, SLC19A2. We demonstrated that fibroblasts from this patient exhibited defective thiamine transport. These data confirm that the SLC19A2 gene is the high-affinity thiamine carrier and that this novel mutation is responsible for TRMA syndrome.

A novel mutation in the thiamine responsive megaloblastic anaemia gene SLC19A2 in a patient with deficiency of respiratory chain complex I

The thiamine transporter gene SLC19A2 was recently found to be mutated in thiamine responsive megaloblastic anaemia with diabetes and deafness (TRMA, Rogers syndrome), an early onset autosomal recessive disorder. We now report a novel G1074A transition mutation in exon 4 of the SLC19A2 gene, predicting a Trp358 to ter change, in a girl with consanguineous parents. In addition to the typical triad of Rogers syndrome, the girl presented with short stature, hepatosplenomegaly, retinal degeneration, and a brain MRI lesion. Both muscle and skin biopsies were obtained before high dose thiamine supplementation. While no mitochondrial abnormalities were seen on morphological examination of muscle, biochemical analysis showed a severe deficiency of pyruvate dehydrogenase and complex I of the respiratory chain. In the patient's fibroblasts, the supplementation with high doses of thiamine resulted in restoration of complex I activity. In conclusion, we provide evidence that thiamine deficiency affects complex I activity. The clinical features of TRMA, resembling in part those found in typical mitochondrial disorders with complex I deficiency, may be caused by a secondary defect in mitochondrial energy production.

The spectrum of mutations, including four novel ones, in the thiamine-responsive megaloblastic anemia gene SLC19A2 of eight families

Thiamine responsive megaloblastic anemia (TRMA) is an autosomal recessive disorder with a triad of symptoms: megaloblastic anemia, deafness, and non-type 1 diabetes mellitus. Occasionally, cardiac abnormalities and abnormalities of the optic nerve and retina occur as well. Patients with TRMA often respond to treatment with pharmacological doses of thiamine. Recently, mutations were found in patients with TRMA in a thiamine transporter gene (SLC19A2). We here describe the mutations found in eight additional families. We found four novel mutations and three that were previously described. Of the novel ones, one is a nonsense mutation in exon 1 (E65X), two are missense mutations in exon 2 (S142F, D93H), and another is a mutation in the splicing donor site at the 5' end of intron 4 (C1223+1G>A). We also summarize the state of knowledge on all mutations found to date in TRMA patients. SLC19A2 is the first thiamine transporter gene to be described in humans. Reviewing the location and effect of the disease causing mutations can shed light on the way the protein functions and suggest ways to continue its investigation.

Mutations in a new gene encoding a thiamine transporter cause thiamine-responsive megaloblastic anaemia syndrome

Thiamine-responsive megaloblastic anaemia syndrome (TRMA; MIM 249270) is an autosomal recessive disorder with features that include megaloblastic anaemia, mild thrombocytopenia and leucopenia, sensorineural deafness and diabetes mellitus. Treatment with pharmacologic doses of thiamine ameliorates the megaloblastic anaemia and diabetes mellitus. A defect in the plasma membrane transport of thiamine has been demonstrated in erythrocytes and cultured skin fibroblasts from TRMA patients. The gene causing TRMA was assigned to 1q23.2-q23.3 by linkage analysis. Here we report the cloning of a new gene, SLC19A2, identified from high-through-put genomic sequences due to homology with SLC19A1, encoding reduced folate carrier 1 (refs 8-10). We cloned the entire coding region by screening a human fetal brain cDNA library. SLC19A2 encodes a protein (of 497 aa) predicted to have 12 transmembrane domains. We identified 2 frameshift mutations in exon 2. a 1-bp insertion and a 2-bp deletion, among four Iranian families with TRMA. The sequence homology and predicted structure of SLC19A2, as well as its role in TRMA, suggest that its gene product is a thiamine carrier, the first to be identified in complex eukaryotes.

Localization of the thiamine-responsive megaloblastic anemia syndrome locus to a 1.4-cM region of 1q23

Thiamine-responsive megaloblastic anemia (TRMA) is a rare autosomal recessive syndrome characterized by megaloblastic anemia, deafness, and diabetes mellitus. A genome scan previously established linkage of this disorder to 1q23 and haplotype analysis defined a 16-cM critical region. Molecular genetic analyses of four unrelated multiplex Iranian families inheriting TRMA confirmed linkage to the same region and identified recombinant chromosomes which permitted refinement of the critical region to a narrow 1.4-cM interval. The haplotypes of the families differed, consistent with at least two independent mutational events. This refinement of the TRMA locus to less than 10% of that previously published should markedly facilitate the identification and evaluation of positional candidate and novel genes which may cause this disorder.

Thiamine-responsive myelodysplasia

The triad of thiamine-responsive anaemia, diabetes mellitus and deafness has been reported in 15 patients with macrocytic anaemia, sometimes associated with moderate thrombocytopenia. The bone marrow aspirate usually shows megaloblastic changes and ringed sideroblasts. However, tri-lineage myelodysplasia has never been reported. We describe two patients who presented with diabetes, deafness and thiamine-responsive pancytopenia. Bone marrow aspirate and biopsy were typical of tri-lineage myelodysplasia. These findings suggest that thiamine may have a role in the regulation of haemopoiesis at the stem cell level. We propose the term 'thiamine-responsive myelodysplasia' rather than that of thiamine-responsive anaemia.

Localization of the gene for thiamine-responsive megaloblastic anemia syndrome, on the long arm of chromosome 1, by homozygosity mapping

Thiamine-responsive megaloblastic anemia, also known as "TRMA" or "Rogers syndrome," is an early-onset autosomal recessive disorder defined by the occurrence of megaloblastic anemia, diabetes mellitus, and sensorineural deafness, responding in varying degrees to thiamine treatment. On the basis of a linkage analysis of affected families of Alaskan and of Italian origin, we found, using homozygosity mapping, that the TRMA-syndrome gene maps to a region on chromosome 1q23.2-23.3 (maximum LOD score of 3.7 for D1S1679). By use of additional consanguineous kindreds of Israeli-Arab origin, the putative disease-gene interval also has been confirmed and narrowed, suggesting genetic homogeneity. Linkage analysis generated the highest combined LOD-score value, 8.1 at a recombination fraction of 0, with marker D1S2799. Haplotype analysis and recombination events narrowed the TRMA locus to a 16-cM region between markers D1S194 and D1S2786. Several heterozygote parents had diabetes mellitus, deafness, or megaloblastic anemia, which raised the possibility that mutations at this locus predispose carriers in general to these manifestations. Characterization of the metabolic defect of TRMA may shed light on the role of thiamine deficiency in such common diseases.

Further studies on erythrocyte thiamin transport and phosphorylation in seven patients with thiamin-responsive megaloblastic anaemia

Erythrocyte thiamin metabolism and transport were investigated in 7 patients from Brazil, Israel and Italy suffering from thiamin-responsive megaloblastic anaemia (TRMA) associated with diabetes mellitus and sensorineural deafness. All patients discontinued thiamin therapy for 4-7 days before the investigation. TRMA patients showed invariably reduced total thiamin levels in erythrocytes (percentage reduction compared with healthy controls, -46.8 +/- 3%; mean +/- SEM). The proportions of individual thiamin compounds, expressed as a percentage of total thiamin content, were within the normal range, whereas their absolute amounts were significantly decreased in the following order: thiamin monophosphate > thiamin pyrophosphate > thiamin. Thiamin pyrophosphokinase activity was also reduced as compared with controls (mean reduction +/- SEM, -25.9 +/- 1%). The saturable, specific component of thiamin uptake, which normally prevails at physiological concentrations of thiamin (< 2 mumol/L), was absent in erythrocytes obtained from TRMA patients, while the non-saturable (diffusive) component of uptake was normally present. These results confirm observations made previously in two patients and demonstrate that TRMA is consistently associated with a state of thiamin deficiency, which is presumably secondary to reduced thiamin cellular transport and absorption (caused by lack of a membrane-specific carrier), and to impaired intracellular pyrophosphorylation.

Thiamine-responsive megaloblastic anemia with diabetes mellitus and sensorineural deafness

This study introduces a patient who has thiamine and thiamine pyrophosphokinase (TPKase) enzyme deficiency associated with diabetes mellitus, sensorineural deafness and thiamine-responsive megaloblastic anemia. Diabetes mellitus was diagnosed when she was 20 months old. After 1 year, macrocytic anemia developed and the thiamine therapy was started at 75 mg/day. During the follow-up, the insulin requirement decreased and even ceased, and macrocytic anemia improved with thiamine treatment. After thiamine therapy was ceased an increase in insulin requirement was observed and macrocytic anemia developed again.

Thiamine-responsive anemia in DIDMOAD syndrome

Two children with the DIDMOAD syndrome (diabetes insipidus, diabetes mellitus, optic atrophy, deafness) developed a megaloblastic and sideroblastic anemia, neutropenia, and borderline thrombocytopenia. Plasma thiamine concentration was low in one patient and normal in the other; in both children, thiamine pyrophosphate in erythrocytes and thiamine pyrophosphokinase activity were lower than the lowest values observed in control subjects. A month after institution of treatment with thiamine, the hematologic findings had returned to normal and the insulin requirements had decreased. Withdrawal of thiamine repeatedly induced relapse of the anemia and an increase in insulin requirements. We propose that an inherited abnormality of thiamine metabolism is responsible for the multisystem degenerative disorder known as DIDMOAD syndrome.

Studies on thiamine metabolism in thiamine-responsive megaloblastic anaemia

We have investigated thiamine metabolism and transport in the erythrocytes of two patients from unrelated families with thiamine responsive megaloblastic anaemia associated with diabetes mellitus and sensorineural deafness. Both patients had low concentrations of thiamine compounds in plasma and red blood cells. When erythrocytes were incubated with thiazole-[2-14C]-thiamine or [35S]-thiamine in vitro, the concentration of label within the cells was markedly reduced compared with controls. In addition, thiamine pyrophosphokinase activity was deficient in haemolysates prepared from the patients. Some relatives of the patients showed abnormal parameters of thiamine status and transport. In both patients treatment with a lipophilic compound corrected the haematological abnormalities and diabetes and in one patient has so far prevented the progression of deafness. We propose that the disorder is caused by an inherited defect of thiamine transport, possibly related to deficient pyrophosphokinase activity, leading to intracellular depletion of active thiamine metabolite derivatives.

Diabetes mellitus, thiamine-dependent megaloblastic anemia, and sensorineural deafness associated with deficient alpha-ketoglutarate dehydrogenase activity

Three brothers with diabetes mellitus, thiamine-responsive megaloblastic anemia, and sensorineural deafness are reported. Two had, in addition, congenital septal defects. The activities of thiamine-dependent enzymes were determined in one patient, revealing low alpha-ketoglutarate dehydrogenase activity, which could have caused a sideroblastic anemia with secondary megaloblastic changes. The anemia was thiamine dependent. The cause of the diabetes mellitus was not known, but it was not type 1.

Thiamine-responsive inborn errors of metabolism

Three different inherited disorders are known in which thiamine may exert a beneficial effect: maple syrup urine disease (MSUD), lactic acidaemia and the syndrome of megaloblastic anaemia with sensorineural deafness and diabetes mellitus. The amounts of thiamine which were used for long-term treatment varied from 20 to 2400 mg day-1. Additional treatment, such as the reduction of dietary branched chain amino acids in MSUD, could not be omitted in some cases. It has been shown that the vitamin improves the stability of the branched chain ketoacid decarboxylase, although some weeks may be needed to observe the in vivo effect of treatment. A prolonged trial with high doses of thiamine should always be given.

Thiamine responsive anaemia: a study of two further cases

A brother and sister of Pakistani origin suffered from sensorineural deafness, diabetes mellitus and a macrocytic anaemia. Their bone marrows showed megaloblastic erythropoiesis and contained many ringed sideroblasts. Electron microscope studies of the bone marrow revealed (1) iron-laden mitochondria in many erythroblasts, (2) non-specific abnormalities indicative of dyserythropoiesis in some erythroblasts, and (3) evidence of ineffective erythropoiesis. The deoxyuridine suppression test indicated that the megaloblastic changes were not caused by an impairment of the methylation of deoxyuridylate. Studies of nucleic acid synthesis in the bone marrow cells showed that the rate of incorporation of [3H]thymidine into DNA was increased and that the rates of incorporation of [14C]glycine and [14C]adenine into both DNA and RNA were essentially within the normal range. The anaemia did not respond to therapy with hydroxocobalamin, folic acid or pyridoxine but responded to 25 mg thiamine, daily, by mouth. In one of the cases a post-thiamine marrow aspirate showed a considerable improvement in both the megaloblastic and sideroblastic changes.