Deletion of SLC19A2, the high affinity thiamine transporter, causes selective inner hair cell loss and an auditory neuropathy phenotype

Mutations in the gene coding for the high-affinity thiamine transporter Slc19a2 underlie the clinical syndrome known as thiamine-responsive megaloblastic anemia (TRMA) characterized by anemia, diabetes, and sensorineural hearing loss. To create a mouse model of this disease, a mutant line was created with targeted disruption of the gene. Cochlear function is normal in these mutants when maintained on a high-thiamine diet. When challenged with a low-thiamine diet, Slc19a2-null mice showed 40-60 dB threshold elevations by auditory brainstem response (ABR), but only 10-20 dB elevation by otoacoustic emission (OAE) measures. Wild-type mice retain normal hearing on either diet. Cochlear histological analysis showed a pattern uncommon for sensorineural hearing loss: selective loss of inner hair cells after 1-2 weeks on low thiamine and significantly greater inner than outer hair cell loss after longer low-thiamine challenges. Such a pattern is consistent with the observed discrepancy between ABR and OAE threshold shifts. The possible role of thiamine transport in other reported cases of selective inner hair cell loss is considered.

Thiamine-responsive megaloblastic anemia syndrome: a disorder of high-affinity thiamine transport

Thiamine-responsive megaloblastic anemia (TRMA) syndrome (OMIM No. 249270) comprises a distinctive triad of clinical features: megaloblastic anemia with ringed sideroblasts, diabetes mellitus, and progressive sensorineural deafness. The TRMA gene has been mapped and cloned. Designated "SLC19A2" as a member of the solute carrier gene superfamily, this gene is mutated in all TRMA kindreds studied to date. The product of the SLC19A2 gene is a membrane protein which transports thiamine (vitamin B1) with sub-micromolar affinity. Cells from TRMA patients are uniquely sensitive to thiamine depletion to the nanomolar range, while pharmacologic doses of vitamin B1 ameliorate the anemia and diabetes. Here we review the current status of studies aimed at understanding the pathophysiology of this unique transport defect.

Characterization of a murine high-affinity thiamine transporter, Slc19a2

Thiamine-responsive megaloblastic anemia with deafness and diabetes (TRMA) is a rare autosomal recessive disorder of thiamine transport. Previous studies have demonstrated that the disease is caused by mutations in the SLC19A2 gene encoding a high-affinity thiamine transporter. We hypothesize that thiamine transport, mediated by SLC19A2, plays a role in the development and or maintenance of several organ systems, in particular the erythropoietic, auditory, and glucose homeostasis systems. To investigate the transporter further, we cloned the murine Slc19a2 locus and characterized the resulting protein. Murine Slc19a2 is a 498 amino acid protein, with 12 predicted transmembrane domains. The gene spans approximately 13kb with 6 exons, structurally identical to that of the human homolog. We localized the Slc19a2 gene to mouse chromosome 1, a region syntenic to human chromosome 1q23 that contains the TRMA locus. Transient expression of Slc19a2 in HEK293T cells resulted in specific uptake of [3H] thiamine, confirming a thiamine transporter function. Western blot analysis of mouse tissues reveals a wide distribution of Slc19a2 protein. Immunohistochemistry studies indicate that Slc19a2 is expressed on the cell surface and intracellularly, and is specifically localized to a subpopulation of cells in cochlea, small intestine, and pancreas.

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.

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

Thiamine-responsive megaloblastic anaemia with diabetes and deafness (TRMA; MIM 249270) is an autosomal recessive disease thought to be due to a defect in thiamine (vitamin B1) transport. Pharmacological doses of thiamine correct the anaemia, and in some cases improve the diabetes, although progressive sensorineural deafness is irreversible. Previous studies localized the TRMA gene to a 4-cM region on chromosome 1q23.3 (ref. 5), and fine-mapping has recently narrowed that region further. We have previously demonstrated that fibroblasts from people with TRMA lack high-affinity thiamine transport. Expression of a gene encoding a known yeast thiamine transporter, THI10 (refs 8-10), in TRMA mutant cells prevents apoptotic cell death in thiamine-depleted medium. On the basis of these studies, we hypothesized that a defective thiamine transporter causes TRMA. We undertook a candidate gene approach to identify putative thiamine transporters in the 1q23.3 critical region. Here we present evidence that the gene SLC19A2 (for solute carrier family 19 (thiamine transporter), member 2) encodes the first known mammalian thiamine transporter, which we designate thiamine transporter-1 (THTR-1).

A sensitive detection of neutrophil activation by fluorescence quenching of membrane inserted singlet oxygen probe

Fluorescence emission intensity of 1,3-diphenylisobenzofuran incorporated in polymorphonuclear granulocytes plasma membranes was investigated in basal conditions and during stimulation with different activators. Phorbol myristate acetate, known as the most effective "oxygen burst" inducer, produced a larger decrease in 1,3-diphenylisobenzofuran fluorescence intensity than 1-alkyl-2-acetyl-sn-glycero-3-phosphocholine (platelet activating factor) and N-formyl-methionyl-leucyl-phenylalanine, known as weak stimulants of oxygen uptake. Diphenyl iodonium an inhibitor of leukocyte NADPH oxidase, and the singlet oxygen selective trap alpha-terpinene inhibited the quenching effect of phorbol myristate acetate on 1,3-diphenylisobenzofuran fluorescence. These data suggest formation of singlet oxygen in activated leukocytes and demonstrate that measurement of 1,3-diphenylisobenzofuran fluorescence intensity provides a new sensitive method of detection of neutrophils activation.

The interaction of phospholipid bilayers with pig heart AMP deaminase: Fourier-transform infrared spectroscopic and kinetic studies

The interaction of pig heart AMP deaminase with different chemical species of phosphatidylcholine and with natural plasma membranes has been investigated. Phospholipids added to the system either as natural biological membranes (plasma membrane vesicles) or in the form of liposomes containing unsaturated phosphatidylcholine considerably enhanced AMP deaminase activity. The secondary structure of pig heart AMP deaminase in the absence and in the presence of dioleoyl phosphatidylcholine and dipalmitoyl phosphatidylcholine liposomes was investigated by Fourier-transform infrared spectroscopy. Quantitative analysis of the amide I band showed that the enzyme contains 45% beta-sheets, 28% alpha-helix, 16% turns and 11% non-ordered structure. In the presence of dioleoyl phosphatidylcholine liposomes, the beta/alpha content ratio decreased; this decrease was dependent on the amount of lipid added. This phenomenon was not observed in the case of dipalmitoyl phosphatidylcholine liposomes. These data suggest a possible role for membrane phospholipids in the regulation of AMP deaminase activity.