DiagnosisClinical DiagnosisThe diagnosis of thiamine-responsive megaloblastic anemia syndrome (TRMA) is based on an obligate triad of clinical features:Megaloblastic anemia occurring between infancy and adolescence: Examination of the bone marrow reveals megaloblastic changes with erythroblasts often containing iron-filled mitochondria (ringed sideroblasts). The anemia is corrected with pharmacologic doses of thiamine (vitamin B₁) (25-75 mg/day compared to US RDA of 1.5 mg/day). However, the red cells remain macrocytic, suggesting a persistent erythropoietic abnormality [Haworth et al 1982, Neufeld et al 1997]. Anemia can recur when thiamine is withdrawn. Progressive sensorineural deafness. Hearing loss has generally been early and can be detected in toddlers. Whether hearing loss is congenital (prelingual) is unknown. Some affected individuals do not show hearing loss when they are younger, while symptoms of megaloblastic anemia and diabetes are present. There are reports that thiamine treatment may not prevent development of hearing loss [Borgna-Pignatti et al 2009, Akın et al 2011] Diabetes mellitus that is non-type I in nature, with age of onset from infancy to adolescence. Insulin secretion is present but defective [Valerio et al 1998]. In some cases, insulin requirements are reduced with thiamine therapy [Neufeld et al 1997]. TestingEven without thiamine supplementation, serum thiamine concentrations are normal; there is no evidence of acidosis or aciduria.In vitro cell-based assays can assist in the diagnosis of TRMA. Studies of [³H]-thiamine uptake by control fibroblasts reveal a saturable process with apparent Km 400-550 nmol/L, while fibroblasts from individuals with TRMA entirely lack this high-affinity component [Stagg et al 1999]. TRMA cells, but not control cells, die of apoptosis in thiamine-depleted medium; accumulated organic acids suggest thiamine starvation [Stagg et al 1999]. Subtle thiamine transport defects at relatively high concentrations have been observed in red blood cells of individuals with TRMA [Rindi et al 1992, Rindi et al 1994]. Neither anti-insulin nor anti-islet cell antibodies have been found in individuals with TRMA [Neufeld et al 1997], but pancreatic histopathology has not been investigated. Molecular Genetic TestingGene. SLC19A2, which encodes the high-affinity thiamine transporter, is the only gene in which mutations are known to cause TRMA. Clinical testing Table 1. Summary of Molecular Genetic Testing Used in Thiamine-Responsive Megaloblastic Anemia SyndromeView in own windowGene SymbolTest MethodMutations DetectedMutation Detection Frequency by Test Method ^{1Test AvailabilitySLC19A2Sequence analysis of coding region Sequence variants 2100% 3Clinical 1. The ability of the test method used to detect a mutation that is present in the indicated gene2. Examples of mutations detected by sequence analysis may include small intragenic deletions/insertions and missense, nonsense, and splice site mutations; typically, exonic or whole-gene deletions/duplications are not detected.3. All individuals with the diagnostic phenotypic triad evaluated by sequence analysis have identifiable mutations in SLC19A2. To date, over 40 families with 33 distinct mutations have been identified [Diaz et al 1999, Raz et al 2000, Scharfe et al 2000, Gritli et al 2001, Neufeld et al 2001, Ozdemir et al 2002, Lagarde et al 2004, Ricketts et al 2006, Bergmann et al 2009, Onal et al 2009, Pichler et al 2012, Shaw-Smith et al 2012, Yilmaz Agladioglu et al 2012]. Homozygosity by descent has been the most common finding. Seven individuals from six families were compound heterozygotes with distinct mutations from each parent [Bergmann et al 2009, Pichler et al 2012, Shaw-Smith et al 2012]. Interpretation of test results. For issues to consider in interpretation of sequence analysis results, click here.Information on specific allelic variants may be available in Molecular Genetics (see Table A. Genes and Databases. and/or Pathologic allelic variants).Testing Strategy To confirm/establish the diagnosis in a probandA clinical diagnosis of TRMA should be considered in individuals with megaloblastic anemia with normal vitamin B₁₂/folic acid levels, with or without diabetes or hearing loss; response to oral thiamine makes the diagnosis highly likely. Identification of two SLC19A2 mutations by sequence analysis confirms the diagnosis. Carrier testing for at-risk relatives requires prior identification of the disease-causing mutations in the family.Note: Carriers are heterozygotes for this autosomal recessive disorder and are not at risk of developing the disorder.Prenatal diagnosis and preimplantation genetic diagnosis (PGD) for at-risk pregnancies require prior identification of the disease-causing mutations in the family.Genetically Related (Allelic) DisordersNo other phenotypes are known to be associated with mutations in SLC19A2.}

Natural HistoryTRMA is characterized by megaloblastic anemia, sensorineural hearing loss, and diabetes mellitus.The earliest findings of significant bone marrow problems have been in the first year of life and the latest in teenage years.Hearing defects may also be present at an early age. Progressive sensorineural hearing loss is irreversible and may not be prevented by thiamine treatment. The basis of the sensorineural deafness is obscure; it is not known if the deafness is caused by abnormalities of the cochlea or of the auditory nerve. However, animal studies suggest that selective inner hair cell loss in the cochlea could be the cause of hearing defects in TRMA [Liberman et al 2006]. Non-type I diabetes mellitus has appeared before school age in many, but not all, individuals. High-dose thiamine supplementation may delay onset of diabetes mellitus, and high-dose thiamine invariably improves the hematologic picture. Whether hearing can be improved or hearing loss delayed by high-dose thiamine has been difficult to study, and the answer is unclear; however, some reports have found that thiamine did not prevent the development of hearing loss in infants with TRMA [Borgna-Pignatti et al 2009, Akın et al 2011]. In addition to the triad of clinical features that characterize TRMA, other findings have been observed, each in only a subset of individuals:Optic atrophy, when commented upon in case reports, seems common. Abnormal appearance of the retina and functional retinal dystrophy have been reported [Meire et al 2000, Kipioti et al 2003, Lagarde et al 2004]. The kindred reported as having DIDMOAD (diabetes insipidus, diabetes mellitus, optic atrophy, and deafness) by Borgna-Pignatti et al [1989] has in retrospect been shown by genetic analysis to have TRMA; optic atrophy was not known to be a finding of TRMA at the time (see Differential Diagnosis). Cardiovascular abnormalities, including sudden death, stroke, high-output heart failure, paroxysmal atrial tachycardia, atrial standstill, and congenital heart defects have been reported [Villa et al 2000, Lorber et al 2003, Lagarde et al 2004, Aycan et al 2011]. Significant neurologic deficit including stroke and focal or generalized epilepsy have been reported in 27% of individuals with TRMA [Shaw-Smith et al 2012].

Genotype-Phenotype CorrelationsNo genotype-phenotype correlation has been discerned. Homozygous null mutations in SLC19A2, regardless of position within the gene sequence, result in TRMA, as do all of the reported missense mutations (however, bias of ascertainment may have occurred).

Differential DiagnosisTable 2. Thiamine-Responsive Dysfunction Syndrome: OMIM Phenotypic Series View in own windowPhenotypePhenotype
MIM NumberGene/LocusGene/Locus
MIM NumberMicrocephaly, Amish type 607196 SLC25A19, DNC, MUP1, MCPHA, THMD3, THMD4 606521 Thiamine-responsive megaloblastic anemia syndrome 249270 SLC19A2, THTR1, TRMA, THMD1 603941 Thiamine metabolism dysfunction syndrome 2 (biotin- or thiamine-responsive encephalopathy type 2) 607483 SLC19A3, THMD2, BBGD 606152 Thiamine metabolism dysfunction syndrome 4 (progressive polyneuropathy type) 613710 SLC25A19, DNC, MUP1, MCPHA, THMD3, THMD4 606521 Thiamine metabolism dysfunction syndrome 5 (episodic encephalopathy type) 614458 TPK1, THMD5 606370 Data from Online Mendelian Inheritance in ManThe combination of megaloblastic red cell changes and ringed sideroblasts in individuals with thiamine-responsive megaloblastic anemia syndrome (TRMA) is unique among anemias influenced by metabolic or nutritional causes. Among acquired anemias, this combination is most suggestive of myelodysplastic syndromes in which megaloblastosis and sideroblasts are often noted. TRMA should not be confused with myelodysplastic disorders of premalignant potential.Phenotypic overlap exists between TRMA and Wolfram syndrome, or DIDMOAD (diabetes insipidus, diabetes mellitus, optic atrophy, and deafness), including diabetes mellitus, optic atrophy, and deafness. Notably missing in Wolfram syndrome is megaloblastic anemia and thiamine responsiveness. Wolfram syndrome; is caused by mutations in WFS1. The encoded protein is a novel transmembrane glycoprotein of 100 kd located in the endoplasmic reticulum, where it is thought to play a role in membrane trafficking, protein processing, or regulation of calcium homeostasis. The combination of diabetes mellitus and deafness calls to mind mitochondrial disorders [Fischel-Ghodsian 1999], but the macrocytic anemia, megaloblastic bone marrow, and response to thiamine distinguish TRMA from these disorders (see Mitochondrial Disorders Overview). Inheritance in TRMA is unequivocally autosomal recessive, which sets it apart from disorders with apparent maternal transmission and mitochondrial inheritance.Note to clinicians: For a patient-specific ‘simultaneous consult’ related to this disorder, go to , an interactive diagnostic decision support software tool that provides differential diagnoses based on patient findings (registration or institutional access required).

ManagementEvaluations Following Initial DiagnosisTo establish the extent of disease in an individual diagnosed with thiamine-responsive megaloblastic anemia syndrome (TRMA), the following evaluations are recommended:Peripheral blood count and bone marrow analysis for evidence of megaloblastic anemia Serum folate concentration, serum vitamin B₁₂ concentration, and serum iron studies to exclude other entities Fasting serum glucose concentration, oral glucose tolerance test (OGTT), and urinalysis to diagnose diabetes mellitus Hearing test Ophthalmologic evaluation Cardiac evaluation, including echocardiography Medical genetics consultationTreatment of ManifestationsManagement focuses on lifelong use of pharmacologic doses (25-75 mg per day) of thiamine (vitamin B₁) in affected individuals. High-dose thiamine may ameliorate diabetes mellitus in the short term and perhaps even for decades [Valerio et al 1998]. Whether treatment with thiamine from birth, or even prenatally, could reduce the hearing defect is a matter of conjecture. See Deafness and Hereditary Hearing Loss Overview.Prevention of Primary ManifestationsEarly administration of pharmacologic doses of oral thiamine (vitamin B₁) (25-75 mg/day compared to US RDA of 1.5 mg/day) ameliorates the megaloblastic anemia and the diabetes mellitus. It may prevent further deterioration of hearing function.SurveillanceThe following are recommended to monitor the efficacy of the oral thiamine therapy as well as disease progression and should be performed at least yearly:Hematologic tests: CBC, reticulocyte count Assessment for glucose intolerance: fasting serum glucose concentration, OGTT, urinalysis Hearing test Ophthalmologic evaluation Cardiac evaluation Pregnancy ManagementWhile there are no published studies evaluating pregnancy outcome in affected women, good diabetic control prior to and during pregnancy is recommended.Evaluation of Relatives at RiskSupplementation with pharmacologic doses of thiamine (vitamin B₁) (25-75 mg/day compared to US RDA of 1.5 mg/day) is recommended as early as possible for at-risk sibs until their genetic status can be determined.See Genetic Counseling for issues related to testing of at-risk relatives for genetic counseling purposes.Therapies Under InvestigationIf the insulin release defect is intrinsic to the SLC19A2-defective islet cells, one could expect islet cell transplantation and cochlear transplantation to be potential cures for TRMA-related diabetes and hearing loss, respectively. Search ClinicalTrials.gov for access to information on clinical studies for a wide range of diseases and conditions.OtherStem cell transplantation could potentially be an effective treatment for the marrow findings of TRMA; however, vitamin therapy alone is satisfactory, as the risk outweighs the benefit of possible restoration of the transporter to the marrow.

Molecular GeneticsInformation in the Molecular Genetics and OMIM tables may differ from that elsewhere in the GeneReview: tables may contain more recent information. —ED.Table A. Thiamine-Responsive Megaloblastic Anemia Syndrome: Genes and DatabasesView in own windowGene SymbolChromosomal LocusProtein NameLocus SpecificHGMDSLC19A21q24​.2Thiamine transporter 1SLC19A2 homepage - Mendelian genesSLC19A2Data are compiled from the following standard references: gene symbol from HGNC; chromosomal locus, locus name, critical region, complementation group from OMIM; protein name from UniProt. For a description of databases (Locus Specific, HGMD) to which links are provided, click here.Table B. OMIM Entries for Thiamine-Responsive Megaloblastic Anemia Syndrome (View All in OMIM) View in own window 249270THIAMINE-RESPONSIVE MEGALOBLASTIC ANEMIA SYNDROME; TRMA 603941SOLUTE CARRIER FAMILY 19 (THIAMINE TRANSPORTER), MEMBER 2; SLC19A2Molecular Genetic PathogenesisDefect of a high-affinity thiamine transporter, SLC19A2, causes TRMA; however, it is still unclear how the absence of SLC19A2 expression results in the seemingly divergent disorders of megaloblastic anemia, diabetes mellitus, and deafness. Biochemical studies on fibroblasts or erythrocytes from individuals with TRMA showed that absence of the high-affinity component of thiamine transport results in low intracellular thiamine concentrations [Rindi et al 1992, Stagg et al 1999]. Defective RNA ribose synthesis caused by intracellular thiamine deficiency is thought to be the cause of megaloblastic changes in TRMA [Boros et al 2003]. Slc19a2 knockout mouse models have been developed [Oishi et al 2002, Fleming et al 2003]; the animal models manifest megaloblastic changes, diabetes mellitus, and sensorineural deafness, the main features of TRMA, when dietary thiamine levels are decreased [Oishi et al 2002]. While the mechanism of megaloblastic changes is still to be elucidated, these models showed defects in insulin secretion and selective loss of inner hair cells in cochlea [Oishi et al 2002, Liberman et al 2006].Questions regarding TRMA disease pathogenesis that still require explanation include why individuals with TRMA do not have manifestations seen in dietary thiamine deficiency [Mandel et al 1984, Poggi et al 1984, Abboud et al 1985] and why the findings in TRMA are organ specific. Recent studies showed that a second high-affinity thiamine transporter, encoded by SLC19A3, has major roles in intestinal thiamine uptake using mouse models, accounting for the absence of overt thiamine deficiency in persons with TRMA [Reidling et al 2010]. In support of this, two Japanese brothers with a Wernicke’s-like encephalopathy were reported to have compound heterozygous mutations in SLC19A3 [Kono et al 2009]. In addition, the difference in distribution of expression of the two thiamine transporters is critical in TRMA: in pancreatic endocrine cells, the expression of SLC19A2 is much higher than that of SLC19A3 and TRMA-associated SLC19A2 mutants disrupt thiamine uptake significantly [Mee et al 2009]. Similarly, it is hypothesized that in TRMA, the other affected tissues (namely, bone marrow and cochlea) do not express or minimally express SLC19A3 [Eudy et al 2000, Rajgopal et al 2001].Normal allelic variants. SLC19A2 is encoded by six exons spanning approximately 22.5 kb. Pathologic allelic variants. SLC19A2 mutations are distributed throughout the gene with no apparent clustering or mutation hot spots. The majority of SLC19A2 mutations known to date are predicted to be null for protein because of nonsense or frameshift mutations. Ten missense mutations have been reported. Such mutations would likely severely disrupt the folding and membrane targeting of the transporter. Consistently, Balamurugan & Said [2002] showed that introducing several of these mutations into transfected HeLa cells resulted in impaired thiamine uptake [Balamurugan & Said 2002]. Normal gene product. The 497-amino acid protein, the high-affinity thiamine transporter 1, is predicted to have 12 transmembrane domains. Abnormal gene product. Mutations result in either a truncated protein from a premature stop codon or aberrantly folded protein caused by missense mutations in transmembrane domains.