Thiamine-responsive megaloblastic anemia syndrome represents a rare autosomal recessive condition originating from mutations in the SLC19A2 gene. It is characterized by a classical triad of megaloblastic anemia, insulin-dependent diabetes mellitus, and sensorineural hearing loss. We present the case of a woman diagnosed with thiamine-responsive megaloblastic anemia, with no history of consanguinity, in whom genetic testing revealed novel SLC19A2 mutations and an unreported clinical manifestation. This case describes an 18-year-old mestizo female patient who presented with a medical history of diabetes starting in infancy, bilateral sensorineural hearing loss, and megaloblastic anemia. The diagnosis of thiamine-responsive megaloblastic anemia was confirmed by genetic testing, which detected compound heterozygous mutations in the SLC19A2 gene that included a pathogenic frameshift mutation (c.620_624dup; p.Pro209Phefs*21) and a missense variant (c.170 T > C; p.Leu57Pro). The patient also experienced thrombotic events, including deep vein and mesenteric thrombosis, previously unreported findings in thiamine-responsive megaloblastic anemia. High-dose thiamine treatment resulted in improved hematologic and glycemic control. This case broadens the genetic and clinical spectrum of thiamine-responsive megaloblastic anemia, highlighting the importance of genetic testing in young patients with the classic triad and showing that early thiamine therapy can markedly improve outcomes.

Molecular diagnostic rates for hereditary hearing loss vary by genetic ancestry, highlighting the importance of population-specific studies. In Pakistan, where consanguineous marriages are prevalent, genetic research has identified many autosomal recessive genes, advancing understanding of rare and novel hearing loss mechanisms. This study aimed to identify pathogenic genetic variants in 31 families from Azad Kashmir, Pakistan, presenting non-syndromic hearing loss. We conducted exome sequencing and bioinformatics analysis, and targeted gene sequencing on 31 Pakistani families with hearing loss. We identified ten pathogenic, three likely pathogenic variants, and one variant of uncertain significance, comprising six nonsense, four missense, three frameshift, and one deep intronic variant, across ten hearing loss-associated genes (MYO15A, GJB2, SLC26A4, TMC1, HGF, TMIE, SLC19A2, KCNE1, ILDR, PCDH15 and MYO6) in 25 families. The overall diagnostic rate, including families with pathogenic and likely pathogenic variants, was 77.4%. GJB2 was the most frequently affected gene, identified in seven families. Thirteen out of 14 identified variants were homozygous. Notably, we identified two novel variants: MYO15A (NM_016239.4, DFNB3) c.870C>G, p.(Tyr290*) and MYO6 (NM_016239.4, DFNB37) c.3465del, p.(Pro1156Leufs*9). Additionally, we identified c.10475dupA, p.(Leu3493Alafs*25) in MYO15A (NM_016239.4, DFNB3) and c.617T>A, p.(Leu206*) in SLC26A4 (NM_000441.2, DFNB4), previously documented in ClinVar but unpublished. We also propose SLC19A2 as a candidate gene presenting as non-syndromic hearing loss, despite its association with thiamine-responsive megaloblastic anemia syndrome. Our work expands the genotypic and phenotypic spectrum of hearing loss by emphasizing the importance of investigating under-represented groups to identify unique genetic variants and clinical characteristics. Such efforts deepen understanding of genetic diversity in under-represented populations to improve diagnosis and treatment strategies.

Thiamine-Responsive Megaloblastic Anemia (TRMA) syndrome, caused by biallelic variants in the SLC19A2 gene, typically presents with a triad of megaloblastic anemia, diabetes mellitus, and sensorineural hearing loss. This study aims to determine the genetic etiology and clinical phenotype of a patient who presented with severe sensorineural hearing loss as the initial symptom, and to expand our understanding of the SLC19A2 variant spectrum. Proband-only whole-exome sequencing was performed to screen the candidate variants, which were subsequently validated by Sanger sequencing within the family. cDNA sequencing based on RT-PCR and TA cloning analysis was used to determine the effect of splicing variants on mRNA processing of SLC19A2 gene. Detailed clinical features were evaluated by a diagnostic hearing test, laboratory and imaging examination. A 2-year-5-month-old Chinese girl was diagnosed with diabetes mellitus and severe sensorineural hearing loss, without abnormal hemoglobin. DNA sequencing revealed a novel compound heterozygous variant of c.808-1G > A and c.1228C > T (p.Gln410*) in the SLC19A2 gene. Both variants were previously unreported. The c.808-1G > A splicing variant is located in intron 2 of SLC19A2, and is predicted to cause exon 3 skipping. The cDNA experiment confirmed this biological event, further indicating that the splicing variant can cause amino acid frameshift alteration (p.Glu270Valfs*10) in SLC19A2. We report a patient with TRMA syndrome (without anemia) caused by a novel compound heterozygous variant in SLC19A2 gene. This study suggests that the possibility of TRMA syndrome should be considered when encountering patients with early-onset severe sensorineural hearing loss in clinical practice. Level 4.

Thiamine-responsive megaloblastic anaemia syndrome (TRMA) is a rare genetic disease caused by mutations in the SLC19A2 gene that encodes thiamine transporter 1 (THTR-1). The common manifestations are diabetes, anaemia, and deafness. The pathogenic mechanism has not yet been clarified. Rat pancreatic islet tumour cells INS.1 were used to construct cell lines stably overexpressing wild-type SLC19A2 and SLC19A2 (c.1409insT) mutants. The mRNA and protein expressions of THTR-1 and endoplasmic reticulum stress (ERS)-associated factors were detected by real-time fluorescence quantitative polymerase chain reaction (PCR) and western blot methods, respectively. Flow cytometry and cell counting kit-8 were used to analyse the effects of SLC19A2 (c.1409insT) mutation on cell apoptosis and proliferation, respectively. 4-Phenylbutyric acid (4-PBA), an ERS inhibitor, was administered to SLC19A2 (c.1409insT)-mutated INS.1 cells, and then the mRNA and protein expressions of ERS-related factors in cells were detected. Mutations in the SLC19A2 (c.1409insT) promote apoptosis and inhibit cell proliferation, thereby upregulating the mRNA and protein levels of ERS-associated factors glucose-regulated protein 78, protein kinase R-like endoplasmic reticulum kinase, C/EBP homologous protein, and activating transcription factor 4. 4-PBA could inhibit ERS caused by SLC19A2 (c.1409insT) mutations, downregulate mRNA and protein expression levels of GRP78, CHOP, and phosphorylated eukaryotic initiation factor 2α, and protect pancreatic islet β-cells. THTR-1 deficiency triggers diabetes in TRMA patients through ERS, and 4-PBA protects pancreatic islet β-cells by inhibiting ERS, which provides new ideas and intervention targets for the prevention and treatment of TRMA and diabetes.