MNV-aware molecular characterization of a rare homozygous TTPA complex allele in ataxia with vitamin E deficiency.

Abstract

Ataxia with vitamin E deficiency (AVED) is a rare autosomal-recessive neurological disorder caused by biallelic pathogenic variants in TTPA. Early vitamin E supplementation may prevent or limit irreversible neurological damage, but diagnosis is often delayed. Multi-nucleotide variants (MNVs) in TTPA have rarely been described and may be misinterpreted when adjacent substitutions are evaluated independently. We investigated a 34-year-old woman with childhood-onset progressive ataxia using clinical, biochemical, neuroimaging, and electrophysiological assessments. Serum vitamin E levels were measured longitudinally during supplementation. Whole-exome sequencing, read-level inspection, and Sanger sequencing were used to identify and confirm a homozygous TTPA complex allele, NM_000370.3:c. 296G > A;299 A > C, predicted to result in NP_000361.1:p. Gly99_Tyr100delinsAspSer, and to assess familial segregation. Population-database review, in silico prediction, and exploratory structure-based analysis were performed to evaluate its potential clinical relevance. The patient had markedly reduced baseline serum vitamin E levels of 0.8 µg/mL, which increased to 7.5 µg/mL after 12 months of supplementation. This biochemical correction was temporally accompanied by qualitatively observed improvements in gait stability, coordination, speech, and fine motor performance. Read-level analysis supported the presence of both substitutions on the same allele, and Sanger sequencing confirmed the homozygous complex allele in the patient and heterozygous carrier status in both parents. The affected residues are conserved and located within the CRAL-TRIO domain of α-tocopherol transfer protein. Exploratory structure-based analysis suggested altered local residue interactions; however, no functional assay was performed, and effects on protein stability, α-tocopherol binding, or transfer could not be established. This report expands the molecular spectrum of AVED by describing a homozygous TTPA complex allele and highlights the importance of MNV-aware interpretation of closely spaced substitutions. Vitamin E supplementation resulted in biochemical correction and was accompanied by possible partial clinical improvement despite initiation in adulthood. Functional studies are required to determine the precise effect of this allele on α-tocopherol transfer protein function.