Identification and protein modelling analysis of a homozygous <i>FUT1*01W.09</i> variant in a Thai patient with the para‐Bombay phenotype

OBJECTIVE: To characterise a Thai patient with a para-Bombay phenotype associated with homozygosity for a variant FUT1 allele and explore the underlying mechanism of reduced enzymatic activity. BACKGROUND: The para-Bombay phenotype is a rare red blood cell phenotype characterised by absent or markedly reduced ABH antigens on erythrocytes, with preserved expression in secretions, typically resulting from FUT1 mutations affecting α1,2-L-fucosyltransferase 1 (α1,2FucT1). MATERIALS AND METHODS: Serological testing was performed using standard methods. ABO, FUT1 and FUT2 genotypes were determined by whole-exome and Sanger sequencing. Structural modelling of the FUT1 variant was conducted using AlphaFold3 and ChimeraX, and functional effects were predicted using ProtVar. RESULTS: phenotype with anti-HI and autoantibodies. Genotyping revealed homozygosity for ABO*B.01, FUT1 c.658C>T (p.Arg220Cys; rs574691621; FUT1*01W.09), and FUT2 c.390C>T (a silent variant, p.Asn130). Structural modelling suggested that the p.Cys220 substitution does not alter overall protein conformation or stability but may impair substrate binding and local intermolecular interactions. CONCLUSION: phenotype associated with homozygosity for the weakened FUT1*01W.09 allele. In silico modelling predicted that the variant impairs α1,2FucT1 activity and substrate binding without compromising protein stability, supporting safe transfusion practice.