A DOB allele encoding an amino acid substitution (Phe62Ser) resulting in a Dombrock null phenotype

The gene polymorphisms responsible for the antigens Doa, Dob, Hy, and Joa in the Dombrock (Do) blood group system have been identified. Four different mutations have been reported to cause the Dombrock null [Gy(a-)] phenotype. These include splice mutations, an eight-nucleotide deletion, and insertion of a stop codon. Here a Dombrock null caused by a single-amino-acid substitution in the full-length protein is reported.

DOA and DOB were determined by polymerase chain reaction-restriction fragment length polymorphism, and DO (ART4) exons and flanking regions were sequenced from genomic DNA. Expression analysis was performed by transfection of wild-type and mutant cDNAs into HEK 293T cells followed by flow cytometry and immunoblotting. Homology modeling was used to map the mutation on the protein structure.

The patient's sample carried nt 793G/G, indicating a DOB/DOB background. Exon 2 sequencing showed the sample carried a new mutation, nt 185T>C, causing a Phe62Ser substitution. This variant Do was not expressed on the surface of transfected HEK 293T cells. The mutation maps to a highly conserved FDDQY motif located between the beta1-strand and alpha1-helix near the COOH terminus in the native molecule.

The Dombrock null reported here is due to a single Phe62Ser mutation. The expression data confirmed that 62Ser is responsible for lack of cell surface Do, and protein modeling suggests the mutation disrupts important aromatic side chain interactions between Phe62 and His160. Production of an antibody to a high prevalence Dombrock antigen (anti-Gya) in this patient was consistent with complete absence of Dombrock/ART4 protein.