A silenced allele in the Colton blood group system

Background The antigens of the Colton blood group system, Coa and Cob, are encoded by a single gene that produces the aquaporin-1 (AQP1) protein, a water channel-forming protein, and are characterized by a single nucleotide polymorphism (SNP). A healthy Caucasoid blood donor originally typed as Co(a-b-) with commercial anti-Cob typed Co(a-b+) when retested with another anti-Cob. Retyped with two different molecular biology methods, the sample came out Coa/Cob. With the aim of understanding these discrepancies, serological, cytometric and molecular biology tests were carried out. Methods Absorption/elution studies with propositus red cells and controls were performed. The region spanning exon 1 to exon 4 of the Colton gene was sequenced, and flow cytometry analyses were carried out. Results Absorption/elution studies showed the absence of Coa and a weak expression of Cob. DNA sequencing confirmed a CT heterozygosity at nucleotide position 134 (i.e. Coa/Cob), and an additional heterozygous CT was found at position 112. The presence of the Cob allele that encodes for the Cob antigen was confirmed. The new allele has the base cytosine at nucleotide 134 (Coa), in cis with the new nucleotide 112T. The nucleotide substitution 112C > T causes a missense mutation leading to an amino acid change from proline (CCG) to serine (TCG) at codon 38. Conclusion The substitution found at codon 38 results in a modified AQP1 protein which explains the Co(a-b+) phenotype and possibly the weak expression of Cob.