Background: More than 20 years ago, two probands were described whose red blood cells (RBCs) typed Sc:1,-2,3. Their serum samples contained alloantibodies reactive with all RBCs tested except those of the Sc:-1,-2,-3 phenotype. Cloning of the Scianna gene allowed us to determine the molecular bases of these samples.
Study design and methods: In a collaborative effort, the two probands' samples and also two Sc:-1,-2,-3 samples were obtained from frozen storage. All 11 SC (ERMAP) exons and their flanking regions were sequenced.
Results: The two probands with antibodies to Scianna-related antigens were homozygous, respectively, for an ERMAP(R81Q) allele caused by a G to A substitution at nucleotide 242 in the ERMAP gene and for an ERMAP(H26Y,G35S) allele, in which the G35S substitution was caused by a G to A substitution at nucleotide 103. Two patients with the Sc:-1,-2,-3 phenotype both carried ERMAP(R332X) alleles caused by a C to T substitution at nucleotide 994 that differed at one nucleotide position in the noncoding region of exon 11. In eight samples carrying orphan low-prevalence antigens, no ERMAP variants were detected that could be implicated in Scianna antigen expression.
Conclusion: SCER and SCAN expanded the Scianna blood group system to seven antigens, have been assigned the ISBT numbers 013.006 (Sc6) and 013.007 (Sc7), and were associated with ERMAP(R81Q) and ERMAP(G35S) proteins, respectively. ERMAP(R332X) is a second molecular basis for the Sc(null) phenotype. The eight low-prevalence antigens By, To(a), Pt(a), Re(a), Je(a), Li(a), SARA, and Sk(a) do not belong to the Scianna blood group system.