Three novel Er blood group system alleles and insights from protein modeling

The Er blood group system was recently shown to be defined by PIEZO1. The system consists of high prevalence antigens Er^a, Er3, ERSA, and ERAMA; and low prevalence antigen Er^b. Er^a/Er^b are antithetical with Er(a-b+) defined by the ER*B allele [c.7180G>A p.(Gly2394Ser)]. A nonsense variant c.5289C>G p.(Tyr1763*) is associated with a predicted Er_null phenotype, and a missense variant c.7174G>A p.(Glu2392Lys) in close proximity to p.2394 causes loss of both Er^a and Er^b expression.

We investigated PIEZO1 in four Er(a-) individuals who presented with anti-Er^a. Whole genome sequencing (WGS) and Sanger sequencing were performed. The location and structural differences of predicted protein changes were visualized using the predicted 3-D structure of Piezo1 created using AlphaFold2.

One individual was homozygous for the reported ER*B. A second had a novel heterozygous nonsense variant c.3331C>T p.(Gln1111*), but a second allelic variant was not found. In the remaining two individuals, two different heterozygous novel missense variants, c.7184C>T p.(Ala2395Val) or c.7195G>A p.(Gly2399Ser), were in trans to the reported c.7180G>A variant, ER*B. AlphaFold2 protein modeling showed that each of the missense variants is predicted to encode an altered structural conformation near Er^a and Er^b.

Investigation of archived samples resulted in the identification of three novel PIEZO1 alleles including a predicted Er_null and two missense variants. Structural modeling suggests that the missense changes potentially alter Er^a/Er^b epitope expression with p.2399Ser resulting in a small increase in the negative electrostatic potential.