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

Background: 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.

Study design and methods: 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.

Results: 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.

Conclusions: 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.