SCAR: The high-prevalence antigen 013.008 in the Scianna blood group system

Two new Scianna variants causing loss of high prevalence antigens: ERMAP model and 3D analysis of the antigens

BACKGROUND

Scianna (Sc) antigens, seven high and two of low prevalence, are expressed on erythrocyte membrane-associated protein (ERMAP). We investigated SC (ERMAP) in individuals who made antibodies to high prevalence Scianna antigens, and propose a 3D model for ERMAP to precisely localize the residues associated with the known antigens.

METHODS

Serological testing and DNA sequencing was performed by standard methods. A 3D structural model was built using a multi-template homology approach. Protein structures representing missense variants associated with the loss or gain of an antigen were generated. Residue accessibility and intraprotein interactions were compared with the wild-type protein.

RESULTS

Two new SC alleles, one with c.349C > T (p.Arg117Cys) in a woman from South India with anti-Sc3 in her plasma, and a c.217_219delinsTGT (p.Arg73Cys) in an African-American woman with an antibody to a new high prevalence antigen, termed SCAC, were identified. Six structural templates were used to model ERMAP. 3D analysis showed that residues key for Scianna antigen expression were all exposed at the surface of the extracellular domain. The p.Arg117Cys change was predicted to abolish interactions between residues 93 and 117, with no compensating interactions.

CONCLUSION

We confirm the extracellular location of Scianna residues responsible for antigen expression which predicts direct accessibility to antibodies. Loss of intraprotein interactions appear to be responsible for a Sc null and production of anti-Sc3 with p.117Cys, SC*01 N.03, and for loss of a high prevalence antigen with p.73Cys, termed SCAC for Sc Arg to Cys. Comparative modeling aids our understanding of new alleles and Scianna antigen expression.

International Society of Blood Transfusion Working Party on Red Cell Immunogenetics and Blood Group Terminology Report of Basel and three virtual business meetings: Update on blood group systems

BACKGROUND AND OBJECTIVES

Under the ISBT, the Working Party (WP) for Red Cell Immunogenetics and Blood Group Terminology is charged with ratifying blood group systems, antigens and alleles. This report presents the outcomes from four WP business meetings, one located in Basel in 2019 and three held as virtual meetings during the COVID-19 pandemic in 2020 and 2021.

MATERIALS AND METHODS

As in previous meetings, matters pertaining to blood group antigen nomenclature were discussed. New blood group systems and antigens were approved and named according to the serologic, genetic, biochemical and cell biological evidence presented.

RESULTS

Seven new blood group systems, KANNO (defined numerically as ISBT 037), SID (038), CTL2 (039), PEL (040), MAM (041), EMM (042) and ABCC1 (043) were ratified. Two (039 and 043) were de novo discoveries, and the remainder comprised reported antigens where the causal genes were previously unknown. A further 15 blood group antigens were added to the existing blood group systems: MNS (002), RH (004), LU (005), DI (010), SC (013), GE (020), KN (022), JMH (026) and RHAG (030).

CONCLUSION

The ISBT now recognizes 378 antigens, of which 345 are clustered within 43 blood group systems while 33 still have an unknown genetic basis. The ongoing discovery of new blood group systems and antigens underscores the diverse and complex biology of the red cell membrane. The WP continues to update the blood group antigen tables and the allele nomenclature tables. These can be found on the ISBT website (http://www.isbtweb.org/working-parties/red-cell-immunogenetics-and-blood-group-terminology/).

A mild case of hemolytic disease of the fetus and newborn due to anti-Sc2

We report the case of a newborn girl with jaundice due to increased indirect bilirubin with a positive direct antiglobulin test (DAT) and compensated hemolysis. The result of the newborn's DAT was discrepant with the negative result of the mother's indirect antiglobulin test. The multiparous mother had a previous history of fetal hydrops miscarriage, with no known cause, and no record of the cause was found at the hospital where she was treated. After referring samples from the mother and newborn to a reference laboratory, the rare alloanti-Sc2 was identified in the mother's plasma and in the newborn's eluate. HEA BeadChip genotyping of the newborn's DNA sample predicted the SC:1,2 phenotype.

We report the case of a newborn girl with jaundice due to increased indirect bilirubin with a positive direct antiglobulin test (DAT) and compensated hemolysis. The result of the newborn’s DAT was discrepant with the negative result of the mother’s indirect antiglobulin test. The multiparous mother had a previous history of fetal hydrops miscarriage, with no known cause, and no record of the cause was found at the hospital where she was treated. After referring samples from the mother and newborn to a reference laboratory, the rare alloanti-Sc2 was identified in the mother’s plasma and in the newborn’s eluate. HEA BeadChip genotyping of the newborn’s DNA sample predicted the SC:1,2 phenotype.