Deletions in the MAL gene result in loss of Mal protein, defining the rare inherited AnWj-negative blood group phenotype

• Published in: Blood 2024-12, Vol.144 (26), p.2735-2747
• Main Authors: Tilley, Louise A., Karamatic Crew, Vanja, Mankelow, Tosti J., AlSubhi, Samah A., Jones, Benjamin, Borowski, Abigail, Yahalom, Vered, Finkel, Lilach, Singleton, Belinda K., Walser, Piers J., Toye, Ashley M., Satchwell, Timothy J., Thornton, Nicole M.
• Format: Article
• Language: English
• Subjects: Blood Group Antigens - genetics; Erythrocyte Membrane - genetics; Erythrocyte Membrane - metabolism; Exome Sequencing; Female; Gene Deletion; Humans; Male; Myelin and Lymphocyte-Associated Proteolipid Proteins - genetics; Phenotype; Sequence Deletion
• ISSN: 0006-4971; 1528-0020; 1528-0020
• DOI: 10.1182/blood.2024025099

•The inherited AnWj-negative blood group phenotype is caused by homozygosity for a deletion in MAL, encoding Mal protein.•Mal protein is expressed on red blood cell membranes of AnWj-positive, but not AnWj-negative, individuals. [Display omitted] The genetic background of the high prevalence red blood cell antigen AnWj has remained unresolved since its identification in 1972, despite reported associations with both CD44 and Smyd1 histone methyltransferase. Development of anti-AnWj, which may be clinically significant, is usually due to transient suppression of antigen expression, but a small number of individuals with persistent, autosomally recessive inherited AnWj-negative phenotype have been reported. Whole-exome sequencing of individuals with the rare inherited AnWj-negative phenotype revealed no shared mutations in CD44H or SMYD1; instead, we discovered homozygosity for the same large exonic deletion in MAL, which was confirmed in additional unrelated AnWj-negative individuals. MAL encodes an integral multipass membrane proteolipid, myelin and lymphocyte protein (Mal), which has been reported to have essential roles in cell transport and membrane stability. AnWj-positive individuals were shown to express full-length Mal on their red cell membranes, which was not present on the membranes of AnWj-negative individuals, regardless of whether from an inherited or suppression background. Furthermore, binding of anti-AnWj was able to inhibit binding of anti-Mal to AnWj-positive red cells, demonstrating the antibodies bind to the same molecule. Overexpression of Mal in an erythroid cell line resulted in the expression of AnWj antigen, regardless of the presence or absence of CD44, demonstrating that Mal is both necessary and sufficient for AnWj expression. Our data resolve the genetic background of the inherited AnWj-negative phenotype, forming the basis of a new blood group system, further reducing the number of remaining unsolved blood group antigens. Human blood groups are inherited polymorphisms of erythrocyte surface proteins that can, via transfusion or pregnancy, stimulate the production of alloantibodies. Tilley and colleagues reveal their discovery of the gene underlying the inherited AnWj-negative red cell phenotype, identifying a 6646 base pair deletion at the MAL locus in 8 AnWj-negative individuals but not in AnWj-positive family members. The authors also identified that not all AnWj-negative phenotypes were inherited and that loss of expression