Lack of the multidrug transporter MRP4/ABCC4 defines the PEL-negative blood group and impairs platelet aggregation.

The rare PEL-negative phenotype is one of the last blood groups with an unknown genetic basis. By combining whole-exome sequencing and comparative global proteomic investigations, we found a large deletion in the ABCC4/MRP4 gene encoding an ATP-binding cassette (ABC) transporter in PEL-negative individuals. The loss of PEL expression on ABCC4-CRISPR-Cas9 K562 cells and its overexpression in ABCC4-transfected cells provided evidence that ABCC4 is the gene underlying the PEL blood group antigen. Although ABCC4 is an important cyclic nucleotide exporter, red blood cells from ABCC4null/PEL-negative individuals exhibited a normal guanosine 3',5'-cyclic monophosphate level, suggesting a compensatory mechanism by other erythroid ABC transporters. Interestingly, PEL-negative individuals showed an impaired platelet aggregation, confirming a role for ABCC4 in platelet function. Finally, we showed that loss-of-function mutations in the ABCC4 gene, associated with leukemia outcome, altered the expression of the PEL antigen. In addition to ABCC4 genotyping, PEL phenotyping could open a new way toward drug dose adjustment for leukemia treatment.

A LU:-16 individual with antibodies.

CONCLUSIONS: Antibodies against Lutheran blood group antigens have been observed during first-time pregnancy. Samples from a woman of African descent were tested in our immunohematology laboratory on several occasions since 2001. Her samples were phenotyped as Lu(a+b-), and anti-Lub was suspected but not identified. She was asked to make autologous donations in preparation for her delivery, which she did. In 2010, two antibodies were identified: anti-Lea and -Lub. Six years later, a third investigation was requested. This time, an antibody directed at a high-prevalence Lutheran antigen was found in addition to the anti-Lea and -Lub previously observed. Her serum was compatible with three out of five Lu(a-b-) reagent red blood cells (RBCs). One of the incompatible Lu(a-b-) reagent RBCs was known to be In(Lu) (KLF1 mutation). The genetic background of the other reagent RBC was unknown. The LU cDNA sequence analysis revealed the presence of the c.230G>A (Lua), c.679C>T (LU:-16), and a silent polymorphism c.1227G>T. Anti-Lu16 was highly suspected. This would be the fifth case of LU:-16 with antibodies reported, all within women of African heritage with the Lu(a+b-) phenotype. Hemolytic disease of the fetus and newborn was not noted in these cases.

Weak D type 42 cases found in individuals of European descent.

Patient samples were referred to our immunohematology reference laboratory to investigate the presence of a weak D antigen. In the last 3 years, 26 samples were received. Serology and molecular analyses were performed to identify the weak D variant. RHD mRNA from all patients was reverse transcribed, and cDNA was sequenced. The results were compared with a normal RHD sequence to identify the polymorphisms causing the weak D phenotype. Five different already known RHD variants were observed: weak D type 1 (5 individuals), weak D type 2 (1 individual), weak D type 42 (17 individuals), weak D type 45 (1 individual), and partial D DNB (2 individuals). Surprisingly, weak D type 42 was prevalent in our population, whereas weak D type 1, 2, and 3 are the most prevalent variants elsewhere. Anti-D was found in six cases of weak D type 42. The higher prevalence of weak D type 42 could be the result of a founder effect. Additional studies are needed to estimate the frequency of this variant in the general population.