Missense Mutations in the ABCB6 Transporter Cause Dominant Familial Pseudohyperkalemia

Abstract 3184

Isolated Familial Pseudohyperkalemia (FP) is a dominant red cell trait characterized by cold-induced slow ‘passive leak’ of red cell K⁺ into plasma, first described in a large Scottish family from Edinburgh (Stewart GW, et al., 1979). Although in freshly obtained blood samples plasma [K⁺] was normal, it was increased when measured in blood stored at or below room temperature. This trait was unaccompanied by clinical symptoms or signs except for mild abnormalities of red cell shape. FP Lille was later described in a large Flemish kindred with morphologically normal red cells (Dagher G, et al., 1989; Vantyghem MC, et al., 1991). In this family, red cell K⁺ efflux measured in the presence of ouabain and bumetanide was normal at 37°C, but greatly increased at 22°C and 9°C. FP Lille mapped to 2q35-q36 (Carella M, et al., 2004), whereas FP Edinburgh mapped to 16q23-qter (Iolascon A, et al., 1999). Subsequently, asymptomatic cases FP Chiswick and FP Falkirk with remarkable increased MCV were reported (Haines PG, et al., 2001).

Functional gene mapping and sequencing analysis of the candidate genes within the 2q35-q36 critical interval in three multigenerational FP families with 20 affected individuals identified two novel heterozygous missense mutations in the ABCB6 gene that cosegregated with disease phenotype. The two genomic substitutions altered two adjacent nucleotides within codon 375 of ABCB6, a porphyrin transporter that in erythrocyte membranes bears the Langereis blood group antigen system (Krishnamurthy PC, et al., 2006; Helias V, et al., 2012). Structural modeling predicts subtle changes in protein structure associated with either mutation.

ABCB6 mRNA and protein levels increased during erythroid differentiation of CD34+ erythroid precursors (at 7 and 14 days of EPO induced differentiation), and of HEL and K562 erythroleukemia cells. However, the ABCB6 R375Q mutation altered neither levels of ABCB6 mRNA or protein, nor protein localization in mature erythrocytes or erythroid precursor cells. These data strongly suggest that missense mutations in residue 375 of the ABCB6 polypeptide either mediate the cold-induced K⁺ leak of chromosome 2-linked FP, or activate an independent, cold-induced cation permeability pathway of the red cell.