Familial pseudohyperkalaemia Chiswick: a novel congenital thermotropic variant of K and Na transport across the human red cell membrane

Atypical hereditary spherocytosis phenotype associated with pseudohypokalaemia and a new variant in the band 3 protein

Red blood cell (RBC) membrane disorders are predominantly caused by mutations resulting in decreased RBC deformability and permeability. We present a family in which, the proband and his daughter presented with pseudohypokalaemia. Studies on the temperature dependence of pseudohypokalaemia suggested a maximum decrease in serum potassium when whole blood is stored at 37°C. Routine haematology suggested mild haemolysis with a hereditary spherocytosis phenotype. These two cases present a novel variant in temperature-dependent changes in potassium transport. A new variant was identified in the SLC4A1 gene which codes for band 3 protein (anion exchanger 1) in RBC membrane which may contribute to the phenotype. This is the first report of familial pseudohypokalaemia associated with changes in RBC membrane morphology. The clinical implications of pseudohypokalaemia are that it can lead to inappropriate investigation or treatment. However, many questions remain to be solved and other RBC membrane protein genes should be studied.

The Molecular Basis for Altered Cation Permeability in Hereditary Stomatocytic Human Red Blood Cells

Normal human RBCs have a very low basal permeability (leak) to cations, which is continuously corrected by the Na,K-ATPase. The leak is temperature-dependent, and this temperature dependence has been evaluated in the presence of inhibitors to exclude the activity of the Na,K-ATPase and NaK2Cl transporter. The severity of the RBC cation leak is altered in various conditions, most notably the hereditary stomatocytosis group of conditions. Pedigrees within this group have been classified into distinct phenotypes according to various factors, including the severity and temperature-dependence of the cation leak. As recent breakthroughs have provided more information regarding the molecular basis of hereditary stomatocytosis, it has become clear that these phenotypes elegantly segregate with distinct genetic backgrounds. The cryohydrocytosis phenotype, including South-east Asian Ovalocytosis, results from mutations in SLC4A1, and the very rare condition, stomatin-deficient cryohydrocytosis, is caused by mutations in SLC2A1. Mutations in RHAG cause the very leaky condition over-hydrated stomatocytosis, and mutations in ABCB6 result in familial pseudohyperkalemia. All of the above are large multi-spanning membrane proteins and the mutations may either modify the structure of these proteins, resulting in formation of a cation pore, or otherwise disrupt the membrane to allow unregulated cation movement across the membrane. More recently mutations have been found in two RBC cation channels, PIEZO1 and KCNN4, which result in dehydrated stomatocytosis. These mutations alter the activation and deactivation kinetics of these channels, leading to increased opening and allowing greater cation fluxes than in wild type.

Advances in understanding the pathogenesis of the red cell volume disorders

Genetic defects of erythrocyte transport proteins cause disorders of red blood cell volume that are characterized by abnormal permeability to the cations Na(+) and K(+) and, consequently, by changes in red cell hydration. Clinically, these disorders are associated with chronic haemolytic anaemia of variable severity and significant co-morbidities, such as iron overload. This review provides an overview of recent insights into the molecular basis of this group of rare anaemias involving cation channels and transporters dysfunction. To date, a total of 5 different membrane proteins have been reported to be responsible for volume homeostasis alteration when mutated, 3 of them leading to overhydrated cells (AE1 [also termed SLC4A1], RHAG and GLUT1 [also termed SCL2A1) and 2 others to dehydrated cells (PIEZO1 and the Gardos Channel). These findings are not only of basic scientific interest, but also of direct clinical significance for improving diagnostic procedures and identify potential approaches for novel therapeutic strategies.

Functional characterization of novel ABCB6 mutations and their clinical implications in familial pseudohyperkalemia

Isolated familial pseudohyperkalemia is a dominant red cell trait characterized by cold-induced 'passive leak' of red cell potassium ions into plasma. The causative gene of this condition is ABCB6, which encodes an erythrocyte membrane ABC transporter protein bearing the Langereis blood group antigen system. In this study analyzing three new families, we report the first functional characterization of ABCB6 mutants, including the homozygous mutation V454A, heterozygous mutation R276W, and compound heterozygous mutations R276W and R723Q (in trans). All these mutations are annotated in public databases, suggesting that familial pseudohyperkalemia could be common in the general population. Indeed, we identified variant R276W in one of 327 random blood donors (0.3%). Four weeks' storage of heterozygous R276W blood cells resulted in massive loss of potassium compared to that from healthy control red blood cells. Moreover, measurement of cation flux demonstrated greater loss of potassium or rubidium ions from HEK-293 cells expressing ABCB6 mutants than from cells expressing wild-type ABCB6. The R276W/R723Q mutations elicited greater cellular potassium ion efflux than did the other mutants tested. In conclusion, ABCB6 missense mutations in red blood cells from subjects with familial pseudohyperkalemia show elevated potassium ion efflux. The prevalence of such individuals in the blood donor population is moderate. The fact that storage of blood from these subjects leads to significantly increased levels of potassium in the plasma could have serious clinical implications for neonates and infants receiving large-volume transfusions of whole blood. Genetic tests for familial pseudohyperkalemia could be added to blood donor pre-screening. Further study of ABCB6 function and trafficking could be informative for the study of other pathologies of red blood cell hydration.

Missense mutations in the ABCB6 transporter cause dominant familial pseudohyperkalemia

Familial Pseudohyperkalemia (FP) is a dominant red cell trait characterized by increased serum [K(+)] in whole blood stored at or below room temperature, without additional hematological abnormalities. Functional gene mapping and sequencing analysis of the candidate genes within the 2q35-q36 critical interval identified-in 20 affected individuals among three multigenerational FP families-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. The ABCB6 R375Q mutation did not alter the levels of mRNA or protein, or protein localization in mature erythrocytes or erythroid precursor cells, but it is predicted to modestly alter protein structure. ABCB6 mRNA and protein levels increase during in vitro erythroid differentiation of CD34(+) erythroid precursors and the erythroleukemia cell lines HEL and K562. These data suggest that the two missense mutations in residue 375 of the ABCB6 polypeptide found in affected individuals of families with chromosome 2-linked FP could contribute to the red cell K(+) leak characteristic of this condition.

Cryohydrocytosis: increased activity of cation carriers in red cells from a patient with a band 3 mutation

BACKGROUND

Cryohydrocytosis is an inherited dominant hemolytic anemia characterized by mutations in a transmembrane segment of the anion exchanger (band 3 protein). Transfection experiments performed in Xenopus oocytes suggested that these mutations may convert the anion exchanger into a non-selective cation channel. The present study was performed to characterize so far unexplored ion transport pathways that may render erythrocytes of a single cryohydrocytosis patient cation-leaky.

DESIGN AND METHODS

Cold-induced changes in cell volume were monitored using ektacytometry and density gradient centrifugation. Kinetics, temperature and inhibitor-dependence of the cation and water movements in the cryohydrocytosis patient's erythrocytes were studied using radioactive tracers and flame photometry. Response of the membrane potential of the patient's erythrocyte membrane to the presence of ionophores and blockers of anion and cation channels was assessed.

RESULTS

In the cold, the cryohydrocytosis patient's erythrocytes swelled in KCl-containing, but not in NaCl-containing or KNO(3)-containing media indicating that volume changes were mediated by an anion-coupled cation transporter. In NaCl-containing medium the net HOE-642-sensitive Na(+)/K(+) exchange prevailed, whereas in KCl-containing medium swelling was mediated by a chloride-dependent K(+) uptake. Unidirectional K(+) influx measurements showed that the patient's cells have abnormally high activities of the cation-proton exchanger and the K(+),Cl(-) co-transporter, which can account for the observed net movements of cations. Finally, neither chloride nor cation conductance in the patient's erythrocytes differed from that of healthy donors. Conclusions These results suggest that cross-talk between the mutated band 3 and other transporters might increase the cation permeability in cryohydrocytosis.

The molecular basis of hereditary red cell membrane disorders

The red cell membrane is one of the best known membranes in terms of structure, function and genetic disorders. As any plasma membrane it mediates transport functions. It also provides the erythrocytes with their resilience and deformability. Many of the proteins and the genes performing these functions are known in great detail, although some disease-responsible genes are yet to be elucidated. Basic knowledge has shed light on important groups of genetic disorders. The latter include (i) the disorders of the red cell mechanics: hereditary spherocytosis, hereditary elliptocytosis and poikilocytosis, and (ii) the disorders of the passive flux of the monovalent cations across the membrane: the stomacytoses and allied conditions. Reciprocally, many information have come from genetics abnormalities. We will review the mutation-disease relationship. A number of points will be underscored: widespread weak alleles modulate the expression of the SPTA1 gene, encoding the alpha-chain of spectrin; mutations in the anion exchanger can give rise to an array of distinct nosological entities, including a renal condition; splenectomy is banned in the stomatocytoses; a variety of stomatocyosis is part of a pleiotropic syndrome that may includes perinatal fetal liquid effusions. The diagnosis, follow-up and treatment of the involved diseases have gradually improved.

The hereditary stomatocytoses: genetic disorders of the red cell membrane permeability to monovalent cations

The hereditary stomatocytoses are mostly accounted for by genetic disorders of red cell membrane permeability to monovalent cations. These conditions, all very rare, are comprised of a hemolytic anemia, frequently macrocytosis, and the presence of abnormally shaped red blood cells. The key test for diagnosis is osmotic gradient ektacytometry, which measures the osmotic resistance and hydration of the red blood cell; the curve depicting the temperature dependence of the cation leak is also important. Syndromes include familial pseudohyperkalemia (FP), which is devoid of hematological features, dehydrated hereditary stomatocytosis (DHS), and overhydrated hereditary stomatocytosis (OHS). Some forms of DHS may be a pleiotropic, showing pseudohyperkalemia and/or perinatal edema. Perinatal edema, if not properly treated, may be lethal but may also resolve spontaneously prior to or shortly after birth and never reappear. Hereditary cryohydrocytosis, type 1 (CHC 1) is characterized by a dramatic resumption of the leak in vitro as the temperature approaches 0 degrees C; cell hydration seems unaltered. In OHS, stomatin, a membrane protein, is sharply reduced; however, this is a secondary event and the primarily mutated protein remains unknown. Hereditary cryohydrocytosis, type 2 (CHC 2) presents similar to OHS, except that the leak dramatically increases close to 0 degrees C. In addition, hematological manifestations are associated with neurological disorders. Of critical practical importance is that splenectomy in DHS or OHS causes thromboembolic events that may be fatal. The genes involved in hereditary stomatocytoses have yet to be identified. Apart from the 16q24-qter locus, related to subsets of DHS and FP, and a chromosome 2 locus assigned to a single case of FP, gene mapping has been difficult. The eventual discovery of individual genes will clarify complicated classification of the stomatocytoses, now based solely on phenotype.

A second locus mapping to 2q35-36 for familial pseudohyperkalaemia

Familial pseudohyperkalaemia (FP) is a symptomless, dominantly inherited red cell trait, which shows a 'passive leak' of K+ cations into the plasma upon storage of blood at room temperature (or below). There are no haematological abnormalities. The loss of K+ is due to a change in the temperature dependence of the leak. The Scottish case initially described, FP Edinburgh, maps to 16q23-qter. Here we studied a large kindred of Flemish descent with FP, termed FP Lille, which was phenotypically identical to the Edinburgh FP. In FP Lille, however, the responsible locus mapped to 2q35-36, with a Lod score of 8.46 for marker D2S1338. We infer that FP Edinburgh and FP Lille, although they are phenocopies of one another, stem from two distinct loci, FP1 (16q23-qter) and FP2 (2q35-36), respectively. This duality hints at the possibility that the protein mediating the leak might be a heterodimer. No mutation was found in three plausibly candidate genes: the KCNE4 gene, the TUBA1 gene and a predicted gene located in genomic contig NT_005403.

Hemolytic disease due to membrane ion channel disorders

PURPOSE OF REVIEW

To summarize recent findings in the study of the 'hereditary stomatocytoses and allied disorders', diseases in which the red cell membrane leaks Na and K, disturbing the osmotic homeostasis of the cell.

RECENT FINDINGS

Recent work has emphasized the diversity of these conditions, especially evident in the variations in temperature dependence of the cation leak. The association between the dehydrated, xerocytic form that maps to chromosome 16, with perinatal ascites is confirmed. Two cases that may represent a new hematoneurologic syndrome have been recognized.

SUMMARY

These leaky-membrane diseases fall into three main categories. The 'dehydrated' or xerocytic form maps to chromosome 16 and shows a minimal leak, and can show an excess of phosphatidylcholine in the membrane. Some of these xerocytic cases show a syndrome of self-limiting perinatal ascites of unknown cause. A second group shows very variable temperature dependence in the cation leak. The most severe 'overhydrated' form shows very leaky cells and the 32 kD stomatin protein is missing, although the gene is not mutated. This deficiency seems to be the result of a trafficking problem. The protein is associated with cholesterol and sphingomyelin-rich 'rafts' and may be some kind of partner protein for a membrane-bound proteolytic system.

Four pedigrees of the cation-leaky hereditary stomatocytosis class presenting with pseudohyperkalaemia. Novel profile of temperature dependence of Na+-K+leak in a xerocytic form

We report four pedigrees of the group of Na(+)-K(+)-leaky red cell disorders of the 'hereditary stomatocytosis' class. Each showed pseudohyperkalaemia because of temperature-dependent loss of K(+) from red cells on storage of whole blood at room temperature. All pedigrees showed an abnormality in the temperature dependence of the 'passive leak' of the membrane to K(+). Two pedigrees, both of which showed a compensated haemolytic state with dehydrated red cells and target cells on the blood film, showed a novel pattern, in which the profile was flat between 37 degrees C and about 32 degrees C then dropped as the temperature was reduced to zero. The third showed the 'shallow slope' profile, with stomatocytes on the blood film and very markedly abnormal intracellular Na(+) and K(+) levels. Minimal haemolysis was present. The fourth pedigree, of Asian origin, showed the shoulder pattern (minimum at 32 degrees C, maximum at 12 degrees C) with essentially normal haematology. Both of these latter two forms have previously been seen in other pedigrees. The first variant represents a novel kind of temperature dependence of the passive leak found in these pedigrees presenting with pseudohyperkalaemia.

ATP-dependent vesiculation in red cell membranes from different hereditary stomatocytosis variants

The hereditary stomatocytoses are a group of dominant haemolytic anaemias that show two main features: invaginated, 'stomatocytic' morphology; and a membrane leak to the univalent cations Na and K. A patient with the most severe variant of these conditions was reported to show a defect in an in vitro process of ATP-dependent endocytic vesiculation (ADEV), which is found in normal red cells. We have examined this endocytosis process in 11 leaky red cell pedigrees available to us in the UK. ADEV in broken membranes was absent only in the two most severely affected, 'overhydrated' pedigrees studied, both of which showed a deficiency in the membrane raft protein, stomatin. The process was present, although typically diminished by about 10-20% compared with normal red cells, in all others. The cross-linker dimethyl adipimate (DMA), which could correct the cation leak in some of these patients, also corrected the ADEV defect in the same patients. In those patients in whom DMA had no effect on the ion leak, ADEV was not absent. In normal cells, this process of vesiculation was inhibited by inhibitors of membrane 'raft' function, by an antistomatin antibody and by vanadate and N-ethyl maleimide, but not by inhibitors of a number of kinases. These data highlight the heterogeneity of these conditions. A mechanism is discussed by which a defect in raft-based endocytosis could lead to the exaggerated surface exposure of an ion channel, which could then function constitutively, i.e. 'leak'.

Familial pseudohyperkalaemia Cardiff: a mild version of cryohydrocytosis

We have investigated a Welsh pedigree showing the 'familial pseudohyperkalaemia' phenotype of dominantly inherited, red-cell-based, temperature-dependent pseudohyperkalaemia associated with normal haematology. The 'passive leak' to K across the membrane of these abnormal red cells showed a 'U-shaped' temperature dependence, with a minimum at about 23 degrees C, qualitatively similar to that seen in the frankly haemolytic 'cryohydrocytosis' variant of the hereditary stomatocytosis group. Like three previous pedigrees with cryohydrocytosis, these patients show an excess of ether lipids in the membrane. However, these patients differ from other 'familial pseudohyperkalaemia' pedigrees, in which the leak showed different temperature profiles.