Two further British families with the 'cryohydrocytosis' form of hereditary stomatocytosis

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.

ICSH guidelines for the laboratory diagnosis of nonimmune hereditary red cell membrane disorders

INTRODUCTION

Hereditary spherocytosis (HS), hereditary elliptocytosis (HE), and hereditary stomatocytosis (HSt) are inherited red cell disorders caused by defects in various membrane proteins. The heterogeneous clinical presentation, biochemical and genetic abnormalities in HS and HE have been well documented. The need to raise the awareness of HSt, albeit its much lower prevalence than HS, is due to the undesirable outcome of splenectomy in these patients.

METHODS

The scope of this guideline is to identify the characteristic clinical features, the red cell parameters (including red cell morphology) for these red cell disorders associated, respectively, with defective cytoskeleton (HS and HE) and abnormal cation permeability in the lipid bilayer (HSt) of the red cell. The current screening tests for HS are described, and their limitations are highlighted.

RESULTS

An appropriate diagnosis can often be made when the screening test result(s) is reviewed together with the patient's clinical/family history, blood count results, reticulocyte count, red cell morphology, and chemistry results. SDS-polyacrylamide gel electrophoresis of erythrocyte membrane proteins, monovalent cation flux measurement, and molecular analysis of membrane protein genes are specialist tests for further investigation.

CONCLUSION

Specialist tests provide additional evidence in supporting the diagnosis and that will facilitate the management of the patient. In the case of a patient's clinical phenotype being more severe than the affected members within the immediate family, molecular testing of all family members is useful for confirming the diagnosis and allows an insight into the molecular basis of the abnormality such as a recessive mode of inheritance or a de novo mutation.

The GPA-dependent, spherostomatocytosis mutant AE1 E758K induces GPA-independent, endogenous cation transport in amphibian oocytes

The previously undescribed heterozygous missense mutation E758K was discovered in the human AE1/SLC4A1/band 3 gene in two unrelated patients with well-compensated hereditary spherostomatocytic anemia (HSt). Oocyte surface expression of AE1 E758K, in contrast to that of wild-type AE1, required coexpressed glycophorin A (GPA). The mutant polypeptide exhibited, in parallel, strong GPA dependence of DIDS-sensitive (36)Cl(-) influx, trans-anion-dependent (36)Cl(-) efflux, and Cl(-)/HCO(3)(-) exchange activities at near wild-type levels. AE1 E758K expression was also associated with GPA-dependent increases of DIDS-sensitive pH-independent SO(4)(2-) uptake and oxalate uptake with altered pH dependence. In marked contrast, the bumetanide- and ouabain-insensitive (86)Rb(+) influx associated with AE1 E758K expression was largely GPA-independent in Xenopus oocytes and completely GPA-independent in Ambystoma oocytes. AE1 E758K-associated currents in Xenopus oocytes also exhibited little or no GPA dependence. (86)Rb(+) influx was higher but inward cation current was lower in oocytes expressing AE1 E758K than previously reported in oocytes expressing the AE1 HSt mutants S731P and H734R. The pharmacological inhibition profile of AE1 E758K-associated (36)Cl(-) influx differed from that of AE1 E758K-associated (86)Rb(+) influx, as well as from that of wild-type AE1-mediated Cl(-) transport. Thus AE1 E758K-expressing oocytes displayed GPA-dependent surface polypeptide expression and anion transport, accompanied by substantially GPA-independent, pharmacologically distinct Rb(+) flux and by small, GPA-independent currents. The data strongly suggest that most of the increased cation transport associated with the novel HSt mutant AE1 E758K reflects activation of endogenous oocyte cation permeability pathways, rather than cation translocation through the mutant polypeptide.

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.

Monovalent cation leaks in human red cells caused by single amino-acid substitutions in the transport domain of the band 3 chloride-bicarbonate exchanger, AE1

We identified 11 human pedigrees with dominantly inherited hemolytic anemias in both the hereditary stomatocytosis and spherocytosis classes. Affected individuals in these families had an increase in membrane permeability to Na and K that is particularly marked at 0 degrees C. We found that disease in these pedigrees was associated with a series of single amino-acid substitutions in the intramembrane domain of the erythrocyte band 3 anion exchanger, AE1. Anion movements were reduced in the abnormal red cells. The 'leak' cation fluxes were inhibited by SITS, dipyridamole and NS1652, chemically diverse inhibitors of band 3. Expression of the mutated genes in Xenopus laevis oocytes induced abnormal Na and K fluxes in the oocytes, and the induced Cl transport was low. These data are consistent with the suggestion that the substitutions convert the protein from an anion exchanger into an unregulated cation channel.

Red cell membrane disorders

Disorders of the erythrocyte membrane, including hereditary spherocytosis, hereditary elliptocytosis, hereditary pyropoikilocytosis, and hereditary stomatocytosis, comprise an important group of inherited hemolytic anemias. These syndromes are characterized by marked clinical and laboratory heterogeneity. Recent molecular studies have revealed that there is also significant genetic heterogeneity in these disorders. This is particularly true for the spherocytosis syndromes where each kindred has a private mutation in one of the spherocytosis genes. Treatment with splenectomy is curative in most patients. Splenectomy via a laparoscopic approach has become the surgical method of choice. Growing recognition and understanding of the long-term risks and complications of splenectomy, including cardiovascular disease, thrombotic disorders, and pulmonary hypertension, and the emergence of penicillin-resistant pneumococci, a concern for infection in overwhelming postsplenectomy infection, have led to reevaluation of the role of splenectomy. Recent management guidelines acknowledge these important considerations when entertaining splenectomy and recommend detailed discussion between health care providers, patient, and family.

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.

Guidelines for the diagnosis and management of hereditary spherocytosis

Hereditary spherocytosis (HS) is a heterogeneous group of disorders with regard to clinical severity, protein defects and mode of inheritance. It is relatively common in Caucasian populations; most affected individuals have mild or only moderate haemolysis. There is usually a family history, and a typical clinical and laboratory picture so that the diagnosis is often easily made without additional laboratory tests. Atypical cases may require measurement of erythrocyte membrane proteins to clarify the nature of the membrane disorder and in the absence of a family history, occasionally molecular genetic analysis will help to determine whether inheritance is recessive or non-dominant. It is particularly important to rule out stomatocytosis where splenectomy is contraindicated because of the thrombotic risk. Mild HS can be managed without folate supplements and does not require splenectomy. Moderately and severely affected individuals are likely to benefit from splenectomy, which should be performed after the age of 6 years and with appropriate counselling about the infection risk. In all cases careful dialogue between doctor, patient and the family is essential. Laparoscopic surgery, when performed by experienced surgeons, can result in a shorter hospital stay and less pain.

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 new cases of stomatin-deficient hereditary stomatocytosis syndrome: association of the stomatin-deficient cryohydrocytosis variant with neurological dysfunction

This report concerns congenitally Na(+)-K(+) leaky red cells of the 'hereditary stomatocytosis' class. Three new isolated cases and one new pedigree are described, and one previously reported case is expanded. In all cases, Western blotting of red cell membranes revealed a deficiency in the 32 kDa membrane protein, stomatin. All showed pronounced cation leaks at 37 degrees C with markedly abnormal intracellular Na(+) and K(+) concentrations, like all other such stomatin-deficient cases. Consistent with recent findings in two previously described British pedigrees, immunocytochemistry demonstrated that the deficiency of stomatin was not complete. On typical blood films, some red cells showed positive stomatin immunoreactivity, while most were negative, although in one case only a minority were negative. All platelets and neutrophils were stomatin positive. The cases differed markedly between themselves with regard to the temperature dependence of the passive leak to K(+). Three showed a simple monotonic temperature dependence, while two showed a minimum at around 20-25 degrees C, such that the cells were extremely leaky at 0 degrees C, giving the phenotype known as 'cryohydrocytosis'. These patients are the only two known cases of stomatin-deficient cryohydrocytosis. Both showed a congenital syndrome of mental retardation, seizures, cataracts and massive hepatosplenomegaly, probably defining a new haemato-neurological syndrome.

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.

Cold storage of 'cryohydrocytosis' red cells: the osmotic susceptibility of the cold-stored erythrocyte

'Cryohydrocytosis' is an unusual human haemolytic anaemia of the 'hereditary stomatocytosis' group, in which the red cell membrane is abnormally permeable to Na and K+ at both body and (even more prominently) refrigerator temperatures. If whole cryohydrocytosis blood is anticoagulated in heparin or EDTA and stored on ice overnight, about 50% of the cells will lyse. Citrate phosphate dextrose adenine (CPDa) anticoagulant, empirically verified as an optimal anticoagulant for storage of normal blood before transfusion, very markedly ameliorated this overnight lysis, suggesting that these cells might form an informative model in which cold storage of the red cell could be studied in a short time scale. Accordingly, we conducted studies of ion flux, cell swelling and lysis in different media used historically for blood preservation and compared the experimental data with an 'integrated red cell model', which seeks mathematically to model the osmotic behaviour of red cells under different conditions. Upon experiment, lysis in these cells was reduced by additives that could be regarded as impermeant extracellular solutes (citrate, mannitol) and by low pH, but not by those agents that are regarded as protecting the cell against energy depletion or oxidation (adenine, glucose, nicotinic acid). The protective effects of these extracellular additives were all reproduced by the computer simulation, confirming the validity of this model, although the effect of pH could be simulated only semi-quantitatively, possibly because Na+ permeability itself depends on pH.

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'.

Molecular basis of red cell membrane disorders

We will consider an array of genetic disorders of the red cell membrane. Some affect well-known genes. The mutations of most cases of hereditary spherocytosis (HS) are located in the following genes: ANK1, SPTB, SLC4A1, EPB42 and SPTA1, which encode ankyrin, spectrin beta-chain, the anion exchanger 1 (band 3), protein 4.2 and spectrin alpha-chain, respectively. A dominant form of distal renal tubular acidosis also stems from distinct mutations in the SLC4A1 gene. The mutations responsible for hereditary elliptocytosis (HE) and its aggravated form, poikilocytosis (HP), lie in the SPTA1 and SPTB gene, already mentioned, and in the EPB41 gene encoding protein 4.1. Whereas in HS, the SPTA1 and SPTB gene mutations tend to abolish the synthesis of the corresponding chains, in HE/HP, they hinder spectrin tetramerization. Allele alpha(LELY) is a common polymorphic allele which plays the role of an aggravating factor when it occurs in trans of an elliptocytogenic allele of the SPTA1 gene. Southeast Asian ovalocytosis results from a 27- nucleotide deletion in the SLC4A1 gene. Besides these conditions in which the mutations were reached from known alterations in the proteins, other conditions required a positional cloning approach. Such are the genetic disorders of membrane permeability to monovalent cations. Knowledge is the most advanced as regards dehydrated hereditary stomatocytois (DHS). DHS was shown to belong to a pleiotropic syndrome: DHS + fetal edema + pseudohyperkalemia, which maps to 16q23-24. Concerning DHS and another disease of the same class, overhydrated hereditary stomatocytosis, splenectomy almost certainly appears to elicit thromboembolic accidents.

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.