Familial Pseudohyperkalemia Maps to the Same Locus as Dehydrated Hereditary Stomatocytosis (Hereditary Xerocytosis)

Marked hyperkalemia due to inappropriate blood sample storage in two suspected cases of familial pseudohyperkalemia

We herein report two suspected cases of pseudohyperkalemia who presented with severe hyperkalemia examined at small primary care clinics; however, re-exams at a tertiary care hospital showed normal potassium levels. We reproduced the laboratory examination conditions of the clinics and found that hyperkalemia was due to sampling/storage condition of serum, which is strongly suggestive of familial pseudohyperkalemia (FP). FP is a possible but under-appreciated cause of hyperkalemia, which does not require treatment, so it is important to include FP in the differential diagnosis of hyperkalemia especially in cases with discrepant of serum potassium levels at different settings.

Pseudohyperkalemia and Pseudohyponatremia in Two Children with T-Cell Acute Lymphoblastic Leukemia

Pseudohyperkalemia and pseudohyponatremia are phenomena in which hematologic disorders with high cell counts result in factitious electrolyte measurements that can result in inappropriate treatment. We describe 2 children with leukemia presenting with both disturbances to highlight the importance of correlating electrolyte results from plasma with those from whole blood before intervening.

Congenital Hemolytic Anemias: Is There a Role for the Immune System?

Congenital hemolytic anemias (CHAs) are a heterogeneous group of rare hereditary conditions including defects of erythrocyte membrane proteins, red cell enzymes, and disorders due to defective erythropoiesis. They are characterized by variable degree of anemia, chronic extravascular hemolysis, reduced erythrocyte life span, splenomegaly, jaundice, biliary lithiasis, and iron overload. Although few data are reported on the role of the immune system in CHAs, several immune-mediated mechanisms may be involved in the pathogenesis of these rare diseases. We reported in ~60% of patients with hereditary spherocytosis (HS), the presence of naturally-occurring autoantibodies (NAbs) directed against different membrane proteins (α- and β-spectrin, band 3, and dematin). Positive HS subjects showed a more hemolytic pattern and NAbs were more evident in aged erythrocytes. The latter is in line with the function of NAbs in the opsonization of damaged/senescent erythrocytes and their consequent removal in the spleen. Splenectomy, usually performed to reduce erythrocyte catheresis and improve Hb levels, has different efficacy in various CHAs. Median Hb increase is 3 g/dL in HS, 1.6–1.8 g/dL in pyruvate kinase deficiency (PKD), and 1 g/dL in congenital dyserythropoietic anemias (CDA) type II. Consistently with clinical severity, splenectomy is performed in 20% of HS, 45% of CDAII, and in 60% of PKD patients. Importantly, sepsis and thrombotic events have been registered, particularly in PKD with a frequency of ~7% for both. Furthermore, we analyzed the role of pro-inflammatory cytokines and found that interleukin 10 and interferon γ, and to a lesser extent interleukin 6, were increased in all CHAs compared with controls. Moreover, CDAII and enzymatic defects showed increased tumor necrosis factor-α and reduced interleukin 17. Finally, we reported that iron overload occurred in 31% of patients with membrane defects, in ~60% of CDAII cases, and in up to 82% of PKD patients (defined by MRI liver iron concentration >4 mg Fe/gdw). Hepcidin was slightly increased in CHAs compared with controls and positively correlated with ferritin and with the inflammatory cytokines interleukin 6 and interferon γ. Overall the results suggest the existence of a vicious circle between chronic hemolysis, inflammatory response, bone marrow dyserythropoiesis, and iron overload.

A case of severe pseudohyperkalaemia due to muscle contraction

Introduction

Severe hyperkalaemia is a serious medical condition requiring immediate medical attention. Before medical treatment is started, pseudohyperkalaemia has to be ruled out.

Case description

A 10-month old infant presented to the emergency department with fever and coughing since 1 week. Routine venous blood testing revealed a severe hyperkalaemia of 6.9 mmol/L without any indication of haemolysis. Reanalysis of the plasma sample confirmed the hyperkalaemia (7.1 mmol/L). Based on these results, the clinical pathologist suggested to perform a venous blood gas analysis and electrocardiogram (ECG) which revealed a normal potassium of 3.7 mmol/L and normal ECG, ruling out a potentially life-treating hyperkalaemia. The child was diagnosed with pneumonia. The paediatrician had difficulty to perform the first venous blood collection due to excessive movement of the infant during venipuncture. The muscle contractions of the child in combination with venous stasis most probably led to a local increase of potassium in the sampled limbs. The second sample collected under optimal preanalytical circumstances had a normal potassium. Since muscle contraction typically does not cause severe hyperkalaemia, other causes of pseudohyperkalaemia were excluded. K₃-EDTA contamination and familial hyperkalaemia were ruled out and the patient did not have extreme leucocytosis or thrombocytosis. By exclusion a diagnosis of pseudohyperkalaemia due to intense muscle movement and venous stasis was made.

Conclusion

This case suggests that intense muscle contraction and venous stasis can cause severe pseudohyperkalemia without hemolysis. Once true hyperkalemia has been ruled out, a laboratory work-up can help identify the cause of pseudohyperkalaemia.

Mechanosensitive Piezo1 ion channel protein (PIEZO1 gene): update and extended mutation analysis of hereditary xerocytosis in India

Hereditary xerocytosis (HX), also known as dehydrated stomatocytosis (DHSt) is a dominantly inherited genetic disorder exhibiting red cell membrane dehydration caused by the loss of the monovalent cation K⁺ and water. Variants in mechanosensitive Piezo ionic channels of the PIEZO1 gene are the primary cause of HX. We have utilized high throughput and highly precise next-generation sequencing (NGS) to make a diagnosis and examine the genotype-phenotype relationship in inflexible HX cases. Seven unrelated patients with unexplained hemolytic anemia were scrutinized with a panel probing 8000 genes related to congenital anemia. Targeted next-generation sequencing identified 8 missense variants in the PIEZO1 gene in 7 unrelated Indian patients. Three of the 8 variants are novel (c.1795G > C, c.2915G > A, c.7372 T > C) and the remaining five (c.4082A > G, c.6829C > A, c.7374C > G, c.7381G > A, c.7483_7488dup) are previously reported. The variants have been validated by Sanger sequencing. One patient with autosomal dominant mutation (c.7372 T > C) is associated with iron refractory iron deficiency anemia. Of the 7 patients, one has HX in combination with a novel homozygous variant (c.994G > A) in the PKLR gene causing PK deficiency resulting in severe clinical manifestations with phenotypic variability. In silico prediction using bioinformatics tools were used to study the possible damaging effects of the novel variants. Structural-functional analysis of the novel variants was investigated by molecular modeling software (PyMOL and Swiss PDB). These results encompass the heterogeneous behavior of mechano-sensitive Piezo1 protein observed in HX patients in India. Moreover, NGS imparted a subtle, economical, and quick tool for understanding the genetic cause of undiagnosed cases of congenital hemolytic anemia. NGS grants a potential technology integrating clinical history together with molecular report profiting in such patients and their families.

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.

Disorders of erythrocyte volume homeostasis

Inherited disorders of erythrocyte volume homeostasis are a heterogeneous group of rare disorders with phenotypes ranging from dehydrated to overhydrated erythrocytes. Clinical, laboratory, physiologic, and genetic heterogeneities characterize this group of disorders. A series of recent reports have provided novel insights into our understanding of the genetic bases underlying some of these disorders of red cell volume regulation. This report reviews this progress in understanding determinants that influence erythrocyte hydration and how they have yielded a better understanding of the pathways that influence cellular water and solute homeostasis.

Disorders of red cell volume regulation

PURPOSE OF REVIEW

Regulation of erythrocyte volume homeostasis is critical for survival of the erythrocyte. Inherited or acquired disorders that perturb this homeostasis jeopardize the erythrocyte, leading to its premature destruction. This report reviews recent insights into pathways that influence cellular water and solute homeostasis and cell volume.

RECENT FINDINGS

The molecular and genetic bases of primary disorders of erythrocyte hydration are beginning to be revealed. Recent studies have implicated roles for a new protein PIEZO1, a long sought after mammalian mechanosensory protein; GLUT1, the glucose transporter; SLC4A1, the anion transporter; RhAG, the Rh-associated glycoprotein; and ABCB6, an ATP-binding cassette family member. Secondary disorders associated with perturbed cellular volume and volume regulation include sickle cell disease, thalassemia, and hereditary spherocytosis, in which dehydration contributes to disease pathology and clinical complications. Advances in understanding the mechanisms regulating erythrocyte solute and water content, particularly associated with mechanotransduction pathways, have revealed novel mechanisms controlling erythrocyte hydration. Understanding these processes may provide innovative strategies to maintain normal erythrocyte volume in disorders associated with primary or secondary cellular dehydration.

SUMMARY

Understanding the mechanisms controlling erythrocyte volume regulation will serve as a paradigm for other cells and may reveal new therapeutic targets for disease prevention and treatment.

Hereditary spherocytosis, elliptocytosis, and other red cell membrane disorders

Hereditary spherocytosis and elliptocytosis are the two most common inherited red cell membrane disorders resulting from mutations in genes encoding various red cell membrane and skeletal proteins. Red cell membrane, a composite structure composed of lipid bilayer linked to spectrin-based membrane skeleton is responsible for the unique features of flexibility and mechanical stability of the cell. Defects in various proteins involved in linking the lipid bilayer to membrane skeleton result in loss in membrane cohesion leading to surface area loss and hereditary spherocytosis while defects in proteins involved in lateral interactions of the spectrin-based skeleton lead to decreased mechanical stability, membrane fragmentation and hereditary elliptocytosis. The disease severity is primarily dependent on the extent of membrane surface area loss. Both these diseases can be readily diagnosed by various laboratory approaches that include red blood cell cytology, flow cytometry, ektacytometry, electrophoresis of the red cell membrane proteins, and mutational analysis of gene encoding red cell membrane proteins.

Hyperkalaemia in the age of aldosterone antagonism

Hyperkalaemia is well recognized as a medical emergency. However, with the publication of trials showing benefit with renin-aldosterone axis suppression in heart failure, the epidemiology of patients presenting with hyperkalaemia has changed. The reported incidence of rate of serious hyperkalaemia (>6.0 mEq/l of potassium) ranges from 6 to 12% in patients on spironolactone with congestive cardiac failure (CCF). A rational choice of therapy based on present evidence is different from the traditionally used algorithm, given our understanding of the physiology relevant to this patient group. This article discusses the changing face of hyperkalaemia and the present evidence and discusses options in treatment of hyperkalaemia.

Pseudohyperkalemia from a Pneumatic Tube Transport System: Case Report and Literature Review

Objective

To report a case of pseudohyperkalemia due to a pneumatic tube transport system.

Case Summary

A 75-year-old male presented to the emergency medicine department with chest pain and intermittent vision loss over the previous 2 days. Laboratory studies revealed a potassium value of 9.6 mEq/L and a white blood cell (WBC) count of 262 × 109/L. An electrocardiogram did not reveal changes consistent with hyperkalemia. Emergent treatment for the hyperkalemia was instituted. Repeat plasma potassium values obtained after treatment for the hyperkalemia remained significantly elevated. It was eventually recognized that the hyperkalemia was due to the combination of undiagnosed leukemia causing a significantly elevated WBC count and transport of the patient's specimen to the laboratory via a pneumatic tube transport system. Manual transport of the specimen to the laboratory repeatedly revealed normal or hypokalemic values.

Discussion

Hyperkalemia is a potentially fatal electrolyte abnormality that must be differentiated from pseudohyperkalemia. Pseudohyperkalemia is defined as a spurious elevation of potassium levels usually due to mechanical trauma during venipuncture resulting in hemolysis and release of potassium from the cellular elements of blood. Pneumatic tube transport systems should be listed in the scientific literature as another potential cause of pseudohyperkalemia, especially in patients with high WBC and/or platelet counts.

Conclusion

Pharmacists and other health care providers should be aware of pneumatic tube transport systems potentially causing pseudohyperkalemia, because regular treatments for hyperkalemia for this problem may cause patient harm.

Mutations in the mechanotransduction protein PIEZO1 are associated with hereditary xerocytosis

Hereditary xerocytosis (HX, MIM 194380) is an autosomal dominant hemolytic anemia characterized by primary erythrocyte dehydration. Copy number analyses, linkage studies, and exome sequencing were used to identify novel mutations affecting PIEZO1, encoded by the FAM38A gene, in 2 multigenerational HX kindreds. Segregation analyses confirmed transmission of the PIEZO1 mutations and cosegregation with the disease phenotype in all affected persons in both kindreds. All patients were heterozygous for FAM38A mutations, except for 3 patients predicted to be homozygous by clinical and physiologic studies who were also homozygous at the DNA level. The FAM38A mutations were both in residues highly conserved across species and within members of the Piezo family of proteins. PIEZO proteins are the recently identified pore-forming subunits of channels that mediate mechanotransduction in mammalian cells. FAM38A transcripts were identified in human erythroid cell mRNA, and discovery proteomics identified PIEZO1 peptides in human erythrocyte membranes. These findings, the first report of mutation in a mammalian mechanosensory transduction channel-associated with genetic disease, suggest that PIEZO proteins play an important role in maintaining erythrocyte volume homeostasis.

Leaky cell syndrome: a rare cause of pseudohyperkalaemia

Life-threatening situations of hyperkalaemia are often caused by renal failure, hyperglycaemia or medication. However pseudohyperkalaemia, a falsely elevated potassium concentration, is usually caused by haemolysis, repeated clenching of the fist during venepuncture or abnormal cell numbers. Another rare cause of pseudohyperkalaemia is familial pseudohyperkalaemia, an autosomal dominantly inherited trait, with increased leakage of potassium from erythrocytes. Under normal in vivo conditions, this increased leakage is compensated by augmented activity of the Na+/K+ ATPase pump. However, after venepuncture the blood cools down to room temperature, reducing the activity of the Na+/K+ ATPase pump whereby the increased potassium leakage becomes more apparent. Here, we present a Dutch patient with extreme familial pseudohyperkalaemia. Interestingly, his two children also show increased potassium leakage at room temperature, albeit at a lower level. Despite the low prevalence of familial pseudohyperkalaemia, it can have important clinical implications and rapid recognition is desired.

Dehydrated hereditary stomatocytosis mimicking familial hyperkalaemic hypertension: clinical and genetic investigation

Dehydrated hereditary stomatocytosis (DHS) is a rare dominant form of hereditary haemolytic anaemia. In some families, pseudohyperkalaemia accompanies DHS. Familial hyperkalaemic hypertension (FHHt), a rare autosomal dominant form of arterial hypertension, is associated with genuine hyperkalaemia. We present a large French family in which DHS and FHHt were diagnosed independently in two separate branches. In branch A, mild DHS accompanied by pseudohyperkalaemia was found. In branch B, the proband and her daughter were initially diagnosed with FHHt, based on the coincidence of high blood pressure and hyperkalaemia. After finding out that branches A and B were related, reinvestigation of the affected members of branch B lead to the diagnosis of DHS, yielding the largest DHS kindred known in France. This allowed extensive linkage analysis based on 19 microsatellites markers in 12 affected and 10 unaffected members at 16q24.1qter, where one known DHS locus maps to. A maximal two-point LOD score (4.71 at theta = 0) was obtained for markers D16S3074 and D16S476. Haplotype analysis led to the definition of a new 11.5 cM disease interval with an upper limit at microsatellite D16S3037.

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.

Pseudohyperkalemia in serum: the phenomenon and its clinical magnitude

We investigated in detail the difference between serum and plasma potassium levels in patients with several conditions associated with pseudohyperkalemia. In total, 435 patients with either thrombocytoses, erythrocytoses, leucocytoses, or a mixed-type disorder and 30 healthy controls were included. In each case, the index Dk [serum potassium minus plasma potassium] and the index Dk100 (Dk x 100,000/platelets), which indicates the Dk value that corresponds to platelets of 100,000/mm(3), were estimated. Median Dk was significantly higher in the groups with platelet, erythrocyte, or mixed-type disorders than in the controls (P=0.001). Among these groups, Dk values were significantly higher in patients with thrombocytosis or mixed-type disorders compared with those with erythrocytosis (P<0.001, for both). Furthermore, no significant difference was observed in Dk values between controls and patients with white blood cell disorders (P=0.74). Dk values did not exceed 2.61 mmol/L, whereas Dk100 values were inversely related to platelet counts (r=-0.351, P<0.01). In conclusion, pseudohyperkalemia is mainly present in patients with thrombocytosis or mixed-type disorders, probably as a result of the degranulation of platelets, which offers a potassium load to the surrounding plasma at the time of clot formation in vitro. However, the degree of pseudohyperkalemia does not increase proportionally with the increase of platelet counts, which may be associated with transfer of part of potassium load from the plasma back into red and white blood cells.

Pseudohyperkalemia in Patients with Increased Cellular Components of Blood

OBJECTIVE

We performed a study to investigate the difference between serum and plasma potassium concentration in patients with increase in one or more of the cellular components of blood.

DESIGN AND METHODS

This study was performed in two phases. During the first phase, we performed a cross-sectional comparison of the difference between serum and plasma potassium concentration (Dk) in 341 patients with the various clinical conditions where pseudohyperkalemia has been described, as well as with secondary or spurious erythrocytosis and in 30 normal controls. A cut-off value of Dk discriminating polycythemia vera from other erythrocytoses was estimated. In the second phase we studied the significance of this cut-off value as predictor of polycythemia vera in 90 naive patients who were referred with an elevated hematocrit.

RESULTS

Dk was significantly increased in the groups with platelet, erythrocyte or with a mixed type disorder compared to the controls (P < 0.01). Among these groups, Dk was significantly increased in the groups with thrombocytosis and mixed type disorder, compared to the group with erythrocytosis (both P < 0.01). A cut-off value of Dk discriminating polycythemia vera from other erythrocytoses was estimated (0.70 mmol/L). Dk (> or = 0.70 mmol/L), platelet and white blood cell count were identified as significant independent predictors of polycythemia vera.

CONCLUSIONS

The Dk is increased in patients with erythrocytoses, thrombocytoses or both. This phenomenon is more profound in patients with a mixed type disorder, such as polycythemia vera patients, compared to those with erythrocytoses alone.

Familial pseudohyperkalemia: a rare cause of hyperkalemia

A previously healthy 19-year-old girl was admitted to our hospital because of hyperkalemia. Pseudohyperkalemia was diagnosed because there was a marked difference between levels of serum and plasma potassium. Her plasma potassium level was markedly increased after 6-hour in vitro incubation of blood at room temperature, suggesting excessive potassium release from red blood cells without coagulation. The plasma potassium levels of the patient and her father were markedly elevated in blood specimens incubated in vitro at 4 degrees C, but not at 37 degrees C. These data indicated pseudohyperkalemia syndrome caused by abnormal leakage of potassium from red blood cells at the lower temperatures.

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.

[Congenital xerocytosis]

Hereditary xerocytosis is a genetic disease inherited as an autosomal dominant trait and is a rare cause of hemolytic anemia. It is caused by abnormal erythrocyte membrane permeability: monovalent cation pump activity is increased and the Na/K pump cannot compensate for the K lost. As a consequence, xerocytes dehydrate, becoming rigid and sensitive to metabolic stress and oxidation. Morbility depends on the severity of the hemolytic anemia. Periodic episodes of jaundice are common during mild infections; most patients remain asymptomatic but experience mild-to-moderate hemolytic anemia, which is generally well compensated. The diagnostic clues are a markedly increased flow through the Na/K pump with a decrease in total intracellular cation content and subsequent red cell dehydration. Treatment is based on monitoring for eventual complications and careful observation during infections, which may worsen the anemia. Splenectomy is not useful and for some authors may even be contraindicated. The prognosis is generally very good. We report the case of a patient with episodes of hemolytic anemia during intercurrent infections and positive diagnostic tests for hereditary xerocytosis.

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.

A case of hereditary xerocytosis diagnosed as a result of suspected hypoglycemia and observed low glycohemoglobin

Hereditary xerocytosis is a primary erythrocyte disorder in which a defect in the erythrocyte membrane leads to potassium efflux from the cell. An osmotic shift of water from the intracellular compartment follows, resulting in decreased deformability of the cell, increased membrane rigidity, hemolysis, decreased average duration of erythrocyte survival, and reticulocytosis. The condition is inherited as an autosomal dominant trait. In this publication, we report the case of a patient who presented with a vague history compatible with hypoglycemia and was found to have a low glycohemoglobin level. Further workup led to a diagnosis of hereditary xerocytosis in this patient and in other members of the family. This case illustrates the importance of understanding the underlying variables that affect the results of all commonly used glycohemoglobin assays, including determination of hemoglobin A(1C) and how primary red-cell disorders may alter its value.

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.

Dehydrated hereditary stomatocytosis is associated with neonatal hepatitis

Dehydrated hereditary stomatocytosis (DHSt) is an inherited haemolytic anaemia associated with increased red cell membrane permeability to Na(+) and K(+). It is increasingly recognized that a syndrome of self-limiting perinatal ascites can accompany the haemolysis. The cause of the perinatal ascites is unknown, and it has been argued that this could be due to cardiovascular, hepatic or lymphatic problems. We describe the case of a 16-year-old girl who presented neonatally with abnormal liver function tests and ascites. She was extensively investigated at that time. A liver biopsy showed hepatitis and fatty changes. Her ascites resolved within 6 months. At the age of 15 years, she developed an episode of acute haemolysis and was re-investigated. A diagnosis of DHSt was made. Pseudohyperkalaemia, due to ex vivo loss of K(+) from red cells, was present. This study confirms the previously noted association of DHSt, pseudohyperkalaemia and perinatal ascites, and suggests that the latter is of predominantly hepatic origin.

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.

The "stomatin" gene and protein in overhydrated hereditary stomatocytosis

In overhydrated hereditary stomatocytosis (OHSt), Coomassie- and silver-stained polyacrylamide gels show an apparently complete deficit of the 32-kDa membrane protein, stomatin. We have used an antistomatin antibody to examine peripheral blood films, bone marrow, splenic tissue, and hepatic tissue from these patients by immunocytochemistry. This technique revealed that, in fact, some red cells did show positive stomatin immunoreactivity; and consistent with this result, Western blot analysis of the red cell membranes confirmed that about one twentieth to one fiftieth of the normal amount of stomatin was in fact present. Flow cytometry, combining immunoreactive quantitation of stomatin expression with thiazole orange staining for reticulocytes, showed that in OHSt, it was the young cells that had more stomatin. Magnetic-activated cell separation studies, using beads to which an anti-transferrin receptor antibody was conjugated, confirmed that in OHSt there was a correspondence between expression of stomatin and the transferrin receptor. Immunocytochemistry and Western blotting revealed that in OHSt patients, the protein was present in spleen, liver, neutrophils, platelets, monocytes, and about 50% of the peripheral lymphocytes, with the same distribution as in healthy controls. Neither Southern blots, nor direct sequencing of multiple subclones of the cDNA, nor sequencing of amplicons from genomic DNA revealed any significant abnormality in stomatin gene sequence in these patients. The deficiency of stomatin from red cells appears to be due to a loss of stomatin from these red cells on maturation in the bone marrow and in the circulation.

Sub-lethal hydrops as a manifestation of dehydrated hereditary stomatocytosis in two consecutive pregnancies

Dehydrated hereditary stomatocytosis (DHS) is a rare congenital hemolytic anemia mapping to 16q23-q24. We showed recently that it is part of a pleiotropic syndrome likely to display pseudohyperkalemia and/or different forms of fetal and placental fluid collections. Here, we report a woman with DHS. She had two consecutive pregnancies associated with severe fetal hydrops. Hydrops would probably have been lethal in the absence of appropriate removal of ascites and excess amniotic fluid. In utero exchange transfusion, performed once, was useless, because anemia was not pronounced enough to be the cause of the hydrops. In both newborns, ascites resolved within a week following birth and never recurred. The association of hydrops and hemolytic anemia suggests the possibility of DHS. Symptomatic treatment of the hydrops assists survival until spontaneous resorption occurs.

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.

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

Two families with inherited abnormalities in Na and K transport across the red cell membrane are described. Both presented with 'pseudohyperkalaemia' as a result of loss of K from the red cells on storage at room temperature. Routine haematology was essentially normal, except for macrocytosis in one family. Studies of the temperature dependence of the passive leak to K showed a novel shoulder pattern with a minimum at 25 degrees C, a maximum at 10 degrees C, followed by a further fall. As in other cases of red cell-based pseudohyperkalaemia, the abnormal temperature dependence of this 'leak' flux could be held to account for the loss of K from the cells at room temperature. These cases represent a novel variant of the temperature dependence of the passive leak of K and Na across the red cell membrane, and can be classified as a mild, non-haemolytic form of the group known as the hereditary stomatocytosis and allied disorders'.

Factitious hyperkalemia

Pseudohyperkalemia, or factitious hyperkalemia, constitutes an artificially high plasma potassium level (P(K)) from a variety of possible causes. Occasionally, the cause cannot be elucidated. Three patients who showed unusually large differences between free-flowing and tourniquet (stasis) potassium levels prompted us to investigate the influence of tourniquets in routine phlebotomy in eight healthy volunteers. P(K) showed a consistent but rather small average increase of 0.2 mEq/L (P < 0.001) during tourniquet use; however, the range was 10-fold, from 0.05 to 0.5 mEq/L in our subjects. We suggest there may be large variability leading to an excessive increase in P(K) in some individuals. In the three patients presented, average excessive increases in P(K) of 1.6, 1.3, and 1.7 mEq/L were seen. Although diagnosing and treating true hyperkalemia remains paramount, recognizing factitious hyperkalemia is important to preclude unnecessary investigations and potentially hazardous intervention.

The hereditary stomatocytoses and allied disorders: congenital disorders of erythrocyte membrane permeability to Na and K

The hereditary stomatocytoses and allied disorders are a set of dominantly inherited haemolytic anaemias in which the plasma membrane of the red cell 'leaks' sodium and potassium. There are about 10 different forms of these conditions, ranging from a moderately severe haemolytic anaemia to minor conditions in which the haematology is essentially normal, but where the patients present with pseudohyperkalaemia, due to leakage of K from the red cells on cooling to room temperature. Frequently misdiagnosed as atypical hereditary spherocytosis, these conditions can show marked thrombotic complications after splenectomy, which should be avoided. Laboratory studies of these conditions have drawn attention to a 32 kDa membrane protein, stomatin, which seems to act as a regulator of Na and K transport in human and animal tissues generally, but mutations in this gene do not cause these diseases. Genetic mapping in some kindreds, but not all, points to a mutation locus on chromosome 16.

Temperature effects on cation transport in hereditary stomatocytosis and allied disorders

The conditions known as 'hereditary stomatocytosis and allied syndromes' comprise a group of dominantly inherited human haemolytic anaemias characterized by a plasma membrane 'leak' to the univalent cations Na and K, an example of a small but growing group of diseases where pathology can be directly attributed to abnormal membrane transport. A number of case reports in the different variants have alluded to temperature-related phenomena, including loss of K on storage at room temperature (giving 'pseudohyperkalaemia') and lysis of cells when stored in the cold ('cryohydrocytosis'). This review collects together published studies of these temperature effects, which show very major differences in the 'leak' K transport. Two main variations on normal emerge: a 'shallow slope' type, in which the flux shows an abnormally low dependence on temperature in the range 37-20 degrees C, and 'high minimum', in which the minimum in this flux, which occurs in normal cells at 8 degrees C, is shifted up to 23 degrees C. These temperature studies provide a powerful method for phenotypic characterization.