A genetic syndrome associating dehydrated hereditary stomatocytosis, pseudohyperkalaemia and perinatal oedema

Digenic Inheritance of a FOXC2 Mutation and Two PIEZO1 Mutations Underlies Congenital Lymphedema in a Multigeneration Family

BACKGROUND

The lymphatic system is essential for maintaining the balance of interstitial fluid in tissues and for returning protein-rich fluids (lymph) to the bloodstream. Congenital lymphatic defects lead to accumulation of lymph in peripheral tissues and body cavities, termed primary lymphedema. To date, only a limited number of individual genes have been identified in association with primary lymphedema. However, variability of age of onset and severity of lymphatic abnormalities within some families suggests that multiple mutations or genes may be responsible, thus hampering efforts to identify individual associated genes.

METHODS

Whole exome sequencing (WES) was performed in 4 members of a large multigeneration family with highly variable lymphedema and followed by Sanger sequencing for identified mutations in 34 additional family members. Genotypes were correlated with clinical and lymphangioscintigraphic phenotypes.

RESULTS

WES uncovered 2 different mechanotransducer PIEZO1 mutations and one FOXC2 transcription factor mutation in various combinations. Sanger sequencing confirmed the presence/absence of the 3 variants in affected and unaffected family members and co-segregation of one or more variants with disease. Genetic profiles did not clearly correlate with the highly variable severity of lymphatic abnormalities.

CONCLUSIONS

WES in lymphedema families can uncover unexpected combinations of several lymphedema-associated mutations. These findings provide essential information for genetic counseling and reveal complex gene interactions in lymphatic developmental pathways. These can offer insights into the complex spectrum of clinical and lymphatic lymphedema phenotypes and potential targets for treatment.

PIEZO1-gene gain-of-function mutations with lower limb lymphedema onset in an adult: Clinical, scintigraphic, and noncontrast magnetic resonance lymphography findings

Primary lymphedema, a rare disease, has a genetic cause in ~40% of patients. Recently, loss-of-function mutations in PIEZO1, which encodes the mechanotransducer protein PIEZO1, were described as causing primary lymphedema, when gain-of-function PIEZO1 mutations were attributed to dehydrated hereditary stomatocytosis type-1 (DHS), a dominant red cell hemolytic disorder, with ~20% of patients having perinatal edema. Lymphedema was diagnosed in a 36-year-old man from a three-generation DHS family, with a PIEZO1-allele harboring 3 missense mutations in cis. Four affected family members had severe fetal and neonatal edema, most severe in the proband, whose generalized edema with prevailing ascites resolved after 8 months. Our patient's intermittent lower limb-lymphedema episodes during hot periods appeared at puberty; they became persistent and bilateral at age 32. Clinical Stemmer's sign confirmed lymphedema. Lower leg lymphoscintigraphy showed substantial dermal backflow in both calves, predominantly on the right. Noncontrast magnetic resonance lymphography showed bilateral lower limb lymphedema, dilated dysplastic lymphatic iliac, and inguinal trunks. Exome-sequencing analysis identified no additional pathogenic variation in primary lymphedema-associated genes. This is the first description of well-documented lymphedema in an adult with PIEZO1-DHS. The pathophysiology of PIEZO1-associated primary lymphedema is poorly understood. Whether it infers overlapping phenotypes or different mechanisms of gain- and loss-of-function PIEZO1 mutations deserves further investigation.

Coordination of WNT signaling and ciliogenesis during odontogenesis by piezo type mechanosensitive ion channel component 1

Signal transmission from the mechanical forces to the various intracellular activities is a fundamental process during tissue development. Despite their critical role, the mechanism of mechanical forces in the biological process is poorly understood. In this study, we demonstrated that in the response to hydrostatic pressure (HP), the piezo type mechanosensitive ion channel component 1 (PIEZO1) is a primary mechanosensing receptor for odontoblast differentiation through coordination of the WNT expression and ciliogenesis. In stem cells from human exfoliated deciduous teeth (SHED), HP significantly promoted calcium deposition as well as the expression of odontogenic marker genes, PANX3 and DSPP, and WNT related-genes including WNT5b and WNT16, whereas HP inhibited cell proliferation and enhanced primary cilia expression. WNT signaling inhibitor XAV939 and primary cilia inhibitor chloral hydrate blocked the HP-induced calcium deposition. The PIEZO1 activator Yoda1 inhibited cell proliferation but induced ciliogenesis and WNT16 expression. Interestingly, HP and Yoda1 promoted nuclear translocation of RUNX2, whereas siRNA-mediated silencing of PIEZO1 decreased HP-induced nuclear translocation of RUNX2. Taken together, these results suggest that PIEZO1 functions as a mechanotransducer that connects HP signal to the intracellular signalings during odontoblast differentiation.

Rare Hereditary Hemolytic Anemias: Diagnostic Approach and Considerations in Management

Hereditary hemolytic anemias (HHAs) comprise a heterogeneous group of anemias caused by mutations in genes coding the globins, red blood cell (RBC) membrane proteins, and RBC enzymes. Congenital dyserythropoietic anemias (CDAs) are rare disorders of erythropoiesis characterized by binucleated and multinucleated erythroblasts in bone marrow. CDAs typically present with a hemolytic phenotype, as the produced RBCs have structural defects and decreased survival and should be considered in the differential of HHAs. This article discusses the clinical presentation, laboratory findings, and management considerations for rare HHAs arising from unstable hemoglobins, RBC hydration defects, the less common RBC enzymopathies, and CDAs.

Hereditary stomatocytosis: An underdiagnosed condition

Hereditary stomatocytoses are a wide class of hemolytic anemias characterized by alterations of ionic flux with increased cation permeability that results in inappropriate shrinkage or swelling of the erythrocytes, and water lost or gained osmotically. The last few years have been crucial for new acquisitions in this field in terms of identifying new causative genes and of studying their pathogenetic mechanisms. This review summarizes the main features of erythrocyte membrane transport diseases, dividing them into forms with either isolated erythroid phenotype (nonsyndromic) or extra-hematological manifestations (syndromic), and focusing particularly on the most recent advances regarding dehydrated forms of hereditary stomatocytosis and familial pseudohyperkalemia.

Disorders of erythrocyte hydration

The erythrocyte contains a network of pathways that regulate salt and water content in the face of extracellular and intracellular osmotic perturbations. This allows the erythrocyte to maintain a narrow range of cell hemoglobin concentration, a process critical for normal red blood cell function and survival. Primary disorders that perturb volume homeostasis jeopardize the erythrocyte and may lead to its premature destruction. These disorders are marked by clinical, laboratory, and physiologic heterogeneity. Recent studies have revealed that these disorders are also marked by genetic heterogeneity. They have implicated roles for several proteins, PIEZO1, a mammalian mechanosensory protein; GLUT1, the glucose transporter; SLC4A1, the anion transporter; RhAG, the Rh-associated glycoprotein; KCNN4, the Gardos channel; and ABCB6, an adenosine triphosphate-binding cassette family member, in the maintenance of erythrocyte volume homeostasis. Secondary disorders of erythrocyte hydration include sickle cell disease, thalassemia, hemoglobin CC, and hereditary spherocytosis, where cellular dehydration may be a significant contributor to disease pathology and clinical complications. Understanding the pathways regulating erythrocyte water and solute content may reveal innovative strategies to maintain normal volume in disorders associated with primary or secondary cellular dehydration. These mechanisms will serve as a paradigm for other cells and may reveal new therapeutic targets for disease prevention and treatment beyond the erythrocyte.

Hereditary Xerocytosis due to Mutations in PIEZO1 Gene Associated with Heterozygous Pyruvate Kinase Deficiency and Beta-Thalassemia Trait in Two Unrelated Families

Hereditary xerocytosis (HX) is a rare disorder caused by defects of RBC permeability, associated with haemolytic anaemia of variable degree and iron overload. It is sometimes misdiagnosed as hereditary spherocytosis or other congenital haemolytic anaemia. Splenectomy is contraindicated due to increased risk of thromboembolic complications. We report the clinical, haematological, and molecular characteristics of four patients from two unrelated Italian families affected by HX, associated with beta-thalassemia trait and heterozygous pyruvate kinase deficiency, respectively. Two patients had been splenectomised and displayed thrombotic episodes. All patients had iron overload in the absence of transfusion, two of them requiring iron chelation. The diagnosis of HX was confirmed by LoRRca Osmoscan analysis showing a left-shifted curve. PIEZO1 gene sequencing revealed the presence of mutation p.E2496ELE, showing that this is one of the most frequent mutations in this disease. The concomitant defects did not aggravate the clinical phenotype; however, in one patient, the initial diagnosis of pyruvate kinase deficiency delayed the correct diagnosis of HX for many years and resulted in splenectomy followed by thrombotic complications. The study underlines the importance of a precise diagnosis in HX, particularly in view of splenectomy, and the need of a molecular confirmation of suspected RBC enzymopathy.

Genetic Diseases of PIEZO1 and PIEZO2 Dysfunction

Mutations in the genes encoding the mechanosensitive cation channels PIEZO1 and PIEZO2 are responsible for multiple hereditary human diseases. Loss-of-function mutations in the human PIEZO1 gene cause autosomal recessive congenital lymphatic dysplasia. Gain-of-function mutations in the human PIEZO1 gene cause the autosomal dominant hemolytic anemia, hereditary xerocytosis (also known as dehydrated stomatocytosis). Loss-of-function mutations in the human PIEZO2 gene cause an autosomal recessive syndrome of muscular atrophy with perinatal respiratory distress, arthrogryposis, and scoliosis. Gain-of-function mutations in the human PIEZO2 gene cause three clinical types of autosomal dominant distal arthrogryposis. This chapter will review the hereditary diseases caused by mutations in the PIEZO genes and will discuss additional physiological systems in which PIEZO channel dysfunction may contribute to human disease pathophysiology.

New insights on hereditary erythrocyte membrane defects

After the first proposed model of the red blood cell membrane skeleton 36 years ago, several additional proteins have been discovered during the intervening years, and their relationship with the pathogenesis of the related disorders have been somewhat defined. The knowledge of erythrocyte membrane structure is important because it represents the model for spectrin-based membrane skeletons in all cells and because defects in its structure underlie multiple hemolytic anemias. This review summarizes the main features of erythrocyte membrane disorders, dividing them into structural and altered permeability defects, focusing particularly on the most recent advances. New proteins involved in alterations of the red blood cell membrane permeability were recently described. The mechanoreceptor PIEZO1 is the largest ion channel identified to date, the fundamental regulator of erythrocyte volume homeostasis. Missense, gain-of-function mutations in the PIEZO1 gene have been identified in several families as causative of dehydrated hereditary stomatocytosis or xerocytosis. Similarly, the KCNN4 gene, codifying the so called Gardos channel, has been recently identified as a second causative gene of hereditary xerocytosis. Finally, ABCB6 missense mutations were identified in different pedigrees of familial pseudohyperkalemia. New genomic technologies have improved the quality and reduced the time of diagnosis of these diseases. Moreover, they are essential for the identification of the new causative genes. However, many questions remain to solve, and are currently objects of intensive studies.

Touch, Tension, and Transduction - The Function and Regulation of Piezo Ion Channels

In 2010, two proteins, Piezo1 and Piezo2, were identified as the long-sought molecular carriers of an excitatory mechanically activated current found in many cells. This discovery has opened the floodgates for studying a vast number of mechanotransduction processes. Over the past 6 years, groundbreaking research has identified Piezos as ion channels that sense light touch, proprioception, and vascular blood flow, ruled out roles for Piezos in several other mechanotransduction processes, and revealed the basic structural and functional properties of the channel. Here, we review these findings and discuss the many aspects of Piezo function that remain mysterious, including how Piezos convert a variety of mechanical stimuli into channel activation and subsequent inactivation, and what molecules and mechanisms modulate Piezo function.

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.

Impaired PIEZO1 function in patients with a novel autosomal recessive congenital lymphatic dysplasia

Piezo1 ion channels are mediators of mechanotransduction in several cell types including the vascular endothelium, renal tubular cells and erythrocytes. Gain-of-function mutations in PIEZO1 cause an autosomal dominant haemolytic anaemia in humans called dehydrated hereditary stomatocytosis. However, the phenotypic consequence of PIEZO1 loss of function in humans has not previously been documented. Here we discover a novel role of this channel in the lymphatic system. Through whole-exome sequencing, we identify biallelic mutations in PIEZO1 (a splicing variant leading to early truncation and a non-synonymous missense variant) in a pair of siblings affected with persistent lymphoedema caused by congenital lymphatic dysplasia. Analysis of patients' erythrocytes as well as studies in a heterologous system reveal greatly attenuated PIEZO1 function in affected alleles. Our results delineate a novel clinical category of PIEZO1-associated hereditary lymphoedema.

To shrink or not to shrink

In this issue of Blood, Andolfo and colleagues show that dehydrated hereditary stomatocytosis (DHSt), an inherited red cell disorder, is associated with a number of distinct germline mutations in PIEZO1, a stretch activated cation channel, in 26 affected individuals from 7 families.1

Multiple clinical forms of dehydrated hereditary stomatocytosis arise from mutations in PIEZO1

Autosomal dominant dehydrated hereditary stomatocytosis (DHSt) usually presents as a compensated hemolytic anemia with macrocytosis and abnormally shaped red blood cells (RBCs). DHSt is part of a pleiotropic syndrome that may also exhibit pseudohyperkalemia and perinatal edema. We identified PIEZO1 as the disease gene for pleiotropic DHSt in a large kindred by exome sequencing analysis within the previously mapped 16q23-q24 interval. In 26 affected individuals among 7 multigenerational DHSt families with the pleiotropic syndrome, 11 heterozygous PIEZO1 missense mutations cosegregated with disease. PIEZO1 is expressed in the plasma membranes of RBCs and its messenger RNA, and protein levels increase during in vitro erythroid differentiation of CD34(+) cells. PIEZO1 is also expressed in liver and bone marrow during human and mouse development. We suggest for the first time a correlation between a PIEZO1 mutation and perinatal edema. DHSt patient red cells with the R2456H mutation exhibit increased ion-channel activity. Functional studies of PIEZO1 mutant R2488Q expressed in Xenopus oocytes demonstrated changes in ion-channel activity consistent with the altered cation content of DHSt patient red cells. Our findings provide direct evidence that R2456H and R2488Q mutations in PIEZO1 alter mechanosensitive channel regulation, leading to increased cation transport in erythroid cells.

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.

Prenatal diagnosis and management of fetal xerocytosis associated with ascites

OBJECTIVES

To discuss the prenatal diagnosis and management of fetal xerocytosis associated with ascites.

CASE REPORT

A 29-year-old woman with hereditary xerocytosis was found to present a fetus with severe ascites on the 20-week scan. Cordocentesis showed mild anemia and blood transfusion was discarded. Ascites worsened and 2 weeks later a new cordocentesis showed lower hematocrit values. Blood transfusion was performed but ascites remained unchanged. Cordocentesis was repeated at 28 weeks and albumin was transfused. Fetal hemoglobin was within the normal range. Peak systolic velocity of the middle cerebral artery remained normal and correctly predicted mild anemia. Expectant management was followed. An elective cesarean section was performed at 32 weeks because of breech presentation and preterm labor which did not respond to aggressive tocolysis. A female newborn weighing 2,615 g was delivered and required paracenteses and exchange transfusion. The newborn was discharged at 4 weeks of life and at 2 months of age, the ascites resolved completely.

CONCLUSIONS

The mechanism of development of ascites in fetal xerocytosis remains unanswered. As ascites does not seem to be related to fetal anemia or hypoalbuminemia, does not substantially change after blood transfusion and tends to resolve in late gestation, a conservative management is reasonable if fetal anemia is not severe.

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.

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.

An extreme consequence of splenectomy in dehydrated hereditary stomatocytosis: gradual thrombo-embolic pulmonary hypertension and lung-heart transplantation

Dehydrated hereditary stomatocytosis (DHS) belongs to the heterogeneous class of hemolytic anemias with leaky red cell membranes. Splenectomy is a highly deleterious treatment, because it favors, with virtually no exception, the occurrence of thromboembolic disease. We describe here the extreme case of a patient with DHS and an associated sickle cell trait. Splenectomy was carried out due to a splenic infarction that occurred during an airplane journey. About 12 years later, the patient noticed an exertional dyspnea, which gradually worsened to such a degree that she became severely incapacitated within 5 years. Eventually, the patient developed a cor pulmonale associated with chronic thromboembolic pulmonary hypertension (CTEPH) and successfully underwent a heart-lung transplant operation. This case ranks as one of the most severe examples ever recorded of the effect that splenectomy may have in DHS patients. Nonetheless, it represents the first case to receive a heart-lung transplant.

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.

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.

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

We describe two British families with similar, dominantly-inherited, temperature-related variants of hereditary stomatocytosis, consistent with the original description of 'cryohydrocytosis'. The cells show a 5-6-fold increase in passive permeability at 37 degrees C with abnormal intracellular Na and K levels at 15-20 and 60-65 mmol/(l cells) respectively. Marked temperature effects were evident: lysis of red cells on storage in the cold was blatant and when whole heparinized blood was stored at room temperature, K accumulated in the plasma, producing 'pseudohyperkalaemia'. Studies of the temperature dependence of passive permeability showed that the minimum in the passive permeability, which is seen in normal cells at 8-10 degrees C, was shifted up to 23 degrees C in these abnormal cells, such that the permeability at 0 degrees C exceeded that at 37 degrees C. The abnormal temperature dependence in these genetically abnormal red cells strongly resembles that seen in normal cells when suspended in media in which either Na or Cl has been replaced by an organic cation or anion: it could be said these cells had a genetic mutation that somehow rendered the cell resistant to the stabilizing action of NaCl at low temperatures.

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

Familial pseudohyperkalemia is a “leaky red blood cell” condition in which the cells show a temperature-dependent loss of potassium (K) from red blood cells when stored at room temperature, manifesting as apparent hyperkalemia. The red blood cells show a reduced lifespan in vivo but there is no frank hemolysis. Studies of cation content and transport show a marginal increase in permeability at 37°C and a degree of cellular dehydration, qualitatively similar to the changes seen in dehydrated hereditary stomatocytosis (hereditary xerocytosis). Physiological studies have shown that the passive leak to K has an abnormal temperature dependence, such that the leak is less sensitive to temperature than that in normal cells. We performed genetic mapping on the original family and found that the condition in this kindred maps to the same locus (16q23-ter) that we have previously identified for an Irish family with dehydrated hereditary stomatocytosis, which does not show the same temperature effects.

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

Familial pseudohyperkalemia is a “leaky red blood cell” condition in which the cells show a temperature-dependent loss of potassium (K) from red blood cells when stored at room temperature, manifesting as apparent hyperkalemia. The red blood cells show a reduced lifespan in vivo but there is no frank hemolysis. Studies of cation content and transport show a marginal increase in permeability at 37°C and a degree of cellular dehydration, qualitatively similar to the changes seen in dehydrated hereditary stomatocytosis (hereditary xerocytosis). Physiological studies have shown that the passive leak to K has an abnormal temperature dependence, such that the leak is less sensitive to temperature than that in normal cells. We performed genetic mapping on the original family and found that the condition in this kindred maps to the same locus (16q23-ter) that we have previously identified for an Irish family with dehydrated hereditary stomatocytosis, which does not show the same temperature effects.

Hereditary dehydrated and overhydrated stomatocytosis: recent advances

The hereditary stomatocytoses and allied disorders are genetic defects of the erythrocyte membrane that result in abnormal permeability to the univalent cations Na+ and K+. Although rare, these conditions reflect abnormalities in physiologic mechanisms that are of paramount interest. All cases (as defined here) show increased plasma membrane permeability to Na+ and K+ and, to a greater or lesser degree, stomatocytic morphology. Dehydrated hereditary stomatocytosis, the most common form of hereditary stomatocytosis, is more heterogeneous than previously thought and includes kindreds showing pseudohyperkalemia or perinatal edema, or both. The gene responsible for both dehydrated hereditary stomatocytosis and familial pseudohyperkalemia, a nonhemolytic variant that presents with high plasma K+ levels, has been mapped to 16q23-qter. The cause of overhydrated hereditary stomatocytosis remains elusive despite the manifest lack of the enigmatic protein stomatin in the erythrocyte membrane. In all cases where splenectomy has been performed, this procedure has conferred a marked risk for thrombosis in adult life. This finding stresses the importance of diagnostic distinction between these conditions and hereditary spherocytosis.

A variant of hereditary stomatocytosis with marked pseudohyperkalaemia

A family with an unusual form of hereditary stomatocytosis is described. The affected members showed a mild, dominantly-inherited, haemolytic anaemia with intracellular Na and K levels of 41-48 and 44-53 mmol/(l cells) respectively. This anaemia was associated with marked 'pseudohyperkalaemia': that is, loss of K from red cells on storage at room temperature. At 37 degrees C, 'leak' tracer flux rates (assessed as the ouabain + bumetanide-resistant K fluxes) showed a roughly 5-fold acceleration compared to normal, and an abnormal temperature dependence with a shallow slope between 37 and 20 degrees C (mean Q10 (ratio of reaction rates at temperature T and T - 10) over this interval, 1.6; normal 2.2). The pseudohyperkalaemia could be attributed to the disparity between pump and leak at 20 degrees C. This is an identical mechanism to that previously shown for the haemato logically trivial condition, 'familial pseudohyperkalaemia. No protein or lipid abnormality was found in the membrane of these cells.