Increased Red Cell KCNN4 Activity in Sporadic Hereditary Xerocytosis Associated With Enhanced Single Channel Pressure Sensitivity of PIEZO1 Mutant V598M

Variant spectrum of PIEZO1 and KCNN4 in Japanese patients with dehydrated hereditary stomatocytosis

Hereditary stomatocytosis (HSt) is a type of congenital hemolytic anemia caused by abnormally increased cation permeability of erythrocyte membranes. Dehydrated HSt (DHSt) is the most common subtype of HSt and is diagnosed based on clinical and laboratory findings related to erythrocytes. PIEZO1 and KCNN4 have been recognized as causative genes, and many related variants have been reported. We analyzed the genomic background of 23 patients from 20 Japanese families suspected of having DHSt using a target capture sequence and identified pathogenic/likely pathogenic variants of PIEZO1 or KCNN4 in 12 families.

The role of mechanosensitive Piezo1 channel in diseases

Mechanotransduction is associated with organ development and homoeostasis. Piezo1 and Piezo2 are novel mechanosensitive ion channels (MSCs) in mammals. MSCs are membrane proteins that are critical for the mechanotransduction of living cells. Current studies have demonstrated that the Piezo protein family not only functions in volume regulation, cellular migration, proliferation, and apoptosis but is also important for human diseases of various systems. The complete loss of Piezo1 and Piezo2 function is fatal in the embryonic period. This review summarizes the role of Piezo1 in diseases of different systems and perspectives potential treatments related to Piezo1 for these diseases.

Prenatal exome sequencing and chromosomal microarray analysis in fetal structural anomalies in a highly consanguineous population reveals a propensity of ciliopathy genes causing multisystem phenotypes

Fetal abnormalities are detected in 3% of all pregnancies and are responsible for approximately 20% of all perinatal deaths. Chromosomal microarray analysis (CMA) and exome sequencing (ES) are widely used in prenatal settings for molecular genetic diagnostics with variable diagnostic yields. In this study, we aimed to determine the diagnostic yield of trio-ES in detecting the cause of fetal abnormalities within a highly consanguineous population. In families with a history of congenital anomalies, a total of 119 fetuses with structural anomalies were recruited and DNA from invasive samples were used together with parental DNA samples for trio-ES and CMA. Data were analysed to determine possible underlying genetic disorders associated with observed fetal phenotypes. The cohort had a known consanguinity of 81%. Trio-ES led to diagnostic molecular genetic findings in 59 fetuses (with pathogenic/likely pathogenic variants) most with multisystem or renal abnormalities. CMA detected chromosomal abnormalities compatible with the fetal phenotype in another 7 cases. Monogenic ciliopathy disorders with an autosomal recessive inheritance were the predominant cause of multisystem fetal anomalies (24/59 cases, 40.7%) with loss of function variants representing the vast majority of molecular genetic abnormalities. Heterozygous de novo pathogenic variants were found in four fetuses. A total of 23 novel variants predicted to be associated with the phenotype were detected. Prenatal trio-ES and CMA detected likely causative molecular genetic defects in a total of 55% of families with fetal anomalies confirming the diagnostic utility of trio-ES and CMA as first-line genetic test in the prenatal diagnosis of multisystem fetal anomalies including ciliopathy syndromes.

Hereditary Xerocytosis: Differential Behavior of PIEZO1 Mutations in the N-Terminal Extracellular Domain Between Red Blood Cells and HEK Cells

Hereditary Xerocytosis, a rare hemolytic anemia, is due to gain of function mutations in PIEZO1, a non-selective cation channel activated by mechanical stress. How these PIEZO1 mutations impair channel function and alter red blood cell (RBC) physiology, is not completely understood. Here, we report the characterization of mutations in the N-terminal part of the protein (V598M, F681S and the double mutation G782S/R808Q), a part of the channel that was subject of many investigations to decipher its role in channel gating. Our data show that the electrophysiological features of these PIEZO1 mutants expressed in HEK293T cells are different from previously characterized PIEZO1 mutations that are located in the pore or at the C-terminal extracellular domain of the protein. Although RBC with PIEZO1 mutations showed a dehydrated phenotype, the activity of V598M, F681S or R808Q in response to stretch was not significantly different from the WT channels. In contrast, the G782S mutant showed larger currents compared to the WT PIEZO1. Interestingly, basal activity of all the mutated channels was not significantly altered at the opposite of what was expected according to the decreased water and cation contents of resting RBC. In addition, the features of mutant PIEZO1 expressed in HEK293 cells do not always correlate with the observation in RBC where PIEZO1 mutations induced a cation leak associated with an increased conductance. Our work emphasizes the role of the membrane environment in PIEZO1 activity and the need to characterize RBC permeability to assess pathogenicity to PIEZO1 mutants associated with erythrocyte diseases.

Structural Designs and Mechanogating Mechanisms of the Mechanosensitive Piezo Channels

The evolutionarily conserved Piezo channel family, including Piezo1 and Piezo2 in mammals, serves as versatile mechanotransducers in various cell types and consequently governs fundamental pathophysiological processes ranging from vascular development to the sense of gentle touch and tactile pain. Piezo1/2 possess a unique 38-transmembrane (TM) helix topology and form a homotrimeric propeller-shaped structure comprising a central ion-conducting pore and three peripheral mechanosensing blades. The unusually curved TM region of the three blades shapes a signature nano-bowl configuration with potential to generate large in-plane membrane area expansion, which might confer exquisite mechanosensitivity to Piezo channels. Here, we review the current understanding of Piezo channels with a particular focus on their unique structural designs and elegant mechanogating mechanisms.

Advances in understanding the pathogenesis of red cell membrane disorders

Hereditary erythrocyte membrane disorders are caused by mutations in genes encoding various transmembrane or cytoskeletal proteins of red blood cells. The main consequences of these genetic alterations are decreased cell deformability and shortened erythrocyte survival. Red blood cell membrane defects encompass a heterogeneous group of haemolytic anaemias caused by either (i) altered membrane structural organisation (hereditary spherocytosis, hereditary elliptocytosis, hereditary pyropoikilocytosis and Southeast Asian ovalocytosis) or (ii) altered membrane transport function (overhydrated hereditary stomatocytosis, dehydrated hereditary stomatocytosis or xerocytosis, familial pseudohyperkalaemia and cryohydrocytosis). Herein we provide a comprehensive review of the recent literature on the molecular genetics of erythrocyte membrane defects and their reported clinical consequences. We also describe the effect of low-expression genetic variants on the high inter- and intra-familial phenotype variability of erythrocyte structural defects.

Erythrocyte ion content and dehydration modulate maximal Gardos channel activity in KCNN4 V282M/+ hereditary xerocytosis red cells

Hereditary xerocytosis (HX) is caused by missense mutations in either the mechanosensitive cation channel PIEZO1 or the Ca²⁺-activated K⁺ channel KCNN4. All HX-associated KCNN4 mutants studied to date have revealed increased current magnitude and red cell dehydration. Baseline KCNN4 activity was increased in HX red cells heterozygous for KCNN4 mutant V282M. However, HX red cells maximally stimulated by Ca²⁺ ionophore A23187 or by PMCA Ca²⁺-ATPase inhibitor orthovanadate displayed paradoxically reduced KCNN4 activity. This reduced Ca²⁺-stimulated mutant KCNN4 activity in HX red cells was associated with unchanged sensitivity to KCNN4 inhibitor senicapoc and KCNN4 activator Ca²⁺, with slightly elevated Ca²⁺ uptake and reduced PMCA activity, and with decreased KCNN4 activation by calpain inhibitor PD150606. The altered intracellular monovalent cation content of HX red cells prompted experimental nystatin manipulation of red cell Na and K contents. Nystatin-mediated reduction of intracellular K⁺ with corresponding increase in intracellular Na⁺ in wild-type cells to mimic conditions of HX greatly suppressed vanadate-stimulated and A23187-stimulated KCNN4 activity in those wild-type cells. However, conferral of wild-type cation contents on HX red cells failed to restore wild-type-stimulated KCNN4 activity to those HX cells. The phenotype of reduced, maximally stimulated KCNN4 activity was shared by HX erythrocytes expressing heterozygous PIEZO1 mutants R2488Q and V598M, but not by HX erythrocytes expressing heterozygous KCNN4 mutant R352H or PIEZO1 mutant R2456H. Our data suggest that chronic KCNN4-driven red cell dehydration and intracellular cation imbalance can lead to reduced KCNN4 activity in HX and wild-type red cells.