Diagnosing dehydrated hereditary stomatocytosis due to a KCNN4 Gardos channel mutation: understanding challenges through study of a multi‐generational family

To the Editor, Dehydrated Hereditary Stomatocytosis (DHSt) is autosomal dominant hemolytic anemia with an estimated incidence of 1:50,000 births. DHSt is characterized by largely compensated hemolysis and splenomegaly and is attributed primarily to mutations in either the PIEZO1 or KCNN4 genes [1]. The majority of described DHSt cases result from gain-of-function mutations in the mechanosensitive cation channel gene PIEZO1, leading to an inappropriate increase in calcium influx. This, in turn, activates the calcium-sensitive, potassiumselective Gardos channel which mediates potassium conductance and regulates red cell volume [2]. Less frequently, gain-of-function mutations have been identified in the Gardos channel gene, KCNN4, that alter calcium sensitivity and result in amore active channel. Therefore, DHSt due to KCNN4mutation is aptly termed a Gardos channelopathy [2–4]. Though a complex compensatorymechanism has been proposed in those with KCNN4 mutations, the increased cation leak across the red blood cell membrane in cases with either PIEZO1 or KCNN4mutations is accompanied by intracellular dehydration and the formation of stomatocytes [4]. Since the first reports in 2015, to our knowledge, KCNN4mutations have been identified in ten DHSt families [2–6]. PIEZO1 mutations in DHStaremoreprevalent,with at least eight timesasmanycases identified [2]. Here we report the eleventh family with DHSt due to a KCNN4 substitution mutation that results in p.R352H. This family highlights

births. DHSt is characterized by largely compensated hemolysis and splenomegaly and is attributed primarily to mutations in either the PIEZO1 or KCNN4 genes [1]. The majority of described DHSt cases result from gain-of-function mutations in the mechanosensitive cation channel gene PIEZO1, leading to an inappropriate increase in calcium influx. This, in turn, activates the calcium-sensitive, potassiumselective Gardos channel which mediates potassium conductance and regulates red cell volume [2]. Less frequently, gain-of-function mutations have been identified in the Gardos channel gene, KCNN4, that alter calcium sensitivity and result in a more active channel. Therefore, DHSt due to KCNN4 mutation is aptly termed a Gardos channelopathy [2][3][4]. Though a complex compensatory mechanism has been proposed in those with KCNN4 mutations, the increased cation leak across the red blood cell membrane in cases with either PIEZO1 or KCNN4 mutations is accompanied by intracellular dehydration and the formation of stomatocytes [4].
Since the first reports in 2015, to our knowledge, KCNN4 mutations have been identified in ten DHSt families [2][3][4][5][6]. PIEZO1 mutations in DHSt are more prevalent, with at least eight times as many cases identified [2]. Here we report the eleventh family with DHSt due to a KCNN4   Previous clinical reports highlight phenotypic variability both within and between the ten KCNN4 families [2][3][4][5][6]. It is possible that some differences are due to the specific underlying mutation; seven families have p.R352H, whereas the other families had novel mutations of p.V282E, p.V282M, and a 28 bp deletion (c.1109_1119 + 17del). One large family with the p.V282M mutation had well-compensated anemia with near-normal hemoglobin levels [5]. In contrast, the proband of another family with p.R352H required transfusion in utero and after preterm birth [3]. Other cases, including those with p.R352H, had variable transfusion requirements during childhood, typically without the ongoing need for transfusion support during adulthood. The majority developed cholelithiasis requiring cholecystectomy. Iron overload was relatively common, though variable within and between families. As in our family, many reported cases underwent splenectomy before a formal diagnosis was made, and splenectomy failed to improve the disease features [2,5].
This is particularly the case with regard to the frequency of iron In the family reported here, as well as others, stomatocytes were infrequent, no xerocytes were identified to assist in diagnosis, and the MCHC was normal. Furthermore, the typical features described for the disease process of DHSt are largely influenced by the disparity in case numbers, which are heavily weighted toward PIEZ01 mutations.
As DHSt due to a Gardos channel KCNN4 mutation is likely to be much more prevalent than reported, confirmation is critical to counsel patients and families appropriately, avoid unnecessary splenectomy and recognize iron overload early in the course of this disorder. Increased recognition of this disease process could also lead to significant treatment advances, such as the use of the selective Gardos channel inhibitor, Senicapoc. This medication has been shown to improve hemolysis in patients with sickle cell disease and might be used to prevent red blood cell dehydration due to a gain of function mutation in KCNN4 [1]. The recognition of this specific disease entity will require a high level of clinical suspicion, plus the inclusion of the KCNN4 gene mutation in commercially available hemolytic anemia panels to appropriately investigate those patients with chronic non-spherocytic hemolytic anemia.

CONFLICT OF INTEREST
The authors have declared no conflict of interest.

ETHICS APPROVAL
This study was performed with IRB approval (Study 00009292), informed consent, and ethics approval as specified by the Human Subjects Protection Office of the Milton S Hershey Medical Center of the Pennsylvania State University.

DATA AVAILABILITY STATEMENT
Supplemental exome sequence data are available on reasonable request to Dr. Carrel.