Page 281 - Haematologica Vol. 107 - September 2022

CASE REPORT
Hereditary anemia caused by multilocus inheritance of PIEZO1, SLC4A1 and ABCB6 mutations: a diagnostic and therapeutic challenge
Red blood cell (RBC) membrane disorders encompass a vast group of hemolytic anemias that differ widely in their clinical, morphologic, laboratory, and molecular features. Subtypes include (i) RBC disorders caused by altered membrane structural organization and (ii) RBC disorders caused by altered membrane transport function.1 The most common anemia among the first group is hereditary spherocytosis (HS), with a prevalence from 1:2000 to 1:5000 births. HS manifests clinically as hemolytic anemia with jaundice, reticulocytosis, splenomegaly, and choleli- thiasis.1 Five genes encoding cytoskeleton and transmem- brane proteins are most commonly associated with HS: ankyrin‐1 (ANK1, 8p11.21), erythrocytic a‐ and b-spectrin chains (SPTA1, 1q21; SPTB, 14q23.3), band 3 anion transport protein (SLC4A1, 17q21.31), and erythrocyte membrane pro- tein band 4.2 (EPB42, 15q15‐q21).2 Disorders of altered membrane transport function include a wide spectrum of hemolytic disorders in which the erythrocyte membrane cation permeability is altered, with dehydrated hereditary stomatocytosis (DHS) the most frequently encountered. The prevalence of DHS remains uncertain, as this disease is often misdiagnosed.3 DHS exhibits alterations of RBC membrane permeability to monovalent cations Na+ and K+, with consequent alterations of intracellular cation, water content, and cell volume. Patients present with hemolytic anemia, typically macrocytic, with increased mean corpuscular hemoglobin (MCH) and mean corpus- cular hemoglobin concentration (MCHC), high reticulocyte count, and jaundice. Blood films may show stomatocytes, usually <10% of total erythrocytes.3 Splenectomy is con- traindicated in patients affected by DHS due to the risk of severe thrombotic events.4 The two causative genes of DHS are PIEZO1 (16q24.3) for DHS type 1 (DHS1) and KCNN4 (19q13.31) for DHS type 2.3,5,6 DHS is also frequently as- sociated with iron overload, which frequently leads to he- patosiderosis.3 Indeed, recent findings highlight a role for PIEZO1 in the regulation of iron metabolism.7 DHS patients can exhibit hyperferritinemia (and even life-threatening hemosiderosis) accompanied by very low values of plasma hepcidin. Overexpression and pharmacological activation of the R2456H and R2488Q PIEZO1 gain-of-function (GoF) mutants in hepatoma cell lines induced decreased ex- pression of the hepcidin-encoding gene, HAMP.7 PIEZO1 in- volvement in iron metabolism was further confirmed in constitutive and in macrophage-specific transgenic PIEZO1 GoF mice. By 1 year of age, these mice develop se-
vere hepatic hemosiderosis with elevated serum ferritin and transferrin saturation, accompanied by increased ery- throphagocytosis, erythropoiesis, and erythroferrone ex- pression.8
DHS may also present as a syndromic form with pseu- dohyperkalemia and/or perinatal edema.2 Familial pseu- dohyperkalemia (FP) can also manifest as an isolated autosomal dominant red cell trait characterized by loss of red cell potassium at low temperatures (<37°C).5 It is caused by mutations in the ABCB6 gene (2q35) encoding the ABCB6 protein, a member of the family of ATP‐binding cassette (ABC) solute transporters that translocate many types of metabolites across intra‐ and extracellular mem- branes.9 The prevalence of FP is probably underestimated, as reflected in the high frequency of certain ABCB6 mis- sense variants in population databases and in two large cohorts of blood donors.5,10
We describe here a 54-year-old female proband followed for 16 years by our clinical genetics unit for severe anemia and iron overload (Figure 1A). The proband presented at age 24 with moderate anemia (hemoglobin [Hb] 9-11 g/dL), jaundice, gallstones, hepatomegaly, and severe spleno- megaly (>18 cm craniocaudal length with weight up to 700 g). Hemoglobinopathies and enzymopathies were ex- cluded. Coombs’s test was negative. Blood smear revealed the presence of numerous stomatocytes, target cells, and rare ovalocytes and erythroblasts (Figure 1B). Osmotic fra- gility was decreased at 2 hours (h) and 24 h post-vene- section. Pink and acidified glycerol lysis tests (AGLT) were positive, leading to an initial clinical diagnosis of HS treated by total splenectomy. After surgery, the proband experienced portal vein thrombosis treated with heparin. Worsening anemia to Hb values of 6-7 g/dL required multiple transfusions. Complete red cell count showed macrocytic anemia with MCV between 110 and 147 fL (Table 1). Mean red blood cell (RBC) half-life (measured by in vitro calcein fluorescence assay) was 21 days, with pro- bable hepatosplenic destruction of RBC.
At the age of 34 years the proband was diagnosed with severe iron overload and hepatosiderosis, and deferoxa- mine treatment was initiated. At age 38 pulmonary emboli were diagnosed, and she was treated with warfarin. At this time the proband also had hyperkalemia (5.0-6.8 mM). Studies of RBC cation content revealed elevated intracel- lular [Na] and low intracellular [K], with increased isotopic fluxes at 37°C (Table 2). Temperature-dependence studies
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