The Flow Cytometric Osmotic Fragility Test Is an Effective Screening Method for Dehydrated Hereditary Stomatocytosis

Introduction: Dehydrated hereditary stomatocytosis (DHSt) or hereditary xerocytosis (HX) is a form of congenital hemolytic anemia characterized by red blood cell (RBC) dehydration. Heterozygous mutations in PIEZO1, a mechanically-activated ion channel, cause DHSt. Recently, KCNN4, which encodes the Gardos channel, has been found to be the second pathogenic gene for DHSt. DHSt is characterized by an alteration in the RBC morphology in target cells, stomatocytes, and/or echinocytes, and RBC deformability assessments by ektacytometry as well as RBC ion flux measurements are currently the standard laboratory tests for DHSt, but their use in laboratories is limited. The flow cytometric osmotic fragility (FCM-OF) test is a useful diagnostic test for hereditary spherocytosis (HS) and also for hereditary elliptocytosis (HE). In this study, we showed that the FCM-OF test could also successfully diagnose DHSt.

Subjects: A total of 46 cases of RBC membrane disorders were examined, and tentative diagnoses were made based on the RBC morphology, acid glycerol lysis time, and eosin 5'-maleimide binding tests, resulting in HS (n=31), HE (n=6), and DHSt (n=9).

Methods: The number of RBCs in isotonic and hypotonic buffers were measured by flow cytometry. The degree of osmotic fragility was expressed as the "percentage residual RBCs (%RRC)". We confirmed the DHSt diagnosis by the massively paralleled sequencing using our custom panels targeting 68 hemolytic anemia-related genes with the next-generation sequencer.

Results: Both HS and HE patients showed a decrease in %RRC; HS (18.0±8.9%, p<0.001) and HE (41.8±15.7%, p<0.001) compared to normal control (66.7±1.5%). DHSt patients showed a significant increase (112.6±34.5%, p<0.001) in FCM-OF. Additionally, next-generation sequencing revealed consistent causative gene mutations for DHSt; PIEZO1 (p.R2488Q and p.E2496ELE) or KCNN4 (p.P204R, p.A279T and p.R352H).

Discussion: We examined 77 patients with congenital hemolytic anemia recently, and 59 cases were confirmed by diagnostic tests (76.6%). The results were as follows: 48 cases of RBC membrane abnormality (62.3%), 6 cases of RBC enzymopathy (7.8%), and 5 cases of hemoglobinopathy (6.5%). Of the cases of RBC membrane disorders, 31 cases of HS, 9 cases of DHSt, and 8 cases of HE were identified. These observations suggest that DHSt is the second-most common RBC membranopathy in Japan, and that the FCM-OF test and targeted sequencing efficiently discriminate DHSt from other RBC membrane disorders.