Mouse Models of Hereditary Xerocytosis-Associated PIEZO1 Mutations Reveal Genotype-Phenotype Differences in Hematological Defects and Iron Loading

Gain-of-function mutations in the mechanosensitive ion channel PIEZO1 are a primary cause of hereditary xerocytosis (HX), a disorder of erythrocyte dehydration characterized by a compensated hemolytic anemia and a propensity for iron overload. The PIEZO1 R2456H and M2225R mutations, which were identified in two large kindreds with HX and affect evolutionarily conserved residues, result in increased channel inactivation time, with greater mean prolongation reported for R2456H. Understanding whether different gain-of-function PIEZO1 mutations differentially impact HX-associated pathophysiology is complicated by marked clinical heterogeneity in HX. To address this, we generated two novel mouse models of the analogous murine mutations (R2482H and M2241R) by CRISPR/Cas genome editing of the endogenous Piezo1 locus on a pure C57BL/6 genetic background. For each strain, we intercrossed heterozygotes to generate wild-type, heterozygous, and homozygous offspring, which were raised on a 200 p.p.m. iron diet and phenotyped at 7-8 weeks of age. Statistical comparisons between sex-matched mice were conducted by 1-way ANOVA with Sidak's posthoc test (p<0.05 considered significant).

Complete blood count analyses in the Piezo1 R2482H strain revealed that compared to wild-type (2482^R/R ) littermates, mice heterozygous for the R2482H allele (2482^R/H) showed significantly elevated reticulocytes (%RETIC), but showed similar hemoglobin (HGB), hematocrit (HCT), red blood cell count (RBC), mean corpuscular volume (MCV), mean corpuscular hemoglobin (MCH), mean corpuscular hemoglobin concentration (MCHC), and red cell distribution width (RDW), compatible with a compensated hemolytic defect. Mice homozygous for the R2482H allele (2482^H/H) showed more severe hematological abnormalities than 2482^R/H mice. Reticulocytes and RDW in 2482^H/H mice were significantly higher than in 2482^R/H mice, and 2482^H/H mice showed mean HGB and HCT that were lower than 2482^R/R controls.

Compared to either 2482^R/R or 2482^R/H mice, 2482^H/H mice also showed significantly greater spleen weight/body weight ratio, which was accompanied by histological evidence of increased extramedullary hematopoiesis (EMH) in spleen. Total non-heme iron content of spleen (calculated as spleen non-heme iron concentration multiplied by spleen weight) was significantly higher in 2482^H/H mice compared to 2482^R/R controls. Compared to wild-type spleens, both 2482^R/H and 2482^H/H spleens showed qualitatively greater non-heme iron in red pulp macrophages by Prussian blue stain, compatible with greater red blood cell turnover. Additionally, in males, liver non-heme iron concentration was significantly higher in 2482^H/H mice than in either 2482^R/R or 2482^R/H mice. Heart weight/body weight ratio was significantly higher in 2482^H/H mice than in 2482^R/R controls.

Characterization of the M2241R mouse strain revealed that compared to wild-type (2241^M/M) littermates, 2241^M/R heterozygotes showed significantly elevated %RETIC and maintained normal HGB, HCT, and red cell indices. Mice homozygous for the M2241R allele (2241^R/R) showed significantly higher %RETIC than 2241^M/R mice. However, %RETIC, spleen weight/body weight ratio, and splenic total non-heme iron content in 2241^R/R mice remained significantly lower than their respective levels in mice homozygous for the R2482H allele (2482^H/H). Additionally, unlike 2482^H/H males, 2241^R/R males did not show liver iron loading. No significant effects of the M2241R or R2482H allele were detected on serum transferrin saturation or serum hepcidin levels.

In summary, we demonstrate that different gain of function alleles of Piezo1 produce different degrees of hematological defects and iron loading in liver and spleen. Interestingly, in contrast to our results, elevated liver iron content was not detected in young adult mice homozygously expressing a conditional Piezo1 R2482H allele under the constitutive Cmv-Cre in a prior study (Ma S, Cell 2021), which might reflect differences in iron quantification methods or in the different Piezo1 alleles themselves. Future studies will be of interest to determine if the phenotypic differences observed between the R2482H and M2241R strains reflect mutation-dependent differences in PIEZO1 function in erythrocytes, macrophages, hepatocytes, and/or other cell types.