Targeting of Calbindin 1 (CALB1) Rescues Erythropoiesis in a Human Model of Diamond Blackfan Anemia: Implications for Novel Therapies

Diamond Blackfan anemia (DBA) is an inherited bone marrow failure syndrome characterized by red cell aplasia, congenital anomalies and a predisposition to cancer. The erythroid defects of DBA include macrocytosis, reticulocytopenia, and a selective decrease or absence of erythroid precursors in the bone marrow. It was the first disease linked to ribosomal dysfunction and >70% of patients have since been shown to have a haploinsufficiency of a ribosomal protein (RP) gene. Among these RP mutations, RPS19 mutations were the first to be identified and are the most common, found in up to 25% of DBA patients. Current therapies for DBA include corticosteroids, chronic transfusions, and/or hematopoietic stem cell transplantation. However, each of these therapies have potential for significant side effects and as such there is an urgent need to develop new therapies for the management of these patients. In addition there is a need for improved understanding of the mechanistic basis for the wide range of phenotypic expression and responses to therapy between patients with the same mutation, which implies a key role for modifier genes in this disease.

To identify potential modifiers in the phenotype of patients and for developing potential novel therapeutic targets, we performed a genome wide CRISPR screen using A549 cells with RPS19 haploinsufficiency as a cellular model for DBA, which enabled large-scale screening. As the p21 pathway is known to be central to the pathophysiology of DBA, we identified hits that resulted in higher and lower levels of p21 expression in the DBA model vs. control, respectively. Among these hits, we consistently identified Calbindin 1 (CALB1), a member of the calcium-binding superfamily that includes calmodulin and buffers entry of calcium into cells upon stimulation of glutamate receptors.

Given the recent findings that calmodulin inhibition can rescue anemia in DBA models by blocking the p90 ribosomal S6 kinase (RSK)-p70 (p70S6K) pathway and the role for glucose and glutamine metabolism in the regulation of human HSC lineage specification, we decided to pursue CALB1 as a potential target of interest. We used peripheral blood derived-CD34+ cells placed in erythroid stimulating media following knockdown of RPS19, and investigated the effects of CALB1 on cell cycle progression and erythroid differentiation by flow cytometry. We observed that knockdown of CALB1 significantly improved cell viability, increased G0/G1 population and promoted erythroid maturation with increased expression of CD36 and glycophorin A in the context of RPS19 deficiency. Together, these data suggest that the knockdown of CALB1 rescues human erythropoiesis in a model of DBA.

We are investigating the effects of CALB1 knockdown on signaling pathways and genes involved in DBA due to RPS19 mutations notably GATA-1/HSP70 and TNF-alpha. We anticipate that the identification of a role for CALB1 in the pathophysiology of DBA will provide a new therapeutic target for DBA either alone or in combination with steroids or calmodulin inhibition.