Oncogenic signaling triggered by mutant calreticulins
Acquired hematopoietic stem cell mutations of calreticulin (CALR) turn a chaperone ER resident protein into a rogue ligand for the thrombopoietin receptor (MPL) - a cytokine receptor expressed in the stem cells and the megakaryocytic lineage. Assembly of the mutant CALR/MPL complex in myeloid cells results in thrombopoietin-independent oncogenic signaling and myeloproliferative phenotype. The Kralovics group studies the CALR/MPL signaling in model cell lines and mouse models aiming to exploit its distinctive features for therapeutic interventions.
Several approaches were undertaken to uncover vulnerabilities of CALR mutated cells. 1) High-throughput drug screening identified several pathways that could be targeted in CALR mutated MPNs. 2) Genome-wide CRISPR/Cas9 mutagenesis revealed gene-gene dependencies either enhancing fitness of CALR mutant cells or causing their death (synthetic lethality). All these new leads are currently explored to identify new drug targets for MPNs.
Development of CAR-T cell therapy for CALR mutated MPN
Since the oncogenic CALR mutations in MPN are all exclusively frameshift mutations in the last exon of the gene, all patients carry a unique C-terminal protein sequence that is missing in healthy cells. This feature in combination with the cell surface expression of the CALR/MPL oncogenic complex makes it an ideal target for antibody-based immunotherapy.
The group generated specific anti-mutant CALR monoclonal antibodies that bind the mutant CALR also on the cell surface in complex with MPL (Figure). These antibodies serve as antigen binding domains for chimeric antigen receptors (CARs) that the group utilizes for engineering T cells that selectively kill CALR mutant cells.