Introduction and Genetic Correction of Calreticulin Mutations in Human Hematopoietic Stem and Progenitor Cells Sheds Light on MPN Pathogenesis
Authors: Foßelteder Johannes , Schlacher Angelika , Pabst Gabriel , Amtmann Bettina , Schöll Wolfgang , Kashofer Karl , Beham-Schmid Christine , Schlenke Peter , Greinix Hildegard T. , Sill Heinz , Woelfler Albert , Zebisch Armin , Reinisch Andreas 
Published: November 2021

Abstract
Introduction:
Recurrent mutations in calreticulin (CALR) are present in 70% to 80% of essential thrombocythemia (ET) and primary myelofibrosis (PMF) patients without a JAK2 or MPL mutation. Despite recent advances in understanding mutant CALR, the detailed mechanisms are not fully elucidated, and current knowledge is mainly based on transgenic mouse models or human cancer cell lines. Thus, to more faithfully model MPN pathogenesis, we first aimed to introduce heterozygous type-1 and type-2 CALR mutations into healthy human hematopoietic stem and progenitor cells (HSPCs) via targeted CRISPR/Cas9-mediated gene knock-in (KI) and investigate its impact on HSPC function in vitro and in vivo. Second, we aimed to correct CALR mutations in patient-derived HSPCs to study their dependence on the initial driver event to exert an MPN phenotype.
Methods:
We used CRISPR/Cas9 to introduce heterozygous CALR mutations into the endogenous gene locus of healthy cord blood-derived HSPCs. Our approach is based on homologous recombination using DNA repair templates delivered by adeno-associated virus serotype 6 (AAV6). Briefly, Cas9-sgRNA ribonucleoprotein (RNP) was used to cut the DNA. Simultaneously AAV6, carrying either a mutation-bearing or a wildtype control cDNA, was co-delivered to allow for targeted in-frame integration. This way, mutant CALR remains under the control of the endogenous promoter. Concurrent integration of a fluorescent reporter downstream of the mutated exon, enabled purification and tracking of modified cells via flow cytometry. Purified CRISPR-modified HSPCs were used for in vitro collagen-based colony-forming assays, proliferation and differentiation assays in liquid culture, and intrafemoral transplantation into immunodeficient NSG mice to assess their pathogenic potential.
Results:
Our CRISPR/Cas9 KI strategy enabled us to efficiently generate and enrich for heterozygous CALR mutant human HSPCs. Modified cells harbor the mutation at the endogenous CALR locus with intact gene regulatory regions. Correct integration and transcript expression were confirmed on DNA and RNA level by sanger sequencing. Additionally, CALR mutant protein expression was confirmed via immunohistochemistry using a diagnostically approved mutant-specific antibody.
Type-1 and type-2 CALR mutations led to TPO-independent growth of CD34 + HSPC-derived cells and a two-fold (p&lt;0.01) increase of megakaryocyte colonies in collagen-based media compared to wildtype control KI. These findings were corroborated by significantly enhanced CD41 + CD42b + megakaryocyte formation of CALR mutant HSPCs upon liquid culture differentiation.
When transplanted into sublethally irradiated immunodeficient NSG mice, CALR mutant HSPCs showed robust engraftment in the bone marrow with a myeloid lineage skewing, outcompetition of wildtype cells and increased formation of CALR mutant CD41 + megakaryocyte progenitors.
To investigate, if removal of type-1 and type-2 CALR mutations can ameliorate MPNs, we utilized our KI strategy to correct both CALR mutations in MPN patient-derived HSPCs by replacing them with wildtype sequences. A successful correction was confirmed on DNA and RNA level and by the absence of mutant CALR protein. Opposite to the results from introducing CALR mutations, correcting the mutations led to a two-fold decrease in megakaryocyte colony formation. Interestingly this was only seen in ET and post-ET MF samples, whereas primary MF samples were unaffected, underscoring the importance of other secondary genetic driver events in the pathogenesis of primary MF.
Conclusion:
Our system allows us to investigate human MPN pathogenesis prospectively and shed light on the transforming mechanisms of mutant CALR in primary HSPCs. We could show that CALR mutations prime HSPCs toward the formation of platelet-producing megakaryocytes. Genetic correction of CALR mutations in MPN patient-derived HSPCs revealed a dependence on the oncogenic mutant CALR driver event in ET and post-ET MF patients, opening the possibility of an ex vivo gene correction approach to remove mutant CALR in patient-derived HSPCs . Lastly, since MPN patient-derived cells have notoriously low engraftment potential in mice, our CRISPR/Cas9-engineered CALR mutant model also provides a powerful new strategy to generate MPN xenotransplants with defined genotypes for the evaluation of novel therapies.

Disclosures
Greinix: Celgene: Consultancy; Therakos: Consultancy; Takeda: Consultancy; Sanofi: Consultancy; Novartis: Consultancy. Sill: Astellas: Consultancy, Membership on an entity's Board of Directors or advisory committees; Celgene: Consultancy, Membership on an entity's Board of Directors or advisory committees; AbbVie: Consultancy, Membership on an entity's Board of Directors or advisory committees. Zebisch: Novartis: Consultancy; AbbVie: Consultancy; Celgene: Consultancy, Honoraria. Reinisch: Celgene: Research Funding; Pfizer: Consultancy.