Calreticulin haploinsufficiency augments stem cell activity and is required for onset of myeloproliferative neoplasms

Abstract

Mutations in JAK2, MPL, or CALR are detected in more than 80% of myeloproliferative neoplasm (MPN) patients and are thought to play a driver role in MPN pathogenesis via autosomal activation of the JAK-STAT signaling cascade. Mutant CALR binds to MPL, activates downstream MPL signaling cascades, and induces essential thrombocythemia in mice. However, embryonic lethality of Calr-deficient mice precludes determination of a role for CALR in hematopoiesis. To clarify the role of CALR in normal hematopoiesis and MPN pathogenesis, we generated hematopoietic cell-specific Calr-deficient mice. CALR deficiency had little effect on the leukocyte count, hemoglobin levels, or platelet count in peripheral blood. However, Calr-deficient mice showed some hematopoietic properties of MPN, including decreased erythropoiesis and increased myeloid progenitor cells in the bone marrow, and extramedullary hematopoiesis in the spleen. Transplantation experiments revealed that Calr haploinsufficiency promoted the self-renewal capacity of hematopoietic stem cells. We generated CALRdel52 mutant transgenic mice with Calr haploinsufficiency as a model that mimics human MPN patients and found that Calr haploinsufficiency restored the self-renewal capacity of hematopoietic stem cells damaged by CALR mutations. Only recipient mice transplanted with Lineage－Sca1+c-kit+ cells harboring both CALR mutation and Calr haploinsufficiency developed MPN in competitive conditions, showing that CALR haploinsufficiency was required for the onset of CALR-mutated MPNs.

Calreticulin del52 and ins5 knock-in mice recapitulate different myeloproliferative phenotypes observed in patients with MPN

“…Alternatively, it is also possible that the amplification of HSCs observed in KI mice may correspond to an increase of a sub-population of active HSCs with more proliferating capacities, that would result in milder secondary engraftment. Interestingly, the group of K Shimoda showed that CALR wt is able to inhibit the CALR del52-induced competitive advantage, thus it is possible that the total absence of CALR wt in homozygous del52 or ins5 HSCs might explain their advantage on their heterozygous counterparts 27 . With age, preferentially homozygous del52 KI mice developed splenomegaly accompanied with a decrease in BM cellularity, while blood parameters remained constant.…”

Calreticulin mutations affect its chaperone function and perturb the glycoproteome

“…39,40 Moreover, CALR haploinsufficiency in a CALR-mutated mouse model augmented hematopoietic stem cell activity in a JAK-STAT-signaling-independent manner, which was paramount for MPN onset. 41 Since the UPR pathways rely on protein degradation pathways, it will be of interest to elucidate whether the targeting of protein degradation pathways may be strategically combined with existing treatment modalities for CALR-mutated MPNs.…”

Extramedullary Hematopoiesis of the Liver and Spleen

“…Among the MPL mutations, the most common are MPL W515L and MPL W515K [65]. Abnormal proliferation of hematopoietic stem cells results from spontaneous activation of the JAK-STAT pathway by MPL mutations [66,67]. Determining this gene is important because it can differentiate patients with a negative JAK2V617F mutation and those with a positive BCR-ABL mutation.…”