Calreticulin (CALR)

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Calreticulin (CALR) is a multifunctional protein of the endoplasmic reticulum (ER). Contrary to JAK2 and MPL, it is not directly involved in the signaling of cytokines receptors. It has two major functions. By binding to glycoproteins in synthesis, it has a role in the quality control of secreted glycoproteins. Then, its C-terminal domain allows the binding of calcium with a high capacity and thus CALR regulate the calcium homeostasis.¹ Other functions have been described, as a role in anti-cancer immunity, cell adhesion or wound healing.^2,3

CALR mutations have been discovered by two European teams in December 2013.^4,5 They have been observed in BCR-ABL negative myeloproliferative neoplasms (MPNs), especially in essential thrombocythaemia (ET) and primary myelofibrosis (PMF) with a frequency of occurrence of 25-30% and 20-30%, respectively.^6,7,4,5,8-10 They have also been described at a lower frequency in myelodysplastic syndromes (0-3,4%)^4,11–13, refractory anemia with ring sideroblasts and thrombocytosis (1%),¹¹ chronic myelomonocytic leukemia (0-3%)^5,13, and in two cases of JAK2-negative polycythaemia vera.¹⁴ Their presence in splanchnic thrombosis seem to be rarer than the presence of JAK2V617F.^15,16

CALR mutations always affect the 9th exon. They consist in deletion and/or insertions, associated with a frameshift of 1 base pair (bp). These mutations lead to an important modification of the C-terminal domain, which become basic, and to the loss of the KDEL motif involved in the retention of the protein in the ER.^4,5 Today, more than 50 mutations have been described, but the two most frequent ones (c.1092_1143del, p.L367fs*46, designated as type 1, and c.1154_1155insTTGTC, p.K385fs*47, called type 2) represent 80-85% of cases.^4,5 The frequency of the other mutations is less than 2%.^4,5

Like JAK2V617F and MPL mutations, CALR mutations make cells more sensitive to hematopietic cytokines and seem to be associated with activation of the Jak2/Stat5 pathways.⁴ They seem to appear early in disease chronology, suggesting an important role in the physiopathology of MPNs. Of note, CALR mutations and JAK2V617F/MPLW515 mutations are generally mutually exclusive.^4,5 Nevertheless rare coexistence of CALR and JAK2 or MPL mutations have been described, but without proof that they occurred in the same clone.^8,11,17,18 CALR mutations are generally heterozygous, but homozygosity is possible, at least for type 2 mutations.⁴

In both ET and PMF, CALR mutations are associated with a distinct presentation and a more favorable prognosis, compared to JAK2V617F-mutated ET and PMF. Overall, CALR-mutated patients are younger et present with a myeloproliferation more specific of the megakaryocytic lineage (thus, they present with a more pronounced thrombocytosis and lesser abnormalities of other myeloid lineages).^4–9,17,19 Furthermore, CALR+ patients present with less frequent thrombotic complications in ET, and a longer survival in PMF.^4,6-9,20-24

Different techniques have been described to detect CALR mutations, including sequencing (by Sanger or Next Generation Sequencing (NGS))^4,5, but also screening techniques like fragment length analysis⁴ or high resolution melting (HRM) curve analysis.¹⁴

References

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   4. Klampfl, T. et al. Somatic mutations of calreticulin in myeloproliferative neoplasms. N. Engl. J. Med. 369, 2379–2390 (2013).
   5. Nangalia, J. et al. Somatic CALR mutations in myeloproliferative neoplasms with non mutated JAK2. N. Engl. J. Med. 369, 2391–2405 (2013).
   6. Rumi, E. et al. JAK2 or CALR mutation status defines subtypes of essential thrombocythemia with substantially different clinical course and outcomes. Blood (2013). doi:10.1182/blood-2013-11-539098
   7. Rotunno, G. et al. Impact of Calreticulin Mutations on Clinical and Hematological Phenotype and Outcome in Essential Thrombocythemia. Blood (2013). doi:10.1182/blood-2013-11-538983
   8. Tefferi, A. et al. CALR vs JAK2 vs MPL mutated or triple-negative myelofibrosis: clinical, cytogenetic and molecular comparisons. Leukemia (2014). doi:10.1038/leu.2014.3
   9. Tefferi, A. et al. Calreticulin mutations and long-term survival in essential thrombocythemia. Leukemia (2014). doi:10.1038/leu.2014.148
   10. Tefferi, A. et al. Type 1 versus Type 2 calreticulin mutations in essential thrombocythemia: a collaborative study of 1027 patients. Am. J. Hematol. 89, E121–124 (2014).
   11. Broséus, J. et al. Low rate of calreticulin mutations in refractory anaemia with ring sideroblasts and marked thrombocytosis. Leukemia (2014). doi:10.1038/leu.2014.49
   12. Patnaik, M. M. et al. CALR mutations are infrequent in WHO-definedrefractoryanemiawith ring sideroblasts. Leukemia 28, 1370–1371 (2014).
   13. Hou, H.-A., Kuo, Y.-Y., Chou, W.-C., Chen, P.-H., Tien, H.-F. Calreticulin mutation was rarely detected in patients with myelodysplastic syndrome. Leukemia (2014). doi:10.1038/leu.2014.71
   14. Broséus, J., Park, J.-H., Carillo, S., Hermouet, S., Girodon, F. Presence of calreticulin mutations in JAK2- negative polycythemia vera. Blood (2014). doi:10.1182/blood-2014-06-58316
   15. Turon, F. et al. Role of calreticulin mutations in the etiological diagnosis of splanchnic vein thrombosis. J. Hepatol. (2014). doi:10.1016/j.jhep.2014.08.032
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   17. Shen, H. et al. CALR and ASXL1 mutation analysis in 190 patients with essential thrombocythemia. Leuk. Lymphoma 1–9 (2014). doi:10.3109/10428194.2014.939963
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   20. Rumi, E. et al. CALR exon 9 mutations are somatically acquired events in familial cases of essential thrombocythemia or primary myelofibrosis. Blood 123, 2416–2419 (2014).
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   22. Panagiota, V. et al. Prognostic effect of calreticulin mutations in patients with myelofibrosis after allogeneic hematopoietic stem cell transplantation. Leukemia (2014). doi:10.1038/leu.2014.66
   23. Andrikovics, H. et al. Distinct clinical characteristics of myeloproliferative neoplasms with calreticulin mutations. Haematologica 99, 1184–1190 (2014).
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