Calreticulin (CALR): A new player in the mutational landscape of myeloproliferative disorders

Published - Written by Mounica Vallurupalli

Advances in molecular biology and sequencing technology have expanded our understanding of myeloproliferative disorders while aiding in the identification of the molecular defects that underlie these diseases. The first among these discoveries was the identification of the oncogenic fusion protein Bcr-abl in CML. Subsequent work lead to the discovery of a small molecule inhibitor, imatinib that has revolutionized the treatment of this disease. Then in 2005, a landmark paper published in the NEJM demonstrated that a common gain-of-function mutation, JAK2 V617F was present in many patients with polycythemia vera, essential thrombocythemia, and idiopathic myelofibrosis. The identification of this mutation revealed the importance of the JAK-STAT signaling pathway in these cancers, opening up new avenues for treating these disorders.

But what can be said of the sizable proportion of patients with a JAK2 wild-type myeloproliferative disorder? In this week’s NEJM, two independent teams of investigators report the results of whole-exome sequencing of patients with myeloproliferative disorders and the identification of a novel mutation in the calreticulin (CALR) gene.

Kampfl et al. performed whole exome sequencing on six patients with JAK2 wild type primary myelofibrosis and identified mutations in the same region of the CALR gene in all of the patients. They were able to confirm these findings in a larger cohort of 1107 patients with myeloproliferative neoplasms, demonstrating that 72% of patients with JAK2 unmutated essential thrombocythemia and myelofibrosis had mutations in CALR. Notably, none of the patients in this study with polycythemia vera had mutated CALR. Further, this study demonstrates that patients with myelofibrosis or essential thrombocytemia containing CALR mutations had better overall survival as compared to patients with JAK2 mutated disease.

Nangalia et al. completed whole exome sequencing on a large cohort of 151 patients with myeloproliferative neoplasms and identified CALR mutations in the majority of JAK2 unmutated cases. They further investigated the presence of this mutation in an expanded set of patients with hematologic malignancies and other cancers as well as normal controls. The authors found that while this mutation occurs at a low rate in myeloid cancers, it is not at all present in normal controls or patients with other cancers. They also showed that patients with the CALR mutations had significantly higher platelet counts and lower baseline hemoglobin when compared to patients with JAK2 mutated essential thrombocytemia or myelofibrosis.

Both studies note that the mutations identified in CALR occur in exon 9 of the gene and result in a frameshift mutation that leads to the loss of an endoplasmic reticulum localizing protein sequence. While Kampfl et al. show that the CALR mutation can be oncogenic, resulting in increased cell proliferation independent of cytokines like IL-3 and increased basal activation of a protein in the STAT pathway, the functional importance of calcreticulin in myeloid cells requires further investigation. It remains unclear why one patient with a CALR mutation will have essential thrombocythemia and another will have myelofibrosis.

In an accompanying editorial, Dr. Ross Levine of the Memorial Sloan-Kettering Cancer Center writes, “ As has been the case in many genetic diseases and in other human cancers, exome sequencing has led to an important new insight into the genetic basis of myeloproliferative neoplasms,” and concludes by saying, “Most importantly, Klampfl et al. and Nangalia et al. fill a gap in our knowledge by identifying the most common mutation in JAK2/MPL-wildtype patients, and allow us to better appreciate the genetic complexity of the different myeloproliferative syndromes.”

NEJM Deputy Editor, Dr. Dan Longo states, “These two papers have made it clear that the myeloproliferative syndromes are disorders of two main pathways. All of polycythemia vera and half of essential thrombocythemia and myelofibrosis cases involve activation of JAK2 either by mutation or activation of an upstream receptor (Mpl). Most of the remaining half of patients with essential thrombocythemia and myelofibrosis have mutations in calreticulin. Calreticulin has many important roles in human biology; thus, it is not entirely clear that its inhibition will be associated with a therapeutic index and be exploitable clinically. But the hope is there and we now know a pathway to target.”

Mounica Vallurupalli is a student at Harvard Medical School, and author of a Perspective article in our Becoming a Physician series, “Mourning on Morning Rounds.”