ASH 2013 - Myeloproliferative Neoplasms (MPN)

Myeloproliferative Neoplasms

ASH 2013 LBA-1: Frequent Mutations in the Calreticulin Gene CALR in Myeloproliferative Neoplasms. Klampfl Thorsten et al.

The classical, BCR-ABL1 negative myeloproliferative neoplasms (MPN) are polycythemia vera (PV), essential thrombocythemia (ET) and primary myelofibrosis (PMF). The most common genetic alteration in MPN is the JAK2-V617F mutation detected in 95% of PV patients and in 50–60% of patients with ET or PMF. Mutations in exon 12 of JAK2 and in the thrombopoietin receptor gene MPL are found in an additional 5–10% of the cases. In recent years a number of other genes were shown to be affected in MPN. However, these mutations are not mutually exclusive with JAK2 and MPL mutations and are also found in other myeloid malignancies. A specific molecular marker for the remaining 40% of ET or PMF patients with wild type JAK2 and MPL is still unknown.

We used whole-exome sequencing to identify novel mutations in PMF patients with wild type JAK2 and MPL. The analysis revealed recurrent somatic insertions and deletions in CALR encoding for calreticulin. All detected mutations resulted in a frameshift and clustered in the last exon (exon 9) of the gene. Following up on this finding we developed a PCR based assay to screen 1107 MPN patients for insertion/deletion mutations in exon 9 of CALR. No mutations were detected in PV. In ET and PMF CALR mutations were mutually exclusive with mutant JAK2 and mutant MPL. Of the patients with wild type JAK2 and MPL, 67% ET and 88% PMF had mutant CALR. We also tested 19 patients with wild type CALR-exon 9 for mutations in the other exons of the gene, but all were negative. Furthermore we did not find CALR-exon 9 mutations in 254 patients with de novo acute myeloid leukemia, 45 with chronic myeloid leukemia, 73 with myelodysplastic syndrome or 64 with chronic myelomonocytic leukemia. Out of 24 patients with refractory anemia with ringed sideroblasts associated with marked thrombocytosis (RARS-T), three patients carried CALR mutations. These patients were wild type for JAK2 and MPL.

In total we detected 36 different types of mutations in CALR. A 52 bp deletion and a 5 bp insertion were the most prominent types found in 53% and 32% of all cases with mutant CALR. All 36 types of mutations result in a frameshift to the same alternative reading frame, generating a novel C-terminus of the mutated protein. The wild type C-terminal region of CALR contains a high-capacity calcium-binding domain and is highly negatively charged. As a consequence of the frameshift mutations the negatively charged amino acids are replaced by both neutral and positively charged amino acids. In addition, an endoplasmic reticulum retention signal present in the wild type protein is lost in the mutant variants. Expression in HEK cells showed that wild type CALR localizes in the endoplasmic reticulum, whereas this localization is less prominent in cells expressing mutant CALR. This observation is in line with the loss of the endoplasmic reticulum retention signal in the mutant protein.

Overexpression of the most common CALR mutation (a 52 bp deletion) in interleukin-3 (IL-3) dependent Ba/F3 cells led to IL-3-independent growth and hypersensitivity to IL-3. Cells overexpressing the mutant were sensitive to the JAK-family kinase inhibitor SAR302503 and showed elevated STAT5 phosphorylation in absence of IL-3. This indicates that JAK-STAT signaling is involved in the observed cytokine independent growth of mutant CALR expressing Ba/F3 cells.

ET and PMF patients with mutant CALR present with lower white blood cell counts (p < .001 for ET, p = .027 for PMF) and elevated platelet levels (p < .001 in both entities) compared to patients with mutant JAK2. In both disease entities patients with mutant CALR show significantly better overall survival than patients with mutant JAK2 (p = .043 in ET, p < .001 in PMF). ET patients with mutant CALR had a lower risk of thrombosis in comparison to those with mutant JAK2 (p = .003). Mutant CALR is a novel, specific molecular marker detected in the majority of MPN patients negative for JAK2 and MPL mutations. Use of this marker in the clinic is expected to improve diagnostic and therapeutic decision-making in MPN.

ASH 2013 LBA-2: The Genomic Landscape of Myeloproliferative Neoplasms: SomaticCALR Mutations in the Majority of JAK2-Wildtype Patients. Jyoti Nangalia et al.

Introduction: BCR-ABL-negative myeloproliferative neoplasms (MPNs), such as polycythemia vera (PV), essential thrombocythemia (ET) and myelofibrosis (MF) are chronic myeloid malignancies characterized by overproduction of hematopoietic cells.  JAK2 mutations are found in most patients with PV, and in only 50–60% of patients with ET and MF. JAK2 mutation testing has greatly simplified MPN diagnosis, but distinguishing JAK2-wildtype ET from reactive thrombocytosis remains a diagnostic challenge. Mutations in signalling pathways (MPL, LNK) and epigenetic regulators (TET2, DNMT3A, IDH1/2, EXH2, ASXL1) have been found in a minority of MPNs. However genome-wide data are lacking and the pathogenesis of MPNs that do not harbor JAK2 or MPL mutations remains obscure. 

Conclusions: We describe the mutational landscape of BCR-ABL negative MPNs and demonstrate that somatic mutations in the endoplasmic reticulum chaperone CALR are found in the majority of patients with JAK2-unmutated MPNs. These results reveal a novel biological pathway as a target for tumorigenic mutations and will simplify diagnosis of MPN patients.

Results can be summarized as such:

  • CALR and JAK2/MPL mutations were mutually exclusive
    • CALR mutations in:
      • 71% of ET (80/112)
      • 56% of idiopathic MF (18/32)
      • 86% of post ET-MF (12/14)
      • 8% of myelodysplasia (10/115)
      • NOT in other myeloid, lymphoid or solid cancers.
    • HOT SPOTS: exon 9, with two common variants
      • L367fs*46 (52 bp deletion)
      • K385fs*47 (5 bp insertion)
  • Compared to JAK2-mutated MPNs, those with CALR mutations:
    • Higher platelet counts (Wilcoxon rank-sum, p = .0003)
    • Lower hemoglobin levels (Student’s t test, p = .02)
    • Higher incidence of transformation to MF (Fishers p = .03).
These two abstracts provide an excellent insight into the role of molecular diagnostics. MPN have long been thought to be the different forms of the same disease. The discovery of the JAK2 V617F mutation indeed demonstrated that it is the principal common clonal event in MPN, but does not explain all cases of MPN, as BCR-ABL translocation does in CML (chronic myelogeneous leukemia). The results presented here should lead the molecular biology laboratories to add exon 9 deletions in calreticulin in cases of presumed MPN without the classical JAK2 V617F mutation. This will tremendously help clinicians in the diagnosis of such conditions. Differences in the disease characteristics between patients with different clonal events suggests that disease severity might be impacted and that the diagnostic clonal event could actually have a predictive value. Moreover, the advent of effective targeted therapies against JAK2 suggests that targeting other clonal events in MPN is of interest.
— CARE Hematology Faculty