Synopsis of: An overview on CALR and CSF3R mutations and a proposal for revision of WHO diagnostic criteria for Myeloproliferative Neoplasms
by Dr. Angela Fleischman
We are rapidly learning more about the mutations acquired by cancer cells, this is particularly evident in blood malignancies. Most recently, calreticulin mutations have been identified in the majority of MPN patients without the JAK2V617F mutation. In addition, mutations in CSF3R, the cell surface receptor which allows cells to respond to the granulocytic growth factor G-CSF, has been identified in patients with chronic neutrophilic leukemia (CNL).
These mutations should be incorporated into the diagnostic criteria for these diseases without undermining the importance of bone marrow pathology and clinical presentation as key diagnostic criteria.
This article by Tefferi et al discusses the identification and relevance of calreticulin and CSF3R mutations in MPN and CNL and proposes how these mutations should be incorporated into the current WHO diagnostic criteria for myeloproliferative neoplasm.
CALRETICULIN (gene is designated as CALR, protein is designated as CRT)
The identification of calreticulin mutations in MPN was somewhat unexpected, as calreticulin had no previous known role in myeloid malignancies. Calreticulin is known for its role in cardiac development and serves as an “eat me” signal on stressed and dying cells for their rapid engulfment by macrophages (immune cells for fighting infection).
Two groups concurrently identified calreticulin mutations, their findings were presented back-to-back at this year’s ASH meeting in the late breaking abstract section and were both published online in the New England Journal of Medicine that same morning. Like JAK2V617F, mutations in calreticulin are acquired in a blood stem cell.
Calreticulin mutations are present in about 25% of patients with ET and about 35% of patients with PMF. Calreticulin mutations are not present in patients with the JAK2V617F mutation, this explains why it is not seen in PV. Among JAK2 negative patients calreticulin mutations are present in 67-88%. Now almost all MPN patients have an identifiable mutation.
There are some clinical differences that segregate calreticulin and JAK2 mutated MPN. Mutliple groups have found that calreticulin mutated patients had higher platelet counts and lower hemoglobin and have a younger age of diagnosis. Calreticulin mutated patients may also have a lower risk of blood clots. Many groups are also investigating whether there are survival differences and risk of transformation to myelofibrosis in calreticulin mutated versus non-mutated patients.
CSF3R (AKA G-CSFR or Granulocyte Colony-Stimulating Factor Receptor)
CSF3R is the cell surface receptor that allows cells to respond to Granulocyte colony-stimulating factor (G-CSF) leading to growth of granulocytes (aka neutrophils) from their progenitor cells. This growth hormone is used therapeutically to stimulate the growth of neutrophils in conditions such as severe congenital neutropenia (SCN) in which patients have severely low numbers of neutrophils. Use of G-CSF saves SCN patients from deadly infection s but unfortunately many of these SCN patients develop leukemia (AML) with mutations in CSF3R. Inherited mutations in CSF3R (meaning these individuals are born with the mutation) have also been described previously in families with elevated neutrophils and have also been described as acquired mutations in AML. In 2013, mutations in CSF3R were identified in a sizeable percentage of patients with chronic neutrophilic leukemia (CNL) and atypical CML (aCML). Two classes of mutations were identified (termed membrane proximal and truncating). Membrane proximal mutant cells are sensitive to the JAK inhibitor ruxolitinib, whereas truncating mutations were sensitive to dasatinib, a drug used in CML. This highlights that identifying which particular mutation a patient is important to help guide drug choices for that patient.
PROPOSALS FOR REVISIONS OF DIAGNOSTIC CRITERIA FOR PV, ET, PMF
The JAK2 mutation has been part of the diagnostic criteria for MPNs since 2008. Presence of JAK2V617F is a major criterion for the diagnosis of PV, and a minor criterion for the diagnosis of ET or PMF. Bone marrow morphology was included as a minor criterion in PV and a major criterion in ET and PMF. The authors argue that presence of calreticulin mutations should not take the place of bone marrow morphology, but should be added to the list of “clonal markers”.
For routine clinical practice, the authors suggest that screening should start with JAK2V617F. If this is found to be negative, then JAK2 exon 12 mutations should be tested in patients suspected of having PV (provided serum Epo is low). In patients suspected of having ET or PMF, the next step if JAK2V617F testing is negative should be calreticulin mutation screening. If a patient’s clinical situation is very consistent with PV and they have the JAK2V617F mutation, bone marrow biopsy may not be absolutely necessary for the diagnosis. In the case of ET and PMF bone marrow biopsy should always be performed.
An overview on CALR and CSF3R mutations and a proposal for revision of WHO diagnostic criteria for Myeloproliferative Neoplasms (Clinical Version)
Disease-specific mutations facilitate diagnostic precision and drug target discovery. In myeloproliferative neoplasms (MPN), this is best exemplified by the chronic myeloid leukemia-associated BCR-ABL1. No other mutation in MPN has thus far shown a similar degree of diagnostic accuracy or therapeutic relevance. However, JAK2 and KIT mutations are detected in more than 90% of patients with polycythemia vera and systemicmastocytosis, respectively, and are therefore used as highly sensitive clonal markers in these diseases. JAK2 and MPL mutations also occur in essential thrombocythemia (ET) and primary myelofibrosis (PMF) but their diagnostic value is limited by suboptimal sensitivity and specificity.
The molecular diagnostic gap in JAK2/MPL-unmutated ET/PMF is now partially addressed by the recent discovery of calreticulin (CALR) mutations in the majority of such cases. However, bone marrow morphology remains the central diagnostic platform and is essential for distinguishing ET from prefibrotic PMF and diagnosing patients that do not express JAK2, MPL or CALR (triple-negative). The year 2013 was also marked by the description of CSF3R mutations in the majority of patients with chronic neutrophilic leukemia (CNL). Herein, we argue for the inclusion of CALR and CSF3R mutations in the World Health Organization classification system for ET/PMF and CNL, respectively.Leukemia accepted article preview online, 20 January 2014. doi:10.1038/leu.2014.35.Link to Abstract
Rethinking the diagnostic criteria of Polycythemia Vera
The aim of this review is to critically address the validity and clinical applicability of three major diagnostic classification systems for polycythemia vera (PV), that is, those proposed by the Polycythemia Vera Study Group (PVSG), the British Committee for Standards in Haematology (BCSH) and the World Health Organization (WHO). Special focus is on which one of the three red cell parameters (hemoglobin-HB, hematocrit-HCT and red cell mass-RCM) should be used as the diagnostic hallmark of PV. The revised BCSH employed a persistently raised HCT level as the first diagnostic criterion in combination with the presence of a JAK2V617F mutation. On the other hand, the WHO classification used a raised HB value as a surrogate for increased RCM in association with molecular markers and for the first time, the bone marrow (BM) morphology was included as a minor criterion.
Ongoing controversy and discussion regards the use of certain threshold values for HCT and HB as surrogates for RCM as well as the existence of prodromal-latent disease, so-called masked PV (mPV). It has been shown that mPV can be recognized in patients not meeting the required HB or HCT threshold levels by both the WHO and BCSH criteria. These cases present with the same baseline clinical features as overt PV but present worsened survival. A critical reappraisal of the WHO criteria may suggest either to reduce the thresholds for HB or to consider HCT values as major diagnostic criterion, as in the BCSH, in association with JAK2V617F mutation. The clinical utility of using HCT as reference variable is supported also by results of clinical trials which explicitly recommend to use the HCT threshold for monitoring treatment. In questionable cases as in mPV, BM biopsy examinations should be mandated together with mutation analysis.Leukemia advance online publication, 17 January 2014; doi:10.1038/leu.2013.380.
Author Information (both abstracts)
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