Proposed Revision of WHO Diagnostic Criteria for MPNs

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.

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An overview on CALR and CSF3R mutations and a proposal for revision of WHO diagnostic criteria for Myeloproliferative Neoplasms (Clinical Version)

Abstract