Novel insights into the biology and treatment of chronic myeloproliferative neoplasms

Abstract

Myeloproliferative neoplasms (MPNs) are clonal disorders of hematopoiesis characterized by a high frequency of genetic alterations, and include chronic myeloid leukemia (CML) and the BCR-ABL1-negative MPNs. Herein we summarize recent advances and controversies in our understanding of the biology and therapy of these disorders, as discussed at the 8th post-American Society of Hematology CML-MPN workshop. The principal areas addressed include the breakthrough discovery of CALR mutations in patients with JAK2/MPL wild type MPN, candidate therapies based on novel genetic findings in leukemic transformation and new therapeutic targets in MPNs, and an appraisal of bone marrow histopathology in MPNs with a focus on the potential new clinical entity of "masked" polycythemia vera. An update on clinical trials of Janus kinase (JAK) inhibitors is presented as well as current understanding regarding the definitions and mechanisms of resistance to JAK inhibitors, and updated information on the safety and efficacy of discontinuation of tyrosine kinase inhibitors in patients with CML.

Keywords: Myeloid leukemias and dysplasias; molecular genetics; myeloproliferative disorders; prognostication; signaling therapies.

Figure 1

CALR gene. (A) Calreticulin (CALR) domain structure. CALR protein has three distinctive domains (N, P, C). The P-domain is involved in the chaperone function. The C-domain is rich in acidic amino acids and contains the high capacity, low affinity calcium binding site. The last four C-terminal amino acids (KDEL) are the endoplasmic reticulum retention signal. (B) Variability of CALR exon 9 insertion/deletion mutations found in MPN. Type 1 and type 2 mutations are most frequent and together they are present in over 85% of CALR mutated cases. (C) Impact of MPN associated mutations on CALR protein structure. Despite the diversity of mutations found in the CALR gene exon 9, the impact on the protein structure is very similar. The two most common CALR insertion/deletion mutations found in MPN (type 1 and type 2) are shown, which frameshift to the same alternative reading frame. This results in a novel C-terminus of the mutant CALR that turns into a positively charged peptide lacking the calcium binding and KDEL regions.

Figure 2

Proposed pathophysiology of the BCR – ABL1 negative classic myeloproliferative neoplasms (MPNs) based on new genetic insights. In this model, proposed by Mario Cazzola and Robert Kralovics, all MPNs originate in a state most consistent with essential thrombocythemia (ET). This occurs in patients with heterozygous JAK2 V617F mutations, CALR mutations or MPL mutations. Patients with heterozygous JAK2 V617F mutations that undergo copy neutral loss-of-heterozygosity (CN-LOH) of the locus of the JAK2 V617F mutation (on chromosome 9p) undergo a phenotypic switch from an ET phenotype to a phenotype most consistent with polycythemia vera (PV). Further genetic and/or epigenetic alterations may then result in further evolution from PV to myelofibrosis (MF). In contrast, patients with a MPL or CALR mutation either remain with a phenotype consistent with ET or undergo evolution directly to MF. Genetic alterations known to be associated with transition from ET to MF include CN-LOH of the locus of MPL mutation (on chromosome 1p). Additionally, genetic data suggest that progressive expansion of CALR mutant clones is associated with transformation from ET to MF.

Figure 3

Model of current understanding of genetic events responsible for leukemic transformation of chronic BCR – ABL1 negative myeloproliferative neoplasms (MPNs). Although JAK2 V617F mutations are sufficient for development of MPN phenotype, a large amount of evidence suggests that earlier genetic events predate development of the JAK2 V617F mutations to establish a “pre-leukemic” MPN initiating cell. Mutations in TET2 as well as DNMT3A have been most frequently described as predating JAK2V617F mutations in patients with MPN. Acquisition of the JAK2 V617F mutation then results in overt MPN clinical disease. Later, acquisition of further mutations, either in a cell bearing the JAK2 mutation or a JAK2 wild type cell results in transformation to acute leukemia. Currently, few studies regarding leukemic transformation of CALR-mutant chronic MPN patients have been described.

Figure 4

Bone marrow morphology in early/prefibrotic PMF is characterized by hypercellular BM displaying a prominent megakaryocytic and granulocytic myeloproliferation. (A) Megakaryocytes are frequently endosteal-paratrabecularly dislocated and show small to large dense clusters. There is a high variability in size ranging from small to giant forms. Other prominent features are signify cant aberrations of nuclear organization such as marked hypolobulation, irregular foldings and condensed chromatin pattern thus creating bulbous or so-called cloud-like/balloon-shaped nuclei with increased nuclear-cytoplasmic ratio. (B) Hematoxylin and eosin staining.

Figure 5

Bone marrow morphology in ET showing an increase in number of megakaryocytes with a random distribution throughout the marrow space or loose clustering of large to giant cells. (A) Megakaryocytes reveal deeply folded nuclei surrounded by a corresponding area of mature cytoplasm, i.e. no evidence of nuclear-cytoplasmic abnormalities. (B) Periodic acid – Schiff staining.

Figure 6

Survival with CML over time: the German CML-study group experience. The data comprise 3682 patients from five randomized trials over the period 1983 – 2013.

Figure 7

Cumulative incidence of deep molecular response (MR 4.5) dependent on BCR – ABL1 transcript levels at 3, 6, 12 and 18 months. The fastest and highest level of MR 4.5 is achieved in patients who have achieved a major molecular response (MMR, <0.1% IS) at the respective time points.