Modulation of JAK2 V617F allele burden dynamics by hydroxycarbamide in polycythaemia vera and essential thrombocythaemia patients

Molecular Genetics of Thrombotic Myeloproliferative Neoplasms: Implications in Precision Oncology

Classical BCR-ABL-negative myeloproliferative neoplasms (MPN) include polycythaemia vera, essential thrombocythaemia, and primary myelofibrosis. Unlike monogenic disorders, a more complicated series of genetic mutations are believed to be responsible for MPN with various degrees of thromboembolic and bleeding complications. Thrombosis is one of the early manifestations in patients with MPN. To date, the driver genes responsible for MPN include JAK2, CALR, MPL, TET2, ASXL1, and MTHFR. Affords have been done to elucidate these mutations and the incidence of thromboembolic events. Several lines of evidence indicate that mutations in JAK2, MPL, TET2 and ASXL1 gene and polymorphisms in several clotting factors (GPIa, GPIIa, and GPIIIa) are associated with the occurrence and prevalence of thrombosis in MPN patients. Some polymorphisms within XRCC1, FBG, F2, F5, F7, F12, MMP9, HPA5, MTHFR, SDF-1, FAS, FASL, TERT, ACE, and TLR4 genes may also play a role in MPN manifestation. This review aims to provide an insightful overview on the genetic perspective of thrombotic complications in patients with MPN.

Assessment of the interlaboratory variability and robustness of JAK2V617F mutation assays: A study involving a consortium of 19 Italian laboratories

To date, a plenty of techniques for the detection of JAK2^V617F is used over different laboratories, with substantial differences in specificity and sensitivity. Therefore, to provide reliable and comparable results, the standardization of molecular techniques is mandatory.

A network of 19 centers was established to 1) evaluate the inter- and intra-laboratory variability in JAK2^V617F quantification, 2) identify the most robust assay for the standardization of the molecular test and 3) allow consistent interpretation of individual patient analysis results. The study was conceived in 3 different rounds, in which all centers had to blindly test DNA samples with different JAK2^V617F allele burden (AB) using both quantitative and qualitative assays.

The positivity of samples with an AB < 1% was not detected by qualitative assays. Conversely, laboratories performing the quantitative approach were able to determine the expected JAK2^V617F AB. Quantitative results were reliable across all mutation loads with moderate variability at low AB (0.1 and 1%; CV = 0.46 and 0.77, respectively). Remarkably, all laboratories clearly distinguished between the 0.1 and 1% mutated samples.

In conclusion, a qualitative approach is not sensitive enough to detect the JAK2^V617F mutation, especially at low AB. On the contrary, the ipsogen JAK2 MutaQuant CE-IVD kit resulted in a high, efficient and sensitive quantification detection of all mutation loads. This study sets the basis for the standardization of molecular techniques for JAK2^V617F determination, which will require the employment of approved operating procedures and the use of certificated standards, such as the recent WHO 1st International Reference Panel for Genomic JAK2^V617F.

Mutations in myeloproliferative neoplasms - their significance and clinical use

INTRODUCTION

Clonal hematologic diseases of the blood such as polycythemia vera, essential thrombocythemia and primary myelofibrosis belong to the BCR-ABL negative Myeloproliferative Neoplasms (MPN). These diseases are characterized by clonal expansion of hematopoietic precursor cells followed by increased production of differentiated cells of the myeloid lineage. Initiation of clonal hematopoiesis, formation of a clinical phenotype as well as disease progression form part of MPN disease evolution. The disease is driven by acquired somatic mutations in critical pathways such as cytokine signaling, epigenetic regulation, RNA splicing, and transcription factor signaling. Areas covered: The following review aims to provide an overview of the mutational landscape of MPN, the impact of these mutations in MPN pathogenesis as well as their prognostic value. Finally, a summary of how these mutations are being used or could potentially be used for the treatment of MPN patients is presented. Expert commentary: The genetic landscape of MPN patients has been successfully dissected within the past years with the advent of new sequencing technologies. Integrating the genetic information within a clinical setting is already benefitting patients in terms of disease monitoring and prognostic information of disease progression but will be further intensified within the next years.

Monitoring Minimal Residual Disease in the Myeloproliferative Neoplasms: Current Applications and Emerging Approaches

The presence of acquired mutations within the JAK2, CALR, and MPL genes in the majority of patients with myeloproliferative neoplasms (MPN) affords the opportunity to utilise these mutations as markers of minimal residual disease (MRD). Reduction of the mutated allele burden has been reported in response to a number of therapeutic modalities including interferon, JAK inhibitors, and allogeneic stem cell transplantation; novel therapies in development will also require assessment of efficacy. Real-time quantitative PCR has been widely adopted for recurrent point mutations with assays demonstrating the specificity, sensitivity, and reproducibility required for clinical utility. More recently, approaches such as digital PCR have demonstrated comparable, if not improved, assay characteristics and are likely to play an increasing role in MRD monitoring. While next-generation sequencing is increasingly valuable as a tool for diagnosis of MPN, its role in the assessment of MRD requires further evaluation.

The JAK2 V617F mutational status and allele burden may be related with the risk of venous thromboembolic events in patients with Philadelphia-negative myeloproliferative neoplasms

INTRODUCTION

Patients with Philadelphia-negative myeloproliferative neoplasms (Ph(-) MPNs) are at increased risk of thromboembolic and hemorrhagic complications. The aim of the study was to determine the relationship between JAK2 V617F mutational status, JAK2 V617F allele burden and the risk of vascular complications occurrence.

MATERIALS AND METHODS

Analysis was performed in a cohort of 186 patients diagnosed with polycythemia vera (53), essential thrombocythemia (114), primary myelofibrosis (11), and unclassified MPN (8). The risk of vascular complications development was analyzed in 126 JAK2 V617F-positive patients with respect to allele burden assessed with allele-specific 'real-time' quantitative polymerase chain reaction (AS RQ-PCR).

RESULTS

Overall prevalence of any vascular complications was 44.6%. Arterial thrombosis occurred in 20.4%, venous thromboembolism (VTE) in 11.3%, bleeding episodes in 24.7% of patients. Individuals harboring JAK2 V617F mutation, regardless of MPN type, were at higher risk of VTE (OR=5.15, 95%CI: 1.16-22.90, P=0.024), mainly deep vein thrombosis (DVT). JAK2 allele burden higher than 20% identified patients with 7.4-fold increased risk of VTE (95%CI: 1.6-33.7, P=0.004), but not of arterial thrombosis, neither of bleeding complications, and remained the only significant VTE risk factor in multivariate logistic regression. High allele burdens (over 50%) were strikingly associated with proximal DVT cases, but not with distal DVT.

CONCLUSIONS

The group of MPN patients with JAK2 V617F allele burden higher than 20% may benefit the most from vigilant monitoring and appropriate prophylaxis against vascular events. Inclusion of JAK2 V617F mutant allele burden in new risk stratifications seems to be justified and requires controlled prospective trials.

JAK2V617F monitoring in polycythemia vera and essential thrombocythemia: clinical usefulness for predicting myelofibrotic transformation and thrombotic events

The JAK2V617F allele burden has been identified as a risk factor for vascular events and myelofibrotic transformation in polycythemia vera (PV) and essential thrombocythemia (ET). However, all previous studies have evaluated a single time point JAK2V617F measurement. Therefore, the frequency and the clinical significance of changes in the JAK2V617F mutant load occurring during the disease evolution remain unknown. In the present study, JAK2V617F monitoring was performed during the follow-up of 347 patients (PV = 163, ET = 184). According to their JAK2V617F evolutionary patterns, patients were stratified as stable < 50% (n = 261), stable ≥50% (n = 52), progressive increase (n = 24) and unexplained decrease (n = 10). After a 2,453 person-years follow-up, a total of 59 thrombotic events, 16 major hemorrhages, and 27 cases of myelofibrotic transformations were registered. At multivariate analyses, patients with a persistently high (≥50%) or unsteady JAK2V617F load during follow-up had an increased risk of myelofibrotic transformation (Incidence rate ratio [IRR]: 20.7, 95% CI: 6.5-65.4; P < 0.001) and a trend for a higher incidence of thrombosis (IRR: 1.7, 1-3.3; P = 0.05) than patients with a stable allele burden below 50%. In conclusion, JAK2V617F monitoring could be useful in patients with PV and ET for predicting disease's complications, especially myelofibrotic transformation.

Emerging drugs for polycythemia vera

INTRODUCTION

Polycythemia vera (PV) patients suffer from disease-related constitutional symptoms, cardiovascular complications and risk of transformation into myelofibrosis and acute leukemia.

AREAS COVERED

Clinical and molecular aspects and current therapies will be described to provide clinical and molecular background to understand the natural history and treatment strategies in PV. Pertinent ongoing research questions, challenges arising out of the specific disease course and biology of PV as well as challenges and opportunities for new agents in PV are addressed. A focus is placed on pegylated interferon-α formulations (PEG-INFa2a) and JAK2 inhibitors. Newest data on symptom burden and incidence and prevalence of PV and MPNs are highlighted in the context of development of PV therapies.

EXPERT OPINION

Therapeutic goals in PV are to prevent vascular events, reduce symptoms and for future therapies delay/prevent disease progression. Currently available treatments such as phlebotomy, antiplatelet therapy, managing risk factors and cytoreductive therapies such as hydroxyurea (HU) and PEG-INFa2a are effective. JAK2 inhibitors recently have shown promising activity in reducing PV symptoms and spleen size and improving blood counts. Yet the influence of long-term outcome and delaying disease progression is unknown. Thus, there still remains an unmet medical need for improved therapy and symptom management in PV.

Clonal analyses reveal associations of JAK2V617F homozygosity with hematologic features, age and gender in polycythemia vera and essential thrombocythemia

Subclones homozygous for JAK2V617F are more common and larger in patients with polycythemia vera compared to essential thrombocythemia, but their role in determining phenotype remains unclear. We genotyped 4564 erythroid colonies from 59 patients with polycythemia vera or essential thrombocythemia to investigate whether the proportion of JAK2V617F -homozygous precursors, compared to heterozygous precursors, is associated with clinical or demographic features. In polycythemia vera, a higher proportion of homozygous-mutant precursors was associated with more extreme blood counts at diagnosis, consistent with a causal role for homozygosity in polycythemia vera pathogenesis. Larger numbers of homozygous-mutant colonies were associated with older age, and with male gender in polycythemia vera but female gender in essential thrombocythemia. These results suggest that age promotes development or expansion of homozygous-mutant clones and that gender modulates the phenotypic consequences of JAK2V617F homozygosity, thus providing a potential explanation for the long-standing observations of a preponderance of men with polycythemia vera but of women with essential thrombocythemia.

Essential thrombocythemia: a rare disease in childhood

Essential thrombocythemia is an acquired myeloproliferative disorder characterized by the proliferation of megakaryocytes in bone marrow, leading to a persistent increase in the number of circulating platelets and thus increasing the risk for thrombotic and hemorrhagic events. The disease features leukocytosis, splenomegaly, vascular occlusive events, hemorrhages and vasomotor disorders. The intricate mechanisms underlying the molecular pathogenesis of this disorder are not completely understood and are still a matter of discussion. Essential thrombocythemia is an extremely rare disorder during childhood. We report on a case of essential thrombocythemia in a child and discuss the diagnostic approach and treatment strategy.

[Treatment of essential thrombocythemia]

Essential thrombocythemia is a chronic myeloproliferative neoplasm characterized by sustained thrombocytosis, bone marrow megakaryocytic hyperplasia and an increased risk of thrombosis and hemorrhage. The goal of treatment is to prevent the development of vascular complications without increasing the risk of transformation. Patients aged>60 years or a history of thrombosis have a high risk of thrombosis while those with a platelet count>1,500 x 10(9)/l have a higher risk of hemorrhage. Patients with low-risk essential thrombocythemia can be managed appropriately with low-dose of acetylsalicylic acid or even observation only, while patients with a high-risk disease are candidates to receive cytoreductive treatment, hydroxyurea being the first choice therapy. Anagrelide is the most suitable option for patients with resistance or intolerance to hydroxyurea. All patients must be submitted to a rigorous control of cardiovascular risk factors.

Leukemic transformation driven by an ASXL1 mutation after a JAK2V617F-positive primary myelofibrosis: clonal evolution and hierarchy revealed by next-generation sequencing

We have characterized the molecular changes underlying the transformation of a JAK2V617F⁺-myelofibrosis with trisomy 8, into a JAK2V617F-negative leukemia. Leukemic clone did not carry JAK2V617F mutation, but showed ASXL1 mutation (R693X). This mutation was identified in a low percentage at diagnosis by next-generation sequencing. Using this technology in serial specimens during the follow-up, we observed a progressive expansion of the ASXL1-mutated minor clone, whereas the JAK2V617F⁺-clone carrying trisomy 8 decreased. Hematologic progression occurred simultaneously with an ASXL1-R693X-negative lung-cancer. This is the first report showing a clear association between the expansion of an ASXL1-mutated clone and the leukemic transformation of myelofibrosis.

Genetic predisposition to molecular response in patients with myeloproliferative neoplasms treated with hydroxycarbamide

JAK2V617F allele burden was prospectively measured in polycythemia vera (PV, n=52) and essential thrombocythemia (ET, n=39) patients receiving hydroxycarbamide (HC) and analyzed according to JAK2 46/1 haplotype and genotype of SLC14A1, SLC14A2 and ARG2 urea transporters. Molecular response (MR) was obtained in 68.7% and 38.9% of PV patients with GG and AA or GA genotype in SLC14A2, respectively (p=0.07). No significant differences were observed neither in PV nor in ET according to JAK2 46/1 haplotype, SLC14A1 and ARG2. In conclusion, JAK2 46/1 haplotype does not influence MR in HC treated patients and urea transporters polymorphisms display a minimal effect.

Differential effects of hydroxyurea and INC424 on mutant allele burden and myeloproliferative phenotype in a JAK2-V617F polycythemia vera mouse model

JAK2-V617F cells show a competitive advantage over wild-type cells in BM transplantation assays. A preclinical mouse model allows the examination of the effects of therapeutic agents on blood parameters and JAK2-V617F mutant allele burden.

JAK2 Inhibition: Reviewing a New Therapeutical Option in Myeloproliferative Neoplasms

JAK2 is a tyrosine kinase gene that plays an essential role in the development of normal haematopoiesis. Hyperactivation of JAK2 occurs in myeloproliferative neoplasms by different mechanisms. As a consequence, JAK2 inhibitors have been designed to suppress the cytokine signalling cascade caused by the constitutive activation of JAK2. In clinical trials, JAK2 inhibitors are efficient in decreasing spleen size, controlling clinical symptoms, and improving quality of life in patients with myeloproliferative neoplasms. However, JAK2 inhibitors are unable to target uncommitted hematopoietic progenitors responsible of the initiation of the myeloproliferative disease. It is expected that, in order to cure the myeloproliferative disease, JAK2 inhibitors should be combined with other drugs to target simultaneously different pathways and to target the initiator hematopoietic cell population in myeloproliferative disorders. Taking advantage of the inhibition of the cytokine cascade of JAK2 inhibitors, these compounds are going to be used not only to treat patients with hematological neoplasms but may also be beneficial to treat patients with rheumatoid arthritis or other inflammatory diseases.

Decrease in JAK2 V617F allele burden is not a prerequisite to clinical response in patients with polycythemia vera

BACKGROUND

Although reduction in the JAK2(V617F) allele burden (%V617F) has been suggested as a criterion for defining disease response to cytoreductive therapy in polycythemia vera, its value as a response monitor is unclear. The purpose of this study is to determine whether a reduction in %V617F in polycythemia vera is a prerequisite to achieving hematologic remission in response to cytoreductive therapy.

DESIGN AND METHODS

We compared the clinical and hematologic responses to change in %V617F (molecular response) in 73 patients with polycythemia vera treated with either interferon (rIFNα-2b: 28, Peg-rIFNα-2a: 18) or non-interferon drugs (n=27), which included hydroxyurea (n=8), imatinib (n=12), dasatinib (n=5), busulfan (n=1), and radioactive phosphorus (n=1). Hematologic response evaluation employed Polycythemia Vera Study Group criteria, and molecular response evaluation, European Leukemia Net criteria.

RESULTS

Of the 46 treated with interferon, 41 (89.1%) had a hematologic response, whereas only 7 (15.2%) had a partial molecular response. Of the 27 who received non-interferon treatments, 16 (59.3%) had a hematologic response, but only 2 (7.4%) had a molecular response. Median duration of follow up was 2.8 years. Statistical agreement between hematologic response and molecular response was poor in all treatment groups.

CONCLUSIONS

Generally, hematologic response was not accompanied by molecular response. Therefore, a quantitative change in %V617F is not required for clinical response in patients with polycythemia vera.

Influence of JAK2 46/1 haplotype in the natural evolution of JAK2V617F allele burden in patients with myeloproliferative neoplasms

JAK2V617F allele burden was prospectively measured in untreated patients with polycythaemia vera (PV, n=26) or essential thrombocythaemia (ET, n=36) and compared according to JAK2 46/1 haplotype status. The mean increase in JAK2V617F allele burden per year was 1%, 0.8% and 6% for PV patients with the JAK2 46/1 haplotype in negative, heterozygous and homozygous status, respectively (p<0.001). The JAK2 46/1 haplotype had no influence in JAK2V617 allele burden in ET. In conclusion, untreated PV patients homozygous for the JAK2 46/1 haplotype show a progressive increase in the JAK2V617F allele burden during the evolution of the disease.

Differences in the JAK2 and MPL mutation status in the cell lineages of the bcr/abl-negative chronic myeloproliferative neoplasm subtypes

OBJECTIVE

While the somatic mutation of Janus Kinase 2 (JAK2) and the thrombopoietin receptor (c-MPL) gene are thought to affect the pathogenesis of bcr/abl negative chronic myeloproliferative neoplasm (MPN), the relationship between the mutation and the clinical features remain obscure.

METHODS

The mutation status of these genes in granulocytes, platelets, T-cells, and erythroid colonies (BFU-E) was obtained from 115 MPN patients, and then the clinical features of the MPN subtypes were compared.

RESULTS

The JAK2-V617F mutation was observed in three lineages of granulocytes, platelets, and BFU-E in almost all polycythemia vera (PV) and primary myelofibrosis (PMF) patients. In contrast, 68% of essential thrombocythemia (ET) patients have the JAK2-V617F mutation in at least one of the lineages, of which 70% of these patients have the JAK2-V617F mutation in three lineages; the remaining ET patients with the JAK2-V617F mutation only exhibited the mutation in one or two lineages. Further, the ET patients that exhibited the JAK2-V617F mutation in three lineages had higher WBC and granulocyte counts as compared to the ET patients that did not have the JAK2-V617F mutation or only had the mutation in one or two lineages. Concerning the MPL gene, two ET patients had the MPL-W515L gene mutation in their platelets, although the lineage of the JAK2-V617F mutation involved differed from case to case.

CONCLUSION

The progenitor cells that are involved with the JAK2-V617F mutation in MPNs are different in each subtype and this difference may also affect the clinical features of MPNs.