Chronic myelomonocytic leukemia genomic signature correlates with the degree of bone marrow fibrosis: A single-institutional retrospective study.

7559 Background: Chronic myelomonocytic leukemia (CMML) has features of both a myeloproliferative neoplasm and a myelodysplastic syndrome. The median overall survival (OS) in most series is 20-40 months. CMML is a relatively rare entity, and there is limited understanding of prognostic molecular markers. CMML associated with bone marrow fibrosis grade 1, appear to have shorter progression free survival. In this study we investigated the correlation of mutations with bone marrow reticulin fibrosis in patients with CMML. Methods: We investigated a cohort of 41 consecutive patients diagnosed with CMML 0, 1, 2, and CMML-AML from 2014 to 2019 at our institute. The median age of 41 patients was 68 years (range, 34-82). 27% were females and 73% were males. This cohort consists of 8 (20%) patients with CMML0, 19 (46%) with CMML1, 8 (20%) with CMML2 and 6 (15%) with CMML-AML. Genomic DNA was extracted from the bone marrow aspirates and targeted mutation NGS libraries were prepared from 200 ng of genomic DNA using the SureSelect target enrichment system (Agilent Technologies Inc.). The gene panel consists of 73 genes focused on myeloid neoplasms. The data were curated on the basis of our molecular pathology and national databases. Additionally, clinical data and bone marrow (BM) reticulin fibrosis grades (n = 27 available) were retrieved from the patient’s medical record. Results: The mutational profile frequency in our cohort showed that the most common mutations were TET2 (31%), ASXL1 (31%), and SRSF2 (23%), with frequencies very similar to those reported in the literature. Of the 27 cases with an available reticulin stain, only low grade fibrosis (MF-0, n = 18. MF-1, n = 9) were identified in our cohort. The frequencies of mutations in ASXL1, U2AF1, TP53, JAK2 and RUNX1, positively correlated with low grade fibrosis (MF-1). Additionally, patients with higher frequencies of SRSF2, TET2, and SETBP1 mutations showed no fibrosis (MF-0). Conclusions: This study is the first to correlate the degree of fibrosis with the frequency of mutations in CMML. We found similar mutations spectrum reported in the literature in patients with CMML. The mutational profile associated with CMML cases appears to affect the degree of the bone marrow fibrosis at the time of diagnosis.

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Bone marrow fibrosis in chronic myelomonocytic leukemia is associated with increased megakaryopoiesis, splenomegaly and with a shorter median time to disease progression

Oncotarget ◽

10.18632/oncotarget.21870 ◽

2017 ◽

Vol 8 (61) ◽

pp. 103274-103282 ◽

Cited By ~ 7

Author(s):

Kseniya Petrova-Drus ◽

April Chiu ◽

Elizabeth Margolskee ◽

Sharon Barouk-Fox ◽

Julia Geyer ◽

...

Keyword(s):

Bone Marrow ◽

Disease Progression ◽

Median Time ◽

Chronic Myelomonocytic Leukemia ◽

Bone Marrow Fibrosis ◽

Myelomonocytic Leukemia ◽

Marrow Fibrosis

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Impact of Bone Marrow Fibrosis and Early Response on Outcome after Azacitidine Therapy in 94 Patients with Myelodisplastic Syndromes, Chronic Myelomonocytic Leukemia and Acute Myeloid Leukemia

Blood ◽

10.1182/blood.v128.22.3187.3187 ◽

2016 ◽

Vol 128 (22) ◽

pp. 3187-3187

Author(s):

Gianluigi Reda ◽

Marta Riva ◽

Ramona Cassin ◽

Bruno Fattizzo ◽

Martina Pennisi ◽

...

Keyword(s):

Bone Marrow ◽

Acute Myeloid Leukemia ◽

High Risk ◽

Myeloid Leukemia ◽

Chronic Myelomonocytic Leukemia ◽

Severe Neutropenia ◽

Bone Marrow Fibrosis ◽

Myelomonocytic Leukemia ◽

Marrow Fibrosis ◽

Acute Myeloid

Abstract Azacitidine (AZA) is effective in high risk myelodysplastic syndromes (MDS), chronic myelomonocytic leukemia type 2 (CMML-2) and low blast count acute myeloid leukemia (AML) patients not suitable for more intensive treatment. Factors that may influence response to AZA are still under investigation. Bone marrow fibrosis is a potentially negative prognostic marker on overall survival (OS), but its clinical significance in this setting of patients remains to be clarified. We evaluated clinical predictors of OS and overall response rate (ORR; complete/partial response CR/PR; stable/progressive disease SD/PD) to AZA in a real life cohort. We studied 94 consecutive patients, treated at two Institutions from June 2009 till February 2016 with AZA subcutaneously (5+2+2 schedule) every 28 days, outside clinical trials. We analyzed data from routine laboratory analysis, bone marrow histology, morphology and cytogenetics at diagnosis. OS was measured from the starting of AZA treatment. Table 1 shows the clinical characteristics pre- and post-AZA: most patients (68%) were AREB1 or AREB2, 13% RCUD/RCMD or MDS NOS according to WHO 2008 classification, 17% AML, 2% CMML. At the onset of AZA therapy the majority of MDS cases (68%) showed an intermediate-2 risk, according to the International Prognostic Scoring System (IPSS) and high/very high risk (78%) according to IPSS-revised. Secondary and de novo cases, as well as cytogenetics risk groups, were equally represented; 50% of patients were transfusion dependant and moderate to severe neutropenia or thrombocytopenia were present in roughly 50/70% of cases respectively. As expected, bone marrow biopsies pre-AZA showed hypercellularity in most patients (65%). Remarkably, 47,5% of cases showed bone marrow fibrosis of ≥1 grade before AZA initiation. These findings were mostly unchanged at post-AZA evaluation. On the whole, 93 patients receiving > 4 cycles of therapy were available for response evaluation according to International Working Group 2006 criteria. After a median of 6 cycles (4-44), ORR was 41.9% (CR 18.3%, PR 11.8%, SD with hematologic improvement HI 11.8%), SD was 21.5%, PD 10.7% and 25.8% failed to achieve a response. Thirteen percent of patients reached at least partial cytogenetic response and 50% a HI. ORR was not influenced by monocytosis, neutropenia or IPSS cytogenetic risk category. Interestingly, pre-AZA marrow blast percentage, cytogenetic risk, time from diagnosis to AZA and the interval from 1st to 6th cycle had no impact on response. As regards marrow characteristics, patients with MF-0 pre-AZA displayed significantly lower PD rate and higher ORR, SD and HI than those with any grade of fibrosis (21.4% vs 51.4% and 78.6% vs 48,6%, respectively p=0.006, Fig1). This observation was also confirmed at marrow evaluation after AZA (22% versus 48% for PD and 78% versus 52% for ORR/SD/HI, p=0.05, Fig1). Regarding cellularity pre- and post-AZA, higher ORR,SD and HI and lower PD were observed for patients with normo/hypo compared to those with hyper-cellularity (Fig1) although not significantly. Forty-one percent of cases presented a hematologic toxicity (33% neutropenia and 18% thrombocytopenia of any grade) occurring after a median of 2 (1-18) AZA cycles. Moreover 28.6% of patients had an infection during AZA treatment, not related to neutropenia degree. Of note, toxicities did not affect median time from the 1st to the 6th AZA cycle (170,115-240 days), nor ORR. Median OS from the beginning of therapy was 18.5 months (12.7-24.4, 95% CI). IPSS high category [HR 2.24 (1.19-4.20) p=0.01], poor cytogenetics [2.19 (1.27-3.78) p=0.005], and lower ORR [0.46 (0.26-0.80) p=0.006] significantly affected OS. Unexpectedly, a response obtained after less than 4 cycles negatively impact OS [HR 0.86 (0.80-0.92) p<0.0001]. Notably, cases with pre-AZA fibrosis ≥MF-1 showed lower OS [2.26 (1.28-3.99) p=0.005]. In conclusion we provide evidence of no relationship between neutropenia and infections and of no impact of toxicities on dose-density and ORR to AZA treatment. Moreover, high marrow fibrosis and hypercellularity may affect response to AZA therapy. Further studies are needed to disclose the clinical/biological significance of marrow fibrosis/cellularity in the era of hypomethylating agents. Disclosures Reda: Roche: Membership on an entity's Board of Directors or advisory committees; Gilead: Research Funding.

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Association of bone marrow fibrosis with inferior survival outcomes in chronic myelomonocytic leukemia

Annals of Hematology ◽

10.1007/s00277-018-3289-6 ◽

2018 ◽

Vol 97 (7) ◽

pp. 1183-1191 ◽

Cited By ~ 6

Author(s):

Maliha Khan ◽

Tariq Muzzafar ◽

Hagop Kantarjian ◽

Ifra Badar ◽

Nicholas Short ◽

...

Keyword(s):

Bone Marrow ◽

Chronic Myelomonocytic Leukemia ◽

Survival Outcomes ◽

Bone Marrow Fibrosis ◽

Myelomonocytic Leukemia ◽

Marrow Fibrosis

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Clinical Experience with Azacitidine In Chronic Myelomonocytic Leukemia (CMML) in Spain

Blood ◽

10.1182/blood.v118.21.5040.5040 ◽

2011 ◽

Vol 118 (21) ◽

pp. 5040-5040

Author(s):

Pablo Gonzalez Navarro ◽

Regina García Delgado ◽

Alicia Bailén Garcia ◽

Juan Antonio Múñoz Múñoz

Keyword(s):

Bone Marrow ◽

Overall Survival ◽

Clinical Trials ◽

Clinical Experience ◽

Peripheral Blood ◽

Progression Free Survival ◽

Complete Response ◽

Chronic Myelomonocytic Leukemia ◽

Free Survival ◽

Myelomonocytic Leukemia

Abstract Abstract 5040 Clinical Experience with Azacitidine In Chronic myelomonocytic leukemia (CMML) in Spain Pablo González Navarro 1*, Regina García Delgado 2*, Alicia Bailén Garcia 3*, Juan Antonio Muñoz Muñoz 4* 1MD, PhD. Hospital San Cecilio, 18014 Granada, Spain, Teléfono: 958023600 [email protected]; 2Hospital Virgen De La Victoria, Málaga, Spain; 3Hospital Carlos Haya, Málaga, Spain; 4MD, PhD. Hospital Universitario Puerta del Mar, Cádiz, Spain Introduction: Chronic myelomonocytic leukemia (CMML) is a clonal disorder of hematopoietic stem cells often occurring in elderly patients. In the new WHO classification, CMML has been reclassified as a myelodysplastic/myeloproliferative disease. CMML has been subdivided in two subclasses: CMML-1:<5% blasts in peripheral blood and 5–9% blasts in bone marrow, and CMML-2: <10% blasts in peripheral blood and 10–19% blasts in bone marrow (Greco et al. Mediterr J Hematol Infect Dis.2011). Azacitidine (AZA) is an hypomethylating agent approved in Europe for the treatment of myelodysplastic syndromes, with an intermediate to high risk of progressing to AML or death; chronic myelomonocytic leukemia (CMML) and AML that has developed from a myelodysplastic syndrome (prescribing information EMEA 2011). Until its approval in May 2009, AZA was used in Spain under compassionate use in clinical trials. AZA produce a direct decrease of DNA methyltransferase activity, reverting aberrant DNA methylation and increasing the expression of silenced genes, leading to celular differentiation and/or apoptosis (Greco et al. Mediterr J Hematol Infect Dis. 2011). Materials and Methods: We report the results of a retrospective, longitudinal, multicenter Spanish study of 27 patients to assess the effectiveness of AZA to treat CMML. We present results of: Response, Overall Response, Overall Survival and Progression Free Survival. Results: Eighteen of the patients (69.23%) had Chronic Myelomonocytic Leukemia (CMML) type 1 and nine (30.77%) CMML type 2. Median age at diagnosis was 69 years. Male/female ratio: 19/8. ECOG performance status score 1–2 was 78%, twenty patients (74%) received an initial dose of 75 mg/m2 of AZA, whereas three patients (11%) received 50mg/ m2. The mean number of cycles received was 8.32, 95%IC (5.91; 10.73). Overall response to treatment was 53% (CR+PR+HI+mCR): 14.81% complete response, 7.4% partial response, 3,7% Medular complete response and 29,62% Hematological Improvement. In addition, 18,51% had stable disease. Thirty-six percent of patients were alive at the end of treatment with AZA. Median Overall Survival and Progression Free Survival were 17.47 months (95%CI 9.33, upper limit not reached) and 10.97 (95%IC 3.97, 17.47) respectively (Figure 1, 2). Conclusion: Our results show that AZA is an active drug in the treatment of patients with CMML, with similar response rates in the published literature. More data from this study and further investigation with different clinical trials are needed to confirm these outcomes as well as safety and effectiveness of this treatment. Disclosures: García Delgado: Celgene and Novartis: Speakers Bureau.

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Extramedullary Hematopoiesis in a Sentinel Lymph Node as an Early Sign of Chronic Myelomonocytic Leukemia

Case Reports in Pathology ◽

10.1155/2015/594970 ◽

2015 ◽

Vol 2015 ◽

pp. 1-4 ◽

Cited By ~ 2

Author(s):

Joslin M. Bowen ◽

Anamarija M. Perry ◽

Erin Quist ◽

Mojtaba Akhtari

Keyword(s):

Bone Marrow ◽

Lymph Node ◽

Sentinel Lymph Node ◽

Myeloproliferative Neoplasm ◽

Extramedullary Hematopoiesis ◽

Lymph Node Involvement ◽

Chronic Myelomonocytic Leukemia ◽

Hematopoietic Malignancy ◽

Hematologic Disorder ◽

Myelomonocytic Leukemia

Chronic myelomonocytic leukemia (CMML) is a clonal hematopoietic malignancy with features of both a myeloproliferative neoplasm and a myelodysplastic syndrome. Even though extramedullary leukemic infiltration is common in CMML patients, lymph node involvement has rarely been reported in the literature. We present an unusual case of a 72-year-old female who was found to have extramedullary hematopoiesis (EMH) in a sentinel lymph node that was excised during mastectomy for lobular breast carcinoma. One year later bone marrow biopsy was performed due to persistent anemia, thrombocytopenia, and monocytosis and the patient was diagnosed with CMML. Our case illustrates the importance of recognizing EMH in a lymph node during routine histological examination, especially in adults. Proliferation of bone marrow elements in a lymph node, in a patient with no known hematologic disorder, should trigger immediate bone marrow evaluation, as this could be the first clue in diagnosing underlying bone marrow disorder.

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A Phase 3, Double-Blind, Placebo-Controlled, Randomized Study Evaluating Navitoclax in Combination with Ruxolitinib in Patients with Myelofibrosis (TRANSFORM-1)

Blood ◽

10.1182/blood-2020-139758 ◽

2020 ◽

Vol 136 (Supplement 1) ◽

pp. 4-4

Author(s):

Jalaja Potluri ◽

Jason Harb ◽

Abdullah A. Masud ◽

Jessica E. Hutti

Keyword(s):

Bone Marrow ◽

Platelet Count ◽

Stock Options ◽

Myeloproliferative Neoplasm ◽

Bone Marrow Fibrosis ◽

Double Blind ◽

Starting Dose ◽

Secondary Endpoints ◽

Marrow Fibrosis

Background: Myelofibrosis (MF) is a rare myeloproliferative neoplasm with poor clinical outcomes. It is characterized by bone marrow fibrosis and an array of constitutional symptoms that impair quality of life. Allogeneic hematopoietic stem cell transplantation (allo-HSCT) remains the only potential curative therapy for MF, but HSCT is only accessible to a minority of patients and is associated with high morbidity and high rates of transplant-related mortality. JAK inhibitors (JAKi), including the JAK1/2i ruxolitinib and JAK2i fedratinib, are approved for the treatment of primary and secondary MF based on reduction in splenomegaly and disease-related symptoms; however, they have little impact on bone marrow fibrosis and are not effective at managing all clinical manifestations of MF. Therefore, a substantial clinical need for novel therapies to improve the disease course of MF exists. Navitoclax is an oral, potent, small-molecule inhibitor of the antiapoptotic B-cell lymphoma 2 (BCL-2) family proteins BCL-XL, BCL-2, and BCL-w and has demonstrated cell-killing activity in myeloproliferative neoplasm-derived cell lines and primary specimens ex vivo. Preliminary data from a Phase 2 study (NCT03222609) of ruxolitinib-experienced patients with primary or secondary MF have shown favorable spleen responses and tolerability with navitoclax plus ruxolitinib (Harrison et al. EHA 2020. EP1081). TRANSFORM-1 aims to evaluate the combination of navitoclax and ruxolitinib vs placebo and ruxolitinib in adults with primary or secondary MF who have not previously received a JAK2i. Study Design and Methods: In this Phase 3, double-blind, placebo-controlled study (NCT04472598), patients aged ≥18 years with intermediate-2 or high-risk MF with measurable splenomegaly, evidence of MF-related symptoms, no prior treatment with JAK2i, and Eastern Cooperative Oncology Group Performance Score ≤2 will be eligible for enrollment. Candidates for allo-HSCT and those who have received prior treatment with a BH3-mimetic compound or BET inhibitor will be excluded. Patients will be enrolled across 130 sites in approximately 17 countries. Planned target enrollment is 230 patients. Patients will be randomized 1:1 to receive navitoclax or placebo, plus ruxolitinib. Randomization stratification factors include intermediate-2 vs high-risk MF and platelet count ≤200 × 109/L vs >200 × 109/L. Navitoclax will be administered orally at a starting dose of 200 mg (platelet count >150 × 109/L) or 100 mg escalated to 200 mg once daily if tolerated after ≥7 days (platelet count ≤150 × 109/L). Ruxolitinib will be administered orally at a starting dose of 20 mg (platelet count >200 × 109/L) or 15 mg (platelet count 100-200 × 109/L) twice daily. Treatment may continue until the end of clinical benefit, unacceptable toxicity, or discontinuation criteria have been met. Patients who discontinue without progression will enter post-treatment follow-up; after disease progression or initiation of post-treatment cancer therapy, patients will enter survival follow-up. The primary endpoint of the study is ≥35% reduction in spleen volume from baseline (SVR35) at Week 24, as measured by magnetic resonance imaging or computed tomography, per International Working Group (IWG) criteria. Secondary endpoints include ≥50% reduction in total symptom score from baseline at Week 24 (measured by Myelofibrosis Symptom Assessment Form v4.0), duration of SVR35, change in fatigue from baseline, time to deterioration of physical functioning, anemia response per IWG criteria, SVR35 per IWG, reduction in grade of bone marrow fibrosis from baseline, overall survival, leukemia-free survival, and overall response and composite response per IWG criteria. Exploratory endpoints include progression-free survival. Safety will be assessed throughout the study via adverse event (AE) monitoring, physical examinations, vital sign measurements, electrocardiogram variables, and clinical laboratory testing. AEs will be graded per National Cancer Institute Common Terminology Criteria for AEs v5.0. The primary statistical analysis will be conducted using a stratified Cochran-Mantel-Haenszel test, and time-to-event secondary endpoints will be analyzed using a stratified log-rank test and Kaplan-Meier methodology. Hazard ratios will be estimated using stratified Cox proportional hazards model. Disclosures Potluri: AbbVie: Current Employment, Other: may hold stock or stock options. Harb:AbbVie: Current Employment, Other: may hold stock or stock options. Masud:AbbVie: Current Employment, Other: may hold stock or stock options . Hutti:AbbVie Inc.: Current Employment, Other: may hold stock or stock options. OffLabel Disclosure: Navitoclax is an investigational drug for the treatment of myelofibrosis

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MO802DOES ETELCALCETIDE REVERSE MYELOFIBROTIC BONE CHANGES DUE TO HYPERPARATHYROIDISM? A CASE REPORT

Nephrology Dialysis Transplantation ◽

10.1093/ndt/gfab096.0011 ◽

2021 ◽

Vol 36 (Supplement_1) ◽

Author(s):

Vincenzo Antonio Panuccio ◽

Rocco Tripepi ◽

Silvia Lucisano ◽

Adele Postorino ◽

Giovanna Parlongo ◽

...

Keyword(s):

Bone Marrow ◽

Alkaline Phosphatase ◽

Kidney Disease ◽

Primary Hyperparathyroidism ◽

Bone Marrow Biopsy ◽

Low Grade ◽

Bone Marrow Fibrosis ◽

Paracrine Factors ◽

Short Period ◽

Marrow Fibrosis

Abstract Background and Aims A 21 year old boy with a diagnosis of Autosomal Recessive Polycystic Kidney Disease and Caroli disease reached a final stage of chronic kidney disease (CKD) and started haemodialysis. Method After 3 years in haemodialysis he underwent a kidney transplant from a cadaveric donor. His transplanted kidney worked fairly well until the patients was 31 year old when he developed graft dysfunction (serum creatinine from 2.7 to 5 mg/dL in a very short period). There was a concomitant increase in serum phosphate levels (8.3 mg/dl) and iPTH that progressively increased to 1032 pg/ml despite a traditional therapy. Results At age 32, the patient returned to haemodialysis treatment thrice weekly with a concomitant a progressive worsening of hyperparathyroidism with bone pain . Cinacalcet at a dosage of 30 mg daily treatment was initiated, then it was increased to 120 mg daily without any benefit. Two years later, being the clinical situation without any improvement and being the patient scarcely compliant to the therapy and because a further enlargement of parathyroid glands was observed, a Parathyroidectomy (PTX) was advised. Nevertheless, PTX was not performed because of patient’s refusal. Furthermore, despite Erythropoietic Stimulating Agent (ESA) therapy, he developed severe anemia that required regular and frequent blood transfusions. iPTH increased to 4500 pg/ml [Fig.1] with a parallel rise in alkaline phosphatase >600 UI/L [Fig.2]. A Computed Tomography scan showed multiple bone-thickening lesions. He thus initiated Etelcalcetide 5 mg e.v. 3 times a week, after the HD session but without any benefit. The dosage was then increased to 7.5 mg but the patient gradually became frail and developed pancytopenia and low-grade fever. Hematological evaluation with bone marrow biopsy was performed in December 2019. Bone marrow histology showed severe fibrosis [Myelofibrosis (MF) grade 3] with normal bone marrow cytogenetics. Blood samples for mutations in JAK-2, CALR, and MPL and BCR-ABL rearrangement were negative. There was no evidence for a myeloproliferative neoplasm (MPN) or metastatic lesions. During the following months, while on a 7,5 mg dose of etalcalcetide, there was a gradual reduction in iPTH [Fig.1] and serum alkaline phosphatase [Fig.2], up to 500 pg/dl and 200 IU/L respectively. The patient developed asymptomatic, often severe, hypocalcemia which was managed with therapy. The patient’s clinical conditions gradually improved, anemia responded to lower doses of ESAs. A bone marrow biopsy was repeated after one year (December 2020) and it showed a reduction in fibrosis (MF grade varying from 1-2). Then etelcalcetide dosage was reduced while serum calcium and phosphate levels were in the normal limit. Conclusion Myelofibrosis secondary to renal osteodystrophy is an uncommon complication. It has been rarely reported and usually is associated with primary hyperparathyroidism. Marrow fibrosis and pancytopenia is related to the excessive iPTH that upregulates production of cytokines and paracrine factors in the bone marrow (IL-1a, IL-6, FNF-a, TGF–b, and platelet-derived growth factor) and it has an important stimulatory effect on fibroblast proliferation. It is known that surgical parathyroidectomy is associated with a reduction of bone marrow fibrosis in primary hyperparathyroidism. To our knowledge, this is probably the first case of tertiary hyperparathyroidism in which the effect of etelcalcetide is comparable to parathyroidectomy as far as on calcium-phosphate balance, and a significant improvement in bone marrow fibrosis and hemoglobin. In conclusion, etelcalcetide at least in this patient seems as effective as PTX on bone balance, bone marrow and anemia.

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Bone Marrow-Specific Loss of ABI1 Induces Myelofibrosis through a Mechanism Involving Activation of NFκB

Blood ◽

10.1182/blood.v128.22.1203.1203 ◽

2016 ◽

Vol 128 (22) ◽

pp. 1203-1203 ◽

Cited By ~ 1

Author(s):

Anna Dorota Chorzalska ◽

John Morgan ◽

Diana Olguta Treaba ◽

Adam J Olszewski ◽

Nathan Kingston ◽

...

Keyword(s):

Stem Cells ◽

Bone Marrow ◽

Hematopoietic Stem Cells ◽

Myeloid Leukemia ◽

Myeloproliferative Neoplasm ◽

Poly I:C ◽

Bone Marrow Fibrosis ◽

Hematopoietic Stem ◽

Organ Specific ◽

Marrow Fibrosis

Abstract Primary myelofibrosis (PMF) is an age-related myeloproliferative neoplasm (MPN) characterized by progressive bone marrow fibrosis, increasingly ineffective hematopoiesis, vascular and inflammatory complications, and progression to acute myeloid leukemia (AML) in 20% of cases. PMF is the rarest and the most severe of MPNs, with unfavorable prognosis and challenging treatment. Constitutive activation of the JAK-STAT pathway is an essential contributor to the pathogenesis of PMF. Nonetheless, clinically available JAK inhibitors do not show consistent disease-modifying effects at the molecular and malignant stem cell levels, underscoring the need to identify other targetable pathogenic mechanisms in PMF and related MPNs. We recently described a new signaling cascade involved in the maintenance of the malignant phenotype of stem cells in chronic myeloid leukemia that affects their adhesiveness, interaction with the microenvironment and resistance to imatinib (Chorzalska et al., Leukemia 2014). These phenotypic changes were caused by loss of Abelson interactor-1 (Abi-1) which regulates integrin α4 and the Src family kinases (SFKs) signaling pathways. To further delineate how organ-specific absence of Abi-1 affects hematopoiesis and hematopoietic stem cells, we generated a conditional, inducible bone marrow-specific Abi-1 transgenic mouse knockout model. For this purpose, we crossed conditional Abi-1(fl/fl) knockout mice with transgenic Tg(Mx1-cre+/-) mice (B6.Cg-Tg(Mx1-cre+ 1Cgn/J) that express Cre recombinase under the control of the polyinosinic:polycytidylic acid (poly(I:C)) or interferon-inducible Mx1 promoter. Obtained Abi-1(fl/fl);Tg(Mx1- cre(+)) animals were subjected to bone marrow-specific Abi-1 deletion induced by intraperitoneal administration of poly(I:C), resulting in organ-specific Abi-1 knockout (Abi-1 KO). Abi-1(fl/fl);Tg(Mx1- cre(-)) animals subjected to the same poly(I:C) treatment (Abi-1 WT) were used as controls in all experiments (Figure 1A). We confirmed the absence of Abi-1 protein in the Abi-1 KO bone marrow by immunoblotting (Figure 1B). Abi-1 KO mice developed deranged hematopoiesis with progressive anemia, thrombocythemia, megakaryocytosis, and myeloid hyperplasia (Figure 1C). They also developed progressive splenomegaly with 1.4x, 2.4x, and 4.8x enlargement of the spleen at 4, 28, and 50 weeks after recombination, respectively, and progressive femur pallor (Figure 1D) with bone marrow fibrosis confirmed by trichrome and silver staining (Figure 1D). Using fluorescence-activated cell sorting, we determined that Abi-1 KO resulted in more than two-fold expansion of the most primitive long-term hematopoietic stem cells (Lin-CD34-Sca-1+c-Kit+ CD135-) in the bone marrow and 3.75-fold increase of LSK cells (Lin- Sca-1+c-Kit+) in the peripheral blood compared with controls (Figure 1E). Increased activities of the SFKs and NFκB were observed in Abi-1 KO bone marrow, and confirmed in human CD34+ cells isolated from the marrow of patients with PMF or AML (Figure 1F,G). Furthermore, CD34+ hematopoietic progenitors and granulocytes from PMF patients, as well as CD34+ hematopoietic progenitors from AML patients, showed decreased levels of ABI1 transcript (Figure 1H). Taken together, our results indicate that a targeted deactivation of Abi-1 in the bone marrow causes expansion of hematopoietic stem cells, myeloid hyperplasia with thrombocytosis and overproduction of collagen and reticulin fibers. The Abi-1 KO murine phenotype meets the Mouse Models of Human Cancers Consortium criteria for a myeloproliferative neoplasm with features resembling human PMF (Kogan et al., Blood 2002). These data indicate that loss of Abi-1 is sufficient for initiation of bone marrow fibrosis, and that Abi-1-SFKs-NFκB cross-talk may represent a new molecular target for the treatment of PMF. Bone marrow-specific, conditional Abi-1 KO animals represent a new model of PMF offering a unique opportunity to interrogate the molecular details of the Abi-1-driven mechanism that leads to the development of stem cell-derived MPNs, and to elucidate potential novel therapeutic approaches that directly address their pathogenesis. Disclosures Olszewski: Bristol-Myers Squibb: Consultancy; Genentech: Research Funding; TG Therapeutics: Research Funding. Reagan:Alexion: Honoraria; Seattle Genetics: Membership on an entity's Board of Directors or advisory committees.

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