Molecular and Cytogenetic Evaluation in Systemic Mastocytosis

Systemic mastocytosis (SM) is a rare hematological neoplasm characterized by pleomorphic symptoms, related to mediators release from pathological mast cells. Tissue infiltration of liver, spleen, gastro-intestinal (GI) tract and osteolysis are present in aggressive disease. As KIT D816V mutation can be found in approximately 80% of patients, several additional somatic mutations of prognostic significance have been detected in a subset of patients.

We report on clinical, cytogenetic and molecular characteristic of 160 patients with SM diagnosed in the last 15 years at our center. Fourteen (8.9%) patients had cutaneous mastocytosis, 85 (53.5%) had indolent SM (ISM), 12 (7.5%) had smoldering SM (SSM), 27 (17%) had aggressive SM (ASM), 17 (10.5%) had SM with an associated hematological neoplasm (AHN-SM) and 1 (0.6%) had mast cell leukemia. Symptoms were mostly related to mediators release: itching and blistering (69, 43%), anaphylaxis (49, 30%), diarrhea/bloating (19, 12%), osteoporosis/bone fracture (10, 6%), constitutional symptoms (8, 5%), bone pain and myalgia (16, 10%), anaphylaxis from Hymenoptera venom (24, 15%). While SM patients were mostly treated with antihistamines and membrane stabilizers, 29 patients needed a disease modifying therapy. Four patients with ISM were treated with Interferon (IFN) for bone lytic alterations, while 1 patient with ISM started midostaurin for uncontrolled symptoms. Other 11 patients with ASM were treated with midostaurin: 4 responded, while 7 needed multiple lines of therapy including allogeneic transplant in 4 cases. Eleven patients with ASM were treated with tyrosin kinase inhibitors (TKI) before midostaurin became available, while 2 elderly patients were treated with low dose cytarabine and TKI for concomitant acute leukemia.

KIT D816V mutation was found in 111 of 131 (85%) evaluable patients with SM. Cytogenetics was normal in all but 2 patients with AHN-SM in which a chromosomal gain was reported. Among 6 patients with ISM, KIT mutation was found in only 2 cases. One patient had isolated KIT mutation and was treated with IFN for skin symptoms and bone fracture, and another patient had a DNMT3A associated mutation and was treated with imatinib. Moreover, 1 patient presented both DNMT3A and TET2 mutations and was treated with IFN for bone fracture. Among 12 ASM patients, 1 was negative for significant mutations: no disease control was achieved after target therapy, and bone marrow function slowly recovered after allogeneic transplant. The most frequently recorded mutations were SRSF2 and TET2: they were present in 4 patients each and recorded together in 3 cases. Two patients with SRSF2 and TET2 mutations died after multiple lines of therapy: 1 patient for infection and graft versus host disease after allogeneic transplant and 1 for progressive disease. Moreover, the latter had additional RUNX1 mutation. Two patients with concomitant SRSF2 and IDH1 mutations and TET2 mutation respectively responded to midostaurin, while 1 with DNMT3A mutation received multiple lines of therapy and died for pneumonia. One patient with ASXL1 mutation responded to chemotherapy after 2 previous lines of treatment. Among 4 patients with isolated KIT mutation, 2 responded to midostaurin, 1 obtained a partial control of symptoms after 4 lines of treatment and 1 suddenly died after achieving stable disease after 2 lines of therapy. Among 3 patients with SM associated to myelodyplastic syndrome, 1 was negative for KIT but positive for TP53 mutations and died for natural event after symptoms directed therapy and continuing recombinant erythropoietin. The other 2 KIT positive patients presented multiple concomitant mutations. Additional TET2 and SRSF2 mutations were recorded in a patient who died few weeks after diagnosis, while additional TET2, SRSF2, RUNX1 and KRAS mutations were recorded in a patient treated with midostaurine and 2CDA before successful allogeneic transplant.

Molecular characterization of SM has uncovered a complex landscape of somatic mutations other than KIT D816V. Although the prognostic significance of some recurrent mutations has been reported, prospective correlation with clinical response may highlighted their role in the pathogenesis of disease and possibly elucidate mechanism of resistance to treatment. On the other hand, cytogenetics might no be considered mandatory except for AHN-SM