Familial Mastocytosis: Identification Of KIT K509I Mutation and Its In Vitro Sensitivity To Imatinib, Dasatinib and PKC412

Mast cell diseases are myeloproliferative neoplasms characterized by an abnormal proliferation and accumulation of mast cells in different tissues. The clinical presentation of mastocytosis is heterogeneous, ranging from skin-limited disease to more aggressive variants that may be associated with multiorgan dysfunction/failure and shortened survival. In a relatively high proportion of cases, the clonal nature of the disease can be established on the basis of the demonstration of gain-of-function mutations involving the tyrosine kinase (TK) domain of KIT in skin lesions and BM cells and by the factor-independent proliferation and transforming abilities of these mutations. The tyrosine kinase inhibitor Imatinib is a treatment available for mastocytosis patients; however, some KIT mutations, specially KIT D816V, confer resistance to this drug.

To characterize the clinical phenotype and molecular mutations of 2 relatives with diagnosis of systemic mastocytosis (WHO 2008). We also aimed to test the in vitro sensitivity of primary bone marrow (BM) cells from both patients to tyrosine kinase inhibitors.

Four individuals were included in the study; two patients (case 1 [mother], and case 2 [daughter]), and the parents of case 1. DNA samples were obtained from total BM cells, CD3⁺ BM cells and oral mucosa of patients, and from peripheral blood of all individuals. KIT (exons 1 to 21) was submitted for Sanger sequencing analysis. Primary bone marrow cells (5X10⁴) from the 2 patients were cultured and treated with Imatinib (5uM), Dasatinib (80nM) and PKC 412 (100nM) or with vehicle only (control cells) and submitted for proliferation (MTT) and apoptosis assays (Annexin-V/PI) at days 4, 8 and 12 of culture.

Case 1 was a 33 year-old woman with a chronic history of pruritic skin rash who was referred to our outpatient service for evaluation of massive splenomegaly (25 centimeters in length) and pancytopenia. She had neither comorbidities nor any familial history of hematological malignancies. The patient had no siblings and had only one daughter (case 2). At biopsy, she showed extensive skin and bone marrow infiltration by mast cells. During follow up, the patient presented with spontaneous splenic rupture and had to undergo splenectomy, which led to the resolution of pancytopenia. She was diagnosed with Aggressive Systemic Mastocytosis. Her daughter (case 2), a 17 year-old woman, was also evaluated for an insidious history of diffuse skin rash. Skin and bone marrow biopsies showed massive infiltration by atypical mast cells and a diagnosis of Indolent Systemic Mastocytosis was made. The rare KIT K509I mutation was found in all DNA samples obtained from both patients, but not from the parents of case 1. This suggests that the KIT K509I was a germ line mutation acquired de novo by patient 1 that was subsequently transmitted to her daughter (patient 2). In vitro treatment of primary bone marrow cells harboring the KIT K509I mutation from patients 1 and 2 resulted in variable clinical response rates according to the drug used and the treatment duration. Imatinib treatment resulted in a significant reduction in proliferation (days 4, 8 and 12 of culture) and an increase in apoptosis (days 8 and 12) in cases 1 and 2 (all p≤0.03). Although Dasatinib resulted in decreased proliferation in both patients at day 12 (all p≤0.008), a significantly higher apoptosis ratio was observed only for patient 1 at day 12 of culture (p=0.03). PKC412 had a negative effect over cell growth in patient 1 (days 4 and 8) and in patient 2 (day 4) (all p≤0.03); however, no effect in apoptosis ratio was seen.

We herein provide a report of a KIT K509I mutation in familial mastocytosis. This mutation has been previously described in the literature in one case of familial mastocytosis. Although rare, the screening for KIT K509I mutation should be considered in all cases of familial mastocytosis. Based on in vitro studies, mastocytosis patients harboring the KIT K509I mutation could benefit from treatment with Imatinib, Dasatinib and PKC 412. However, Imatinib may be more effective in inducing neoplastic mast cells apoptosis. Both patients described were started on Imatinib in June 2013.

No relevant conflicts of interest to declare.