Prognotic Impact of Mutations in Systemic Mastocytosis

Introduction: Systemic mastocytosis (SM) is an heterogeneous myeloproliferative neoplasm characterized by uncontrolled growth of cKIT mutated clonal mast cells, with clinical manifestations ranging from indolent forms (ISM) with only skin lesions and/or anaphylactic shock to smouldering SM (SSM) with evidence of organ involvement to aggressive forms (ASM) with extensive organ damage. SM with an associated hematological neoplasm (SM-AHN) is a subtype with adverse prognosis possibly related to the associated neoplasm rather than SM. Recent data indicate that mutations in genes associated to myeloid malignancies correlate with survival and disease progression but information are still scanty and not completely concordant, and mutations apparently did not correlate with disease manifestations; furthermore, most patients analyzed were SM-AHN, therefore the contribution of mutations associated with the non-mast cell clone could not be ascertained clearly. The aim of this study was to address the prognostic relevance of mutations at diagnosis in a series of 70 pts with SM, excluding SM-AHD.

Methods: There were 58 ISM, 4 SSM, 7 ASM and 1 mast cell leukemia (MCL) pts. cKITD816V mutation was assessed by RTQ-PCR. Mutations in myeloid-neoplasm associated genes were assessed by next generation sequencing (NGS) with Ion Torrent PGM platform; analyzed genes included ASXL1, CBL, cKIT, ETNK1, EZH2, IDH1, IDH2, SRSF2, RUNX1, TET2. We also evaluated the cKITD816V variant allele frequency (VAF) by NGS. Kruskal-Wallis test and Chi-squared test were used for analysis of continuous and categorical variables, respectively. Cox regression model was used for survival analysis; variables included were WHO subgroup, cKITD816V >2% VAF, mutations in additional genes, age at diagnosis >60 years, serum tryptase >200 ng/mL, alkaline phosphatase and white blood cells greater than upper normal limit. We did not considered as single entities the features included in B and/or C findings.

Results: Median age at diagnosis was 45 years (y)(range 17-76) for ISM, 67 y (53-74) for SSM, 73 (41-81) for ASM (p<0.001).

At last follow up (median 2 years, range 0.2-14) 2 pts with ASM evolved to acute myeloid leukemia (AML, 2.8% of all series, 28% of ASM), 1 ISM pts developed organ damage and shifted to ASM; 4 pts died (4/70, 5.7%). Skin involvement was found in 80.6% of pts, 31.3% had history of anaphylaxis and 27.8% osteoporosis. Median serum tryptase levels were 28.7 ng/mL (range 3-192) for ISM, 190 (60-591) for SSM, 64 (23-300) for ASM and 2000 in MCL.

All but 3 pts (2 ISM, 1 ASM) were cKITD816V mutated by RT-PCR in bone marrow aspirate (BM) or peripheral blood (PB) (in pts with punctio sicca). 54/67 (80%) had a cKITD816V mutation detectable in PB, underlying that even with high sensitivity methods PB cannot replace BM analysis for diagnosis. Median VAF was 0.34% (range 0.03-38.4) in ISM, 17.2% (0.3-45) in SSM, 26.7% (0.9-81.7) in ASM and 71.7% in MCL.

Pts with at least 1 additional mutation were 6.9% (4/54) ISM, 50% (2/4) SSM and 57% (4/7) ASM(p<0.001). Five pts (7%), 4 ASM (57%) and 1 MCL, had more than 1 additional mutations (p<0.001). TET2 was the most frequent mutated gene, in 9/70 cases (12.9%), followed by ASXL1 and SRSF2 in 2 pts (2.9% each) and EZH2, IDH2, CBL and RUNX1 in 1 pts each (1.4%). No mutation was found in IDH1 and ETNK1. No recurrent mutation types were found. All pts with more than one additional mutations other than cKITD816V carried a TET2 mutation combined with ASXL1 and SRSF2 mutation in two cases each and CBL in one case. One pt had a third additional EZH2 mutation. Karyotype was normal in all the 58 pts evaluated, including 5 ASM and 3 SSM.

In univariate analysis the following parameters were associated with shorter survival: one or more additional mutations (HR 19.2, CI 2-185, p<0.001), TET2 mutation (HR 28, CI 2.9-271, p=0.004) (mutations in other single genes were not evaluated due to low number of cases) and ASM variant (HR 5.3, CI 1.7-16.1, p=0.003). Pts older than 60 y at diagnosis had higher frequency of additional mutations (35% versus 8% in pts <60 y, p=0.005) and of cKITD816V VAF >2% (68.8% versus 8.3%, P<0.001).

None of the analyzed molecular features correlated with anaphylaxis, osteoporosis or skin involvement.

Conclusions: With the limitations due the small number of events recorded in this series, these data suggest that knowledge of molecular asset in pts with SM might provide prognostically relevant information.