About 17% of AML with NPM1 mutations Show a Specific Pattern of Chromosome Aberrations but These Cases Do Not Differ Prognostically from AML with NPM1 Mutations Carrying a Normal Karyotype

AML with mutated nucleophosmin gene (AML NPM1mut) usually carries a normal karyotype and will be suggested as a provisional entity in the new WHO classification. Thus far, the impact of chromosome aberrations in AML NPM1mut has not been evaluated in detail. Aim of this study was to determine the incidence and prognostic impact of clonal chromosome aberrations in NPM1mut. We further compared this pattern to additional aberrations in AML with recurrent genetic aberrations: t(8;21)(q22;q22), inv(16)(p13q22)/t(16;16)(p13;q22), t(15;17)(q22;q12) and 11q23-abnormalities leading to an MLL-rearrangement. In total 415 AML (de novo AML: 392, s-AML: 11, t-AML: 12) showing an NPM1 mutation were analyzed by chromosome banding analysis. 71 of these showed clonal chromosome aberrations (17.1%; de novo AML: 63 (16.1%), s-AML: 5 (45.5%), t-AML: 3 (25%); de novo AML vs. s-AML: p=0.024). Overall, 111 chromosome aberrations were observed. The most frequent abnormalities were +8 (n=30), −Y (n=10), +4 (n=9), del(9q) (n=5), +21 (n=4), −7 (n=3), +5 (n=2), +10 (n=2), +13 (n=2),+18 (n=2), del(12p) (n=2), del(20q) (n=2), other non-recurrent balanced aberrations (n=6), other non-recurrent unbalanced aberrations (n=32). For comparison 63 AML with t(8;21), 37 cases with inv(16)/t(16;16), 83 patients with t(15;17) and 83 AML showing a 11q23/MLL-rearrangement were evaluated. 44 (69.7%), 13 (35.1%), 39 (47%), and 28 (43.1%) cases showed clonal chromosome aberrations in addition, respectively. Therefore, additional chromosomal aberrations are more frequent in all these subgroups than in the AML NPM1mut. Similar to NPM1mut cases +8 (n=2), −X/Y (n=32), +4 (n=2), and del(9q) (n=10) were observed. The only other recurrent additional aberrations was del(11q) (n=2). In inv(16)/t(16;16) we also found +8 (n=5) and −Y (n=1). The only other recurring additional aberrations were +22 (n=6) and del(7q) (n=2). In AML with t(15;17) recurring additional abnormalities were +8 (n=12), −Y (n=3), del(9q) (n=2), ider(17)(q10) t(15;17) (n=7). AML with 11q23/MLL-rearrangement showed +4 (n=2), +8 (n=8), +13 (n=2), +19 (n=4), +21(n=4), +22 (n=2), −Y (n=1). Thus, chromosome aberration in AML NPM1mut share many overlaps to those in AML with recurrent aberrations. Furthermore, the prognostic impact of chromosome aberrations in AML NPM1mut was evaluated. No difference with respect to overall survival (OS) and event-free survival (EFS) was observed between AML NPM1mut with a normal (n=344) and an aberrant karyotype (n=71) (OS at 2 yrs 78% vs. 81%, p=0.969; EFS at 2 yrs 40% vs. 50%, median EFS 544 days vs. 522 days, p=0.253). The FLT3-ITD status was available in 400 cases. 127 (38%) of 334 cases with a normal karyotype showed a FLT3-ITD, while in only 16 (24%) of 66 cases with an aberrant karyotype a FLT3-ITD was observed (p=0.035). While the negative prognostic impact of additional FLT3-ITD was confirmed in our cohort, the presence of chromosome aberrations did not influence prognosis neither in the FLT3-ITD negative nor in the FLT3-ITD positive subgroup. In addition, 31 patients with AML NPM1mut were analyzed by chromosome banding analysis at diagnosis and at relapse (median time diagnosis to relapse: 301 days (range: 71–986). 22 cases (71%) showed a normal karyotype both at diagnosis and relapse. In 4 cases a normal karyotype was observed at diagnosis and an aberrant karyotype at relapse (del(9q) (n=2), t(2;11) (n=1), inv(12) (n=1)). One case with +8 at diagnosis showed +8 also at relapse. One case with +4 at diagnosis showed +4 and additional aberrations at relapse. In 1 case clonal regression was observed (+21 -> normal). One case with an unbalanced 1;3-translocation at diagnosis showed a der(17;18) (q10;q10) at relapse and one case with −Y at diagnosis showed a del(3p) at relapse. In conclusion: 1. Frequency of additional chromosome aberrations is low in AML NPM1mut as compared to other genetically defined WHO entities. 2. The pattern of additional chromosome aberrations is overlapping between the 5 groups analyzed. 3. Chromosome aberrations observed at diagnosis in AML NPM1mut do not influence prognosis in comparison to AML NPM1mut with normal karyotype. 4. The karyotype is stable in most AML NPM1mut patients at diagnosis and at relapse. These results point to chromosomal aberrations occurring in AML NPM1mut as secondary events and further support inclusion of AML NPM1mut as a provisional entity in the new WHO classification.