Higher Percentages of Ring Sideroblasts and SF3B1 Mutations in Patients with AML Correlate with Mutations in TP53 and Adverse Cytogenetics, but Have No Independent Impact on Outcome

Introduction

Ring sideroblasts (RS) are common findings in myeloid malignancies as MDS or MDS/MPN, especially in RARS-T, and are associated with SF3B1 mutations. However, the incidence of both RS and SF3B1 mutations have not yet been assessed properly in acute myeloid leukemia (AML).

Aim

Determine the frequency of RS and SF3B1 mutations in 1857 patients with de novo and therapy-related AML (t-AML). Define their impact on survival and association with other frequently mutated genes, as well as with cytogenetic abnormalities.

Patients and Methods

From a total of 1857 AML patients (excluding those with cytogenetically-defined entities according to WHO), bone marrow assessment revealed 473 (25%) with RS ≥ 1% and thereof 183 (10% of all) with ≥15 RS. Sequencing or melting curve analysis were performed in a subcohort of 340/473 patients for the detection of mutations in: SF3B1, ASXL1, DNMT3A, FLT3-TKD, IDH1R132, IDH2R140, IDH2R172, KRAS, NPM1, NRAS, RUNX1, TET2, FLT3-ITD and MLL-PTD. These 340 cases were subject to the study. Out of these 340, 141 cases (42%) had RS ≥ 15% and the remaining 199/340 (59%) had RS ≥1 to <15%. The cohort consisted of 303 (89%) de novo AML (FAB: M0 n=18, M1 n=67, M2 n=165, M4 n=30, M5 n=3, M6 n=20) and 37 (11%) t-AML. 148 were female and 192 were male, with median age 74 years, range: 20-93 years. Chromosome banding analysis (assisted by FISH if needed) was performed in all 340 cases.

Results

The percentage of bone marrow blasts correlated inversely with the percentage of RS present (r 0.213, p<0.001). 136 (40%) patients had a normal karyotype. Intermediate cytogenetics according to MRC criteria were found in 193 (57%), and adverse in 147 (43%). Patients with RS ≥15% more frequently had adverse cytogenetics in comparison to those with RS between 1-14% (54% vs 36%, p=0.001).

The frequencies of gene mutations were as follows: TP53 103/331 (31%), RUNX1 84/315 (26%), DNMT3A 86/337 (25%), TET2 68/330 (20%), ASXL1 58/334 (17%), IDH2R140 53/338 (15%), NPM1 43/340 (12%), SF3B1 34/334 (10%), FLT3-ITD 33/340 (10%), NRAS 29/340 (8%), IDH1R132 21/339 (6%), MLL-PTD 22/337 (6%), FLT3-TKD 18/333 (5%), KRAS codon 12 13/299 (4%), IDH2R172 13/338 (3%) and KRAS codon 61 3/299 (1%). Moreover, in total 30 variants in 28 patients were identified in DNMT3A, RUNX1, TET2 and TP53, which according to current knowledge cannot be assigned to mutations or SNPs yet.

Patients with ≥15% more frequently had TP53 mutations (mut) (44% vs 22%, p<0.001) and less frequently IDH2R140 mut (11% vs 19%, p=0.094) and MLL-PTD (2% vs 6%, p=0.006). Accordingly, patients with TP53 mutations had higher percentages of RS as compared to those without (28% vs 16%, p<0.001) and patients with IDH2R140 mut and MLL-PTD, respectively, had lower percentages of RS as compared to those without (15% vs 21%, p=0.043 and 11% vs 21%, p=0.025, respectively). Furthermore, patients with mutations in the following genes had fewer RS than patients with the respective wild-types: ASXL1 (15% vs 21%, p=0.040), FLT3-ITD (14% vs 21%, p= 0.049), IDH2R140 (15% vs 21%, p=0.043), MLL-PTD (11% vs 21%, p=0.025), NPM1 (13% vs 21%, p=0.018) and KRAS codon 61  (3% vs 20%, p<0.001). Conversely, patients with mutated SF3B1 had more RS than patients with wild type (27% vs 19%, p=0.054). However, the number of RS did not translate into an increase in the mutational burden of SF3B1.

Given the limited degree of overlap between mutations of the four most frequently mutated genes, we hierarchically separated the patients into 5 groups: TP53mut, ASXL1mut, NPM1mut, SF3B1mut and patients without any of these mutations. Interestingly, the percentage of RS was very similar in the two groups, TP53mut and SF3B1mut, and significantly higher as compared to all other groups (TP53mut/SF3B1mut: 28% vs ASXL1mut/NPM1mut/others 14%, p<0.001).

The number of RS did not have an impact on the overall survival (OS) and event free survival (EFS) of patients.

Conclusion

1.     Ring sideroblasts ≥ 15% are present in 10% of de novo and t-AML.

2.     The blast count correlates inversely with the number of RS.

3.     Patients with ≥ 15% RS more frequently carry TP53 mutations and adverse cytogenetics.

4.     Although patients with ≥ 15% RS have worse molecular (TP53mut) and cytogenetic features, there is no statistically significant impact on survival when compared to patients with <15% RS.

5.     Patients with mutated ASXL1, FLT3-ITD, IDH2R140, MLL-PTD and NPM1 have less RS than wild type patients while those with TP53 or SF3B1 mutations have higher RS.