Recent development in sequencing technologies with deep coverage for mutation analysis has enabled the identification of clonal architecture in some cancers. RAS mutations are observed in a large proportion of MLL leukemias. Our hypothesis is that determination of RAS mutation status in MLL leukemias should provide insights into the clonal make up of this disease and clues about the nature of clones that overcome therapy.
We combined exome and transcriptome sequencing in 32 adult MLL leukemias and results were compared to our cohort of 48 normal karyotype (NK) AML. Exome capture and paired-end sequencing (2 x 100bp, Illumina HiSeq 2000) were performed using TruSeq (Illumina) protocols. Mean coverage was 165X for transcriptome and 42X for exome. Initial analysis was focused on 25 known AML-associated genes and excluded all other novel mutations. Average transcriptome and exome coverage for N/KRAS alleles was 287X (25-846) and 42X (9-151), respectively. Clones were defined based on the identification of N/KRAS mutations in at least 1% of the reads.
Figure 1 shows mutation status, MLL partners and FAB classification for each MLL leukemia. No mutations were observed in NPM1, FLT3 (ITD), CEBPA (biallelic), RUNX1, DNMT3A, IDH1, KIT, BCOR, SF3B1, U2AF1 or RAD21. On average, 1 mutated gene (range: 0-4) per sample was found compared to 3 (range 0-5) in NK-AML (p < 0.0001). We observed that 13/32 MLL leukemias (which include 2 paired samples) harbored N/KRAS mutations. There were no association between RAS mutation status and MLL partner, FAB classification, age, white blood cell count and overall survival. RAS mutations were found in 15% of NK-AML which contained on average 2.3 additional mutations in leukemia-associated genes compared to only 0.3 (p<0.0001) in MLL leukemias.
Excluding 2 paired relapse specimens, a total of 24 N/KRAS mutated clones were identified in 11 of the 30 MLL leukemias. The first sample included 5 clones each containing different NRAS mutations (e.g. G13R, G13D, etc.) contributing to 17, 9, 4, 2 and 2 % of the reads. Since RAS mutations are mostly heterozygous, we estimated that the contribution of each clones varied between 34 (i.e. 17% x 2) to 4%. A similar analysis revealed 4 clones in another specimen, contributing to 22, 12, 12 and 4 % of the cells. In 4 additional samples, the proportions of N/KRAS mutated clones were 1) 42, 38 and 8% 2) 92, 4 and 2 %, 3) 78 and 12% and 4) 52 and 32%, establishing that 20% (6/30) of these MLL leukemias were oligo- to polyclonal. In comparison, our NK-AML cohort of 48 patients included 7 specimens mutated for N/KRAS in which a total of 10 different clones were identified for an average of 0.2 RAS mutated clones per NK-AML versus 0.8 in MLL leukemias (p=0.007). This result further strengthens the hypothesis that RAS and MLL-fusion genes are strong collaborators in human AML.
Grossmann et al recently showed that RAS mutated clones can be lost at relapse (Leukemia, 2013), possibly suggesting that other genes are at play in collaborating with MLL-fusions and causing drug resistance. To identify such genes, we further analyzed paired diagnosis and relapse samples in 2 patients. In the first patient, the KRAS mutation that was found in 66% of the cells at diagnosis was identified in all cells at relapse. In the second patient, while KRAS G12V and G12D mutations were found in 78% and 12 % of the cells at diagnosis, only the G12V clone was detected in 100% of the cells at relapse indicating in vivo clonal selection in both cases. We then performed a comparative analysis of mutated/wild type allele ratios for other coding genes. This analysis enabled us to identify a subset of mutations in candidate genes that are present at relapse in the dominant clone but that were undetectable or at lower frequency at presentation, indicating they might be specifically involved into occurrence of relapse (i.e. drug resistance).
NRAS and KRAS are mutated in 37% of MLL leukemias in this cohort. In contrast to NK-AML, these leukemias are frequently oligo- to polyclonal and contain few additional mutations suggesting that RAS activation may be sufficient to induce AML in the presence of MLL fusions. Evidence from our limited number of relapse patients, and that of others, suggests that RAS does not confer drug resistance which could be explained by novel mutations in genes that were specifically detected in the dominant clones at relapse.
No relevant conflicts of interest to declare.
Asterisk with author names denotes non-ASH members.