Whole Exome Sequencing Reveals That DNMT3A and ASXL1 Mutation Are Involved in the Development of Tyrosine Kinase Inhibitor Resistance in Chronic Myeloid Leukemia Patients without ABL1 Tyrosine Kinase Domain Mutation

BACKGROUND:

Tyrosine kinase inhibitor (TKI) resistance is the most relevant event during the treatment of chronic myeloid leukemia (CML), which correlates with high risk of treatment failure, disease progression and death, explaining half of treatment failed CML patients. However, the remaining half with TKI resistance does not show any ABL1 tyrosine kinase domain (TKD) mutation indicating the presence of alternative pathogenic pathways behind TKI resistance. Thus we hypothesized that the novel mutation besides ABL1-TKD mutation occurs during the development of TKI resistance. Using whole exome sequencing, we screened 13 pairs of CML cases with TKI resistance, but without ABL1-TKD mutation. The present study attempts to: 1) explore novel mutation(s) developing TKI resistance to CML treatment and 2) validate the somatic variants in an independent cohort of CML patients (n=100).

METHODS:

Thirteen CML cases with TKI resistance but not having ABL1 TKD mutation were included prospectively. Reason for TKI resistance includes Loss of MCyR (n=7) with (n=2) or without additional cytogenetic abnormality (ACA; n=5), progression to blastic crisis (n=3), development of ACA (n=1), development of clonal evolution in Ph neg clone (n=1), primary cytogenetic resistance (n=1). TKI resistance were demonstrated to imatinib (n=12), dasatinib (n=5), nilotinib (n=4) or ponatinib (n=2). The latest treatment includes ponatinib (n=3), dasatinib (n=8) alone (n=4), with smoothen inhibitor (n=2), or with after systemic chemotherapy (n=2), omacetaxine (n=1), and nilotinib (n=1). Disease stage at the time of exome sequencing was chronic phase (n=10) or blastic crisis (n=3). Germline and tumor samples at the time of TKI resistance were compared using whole exome sequencing (Illumina TruSeq kit, HiSeq 2000). Targeted sequencing for selected variants was performed to validate the result. All patients were confirmed the absence of ABL1-TKD mutations using Sanger sequencing.

RESULTS:

1) Exome sequencing (Illumina Truseq kit) was performed as per the manufacturer's protocol using an Illumina HiSeq 2000 sequencer. DNA from buccal mucosa was used as a control for variant calling. Exome sequencing reads processing includes mapping to human genome hg19, marking PCR duplicates, realignment of indels, fixing mate information, and discard the reads with more than 2 mismatches to increase the true positive rate. In the end, we have on-target-coverage of 57x. Lastly, 72% of target positions are mapped more than 30x.

2) One hundred nineteen somatic variants were identified in 13 patients in 108 genes. Among them 5 genes have variants in multiple patients including DNMT3A (n=3), ASXL1 (n=2), NPIPB5 (n=2), ATXN3 (n=2) and EFEMP1 (n=2) . We also found at least 1 mutation in well-known driver genes in 6 patients (6/13 = 46%).

3) Three out of 4 patients with ACA carry variants at least one of DNMT3A (n=2), ASXL1 (n=2), and SETBP1 (n=1). Also, 2 out of 3 cases progressed to blastic crisis demonstrate variants in DNMT3A (n=1) and IDH1 (n=1).

4) Interestingly, in one patient, exome sequencing reveals ABL1-TKD mutation (T315I), which was not detected at the initial screening by Sanger sequencing.

5) The result of targeted sequencing in an independent cohort of CML patients (n=100) will be presented in the annual meeting of American Society of Hematology in Dec 2015.

CONCLUSION:

Our study suggest that DNMT3A and ASXL1 mutations seem to be the driver mutations involved in the development of TKI resistance/progression, independent of ABL1-TKD mutation. Also, exome sequencing can detect ABL1-TKD mutations including T315I prior to be detected by initial Sanger sequencing.