In CML Patients with Good Response to TKIs Other Gene Mutations Are Frequently (37%) Present in Addition to Philadelphia Negative, Cytogenetically Aberrant Clones but Are Rare (4%) in Cases with MMR and Normal Karyotype

Background: In chronic myeloid leukemia (CML) clonal chromosome aberrations in metaphases not carrying a t(9;22)(q34;q11) have been described during treatment with tyrosine kinase inhibitors (TKI), so-called Philadelphia-negative (Ph-) clones. Very rarely transformation to MDS was observed in patients carrying such Ph- clones but mainly restricted to patients harboring -7. Overall, the clinical significance of this phenomenon remains obscure.

Aim: 1) Analyze in a large cohort of TKI-treated CML patients who developed Ph- clones the presence and occurrence of molecular mutations over time. 2) Evaluate whether molecular mutations are also present in CML patients who were at least in major molecular remission (MMR) and presented with a normal karyotype.

Patients and Methods: First Cohort: 51 CML patients (pts, 23 males, 28 females; median age: 60 yrs, range: 37-84 yrs) with response to TKI (imatinib only: n=32, nilotinib only: n=2, imatinib and dasatinib or nilotinib: n=11, all three TKIs: n=6) who developed Ph- clones. Cytogenetics in these pts were as follows: +8 sole (n=24), -Y (n=8), -7 sole (n=4), +9 (n=2), other trisomies (n=4), 9 had other aberrations including some with combinations of two different clones (n=4). In median these abnormalities were present in 30% (range 8-100%) of analyzed metaphases. BCR-ABL1 levels at the time point of analysis were between 0 and 3.8 (median: 0.023) according to international scale. Second Cohort: 50 CML pts (24 males, 26 females; median age: 56 yrs, range: 21-83 yrs), who were at least in MMR and without development of any cytogenetic aberration after 3 years of imatinib treatment. Median time from start of therapy to analysis was 2.6 years (range 3 months to 14 yrs).

All cases were analyzed with a pan-myeloid gene panel of 29 genes: ASXL1, BCOR, BRAF, CBL, DNMT3A, ETV6, EZH2, FLT3 (TKD), IDH1, IDH2, JAK2, KIT, KRAS, MLL-PTD, NOTCH1, NPM1, NRAS, PRPF40B, PTPN11, SF1, SF3A1, SF3B1, SRSF2, TET2, TP53, U2AF1, U2AF2 and ZRSR2. Either complete coding genes or hotspots were first amplified by a microdroplet-based assay (RainDance, Billerica, MA) and subsequently sequenced with a MiSeq instrument (Illumina, San Diego, CA). In addition, RUNX1 was sequenced on the 454 NGS platform (454 Life Sciences, Branford, CT).

Results: In the first cohort 28 mutations were found in 19 patients, as 5 patients had 2 and 2 patients even 3 mutations.Thus,in 19/51 pts (37.3%) ≥1 mutation was detected. Median mutation load was 11.5% (range: 2-56%). In detail, mutations in the following genes were detected: ASXL1 (n=9), DNMT3A (n=7), RUNX1 (n=3), NRAS (n=2), TET2 (n=2) and one each in CBL, EZH2, IHD1, PRPF40B, and TP53. Subsequently, these mutations were evaluated in samples from earlier or later time points (18 pts with a total of 235 samples, range: 3-20 samples/pt). In 12 cases a sample from diagnosis of CML was available. In 2 cases a CBL and an ASXL1 mutation were already detectable at low levels, 1.4% and 2%, respectively, at the time of diagnosis and later increased with decreasing BCR-ABL1 levels. In all other 10 cases the mutations were not detectable at diagnosis and were for the first time detectable during TKI treatment (in median after 24 months after diagnosis, range 2-73 months). In the remaining 6 cases date of occurrence could not be determined by backtracking as all earlier samples available were positive for the respective mutation. However, the over time mutation levels were inversely related to BCR-ABL1 expression indicating the presence in independent clones.

Within the second cohort with cases in MMR that remained cytogenetically normal only in 2 of the 50 pts (4%) mutations were detected. In one patient a DNMT3A mutation was detected that could be monitored for 8 years with constant low mutation load (3-6%). This was not detectable at diagnosis and occurred after 6 months on imatinib. Very similarly, in the second case a TET2 mutation was first detected after 6 months on imatinib with a mutation load of 2% that very slowly increased to 7% within 8 years.

Conclusions: 1) In CML patients that develop Ph- clones other mutations occur in 37.3%. 2) In contrast, in randomly selected CML pts with MMR that are cytogenetically normal, molecular mutations can be detected in only 4%. 3) The clinical importance of molecular mutations in CML in MMR remains unclear. 4) However, these results implicate that chromosomal aberrations are an indicator for genomic instability, also at the molecular level.