In newly diagnosed chronic phase Chronic Myeloid Leukemia and Acute Lymphoblastic Leukemia patients, imatinib-resistant BCR-ABL mutations are already detectable at low levels but do not correlate with subsequent clinical outcome – A study by the GIMEMA ALL and GIMEMA CML Working Parties.

Bcr-Abl kinase domain (KD) mutations may cause resistance to tyrosine kinase inhibitor (TKI) therapy in Ph+ acute lymphoblastic leukemia (ALL) and chronic myeloid leukemia (CML). Mutations in rare Ph+ cells have been detected in some imatinib-naïve advanced-phase CML patients (pts), but it is unclear whether low level mutations may already be present at diagnosis in chronic-phase (CP) CML as well as in Ph+ ALL, and whether their detection predicts for subsequent treatment failure. We analyzed cDNA samples from 24 newly diagnosed pts with Ph+ ALL (n=13) or CP-CML (n=11) who subsequently received TKI therapy. Screening for low level mutations was performed by cloning the Bcr-Abl KD (a.a. 206-524) in a bacterial vector and sequencing 200 clones for each pt. All pts had evidence of aberrant KD sequences. Three to twelve different mutations were detected in each pt. A total of 115 mutations (41 silent, 5 nonsense, 69 missense mutations) were observed. The majority (107/115, 93%) have never been reported in association with TKI resistance and are likely not to confer any advantage under TKI selective pressure. Interestingly, 103/115 (90%) mutations were transitions: G>A (n=30), A>G (n=25), C>T (n=25), T>C (n=23). One of the eleven CP-CML pt received hydroxyurea for 6 months before starting imatinib therapy. In this pt, high-sensitivity mutation screening was performed again immediately before imatinib start and showed further accumulation of mutations. Eight Ph+ ALL pts and three CML pts subsequently relapsed with mutations, but only two with a mutation (T315I) already detectable at diagnosis. The remaining thirteen pts are in persistent remission (follow-up, 12-52 months), although four of them were harbouring known imatinib-(H396P, D276G, E355G) or dasatinib-(F317L) resistant mutations at low levels. We can conclude that: a) mutations can probably be found at diagnosis in all CP-CML and Ph+ ALL pts; b) mutations seem to arise randomly and most of them are silent/not conferring any growth advantage; c) generation of mutations seems to be linked to Bcr-Abl-driven genetic instability; d) TKI-resistant mutations present at low levels at diagnosis do not always outgrow and lead to relapse, probably because some of them arise in cell clones with limited self-renewal capacity. This warns against high-sensitivity mutation screening of all pts before the start of therapy.