Clinical Relevance of Low Burden BCR-ABL1 Mutations Detectable By Amplicon Deep Sequencing (DS) in Philadelphia-Positive (Ph+) Acute Lymphoblastic Leukemia (ALL) Patients (pts): The Type of Mutation Matters

Background and Aims- In Ph+ ALL pts treated with tyrosine kinase inhibitors (TKIs), the likelihood of acquiring TKI-insensitive mutations and the striking incidence of highly resistant T315I and compound mutants underscore the importance of BCR-ABL1 kinase domain (KD) sequence surveillance for timely and rational therapeutic reassessment.

We used an amplicon DS strategy of the BCR-ABL1 KD to assess the following issues:

i) whether DS allows earlier detection of emerging TKI-insensitive mutations in pts undergoing BCR-ABL1 KD mutation screening for minimal residual disease (MRD) persistence;

ii) whether TKI-insensitive low burden mutations can be identified in relapsed pts with negative conventional sequencing results;

iii) whether TKI-insensitive low burden mutations are necessary and sufficient to predict for treatment failure in all cases.

Methods- This study was conducted in a total of 56 Ph+ ALL pts who received TKI-based therapies at our or collaborating institutions and were referred to our laboratory for MRD follow-up monitoring by RQ-PCR and for BCR-ABL1 KD mutation analysis in case of MRD positivity. These pts were divided into three different cohorts:

i) 10 de novo and 24 advanced Ph+ ALL pts who relapsed and developed BCR-ABL1 KD mutations on TKI-based therapy administered 1st-line or for recurrent disease, respectively. To reconstruct the dynamics of mutation emergence, longitudinal re-analysis of monthly-collected samples from the time of hematologic relapse backwards was performed by DS. Whenever samples were available, the analysis was done back to the time of diagnosis (n=10/10) or back to the time of first or former relapse (n=15/24), respectively. Two to 6 samples were analyzed for each pt, for a total of 109 samples.

ii) 14 Ph+ ALL pts who were known to be negative for mutations at the time of hematologic relapse as assessed by conventional sequencing. Relapse samples were reanalyzed by DS.

iii) 8 Ph+ ALL pts with long-term relapse-free survival despite persistent or intermittent MRD positivity at multiple timepoints. Up to 5 samples were analyzed for each pt, for a total of 28 samples.

DS was performed on a Roche GS Junior. Lower mutation detection limit of DS was 1%.

Results- In the 34 de novo or advanced Ph+ ALL pts who were known to have acquired TKI-insensitive mutations at the time of relapse on tyrosine kinase inhibitor (TKI) therapy, longitudinal retrospective reanalysis by DS allowed mutation backtracking in 13 (41%) cases. One patient was found to harbour a low burden Y253H at diagnosis. In 3 imatinib (IM)-resistant pts who switched to dasatinib (DAS), a low burden T315I mutation was already detectable at baseline.

In the 14 pts with no mutations detectable by conventional sequencing at the time of relapse on IM or DAS, low burden TKI-insensitive mutations were detected by DS in 6 (43%) cases. In 2 cases who had relapsed on DAS, a T315I and an F317L mutation, respectively, were present just below the lower detection limit of conventional sequencing (15.9% and 12.4%, respectively); in the remaining 4 pts, DS identified multiple (2 to 3) low burden mutations, all of which known to confer a moderate to high degree of insensitivity to the ongoing TKI.

In the 8 pts with persistently or transiently detectable BCR-ABL1 transcripts at multiple timepoints despite stable hematologic remission, DS detected low burden mutations in 9 samples from 4 pts. However, no mutation known to be truly insensitive to the ongoing TKI could be recognized.

Conclusions

• MRD persistence in Ph+ ALL pts may hide emerging TKI-insensitive BCR-ABL1 KD mutations that DS may identify earlier than conventional sequencing - allowing a greater leeway before overt hematologic relapse occurs;

• polyclonal resistance sustained by multiple TKI-insensitive low burden mutations may explain relapse in a proportion of cases with unmutated BCR-ABL1 KD sequences as assessed by conventional sequencing;

• the type of mutation matters: detection of low burden mutations insensitive to the ongoing TKI was always found to predict/correlate with treatment failure. Detection of low burden mutations with low/unknown IC50 might explain low level MRD but does not predict for an impending relapse;

• MRD-triggered, BCR-ABL1 KD mutation screening by DS may be precious for earlier and more effective use of preemptive rescue therapies.

Supported by ELN, AIL, AIRC, FP7 NGS-PTL project, Progetto Regione-Università 2010-12 (L. Bolondi)