Imatinib (IM) is the first therapeutic option for patients with newly diagnosed CML chronic phase (CML-CP). As alternative therapeutic options are available, identification of predictive factors of IM resistance may be of interest.

To identify specific early predictive biomarkers of IM resistance, we performed pangenomic microarray analysis in four patients with IM secondary resistance. All the four patients were in complete hematologic response at the time of analysis and RNA were extracted from peripheral mononuclear cells. Comparison of gene profile expression -was done between a sample corresponding to -BCR-ABL transcript decrease preceding the best response to IM and a sample corresponding at the time of IM resistance. BCR-ABL transcript levels were equivalent between the two points. Each patient was analyzed independently in order to limit genetic variability. We identified 22 genes differentially expressed at the time of resistance (4 up regulated, 18 down regulated). Among them, the defensin alpha 1 (DEFA1) and alpha 4 (DEFA4) genes were strongly over expressed at the time of IM resistance, 63-fold and 30-fold respectively.

Using Q-RT/PCR we next performed a prospective analysis of DEFA1 expression in 15 patients treated with standard dose (400mg/day) of IM. Nine of them were IM responders while 6 exhibited IM secondary resistance. IM responders were characterized by a strong and durable decrease in DEFA1 transcript level with a DEFA1-β actin ratio above 103. Patients resistant to IM were characterized by a lower DEFA1 expression before IM start and a dramatic increase of DEFA1 expression. Moreover the increasing of DEFA1 expression in IM secondary resistant patients occured while BCR-ABL transcript level continue to decrease. The median time between the increase of DEFA1 and the increase of BCR-ABL trancript level was 6 months (range from 0 to 23 months).

Finally we investigated DEFA1 expression in 39 CML-CP (19 early chronic phase, 20 late chronic phase)-. before IM started-. Twenty eight patients were IM responders and 11 were IM resistants (primary resistance in 2, secondary resistance in 9). DEFA1 expression before IM start was significantly lower in the IM resistant group (p=0.03) than IM responder group.

In conclusion our data suggest that variation of DEFA1 expression is early associated with IM resistance while low expression of DEFA1 at the beginning of IM treatment may be predictive of IM resistance independently of BCR-ABL level.

Disclosures: No relevant conflicts of interest to declare.

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