Heterogeneous Responses Among Ph⁺-B-ALLs to Imatinib with NRAS Mutant Being One Mechanism of Complete Resistance

Background. Philadelphia Chromosome (Ph⁺) characterized by BCR-BAL fusion genes are one of the major leukemogenic drivers for a number of leukemia, including acute lymphoid leukemia (ALL) and chronic myeloid leukemia (CML). Experimental models are important to study disease mechanisms and assess pharmaceuticals, including drug resistances/mechanisms. Patient derived xenografts (PDXs) are stem cell driven diseases and considered to be the most closely resembling the original patients including leukemia [1], in terms of histo-/molecular pathology and pharmacology [2]. As far as we know, there has yet to be reported of PDXs of Ph⁺ leukemia, and thus such models could be particular useful for drug evaluation and study drug resistance.

Methods. Different B-lineage acute lymphoid leukemia (B-ALL) adult and pediatric patient bone marrow (BM) cells were engrafted into NOD/SCID mice or NSG-like mice, via IV injection (also intra-BM). The disease development is performed by monitoring peripheral blood for human CD45⁺ cells and cage-side clinical observations. At the terminal leukemia stage, the mice were gross-/histo-pathologically analyzed and immune-phenotyped for different affected tissues. The diseases are passed in mice by serially transplantations, and are by transcriptome sequenced-/pharmacologic-tested, enabling biomarker discovery.

Results. We achieved aggressive and serially transferrable full-blown ALL in mice for a number of patient samples that cause 100% mortality. The diseases of the established B-ALL-PDXs (patient-derived xenografts) start at BM followed by the quick expansion to peripherals, e.g. spleen and blood. The leukemic cells in mice have similar morphology, phenotypes and genotypes, including five ALLs with different forms of BCR-ABL fusion as seen in the original patients. The treatments of these five Ph⁺-ALLs with imatinib, a TKI targeting BCR-ABL fusion, caused heterogeneous remissions in peripheral blood, spleen and bone among the five PDXs, along with the correspondingly relief of the symptoms and extension of life. This is largely as anticipated and confirms that BCR-ABL fusion are likely key leukemogenic drivers to the majority of these PDXs. One of the five Ph⁺-ALLs, AL7267, displayed complete resistance to imatinib, which could potentially be explained by the identification of NRAS-G12S mutation that is absent in other 4 BCR-ABL bearing PDXs and likely responsible for AL7267's non-response to imatinib.

Conclusions. Our observations in the mouse trial of the five different Ph⁺ B-ALL-PDXs would suggest that individual patients of the same diseases could be greatly different in their response to TKIs targeting BCR-ABL, and NRAS mutation could be one of the important resistant mechanisms. These models could be useful tools to investigate drug resistance and the combination therapy to overcome the resistance.