Global Gene Expression and Mutation Signatures Are Preserved in PDX Models of Pediatric AML and Aid Discovery of Targeted Therapy for Cases with CBFA2T3/GLIS2 Rearrangement

Therapies for pediatric acute myeloid leukemia (AML) remain unsatisfactory and generally do not incorporate molecularly-targeted agents aside from FLT3 inhibitors outside of the relapse setting. Patient-derived xenograft (PDX) models of AML are increasingly accessible for the preclinical evaluation of targeted therapies, though the degree to which these systems recapitulate the disease state as found in patients has not been well defined for AML. Gene expression profiling of patient blasts has been successfully used to discriminate distinct subtypes of AML, to uncover sub-type specific vulnerabilities, and to predict response to therapy and outcomes. We sought to systematically examine PDX models of pediatric AML for their ability to replicate global gene expression patterns and preserve mutational signatures found in patients. In addition, we conducted in-depth bioinformatic analyses of samples with cryptic CBA2T3-GLIS2 fusion generated by the inv(16)(p13.3q24.3) for identification of potential novel targeted therapies.

We performed detailed analyses of RNA sequencing data from a diverse series of 24 pediatric AML PDX models established from samples obtained from patients with relapse and refractory disease. Initially we compared our PDX data against 49 selected relapse and refractory patient sample data files found in the NCI TARGET dataset of pediatric AML. When applying unsupervised hierarchical clustering to the PDX samples, we found that clustering was associated with MLL status. Clustering of the combined sets of samples by MLL status showed integration of samples according to mutation profile, regardless of data source (PDX or patient). The expression levels of all detectable transcripts were highly conserved between PDX and patient MLL-r samples. Separate analysis of each dataset yielded MLL specific gene lists that included a subset of overlapping genes which may point to a unique relapse and refractory pediatric MLL-r signature. This list contains several interesting new targets for further study.

A subset of 12 PDX models were compared directly to the matched patient sample from which they were established. This analysis revealed strong similarity, with each PDX most closely related to its matched patient sample, suggesting retention of sample-specific gene expression in immune deficient mice.

We set up our PDX models in NSG mice with transgenic expression of human myelo-supportive cytokines SCF, GM-CSF, and IL-3 in order to promote the most efficient and robust engraftment of precious patient material. In order to detect any skewing effects due to the host mouse strain, we compared NSGS PDX RNA sequencing data to 10 matched NSG PDX models. This comparison revealed consistent differences in only 9 transcripts, which were almost entirely related to increased JAK/STAT signaling and macrophage activation pathways in NSGS mice relative to NSG mice.

Interestingly, during this analysis we observed a distinct PCA-driven clustering of a pair of PDX samples with previously clinically unidentified driver mutations. Reanalysis of the RNA sequencing data revealed evidence of a cryptic GLIS2 rearrangement (found in ~1% of pediatric AML cases) as the driver mutation, which was subsequently confirmed by RT-PCR in both samples. The unique CBFA2T3/GLIS2 RNA signature was mined to guide the composition of a focused 75-molecule in vitro drug screen against ex vivo PDX samples with an emphasis on the SHH, WNT, and BCL2 pathways. This screen identified the Wnt-C59 PORCN inhibitor as having specific activity against CBFA2T3/GLIS2+ AMLs. Further testing of C-59 in combinatorial studies revealed enhanced effects with the addition of the BCL2 inhibitor, venetoclax. In vivo experiments are currently underway to determine the pre-clinical efficacy of this novel combination.

In summary, we found highly significant fidelity of gene expression in PDX models of relapse and refractory pediatric AML. Analysis of this dataset has led to several insights, including potential targeted therapies, highlighting how this system could be a valuable tool for discovery of novel targeted therapies, especially for very rare, distinct subtypes of disease.