Oral Presentation 35th Lorne Cancer Conference 2023

Immune-deficient MISTRG mice support expansion of relapse-inducing leukaemia-initiating cells in paediatric acute myeloid leukaemia (#44)

Patrick Connerty 1 2 3 , Angela Xie 1 2 3 , Yashar Mesbahi 1 2 3 , Toby N Trahair 1 3 4 , Nisitha Jayatilleke 1 2 3 , Chelsea Mayoh 1 2 3 , Vanessa Tyrrell 1 2 3 , Richard B Lock 1 2 3
  1. Childrens Cancer Institute Australia, Randwick, NSW, Australia
  2. Centre for Childhood Cancer Research, Faculty of Medicine, University of New Soth Wales, Sydney, NSW, Australia
  3. School of Clinical Medicine, UNSW Medicine & Health, UNSW Sydney, NSW, Australia
  4. Kids Cancer Centre, Sydney Children’s Hospital, Randwick, NSW, Aus

Background: Two humanised immune-deficient mouse strains, NRGS and MISTRG, are reportedly receptive to engraftment of human myeloid disorders. However, no studies to date have compared the two strains for their ability to recapitulate paediatric acute myeloid leukaemia (AML) in vivo.

Hypothesis: NRGS and MISTRG mice exhibit differences in their ability to propagate leukaemia initiating cells (LICs) in paediatric AML patient-derived xenografts (PDXs).

Aims: To investigate the differences in engraftment, clonal selection and serial transplantation of AML PDXs in NRGS and MISTRG mice.

Methods: Primary patient samples were transplanted into NRGS or MISTRG mice and leukaemia engraftment was assessed by flow cytometry. PDXs were subjected to DNA-seq and variant allele frequency (VAF) was assessed between patient samples and PDXs. Four samples were selected for limiting dilution assays to assess serial transplantation ability and LIC frequency in both strains. Both MISTRG and NRGS mice were treated with AML standard of care (cytarabine and daunorubicin) and the time to disease re-initiation was monitored.

Results: We show that 10/11 and 9/11 of primary AML samples engrafted in NRGS and MISTRG, respectively, indicating that both strains support the engraftment of primary paediatric AML. DNA-seq analysis demonstrated that >90% of VAFs were preserved (<1.2-fold change) from the patient to the PDXs in both strains, suggesting no difference in clonal selection between strains. However, serial transplantation assays demonstrated that MISTRG mice were superior for subsequent transplantation with 3/4 samples successfully engrafting at second passage compared to 1/4 in NRGS. Furthermore, limiting dilution assays revealed that MISTRG-derived PDXs exhibit 10-fold higher LIC frequency compared to NRGS-derived PDXs (LIC frequency: 1 in 1.1x105 vs 1 in 10.1x105). This finding was further supported by a translatable relapse model which showed that MISTRG mice relapse twice as quickly as NRGS mice (16.5 vs 31.0 days) following standard of care treatment.

Conclusion: These results have significant implications for the use of MISTRG or NRGS mice for future preclinical testing of LIC-targeted treatments.