Doxorubicin sensitivity and acquired resistance of P-gp- and BRCA1-deficient mouse mammary tumors

P-glycoprotein (P-gp) is the most studied ATP-binding cassette transporter, known to be able to cause multidrug resistance. The relevance of P-gp for clinical drug resistance remains controversial, however. Using our K14cre;Brca1F/F;p53F/F conditional mouse model of hereditary breast cancer, we previously found that modestly elevated expression levels of the mouse P-gp genes Mdr1a (Abcb1a) and/ or Mdr1b (Abcb1b) are sufficient to cause resistance to the topoisomerase II (TOP2) inhibitor doxorubicin. To study P-gp-independent resistance mechanisms, we bred our model onto a P-gp-deficient background. When transplanted into P-gp-proficient mice, these tumors were hypersensitive to the maximum tolerable doxorubicin dose and usually did not acquire drug resistance. Only when we lowered the dose to 50%, resistance eventually occurred. Tumors that were resistant to the 50% dose showed a stable resistance phenotype upon transplantation into new recipient animals and subsequently also acquired resistance to the full dose. As a known mechanism of resistance we identified low TOP2� transcript and protein levels in about half of the 50% dose-resistant tumors. Some tumors without TOP2� downregulation were nevertheless cross-resistant to the TOP2 inhibitor etoposide, but not to the TOP1 inhibitor topotecan. RNA sequencing analysis of these tumors did not identify Top2 mutations, but instead revealed lower Top2β RNA levels as a possible alternative resistance mechanism for some of the tumors. Our results suggest that low expression and not mutation of the drug target TOP2 explains many cases of P-gpindependent doxorubicin resistance in our mouse model. Doxorubicin sensitivity and acquired resistance of P-gpand BRCA1-deficient mouse mammary tumors