Implications of TP53 alterations for Therapy Response in Multiple Myeloma

Deletion of the tumor suppressor gene TP53is significantly associated with an unfavorable clinical course of Multiple Myeloma (MM). In addition, point mutations that abrogate p53 function similarly shorten survival. Most recently 'double hit', bi-allelic TP53inactivated MM was identified as an ultimate high risk feature of MM, affecting 3.7% of newly diagnosed MM patients (NDMM, Walker et al., Leukemia 2018).

For TP53genetic analysis we combined data from two targeted sequencing M³P cohorts with targeted sequencing and FISH data, one of NDMM (n=142, Kortüm et al. Blood Cancer J. 2016), and one of multirefractory patients (rMM, n=40; Kortüm et al. Blood 2016). We also included two independent cohorts with paired NDMM and rMM, one with mutational data (n=43; Corre J et al 2017) and one with paired FISH analysis (Merz M et al. Haematologica 2017). We confirmed an increase of mutations in TP53 (8% NDMM / 16.9% rrMM), as well as del17p (12.6% / 22%). Similarly, mono- (12.7% / 22.5%) and bi-allelic events (5.6% / 17.5%) both demonstrated a significant increase over time (Figure, top). Importantly we observed deletion after mutation and vice versa in our cohorts.

Next we established an AMO-1 MM cell line model (TP53+/+) mimicking mono and bi-allelic TP53-inativation. After CRISPR/Cas9-mediated TP53destruction, we introduced a modified Sleeping Beauty (SB) vector with two separate expression cassettes (p53 wt / wt and mutant p53 (R282W or R175H). Functionality of the p53 system was confirmed using nutlin-3, as inhibitor of MDM2-p53 interaction, as described elsewhere.

Results: Doxorubicin and Melphalan are commonly used compounds in the treatment of MM. The IC50 of Melphalan in AMO1 naïve cells was 5µM and 5.5 µM after the reintroduction of 2 copies of wt TP53 in the KO model, with cell viability at 10µM of 30% and 31% respectively, measured by AlamarBlue Assay. Strikingly, we observed significant resistance induction in our hemizygous systems (del/wt p53; cell viability at 10µM 60% and mut/wt (62%). This furthermore increased within our homozygous models (TP53 del/del (85%); TP53 mut/del 80%) (Figure, bottom). Similar results were observed under doxorubicin treatment. Remarkably, this effect was absent against proteasome inhibition.

Conclusions

Here we present first evidence of TP53 inactivation impacting drug response to Melphalan and Doxorubicin, which might lead to the clonal selection of MM subclones harboring increased risk. The fact, that response to proteasome inhibition was not affected in our model might, at least in part, might explain their ability to confine high risk in MM.

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