PhD Scientific Days 2023

Molecular Sciences - Posters L

Significance of CDK12 expression in the resistance to PARP inhibitors in BRCA1/2 mutant breast cancers

Text of the abstract

Introduction
Poly-ADP ribose polymerase (PARP) enzyme is involved in the single-strand DNA breaks and homologous recombination repair processes. PARP inhibitors (PARPi) increase the mutational rate that induces cell death due to the lack of DNA repair in BRCA1/2 mutant breast tumors. PARPi treatment resistance is challange. Increased cyclin-dependent kinase 12 (CDK12) expression has been described in cases of PARPi resistance. CDK12 promotes the transcription of several genes responsible for DNA repair that may play a role in PARPi resistance.
Aim
To determine the CDK12 expression in PARPi sensitive and resistant BRCA1/2 mutated breast cancer cells.
Methods
We conducted our experiments on two breast tumor cell lines carrying BRCA1/2 mutations (MDA-MB-436 and BT-474). We determined the copy number of the CDK12 gene using DNA sequencing and then confirmed the expression of the CDK12 protein with Western blot. We then examined the response of the cells to the PARPi rucaparib using a luminescent cell viability assay, the endpoint was the inhibitory effect 50 (IC50) value and the area under the curve (AUC). Furthermore, we determined the inhibitory response of BSJ-4-116 a CDK12 protein degrader agent to examine the effect of CDK12 depletion.
Results
DNA sequencing resulted in normal (n=2) CDK12 gene copy numbers for the MDA-MB-436 cell line, while an increased (n=27) copy number was found for BT-474. Concurrently, CDK12 protein overexpression was observed in the BT-474 line compared to MDA-MB-436 (MDA-MB-436: 1.00±0.13 a.u. vs. BT-474: 1.71±0.08* a.u.; t-test; *p<0.05). Furthermore, the BT-474 line showed decreased sensitivity to rucaparib PARPi compared to MDA-MB-436 (IC50MDA-MB-436: 52.04 μM vs. IC50BT-474: 210.00 μM; AUCMDA-MB-436: 269 a.u. vs. AUCBT-474: 328 a.u.). The BSJ-4-116 CDK12 degrader resulted MDA-MB-436 (IC50MDA-MB-436: 0.0014 μM and BT474 (IC50BT-474: 0.100 μM) while the AUC were the following AUCMDA-MB-436: 108 a.u. and AUCBT-474: 211 a.u.).
Conclusion
In our study, the decreased rucaparib PARPi efficacy in BT-474 cell line was associated with increased CDK12 gene copy number and protein expression. Based on these results, CDK12 overexpression may contribute to the reduced PARPi efficacy. The inhibitory effect of BSJ-4-116 CDK12 degrader was effective in the elimination of the BRCA mutated breast cancer that seemed independent of CDK12 expression.