6. PARP Inhibitors in Clinical Use Induce Genomic Instability in Normal Human Cells

Ito S., et al., 2016.

Poly(ADP-ribose) polymerase 1  (PARP1) and PARP2 are quickly activated in response to DSB to produce PAR. Mice lacking PARP1 or PARP2 are hypersensitive to IR and alkylating agents and express genomic instability with increased sister chromatid exchanges (SCEs). "Neither PARP1 or PARP2 alone can compensate completely for the loss of other." PARP inhibitors (PARPi) were known to increase SCEs 2 folds higher in normal human cells at high doses. PARPi monotherary has been use to treat cancer in patients lacking BRCA1 and BRCA2 function where DNA repair is impaired leading to tumor reduction. Inhibiting nuclear PAR in proliferating cells by PARPi can result in DNA damage accumulation even in repair-proficient cells and cause genotocixity. An increase in SCEs can potent mutants and induce loss of heterozygosity.

The experiments were done on non-transformed cells (MCF-10, HMEC-hTERT) under the treatment of olaparib, which is an effective inhibitor of PARP1 and PARP2. The result show a significant elevation of SCE frequency in normal proliferating cells in response to 1uM olaparib, leading to genomic instability from hyperrecombination.

It was hypothesize that highly induced SCEs only occurs when PARPi is present, which is contrary with the known fact that DNA damaging agents usually result in persistent effect in structural DNA alteration by increasing SCEs.  Therefore, after exposure to 1uM olaprarib for 24 hours, it was demonstrated that olaparib exposure greatly induce chromatid-type aberrations, decreasing genomic integrity.

SCEs is also dose-dependent to olaparib and veliparib with doses even below IC10 . The MCF10A was able to keep the survival up to 95% but elevated SCEs up to 4 folds. SCEs are elevated at low doses of olaparib and veliparib but also further induced without increasing cytotoxicity as residual PARP activity is inhibited.

BRCA1 and BRCA2 deficient cells are highly sensitive to olaparib and veliparib in mouse emryonic stem (mES) cells. DNA damage was induced with chemotherary while inhibiting SSB repair with PARPi. Cells were treated with cisplatin and olaparib to compared their combined effect to that of individual agent alone. They showed additive effect in increasing the SCEs to 1.8-1.9 folds in MCF10A (even with non-cytotoxic concentration of cisplatin), potentially leading to chromatin-type aberrations.

Two cytogenetic biomarkers, SCE and chromatid-type aberrations were analyzed to confirm that continuous exposure to PARPi, especially when combining with cisplatin, can cause genomic instability in proliferating cells. The increased chromosome aberration should be carefully considered when using "these agents in prevention and early stage cancer and non-oncologic indications."

"The hyper recombination with elevated SCEs is agreeable with unresolved breaks persisting and disturbing normal DNA replication. An SCE is  the cell's "next best" recourse when presented with DNA damage at a stalled replication fork. Yet, the increase in chromatid aberrations confirms that the increase in SCEs does not fully succeed in rescuing all the DNA breaks occurring with PARP inhibition."