PARP-1 inhibitors: are they the long-sought genetically specific drugs for BRCA1/2-associated breast cancers?

Abstract

Recent studies demonstrated that PARP-1 [poly(ADP-ribose) polymerase-1] inhibitors kill breast cancer associated gene-1 and -2 (BRCA1/2) deficient cells with extremely high efficiency while BRCA+/- and BRCA+/+ cells are relatively non-responsive to the treatment. It was therefore proposed that PARP-1 inhibitors might be the long-sought genetically specific drugs that are both safe and effective for treating BRCA1/2-associated breast cancers. However, a report published in a recent issue of the International Journal of Biological Sciences revealed that PARP-1 inhibitors, although able to kill naïve BRCA1 mutant cells with high specificity both in vitro and in vivo, exhibit minimal specificity in inhibiting the growth of mouse mammary tumor cells irrespective of their BRCA1 status in allograft nude mice. Non-specific inhibition in human BRCA1+/+, BRCA1+/-, and BRCA1-/- breast cancer cells by PARP-1 inhibitors was also observed. Additional mutations occurring during cancer progression may be a culprit, although the exact cause for the resistance of BRCA1-/- breast cancer cells to PARP-1 inhibitors remains elusive. These findings suggest that PARP inhibition may serve as an approach for the prevention of BRCA related breast cancer and may be useful in combination with other chemotherapeutic agents in the treatment of breast cancer.

Figure 1

A model showing DSB formation and DNA rearrangement in the absence of BRCA and PARP1. (A) PARP-1 binds and repairs single strand breaks (SSBs) in DNA. These SSBs result in formation of double strand breaks (DSBs) when they meet replication forks. (B) In the presence of wild type BRCA1 and BRCA2, DSBs can be efficiently repaired by RAD51 mediated homologous recombination (HR). (C) BRCA1 or BRCA2 mutant cells cannot repair DNA DSBs properly upon the inhibition of PARP, leading to DNA replication arrest and illegitimated DNA ends joining.

Figure 2

A model showing how Ku70 or ligase IV deficiency represses lethality caused by DSB inducing agents in PARP-1-/- DT40 cells. (A) Treatment of DSB inducing agents in PARP-1-/- DT40 cells decreases ability of HR (HR*) and causes DSBs. (B) Ku-70 protein competes with HR machinery and binds to DSBs, directing the repair toward the non-homologous end joining (NHEJ), where ligase IV (LIG4) also plays an important role. This results in lethality of PARP-1-/- cells. (C) The absence of Ku70 or Ligase IV prevents illegitimated DNA end ligation of DSBs through NHEJ in PARP-1-/- cells. This allows more time for PARP-1 deficient cells to repair DSBs, therefore, preventing their lethality.

Figure 3

Effect of a potent PARP-1 inhibitor, AG14361, on BRCA1 mutant and control cancer cells. (A) Clonogenic survival assay. Five hundred 780 (BRCA1Δ11/Δ11), and 525 (BRCA1-/-), and NEU (BRCA1+/+) cells were exposed to AG14361 for 10 consecutive days, 18 hours after plating. (B,C) Response of in vivo allograft of BRCA1-/- (B), and BRCA1+/+ (C) mammary tumor cells to AG14361. The recipient nude mice were treated intra-peritoneally with AG14361 (30 mg/kg) 24 hours after transplantation of 1 X 10⁶ cells for three consecutive days starting on days 2, 9 and 16 (arrows). Our data indicated that the treatment failed to achieve a more than marginal specific killing of BRCA1 mutant cancer cells as both BRCA1-/- and BRCA1+/+ cells showed about a 30-40% reduction in growth. (D) Clonogenic assay. MCF-7, MDA-MB-231 and HCC1937 cells were treated and exposed to AG14361 continuously for 10 days.

Figure 4

A model depicting BRCA1 associated tumorigenesis in regarding to the development of PARP resistance. In familial cancer, one mutant BRCA1 allele is inherited from the germline. One additional mutation in the BRCA1 locus (LOH) results in a BRCA1 deficiency, which could result in genetic instability, activation of p53-dependent cell cycle checkpoints and apoptosis pathways, leading to growth arrest and cell death. However, genetic instability triggered by BRCA1 loss could facilitate mutations in general. When permissive mutations, such as ATM, CHK2 or p53 heterozygous or homozygous mutations, occur, BRCA1-/- cells survive. Cells in this phase may be sensitive to PARP inhibition. However, PARP resistance may develop when further genetic alterations occur leading to full growth of breast cancers.