6-Thioguanine Selectively Kills BRCA2-Defective Tumors and Overcomes PARP Inhibitor Resistance

Abstract

Familial breast and ovarian cancers are often defective in homologous recombination (HR) due to mutations in the BRCA1 or BRCA2 genes. Cisplatin chemotherapy or poly(ADP-ribose) polymerase (PARP) inhibitors were tested for these tumors in clinical trials. In a screen for novel drugs that selectively kill BRCA2-defective cells, we identified 6-thioguanine (6TG), which induces DNA double-strand breaks (DSB) that are repaired by HR. Furthermore, we show that 6TG is as efficient as a PARP inhibitor in selectively killing BRCA2-defective tumors in a xenograft model. Spontaneous BRCA1-defective mammary tumors gain resistance to PARP inhibitors through increased P-glycoprotein expression. Here, we show that 6TG efficiently kills such BRCA1-defective PARP inhibitor-resistant tumors. We also show that 6TG could kill cells and tumors that have gained resistance to PARP inhibitors or cisplatin through genetic reversion of the BRCA2 gene. Although HR is reactivated in PARP inhibitor-resistant BRCA2-defective cells, it is not fully restored for the repair of 6TG-induced lesions. This is likely to be due to several recombinogenic lesions being formed after 6TG. We show that BRCA2 is also required for survival from mismatch repair-independent lesions formed by 6TG, which do not include DSBs. This suggests that HR is involved in the repair of 6TG-induced DSBs as well as mismatch repair-independent 6TG-induced DNA lesion. Altogether, our data show that 6TG efficiently kills BRCA2-defective tumors and suggest that 6TG may be effective in the treatment of advanced tumors that have developed resistance to PARP inhibitors or platinum-based chemotherapy. Cancer Res; 70(15); 6268-76. ©2010 AACR.

Oxidation-Mediated DNA Cross-Linking Contributes to the Toxicity of 6-Thioguanine in Human Cells

“…8). MMR-deficient cells do, however, retain a susceptibility to killing by high thiopurine concentrations, indicating the existence of MMR-independent thiopurine cytotoxicity (9). DNA 6-TG is also implicated in a cytotoxic pathway, which involves oxidative DNA damage.…”

“…Cell killing reflects the formation of potentially lethal DNA lesions that inhibit replication. Its effects are particularly marked in cells with defects in the Fanconi anemia (FA) and homologous recombination (HR) pathways and cells with defects in either pathway are extremely sensitive to 6-TG (9,14,16). Together the FA and HR pathway protect cells against DNA damage that arrests replication (reviewed in ref.…”

BRCA1, PARP, and 53BP1: conditional synthetic lethality and synthetic viability

“…Development of PARP inhibitors that are not substrates for MDR-mediating efflux pumps may decrease this potential mechanism of resistance. 6-Thioguanine has also been reported to be effective in killing PARP-resistant BRCA1 2/2 tumor cells that have p-glycoprotein mediated resistance to olaparib (Issaeva et al, 2010).…”

Poly(ADP-Ribose) Polymerase-1 Inhibition: Preclinical and Clinical Development of Synthetic Lethality

“…In preclinical models, 6-thioguanine selectively kills BRCA2-defective tumors resistant to the PARP inhibitor AG014699 by induction of double-strand breaks, which require homologous repair for survival (84). 6-Thioguanine is also active in BRCA1 cells resistant to PARP inhibitors by virtue of overexpression of p-glycoprotein (84).…”