High Activity of Poly(ADP-Ribose) Polymerase in Chronic Lymphocytic Leukemia

Abstract 1788

Current treatment protocols for Chronic lymphocytic leukemia (CLL), including FCR (fludarabine, cyclophosphamide and rituximab) have improved response rates and newer approaches targeting the B-cell receptor show promise in early clinical trials. Despite these advances, CLL remains incurable, and since CLL patients are predominantly elderly, many cannot tolerate more aggressive therapies. Therefore, development of biomarkers to stratify subgroups of patients for personalised medicine is paramount to achieve the best therapeutic outcomes for these patients.

Chemo-resistance in CLL frequently occurs due to loss of function of either p53 or ataxia telangiectasia mutated kinase (ATM) (del(17p) and del(11q) respectively). p53 and ATM mutation and/or deletion results in defective DNA double-strand break (DSB) response pathways, which confer greater genomic instability, poorer response and shorter overall survival (OS). ATM is a key DSB signaling kinase with roles in homologous recombination repair and cell cycle checkpoint activation, which facilitate DNA repair. We hypothesized that as well as defects in DSB-activated enzymes, genomic instability arising from unresolved single-strand DNA breaks may play a role in disease progression and resistance in CLL.

Poly (ADP-ribose) polymerase (PARP) is activated by DNA damaging agents that induce single strand breaks (e.g. alkylating agents) and is pivotal in the signaling to direct repair of such DNA damage. Therefore, inhibitors of PARP sensitise cells to certain classes of chemotherapeutic agents by inhibiting DNA repair. PARP is also the first line of defence against oxidative stress, which is associated with poor prognosis in CLL. We developed the first clinically-used PARP inhibitor (PARPi) and subsequent studies identified the synthetic lethality of PARPi monotherapy in homologous recombination repair defective (HRD) cancer, which selectively kills HRD tumour cells, but is non-toxic to the HR-competent normal tissues. Importantly, PARP activity is reported to be higher in HRD cells, which may reflect increased genomic instability and oxidative stress in these cells. Currently, there are several PARPi undergoing clinical evaluation, including two studies in CLL patients.

Here, we analysed the activity of PARP in CLL, to determine whether it could be used as a biomarker of response to conventional therapy and a means to stratify patients for novel therapies, including PARP inhibitor (PARPi) monotherapy. Using the clinically validated PARP activity assay, we found that PARP activity in PBMCs from healthy volunteers was in the same range as we had previously observed (Zaremba T, Biochem J 2011). However when we measured PARP activity in CLL cells (n=33 cases) it was up to 100-fold higher than the activity in PBMCs from healthy volunteers. Activity ranged from 100-50, 000 pmoL/10⁶ cells and was higher in those patients with Binet stage C, or progressive disease. When patients were stratified according to ATM functional status (determined by measuring ATM activity) PARP activity was significantly higher in cases with confirmed ATM dysfunction (p=0.02), compared to those with functional ATM. We then designated cases as having PARP activity that was either higher than, or lower than the median (12,400 pmoL/10⁶cells) and performed Kaplan Meier analysis. There was a trend towards high PARP activity being associated with shorter OS.

These results could have significant ramifications: recent reports support the concept that ATM-defective CLL can call be selectively targeted by PARPi (Weston VJ, Blood 2010) and therefore current phase I clinical trials are investigating the use of PARPi in HRD (including ATM-defective) CLL.These trials are stratifying patients by del(11q) and ATM status (since ATM plays a key role in mediating successful HR) with the aim of selectively killing ATM-defective CLL cells with the PARPi. However, ATM status alone may not be sufficient for patient stratification. We propose that CLL cells with high PARP activity are “addicted” to PARP by virtue of its role protecting against increased DNA damage, and that PARP activity may be a key determinant of patient sensitivity to PARP inhibitors. Additionally, the underlying mechanism for high PARP activity in CLL requires further investigation, since this may reveal new therapeutic options, and strategies for patient stratification prior to clinical trial.