Inhibition of Poly(ADP)-Ribose Polymerase as a Therapeutic Strategy for Breast Cancer

As knowledge increases about the processes underlying cancer, it is becoming feasible to design “targeted therapies” directed toward specific pathways that are critical to the genesis or maintenance of the malignant phenotype. Poly(ADP-ribose) polymerase (PARP) inhibitors are an example of this new framework. DNA damage repair is a complex and multifaceted process that is critical to cell survival. Members of the PARP family are central to specific DNA damage repair pathways, particularly the base excision repair (BER) pathway. PARP inhibition, with subsequent impairment of the BER mechanism, may enhance the cytotoxicity of agents that generate single-strand breaks in DNA, such as radiation and certain chemotherapy drugs. In addition, PARP inhibitors may induce death through “synthetic lethality” if the DNA repair mechanisms that rescue BER-deficient cells are themselves impaired. This mechanism is thought to underlie the impressive results of PARP inhibition in BRCA-associated breast and ovarian cancer, and may also account for the reported benefit of this approach in “triple-negative” breast cancer. This review will examine the current understanding of PARP inhibition as a treatment for breast cancer, ongoing clinical trials, and future directions for this new approach.

Poly-(ADP-ribose) polymerases (PARPs) are a family of enzymes that catalyze the transfer of ADP-ribose from nicotinamide adenine dinucleotide (NAD+) to acceptor proteins.[1,2] This transfer results in the creation of long, negatively charged, branched polymers on the acceptor proteins. The first PARP (PARP1) was described in 1963.[3] The family has since been expanded to include at least 17 members.[2] PARP1 and PARP2 are the most abundant members of the family, and appear to be the only ones localized to the nucleus. PARP1, the major nuclear PARP, is a 116-kDA protein with two N-terminal zinc-finger DNA-binding domains, a nuclear localization signal domain, and a BRCT-repeat automodification domain.[4] At the C-terminus, a catalytic domain containing the “PARP signature” identifies it as a member of the PARP superfamily.

Role of PARPs

PARPs play an important role in a number of cellular damage response pathways.[5] PARPs are involved in inflammation and, through NAD+-depletion, in triggering cell death in response to stresses such as ischemia. However, the function of PARPs that is most relevant to oncology is their role in DNA damage repair. PARP1 and PARP2 are the family members that participate in this process. Most is known about the action of PARP1. PARP2 appears to participate in the same processes, but it likely also has its own unique functions.[6]

Maintenance of genomic integrity is a critical cellular function. A number of exogenous and endogenous insults can result in DNA damage. To address these various forms of potentially lethal injury, cells have evolved a number of different repair pathways.[7] PARP1 is a major component of the base excision repair (BER) pathway, which is directed toward the repair of certain types of damage to the component bases of DNA and single-strand breaks in the DNA structure.[8,9] PARP1 senses and binds to “nicked” DNA through its N-terminal zinc-finger DNA-binding domains, after which it forms a homodimer. After binding, the catalytic activity is engaged and PARP1 automodifies by adding ADP-ribose moieties to the BRCT repeat domain. The extended negatively charged chain appears both to protect the break from further degradation (“anti-recombinogenic”) and to serve as a “beacon” for recruitment of BER effector proteins, such as XRCC1.

PARP1 also appears to modify local histone proteins (especially H1 and H2B) by poly(ADP)-ribosylation. The consequent negative charge may contribute to modification of local chromatin structure, resulting in improved access for repair proteins.[10] Chromatin modification may also be mediated through the action of other proteins, such as ALC1.[11]

PARP1 is not essential for survival. PARP1 knockout mice are viable and fertile.[12] However, these animals are sensitive to DNA-damaging agents such as N-methyl-N-nitrosourea (MNU) and ionizing radiation. Residual PARP2 activity may be compensating for the loss of PARP1, as PARP1/PARP2 double knockout mice are not viable.[13] Other DNA damage repair pathways are also likely to be involved in the rescue of cells with impaired base excision repair.

Single-strand breaks may result in unattached double-strand ends if they are unrepaired when encountered by a replication fork. To avoid this result, the cell engages repair mechanisms that are involved in the repair of double-strand DNA breaks, such as homologous recombination repair and nonhomologous end-joining.[14] Homologous recombination repair is a high-fidelity mechanism that is activated during later S phase, when a sister chromatid is available to serve as a template for resolving the double-strand break. Nonhomologous end-joining is less accurate, and therefore more likely to result in genomic instability, but it is available throughout the cell cycle. Bone marrow cells from PARP1 knockout mice demonstrate an increased prevalence of sister chromatid exchanges under both baseline and stress conditions, indicating that homologous recombination repair mechanisms are engaged in response to endogenous and exogenous DNA damage when PARP1 is absent.[12] The availability of this repair mechanism is thought to be responsible for the continued viability of cells in which PARP1 function is lost.

Synthetic Lethality

FIGURE 1

Schematic Illustration of Synthetic Lethality

As above, double-strand break repair pathways, especially homologous recombination repair, appear to be critical to rescuing cells that are lacking PARP. Therefore, one would predict that cells with defects in homologous recombination repair would be sensitive to loss of single-strand repair capability through inhibition of PARP activity.[15] Conversely, cells that are competent in repairing double-strand breaks would, like PARP1 knockout mice, experience little effect from loss of PARP function, at least under normal conditions.

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