Kelly McCann, MD, PhD
PARP inhibitors have made a substantial impact on the treatment of ovarian cancer in recent years. The DNA damage sensor PARP has wide-reaching roles in several pathways and processes of DNA repair, said Kelly McCann, MD, PhD. However, ovarian tumors continue to develop resistance to these therapies.
"The DNA damage response is a very complex signaling network. PARP1 and PARP2—the main enzymes that we are targeting with PARP inhibitors—are DNA damage sensors, so they are far upstream of all of the processes that need to occur in order to prepare DNA," explained McCann. "That not only includes the DNA repair enzymes, but also halts the cell cycle. If the cells can't repair their DNA, ideally, they undergo apoptosis."
In a presentation during the 2018 OncLive®
State of the Science Summit™ on Ovarian Cancer, McCann, a medical oncologist in the Breast Cancer Research Group at the University of California, Los Angeles, discussed the process of DNA damage and repair, resistance to PARP inhibitors, and rational combination strategies to overcome resistance.
Understanding DNA Damage
PARP1 and PARP2 both bind DNA at sites of damage, and they undergo a conformational change to activate their catalytic activity, McCann explained. The types of damage recognized are single-strand DNA breaks, DNA double-strand breaks, and DNA crosslinks, supercoils and stalled replication forks. When they bind to this kind of DNA damage, they use nicotinamide adenine dinucleotide (NAD+) as a substrate in order to add ADP ribose moieties onto the target proteins, added McCann. They add these in a linear fashion, as well as in chains in a process called PARylation.
PARylation has many roles to play in DNA repair proteins. All of the major pathways have PAR-binding motifs, so it can recruit DNA repair enzymes and hold them in place to repair DNA.
“PARylation can cause the chromatin to relax, which allows access to the DNA,” said McCann. “It can inhibit RNA polymerase I and II, and there is a role to play in the G2/M checkpoint, as well as in cell death. Not just apoptosis, but with parthanatos, which is a mechanism by which the NAD+ is used up and the cells die due to a cellular respiration disaster.”
In the presence of a PARP inhibitor, PARP1 cannot PARylate proteins to transduce a DNA damage signal, and is trapped on the DNA, which is referred to as “PARP trapping.”
Currently, PARP inhibitors are approved for the treatment of patients with deleterious mutations in BRCA1/2
. McCann said that it is important to understand the cell cycle as it relates to DNA repair.
"Most of our cells are in G0 or G1, and the predominant mechanism to repair DNA double-strand break is nonhomologous end-joining. At the site of a DNA double-strand break, the ends are processed, and the chromosome is ligated back together, so you have some loss of sequence," said McCann.
When the cells undergo replication, the DNA is replicated so there is the identical chromatin held together by a centromere. Then there are sister chromatids, which are identical sequences. However, McCann added that homologous recombination is more of a viable strategy for DNA repair.
"In S phase, DNA damage is recognized because the chromatin is open, so PARP inhibitors are playing a greater role in replicating cells, and that is when homologous recombination is most active," McCann explained.
Beyond germline BRCA1/2
, other methods can identify homologous recombination deficiency (HRD). Mechanisms include gene sequencing, defects in double-strand break repair, a patient’s clinical history, and response to platinum agents—which can predict response to PARP inhibitors, said McCann.
Trials Designed to Overcome PARP Resistance
There are multiple clinical trials assessing combinations with PARP inhibitors in order to expand their use in the treatment of ovarian cancer. These trials are addressing multiple questions, including the ability to cause and inhibit DNA damage repair, forced apoptosis through dual DNA repair blockade, inducing BRCA
-ness, and overcoming PARP inhibitor resistance.
The current strategies that clinical trials of combinations with PARP inhibitors are implementing are meant to cause DNA damage with chemotherapy, target HRD, interfere with multiple DNA repair processes, or abrogate possible PARP inhibitor resistance mechanisms, according to McCann.