Olaparib significantly delays photoreceptor loss in a model for hereditary retinal degeneration

The enzyme poly-ADP-ribose-polymerase (PARP) mediates DNA-repair and rearrangements of the nuclear chromatin. Generally, PARP activity is thought to promote cell survival and in recent years a number of PARP inhibitors have been clinically developed for cancer treatment. Paradoxically, PARP activity is also connected to many diseases including the untreatable blinding disease Retinitis Pigmentosa (RP), where PARP activity appears to drive the pathogenesis of photoreceptor loss. We tested the efficacy of three different PARP inhibitors to prevent photoreceptor loss in the rd1 mouse model for RP. In retinal explant cultures in vitro, olaparib had strong and long-lasting photoreceptor neuroprotective capacities. We demonstrated target engagement by showing that olaparib reduced photoreceptor accumulation of poly-ADP-ribosylated proteins. Remarkably, olaparib also reduced accumulation of cyclic-guanosine-monophosphate (cGMP), a characteristic marker for photoreceptor degeneration. Moreover, intravitreal injection of olaparib in rd1 animals diminished PARP activity and increased photoreceptor survival, confirming in vivo neuroprotection. This study affirms the role of PARP in inherited retinal degeneration and for the first time shows that a clinically approved PARP inhibitor can prevent photoreceptor degeneration in an RP model. The wealth of human clinical data available for olaparib highlights its strong potential for a rapid clinical translation into a novel RP treatment.Peer reviewe

Progression of cell death and PARP activity in the early post-natal rd2 retina.

<p>The TUNEL assay for dying cells showed significantly increased numbers of positive cells between P14 and P24, and peaked at P18 in <i>rd2</i> retinae (A,B,G). The quantification of PARP activity positive cells over time identified significantly higher numbers of positive cells at P16 and P18 in <i>rd2</i> ONL (C,D,G). Similar to PARP activity, immunohistochemical analysis of PARylated proteins revealed significantly increased numbers of PAR positive cells at P16 and P18 in <i>rd2</i> retina (E,F,G). Remarkably, while PARP activity and PARylation of proteins peaked at P16, the peak of cell death occurred only at P18. Western blot analysis confirmed increased levels of PARylated proteins in <i>rd2</i> retina at P18. The images shown in A-F are representative for observations on at least three different specimens for each genotype. Data shown in G is based on marker quantifcations in three different wt and <i>rd2</i> animals per time-point; note that the wt datasets were in part published previously in [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0181374#pone.0181374.ref009" target="_blank">9</a>,<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0181374#pone.0181374.ref020" target="_blank">20</a>].</p

Drug repurposing studies of PARP inhibitors as a new therapy for inherited retinal degeneration

The enzyme poly-ADP-ribose-polymerase (PARP) has important roles for many forms of DNA repair and it also participates in transcription, chromatin remodeling and cell death signaling. Currently, some PARP inhibitors are approved for cancer therapy, by means of canceling DNA repair processes and cell division. Drug repurposing is a new and attractive aspect of therapy development that could offer low-cost and accelerated establishment of new treatment options. Excessive PARP activity is also involved in neurodegenerative diseases including the currently untreatable and blinding retinitis pigmentosa group of inherited retinal photoreceptor degenerations. Hence, repurposing of known PARP inhibitors for patients with non-oncological diseases might provide a facilitated route for a novel retinitis pigmentosa therapy. Here, we demonstrate and compare the efficacy of two different PARP inhibitors, BMN-673 and 3-aminobenzamide, by using a well-established retinitis pigmentosa model, the rd1 mouse. Moreover, the mechanistic aspects of the PARP inhibitor-induced protection were also investigated in the present study. Our results showed that rd1 rod photoreceptor cell death was decreased by about 25-40% together with the application of these two PARP inhibitors. The wealth of human clinical data available for BMN-673 highlights a strong potential for a rapid clinical translation into novel retinitis pigmentosa treatments. Remarkably, we have found that the efficacy of 3 aminobenzamide was able to decrease PARylation at the nanomolar level. Our data also provide a link between PARP activity with the Wnt/beta-catenin pathway and the major intracellular antioxidant concentrations behind the PARP-dependent retinal degeneration. In addition, molecular modeling studies were integrated with experimental studies for better understanding of the role of PARP1 inhibitors in retinal degeneration

PJ34 treatment partially restores rhodopsin expression in rd2 photoreceptors.

<p>Immunostaining for rhodopsin showed a decreased expression level and dramatically shortened outer segments in P18 <i>rd2</i> retina, when compared with age-matched <i>wt</i> (A, B). Rhodopsin expression and OS length appeared to increase in <i>rd2</i> cultures that were treated with 1.5 μM PJ34 (C, D). The images shown are representative for observations on at least three different specimens for each genotype/treatment condition.</p