Putative electron-affinic radiosensitizers and markers of hypoxic tissue: Synthesis and preliminary in vitro biological characterization of C3-amino-substituted benzotriazine dioxides (BTDOs)

INTRODUCTION

The redox characteristics of 1,2,4-benzotriazine-1,4-dioxides (BTDOs) make them potential radiosensitizing agents for hypoxic cells in solid human cancers. Tirapazamine (TPZ) is the most clinically tested BTDO radiosensitizer, despite its toxicity at effective doses. To date, no BTDOs have been developed as diagnostic markers of tissue hypoxia.

HYPOTHESIS

TPZ analogues with appropriate reporting groups can act as potential radiosensitizers and hypoxia selective diagnostics.

EXPERIMENTAL AND RESULTS

3-Chloro-1,2,4-benzotriazine 1-oxide was substituted at the C3 position to afford 3-(2-hydroxyethoxyethyl)-amino-1,2,4-benzotriazine-1-oxide, which was oxidized to 3-(2-hydroxyethoxyethyl)-amino-1,2,4-benzotriazine-1,4-dioxide (HO-EOE-TPZ) or converted to 3-(2-tosyloxyethoxyethyl)-amino-1,2,4-benzotriazine-1,4-dioxide (Tos-EOE-TPZ). Tos-EOE-TPZ was intended for use as a synthon for preparing 3-(2-azidoethoxyethyl)-amino-1,2,4-benzotriazine-1,4-dioxide (N₃-EOE-TPZ) and 3-(2-iodoethoxyethyl)-amino-1,2,4-benzotriazine-1,4-dioxide (I-EOE-TPZ). The logP values (-0.69 to 0.61) for these molecules bracketed that of TPZ (-0.34). Cell line dependent cytotoxicities (IC₅₀) in air were in the 10-100 μM range, with Hypoxia Cytotoxicity Ratios (HCR; IC₅₀-air/IC₅₀-hypoxia) of 5-10. LUMO calculations indicated that these molecules are in the optimal redox range for radiosensitization, offering cell-line-specific Relative Radiosensitization Ratios (RRSR; SER/OER) of 0.58-0.88, compared to TPZ (0.67-0.76).

CONCLUSION

The LUMO, IC₅₀, HCR and RRSR values of 3-(2-substituted ethoxyethyl)-amino-1,2,4-benzotriazine-1,4-dioxides are similar to the corresponding values for TPZ, supporting the conclusion that these TPZ analogues are potentially useful as hypoxia-activated radiosensitizers. Further studies into their biodistributions in animal models are being pursued to determine the in vivo potential in hypoxia management.

Domain analysis of PNKP-XRCC1 interactions: Influence of genetic variants of XRCC1

Polynucleotide kinase/phosphatase (PNKP) and X-ray repair cross-complementing 1 (XRCC1) are key proteins in the single-strand DNA break repair pathway. Phosphorylated XRCC1 stimulates PNKP by binding to its forkhead-associated (FHA) domain, whereas nonphosphorylated XRCC1 stimulates PNKP by interacting with the PNKP catalytic domain. Here, we have further studied the interactions between these two proteins, including two variants of XRCC1 (R194W and R280H) arising from single-nucleotide polymorphisms (SNPs) that have been associated with elevated cancer risk in some reports. We observed that interaction of the PNKP FHA domain with phosphorylated XRCC1 extends beyond the immediate, well-characterized phosphorylated region of XRCC1 (residues 515-526). We also found that an XRCC1 fragment, comprising residues 166-436, binds tightly to PNKP and DNA and efficiently activates PNKP's kinase activity. However, interaction of either of the SNP-derived variants of this fragment with PNKP was considerably weaker, and their stimulation of PNKP was severely reduced, although they still could bind DNA effectively. Laser microirradiation revealed reduced recruitment of PNKP to damaged DNA in cells expressing either XRCC1 variant compared with PNKP recruitment in cells expressing WT XRCC1 even though WT and variant XRCC1s were equally efficient at localizing to the damaged DNA. These findings suggest that the elevated risk of cancer associated with these XRCC1 SNPs reported in some studies may be due in part to the reduced ability of these XRCC1 variants to recruit PNKP to damaged DNA.

Persistent 3'-phosphate termini and increased cytotoxicity of radiomimetic DNA double-strand breaks in cells lacking polynucleotide kinase/phosphatase despite presence of an alternative 3'-phosphatase

Polynucleotide kinase/phosphatase (PNKP) has been implicated in non-homologous end joining (NHEJ) of DNA double-strand breaks (DSBs). To assess the consequences of PNKP deficiency for NHEJ of 3'-phosphate-ended DSBs, PNKP-deficient derivatives of HCT116 and of HeLa cells were generated using CRISPR/CAS9. For both cell lines, PNKP deficiency conferred sensitivity to ionizing radiation as well as to neocarzinostatin (NCS), which specifically induces DSBs bearing protruding 3'-phosphate termini. Moreover, NCS-induced DSBs, detected as 53BP1 foci, were more persistent in PNKP ^-/- HCT116 cells compared to their wild-type (WT) counterparts. Surprisingly, PNKP-deficient whole-cell and nuclear extracts were biochemically competent in removing both protruding and recessed 3'-phosphates from synthetic DSB substrates, albeit much less efficiently than WT extracts, suggesting an alternative 3'-phosphatase. Measurements by ligation-mediated PCR showed that PNKP-deficient HeLa cells contained significantly more 3'-phosphate-terminated and fewer 3'-hydroxyl-terminated DSBs than parental cells 5-15 min after NCS treatment, but this difference disappeared by 1 h. These results suggest that, despite presence of an alternative 3'-phosphatase, loss of PNKP significantly sensitizes cells to 3'-phosphate-terminated DSBs, due to a 3'-dephosphorylation defect.

Nanoencapsulation of Novel Inhibitors of PNKP for Selective Sensitization to Ionizing Radiation and Irinotecan and Induction of Synthetic Lethality

There is increasing interest in developing and applying DNA repair inhibitors in cancer treatment to augment the efficacy of radiation and conventional genotoxic chemotherapy. However, targeting the inhibitor is required to avoid reducing the repair capacity of normal tissue. The aim of this study was to develop nanodelivery systems for the encapsulation of novel imidopiperidine-based inhibitors of the DNA 3'-phosphatase activity of polynucleotide kinase/phosphatase (PNKP), a DNA repair enzyme that plays a critical role in rejoining DNA single- and double-strand breaks. For this purpose, newly identified hit compounds with potent PNKP inhibitory activity, imidopiperidines A12B4C50 and A83B4C63 were encapsulated in polymeric micelles of different poly(ethylene oxide)- b-poly(ε-caprolactone) (PEO- b-PCL)-based structures. Our results showed efficient loading of A12B4C50 and A83B4C63 in PEO- b-PCLs with pendent carboxyl and benzyl carboxylate groups, respectively, and relatively slow release over 24 h. Both free and encapsulated inhibitors were able to sensitize HCT116 cells to radiation and the topoisomerase I poison, irinotecan. In addition, the encapsulated inhibitors were capable of inducing synthetic lethalilty in phosphatase and tensin homologue (PTEN)-deficient cells. We also established the validity of the peptide GE11 as a suitable ligand for active targeted delivery of nanoencapsulated drugs to colorectal cancer cells overexpressing epidermal growth factor receptor (EGFR). Our results show the potential of nanoencapsulated inhibitors of PNKP as either mono or combined therapeutic agents for colorectal cancer.

Structural and functional characterization of the PNKP-XRCC4-LigIV DNA repair complex

Non-homologous end joining (NHEJ) repairs DNA double strand breaks in non-cycling eukaryotic cells. NHEJ relies on polynucleotide kinase/phosphatase (PNKP), which generates 5΄-phosphate/3΄-hydroxyl DNA termini that are critical for ligation by the NHEJ DNA ligase, LigIV. PNKP and LigIV require the NHEJ scaffolding protein, XRCC4. The PNKP FHA domain binds to the CK2-phosphorylated XRCC4 C-terminal tail, while LigIV uses its tandem BRCT repeats to bind the XRCC4 coiled-coil. Yet, the assembled PNKP-XRCC4–LigIV complex remains uncharacterized. Here, we report purification and characterization of a recombinant PNKP–XRCC4–LigIV complex. We show that the stable binding of PNKP in this complex requires XRCC4 phosphorylation and that only one PNKP protomer binds per XRCC4 dimer. Small angle X-ray scattering (SAXS) reveals a flexible multi-state complex that suggests that both the PNKP FHA and catalytic domains contact the XRCC4 coiled-coil and LigIV BRCT repeats. Hydrogen-deuterium exchange indicates protection of a surface on the PNKP phosphatase domain that may contact XRCC4–LigIV. A mutation on this surface (E326K) causes the hereditary neuro-developmental disorder, MCSZ. This mutation impairs PNKP recruitment to damaged DNA in human cells and provides a possible disease mechanism. Together, this work unveils multipoint contacts between PNKP and XRCC4–LigIV that regulate PNKP recruitment and activity within NHEJ.

The Rev1 interacting region (RIR) motif in the scaffold protein XRCC1 mediates a low-affinity interaction with polynucleotide kinase/phosphatase (PNKP) during DNA single-strand break repair

The scaffold protein X-ray repair cross-complementing 1 (XRCC1) interacts with multiple enzymes involved in DNA base excision repair and single-strand break repair (SSBR) and is important for genetic integrity and normal neurological function. One of the most important interactions of XRCC1 is that with polynucleotide kinase/phosphatase (PNKP), a dual-function DNA kinase/phosphatase that processes damaged DNA termini and that, if mutated, results in ataxia with oculomotor apraxia 4 (AOA4) and microcephaly with early-onset seizures and developmental delay (MCSZ). XRCC1 and PNKP interact via a high-affinity phosphorylation-dependent interaction site in XRCC1 and a forkhead-associated domain in PNKP. Here, we identified using biochemical and biophysical approaches a second PNKP interaction site in XRCC1 that binds PNKP with lower affinity and independently of XRCC1 phosphorylation. However, this interaction nevertheless stimulated PNKP activity and promoted SSBR and cell survival. The low-affinity interaction site required the highly conserved Rev1-interacting region (RIR) motif in XRCC1 and included three critical and evolutionarily invariant phenylalanine residues. We propose a bipartite interaction model in which the previously identified high-affinity interaction acts as a molecular tether, holding XRCC1 and PNKP together and thereby promoting the low-affinity interaction identified here, which then stimulates PNKP directly.

Microhomology-mediated end joining is activated in irradiated human cells due to phosphorylation-dependent formation of the XRCC1 repair complex

Microhomology-mediated end joining (MMEJ), an error-prone pathway for DNA double-strand break (DSB) repair, is implicated in genomic rearrangement and oncogenic transformation; however, its contribution to repair of radiation-induced DSBs has not been characterized. We used recircularization of a linearized plasmid with 3΄-P-blocked termini, mimicking those at X-ray-induced strand breaks, to recapitulate DSB repair via MMEJ or nonhomologous end-joining (NHEJ). Sequence analysis of the circularized plasmids allowed measurement of relative activity of MMEJ versus NHEJ. While we predictably observed NHEJ to be the predominant pathway for DSB repair in our assay, MMEJ was significantly enhanced in preirradiated cells, independent of their radiation-induced arrest in the G2/M phase. MMEJ activation was dependent on XRCC1 phosphorylation by casein kinase 2 (CK2), enhancing XRCC1's interaction with the end resection enzymes MRE11 and CtIP. Both endonuclease and exonuclease activities of MRE11 were required for MMEJ, as has been observed for homology-directed DSB repair (HDR). Furthermore, the XRCC1 co-immunoprecipitate complex (IP) displayed MMEJ activity in vitro, which was significantly elevated after irradiation. Our studies thus suggest that radiation-mediated enhancement of MMEJ in cells surviving radiation therapy may contribute to their radioresistance and could be therapeutically targeted.

Screening of genotoxicity and mutagenicity in extractable organics from oil sands process-affected water

Large volumes of oil sands process-affected water (OSPW) are produced by the oil sands surface mining industry during alkaline hot-water extraction of bitumen. It is well documented that the acid extractable organics (AEOs) in OSPW, a highly complex mixture of acidic and polar neutral substances, are acutely toxic; but few studies have examined the genotoxicity or mutagenicity of this mixture. In the present study, the in vitro SOS Chromotest and the Ames test (TA98 and TA100 strains) were used to evaluate genotoxicity and mutagenicity for whole OSPW AEOs in the presence and absence of biotransformation by rat S9 liver enzymes. Two subfractions were also examined in the same assays: neutral extractable fraction (F1-NE), and the subsequent acid extractable fraction (F2-AE). In the SOS assay, whole AEO was cytotoxic when concentrated 2× (i.e., twice as concentrated as the environmental sample) and showed increasing genotoxic response above 6×. Co-exposure with S9 had a protective effect on the cell SOS-inducing factor and survival but did not eliminate genotoxicity above 6× concentrations. Most of the cytotoxicity was attributable to F2-AE, but both F1-NE and F2-AE had similar genotoxic dose-responses above 6×. In the Ames test without S9, whole AEO was mutagenic in both strains above 10× concentrations. Co-incubation with S9 had little effect on the TA100 strain but with TA98 resulted in bioactivation at midlevel doses (1.5-6.3×) and protection at higher doses (10-25×). The 2 subfractions were mutagenic in both strains but with different dose-responses. Further research in vivo or in more relevant cells is warranted to investigate the carcinogenic risks of OSPW. Environ Toxicol Chem 2017;36:1397-1404. © 2016 SETAC.

Characterization of DNA Substrate Binding to the Phosphatase Domain of the DNA Repair Enzyme Polynucleotide Kinase/Phosphatase

Polynucleotide kinase/phosphatase (PNKP) is a DNA strand break repair enzyme that uses separate 5' kinase and 3' phosphatase active sites to convert damaged 5'-hydroxyl/3'-phosphate strand termini to ligatable 5'-phosphate/3'-hydroxyl ends. The phosphatase active site has received particular attention as a target of inhibition in cancer therapy development. The phosphatase domain dephosphorylates a range of single- and double-stranded substrates; however, structural studies have shown that the phosphatase catalytic cleft can bind only single-stranded substrates. Here we use a catalytically inactive but structurally intact phosphatase mutant to probe interactions between PNKP and a variety of single- and double-stranded DNA substrates using an electrophoretic mobility shift assay. This work indicates that the phosphatase domain binds 3'-phosphorylated single-stranded DNAs in a manner that is highly dependent on the presence of the 3'-phosphate. Double-stranded substrate binding, in contrast, is not as dependent on the 3'-phosphate. Experiments comparing blunt-end, 3'-overhanging, and frayed-end substrates indicate that the predicted loss of energy due to base pair disruption upon binding of the phosphatase active site is likely balanced by favorable interactions between the liberated complementary strand and PNKP. Comparison of the effects on substrate binding of mutations within the phosphatase active site cleft with mutations in surrounding positively charged surfaces suggests that the surrounding surfaces are important for binding to double-stranded substrates. We further show that while fluorescence polarization methods can detect specific binding of single-stranded DNAs with the phosphatase domain, this method does not detect specific interactions between the PNKP phosphatase and double-stranded substrates.

ATPase activity tightly regulates RecA nucleofilaments to promote homologous recombination

Homologous recombination (HR), catalyzed in an evolutionarily conserved manner by active RecA/Rad51 nucleofilaments, maintains genomic integrity and promotes biological evolution and diversity. The structures of RecA/Rad51 nucleofilaments provide information critical for the entire HR process. By exploiting a unique capillary electrophoresis-laser-induced fluorescence polarization assay, we have discovered an active form of RecA nucleofilament, stimulated by ATP hydrolysis, that contains mainly unbound nucleotide sites. This finding was confirmed by a nuclease protection assay and electron microscopy (EM) imaging. We further found that these RecA-unsaturated filaments promote strand exchange in vitro and HR in vivo. RecA mutants (P67D and P67E), which only form RecA-unsaturated nucleofilaments, were able to mediate HR in vitro and in vivo, but mutants favoring the formation of the saturated nucleofilaments failed to support HR. We thus present a new model for RecA-mediated HR in which RecA utilizes its intrinsic DNA binding-dependent ATPase activity to remodel the nucleofilaments to a less saturated form and thereby promote HR.

Characterization of Plasmodium falciparum ATP-dependent DNA helicase RuvB3

BACKGROUND

Malaria is one of the most serious and widespread parasitic diseases affecting humans. Because of the spread of resistance in both parasites and the mosquito vectors to anti-malarial drugs and insecticides, controlling the spread of malaria is becoming difficult. Thus, identifying new drug targets is urgently needed. Helicases play key roles in a wide range of cellular activities involving DNA and RNA transactions, making them attractive anti-malarial drug targets.

METHODS

ATP-dependent DNA helicase gene (PfRuvB3) of Plasmodium falciparum strain K1, a chloroquine and pyrimethamine-resistant strain, was inserted into pQE-TriSystem His-Strep 2 vector, heterologously expressed and affinity purified. Identity of recombinant PfRuvB3 was confirmed by western blotting coupled with tandem mass spectrometry. Helicase and ATPase activities were characterized as well as co-factors required for optimal function.

RESULTS

Recombinant PfRuvB3 has molecular size of 59 kDa, showing both DNA helicase and ATPase activities. Its helicase activity is dependent on divalent cations (Cu²⁺, Mg²⁺, Ni⁺² or Zn⁺²) and ATP or dATP but is inhibited by high NaCl concentration (>100 mM). PfPuvB3 is unable to act on blunt-ended duplex DNA, but manifests ATPase activity in the presence of either single- or double-stranded DNA. PfRuvB3.is inhibited by doxorubicin, daunorubicin and netropsin, known DNA helicase inhibitors.

CONCLUSIONS

Purified recombinant PfRuvB3 contains both DNA helicase and ATPase activities. Differences in properties of RuvB between the malaria parasite obtained from the study and human host provide an avenue leading to the development of novel drugs targeting specifically the malaria form of RuvB family of DNA helicases.

Neurological disorders associated with DNA strand-break processing enzymes

The termini of DNA strand breaks induced by reactive oxygen species or by abortive DNA metabolic intermediates require processing to enable subsequent gap filling and ligation to proceed. The three proteins, tyrosyl DNA-phosphodiesterase 1 (TDP1), aprataxin (APTX) and polynucleotide kinase/phosphatase (PNKP) each act on a discrete set of modified strand-break termini. Recently, a series of neurodegenerative and neurodevelopmental disorders have been associated with mutations in the genes coding for these proteins. Mutations in TDP1 and APTX have been linked to Spinocerebellar ataxia with axonal neuropathy (SCAN1) and Ataxia-ocular motor apraxia 1 (AOA1), respectively, while mutations in PNKP are considered to be responsible for Microcephaly with seizures (MCSZ) and Ataxia-ocular motor apraxia 4 (AOA4). Here we present an overview of the mechanisms of these proteins and how their impairment may give rise to their respective disorders.

Association of ATM activation and DNA repair with induced radioresistance after low-dose irradiation

Mammalian cells often exhibit a hyper-radiosensitivity (HRS) to radiation doses <20 cGy, followed by increased radioresistance (IRR) at slightly higher doses (∼20-30 cGy). Here, the influence of DNA double-strand break repair (DSBR) on IRR was examined. The failure of Ataxia telangiectasia (AT) cells to undergo IRR reported by others was confirmed. Flow cytometric analysis indicated that normal cells fail to show a measurable increase in serine 1981 phosphorylated AT-mutated (ATM) protein after 10 cGy up to 4 h post irradiation, but a two- to fourfold increase after 25 cGy. Similarly, more proficient reduction of phosphorylated histone H2AX was observed 24 h after 25 cGy than after 10 cGy, suggesting that DSBR is more efficient during IRR than HRS. A direct examination of the consequences of inefficient DNA repair per se (as opposed to ATM-mediated signal transduction/cell cycle responses), by determining the clonogenic survival of cells lacking the DNA repair enzyme polynucleotide kinase/phosphatase, indicated that these cells have a response similar to AT cells, i.e. HRS but no IRR, strongly linking IRR to DSBR.

Abstract POSTER-THER-1410: Combining small molecule drugs and standard chemotherapy for treatment of granulosa cell tumour cells

Evasion of apoptosis is a hallmark of cancer, and direct induction of apoptosis without dependence on signaling upstream of Caspase-3 (CASP3) is an attractive target for cancer therapy. CASP3 sits at the hub of apoptotic pathways and it is a primary target for inhibition by anti-apoptotic proteins like the X-linked inhibitor of apoptosis (XIAP).

Granulosa cell tumour (GCT) is a rare form of ovarian cancer, highly lethal in the event of recurrence, and has no standard chemotherapy because it is resistant to most common drugs.

A recently discovered small-molecule drug, procaspase activating compound-1 (PAC1), has been shown to effectively cleave procaspase-3 into its active form by removal of an inhibitory zinc ion, facilitating autocleavage of the zymogen and direct induction of apoptosis.

Initial in vitro experiments in our lab have shown PAC1 capable of significantly reducing viability of GCT cells (represented by the KGN cell line), and combining PAC1 with a drug inhibiting XIAP further increases the killing effect. In addition, combining the small-molecule XIAP inhibitor with carboplatin, a standard chemotherapy agent, displays significant drug interaction while killing GCT cells, in vitro.

Results: GCT cell line KGN was treated with various concentrations of PAC1, for 24 and 48 hour time points, and assessed for cell viability using a metabolic assay. Results showed significant reduction of cell viability (p&lt;0.05) in both time and dose-dependent manners. The dose-response curve for these assays indicate an EC50 of ~10 µM PAC1.

High-content screening of PAC1-treated GCT cells produced quantified imagery that suggests treatment with PAC1 is, in fact, inducing activation of CASP3-mediated apoptosis and an assay inhibiting CASP3 displayed reduction in PAC1-induced killing.

Combining 10 µM PAC1 with selected concentrations of embelin, a monovalent XIAP-inhibitor, showed strong additive effect in initial experiments, indicating it is an area worthy of further research. Combination of embelin with carboplatin also displayed significant drug interaction as well as single drug effect (p&lt;0.05).

This presentation suggests that PAC1 reduces viability of GCT cells in a time and dose-dependent manner, in vitro, and the mechanism of that loss of viability is caspase-mediated apoptosis. Furthermore, combining PAC1 with drugs that inhibit XIAP enhances the killing effect.

Combining small molecule drugs that promote apoptosis display strong interaction with carboplatin, a standard chemotherapeutic agent, and may have the potential to allow carboplatin dose reduction. Further research to determine activity in animal models and/or primary human GCT explants is warranted and planned.

Citation Format: Powel Crosley, Kate Agopsowicz, Michael Weinfeld, Mary Hitt. Combining small molecule drugs and standard chemotherapy for treatment of granulosa cell tumour cells [abstract]. In: Proceedings of the 10th Biennial Ovarian Cancer Research Symposium; Sep 8-9, 2014; Seattle, WA. Philadelphia (PA): AACR; Clin Cancer Res 2015;21(16 Suppl):Abstract nr POSTER-THER-1410.

Therapeutic and analytical applications of arsenic binding to proteins

Knowledge of arsenic binding to proteins advances the development of bioanalytical techniques and therapeutic drugs.

DNA ligase III acts as a DNA strand break sensor in the cellular orchestration of DNA strand break repair

In the current model of DNA SSBR, PARP1 is regarded as the sensor of single-strand breaks (SSBs). However, biochemical studies have implicated LIG3 as another possible SSB sensor. Using a laser micro-irradiation protocol that predominantly generates SSBs, we were able to demonstrate that PARP1 is dispensable for the accumulation of different single-strand break repair (SSBR) proteins at sites of DNA damage in live cells. Furthermore, we show in live cells for the first time that LIG3 plays a role in mediating the accumulation of the SSBR proteins XRCC1 and PNKP at sites of DNA damage. Importantly, the accumulation of LIG3 at sites of DNA damage did not require the BRCT domain-mediated interaction with XRCC1. We were able to show that the N-terminal ZnF domain of LIG3 plays a key role in the enzyme's SSB sensing function. Finally, we provide cellular evidence that LIG3 and not PARP1 acts as the sensor for DNA damage caused by the topoisomerase I inhibitor, irinotecan. Our results support the existence of a second damage-sensing mechanism in SSBR involving the detection of nicks in the genome by LIG3.

The human polynucleotide kinase/phosphatase (hPNKP) inhibitor A12B4C3 radiosensitizes human myeloid leukemia cells to Auger electron-emitting anti-CD123 ¹¹¹In-NLS-7G3 radioimmunoconjugates

INTRODUCTION

Leukemia stem cells (LSCs) are believed to be responsible for initiating and propagating acute myeloid leukemia (AML) and for causing relapse after treatment. Radioimmunotherapy (RIT) targeting these cells may improve the treatment of AML, but is limited by the low density of target epitopes. Our objective was to study a human polynucleotide kinase/phosphatase (hPNKP) inhibitor that interferes with DNA repair as a radiosensitizer for the Auger electron RIT agent, ¹¹¹In-NLS-7G3, which recognizes the CD123⁺/CD131⁻ phenotype uniquely displayed by LSCs.

METHODS

The surviving fraction (SF) of CD123⁺/CD131⁻ AML-5 cells exposed to ¹¹¹In-NLS-7G3 (33-266 nmols/L; 0.74MBq/μg) or to γ-radiation (0.25-5Gy) was determined by clonogenic assays. The effect of A12B4C3 (25 μmols/L) combined with ¹¹¹In-NLS-7G3 (16-66 nmols/L) or with γ-radiation (0.25-2Gy) on the SF of AML-5 cells was assessed. The density of DNA double-strand breaks (DSBs) in the nucleus was measured using the γ-H2AX assay. Cellular dosimetry was estimated based on the subcellular distribution of ¹¹¹In-NLS-7G3 measured by cell fractionation.

RESULTS

Binding of (111)In-NLS-7G3 to AML-5 cells was reduced by 2.2-fold in the presence of an excess (1μM) of unlabeled NLS-7G3, demonstrating specific binding to the CD123⁺/CD131⁻ epitope. ¹¹¹In-NLS-7G3 reduced the SF of AML-5 cells from 86.1 ± 11.0% at 33 nmols/L to 10.5 ± 3.6% at 266 nmols/L. Unlabeled NLS-7G3 had no significant effect on the SF. Treatment of AML-5 cells with γ-radiation reduced the SF from 98.9 ± 14.9% at 0.25Gy to 0.03 ± 0.1% at 5 Gy. A12B4C3 combined with ¹¹¹In-NLS-7G3 (16-66 nmols/L) enhanced the cytotoxicity up to 1.7-fold compared to treatment with radioimmunoconjugates alone and was associated with a 1.6-fold increase in DNA DSBs in the nucleus. A12B4C3 enhanced the cytotoxicity of γ-radiation (0.25-0.5Gy) on AML-5 cells by up to 1.5-fold, and DNA DSBs were increased by 1.7-fold. Exposure to ¹¹¹In-NLS-7G3 (66 nmols/L) delivered up to 0.6Gy to AML-5 cells.

CONCLUSIONS

We conclude that A12B4C3 radiosensitized AML cells to the DNA damaging effects of ¹¹¹In-NLS-7G3. Combination treatment may increase the effectiveness for Auger electron RIT of AML targeting the LSC subpopulation.

Synthetic lethal targeting of PTEN-deficient cancer cells using selective disruption of polynucleotide kinase/phosphatase

A recent screen of 6,961 siRNAs to discover possible synthetic lethal partners of the DNA repair protein polynucleotide kinase/phosphatase (PNKP) led to the identification of the potent tumor suppressor phosphatase and tensin homolog deleted on chromosome 10 (PTEN). Here, we have confirmed the PNKP/PTEN synthetic lethal partnership in a variety of different cell lines including the PC3 prostate cancer cell line, which is naturally deficient in PTEN. We provide evidence that codepletion of PTEN and PNKP induces apoptosis. In HCT116 colon cancer cells, the loss of PTEN is accompanied by an increased background level of DNA double-strand breaks, which accumulate in the presence of an inhibitor of PNKP DNA 3'-phosphatase activity. Complementation of PC3 cells with several well-characterized mutated PTEN cDNAs indicated that the critical function of PTEN required to prevent toxicity induced by an inhibitor of PNKP is most likely associated with its cytoplasmic lipid phosphatase activity. Finally, we show that modest inhibition of PNKP in a PTEN knockout background enhances cellular radiosensitivity, suggesting that such a "synthetic sickness" approach involving the combination of PNKP inhibition with radiotherapy may be applicable to PTEN-deficient tumors.

Polynucleotide kinase/phosphatase, Pnk1, is involved in base excision repair in Schizosaccharomyces pombe

We previously reported that Schizosaccharomyces pombe pnk1 cells are more sensitive than wild-type cells to γ-radiation and camptothecin, indicating that Pnk1 is required for DNA repair. Here, we report that pnk1pku70 and pnk1rhp51 double mutants are more sensitive to γ-radiation than single mutants, from which we infer that Pnk1's primary role is independent of either homologous recombination or non-homologous end joining mechanisms. We also report that pnk1 cells are more sensitive than wild-type cells to oxidizing and alkylating agents, suggesting that Pnk1 is involved in base excision repair. Mutational analysis of Pnk1 revealed that the DNA 3'-phosphatase activity is necessary for repair of DNA damage, whereas the 5'-kinase activity is dispensable. A role for Pnk1 in base excision repair is supported by genetic analyses which revealed that pnk1apn2 is synthetically lethal, suggesting that Pnk1 and Apn2 may function in parallel pathways essential for the repair of endogenous DNA damage. Furthermore, the nth1pnk1apn2 and tdp1pnk1apn2 triple mutants are viable, implying that single-strand breaks with 3'-blocked termini produced by Nth1 and Tdp1 contribute to synthetic lethality. We also examined the sensitivity to methyl methanesulfonate of all single and double mutant combinations of nth1, apn2, tdp1 and pnk1. Together, our results support a model where Tdp1 and Pnk1 act in concert in an Apn2-independent base excision repair pathway to repair 3'-blocked termini produced by Nth1; and they also provide evidence that Pnk1 has additional roles in base excision repair.