Drew Murphy

Organic light-emitting materials and devices

An organic light emitting device is provided. The device has an anode, a cathode, and an emissive layer disposed between the anode and the cathode, the emissive layer further comprising an emissive material having the structure: wherein each of the variables are defined herein.

Cyclometalated Ir complexes in polymer organic light-emitting devices

Several new iridium based cyclometalated complexes were investigated as phosphorescent dopants for molecularly doped polymeric organic light-emitting diodes. Specifically, the complexes used in this study were iridium (III) bis(2-phenylpyridinato-N,C2′) (acetylacetonate) [ppy], iridium (III) bis(7,8-benzoquinolinato-N,C3′) (acetylacetonate) [bzq], iridium (III) bis(2-phenylbenzothiazolato-N,C2′) (acetylacetonate) [bt], iridium (III) bis(2-(2′-naphthyl)benzothiazolato-N,C2′) (acetylacetonate) [bsn] and iridium (III) bis(2-(2′-benzo[4,5-a]thienyl)pyridinato-N,C3′) (acetylacetonate) [btp]. Single layer devices of doped polyvinylcarbazole: 2-(4-biphenyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole give maximum external quantum efficiencies that varied from 3.5% for the ppy dopant to 0.4% for the btp dopant. Several different device heterostructure architectures were explored, and the best quantum efficiency of the devices reached 4.2% for the heterostructures.

Base Excision Repair and Cancer

Base excision repair is the system used from bacteria to man to remove the tens of thousands of endogenous DNA damages produced daily in each human cell. Base excision repair is required for normal mammalian development and defects have been associated with neurological disorders and cancer. In this paper we provide an overview of short patch base excision repair in humans and summarize current knowledge of defects in base excision repair in mouse models and functional studies on short patch base excision repair germ line polymorphisms and their relationship to cancer. The biallelic germ line mutations that result in MUTYH-associated colon cancer are also discussed.

Nucleosome Disruption by DNA Ligase III-XRCC1 Promotes Efficient Base Excision Repair

ABSTRACT Each day, approximately 20,000 oxidative lesions form in the DNA of every nucleated human cell. The base excision repair (BER) enzymes that repair these lesions must function in a chromatin milieu. We have determined that the DNA glycosylase hNTH1, apurinic endonuclease (APE), and DNA polymerase β (Pol β), which catalyze the first three steps in BER, are able to process their substrates in both 601- and 5S ribosomal DNA (rDNA)-based nucleosomes. hNTH1 formed a discrete ternary complex that was displaced by the addition of APE, suggesting an orderly handoff of substrates from one enzyme to the next. In contrast, DNA ligase IIIα-XRCC1, which completes BER, was appreciably active only at concentrations that led to nucleosome disruption. Ligase IIIα-XRCC1 was also able to bind and disrupt nucleosomes containing a single base gap and, because of this property, enhanced both its own activity and that of Pol β on nucleosome substrates. Collectively, these findings provide insights into rate-limiting steps that govern BER in chromatin and reveal a unique role for ligase IIIα-XRCC1 in enhancing the efficiency of the final two steps in the BER of lesions in nucleosomes.

Human POLB Gene Is Mutated in High Percentage of Colorectal Tumors

Background: Previous small scale studies indicate that DNA polymerase β variants are present in 30% of human tumors. Results: 40% of samples in a large human colorectal tumor collection harbor coding region variants, many of which exhibit altered function. Conclusion: Aberrant activity or fidelity phenotypes exhibited by variants may contribute to tumorigenesis. Significance: Expression of variants in human tumors plays a role in driving carcinogenesis. Previous small scale sequencing studies have indicated that DNA polymerase β (pol β) variants are present on average in 30% of human tumors of varying tissue origin. Many of these variants have been shown to have aberrant enzyme function in vitro and to induce cellular transformation and/or genomic instability in vivo, suggesting that their presence is associated with tumorigenesis or its progression. In this study, the human POLB gene was sequenced in a collection of 134 human colorectal tumors and was found to contain coding region mutations in 40% of the samples. The variants map to many different sites of the pol β protein and are not clustered. Many variants are nonsynonymous amino acid substitutions predicted to affect enzyme function. A subset of these variants was found to have reduced enzyme activity in vitro and failed to fully rescue pol β-deficient cells from methylmethane sulfonate-induced cytotoxicity. Tumors harboring variants with reduced enzyme activity may have compromised base excision repair function, as evidenced by our methylmethane sulfonate sensitivity studies. Such compromised base excision repair may drive tumorigenesis by leading to an increase in mutagenesis or genomic instability.

Colon Cancer-associated DNA Polymerase β Variant Induces Genomic Instability and Cellular Transformation

Background: Mutations in the POLB gene are present in 40% of human colorectal tumors. Results: The G231D variant is a slow polymerase that induces genomic instability and cellular transformation. Conclusion: The slow G231D variant induces cellular transformation due to its inability to fill in single nucleotide gaps. Significance: Slow pol β variants may drive tumorigenesis. Rapidly advancing technology has resulted in the generation of the genomic sequences of several human tumors. We have identified several mutations of the DNA polymerase β (pol β) gene in human colorectal cancer. We have demonstrated that the expression of the pol β G231D variant increased chromosomal aberrations and induced cellular transformation. The transformed phenotype persisted in the cells even once the expression of G231D was extinguished, suggesting that it resulted as a consequence of genomic instability. Biochemical analysis revealed that its catalytic rate was 140-fold slower than WT pol β, and this was a result of the decreased binding affinity of nucleotides by G231D. Residue 231 of pol β lies in close proximity to the template strand of the DNA. Molecular modeling demonstrated that the change from a small and nonpolar glycine to a negatively charged aspartate resulted in a repulsion between the template and residue 231 leading to the distortion of the dNTP binding pocket. In addition, expression of G231D was insufficient to rescue pol β-deficient cells treated with chemotherapeutic agents suggesting that these agents may be effectively used to treat tumors harboring this mutation. More importantly, this suggests that the G231D variant has impaired base excision repair. Together, these data indicate that the G231D variant plays a role in driving cancer.

The S229L Colon Tumor-associated Variant of DNA Polymerase β Induces Cellular Transformation as a Result of Decreased Polymerization Efficiency

Background: The POLB gene is mutated in 40% of human colorectal tumors. Results: The S229L variant is a slow DNA polymerase that leads to the accumulation of BER intermediates and induces cellular transformation. Conclusion: The S229L variant transforms cells by inducing genomic instability. Significance: Tumor-associated variants of DNA polymerase β can affect DNA repair efficiency and drive cancer. DNA polymerase β (Pol β) plays a key role in base excision repair (BER) by filling in small gaps that are generated after base adducts are excised from the DNA. Pol β is mutated in a large number of colorectal tumors, and these mutations may drive carcinogenesis. In the present study, we wished to determine whether the S229L somatic Pol β variant identified in a stage 3 colorectal tumor is a driver of carcinogenesis. We show that S229L does not possess any defects in binding to either DNA or nucleotides compared with the WT enzyme, but exhibits a significant loss of polymerization efficiency, largely due to an 8-fold decrease in the polymerization rate. S229L participates in BER, but due to its lower catalytic rate, does so more slowly than WT. Expression of S229L in mammalian cells induces the accumulation of BER intermediate substrates, chromosomal aberrations, and cellular transformation. Our results are consistent with the interpretation that S229L is a driver of carcinogenesis, likely as a consequence of its slow polymerization activity during BER in vivo.

High-efficiency organic electrophosphorescent devices

We have fabricated saturated red, orange, yellow and green OLEDs, utilizing phosphorescent dopants. Using phosphorescence based emitters we have eliminated the inherent 25% upper limit on emission observed for traditional fluorescence based systems. The quantum efficiencies of these devices are quite good, with measured external efficiencies > 15% and > 40 lum/W (green) in the best devices. The phosphorescent dopants in these devices are heavy metal containing molecules (i.e. Pt, and Ir), prepared as both metalloporphyrins and organometallic complexes. The high level of spin orbit coupling in these metal complexes gives efficient emission from triplet states. In addition to emission from the heavy metal dopant, it is possible to transfer the exciton energy to a fluorescent dye, by Forster energy transfer. The heavy metal dopant in this case acts as a sensitizer, utilizing both singlet and triplet excitons to efficiently pump a fluorescent dye. We discuss the important parameters in designing electrophosphorescent OLEDs as well as their strengths and limitations. Accelerated aging studies, on packaged devices, have shown that phosphorescence based OLEDs can have very long device lifetimes.

Remote Mutations Induce Functional Changes in Active Site Residues of Human DNA Polymerase β

With the formidable growth in the volume of genetic information, it has become essential to identify and characterize mutations in macromolecules not only to predict contributions to disease processes but also to guide the design of therapeutic strategies. While mutations of certain residues have a predictable phenotype based on their chemical nature and known structural position, many types of mutations evade prediction based on current information. Described in this work are the crystal structures of two cancer variants located in the palm domain of DNA polymerase β (pol β), S229L and G231D, whose biological phenotype was not readily linked to a predictable structural implication. Structural results demonstrate that the mutations elicit their effect through subtle influences on secondary interactions with a residue neighboring the active site. Residues 229 and 231 are 7.5 and 12.5 Å, respectively, from the nearest active site residue, with a β-strand between them. A residue on this intervening strand, M236, appears to transmit fine structural perturbations to the catalytic metal-coordinating residue D256, affecting its conformational stability.

Abstract B37: A DNA polymerase beta gene harboring the colon cancer-associated G231D variant increases the sensitivity of cells to chemotherapeutic agents and induces cellular transformation

Rapidly advancing technology has resulted in the generation of the genomic sequences of several human tumors. These studies have revealed that tumors are heterogeneous and contain many mutations, supporting the mutator phenotype hypothesis of human cancer. Drivers of carcinogenesis are thought to be mutated genes that occur at high frequencies in large numbers of human tumors. However, functional characterization of many of the somatic mutations is lacking. We have identified several mutations of the DNA polymerase beta (POLB) gene in human colorectal cancer. Pol beta is the main polymerase involved in the base excision repair (BER) pathway and aberrant expression of Pol beta has been linked to a cancerous phenotype. In this study, we characterized the homozygous G231D mutation that was identified in a Stage 3 colorectal tumor. We have demonstrated that its expression in epithelial cells induced cellular transformation and increased the rate of proliferation. The transformed phenotype persisted in the cells even once the expression of G231D was extinguished. These data indicated that the expression of G231D caused an inheritable change; this change is most likely due to an increase in genomic instability caused by unfilled gaps proceeding through replication. Expression of G231D was insufficient to rescue Pol beta-deficient cells treated with chemotherapeutic agents suggesting that these agents may be effectively used to treat tumors harboring this mutation. Biochemical analysis of G231D revealed that it was 100-fold slower than WT Pol beta as determined by pre-steady state kinetics. The slow rate was a result of the decreased binding affinity of nucleotides by G231D. Residue 231 of Pol beta lies in close proximity to the template strand of the DNA. Molecular modeling showed that the change from a glycine to a negatively charged amino acid caused a repulsion between the template and residue 231 leading to the disruption of the dNTP binding pocket. Together, these data indicate that a person harboring the G231D Pol beta mutation may have an increased risk of cancer. Moreover, the altered sensitivity to chemotherapeutic drugs in cells carrying the G231D variant may have further implications in how to better improve the therapeutic outcome in patients with this mutation and supports the need for personalized cancer therapies. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the Second AACR International Conference on Frontiers in Basic Cancer Research; 2011 Sep 14-18; San Francisco, CA. Philadelphia (PA): AACR; Cancer Res 2011;71(18 Suppl):Abstract nr B37.