S MALL ‐M OLECULE I NHIBITORS OF DNA P OLYMERASE F UNCTION
D ISSERTATION
zur Erlangung des Akademischen Grades des Doktors der Naturwissenschaften
(Dr. rer. nat.)
vorgelegt von
Tobias Strittmatter
aus Küssaberg‐Kadelburg
an der Universität Konstanz
Mathematisch‐Naturwissenschaftliche Sektion Fachbereich Chemie
2014
Tag der mündlichen Prüfung: 05. Dezember 2014 Prüfungsvorsitz: Prof. Dr. Gerhard Müller 1. Referent: Prof. Dr. Andreas Marx 2. Referent: Prof. Dr. Thomas U. Mayer
„Meinen Eltern”
Die vorliegende Arbeit entstand in der Zeit von November 2009 bis August 2014 in der Arbeitsgruppe von Prof. Dr. Andreas Marx am Lehrstuhl für Organische und Zelluläre Chemie am Fachbereich Chemie der Universität Konstanz.
Nach diesem Zeitraum geprägt von intensiver Forschungsarbeit liegt nun meine Dissertation vor und ein weiteres Kapitel meiner beruflichen Karriere kann nun erfolgreich abgeschlossen werden. Aus diesem Grund ist es jetzt auch an der Zeit, mich bei all denen zu bedanken, die mich während dieser spannenden Zeit begleitet, unterstützt und gefördert haben.
Meinem Doktorvater Prof. Dr. Andreas Marx danke ich ganz herzlich für die Überlassung der sehr interessanten und interdisziplinären Themenstellung, sowie für das in mich gesetzte Vertrauen, welches mir viel Raum für die selbstständige Bearbeitung und kreative Gestaltung des Themas erlaubte. Ihm, wie auch den weiteren Mitgliedern meines „Thesis Committee“, Prof. Dr. Thomas Mayer und Herrn Prof. Dr. Michael Berthold, danke ich zudem für die anregenden wissenschaftlichen Diskussionen und die geleisteten Hilfestellungen. Prof. Dr.
Thomas Mayer danke ich außerdem für die Übernahme des Zweitgutachtens und Herrn Prof.
Dr. Gerhard Müller für die Übernahme des Prüfungsvorsitzes.
Natürlich möchte ich mich auch bei allen früheren und jetzigen Freunden und Kollegen der Arbeitsgruppe Marx und der Graduiertenschule Chemische Biologie für die unvergessliche Zeit, die tolle Arbeitsatmosphäre und die Hilfsbereitschaft bedanken. Besonderer Dank gilt hier Dr. Norman Hardt, Magdalena Grzywa, Matthias Drum und Dr. Nina Blatter für die sehr gute, langjährige Zusammenarbeit und die tolle Laboratmosphäre. Zudem danke ich Dr. Karl‐
Heinz Jung für die interessanten wissenschaftlichen Fachgespräche. Weiterhin danke ich ganz herzlich meinen sehr talentierten Bachelorstudenten Annika Hantusch, Moritz Pott, Joos Aschenbrenner und Melina Hoffmann, sowie allen Mitarbeiterpraktikanten und wissenschaftlichen Hilfskräften für ihre Leistung, ihr Engagement und ihr Interesse.
Bei Dr. Thomas Huhn, Dr. Timo Immel und Malin Bein bedanke ich mich für die Durchführung der ersten Zytotoxizitätsmessungen. Bei Herrn Prof. Dr. Thomas Brunner und Anette Brockmann möchte ich mich besonders für die großartige Hilfe bei komplexen zellbiologischen Fragestellungen und die fruchtbare Zusammenarbeit bei der Durchführung der Zellassays bedanken.
Meinen Lektoren Dr. Norman Hardt, Karin Reichardt, Matthias Drum und Joachim Braun danke ich für die Durchsicht meiner schriftlichen Arbeit.
Meinen Freunden, meiner Familie und ganz besonders meinen Eltern danke ich für die liebevolle und bedingungslose Unterstützung in allen Lebenslagen.
Teile dieser Arbeit sind veröffentlicht in:
T. Strittmatter, B. Bareth, T. A. Immel, T. Huhn, T. U. Mayer & A. Marx
”Small Molecule Inhibitors of Human DNA Polymerase λ”
ACS Chem. Biol. 2011, 6 (4), 314 – 319
T. Strittmatter, J. Aschenbrenner, N. Hardt & A. Marx
”Synthesis of 4′‐C‐alkylated‐5‐iodo‐2′‐deoxypyrimidine nucleosides”
ARKIVOC 2013, (ii) Issue in Honor of Prof. Richard R. Schmidt, 46 – 59
T. Strittmatter, A. Brockmann, M. Pott, A. Hantusch, T. Brunner & A. Marx
”Expanding the Scope of Human DNA Polymerase λ and β Inhibitors”
ACS Chem. Biol. 2014, 9 (1), 282–290.
Weitere Publikationen:
M. Catarinella, T. Grüner, T. Strittmatter, A. Marx & T. U. Mayer
”BTB‐1: A Small Molecule Inhibitor of the Mitotic Motor Protein Kif18A”
Angew. Chem. Int. Ed. 2009, 48, 9072 – 9076 Angew. Chem. 2009, 121, 9236 – 9240
B. Reichmann, M. Drexler, B. Weibert, N. Szesni, T. Strittmatter & H. Fischer
”Amino‐substituted Butatrienes: Unusual η1Ligands Formed by an Unusual Reaction”
Organometallics 2011, 30 (5), 1215 – 1223
O. B. Gutiérrez Acosta, N. Hardt, S. M. Hacker, T. Strittmatter, B. Schink & A. Marx
”TPP stimulates acetone activation by D. biacutus as monitored by a fluorogenic ATP analogue”
ACS Chem. Biol. 2014, 9 (6), 1263 – 1266.
A. Brockmann, T. Strittmatter, S. May, A. Marx & T. Brunner
”Structure‐function relationship of thiazolide‐induced apoptosis in colorectal tumor cells”
ACS Chem. Biol. 2014, 9 (7), 1520 – 1527.
J. Braun, M. M. Möckel, T. Strittmatter, A. Marx, U. Groth & T. U. Mayer
”Synthesis and Biological Evaluation of Optimized Inhibitors of the Mitotic Kinesin Kif18A”
ACS Chem. Biol. (DOI: 10.1021/cb500789h), in press.
Table of Contents
Introduction ... 3
1) Chemical genetics... 3
2) DNA polymerases... 5
2.1) General...5
2.2) DNA polymerases and the polymerisation reaction...6
2.3) DNA polymerase λ and β...8
2.4) Herpes virus DNA polymerase...11
3) DNA polymerases as drug targets... 12
3.1) General...12
3.2) Screening methods for DNA polymerase inhibitors...14
Concepts and objectives...20
Results and discussions ...22
4) Small‐molecule inhibitors of human pol λ and pol β... 22
4.1) Introduction...22
4.2) Biochemical evaluation of the 1st small‐molecule generation...24
4.2.1) Conclusion...29
4.3) Establishment of a 2nd generation compound library of potential active molecules against pol λ and β...29
4.3.1) Design of the 2nd small‐molecule generation SM12, 29‐58...29
4.3.2) Synthesis of 2nd small‐molecule generation SM12, 29‐58...31
4.3.3) Conclusion...34
4.4) Biochemical evaluation 2nd small‐molecule generation...35
4.4.1) Evaluation of the 2nd small‐molecule generation...35
4.4.2) Side‐by‐side comparison of SM1 and SM49 with reported inhibitors...38
4.4.3) Discussion and structure‐activity relationships...39
4.4.4) Conclusion...41
4.5) Cellular investigations of the 1st and 2nd small‐molecule generation...42
4.5.1) Introduction...42
4.5.2) Cell viability measurements of potential rhodanine probes...43
4.5.3) Co‐treatment experiments with genotoxic agents and probes SM1 or SM49...44
4.5.4) Conclusion...47
5) Synthesis of 4’‐C‐alkylated‐5‐iodo‐2’‐deoxypyrimidine nucleosides as potential antiviral drugs and synthetic building blocks... 49
5.1) Introduction...49
5.2) Synthesis of 4‐C‐modified carbohydrate building blocks...52
5.3) Synthesis of 4’‐C‐alkylated‐pyrimidine nucleosides...54
5.4) Synthesis of 4’‐C‐alkylated‐5‐iodo‐2’‐deoxypyrimidine nucleosides...54
5.5) Evaluation of the synthons ‐ exemplified by 3’,5’‐di‐O‐acetyl‐2’‐deoxy‐5‐iodo‐4’‐C‐
propyluridine N12c in a Sonagashira test‐reaction...56
5.6) Discussion and Conclusion...57
Conclusion ...59
Zusammenfassung ...65
Materials and methods ...72
6) Chemistry (nucleosides)... 72
6.1) General...72
6.2) Synthesis of 4‐C‐modified carbohydrate building blocks...73
6.3) Synthesis of 4`C‐alkylated‐pyrimidine nucleosides...75
6.4) Synthesis of 4’‐C‐alkylated‐5‐iodo‐2’‐deoxypyrimidine nucleosides...80
6.5) Sonagashira test‐reaction...87
7) Chemistry (small‐molecules)... 90
7.1) General...90
7.2) Synthesis of precursor aldehydes and acetophenon SM59‐84...90
7.3) Synthesis of small‐molecules SM1, 4, 10, 12, 16, 17, 21, 23, 27‐58...102
8) Molecular biological and biochemical methods...123
8.1) Chemicals, reagents, inhibitors and solvents...123
8.2) Expression and purification of pol λ and pol β...123
8.2.1) Nucleotide‐ and amio acid sequences of pol β and pol λ...124
8.3) SDS‐PAGE...126
8.4) Nucleotides and oligonucleotides...126
8.4.1) DNA oligonucletide purification...126
8.4.2) 5’‐Radioactive labelling of DNA oligonucletides...127
8.5) Radiometric primer extension...127
8.5.1) Pol λ PEX assay with variable small‐molecule concentrations...128
8.5.2) Pol λ PEX assay with variable dNTP concentrations...128
8.5.3) Pol λ‐TdT assay with variable small‐molecule concentrations...128
8.5.4) Pol β PEX assay with variable small‐molecule concentrations...129
8.6) Polyacrylamide gel electrophoresis (PAGE)...129
8.6.1) Quantitative analysis of gel images...129
9) Cell biological methods...131
9.1) Cell cultivation...131
9.2) AllamarBlue assay...131
9.2) MTT assay...132
References ...133
Introduction
1) Chemical genetics
The illustrious quotation from the great chemist Marcelin Berthelot (*1827 ‐ †1907)
“La chimie crée son objet. Cette faculté créatrice, semblable à celle de l'art lui‐même, la distingue essentiellement des sciences naturelles et historiques. Les derniers ont un objet donné d'avance et indépendent de la volonté et de l'action du savant.”1
expresses impressively the capability of a synthetic chemist to create and design molecules with novel molecular structures and, in consequence, novel properties and features.2,3 Because of the creative passion of synthetic chemists, chemistry has been assigned various enabling roles and several of its sister disciplines have grown “chemical” branches such as chemical genetics.2‐4
As result from the melting of organic chemistry with cell biology, Timothy J. Mitchison and Stuart L. Schreiber described for the first time the essential elements of the interdisciplinary chemical genetics approach at the end of the last century.4,5 The young discipline took up the cause of clarifying biological pathways, or rather the functions of genes and gene products, with the aid of small‐molecule probes (molecular weight < 900 g mol‐1)6.4,5,7‐11
Figure 1. Reverse chemical genetics ‐ gene or gene product (e.g. DNA polymerase) to phenotype.
Figure was adapted from literature 9.
In classical genetics, biological processes are elucidated in living cells by the induction of dysfunctions, and analysis of the resulting phenotypes of interest. This can be achieved by manipulation of the genetic information, antibodies, or RNA interference, for example.8‐11 Traditional forward genetics operates from phenotype to genotype and reverse genetics the other way round.8‐11 However, chemical genetics differs fundamentally from the classical methods and has several advantages:4,5,7‐11
Small‐molecules probes show on cells a fast and strong effect.
The biological effect is due to cell metabolism mostly reversible. So, the dynamic analysis of protein regulation and functions are amenable.
The effect can be influenced by small‐molecule concentrations, and hence, different phenotype characteristics can be achieved.
The phenotype can be studied in the organism at any time of its physiological development stage. For example, gene knockouts cannot be examined in adult organisms, when they are lethal to the embryonic stage.
Small‐molecules can differentiate between proteins, coded on the same gene.
And for conserved targets, one and the same small‐molecule probe can be used in various organisms and systems.
As in classical genetics, chemical genetics can be subdivided into two approaches ‐ the
“forward” and the “reverse” approach.4,5,7‐11 Within forward chemical genetics, phenotype inducing small‐molecules are screened on cells up to multicellular organisms. Identified compounds ‐ that induce a phenotype of interest ‐ are selected and the biological targets must be examined in later phases of the study.4,5,7‐11 On the other hand, a selected biological target is screened in the “reverse” approach (Figure 1). Afterwards, the specificity of the found probe is investigated, and lastly, phenotypes are examined in a cellular context.4,5,7‐11 In order to develop powerful molecular probes for dissecting biological processes, diverse small‐molecule libraries are created12‐14 and screened under great technical and scientific effort.4,7‐11 The compounds are natural products (from plants, animals or micro‐organisms) or from synthetic origin.4,8‐11 In both approaches, the effort and complexity of the screening‐
system can vary greatly depending on the type of bio‐assay and method to determine the phenotype.4,7‐11 Due to the fact that chemical genetics is closely related to pharmaceutical research, the discovered molecular tools not only might be of great value for basic science but also may open up novel avenues for the treatment of diseases.2,4,5,7‐13
The main part of the work presented herein is based on a reverse chemical genetics approach. Therefore, DNA polymerases were selected as molecular targets to further elucidate their respective cellular functions (Figure 1).
2) DNA polymerases 2.1) General
The survival and development of each organism relies on the equal distribution of its genome during cell division. Errors in this process can lead to severe developmental defects, cancer, or even death.15‐17 DNA polymerases are key enzymes to pass the exact genomic information down generations. Over 50 years ago, Kornberg et al. discovered in E. coli the first enzyme (DNA polymerase I or Kornberg‐Polymerase), that catalyses the accurate replication of DNA.18,19 Since this and other pioneering discoveries, it was assumed for a long period of time, that only six “classical” DNA polymerases (pol α, β, γ, δ, ε, and terminal deoxynucleotidyl transferase (TdT)) are responsible for DNA replication and repair in all mammalian cells.15,17,20,21
For that reasons, the discovery of several “novel” specialized DNA polymerases was a real sensation in the last decades.15,20,22 So far, at least 15 different human DNA polymerases are known.15,17,20,22
All enzymes share a common 3D‐structure, that is reminiscent of a right hand, and can be subdivided into a finger‐, thumb‐ and a highly conserved palm domain (see also Figure 4).17,20,23,24 With regard to their sequence homology and structural similarity, the 15 enzymes have been subdivided into six DNA polymerase families A, B, C, D, X, and Y (Table 1).20,25,26
The basic functions of the six “classical” DNA polymerases have been elucidated from catalytic properties, and observation of cell physiology. Pol α catalyses the initiation of chromosomal DNA replication at origins of replication and at Okazaki fragments on the lagging‐strand,27,28 pol β is involved in base excision repair (BER),29‐31 pol γ synthesizes mitochondrial DNA,32 pol δ has a role in lagging‐strand synthesis,33,34 pol ε participates in the synthesis of the leading‐strand of chromosomal DNA,35 and TdT facilitates antigen receptor diversity36.15,20
In the course of these and other cellular processes or by environmental conditions, DNA mutations and damages occur.16 To maintain the genetic integrity of the genome, an elaborate set of sophisticated repair mechanisms have evolved. The set includes, amongst others, the “novel” specialized DNA polymerases (pol η, θ, κ, λ, μ, ν, ι, ζ, and REV1).15‐17,20,37
Features of some of these enzymes are known (Table 1), but to understand in depth the task of a particular enzyme stills await clarification in the majority of the cases. For that reason, there is a great demand for appropriate methods and reagents (e.g. small‐molecule probes) to dissect the cellular functions of DNA polymerases.
Table 1. Human DNA polymerasesa
DNA polymerase Gene Protein size (kDa) Family Main Function
pol α (alpha) POLA1 166 B DNA replication priming
pol β (beta)b POLB 38 X DNA repair
pol γ (gamma) POLG1 140 A Mitochondrial DNA replication and repair pol δ (delta) POLD1 124 B DNA replication (lagging‐strand)
pol ε (epsilon) POLE 262 B DNA replication (leading‐strand) TdT DNTT 58 X DNA repair, V(D)J recombination
pol η (eta) POLH 78 Y Bypass synthesis (inserter)
pol ι (iota) POLI 80 Y Bypass synthesis (inserter)
pol κ (kappa) POLK 99 Y Bypass synthesis (inserter/extender) pol λ (lambda)b POLL 63 X DNA repair, V(D)J recombination
pol μ (mu) POLM 55 X DNA repair, V(D)J recombination
pol θ (theta) POLQ 290 A DNA repair
pol ζ (zeta) POLZ 353 B Bypass synthesis (extender)
REV 1 REV1 138 Y Bypass synthesis (inserter)
pol ν (nu) POLN 100 A DNA repair
a table was adapted from literature 15,17,20.
b see also chapter 2.3)
2.2) DNA polymerases and the polymerisation reaction
All previously discovered DNA polymerases share a common, over several steps well‐
coordinated reaction mechanism (Figure 2) for the synthesis of the helical DNA polymer (composed of a sugar phosphate backbone, to which the four heterocyclic bases adenine (A), guanine (G), thymine (T), and cytosine (C) are attached38) (Figure 3).38‐42
E:DNAn
E
E:DNAn:dNTP
E*:DNAn:dNTP
E*:DNAn+1:PPi
E:DNAn+1:PPi E:DNAn+1
DNAn+1
PPi dNTP
DNAn DNA binding
dNTP binding
conformational change
conformational change
pyrophosphyl transfer
hydrolyosis enzyme
dissociation
processive DNA synthesis 1
2
3
4
5 6
8
7 kpol
2 Pi pyrophosphate
release
Figure 2. Mechanism of DNA polymerase catalyzed nucleotide incorporation. After binding of a DNA primer template complex (DNAn), the DNA polymerase (E) binds an incoming dNTP that is afterwards tightly bound and arranged for the chemical step by a conformational change of the enzyme (E*). After bond formation and an other conformational change, pyrophosphate (PPi) is released to start another cycle of catalysis. Figure was adapted from literature 41‐43.
N N N
N NH2 O
O O
PO2-
O
O O
PO2- O
O O
PO2-
O
O O
PO2- N N N
N O
NH N N
HN
O
N N
O
O
N N N H N N
O O
O PO2-
O OH
O O
OH
O N
N N N O
NH
N N HN
O H
H
H H
H H
H H
3`‐end
3`‐end 5`‐end
5`‐end P OP OP O-
O O- O
O- O
O-
template
primer A
A C
C G
G
T
synthesis direction
incoming dNTP
-O2P O
O O
O
HO O
3`‐end 5`‐end
P primer
C
G
incoming dNTP O O O
P P O
-O O O- O- O Mg2+
Mg2+
O H
H O- O
O- O
O- O DNA polymerase
A B
Figure 3. Enzymatic DNA polymerization. (A) Schematic representation of template directed DNA synthesis catalyzed by a DNA polymerase. (B) Schematic representation of the corresponding trigonal bipyramidal transition state in the active site of a DNA polymerase (template not shown). Figure was adapted from literature 24,44.
DNA replication proceeds semiconservative. In doing so, DNA polymerases use one DNA parent strand as a template for synthesis of the exact complementary replica.18,20,45 During the synthesis, the single‐stranded template dictates to the enzyme according to
Watson‐Crick38, in which sequence the four native 2´‐deoxynucleoside‐5`‐triphosphates (dNTPs) have to be connected to the 3`‐OH end of the hybridized primer. Thereby, the primer is always extended in the 5` to 3` direction (Figure 3A).20,42
From a chemical point of view, the addition of a dNTP to the primer is performed according to the mechanism of nucleophilic substitution (SN2). In the active site of the enzyme, the trigonal bipyramidal transition state is stabilized by two metal ions, e.g. magnesium (II) ions.
For that reason, the reaction mechanism is also called the “two metal ion mechanism”
(Figure 3B).24 The SN2 reaction is accompanied by pyrophosphate release. Its subsequent hydrolysis favors DNA synthesis and prevents the reverse reaction. According to the latest findings, the rate limiting step of the reaction is a local reorganization step in the active site of the enzyme.46 Nevertheless, the velocity of the polymerisation (kpol) of correct paired nucleotides achieves 1000 dNTP s‐1, and the catalytic efficiency (Kd kpol‐1) is in the region of diffusion control (~107 M‐1 s‐1).41,42 Due to this facts, DNA polymerases belong to most powerful enzymes.47
2.3) DNA polymerase λ and β
Most of the work presented herein covers the “classical” pol β31,32,48 and the recently discovered “novel” pol λ49. Both nonreplicative human enzymes are members of the DNA polymerase X‐family (Table 1).15,17,20,22,37
The exonuclease‐deficient pol λ (64 kDa) contains all the structural features required for DNA binding, nucleotide binding and selection, and catalysis of DNA polymerization, which are conserved in pol β (39 kDa) ‐ the smallest known human DNA polymerase (Table 1). On this account, the primary sequence and the 3D‐
structure of the catalytic core of both DNA polymerase are highly homologous (Figure 4).49,50 Because of its ability to remove the 5`‐deoxyribose phosphate (dRP) generated after incision by an abasic (AP) endonuclease (dRP‐lyase activity) and its DNA synthesis specificity for short gaps, pol β is the prime DNA polymerase participating in BER (Figure 5).15,17,20,30,37,51
In addition, pol β is able to associate with other downstream enzymes of the BER pathway like DNA ligase I, AP endonuclease, and XRCC1‐DNA ligase III.30,37 Extensive studies show that pol β bypasses several DNA lesions via translesion synthesis (TLS), for example, AP sites52 and cisplatin adducts53.
Figure 4. Family X DNA polymerase λ and β. (A) Schematic representation of pol β (red) and pol λ (green). Pol λ consists of a nuclear localization signal (NLS), a BRCA1‐C terminal (BRCT) domain (residues 36‐132), a proline‐
rich region (residues 133‐243), and a pol β‐like catalytic core region (residues 244‐575), with a helix‐hairpin‐
helix (HhH) and a DNA polymerase X motif.49 (B) Superimposition of the pol β‐like catalytic core region (residues 244‐575) of pol λ (green) and pol β (red). PDB IDs 2PFN and 2FMP (shown without DNA).
Pol λ, the other DNA polymerase of interest, is unique in possessing all the enzymatic activities which are individually present in the other X‐family members.20 Pol λ is capable of synthesizing DNA de novo as well as template‐dependent, and displays dRP‐lyase and TdT activity.54‐60 It is implicated that pol λ is involved in gap filling during nonhomologous end joining36,61,62, TLS52,63,64, and BER54,65,66.
Moreover, studies with chicken DT40 cells67, as well as mammalian fibroblasts68, showed that pol λ has a backup role for pol β in BER. Experiments with reactive oxygen species (ROS) indicate that pol λ protects cells from oxidative damage66,69, and there is also evidence that pol λ is required for cell cycle progression and is functionally connected to the S phase DNA damage response machinery in cancer cells.69
Figure 5. Schematic representation of BER imbalance by targeting pol β. BER is a highly coordinated, multistep pathway, that removes a damaged DNA base and replaces it with the correct base. The genotoxic TMZ induce the formation of a base damage (e.g. 3‐methyladenine (3meA)), which is excised by DNA glycosylase (AAG) to produce an apurinic site (AP). Afterwards, an AP endonuclease (APE) incises the DNA backbone (5´ to the AP site) and generates a single‐strand break. Then, pol β removes the 5´‐dRP moiety through its intrinsic lyase activity and fills in the resulting gap. In the final step, the nick is sealed by DNA ligase (LIG) to finish base excision repair (BER). If BER is inhibited, or downstream steps of BER are limiting, then toxic intermediates accumulate and can lead to cell death. In this way, the dosage of the DNA‐damaging agent can be reduced.
Figure was adapted from literature 16,70.
It is well known that aberrant levels of specialized DNA polymerases might cause genomic instability.15,20,71 A recent investigation of the expression patterns of specialized DNA polymerases in 68 different tumor samples revealed that in more than 45% of these tumors at least one specialized DNA polymerase was two‐fold‐enhanced expressed. Of particular interest was the fact that over 30% of all samples had either pol λ or β overexpressed.72 Consequently, the regulation of both DNA polymerases could be crucial in cancer treatment, since many chemotherapeutic regimes in use depend at least in part on the artificial induction of DNA damage. If effective, their utility is typically limited by the severity of side effects caused by the nonselective targeting of cancerous and healthy tissue in addition to the potential to induce mutagenic events that can actually accelerate disease
development.41,70,73 The clinical efficacy of anticancer drugs like cisplatin and monofunctional alkylating agents (e.g. temozolomid (TMZ)) is often reduced by cellular DNA repair mechanisms.15‐17,20,37,51,70
Consequently, both enzymes specialized for DNA repair are discussed as promising future drug targets, to reduce the dosage of DNA‐damaging agents while improving their activity via targeting of their respective repair pathways (Figure 5).15,17,20,37,51,70,74
To date, several inhibitors of DNA polymerase λ and β were developed and investigated (see also chapter 4.4.4).37,51,74‐79
However, one remaining challenge is still to find novel potent small molecule inhibitors that selectively inhibit one of these enzymes. In addition, a discriminating inhibitor could facilitate the targeting of one of these DNA polymerases over the other, to probe the enzymes’ respective cellular functions.
2.4) Herpes virus DNA polymerase
Viral infections are the leading cause of many critical illnesses. Without exception, all viruses are obligate, intracellular molecular parasites that replicate only inside the cell of a living organism.80,81 For the reproduction of the viral genome, viruses often encode an own DNA polymerase.20 Due to the importance of these enzymes for the amplification of the genetic code, viral DNA polymerases are established targets for current chemotherapies.41,81 These facts are also relevant for the family of the Herpes viruses (see also chapter 5.1), which reproduce their genetic code with their own DNA‐dependent DNA polymerase in the nucleus of a host cell. Among the nine representatives of the morphologically very similar Herpesviridae, herpes simplex virus‐1 (HSV‐1) is the most researched member.20 For that reason, the HSV‐1 DNA polymerase serves herein as model enzyme, which is illustrated briefly. After the HSV‐1 infection of mammalian cells, Keir et al. discovered in 1966 for the first time DNA polymerase activities that deviated from the features of host enzymes.82 Later, Weissbach et al. purified and characterized a viral DNA polymerase with a high molecular weight (180 kDa) from HSV‐1 infected human cells.83 It was found, that the enzyme favoured
‐ very likely due to the composition of the HSV genome (67% GC content)20 ‐ the synthesis of
GC rich DNA.83 Today, the 3D‐structur of the HSV‐1 DNA polymerase is known. The enzyme is a heterodimer, composed of a catalytic subunit (UL30), whose structure is similar to B‐family DNA polymerase structures, and a processivity factor (UL42) that increases the fidelity.84‐86 The catalytic subunit consists of six domains. The N‐terminal part contains a pre‐N‐terminal,
an N‐terminal, and an exonuclease domain, that ensures a high fidelity of DNA replication as well as an RNase H activity.20,87 The C‐terminal part adopts the usual right hand folding in palm, fingers and thump domains.85 Based on sequence alignments, the replicative HSV DNA polymerase (UL30) belongs to the B‐family of DNA polymerases (see also chapter 2.1).20
3) DNA polymerases as drug targets 3.1) General
As foreshadowed in previous chapters, DNA replication serves not only as a target for answering chemical genetic issues, but has been tried and tested for decades as prominent target for life‐saving medicines. Numerous pathological states, like cancer, autoimmune disease, and many bacterial and viral infections can be traced back to uncontrolled DNA metabolism.15‐17,20,37,51,70,79
For that reason, it is one of modern medicine's top priorities to combat those diseases with novel and innovative drugs. If one wants to specifically interfere in DNA metabolism, DNA polymerases of humans, animals, viruses, and bacteria are potential key drug targets, which are responsible for the correct synthesis and repair of the genetic code (see also chapter 2).15‐17,37,41,53,57,80
Due to the fact, that DNA polymerases share a similar 3D‐structure, and reaction mechanism to synthesise DNA, it is very challenging to address a drug molecule for a particular enzyme of interest (see also chapter 2.2). However, if the drug has a poor selectivity in its mode of action, the therapy is limited and inevitably associated with severe side effects. Current medicines employed to target the metabolism of DNA often induce DNA damage (see also Figure 5, and chapter 2.3),70 influence the dNTP pool, that is provided for DNA synthesis by the cell, or inhibit the enzymatic synthesis of DNA.41 For the last‐mentioned therapeutic strategy, which is relevant for this work, nucleoside analogs are a frequently used type of drug. Nucleosides, that are able to enter a cell (Figure 6, see also chapter 5.1), function as so called “prodrugs”, which have to be transformed by specific cellular nucleoside kinases to the actual enzyme substrate analogue;
the nucleoside‐5`‐triphosphates.20,41 Nucleoside‐5`‐triphosphates derivatives that are not/hardly able to enter cells cause often complications. In order to be effective, they have to compete with the natural dNTP substrate pool for the active site of the DNA polymerase target.20,41 On this account, the respective nucleosides must be administered in high
concentrations to be effective, which can result in turn in selectivity and drug resistance problems.20,41 An additional disadvantage is, that nucleotide analogues and nucleoside‐
5`‐triphosphates can also be utilized, modified, or degraded by a multiplicity of cellular pathways or other enzymes.13,15,17,20,37,42,70,74,79,81
Thus, in developing nucleoside analogues, it is necessary to investigate not only the optimisation of their interaction with DNA polymerases, but also their ability to be transformed to the 5`‐triphosphates without being degraded or showing serious side effects.20
Figure 6. Chemical structures of approved modified nucleoside analogues. (A) Current antiviral drugs (see also chapter 5.1). (B) Currant anticancer drugs. Figure was adapted from literature 20,41.
To address these problems and other issues, it is an ongoing concern to develop innovative therapeutic approaches and novel drugs.88‐91 In the following, some for this work relevant examples and perspectives are given.
One current option to open up the way for novel drugs and nucleoside analogues is, for example, to study certain structural features of various known molecules. Afterwards, the interesting features are fused together in one novel drug‐like molecule, bearing all desired properties (see also chapter 5.1).
Other research approaches proceed towards the development of non nucleosidic small‐
molecule inhibitors, which act on the activity of a respective DNA polymerase. Thereby, the
small‐molecule could act directly on the active site of the enzyme, induce conformational changes in further protein domains (e.g. finger or thumb domain) to act indirectly on the enzymes` activity, or block protein‐protein interactions that are important for the processivity.12,13,20,41,92‐94
In general, small‐molecule DNA polymerase inhibitors have important advantages over substrate analogues. They are ideally suited to tune for a high target selectivity, they are capable to enter a cell and do not require intercellular activation.13,20,41
Many chemotherapeutic regimes in use depend at least in part on the artificial induction of DNA damage. The clinical efficacy of anticancer drugs is often reduced by cellular DNA repair mechanisms.15‐17,20,37,51,70,78,79 Consequently, specialized DNA polymerases are discussed as promising future drug targets, to reduce the dosage of DNA‐damaging agents while improving their activity via inhibition of their respective DNA repair pathways (see also chapter 4).17,37,51,70,78,79
3.2) Screening methods for DNA polymerase inhibitors
In the preceding chapters it was described in detail, that there is a great demand for novel DNA polymerases inhibitors in basic as well as in the applied sciences. The discovery process of novel agents against a chosen protein target usually involves high‐throughput screening (HTS) or high‐content screening (HCS) techniques, wherein large compound libraries are screened for the desired effect (see also chapter 1).95 Apart from emerging in silico based methods20,96‐101, one needs for such large‐scale screening campaigns automated, robust, reliable, and cheap in vitro assays.95 DNA polymerase inhibitors generally exert their effects through interference with the enzyme and/or cofactors, or the direct interaction with
DNA.102 While there is no simple way to study DNA replication in vitro, the world‐renowned
polymerase chain reaction (PCR) is an ideally suited enzyme assay, which involves a similar set of DNA replication transactions.103‐105 Classic PCR and the respective electrophoresis techniques104,105 have been applied previously for the in vitro discovery and characterisation of several inhibitors of thermostable DNA polymerases.103,106‐110 However, to screen thousands of molecules, the modern real‐time PCR is considerably more favourable, and hundreds of in vitro reactions could be screened in parallel.111,112 Real‐time PCR instruments are able to monitor the concentration of the arising dsDNA products in multi‐well formats by
measuring for example the emission of sequence‐specific fluorescent oligonucleotide probes or fluorogenic dsDNA binders like SYBR® Green I.111
Because the common mammalian, viral, and bacterial target DNA polymerases are thermolabile, these proteins are not applicable for modern PCR and other screening methods are required that can be performed at moderate temperatures.
The primer extension reaction (PEX) is an effective and useful way to explore thermolabile and thermostable DNA polymerases in the laboratory.43,44,75,76,113‐119
To study the DNA template dependent DNA polymerization function, a short radioactive or fluorescence labeled DNA primer strand gets annealed to a longer DNA template strand, and gets elongated by enzymatic dNMP incorporation (Figure 7A). Noteworthy, to analyze TdT enzyme activities the reaction is performed without the template strand and is named single‐
stranded PEX.59,60,120 However, after a defined period of time, the PEX reactions are quenched and quantitatively analyzed via polyacrylamide gel electrophoresis (PAGE) (Figure 7A).
Figure 7. Generally applicable radioactive PEX assays to characterize and screen small‐molecule inhibitors of thermolabile DNA polymerases (A) Principle of the radioactive PEX assay to analyze the effect of an inhibitor on the DNA template‐dependent polymerisation function of a respective DNA polymerase.75,76 (B) Assay scheme for inhibitor screening via the scintillation method. DNA polymerase polymerizes isotopic phosphate
labelled dNMPs. The exact amount of formed radioactive DNA can be determined by scintillation measurements.
Another common technique to analyse the PEX reaction mixtures is the scintillation method.
Therefore, the reaction is performed with an unlabelled primer template‐complex and isotopic labelled dNTPs as enzyme substrates. After the reaction, the radioactivity of the novel synthesized DNA is quantitatively measured by scintillation (Figure 7B).120‐125
So far, both PEX methods were implemented for the screening and characterisation of inhibitors,75,76,100,120,124,126,127
but to screen huge compound libraries, PAGE analytics and accordingly the usage of radioactive reagents is not practicable. Therefore, novel automatable assay readouts and the respective tailor‐made reagents were evolved. As a first excellent example, Summerer et al. developed a Förster Resonance Energy Transfer (FRET)‐
based assay format that translates the proceeding DNA synthesis into a fluorescent signal in real‐time (Figure 8A).128,129 The fluorescence signal is generated by the DNA polymerase triggering opening of a molecular beacon, by extension of the primer strand.128,129 The resulting distance alteration is reported by FRET between two dyes introduced into the molecular beacon stem and enables the quantitative characterization of inhibitors.128,129 Recently, the elegant real‐time strategy was adopted from others130 and successfully utilised in a further developed fashion for screening campaigns of human DNA polymerases (Figure 8B).131‐133
Figure 8. Screening for inhibitors via real‐time FRET methods. (A) The template probe labelled with donor (grey) and acceptor (brown) has a hairpin extension in closed conformation before start of reaction. While extension proceeds, the DNA polymerase (blue) opens the stem and prevents re‐annealing by DNA duplex formation. The increase in the distance between the two labels is reported by restoration of donor emission.
Figure was adapted from literature 128,129. (B) Strand displacement DNA synthesis assay. DNA polymerase incorporates dNTP thereby extending the primer strand and displacing the downstream reporter strand labelled with a 3`‐fluorophore donor, leading to an increased fluorescence signal. Figure was adapted from literature 131‐133.
To access continuous FRET‐based PEX assays, some researchers focused on the enzymatic turnover of fluorescently labled dUTP substrates. For that reason, Cauchon et al. desined a primer‐template complex, that was labelled at the 5`‐template‐terminus with a donor fluorophore.134 Polymerisation mediated by incubation with a DNA polymerase, dNTPs, and an acceptor labeled dUTP juxtaposes the donor‐acceptor pairs, resulting in donor quenching (Figure 9A).134 On the other hand, Krebs et al. reported an FRET assay that quantifies the incorporation of complementary pairs of fluorescently labeled dUMP into the DNA product, and taking advantage, that the dye‐conjugated dNTP pairs in solution do not interact to produce a FRET signal (Figure 9B).135
Figure 9. Screening for inhibitors using fluorescently labled dUTP substrates. (A) The substrate is a short DNA/DNA primer/template. The template strand is labelled with a donor fluorophore (grey). Polymerization mediated by incubation with DNA polymerase (blue), dNTP, and an acceptor‐labelled dUTP (brown) juxtaposes the donor‐acceptor pairs, resulting in donor quenching. Figure was adapted from literature 134. (B) Dye‐
conjugated nucleotide pairs in solution do not interact to produce a FRET signal. DNA polymerase incorporates the dye‐nucleotides into the DNA. The close proximity of the two dyes in the polymer allows interaction between the dyes causing the generation of a FRET signal proportional to the amount of DNA produced in the sample. Figure was adapted from literature 135.
Other interesting examples amenable to automation are fluorometric PEX techniques that monitor the concentration of the arising dsDNA products.75,136‐138 The increase of the fluorescence signal caused by PicoGreen™136 or SYBR® Green I75,137,138 emitting upon binding to dsDNA was investigated as a fast readout for DNA polymerase activity (Figure 10). PEX resulted in high concentrations of dsDNA when the respective DNA polymerase was not inhibited.75,137,138 On the contrary, when the enzyme was inhibited, the primer was not extended and the fluorescence signal was low in relation to control reactions.75 Using that simple and economical method, diverse compound libraries were screened and potential
inhibitors of bacterial139 and human75 DNA polymerases were identified (Figure 10, see also chapter 4.1). Recently, Dallmann et al. extended the scope of this readout and established an in vitro assay for the parallel multiplicative target HTS against divergent bacterial replicases.140
Figure 10. Principle of the SYBR® Green I (stars) ‐ based HCS assay.75
Of note, a possible approach could also be the measurement of the arising pyrophosphates ions (PPi) that are produced by enzymatic dNTP consumtion (see also Figure 2). Reportedly, PPi release of DNA polymerases was investigated in real‐time for instance by different colorimetric and/or enzyme coupled assays.141‐143
In the recent years, DNA arrayed ultra‐HTS formats for PEX reactions were developed.
Interestingly, these systems are time and cost efficient, and require only minimal amounts of reagents. DNA arrays are based on the spatial separation of on a surfaces immobilized or covalently bound primers.20,53,119,144 The enzymes, templates, screening compounds, buffers, and natural as well as labelled (e.g. radioactive or fluorescent) dNTP substrates can also be applied with spatial separation to perform the PEX reactions.53,119,144 After the reaction, the surfaces are washed several times ‐ whereas the primers remain ‐ and can be analysed by phosphor imaging144 or fluorescents measurements53,119, (Figure 11). By employment of this time and cost efficient concepts, Boudsocq et al. could identify a variety of natural products that inhibit the BER enzyme pol β.53
Figure 11. Principle of DNA arrayed HTS format ‐ using fluorescently labelled dNTP substrates.
As one can see in this overview, many automatable techniques to screen DNA polymerase inhibitors are established and are ready for their application to screen the chemical space.
These methods have indeed the potential to discover novel interesting molecules that not only might be of great value for basic science but also may open up novel avenues for the treatment of diseases related to genome integrity.
Concepts and objectives
The survival and development of each organism relies on the equal distribution of its genome during cell division. DNA polymerases are key enzymes to pass the exact genomic information down generations.15,17,20,22
In the last decades several novel DNA polymerases were discovered, and so, at least 15 different human DNA polymerases are known today.15,17,20,22
Features of some of these enzymes are known, but to understand in depth the task of a particular enzyme stills await clarification in the majority of the cases. For that reason, there is a great demand for appropriate methods and molecular tools to dissect the respective biological functions of DNA polymerases.
The aim of this work (chapter 4) deals with the discovery and development of tailored small‐
molecule probes in order to gain insights into the functions of human pol λ and β. In previous surveys several small‐molecule inhibitors of pol λ and few moderate inhibitors of pol β were discovered.75,76 Importantly, the rhodanine‐based compounds were the most active inhibitor class and some of them could even discriminate between the highly homologous pol λ and β.75,76 Due to this facts, the rhodanine‐based small‐molecules were seen as an appropriate starting point for the development of molecular probes to specifically investigate the biological functions of pol λ and β.75,76 For this purpose, systematic synthetic optimization should be undertaken in order to further expand the chemical diversity and to find novel and more potent small‐molecule inhibitors of pol λ and β. The most promising inhibitors of the first and second generation should be further investigated enzymatically and should be evaluated in comparison to reference inhibitors. Out of the generated in virto data extensive SAR should be established and discussed in detail. With the aim to further develop the molecular probes in a cellular context, the probes should be explored on different human cell lines. Afterwards, the suited molecular probes should be investigated in proof‐of‐concept studies to specifically target the pol λ and β in their respective biological pathways.
In addition, DNA polymerases serve not only as a target for molecular probes to dissect their biological functions, but have been tried and tested for decades as prominent target for life‐
saving medicines.15,17,20,22,37,41
In the last decades, 4’‐C‐modified nucleoside analogues aroused scientific interest, because a couple of derivatives of this interesting compound class showed antiviral activity145‐153, even against multi‐drug resistant pathogens.146,154 Because