Four-color, enzyme-free interrogation of DNA sequences with chemically activated, 3′-fluorophore-labeled nucleotides

Article abstract of DOI:10.1002/anie.200600804

(Figure Presented) Color reveals base: The enzymatic template-directed elongation of primers with fluorophore-labeled nucleoside triphosphates is the most frequently used reaction for determining DNA sequences. Primer extension followed by optical read out can now be achieved without enzymes.

Full text of DOI:10.1002/anie.200600804

Communications
biotechnology. A recent review on DNA sequencing tech-
nologies states: “Current sequencing technologies are too
expensive, labor intensive, and time consuming for broad
application in human sequence variation studies”.[1] The
vision of the “$1000 human genome” calls for significant
improvements over existing sequencing and genotyping
techniques.[2] The benchmark method for interrogating
DNA is Sanger sequencing[3] with fluorophore-labeled
dideoxynucleoside triphosphates[4] and capillary electropho-
resis, monitored by a charge-coupled-device (CCD) camera.
It involves enzymes and substrates that are costly. Further-
more, it produces an oligonucleotide for every position to be
interrogated in reactions that must be run in individual
vessels. An entirely chemical approach to sequencing by
synthesis promises a less-expensive access to sequence
information, particularly if one was to perform a nonenzy-
matic primer extension in a microarray format.
To the best of our knowledge, no enzyme-free protocol for
sequencing by stepwise primer extension currently exists.
Nonenzymatic primer extension has been previously demon-
strated, however. Studies focused on prebiotic chemistry have
shown that RNA strands complementary to a template can be
formed in the absence of enzymes.[5] A number of chemically
activated nucleic acids have been shown to undergo chemical
primer extension,[6] and a number of related reactions have
been shown to occur in a templated fashion.[7] A chemical
primer-extension method for nonenzymatic genotyping of
single-nucleotide polymorphisms was recently reported.[8]
This method employs azaoxybenzotriazolides of 2’-deoxynu-
cleotides (OAt-dNMPs) as activated monomers and short
“helper oligonucleotides” to achieve primer conversion
within hours,[9] but requires mass spectrometry as a read-out
technique.
DNA Recognition
Herein we report that OAt-dNMP-driven primer exten-
sions can be carried out with fluorophore-labeled monomers.
The labeled nucleotides allow for optical read out.[10] The
focus of this study was on genotyping, that is, the extension of
primers by a single nucleotide. Syntheses for all four dNMPs,
each labeled with a different dye, are reported together with a
protocol for their activation. We also report the first activated
monomers in which the fluorophore is attached through a
photolabile linker.[10c] This allows it to be removed after the
optical read out through irradiation with UV light.
DOI: 10.1002/anie.200600804
Four-Color, Enzyme-Free Interrogation of DNA
Sequences with Chemically Activated, 3’-
Fluorophore-Labeled Nucleotides**
Niels Griesang, Kerstin Gießler, Tanja Lommel, and
Clemens Richert*
Our first efforts to prepare activated, fluorophore-labeled
mononucleotides for sequence determination led to an
imidazolide of thymidine 5’-monophosphate with a directly
linked carboxyfluorescein moiety in a lengthy, solution-phase
synthesis.[11] This imidazolide showed low reactivity in primer-
extension assays, prompting us to use azaoxybenzotriazo-
lides[8] for the current study. The current nucleotides (1a, c, g,
t, Scheme 1) also feature a hexaethyleneglycol linker to
minimize steric interference from the dyes. The fluorophores
were appended to the 3’-position to prevent undesired
multiple extension reactions. The ability to decorate the 3’-
position with a substituent is one of the advantages of
chemical primer extension. The active sites of polymerases,
on the other hand, put an a helix on the base pair between the
incoming dNTP and the templating nucleotide[12] leaving little
space for substituents or protecting groups at the 3’-position,
The technology for interrogating nucleic acid sequences
affects the progress of genetics, medical diagnostics, and
[*] Dr. N. Griesang, Dipl.-Chem. K. Gießler, T. Lommel, Prof. C. Richert
Institut für Organische Chemie
Universität Karlsruhe (TH)
76131 Karlsruhe (Germany)
Fax: (+49)721-608-4825
E-mail: cr@rrg.uka.de
Homepage: http://chip.chemie.uni-karlsruhe.de
[**] The authors thank U. Plutowski, C. Ahlborn, and S. Vogel for helpful
discussions. This work was supported by the DFG (grant No. 1063/
1-3) and the Fonds der Chemischen Industrie.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Scheme 1. Synthesis of labeled active esters of mononucleotides 1a, c, g, t. Conditions: a) Coupling cycle with 3, b) coupling cycle with 4a, c, g, t,
c) coupling cycle with 5, d) coupling cycle with phosphoramidites of cyanine dyes, e) NH4OH, f) HOAt, EDC·HCl; pH 5. Bz=benzoyl,
PG=protecting group.
which is the most obvious position for labeling and (rever-
sibly) blocking extensions.[13]
The synthesis of active esters 1a, c, g, t (Scheme 1) started
from controlled pore glass (cpg) loaded with a deoxynucleo-
side presenting a dimethoxytrityl(DMT)-protected hydroxy
group.[14] Four phosphoramidite coupling cycles, including
oxidation, capping, and detritylation, were run on a 2-mmol
scale on a DNA synthesizer using phosphorylating reagent
3,[15] 5’-phosphoramidites 4a, c, g, t,[16,17] hexaethyleneglycol
linker reagent 5,[18] and the phosphoramidite of one of the
four fluorophores chosen (Cy3, 3.5, 5, or 5.5). Nucleotides 7a,
c, g, t were obtained in 17–56% overall yields and were
Figure 1. Fluorescence spectra of 7a, c, g, t. Conditions: 7a (0.39 mm
characterized by NMR spectroscopy, MS, and UV/Vis spec-
in water), excitation wavelength: 585 nm; 7c (0.31 mm in water)
excitation wavelength: 545 nm; 7g (0.23 mm in water) excitation wave-
troscopy. The normalized fluorescence spectra are shown in
Figure 1.
Each of the labeled nucleotides were converted into its
length: 680 nm; 7t (0.18 mm in water), excitation wavelength: 643 nm.
If =fluorescence intensity.
active ester (1a, c, g, t). Best results were obtained with
aqueous buffers containing N’-(3-dimethylaminopropyl)-N-
ethylcarbodiimide hydrochloride (EDC)[19] and hydroxyaza-
benzotriazole.[8] Compounds 1a, c, g, t are (necessarily) labile
and evidence for their formation was obtained by MALDI
MS (see the Supporting Information). Solutions of 1a, c, g, t,
when diluted, were directly used for primer extension.
Primer 8, featuring a 3’-amino-2’,3’-dideoxynucleotide at
the 3’-terminus, was employed for assays (Scheme 2), as
amines are known to be reactive toward chemically activated
nucleotides and nucleic acids.[20] Assays used equimolar
mixtures of OAt-esters 1a, c, g, t (3.6 mm each), a buffer
containing
2-[4-(2-hydroxyethyl)-1-piperazinyl]ethanesul-
fonic acid (HEPES; 0.2m), NaCl (0.4m), and MgCl2 (0.08m)
at pH 8.9, and were performed at room temperature. Four
templates (9a, c, g, t), displaying any of the four nucleobases
at the site to be interrogated, were reacted with OAt-dNMPs
in the presence of downstream-binding helper oligonucleo-
tide 10, which helped to retain nucleotides at the reaction
site.[8,9] First, activated nucleotides without fluorophore labels
were reacted to establish the kinetics of primer extension for
the sequence motif chosen under conditions identical to those
used earlier.[8] Half-life times for primer conversion between
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their surface. After incubation and
washing, scans of the microarray surface
with four different filter settings and
appropriate corrections for the relative
fluorescence intensities of the cyanine
dyes allowed for unambiguous base
calls for each of the templates
(Figure 3). (The filter settings and cor-
rection factors had been established
separately in calibration experiments
with Cy-labeled oligonucleotides; see
the Supporting Information.) Further-
more, primer extension with the unre-
lated template sequence 5’-GGAAT-
CACACGTGCG-3’ and labeled mono-
mers 1a, c, g, t again showed high
selectivity (see the Supporting Informa-
tion).
Finally, we performed exploratory
assays to determine the potential for
reading more than one nucleotide in the
target strands. For this, 13 was prepared,
whose cyanine fluorophore is linked to
the
through
3’-amino-2’,3’-dideoxynucleotide
Scheme 2. Determining nucleotides in DNA templates through non-enzymatic primer extension with
fluorophore-labeled, activated nucleotides 1a, c, g, t. Oligodeoxynucleotide 10 serves as a “helper
oligonucleotide”,[8] generating additional stacking surfaces for the activated nucleotide. The nucleobase in
the template (B’) can be determined through fluorescence (Figure 3).
a
photolabile linker[10c]
(Scheme 3, see also the Supporting
Information). Thymidines, such as 13,
are the most difficult cases for chemical
primer extension, as their nucleobase
0.13 and 2.5 h and sequence-selective reactions were mea-
sured (see the Supporting Information). Next, 90 pmol of
each template was reacted with an equimolar mixture of
fluorescent nucleotides 1a, c, g, t (3.6 mm), and the formation
of extended, fluorophore-labeled primers 11a, c, g, t was
measured.
MALDI mass spectra confirmed that in each case the
correct nucleotide was incorporated opposite the templating
base with high fidelity (Figure 2). The same result was
obtained when the oligonucleotides were bound on micro-
arrays[21] featuring capture oligonucleotides (12) on spots of
Figure 3. Scans of a microarray with the capture oligonucleotide 12
after hybridization with products from primer extensions with tem-
plates 9a, 9c, 9g, or 9t. a) Overlay of false-color fluorescence images,
generated with the settings for each of the fluorophores (Cy3/3.5/5/
5.5) using filters, correction factors, and exposure times established in
calibration experiments (see the Supporting Information). Products
were analyzed in duplicate (two spots vertically above each other).
b) Integration of the fluorescence signal obtained in every channel for
each pair of spots; the small bars accompanying those for the correct
signals are believed to result from mismatch incorporation.
Figure 2. MALDI-TOF spectra of products of primer extension with
labeled nucleotides 1a, c, g, t at a 36 mm concentration of oligonucleo-
tides, after 14 h at room temperature; a) template 9g, b) template 9a,
c) template 9t, d) template 9c.
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Scheme 3. Enzyme-free primer extension with 13, whose fluorophore is linked through a photolabile linker. In a competitive reaction with
equimolar amounts of 16a, c, g and 13 (3.6 mm/18 mm), the residue of 13 is appended to primer 8 to give extended primer 14. The adenosine
base in template 9a was successfully identified through a fluorescence scan on a microarray (not shown). Subsequent irradiation of 14:9a with
UV light cleaves the photolabile linker, removing the Cy5 label and producing the amino-terminal primer 15, which is suitable for a subsequent
primer-extension reaction. Compare Figure 4 for mass spectra.
forms only two hydrogen bonds and the binding site at the
template offers little stacking surface for the pyrimidine. Even
for the unlabeled OAt-dNMPs, primer extension is, therefore,
an order of magnitude slower than for the strongly base
pairing nucleotides (see the Supporting Information). Despite
the odds, which were worsened by the lack of a helper
oligonucleotide in the “multiple-extension case,” 13 reacted
selectively with primer 8 to give 14 in the presence of
competitor nucleotides 16a, c, g (Scheme 3), as seen in
MALDI-TOF mass spectra (Figure 4) and fluorescence
images of a microarray read-out experiment. When the
oligonucleotide complex containing 14 was irradiated with
UV light (254 nm) from a simple, hand-held lamp for 5 min,
the linker with the fluorophore was removed, generating the
extended primer 15, which was suitable for a subsequent
interrogation of the next position in the template (Figure 4).
An exploratory assay with the deoxycytidine analogue of 13
showed that a dual primer extension is feasible (see the
Supporting Information). We are in the process of optimizing
conditions to ensure full primer conversion, even for weakly
base-pairing nucleotides, and developing a capping scheme
that ensures blocking of failure strands.
Together, these results suggest that chemical primer
extension with inexpensive, fluorescent substrates is suitable
for determining nucleotides in DNA sequences, including
read-out in a parallel fashion through microarrays. In
favorable cases, the colors of the fluorophores may be
detected with the naked eye, providing a simplified form of
the current assays (Scheme 2) that might serve as a “lacmus
test” for detecting single-nucleotide polymorphisms. The
more-sophisticated form involving microarrays and a chip
scanner might prove an inexpensive alternative to current
genotyping systems. Modifications in the helper[22] and
primer,[23] as well as adjustments in the nucleotide concen-
tration and activation chemistry might eventually lead to
assays that go to completion in minutes, rather than in hours.
Though far from competitive at the moment, assay formats
involving removable fluorophores might thus become suit-
able for reading short stretches of DNA. Photolabile linkers
have the advantage of enabling the use of light to control
when the next nucleotide is admitted. Unlike most chemical
deprotection agents, light does not induce the dissociation of
primer–template duplexes. We are currently working on
syntheses for nucleotides containing all four nucleobases (A/
C/G/T) with photolably linked fluorophores. Independent of
Figure 4. MALDI-TOF mass spectra of samples drawn from the
reaction shown in Scheme 3. Assay conditions: template (9a; 36 mm)
and primer (8); HEPES buffer solution (0.2m), NaCl (0.4m), and
MgCl2 (0.08m), pH 8.9, 26 h, RT. Please note that longer sequences
give smaller signals as a consequence of lower desorption/ionization
efficiencies.
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Communications
[14] The nucleoside-loaded cpg was chosen as an inexpensive source
of a DMT-protected, alcohol-displaying support for automated
DNA synthesis, even though the deoxynucleoside loaded onto
the support was, in itself, not required. The first nucleoside does
serve as a useful internal standard for HPLCanalyses of crudes,
however.
[15] T. Horn, M. S. Urdea, Tetrahedron Lett. 1986, 27, 4705 – 4708.
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[18] S. Altmann, A. M. Labhardt, D. Bur, C. Lehmann, W. Bann-
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further technical advances, the current results again demon-
strate that nucleic acids in themselves have a strong propen-
sity to undergo spontaneous replication reaction sequence
selectively.
Received: March 1, 2006
Published online: August 22, 2006
Keywords: DNA recognition · DNA replication ·
.
oligonucleotides · primer extension · single-nucleotide
polymorphism
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