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Commissariat à lEnergie Atomique, Département de Radiobiologie et Radiopathologie [M. A., S. C., C. L., S. B., J. P. R.] and UMR217 Centre National de la Recherche Scientifique/Commissariat à lEnergie Atomique [M. A., S. B., J. P. R.], BP6, F92265 Fontenay aux Roses, France; Centre National de Séquençage, F94000 Evry, France [G. G.]; Département dAnatomie et de Cytologie Pathologiques, Hôpital Cochin, F75014 Paris, France [A. V.]; Cytopathologie et Cytométrie Cliniques LRC/CEA #14, Institut Curie, F75248 Paris, France [J. K., P. V., A. K. E. N.]; and Service dOncologie Médicale, Hôtel Dieu, F95004 Paris, France [S. O.]
| ABSTRACT |
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| Introduction |
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Here we analyze a series of 99 clear cell kidney tumors and matched normal tissues for mutations in OGG1 and for LOH in the region of chromosome 3p harboring this gene. The variant alleles of the Ogg1 protein were expressed in Escherichia coli and analyzed for their enzymatic activities.
| Materials and Methods |
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LOH Analysis.
Mapping of the OGG1 gene was performed by amplification from
a hybrid radiation panel as described previously (14)
,
using the following OGG1 cDNA-specific oligonucleotides:
(a) 5'-GCTGTTCAGTGCCGACCT; and (b)
5'-AACATGAGACTGGGTGGGG. For LOH analysis, 0.2 µg of DNA from each
sample was subjected to PCR using primers directed against the
following chromosome 3p highly polymorphic microsatellite markers:
(a) AFM238wb12; (b) AFM295yc9; and (c)
AFM095xc1. The primer sequences and PCR conditions used were those
obtained from the Genome
Database.4
For each pair, one of the primers was labeled with Cy5, a fluorescent
dye. PCR products were separated on 6% denaturing PAGE. Gels were run
and analyzed on an ALFexpress automated sequencer (Amersham Pharmacia
Biotech). Homozygosity was classified as noninformative. LOH was
calculated by comparing the ratios between the peak areas of the two
alleles in the tumor with that in the corresponding normal tissue.
Differences larger than 30% were considered to be a LOH.
Mutation Analysis.
OGG1 mutations were screened by DGGE analysis of
RT-PCR-amplified materials as described previously (15)
.
Samples with an altered DGGE profile were then sequenced. Study of the
OGG1 cDNA melting domain profile showed that five independent RT-PCRs
were mandatory to analyze nucleotides 309-1250. The 3'-end of the cDNA
sequence was analyzed systematically by sequencing because alternative
splicing occurs within this region. For DGGE analysis, a psoralen clamp
was attached at the 5'-end of one of each of the primer pairs, whose
sequences are given in Table 1
. Each RT-PCR was carried out in a final volume of 100 µl using
one-fiftieth of cDNA template obtained from 1 µg of total RNA mixed
with 50 pmol of each forward and reverse primer in 10x PCR buffer
containing 27 mM MgCl2.
After denaturing for 2 min at 94°C, 32 cycles consisting of 50 s
at 94°C, 50 s at the appropriate annealing temperatures (Table 1)
, and 20 s at 72°C were performed using an automatic DNA
thermal cycle (MJ Research). The last cycle was followed by an
additional 10 min at 72°C to complete all of the products, and a
10-µl aliquot was electrophoresed on a 2% agarose gel in 1x
Tris-borate EDTA and examined by ethidium bromide staining to
confirm the presence of the appropriately sized product. Heteroduplex
and homoduplex formations were performed by incubating each PCR product
for 5 min at 94°C and 45 min at 55°C, and stabilization of the
duplexes was obtained by UV irradiation at 365 nm for 20 min, which
provokes a covalent link at the 5'-end of both strands of DNA due to
the presence of the photoactivatable intercalating psoralen group.
Denaturant PAGE of the stabilized duplexes (30 µl) was carried out at
60°C using a thermostated gel apparatus (Prolabo, Paris, France).
Gels contained 7% acrylamide in 1x TEA buffer [40
mM Tris-acetate, 1 mM EDTA
(pH 8.0)] with linearly increasing gradients of the denaturants urea
and formamide (100% denaturant corresponds to 40% formamide and 7
M urea). The respective gradients and running
conditions for each DGGE are given in Table 1
. PCR products of
wild-type and known mutants, obtained using point mutated
oligonucleotides, were systematically included as controls with each
set of test samples. After electrophoresis, the gels were stained with
ethidium bromide and photographed under UV illumination at 354 nm. Each
sample showing an abnormal electrophoresis profile, as compared with
the wild-type migration, was subjected to an independent RT-PCR
amplification using the appropriate primers without the psoralen clamp
and purified by centrifugation through centricon-100 columns (Amicon).
Sequencing reactions were carried out on both strands using the
sequencing kit (Perkin-Elmer) on an ABI prism (Perkin-Elmer). For
analysis of exons 6 and 7, the genomic region was PCR amplified using
the oligonucleotides 5'-GGGCTGGGATGGTAGAAAC and 5'-CTGCTTGATTTGGGGGTG.
PCR reactions were carried out in 50 µl containing 0.2 µg of
genomic DNA, 50 pmol of each primer, 1.7 mM
MgCl2, 0.1 mM each
deoxynucleotide triphosphate, 0.25 unit of Taq DNA polymerase (Roche),
and 1x PCR buffer containing 17 mM
MgCl2. The reaction mixture was denatured for 3 min at
94°C and incubated for 35 cycles of denaturing for 50 s at
94°C, annealing for 50 s at 59°C, and extension for 30 s
at 72°C. The final extension was continued for 5 min at 72°C. The
fragment obtained was purified and sequenced as described above.
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The enzymatic activity of the purified proteins was assayed by monitoring their capacity to cleave a 34-mer duplex oligonucleotide harboring an 8-OxoG:C bp (16) . In each reaction (10 µl, final volume), 50 fmol of 32P-labeled 8-OxoG:C duplex were incubated in 25 mM Tris-HCl (pH 7.6), 2 mM Na2EDTA, and 50 mM NaCl with 14 ng of each of the hOgg1 proteins for 15 min at 37°C. Reactions were stopped by adding 6 µl of formamide dye and subjected to 7 M urea-20% PAGE. Gels were scanned and quantified using a PhosphorImager (Molecular Dynamics).
| Results |
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DNA from both tumor and normal kidney tissues was obtained from 104 patients, and the allelic balance for the three polymorphic markers mentioned above was analyzed. Of the 99 patients, patients 82, 77, and 73 were heterozygous in the normal tissue for markers AFM238wb12, AFM295yc9, and AFM095xc1, respectively. The flanking markers AFM238wb12 and AFM095xc1 showed LOH in 65 of 82 (79%) and 59 of 73 (81%) informative cases. LOH for the central marker, the marker closest to OGG1, was observed in 83% (64 of 77) of the informative cases. Among the noninformative samples for this central marker, 12 cases showed loss of the two flanking markers. Based on the above results, we estimate the LOH for the region harboring OGG1 to occur in 76 of 89 informative samples (85%). Considering the location of the markers used, these data imply a concomitant loss of one VHL allele in the tumor samples.
OGG1 Alterations.
Two complementary strategies were used to screen for mutations in the
coding sequences of OGG1. In the first strategy, total RNA
was extracted from the tissue samples (99 cases) and used as substrates
for cDNA synthesis. The cDNAs were analyzed by PCR-DGGE. In the cases
where an abnormal migration pattern was observed, an independent PCR
was performed, and the product was sequenced. Because of the presence
of alternatively spliced forms of OGG1 mRNA (reviewed in
Ref. 17
), multiple fragments were generated during the
amplification of the sequences spanning exons 6 and 7, thus interfering
with the interpretation of the results. To circumvent this problem,
genomic DNA was isolated, and the region spanning exons 6 and 7 was
amplified by PCR and analyzed for the presence of mutations by
sequencing both strands of the amplification products. The combination
of these approaches led to the identification of variant forms of
Ogg1 cDNA other than the frequent polymorphism at amino acid
326. Indeed, 22 tumor samples displayed altered forms of
OGG1 cDNA. However, in eight cases, a new RT-PCR failed to
detect the alteration. One explanation resides in the possibility of
mutations being present in a small fraction of the tumor tissue.
Analysis of the OGG1 transcript in the normal tissue was
performed in all cases. Five of the 14 confirmed sequence changes were
also present in the normal tissue and corresponded to
polymorphisms. Table 2
displays the polymorphisms of the OGG1 gene found in this
series. For polymorphism at position 326, the allelic frequency of the
Cys form is 25%, which is not statistically different from the
frequencies reported in other pathologies or in a normal population
(18, 19, 20)
. There is no preferential loss of one of the
alleles in the tumors. The amino acid changes at positions 85 and 308
had already been reported in lung and head and neck tumors,
respectively (19
, 21)
. Reconstitution of the mutant
proteins and their expression in bacteria show that the changes coded
by these polymorphic alleles do not significantly affect the enzymatic
activity of the Ogg1 glycosylase (data not shown; Ref.
19
). Two new polymorphisms at codons 220 and 323 were
found in this series. Neither of them produces changes at the amino
acid level.
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Characterization of the Mutant OGG1 Proteins.
To assess the functional status of the proteins coded by the mutant
forms of the OGG1 gene found in the tumor samples,
site-directed mutagenesis was performed on a plasmid coding for a
glutathione S-transferase-OGG1 fusion (16)
. The
proteins carrying each of the four different amino acid substitutions
were subsequently expressed in E. coli and purified. Their
8-OxoG DNA glycosylase activity was tested by determining their
capacity to cleave an oligonucleotide harboring a unique 8-OxoG
residue. Fig. 1
shows the results of a representative experiment. Using this assay, the
mutants in codons 12, 169, and 232 show no difference in their
enzymatic activity when compared with the wild-type form. However, in
the case of the R46Q mutant, the DNA glycosylase/AP lyase activity is
drastically reduced. Its specific activity is 4-fold lower than that of
the corresponding wild-type version, suggesting a strong
impairment in its DNA repair capacity.
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| Discussion |
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The substitution in codon 46 corresponds to an amino acid conserved in all of the eukaryotic Ogg1, from yeast to human. The amino acid change shows a marked effect by impairing the in vitro enzymatic activity of the mutant OGG1 protein when expressed in E. coli. For the tumor in which this mutation was detected, homozygosity prevented LOH analysis of the region harboring the OGG1 gene. On the DGGE analysis of the tumor cDNA, a wild-type band was detected, but it could be attributed to contamination with normal host cells, which presented only the wild-type form of OGG1. Interestingly, the same base substitution was reported in the small cell lung carcinoma cell line NCI-H526 (18) , suggesting that such a mutation could be associated with the carcinogenic process. Recent elucidation of the crystal structure of hOGG1 shows that residue 46 is located in ß-sheet B, not far from Gly-42, which makes direct contact with 8-OxoG (23) .
Mutations in codons 12 and 232 affect nonconserved amino acids. Consistent with this is the lack of effect on the in vitro 8-OxoG DNA glycosylase activity of the mutant proteins. Residue 232 maps to the surface of the protein, whereas the position for residue 12 has not been mapped (23) . However, this mutation potentially alters the proposed mitochondrial localization sequence. In that case, substitution at position 12 would affect the level of DNA repair activity in mitochondria. As for the amino acid at position 169, it is conserved in the mammalian OGG1 genes, but not in the yeast gene. The substitution of arginine for a glutamine found at this position in case R48 does not affect the in vitro enzymatic activity but could interfere with the interaction between OGG1 and other proteins involved in the repair mechanism. Alternatively, the amino acid changes described could have consequences on the stability of the proteins in the cell. A more detailed biochemical characterization of these mutant forms is necessary to evaluate their potential effect on the DNA repair capacity of the cells harboring them.
In this study, several polymorphisms were detected in the OGG1 gene. The amino acid substitution at codon 326 was found at the same frequency reported previously for other types of cancers and in normal individuals. No specific bias was found in the loss of either allele in the RCC cases studied. Two rare polymorphisms leading to amino acid changes at positions 85 and 308 were also detected. These polymorphisms had previously been reported for lung and head and neck cancers, respectively (19 , 21) . In the in vitro assay, the proteins coded by these variants are functional 8-OxoG DNA glycosylases.
In the analysis of the OGG1 mRNA from the tumor samples, several deletion or insertion modifications were detected. Their location, together with the lack of DNA sequence changes in the OGG1 gene, indicates that they are the result of aberrant splicing of the primary transcript. Four of the six cases involved deletions starting at the 5'-end of exon 5. In all cases, the protein encoded by those messengers is likely be inactive. Because several alternatively spliced forms of OGG1 have been reported in cell lines and normal tissues, it remains to be determined whether the alternative spliced forms described here are characteristic of the tumor tissue. Such a situation may imply a defect in the RNA processing machinery specific to the tumor cells.
The dissociation between the high frequency of LOH and the inactivation of the OGG1 gene suggests only a minor impact of alterations in the OGG1 gene on the genesis of renal tumors. Similar results were found for gastric, lung, and head and neck cancers (18 , 19 , 24 , 25) . However, several possibilities for its involvement in the carcinogenic process remain to be investigated. Because there is loss of one of the alleles in 85% of the RCC cases studied, a dosage effect on the 8-OxoG repair cannot be ruled out. This is underscored by recent studies showing that tumors having lost one OGG1 allele have twice as much 8-OxoG in their DNA (20) . This could lead to a weak mutator phenotype. Alternatively, gene silencing could be totally or partially turning off the remaining OGG1 allele through methylation of the promoter CpG island (26) . Finally, because of the finding that some of the mutations detected in the original DGGE screen could not be confirmed by a second RT-PCR, the possibility of ongoing mutations having an effect on the progression of the tumor should be considered.
In conclusion, our analysis of 99 cases of renal cancers has identified four somatic mutations in OGG1, along with several polymorphisms. It is important to note that similar studies for other types of cancer failed to identify any mutation in primary tumors. One of the mutations clearly impairs the enzymatic activity of the gene product, whereas the other mutations could have more subtle effects on the DNA repair mechanism, possibly leading to different levels of mutator phenotypes that could contribute to tumorigenesis.
| ACKNOWLEDGMENTS |
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| FOOTNOTES |
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1 Supported by funding from the Commissariat
à lEnergie Atomique, the Centre National de la Recherche
Scientifique, Electricité de France, and the Association pour la
Recherche sur le Cancer. ![]()
2 To whom requests for reprints should be
addressed, at Département de Radiobiologie et Radiopathologie,
Commissariat à lEnergie Atomique, 60 avenue du
Général Leclerc, F92265 Fontenay aux Roses, France. Phone:
33-1-46-54-88-57; Fax: 33-1-46-54-88-59; E-mail: jpradicella{at}cea.fr ![]()
3 The abbreviations used are: TSG, tumor
suppressor gene, 8-OxoG, 7,8-dihydro-8-oxoguanine; LOH, loss of
heterozygosity; RCC, renal cell carcinoma; RT-PCR, reverse
transcription-PCR; AP, abasic site; DGGE, denaturing gradient gel
electrophoresis. ![]()
Received 3/21/00. Accepted 7/19/00.
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