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Technical Briefs |
1 International Agency for Research on Cancer, Lyon, France;2 Asper Biotech Ltd., Tartu, Estonia;3 Institute of Molecular and Cell Biology, University of Tartu/Estonian Biocentre, Tartu, Estonia;4 Department of Head and Neck Surgery, Institut Gustave-Roussy, Villejuif, France;5 Institute of Carcinogenesis, Cancer Research Center, Moscow, Russia;
aaddress correspondence to this author at: International Agency for Research on Cancer, 150, Cours Albert Thomas, F-69372 Lyon Cedex 08, France; fax 33472738322, e-mail hainaut{at}iarc.fr
Mutations of TP53 (17p13.1; OMIM 191170; PubMed accession number X54156) are common in cancers and are typically missense within exons 4–9, impairing the capacity of p53 to transactivate genes involved in cell cycle arrest, apoptosis, and DNA repair (1). Functionally, mutations may differ according to their nature and position, as well as to the presence of a common polymorphism at codon 72 (arginine or a proline) in the mutant allele (2). Knowing TP53 mutation status has potential applications for cancer prognosis (3)(4) and early diagnosis (5), identification of mutagen "fingerprints" (1)(6), and prediction of therapeutic outcomes (7)(8). To achieve this purpose, sensitive, fast, and cost-effective methods are needed to assess the whole coding sequence plus exon/intron boundaries. Current approaches are based on mutation prescreening with single strand conformational polymorphism analysis, temporal temperature gradient electrophoresis, or denaturing HPLC (DHPLC) combined with direct sequencing of relevant PCR fragments [reviewed in Ref. (9)]. These methods are labor-intensive, difficult to standardize, and in some cases, of limited sensitivity. In recent years, 2 microarray methods for resequencing TP53 have been described: the Affymetrix p53 GeneChip array, described elsewhere (10)(11), and the Arrayed Primer Extension (APEX), based on incorporation of 4 dye terminators into oligonucleotide primers that each identify a base in the target sequence (12). In 2002, we described an APEX array for resequencing TP53 exons 2–9, which contain 95% of known mutations in TP53 (13). Here we compare the sensitivity and detection limits of APEX with a standard method, DHPLC/direct sequencing, and discuss the potential of APEX for application to cancer diagnostic or prognostic purposes.
Specimens in the comparison set included 6 cell lines with mutations in different TP53 exons (see Table 1 of the Data Supplement that accompanies the online version of this Technical Brief at http://www.clinchem.org/content/vol51/issue7/) and 60 tumor samples with TP53 mutations identified by DHPLC (exons 4–9) and direct sequencing (29 from paraffin-embedded lung cancers and 31 frozen specimens of oral cancers). The persons performing the APEX screening (exons 2–9) were blinded to mutation status. Mutations in these specimens were representative of the diversity of TP53 mutations in human cancers. Four specimens positive by DHPLC but with no mutation found by sequencing were also tested. DNA extractions, PCR, DHPLC, and sequencing were performed according to described protocols (see Tables 2 and 3 of the online Data Supplement) (14).
APEX probes were 25mer oligonucleotides with 12-carbon amino linkers at their 5' end covering the sense and antisense wild-type TP53 sequence (Genset) and were spotted on 24 x 60 mm SAL-type aminosilane/phenylene diisothiocyanate-coated microarray slides (15). PCR products were concentrated and purified by use of the Jet quick PCR purification Spin Kit (Genomed). Hybridization and primer extension reactions were performed as published previously (13). Data were analyzed with GenoramaTM 4.2 genotyping software, using clustered signal patterns from 20 wild-type TP53 DNA sequences as statistical reference. For each position, the software calculates the distance (difference) between the sample signal and the signal pattern of the wild-type reference cluster database [algorithm described by Tonisson et al. (13)]. The distance value is used as a measure for calling the given base. Zero distance indicates a perfect match between the given base and the wild-type reference. The default cutoff distance value of the software with cluster analysis is 30 but can be adapted by the user.
A representative analysis of a TP53 exon 7 mutation at codon 236 (TAC>TGC) is shown in Fig. 1
. To evaluate APEX detection limits, defined amounts of PCR products of DNA from 6 cell lines homo- or hemizygous for known mutations were mixed with wild-type PCR products and analyzed in parallel by DHPLC and APEX. An example of DHPLC and APEX detection limits is given in Fig. 1
of the online Data Supplement. With DHPLC, detection limits showed variations depending on mutation type and mutation sequence context (Table 1
). With APEX, all mutations were detected with detection limits of 3.125%–6.25%, consistent values reported previously for codons 273 (CGT>CAT) and 248 (CGG>TGG) (13). In comparison, direct sequencing does not detect most mutations in samples containing less than 25%–30% mutated DNA (16)(17). Of 66 TP53 mutations identified by DHPLC/sequencing in 60 tumors, APEX identified 62 but failed to detect 4 mutations (sensitivity, 94%; see Table 4 of the online Data Supplement). All mutations occurred between exons 4 and 9. Among substitutions, 49 of 52 (94.2%) were correctly identified by APEX. Two of the mutations that escaped APEX detection (codon 91, TGG>TGA and codon 174 AGG>TGG) corresponded to positions generating background signals in one DNA strand. The third one (codon 173 GTG>ATG) was identified on the antisense strand only because of inefficient hybridization at this position. APEX specificity and sensitivity may be further improved by experimentally establishing cutoff values for mutants at each position of the APEX array or by modifying probe design to minimize the formation of secondary structures interfering with DNA hybridization.
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The 14 other mutations detected by DHPLC/sequencing were deletions or insertions. APEX correctly identified 1 insertion (+ 1 bp; codons 62–63) and detected the base change at one extremity in 12 other cases (85.7%), including 4 insertions ranging from 1 to 8 bp, 7 deletions ranging from 1 to 4 bp, and 1 rearrangement at codon 184 that was not clearly identified as an insertion or deletion by sequencing. However, in these 12 cases, APEX did not allow complete identification of the change in DNA sequence. Overall, APEX had a sensitivity of 93% in detecting insertion/deletions, but only 7% in fully identifying their positions and sizes. These limitations are intrinsic to APEX design, which is based on hybridization of wild-type sequences to complementary, immobilized oligonucleotides (18). In theory, the presence of a deletion should lead to incorporation of an unexpected nucleotide at the 5' border of the deletion and corresponding to the first nucleotide flanking the 3' border of the deletion. Presence of an insertion should generate an unexpected signal corresponding to the first inserted base at the 5' border of the insertion. However, these events will be detectable only if the signals they generate differ from expected wild-type signals at these positions. Assuming equal distribution of insertions and substitutions at all base positions, there is a 1 in 4 chance for either strand that the border may be undetectable and a 1 in 16 chance that the sequence alteration escapes detection. However, in practice, the odds for nondetection are higher because insertions and deletions often occur within single- base repeats, making it likely that the modified signal may be identical to the expected one. The nonrandom distribution of insertions/deletions in TP53 may explain the relatively poor performance of APEX in mapping both borders of insertions/deletions. Despite these limitations, APEX compares well with the Affymetrix GeneChip p53 in detecting deletions and insertions (11)(17)(19)(20). Indeed, the latter does not allow detection of sequence alterations unless representative mutant oligonucleotides have been spotted on the array (10)(11).
Of the 4 specimens positive by DHPLC but negative by sequencing, APEX detected base changes in 3, including 1 specimen with 2 mutations (a 1-bp substitution in intron 5 and a silent mutation at codon 147; see Table 4 of the online Data Supplement). Finally, in 5 specimens, APEX generated several signals (2 or 3 per specimen), only one of which was confirmed by sequencing (see Table 4 of the online Data Supplement). The unconfirmed signals by DHPLC/sequencing were detected only on 1 DNA strand by APEX, suggestive of an insertion or deletion as discussed above. Thus, none of these discordant changes were single-base substitutions. We interpreted the discordant signals as "false-positive" APEX signals, which gave an overall positive predictive value for APEX of 92.5% (62 of 67).
We found that both fresh-frozen and formalin-fixed, paraffin-embedded tissues are suitable for mutation detection by APEX. The positive predictive values for APEX were 92% (33 of 36) and 93.5% (29 of 31) for frozen and paraffin-embedded tissues, respectively. Thus, the use of paraffin archives did not lead to a higher number of ambiguous or artifactual signals.
The second base of codon 72 harbors a common polymorphism in TP53 that affects a BstUI digestion site. The concordance between APEX and either restriction digestion or DHPLC/sequencing data was perfect for all 64 specimens analyzed (see Fig. 2 of the online Data Supplement). This polymorphism may play a role in cancer susceptibility (21)(22) and response to therapy (2). Recently, Bonafe et al. (23) reported that preferential loss of the codon 72P allele in breast tumors of heterozygous patients was associated with a significant decrease in disease-free and overall survival. Additional studies have shown an improved response to chemo-radiotherapy and a longer overall survival in patients whose squamous cell carcinomas tumor retained a wild-type 72R allele rather than a wild-type 72P allele (24). Thus, genotyping of TP53 at codon 72, combined with mutation detection, may be relevant for the therapeutic management of cancer. APEX also genotyped 2 other, less frequent polymorphisms at codons 213 (CGA/CGG) and 36 (CCG/CCA) for which concordance between DHPLC and APEX was also perfect.
In conclusion, APEX offers a flexible, sensitive, and low-cost resequencing alternative for large-scale studies involving retrospective analysis of pathology collections as well as for application to studies for which fresh-frozen materials are available.
Acknowledgments
Florence Le Calvez was supported by Special Training Awards from the International Agency for Research on Cancer; Neeme Tonisson was supported as Junior Fellow by Wellcome Trust International Senior Research Grant for Central Europe no. 070191/Z/03/Z and Andres Metspalu by targeted funding from EMER (no. 0518). This research project was supported by European Community (EC) FP6 funding. This publication reflects the authors’ views and not necessarily those of the EC. The Community is not liable for any use that may be made of the information contained herein.
References
The following articles in journals at HighWire Press have cited this article:
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  E. Gormally, P. Vineis, G. Matullo, F. Veglia, E. Caboux, E. Le Roux, M. Peluso, S. Garte, S. Guarrera, A. Munnia, et al. TP53 and KRAS2 Mutations in Plasma DNA of Healthy Subjects and Subsequent Cancer Occurrence: A Prospective Study. Cancer Res., July 1, 2006; 66(13): 6871 - 6876. [Abstract] [Full Text] [PDF]
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