Mutagenesis, Vol. 17, No. 4, 279-280,
July 2002
© 2002 UK Environmental Mutagen Society/Oxford University Press
DISCUSSION FORUM |
Smoking, lung cancers and their TP53 mutations
Male Urological Cancer Research Centre, Institute of Cancer Research, 15 Cotswold Road, Sutton, Surrey SM2 5NG, UK
The TP53 suppressor gene possesses several properties that have facilitated its use as a reporter of genotoxic exposure (Biggs et al., 1993
). In particular, it is mutated in a high proportion of many types of human cancer and it sustains a broad spectrum of mutations that can vary in both their position and type. In lung tumours from smokers, the high frequency of G
T transversions (30% compared with 10% in cancers without tobacco aetiology) has been attributed to DNA adducts of polycyclic aromatic hydrocarbons (PAHs), such as benzo[a]pyrene, that are present in tobacco smoke (Hainaut and Pfeifer, 2001). Benzo[a]pyrene is activated to form benzo[a]pyrene diol epoxide (BPDE), which reacts with DNA predominantly at the N2-position of guanine, and both in vitro and in vivo studies have demonstrated that BP and BPDE can, in common with other PAHs, induce GT transversions (Wei et al., 1991; DeMarini et al., 1995; Schiltz et al., 1999; Wijnhoven et al., 2000).
A single point mutation in the TP53 gene results in a base change in both DNA strands with, for example, a GT mutation in one strand corresponding to a CA change in the complementary strand (G:CT:A). The question then arises whether such a change was induced by modification of G in one strand or of the C in the complementing strand. Recently, Rodin and Rodin (2000) examined the spectra of mutations that they believed were induced by damage of each of the two DNA strands of TP53 and concluded that when each strand was examined individually, there were no significant differences between the spectra in lung adenocarcinomas in smokers and non-smokers. The plot of mutations that represents the core of their argument and around which other arguments and conclusions in their paper are based is shown in Figure 1A, where all 12 possible base pair substitution mutations are grouped into six pairs, each consisting of two complementary substitutions. In this figure the mutations above the line represent the changes presumed to be induced by damage in the non-transcribed strand (NTS) of TP53 whereas those below the line represent the same changes induced in the transcribed strand (TS). For example, on the left-hand side, the GT above the line represents a mutation induced in the NTS, whereas CA below the line represents the GT mutation induced in the TS. (Note that by convention all mutations are referred to by the mutation found in the NTS.) The data as presented show: (i) that above the central line the ratio of the mutation frequency in smokers to that of the same mutations in non-smokers remains approximately constant; (ii) that a similar relationship, albeit with a different value of the ratio, also holds for mutations in the TS shown below the line. Based on these observations, Rodin and Rodin (2000) concluded that it would be premature to attribute the prevalence of GT mutations to a mutational signature left by BPDE.
|
There are, however, significant problems with the presentation of the information in this figure that affect its interpretation. It is, for example, unclear why lung adenocarcinomas alone have been examined when the incidence of squamous cell carcinomas of the lung and non-small cell lung cancers is also known to be smoking related. A particular problem is that the ratio of G
T mutations in lung adenocarcinomas observed in smokers compared with that observed in non-smokers (~30:18% estimated from Rodin and Rodin, 2000; Figure 1A
) is substantially different from the ratio of ~30:10% reported when all lung cancer types are examined and in analyses of larger data sets (see for example Hainaut & Pfeifer, 2001). A ratio of 3:1 would be quite different from the smoker:non-smoker ratio for all other mutations (~1.4:1 for GT, CT, GC and AG) as reported by Rodin and Rodin (2000) and would be inconsistent with the view that all the smokers:non-smokers mutation ratios were the same.
A second problem relates to the justification that Rodin and Rodin (2000) have used to assign the mutations to the TS and NTS of TP53. The authors argue that for GT, GC and CT mutations, the strand with the putative primary lesion can be identified.
In the case of GT and GC transversion mutations, they state that `both presumed exogenous and endogenous sources, e.g. BPDE and oxidative DNA damage, respectively, point to modified Gs as the primary lesion'. I would agree that G is the likely target in these two cases.
For the CT mutations, Rodin and Rodin (2000) propose that this alteration is induced by deamination of the target C in methylated 5-mCpG dinucleotide sequences in the NTS and that the complementary GA mutation is produced by a similar mechanism in the TS. They imply that this mechanism occurs in lung cancers in smokers and non-smokers. It is on the basis of this argument that CT mutations are positioned above the central line in Figure 1A. One concern with this rationale is that in both smokers and non-smokers, only ~40% of CT mutations occur in CpG sequences. No explanation is provided for how the remaining 60% of CT mutations in the TP53 gene are induced.
The proposals of Rodin and Rodin (2000) also seem to ignore the evidence that a major effect of methylation of C is enhancement of the reaction of the adjacent G in CpG dinucleotides with DNA-damaging agents such as BPDE (Denissenko et al., 1997; Chen et al., 1998; Smith et al., 2000). This is particularly significant because CpG sequences at all of the major mutational hot-spots within the TP53 gene are normally methylated (Rideout et al., 1990; Magewu and Jones, 1994; Harris, 1996). Importantly, it is well established that a variety of endogenous and exogenous chemicals, including PAHs such as BPDE, can, through binding to G, induce GA transition mutations in addition to GT and GC transversion mutations (for BPDE see Shukla et al., 1997a,b, 1999; Schiltz et al., 1999).
The overwhelming weight of evidence, therefore, supports the view that GA transition mutations are induced mainly as a consequence of the chemical modification of G, not as a consequence of deamination of 5-mC in the opposite strand. Figure 1A, therefore, needs to be modified by bringing the GA mutations onto the NTS above the central line and by placing CT mutations on the TS below the line. This is shown in Figure 1B. In this representation the GT, GA and GC mutations shown above the line are all induced by modification of G in the NTS, whereas the complementary mutations CA, CT and CG shown below the line are induced in the TS.
Examination of Figure 1B immediately reveals that, in contrast to the claim of Rodin and Rodin (2000), the spectra of TP53 mutations in smokers and non-smokers are not concordant (e.g. compare GT and GA in Figure 1B). For both the NTS and TS, the spectra of TP53 mutations in smokers and non-smokers are different and the data are consistent with the view expressed by many authors that there is an alteration in the mutation spectrum as a consequence of tobacco smoke exposure.
Acknowledgments
Dr Martin Osborne is thanked for his helpful comments. C.S.C. is funded by the Cancer Research U.K.
References
-
Biggs,P.J., Warren,W., Venitt,S. and Stratton,M.R. (1993) Does a genotoxic carcinogen contribute to human breast cancer? The value of mutational spectra in unravelling the aetiology of cancer. Mutagenesis, 8, 275–283.
Chen,J.X., Zeng,Y., West,M. and Tang,M.S. (1998) Carcinogens preferentially bind at methylated CpG in the TP53 mutational hot spots. Cancer Res., 58, 2070–2075.
DeMarini,D.M., Shelton,M.L. and Levine,J.G. (1995) Mutation spectrum of cigarette smoke condensate in Salmonella: comparison to mutations in smoking-associated tumors. Carcinogenesis, 16, 2535–2542.
Denissenko,M.F., Chen,J.X., Tang,M.S. and Pfeifer,G.P. (1997) Cytosine methylation determines hot spots of DNA damage in the human TP53 gene. Proc. Natl Acad. Sci. USA, 94, 3893–3898.
Hainaut,P. and Pfeifer,G. (2001) Patterns of TP53 GT transversions in lung cancers reflect the primary mutagenic signature of DNA-damage by tobacco smoke. Carcinogenesis, 22, 367–374.
Harris,C.C. (1996) TP53 tumor suppressor gene: from the basic research laboratory to the clinic—an abridged historical perspective. Carcinogenesis, 17, 1187–1198.
Magewu,A.N and Jones,P.A. (1994) Ubiquitous and tenacious methylation of the CpG site in codon 248 of the TP53 gene may explain its frequent appearance as a mutational hot spot in human cancer. Mol. Cell. Biol., 14, 4225–4232.
Rideout,W.M., Coetzee,G.A., Olumi,A.F. and Jones,P.A. (1990) 5-Methylcytosine as an endogenous mutagen in the human LDL receptor and TP53 genes. Science, 249, 1288–1290.
Rodin,S.N and Rodin,A.S. (2000) Human lung cancer and TP53: the interplay between mutagenesis and selection. Proc. Natl Acad. Sci. USA, 97, 12244–12249.
Schiltz,M., Cui,X.X., Lu,Y.P., Yagi,H., Jerina,D.M., Zdzienicka,M.Z., Chang,R.L., Conney,A.H. and Wei,S.J.C. (1999) Characterisation of the mutational profile of (+)-7R,8S-dihydroxy-9S,10R-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene at the hypoxanthine (guanine) phosphorisbosyltransferase gene in repair-deficient Chinse hamster V-H1 cells. Carcinogenesis, 20, 2279–2285.
Shukla,R., Jelinsky,S., Lui,T., Geacintov,N.E. and Loechler,E.L. (1997a) How stereochemistry affects mutagenesis by N2-deoxyguanosine adducts of 7,8-dihydroxy-9,10-epoxy-7,8,9,10-tetrahydrobenzo(a)pyrene: configuation of the adduct bond is more important than those of the hydroxyl groups. Biochemistry, 43, 13263–13269.
Shukla,R., Lui,T., Gaecintov,N.E. and Loechler,E.L. (1997b) The major, N2-dG adduct of (+)-anti-B[a]PDE shows a dramatically different mutagenic specificity (predominantly, GA) in a 5'-CGT-3' sequence context. Biochemistry, 36, 10256–10261.[Medline]
Shukla,R., Geacintov,N.E. and Loechler,E.L. (1999) The major, N2-dG adduct of (+)-anti-B[a]PDE induces GA mutations in a 5'-AGA-3' sequence context. Carcinogenesis, 2, 261–268.
Smith,L.E., Denissenko,M.F., Bennett,W.P., Li,H., Amin,S., Tang,M.S. and Pffeifer,G.P. (2000) Targeting of lung cancer mutational hotspots by polycyclic aromatic hydrocarbons. J. Natl Cancer Inst., 92, 803–811.
Wei,S.J., Chang,R.L., Wong,C.Q., Bhachech,N., Cui,X.X., Henning,E., Yagi,H., Sayer,J.M., Jerina,D.M. and Preston,B.D. (1991) Dose-dependent differences in the profile of mutations induced by an ultimate carcinogen from benzo(a)pyrene. Proc. Natl Acad. Sci. USA, 88, 11227–11230.
Wijnhoven,S.W.P., Kool,H.J.M., van Oostrom,C.T.M., Beems,R.B., Mullenders,L.H.F., van Zeeland,A.A., van der Horst,G.T.J., Vrieling,H. and van Steeg,H. (2000) The relationship between benzo(a)pyrene-induced mutagenesis and carcinogenesis in repair-deficient cockayne syndrome group B mice. Cancer Res., 60, 5681–5687.
Received on February 7, 2002; accepted on March 25, 2002.
CiteULike 
Connotea 
Del.icio.us What's this?
This article has been cited by other articles:
|
|
 |
|
  P.D. Lewis, B. Manshian, M.N. Routledge, G.B. Scott, and P.A. Burns Comparison of induced and cancer-associated mutational spectra using multivariate data analysis Carcinogenesis, April 1, 2008; 29(4): 772 - 778. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
D. H. Phillips Smoking-related DNA and protein adducts in human tissues Carcinogenesis, December 1, 2002; 23(12): 1979 - 2004. [Abstract] [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||