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Advances in Brief |
Centre for Environmental Health and Department of Biology, University of Victoria, Victoria, British Columbia, V8W 3N5 Canada
| ABSTRACT |
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T:A transversions and deletions of
G:C bp. The results directly link PhIP-induced mutations with the
earlier observation of PhIP-induced prostate cancer in rats and suggest
that exposure to dietary PhIP could be a risk factor in the incidence
of human prostate cancer. | Introduction |
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| Materials and Methods |
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lacI Mutational Assay.
High molecular weight DNA was recovered from prostate tissue by a
dialysis purification procedure described previously (9)
.
lacI transgenes were recovered from purified rat chromosomal
DNA by an in vitro
packaging reaction, and packaged
phage were plated on the Escherichia coli host strain
following recommended methods (10)
. MFs were calculated by
dividing the total number of lacI mutant plaques by the
total number of mutant plus wild-type plaques.
DNA Sequencing and Data Analyses.
Mutations in the lacI transgene were determined by the PCR
cycle sequencing method described previously (11)
. All 12
spontaneous prostate mutants were sequenced, as were 178 mutants from
PhIP-treated rats. The lacI gene is numbered according to
Farabaugh (12)
.
| Results and Discussion |
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We then determined the mutational specificity of spontaneous and
PhIP-induced mutations in rat prostate tissue. Twelve mutants were
recovered and sequenced from the prostates of the untreated animals,
and 11 independent mutations were identified (Tables 1
2)
. Despite the small sample size, the prostate spontaneous MS was
consistent with those previously determined in lacI
transgenes recovered from a variety of tissues (16
, 17)
.
Specifically, the spontaneous prostate MS was characterized by 55%
G:C
A:T transition mutations (all of which occurred at 5'-CpG-3'
dinucleotide sites), 18% transversions, and 27% non-base substitution
mutations (Table 2)
.
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T:A transversions, and 10% each of
G:C
A:T transitions and G:C
T:A transversions. (In the following
sentence, the numbers in parentheses indicate the numbers of
independent mutations.) With regard to sequence specificity, the PhIP
mutations included five (four) -1 frameshifts of G:C at
lacI nucleotide positions 9092, five (four) base
substitution mutations at position 92, and 11 (four) -1 frameshifts of
G:C at position 877. Of the six independent deletion mutations, five
were dinucleotide deletions involving cytosine and guanine:
(a) CC; (b) CG; (c) CG; (d)
GC; and (e) GC.
The PhIP-induced MS from prostate was consistent with those determined
previously in the rat colon (13)
and mammary gland
(14)
. The large increase in the proportion of -1
frameshifts involving G:C bp and the increase in the proportion of
G:C
T:A transversions in PhIP-treated prostate (Table 2)
are
particularly characteristic of PhIP mutational spectra recovered from
colon and mammary gland.
To our knowledge, this is the first report of spontaneous and induced mutation frequencies and spectra from prostate. The PhIP mutational data from prostate, combined with the previous observation that PhIP (mixed into the diet at a dose of 400 ppm for 52 weeks) causes prostate tumors in the rat (7) , provide convincing evidence that PhIP is a genotoxic carcinogen in the case of rat prostate cancer.
The rats in our mutagenicity study received 200 ppm of PhIP mixed into the diet for 61 days, an exposure sufficient to dramatically elevate the mutation frequency in this organ. During this period, the average daily food consumption and body weights were approximately 13 and 180 g, respectively, giving an estimated daily consumption of PhIP of 14 µg of PhIP per gram of body weight (i.e., 14 ppm PhIP, adjusted for body weight). It has been estimated that daily human dietary intake of heterocyclic amines approaches microgram quantities (3 , 18) . Therefore, assuming that the average human consumes 1 µg of PhIP daily at a body weight of 70 kg [calculated using the data provided in Table 3 in the article by Layton et al. (18) ], the estimated daily human consumption is approximately 1.4 x 10-5 µg of PhIP per gram of body weight (i.e., 1.4 x 10-5 ppm, adjusted for body weight), a difference of approximately 106-fold compared to the rats. However, this fold difference in exposures is approximately 103-fold once the durations of exposures are considered (61 days for the rat mutagenicity study; assumed exposure of 60 years for humans).
Wakabayashi et al. (3) have noted that the carcinogenic effects of various heterocyclic amines appear to be additive or synergistic, whereas Felton et al. (5) have found that the binding of heterocyclic amines to DNA is linear at doses well in excess of the average daily intakes of heterocyclic amines (18) . Felton and colleagues have estimated that the overall risk of cancer due to dietary exposure to heterocyclic amines is approximately 1 x 10-4, with the incremental risk of cancer due to exposure to PhIP being approximately 5 x 10-5 (18) .
Thus the mutagenicity data presented here, combined with the previous demonstration that PhIP is a rat prostate carcinogen, provide additional evidence that humans who consume excessive amounts of PhIP may risk developing prostate cancer. This conclusion is also consistent with the recent demonstration that PhIP appears to be a substrate for N-acetyltransferase activity present in human prostate epithelial cells in vitro (19) and in human prostate tissue implanted into nude mice (20) .
The transgenic rodent model provides a practical opportunity to investigate the mechanisms contributing to cancer in the prostate. Studies including the role of diet, as well as the efficacy of potential chemopreventive therapies, can be undertaken using the approach described.
| Acknowledgments |
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| FOOTNOTES |
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1 To whom requests for reprints should be
addressed, at Centre for Environmental Health, University of Victoria,
P. O. Box 3020 STN CSC, Victoria, British Columbia, V8W 3N5
Canada. E-mail: bwglick{at}uvic.ca ![]()
2 The abbreviations used are: PhIP,
2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine; MF,
mutant frequency; MS, mutational spectrum. ![]()
Received 10/13/99. Accepted 11/30/99.
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