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Laboratory of Cancer Genetics, Institute of Medical Technology, University of Tampere and Tampere University Hospital [N. M., K. S., M. M., J. S., O-P. K., P. A. K.]; Department of Clinical Genetics, Tampere University Hospital [P. A. K.]; and Division of Urology, Tampere University Hospital and Medical School, University of Tampere [T. L. J. T.], 33521 Tampere, Finland; Laboratory of Cancer Genetics, National Center for Human Genome Research, NIH, Bethesda, Maryland 20892-4470 [J. S., O-P. K.]; and Department of Pathology, Erasmus University, 3000 Rotterdam, the Netherlands [J. T.]
| ABSTRACT |
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| INTRODUCTION |
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reductase, vitamin D receptor, and the
AR3
gene (17, 18, 19)
. Because AR protein is a key mediator of growth signaling in the prostate, many investigators have considered AR as a candidate prostate cancer susceptibility gene. The AR gene contains two polymorphic trinucleotide repeat regions in exon A: (a) a CAG repeat (coding for polyglutamine); and (b) a GGC repeat (polyglycine). The length of the CAG repeat sequence is inversely correlated with the transactivational activity of the AR. Irvine et al. (20) proposed that men with short CAG repeats would have an increased risk of prostate cancer. Subsequently, Giovannucci et al. (21) and Stanford et al. (22) conducted a case-control study and found an association between a low number of AR gene CAG repeats and an increased risk of prostate cancer. In particular, short CAG repeat sequence was strongly associated with cancers characterized by extraprostatic extension, distant metastases, or poor histological differentiation. However, not all authors have been able to confirm these findings (23 , 24) .
Numerous specific mutations of the AR gene have also been reported in human prostate cancer patients (see the Androgen Receptor Gene Mutations Database4 ). Many of these mutations occur in regions of the gene coding for the ligand- or DNA-binding domains of the AR, and functional studies have indicated that such mutations often alter the specificity of the transcriptional response of the AR to androgens, antiandrogens, and other steroids. In many cases, the mutations have only been studied from the tumor tissue, with the highest prevalence of AR mutations reported in metastases of patients with hormone-refractory disease (25 , 26) . The germ-line origin of such mutations has been established in only two reports (27 , 28) , but the role of such mutations in the genetic predisposition to prostate cancer has not been firmly established.
Elo et al. (27) described an R726L germ-line mutation of the AR gene in a prostate cancer patient from Northern Finland. We recently found the same mutation in another Finnish prostate cancer patient when screening for AR mutations by single-strand conformational polymorphism in six patients whose cancers appeared during finasteride treatment for benign prostatic hyperplasia (29) . The R726L mutation affects the hormone-binding region in exon E and was reported to lead to activation of the AR not only by dihydrotestosterone and testosterone but also by estradiol (27) . The fact that this mutation has not been found in any published study of the AR gene suggests that it may represent a unique Finnish mutation. Here we analyzed the frequency of the R726L mutation in over 1400 specimens from blood donors, consecutive prostate cancer patients with no family history of prostate cancer, and patients with a positive family history of prostate cancer to explore the frequency of this mutation in the Finnish population, as well as its association with prostate cancer.
| MATERIALS AND METHODS |
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During the sample collection, 559 new prostate carcinomas were diagnosed at TAUH. Twenty-five percent of the patients were excluded from the present study because of a positive family history for prostate cancer or their refusal to participate in the study. One hundred and six prostate cancer families with two or more affected cases were identified through referrals from physicians, family questionnaires sent to patients, and newspaper, radio, and television advertisements. A sample from one randomly chosen affected case from each family was screened for the R726L mutation.
Written informed consent was obtained from all patients and their family members, and research protocols were approved by the Ethical Committee of TAUH. Diagnoses of all prostate cancer patients were confirmed through medical records or the Finnish Cancer Registry. The patients family histories for malignancies were documented from family questionnaires completed by the patients. Prostate cancer was considered "sporadic" when the patient reported no first- or second-degree relatives with prostate cancer.
The population frequency of the R726L mutation was established by analyzing DNA specimens from 778 anonymous, unselected healthy blood donors from the Tampere region (656 men and 122 women). Together, these specimens allowed us to scan for the allele frequency of the R726L mutation on 900 X chromosomes.
ASO Hybridization to Detect R726L.
Genomic DNA was amplified using primers 5'-CCCAACAGGGAGTCAGACTTA-3' and
5'-CCTGGAGTTGACATTGGTGA-3'. One hundred ng of DNA was amplified by 35
cycles of PCR in reactions containing 200 nM of both
primers, 200 µM of each deoxynucleotide triphosphate, 50
mM KCl, 10 mM Tris-HCl (pH 8.3), 1.75
mM MgCl2, 0.001% (w/v) gelatin, and 2.5 units
of AmpliTaqGold DNA polymerase (Perkin-Elmer, Norwalk, CT). After
an initial denaturation step of 10 min at 95°C, the cycle parameters
were as follows: 95°C for 30 s, 57°C for 1 min, and 72°C for 1
min, with a 5-min extension at 72°C after the last cycle. Mutation
detection was done using ASO hybridization as described by Friedman
et al. (30)
with the following exceptions:
(a) filters were prewet and wells were washed with 0.4
M Tris-HCl (pH 7.5); (b) probes were
end-labeled with 32P at 37°C for 3 h by
terminal deoxynucleotidyl transferase (Amersham Life Science Inc.,
Cleveland, OH); and (c) hybridizations were performed at
54°C. ASOs used in hybridizations were 5'-AGGCTTCCGCAACTTACA-3' (wild
type) and 5'-AGGCTTCCTCAACTTACA-3' (mutation). A mutation positive
control as well as a negative control of the PCR reaction was included
in each ASO hybridization. All samples with R726L mutation as well as
42 randomly chosen samples negative for mutation were sequenced with
ABI PRISM 310 Genetic Analyzer (Perkin-Elmer) as recommended by the
manufacturer. Primers used in sequencing were the same as those used
for PCR.
Analysis for CAG Repeats in the AR.
To explore whether the R726L mutation had a common origin, we analyzed
the length of the AR CAG repeat in all mutation carriers. If
the R726L mutation represented an ancient founder mutation, one would
expect to find that neighboring genetic markers on the X chromosome,
such as the CAG repeat, would also be shared between the mutation
carriers. The distribution of CAG repeats in the R726L carriers was
compared with CAG repeats of 811 Finnish blood
donors.5
The fragment containing the CAG repeat was amplified by PCR using
previously published primers (20)
, with the exception that
the other inner primer was labeled with 5'-66-carboxyfluorescein.
Electrophoresis was performed using the ABI 310 Genetic Analyzer
(Perkin-Elmer) according to the manufacturers instructions. Run
results were analyzed by use of the Genescan 2.1 and Genotyper 2.0
(Perkin-Elmer) computer programs.
Statistical Analyses.
Statistical analyses were performed using GraphPad InStat version 2.04a
(GraphPad Software, San Diego, CA). Correlations were made with
the two-tailed Fishers exact test,
2 test,
and
2 test for linear trend. Comparison of the
ages was made using the two-tailed Students t test. In
addition, the OR and 95% confidence intervals were calculated.
| RESULTS |
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Because the R726L mutation was also found in eight prostate cancer patients who reported no family history of prostate cancer, we constructed extended pedigrees from their families based on family questionnaires sent to the patients. Two of these patients turned out to have a maternal relative with prostate cancer.
Clinical features of the R726L mutation-positive prostate cancers
were compared with those of noncarriers (Table 2)
. There were no significant differences in tumor stage, metastasis
stage, or tumor grade between these groups. The average age at prostate
cancer diagnosis was slightly lower in patients harboring the R726L
mutation (65.5 ± 7.0 years; range, 5979 years) as
compared with the rest of the prostate cancer patients (68.4 ± 8.3 years; range, 4892 years), but this difference was not
statistically significant (P = 0.25).
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| DISCUSSION |
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First, the mutation was seen in approximately 2% of the samples from the prostate cancer patients, whereas the population frequency of this allele in blood donors was 0.3%. The results suggest an almost 6-fold overrepresentation of the R726L mutation in the prostate cancer cases as compared with ethnically and geographically matched population controls. Second, the mutation was found in two patients with a positive family history for prostate cancer. Family members of the mutation-positive cases were also screened for the R726L mutation, and the R726L mutation was shared between the prostate cancer patients. Third, the R726L germ-line mutation in the coding region of the AR gene changes an evolutionarily exceptionally well-conserved amino acid in the protein (from a positively charged amino acid to a nonpolar amino acid). In addition, there is prior evidence that the R726L mutation may change the functional characteristics of the AR protein. Elo et al. (27) demonstrated that the R726L mutation did not alter the ligand binding specificity of the AR protein, but its transactivational activity in the presence of estradiol was increased. Although estradiol itself is unlikely to be the target of the mutant receptor, this finding suggests that the transactivational response of the mutated AR gene is altered in response to the ligands. This, in turn, may explain its association with prostate cancer. Additional studies are needed to establish the exact biological significance of the R726L mutation.
Eighty-five percent of the R726L mutation carriers had 26 CAG repeats in the AR gene. The average AR CAG repeat length in the Finnish population is 22, with a wide range from 8 to 30. The population frequency of the 26 CAG repeat length is only 2.7%,5 suggesting that there is a strong linkage disequilibrium between the R726L mutation and the CAG repeat within the AR gene. The 25 and 27 AR CAG repeats seen in two mutation carriers may be explained by the instability of the CAG repeats (20, 21, 22, 23, 24) . Together, the results suggest that the R726L mutation carriers originate from a single ancestral founder.
In conclusion, we have demonstrated an up to 6-fold increased frequency of the AR R726L germ-line mutation in sporadic as well as family-positive prostate cancers. Additional studies are required to elucidate the potential role of the R726L mutation as a marker of genetic predisposition for prostate cancer or as a modifier locus. These kinds of infrequent but potentially cancer-associated variants of the AR gene provide an example of the importance of rare disease-associated single-nucleotide polymorphisms that are often overlooked in single-nucleotide polymorphism-based genetic studies. The role of such events along the AR signaling pathway deserves further study in prostate cancer.
| ACKNOWLEDGMENTS |
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| FOOTNOTES |
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1 Supported by the Medical Research Fund of
Tampere University Hospital, the Sigrid Juselius Foundation, the
Lahtikari Foundation, the Finnish Cancer Institute (P. A. K.),
Duodecim (Finnish College of Physicians; P. A. K.), the Dutch Cancer
Society (J. T.), Pirkanmaa Cancer Society, and Maud Kuistila
Foundation. The prostate cancer family collection was supported in part
by a contract from the National Human Genome Research Institute, NIH
(NO1-HG-55389). ![]()
2 To whom requests for reprints should be
addressed, at Laboratory of Cancer Genetics, Tampere University
Hospital, P. O. Box 2000, FIN-33521 Tampere, Finland. Phone:
358-3-2474128; Fax: 358-3-2474168; E-mail: blpako{at}uta.fi ![]()
3 The abbreviations used are: AR, androgen
receptor; ASO, allele-specific oligonucleotide; OR, odds ratio; TAUH,
Tampere University Hospital. ![]()
4 http://www.mcgill.ca/androgendb. ![]()
5 N. Mononen, M. Matikainen, J. Schleutker,
P. A. Koivisto, T. L. J. Tammela, and O-P. Kallioniemi.
The AR CAG repeat and its relationship to prostate
cancer, manuscript in preparation. ![]()
Received 3/27/00. Accepted 9/15/00.
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