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Molecular Oncology, Markers, Clinical Correlates |
Danish Cancer Society, Department of Experimental Clinical Oncology, Aarhus University Hospital, DK-8000 Aarhus C [J. A., J. O.]; Department of Tumour Cell Biology, Institute of Cancer Biology, DK-2100 Copenhagen Ø [P. G.]; and Departments of Oncology [M. Y.] and Human Genetics [L. L. H.], Aarhus University Hospital, DK-8000 Aarhus C, Denmark
| ABSTRACT |
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| INTRODUCTION |
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Because different missense mutations (i.e., mutations that cause amino acid substitutions) have different effects on the structure and function of the protein (4) , attempts have been made to correlate clinical outcome with the location of the mutation. Thus, Bergh et al. (5) found that mutations in the conserved domains II and V were associated with significantly worse prognosis. Others have compared mutations affecting different structural domains of p53. Børresen-Dale et al. (6) and Gentile et al. (7) have shown that patients with missense mutations in the parts of the protein involving zinc binding have a particularly poor prognosis. Kucera et al. (8) could not confirm the importance of the zinc-binding domain, and finally, Berns et al. (9) found the poorest prognosis associated with missense mutations affecting amino acids directly involved in DNA contact. Although it is still unclear which mutations are associated with the worst prognosis, these studies have demonstrated a heterogeneity in the clinical phenotype of TP53 mutations in breast cancer. Whereas the aggressive phenotype of certain mutations has been seen, it has also been speculated that there might be other missense mutations that display a clinically "silent" phenotype (10) .
We have previously evaluated 294 patients with primary early breast carcinoma for mutations in TP53 (11) . Among these patients, 69 (23%) carry a sporadic TP53 mutation that is significantly associated with increased risk of distant metastasis, disease-free survival, and overall survival in both node-negative and node-positive patients. Also, in a Cox proportional hazards analysis, TP53 mutation independently predicts the occurrence of distant metastasis and death (overall survival), with relative risks of 2.4 (95% confidence interval, 1.53.9) and 2.7 (95% confidence interval, 1.74.2), respectively (11) .
In the present study, tumor characteristics and clinical outcome of these 294 patients are related to the presence, type, and location of TP53 mutations. Also, we present data on 21 additional patients identified within the same period but presenting with either disseminated or bilateral disease or with other neoplastic disease at the time of diagnosis. We demonstrate that a prediction of biochemical phenotype or aggressiveness of different TP53 mutations correlates with the clinical phenotype (risk of disseminated disease and death).
| MATERIALS AND METHODS |
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DGGE Analysis.
DNA was extracted from the pellet left over from estrogen receptor
analysis. The quality of DNA from this material has previously been
assesed by loss of heterozygosity analysis (12)
. The
entire coding region and all exon/intron boundaries of TP53
were analyzed by DGGE.3
Twelve sets of primers for amplification and DGGE analysis of exons
211 (including overlapping amplicons for exons 4 and 5) have been
described previously (13)
. Amplification reactions were
carried out by 38 rounds of thermal cycling (94°C for 20 s,
62°C for 20 s, and 72°C for 20 s) in final volumes of 15
µl containing 10 mM Tris-HCL (pH 8.3), 50
mM KCl, 1.5 mM
MgCl2, 0.02% gelatin, 0.2
mM cresol red, 12% sucrose, 5% DMSO, 100
µM of each deoxynucleotide triphosphate, 0.6
µM of each primer, 100 ng of DNA, and 0.5 unit
of AmpliTaq DNA polymerase (Perkin-Elmer Cetus, Emeryville, CA). For
DGGE analysis, PCR products were run on 6% polyacrylamide gels
containing various gradients of urea and formamide (100% denaturant
consisted of 7 M urea and 40% formamide). As a
modification to a previous approach (13)
, the gradient
ranges were narrowed and adapted to each exon to increase the
separation between mutant and wild-type alleles. The gradient ranges
were 3575% for exons 2, 3, 4, 7, 8, and 10; 2060% for exon 9;
2565% for exon 6; and 4080% for exon 5, respectively. The gels
were run at 160 V for 5 h in 1x TAE buffer at a constant
temperature of 58°C. After electrophoresis, gels were stained in 1x
TAE buffer containing 2 µg/ml ethidium bromide and analyzed
under UV transillumination. Each sample was analyzed once by DGGE.
DNA Sequencing.
Mutant heteroduplex or homoduplex bands were excised and reamplified as
described previously (13)
. Sequencing of PCR products was
performed either with 33P-end-labeled primers
using the ThermoPrime Cycle Sequencing Kit (Amersham, Cleveland, OH) or
with the BigDye DyeTerminator Cycle Sequencing Kit and analyzed on an
ABI 310 (Perkin-Elmer Cetus). Only excised bands were sequenced.
Statistics.
A two-sided
2
test was used to test for an
association between categorial data. The probability of treatment
failure was calculated for the end points of freedom from distant
metastasis, disease-specific survival (death of cancer), and overall
survival by the Kaplan-Meier product-limit analysis. The Mantel-Cox
test was used for comparison, and a test for trend with equal weighing
was performed if more than two groups were compared. All time estimates
were done using the date of primary surgery as the initial value.
A multivariate Cox proportional hazards analysis was used to evaluate prognostic parameters with respect to risk of distant metastasis, disease-specific death, and overall death. Parameters were included in the model using forward selection, and statistical analysis was performed by using the Wald test. The level of statistical significance was set to 5%. The Ps estimated are those for a two-tailed test. The statistical analysis was performed using the BMDP program package (SPSS Inc., Chicago, IL).
The date for evaluation of disease-specific and overall survival was October 1, 1999. This gives a median potential observation time of 77 months (range, 2585 months); all patients were followed for at least 60 months, except for one patient who emigrated after 25 months. The median observation time for evaluation of distant metastasis was 60 months (range, 179 months).
| RESULTS |
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-helix
(amino acids 337355) (14)
.
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In Fig. 1
, the location and number of the mutations are illustrated and
compared with the domain structure of p53. Six mutations were
identified in the tetramerization domain. Four of these were identical
nonsense mutations, Q331X. The remaining two mutations
consisted of a small in-frame deletion and insertion, respectively. In
Fig. 1
, these two mutations are classified as missense mutations.
However, because missense mutations in the tetramerization domain can
destabilize the protein structure without being dominant negative
(15
, 16)
, these two in-frame deletion and insertions are
also expected to produce a nonfunctional protein and will be included
in the null mutations along with the nonsense and frameshift mutations.
With this adjustment, the material includes 47 missense mutations
(64%) and 27 null mutations (36%). Detailed description of these 74
missense or null mutations and of 1 silent mutation is given in Table 1
, together with the clinical details of
the patients.
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21 mm versus 26 of 145 (18%) tumors < 21 mm;
P = 0.03], negative estrogen receptor status
[TP53 mutation was seen in 35 of 86 (41%)
receptor-negative tumors versus 39 of 229 (17%)
receptor-positive tumors; P < 0.001], and, in ductal
carcinomas, high grade of anaplasia [TP53 mutation was seen
in 33 of 75 (44%) grade 3 tumors, 32 of 103 (31%) grade 2 tumors, and
1 of 63 (2%) grade 1 tumors; P < 0.001]. Only
one patient with a grade 1 ductal carcinomas carried a TP53
mutation. The mutation was R280K, which changed an amino acid directly
involved in DNA binding. The patient relapsed after 21 months and died
2 years after diagnosis (11)
; the grading of the sample
has not been reevaluated. The general association with an aggressive
phenotype seemed to be more or less similar for the four different
mutation types (not tested).
Analysis of the outcome for the 294 patients with primary unilateral
disease who had no evidence of distant metastasis at the time of
diagnosis showed distinct clinical phenotypes associated with the
proposed grouping of TP53 mutations (Fig. 2)
. Thus, 60 months after treatment,
disease-specific survival for patients with mutations affecting amino
acids directly involved in DNA or zinc binding was only 35% compared
with 89% for patients without mutations. Patients with null mutations
or with any of the remaining missense mutations within the
conserved/structural domains had an intermediate survival of 67% and
48%, respectively. Finally, the disease-specific survival for patients
with missense mutations outside the domains was 89%. Similar patterns
are found for two other end points, freedom from distant metastasis and
overall survival (data not shown).
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| DISCUSSION |
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Previous attempts to link missense mutations in specific domains or residues within this central part of the p53 molecule to clinical outcome for breast cancer patients have yielded contradictory results. Thus, it is not clear whether particularly poor prognosis is correlated with missense mutations in the conserved domains II and V, the L2 and L3 domains, or those amino acids directly involved in DNA contact (5, 6, 7, 8, 9) .
Here, we suggest a new model for the differentiation of TP53
mutations. Null mutations are considered as one group and include
traditional nonsense and frameshift mutations. Also, we suggest that
missense mutations that are predicted to disrupt the function of the
tetramerization domain be classified as null mutations. In the present
study, the only two in-frame deletions/insertions identified (Table 1)
affect amino acids shown to be essential for tetramerization (15
, 16)
. Missense mutations are divided into three groups:
(a) a small group containing amino acids directly involved
in DNA or zinc binding; (b) a large group containing the
remaining missense mutations within the structural/conserved domains
(as depicted in Fig. 1
); and (c) a small group containing
all mutations affecting amino acids outside these domains.
When correlated with tumor/patient characteristics like tumor size,
estrogen receptor status, nodal status, and (for ductal carcinomas)
grade of anaplasia, all four groups of mutations are associated with a
more aggressive phenotype than tumors from patients without
TP53 mutation (Table 2)
. As a point of interest, it has been
suggested that medullary carcinoma is a distinct genetic subtype with a
very high frequency of TP53 mutations of up to 100% in
typical medullary breast carcinoma (19)
. The medullary
carcinomas in the present study have not been subtyped, but the finding
that seven of eight are wild-type for TP53 indicates that this
correlation might be less strict.
Although all types of TP53 mutations are associated with an
increase in the aggressiveness of the tumor/patient characteristics,
different mutations might still be associated with different effects on
the ability of the tumor to metastasize and/or the ability to respond
to radiation or chemotherapeutic therapy. When TP53 mutation
status is correlated with either freedom from distant metastasis,
disease-specific survival, or overall survival, a strong association is
indeed observed between the different groups of mutations and clinical
outcome (Fig. 3)
. Thus, optimal separation of prognosis, disease
progression, and survival based on TP53 status is obtained
by grouping patients into three groups: (a) wild-type +
neutral, patients with wild-type tumors together with missense
mutations outside structural/conserved domains; (b) very aggressive,
patients with missense mutations affecting amino acids directly
involved in DNA or zinc binding; and (c) aggressive,
patients with the remaining missense mutations and all null
mutations. Dividing patients according to other parameters such
as the presence of missense mutations in L2/L3 or those affecting DNA
binding is also prognostic for the clinical outcome, but not to the
same extent as shown in Figs. 2
and 3
(data not shown).
The very poor prognosis for patients with missense mutations affecting amino acids directly involved in DNA or zinc binding could reflect dominant positive effects [gain of function (20) ] or a particularly strong dominant negative phenotype (21) of these mutations. Among the rest of the missense mutations within the structural/conserved domains, a high percentage might have a dominant negative phenotype. Whether loss of p53 function in general leads to a less aggressive phenotype than dominant positive mutations or whether the intermediate clinical phenotype reflects a heterogeneity within this group of patients is not clear. Null mutations are also associated with an intermediate aggressive phenotype. It is possible that reduction of p53 dosage in itself is sufficient to promote some tumor progression (22) , but without information of the retention of the wild-type allele, the intermediate phenotype in this group could also reflect a patient heterogeneity.
The final group of mutations includes the missense mutations outside any structural or conserved domains. Although they occur in only 9 of 69 mutations found in patients with primary unilateral disease and no evidence of distant metastasis (13% of tumors with mutations or 3% of the patients), they appear to define a novel class of TP53 mutations in breast cancer. Although they are associated with aggressive clinical parameters like large size, negative estrogen receptor status, positive nodal status, and the absence of low-grade ductal carcinomas, these mutations do not seem to confer a worse prognosis on the patients as compared to patients without TP53 mutations. Recently, a detailed analysis of mutations selected in BRCA-associated tumors has identified a class of mutations that is frequent in these tumors but rare in sporadic tumors not associated with mutations in BRCA1 or BRCA2 (23) . These mutations have lost the ability to suppress transformation and are likely to be selected on that basis; however, they still retain a biochemical phenotype close to that of the normal protein. Thus, to a certain degree, these mutant proteins can still transactivate downstream targets, suppress growth, and induce apoptosis (23) . The majority of these BRCA-associated mutations are indeed located outside structural/conserved domains, thus providing a possible link between a novel biochemical phenotype and the lack of effect on clinical outcome for breast cancer patients seen in the present study. In this respect, it might be relevant to note that although BRCA-associated tumors contain a very high frequency of TP53 mutations compared to sporadic tumors without BRCA mutations (24) , survival for these patients is apparently not significantly different (reviewed in Ref. 25 ). Interestingly, in a recent analysis of cases of childhood adrenocortical carcinoma unselected for a family history of cancer, 80% of the children carried a germ-line mutation in TP53 (26) . Adrenocortical carcinoma, together with bone and soft tissue sarcomas and breast and brain tumors, is part the Li-Fraumeni syndrome associated with germ-line mutations in TP53 (27 , 28) . The identified missense mutations were all located outside structural/conserved domains, and several carriers unaffected in their 40s and 50s were identified, demonstrating a low-penetrance phenotype of these mutations (26) .
When stratified according to the biochemical phenotype, TP53
mutations together with nodal status contained all of the significant
prognostic information (with the addition of postmenopausal status
for the end point of overall survival). However, with the number of
patients included in this study, it is not possible to clarify whether
TP53 mutation in breast cancer is a prognostic marker for
distant metastasis or whether it predicts response to specific types of
therapy, e.g., adjuvant chemotherapy. The observation that
three of eight patients with distant metastasis already present at the
time of diagnosis carried a mutation and that two of these three
mutations are of the very aggressive type (Table 1)
suggests
that TP53 mutation can carry prognostic information for
metastatic capability. As a predictive marker, this study lacks the
power to detect a potential difference in outcome from specific types
of therapy. To obtain a sufficient number of patients, a large national
study has recently been initiated by The Danish Breast Cancer
Cooperation Group.
In conclusion, we suggest a new model for the prediction of the clinical phenotype of different TP53 mutations in patients with primary breast cancer and predict two interesting groups of patients. First, if mutational analysis of the p53 gene is included in any clinical decision-making for breast cancer patients, it is important to realize that mutations might be identified that will have no effect on prognosis and/or response to therapy. A second important group of patients identified in this study is the group of patients with missense mutations affecting amino acids that are directly involved in DNA or zinc binding (the very aggressive group). These patients are at a very high risk of developing distant metastasis within 5 years after treatment. Finally, the results presented here demonstrate the necessity of including at least exons 410 in any analysis of TP53 mutations in breast cancer. Although only 1 of the 47 missense mutations occurred outside exons 58, 7 of 27 (26%) of the nonsense mutations, frameshift mutations, or in-frame deletions/insertions were found outside these exons, including four Q331X, which might be a novel hot spot in breast cancer.
| FOOTNOTES |
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1 Supported by Danish Cancer Society Grants
9510057 and 9810017; Danish Medical Research Council Grant 9900767;
Mrs. Astrid Thaysens Foundation; the Clinical Research Unit,
Danish Cancer Society, Oncologic Center, Aarhus University Hospital
(Grant KFE-Aa-132-98); and Frits, Georg og Marie Cecilie Gluds Legat,
Faculty of Health Sciences, University of Aarhus. ![]()
2 To whom requests for reprints should be
addressed, at Danish Cancer Society, Department of Experimental
Clinical Oncology, Aarhus University Hospital, Noerrebrogade 44,
Building 5, 8000 Aarhus C, Denmark. Phone: 45-8949-2626; E-mail: jan{at}oncology.dk ![]()
3 The abbreviation used is: DGGE, denaturing
gradient gel electrophoresis. ![]()
Received 3/17/00; revised 7/17/00; accepted 7/17/00.
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