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From the Department of Pathology*
and the Norris
Comprehensive Cancer Center,
University of
Southern California School of Medicine, Los Angeles, California
| Abstract |
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| Introduction |
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| Materials and Methods |
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The use of human tissue in this study was reviewed and approved by the University of Southern California Institutional Research Committee. Fifty-eight frozen breast biopsy samples with CIS, stored at -186°C under liquid nitrogen and accessioned sequentially between 1988 and 1996 by the USC Breast Tumor and Tissue Bank, were used for this investigation. Twenty-seven cases had only carcinoma in situ and 31 cases had evidence of both invasive and in situ carcinoma. Among the cases with only CIS, 13 (48%) breast CIS were classified as comedocarcinoma, 4 (15%) as cribriform ductal CIS, 3 (11%) as micropapillary ductal CIS, 3 (11%) as intraductal papillary carcinoma, and 4 (15%) as solid ductal CIS. In the cases with invasive disease and CIS, 10 (32%) breast CIS were classified as comedocarcinoma, 8 (26%) as cribriform ductal CIS, 3 (10%) as micropapillary ductal CIS, 1 (3%) as papillary ductal CIS, and 9 (29%) as solid ductal CIS. Frozen tissue sections stained with hematoxylin and eosin (H&E) were used to confirm the histological composition of the specimens. Those cases having microinvasion were classified as CIS with invasive disease. Microinvasion, defined as microscopic invasion of breast stroma beyond the ductal-lobular unit <1 mm in maximal diameter as well as invasive breast carcinoma, was identified in histological frozen sections by a board-certified pathologist (M. F. P.). Only one case in this series had microinvasion (C425). Nuclear grading of the CIS was classified from 1 to 3, with 2 cases classified as nuclear grade 1 (low grade), 32 as nuclear grade 2 (intermediate grade) and 24 as nuclear grade 3 (high grade).
Microdissection and DNA Isolation
The different tissue components of the breast specimens were separated according to histomorphological phenotypes by mechanical microdissection with the assistance of a microscope. Ten to twenty frozen serial sections, 10 µm thick, were cut and fixed in 95% ethanol. The initial section was stained with H&E and subsequent sections for microdissection were stained with ethyl green. Microdissection was performed to separate benign breast epithelium, breast CIS and invasive carcinoma. Care was taken to minimize contamination of epithelial cells with stromal cells. For example, only the central portion of the CIS from the ductal lumen was excised from the ductal epithelium. Any possible area of potential contamination was discarded. After microdissection, the DNA of each component was extracted as previously described.23 In three cases, formalin-fixed, paraffin-embedded tissue was used in addition to frozen tissue. These cases were sectioned and subjected to microdissection and DNA extraction as described previously.23
p53 Primer Design
Polymerase chain reaction (PCR) was used to amplify exons 211. Ten different sets of 19- to 25-mer oligonucleotide primers were designed using the genomic sequence of p53 (GenBank accession numbers U94788 and X54156).24 Primers were designed to span each exon of the p53 open reading frame and sufficient bases of the intronic sequence to ensure the splice donor and splice acceptor sites were included for analysis.24
SSCP and DNA Sequence Analysis
The SSCP technique was initially used as a screen for DNA sequence alterations in p53 exons 2 through 11 as described elsewhere.23 Exons with altered mobility were analyzed for DNA sequence changes as described23 using the CircumVent Thermal Cycle Sequencing Kit (New England Biolabs, Beverly, MA) or the ThermoSequenase Kit (Amersham, Arlington Heights, IL), according to the manufacturers instructions. DNA sequencing was performed twice, from separate PCR reactions, to confirm that the mutation found was not an artifact of PCR.
The term "DNA mutations" is used below to refer to changes in the coding sequence giving rise to a change in amino acid sequence. In contrast, the term "DNA sequence alterations of unknown significance" is used below to refer to base changes outside of the coding region (exon), such as base changes in the intron sequence or in the 3' untranslated region. Finally, the term "DNA polymorphisms" is used to refer to base changes that are either silent and do not code for different amino acids, or base changes which code for amino acid substitutions that have been reported to maintain normal p53 function.
p53 Immunohistochemistry
p53 protein was identified in tissue using the peroxidase anti-peroxidase immunohistochemical technique. Frozen sections 4 µm thick were incubated with anti-human p53 mouse monoclonal antibody (DO-7 or Ab240, Dako Corp., Carpinteria, CA or Zymed Laboratories, South San Francisco, CA, respectively) as previously described.24 The percentage of positively immunostained tumor cell nuclei was determined from the number of nuclei containing immunoreaction product divided by the total number of nuclei, both immunostained and unstained. A minimum of 100 tumor cells were scored. Those breast tissues with p53 immunostaining in at least 10% of the cell nuclei were considered to have p53 overexpression, while those with less than 10% p53 immunostained nuclei were considered to be within the normal range of p53 expression.24
DNA Ploidy Analysis
DNA ploidy analysis was performed on all samples as previously described.25
Statistical Analyses
The association between p53 overexpression and p53 mutations was evaluated using Fishers exact test.
| Results |
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SSCP screening identified altered mobility patterns in 25 of 58
cases for at least one of the exons evaluated (Figure 1)
. Sequencing confirmed the presence of
mutations in 12 cases (Figure 2
and
Tables 1 and 2
), the presence of DNA sequence alterations of unknown
significance in three cases (Tables 1 and 2)
, and the presence of DNA polymorphisms
in 8 cases (Table 3)
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In 10 of 27 (37%) breast CIS cases with no concurrent invasive
carcinoma there were six (22%) mutations (Table 1)
, two (7.5%)
alterations of unknown significance (Table 1)
and two (7.5%)
polymorphisms (Table 3)
. Three of 6 mutations, and both alterations of
unknown significance, had comedocarcinoma histology. One mutation had
papillary histology, one had micropapillary histology, and one had
cribriform histology. One polymorphism had papillary histology and one
had solid histology (Table 3)
. The DNA sequence alterations were
distributed between exons 4 and 11 (Figure 3)
. One deletion of a G in codon 112 was
predicted to result in a shift in the open reading frame and the
introduction of a stop at codon 122 in exon 4. Four mutations, two
Arg248Gln and two Arg249Gly, were found in exon 7. One Ala276Pro
mutation was identified in exon 8 and one Gly325Stop mutation was
identified in exon 9. A C
T polymorphism (CCG
CCA) was found in the
4th exon at codon 36, a C
G polymorphism (CCC
CGC) at codon 72 and
two identical T
A changes, at nucleotide 18717 in the p53 genomic
sequence, were noted in exon 11 after the termination codon (Table 1)
.
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A base change (at nucleotide 18717) after the termination site in
exon 11, did contain the same sequence alteration in adjacent benign
tissue as would be expected for a DNA polymorphism. Analysis of Mutations in Breast CIS and Invasive Disease
Among the 31 cases of breast CIS with invasive disease,
6 (19%) had mutations, 1 had a DNA sequence alteration of unknown
significance, and 6 had DNA polymorphisms. Three of the mutated cases
had comedocarcinoma histology, one had a micropapillary histology, one
had a solid histology, and one had a cribriform histology. The
alteration of unknown significance had a micropapillary histology. The
6 polymorphisms consisted of 1 case with comedocarcinoma, 2 cases
with solid histology, and 3 cases with cribriform histology (Table 2)
.
The DNA alterations occurred between the third intron and the ninth
exon.
The six breast CIS cases with p53 mutations had sequence alterations
distributed from exon 5 though exon 9 (Table 2
and Figure 3
). No
mutations were identified in exons 2, 3, 4, 10, or 11. One case had a
Gln165Stop mutation in exon 5. Two mutations were in exon 6, coding for
a Leu194Ile and a Tyr205Ser change. Two mutations occurred at the DNA
binding codons: an Arg248Trp mutation was found in exon 7 and an
Ala276Pro change in exon 8. Another case had 16 bases deleted in exon
9, a loss of codons 323 to 328. One alteration of unknown significance
occurred in the third intron, a 3 base insertion beginning at base
11992. Among these cases, there were a total of 6 with polymorphisms: 2
in codon 72 of the 4th exon, a C
G transversion (Pro72Arg), and 4 in
codon 213 of the sixth exon, an A
C transversion (Arg213Arg).
Comparisons of the mutations in the CIS and invasive components in all
5 cases with mutations and sufficient tissue available revealed the
same mutation in both components. A sixth case with a DNA mutation did
not have sufficient invasive material for analysis. In cases lacking
p53 mutation in the breast CIS component, no mutations were detected in
the invasive component. As in the cases with only CIS, no mutations of
p53 were found in the benign epithelium of 3 cases with p53 mutations
in both CIS and invasive disease (Table 2)
.
p53 Protein Expression
In the entire cohort, 17 of the 58 cases (29%) showed p53
overexpression by immunohistochemical staining in the carcinoma. Two
cases had p53 overexpression only in benign hyperplastic tissue (Table 1)
. p53 protein was either exclusively or predominantly nuclear (Figure 2)
. Cytoplasmic staining without nuclear staining was not identified.
In frozen tissue sections immunostaining is observed in nuclei of a low
percentage of normal, proliferatively active tissues. Therefore, we
have used 10% immunostained tumor cell nuclei as a value for
separation of normal expression from overexpression.24
The
percentage of cells with nuclear staining for p53 varied from 2 to
90%. Among those cases showing any staining only two cases had less
than 10% of tumor cell nuclei positively immunostained. One of these
had a p53 polymorphism and the other had wild-type p53.
p53 overexpression was noted in 6 of 27 (22%) cases of breast CIS
without invasion (Table 1)
. The percent of nuclei immunostained ranged
from 10% to 47% and the average staining was 32.5%. Among the 6
cases of CIS with p53 overexpression, 3 were classified as
comedocarcinoma, one as solid, one as cribriform and one as
micropapillary CIS. Six cases had sufficient benign epithelium for
interpretation of immunostaining. Four cases with benign ductal
epithelium lacked p53 immunostaining, while two cases exhibited p53
immunostaining in areas of benign ductal hyperplasia. Neither case with
benign hyperplasia had a p53 mutation in the CIS portion. The amount of
hyperplastic breast tissue available from these cases was insufficient
for DNA sequence analysis (Table 1)
. Eleven (35%) of 31 cases of CIS
with invasive disease had p53 overexpression (Table 2)
. Seven cases
with CIS components had comedocarcinoma histology, two had
micropapillary histology, and two had cribriform histology. The percent
of nuclei immunostained ranged from 14% to 90% and the average
staining among these samples was 41.5%. Six cases with p53
overexpression in the CIS had no expression in the normal breast
epithelium. Only one case (C425) with both p53 immunostaining and p53
mutation had sufficient benign DNA for sequencing. The benign tissue
from this case did not have a mutation in 3 separate areas sampled.
Comparison of p53 Mutations with Protein Overexpression
Overall, 11 of the 58 cases (19%) had both p53 overexpression and
either mutations or sequence alterations of undetermined significance,
37 had neither mutations nor overexpression, 6 had overexpression
without mutations and 4 had a p53 mutation but lacked overexpression.
Overexpression of p53 was highly correlated (P
< 0.0001, Fishers exact test) with the presence or absence of
mutations/sequence alterations (Table 4)
.
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| Discussion |
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Little direct evidence of genetic alterations in CIS is available in the literature. Only three studies have examined cohorts of CIS using SSCP as a screening tool followed by DNA sequence analysis to confirm apparent mutations.17-19 Two of these studies screened exons 4 to 9,17,19 and one screened only exons 5, 7, and 8.18 Three other studies screened with either immunohistochemistry20,21 or SSCP of exons 49.22 However, one immunohistochemistry study confirmed only 2 mutations in 8 cases of p53 overexpression,21 and the other immunohistochemical study was able to confirm only 1 mutation among 4 immunohistochemically positive tissues.20 The last report did not attempt to confirm the presence of mutations after a positive SSCP screening.22 Taken together, these screening studies revealed 28 possible mutations; however, only 9 p53 gene mutations were confirmed.17-21 These p53 gene mutations include 7 missense mutations: Arg202His,19 His214Arg,17 Ser215Gly,19 Met237Ile,19 Gly248Asp,21 and two Arg273His mutations.18,20 Two frame shift mutations were noted and both were predicted to result in premature termination codons and truncated p53 protein.17,19 The frequencies of confirmed p53 mutations in the breast CIS cohorts screened were 3%,18,20 4%,21 11%19 and 12%.17
The present work shows a frequency of p53 mutations in breast CIS lacking invasive carcinoma (22%) much higher than has been previously described17-21 and similar to the frequency of p53 mutations in breast CIS with coincident invasive carcinoma (19%). This rate is similar to the 22% rate found in a single large study of p53 mutations in invasive breast cancer.26 The higher percentage of mutations in this study as compared to the previous work is probably related to the more detailed molecular analysis performed in the current cohort. For instance, this is the only study of breast CIS to screen all coding exons of p53. Twenty percent of DNA alterations were located outside of exons 4 to 9 in this study, and 40% of all p53 DNA alterations would have been undetected if only exons 5 to 8 had been screened. This study shows that to assess the p53 gene completely for alterations, all exons should be screened.
There is no evidence of increased p53 overexpression during progression
from CIS to invasive disease within the same tumor (Table 2)
.
Furthermore, although the frequency of p53 overexpression in breast CIS
lacking invasion (22%) was lower than the frequency of p53
overexpression in breast CIS cases having invasion (35%), this
difference was not significant.
Those breast tumors with both a CIS component and an invasive component
with wild-type p53 in the CIS component by DNA sequence analysis also
had wild-type p53 in the invasive component (data not shown). When p53
mutations were identified in a CIS component by DNA sequence analysis,
the identical mutation was identified in the invasive component of the
tumor, suggesting a clonal relationship between CIS and invasive
disease (Table 2
and Figure 2
). In those cases of carcinoma with p53
mutations and sufficient benign ductal tissue for DNA analysis, no
evidence of p53 mutations was found in the benign ductal epithelium. A
recent report showed a similar lack of p53 mutations in benign ductal
epithelium compared to CIS.27
These results suggest that
p53 mutations occur before invasion in the progression of the disease,
primarily in intraductal breast carcinomas.
There were 8 polymorphisms found in this cohort (Table 3)
. Arg72Pro was
the only polymorphism associated with a change in the amino acid
sequence. The codon 72 alteration has recently been implicated in the
pathogenesis of cervical cancer by one group,28
but this
result was recently questioned.29
The effect of this
polymorphism in breast cancer is unclear. The Arg213 alteration does
not code for an amino acid change. This codon is the most frequently
altered non-missense codon30
and is reported to be a
polymorphic site.31
Noncoding DNA sequence alterations
have been reported at high frequency in sporadic breast
tumors32
and certain silent mutations at the wobble
nucleotide in the fibroblast growth factor receptor 2 gene appear to
have phenotypic effects,33
possibly through the
opening of a cryptic splice site. It is not clear whether similar
alterations may have similar effects in p53.
The p53 mutations reported in this study can be divided into three groups. Mutations were found in sites which interact directly with DNA (Arg248Gln, Arg248Trp, Ala276Pro),34 in sites that preserve p53 structural integrity (112delG, Gly165Stop, Leu194Ile, Tyr205Ser, Arg249Gly),35 and in sites outside the core domain (Gly325Stop, del323328, and alterations of unknown significance).36,37 Among the 12 mutations, five interfere directly with p53/DNA binding, six interfere with internal stabilization of p53, and one codes for a termination site in the 3' oligomerization domain.
The alterations of unknown significance are not easily classified. One such alteration effected p53 outside of the core domain but, nonetheless, exhibited p53 overexpression. This C-to-A alteration in the third intron does not appear to effect any cryptic splice sites. However, this alteration was within the putative lariat branch point.38 Loss of this site could impair the proper splicing of this intron. The observed overexpression of p53 in this case (14% of nuclei immunostained for p53 protein) suggests that this alteration may be significant.
The T
A alteration of bp 18717 in p53 (Table 1)
appears to be a
polymorphic site because it exists in the normal epithelium in one
case. However, evidence suggests that this alteration is relevant to
p53 function. First, the two cases in this study, as well as 4 of 5
ovarian carcinomas (Reles A, Wen W-H, Press M, manuscript in
preparation) exhibiting this alteration, overexpress p53 protein. This
indicates a possible mechanism of p53 overexpression that is not
related to mutations in the coding sequence. It has been noted in the
human
-globin mRNA that alterations of the 3' untranslated region
can determine RNA localization, polyadenylation, translation
initiation, and RNA stability.39
The stability of
-globin mRNA is modulated by the binding of a ribonucleoprotein
complex to specific polycytosine sites in the 3' untranslated region of
the gene.40
The sequence of this site in
-globin is
5'-CCTCCCTCCCC-3'. In p53, the site of the bp 18717 T -> A alteration
is 5'-CCTCCCT/ACCCC-3', which is an exact match
for the consensus binding site of
-globin mRNA. Certain
substitutions in the consensus binding site destabilize the
-globin
mRNA; substitutions at the T
A site, however, were not
tested.41
It appears possible that a polycytosine binding
protein may also influence the stability of p53 mRNA. Alterations such
as the T
A substitution in these consensus sequences could possibly
increase translation of p53 protein and lead to the observed p53
overexpression.
In conclusion, this study demonstrated genetic alterations of p53 as well as overexpression of the p53 protein. A number of conclusions can be reached based on this data. First, although the results of prior studies suggested that there was an increased percentage of p53 mutations in invasive disease as compared to CIS, our results did not support this view, suggesting that most p53 mutations in breast occur before invasion of the breast stroma. In combination with the lack of p53 mutations in benign epithelium in cases with p53 mutations in carcinoma components, it appears that p53 may be altered during formation of the CIS lesion in the pathogenesis of breast cancer. Second, identical mutations in both CIS and invasive components of the same tumor support a precursor-product relationship between CIS and invasive disease consistent with a clonal relationship between CIS and invasive disease. Finally, p53 mutations occur in most histological types of intraductal breast cancer. These results suggest that p53 alterations are important in the pathogenesis of early breast neoplasia.
| Acknowledgements |
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| Footnotes |
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Supported in part by grants from the National Cancer Institute (CA48780 and CA58197), the U.S. Army Medical Research and Materiel Command (DAMD 17-96-1-6156 and DAMD 1794-J-4234), and the National Institutes of Health National Center for Research Resources (GCRC MO1 RR-43). J. L. was supported by a predoctoral training grant from the California Breast Cancer Research Program (training grant ITB-0091-L01).
Accepted for publication September 29, 1999.
| References |
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helices help us to understand p53. Cell 1994, 78:543-546[Medline]
-globin mRNP stability complex. EMBO J 1995, 14:4357-4364[Medline]
-globin mRNA stability. Mol Cell Biol 1995, 15:1769-1777[Abstract]
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