(American Journal of Pathology. 2000;156:1767-1771.)
© 2000 American Society for Investigative Pathology
BRCA2 Is Inactivated Late in the Development of Pancreatic Intraepithelial Neoplasia
Evidence and Implications
Michael Goggins,
Ralph H. Hruban and
Scott E. Kern
From the Departments of Pathology and Oncology, Johns Hopkins
Medical Institutions, Baltimore, Maryland
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Abstract
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Patients harboring germline BRCA2 mutations are at
an increased risk of developing pancreatic cancer. We investigated the
prevalence of biallelic inactivation of BRCA2 in the
presumed precursors to invasive pancreatic ductal carcinomas,
pancreatic intraepithelial neoplasia (PanIN). Surgical resection
specimens from three patients with germline BRCA2
mutations who developed pancreatic ductal adenocarcinoma were studied.
Fourteen PanINs were needle-microdissected from paraffin-embedded
tissue. DNA was isolated from these microdissected tissues and
amplified by primer-mediated pre-amplification. Loss of heterozygosity
at the BRCA2 locus was determined by polymerase chain
reaction amplification and cycle sequencing. The presence of the
wild-type alleles was evaluated at the nucleotide positions of the
germline BRCA2 mutations. The K-ras gene
was sequenced at codon 12 and 13 to confirm the efficacy of
microdissection. By histological evaluation the prevalence of PanINs in
these patients was not notably elevated. Loss of the wild-type allele
of BRCA2 was present in one high-grade PanIN (PanIN
3), but in none of 13 low-grade PanINs (PanIN 1). In
contrast, K-ras mutations were detectable in 7
of the 14 PanINs. These results suggest that biallelic inactivation of
the BRCA2 gene is a relatively late event in pancreatic
tumorigenesis. In contrast to classical molecular progression models of
tumorigenesis, the inactivation of the wild-type allele in a
carrier of a recessive tumor susceptibility gene may not always be the
first somatic event during the molecular evolution of a cancer.
The necessity for earlier genetic alterations before biallelic
inactivation of a recessive tumor susceptibility gene such as
BRCA2 may explain why affected carriers have normal
numbers of neoplastic precursor lesions, a relatively low
phenotypic penetrance, and late age of onset of pancreatic and
other cancers.
 |
Introduction
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Hereditary predisposition to
adenocarcinoma of the pancreas is clinically evident in approximately
10% of patients who develop the disease.1-3
Germline
BRCA2 mutations account for a portion of this group, and as
many as 5 to 10% of patients with apparently sporadic pancreatic
cancer harbor germline BRCA2 mutations.4,5
The
lifetime risk of pancreatic cancer in carriers of a BRCA2
germline mutation is, however, probably in the range of
5%.4-8
The genetic and environmental influences that
result in the low penetrance of pancreatic cancer in BRCA2
mutant carriers are poorly understood. A critical genetic event that
would be expected to influence penetrance of pancreatic cancer in these
carriers is the occurrence and timing of inactivation of the wild-type
BRCA2 allele in pancreatic epithelium. For example, in
patients with familial adenomatous polyposis (FAP), the inactivation of
the wild-type allele of APC in colonic epithelial stem cells
is probably the first genetic alteration (the gatekeeper) during the
evolution of a neoplasm. Loss of the wild-type APC allele
occurs in early adenomas both in FAP patients and the MIN
mouse.9-12
In breast and pancreatic cancers that occur in
carriers with germline BRCA2 mutations, the inactivation of
the wild-type allele is usual.4,13
Yet the timing of loss
of the wild-type copy of BRCA2 has not been studied in
neoplastic precursor lesions that develop in patients with germline
BRCA2 mutations, and there are reasons to suspect that
BRCA2 does not follow the gatekeeper model.
To study the timing of BRCA2 alterations in the development
of pancreatic cancer we took advantage of the observation that multiple
neoplastic precursor lesions are often present in the pancreata of
patients with pancreatic ductal adenocarcinomas.14-20
These lesions are called pancreatic intraepithelial neoplasia (PanIN;
see http://www.path.jhu.edu/pancreas_panin). By determining the
frequency of genetic alterations in the PanINs of varying histological
severity, one can establish a progression model for the development of
infiltrating adenocarcinoma of the pancreas. The histological and
genetic analysis of such neoplastic precursors can provide insights
into the genetic progression of pancreatic cancer in individuals with
germline BRCA2 mutations.
We investigated the timing of biallelic inactivation of the
BRCA2 gene in PanINs by analyzing DNA from a series of
microdissected PanINs located in the pancreatic parenchyma adjacent to
invasive pancreatic carcinomas resected from patients with germline
BRCA2 mutations. DNA from these PanINs was analyzed for loss
of the wild-type allele at BRCA2 and for the presence of
K-ras gene mutations.
 |
Materials and Methods
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Patients with germline BRCA2 mutations who developed
pancreatic cancers were identified previously.4
From the
archives of The Johns Hopkins Hospital, slides of pancreatic resection
specimens from these cases with germline BRCA2 mutations
were reviewed. Three cases were selected based on the presence of
infiltrating pancreatic adenocarcinoma of the pancreas and the
availability of archival material adequate for the study of associated
PanINs (Table 1)
. PanINs were classified
using criteria established at the National Cancer Institute-sponsored
Pancreatic Cancer Think Tank in Park City, Utah
(http://www.path.jhu.edu/pancreas_panin). Briefly, PanINs were
classified as grade 1 when duct lesions lacked significant nuclear
abnormalities, as grade 2 when duct lesions had moderate cytological
and architectural atypia, and grade 3 when those lesions showed marked
architectural or cytologic atypia. The histological features
were graded by an experienced pathologist (R. H. H.) familiar
with the Park City classification scheme before the molecular analysis.
Seven-micron sections from formalin-fixed, paraffin-embedded tissue
blocks were stained with hematoxylin and eosin. PanINs, infiltrating
carcinoma, and adjacent normal tissue were dissected under direct
visualization using an inverted microscope and a glass needle attached
to a micromanipulator.21
Mock dissections were performed
on slides lacking tissue, from which the tip of the glass needle was
processed for DNA isolation and polymerase chain reaction (PCR)
amplification to examine for and rule out contamination during the
process of microdissection. DNA was extracted from microdissected
tissue using 500 µg/ml of proteinase K and 0.5% NP40 and
incubated overnight at 56°C. DNA samples were then subjected to whole
genome amplification by primer-mediated pre-amplification (PEP) as
previously described.22
We have previously observed
stochastic errors in PCR when amplifying paraffin-embedded DNA when the
input DNA is less than ~50 cells, particularly if the PCR products
are over 400 bp.21
Hence, PEP was performed on 300 to 600
microdissected cells and amplified using a degenerate 15-mer for 50
cycles of 92° for 30 seconds, 37° for 2 minutes, and 55° for 4
minutes, ramping at 1° every 10 seconds. The reproducibility of PEP
to amplify two alleles equally was determined by PEP amplification of
DNA isolated from normal pancreatic acini and subsequently amplifying
with PCR primers that specifically amplified the region of DNA spanning
the patients known germline BRCA2 mutations. These results
obtained using PEP were also confirmed by direct PCR amplification and
sequencing of microdissected tissues in the absence of PEP. Sequencing
of PCR products confirmed that PEP had amplified both BRCA2
alleles from all 18 samples. PCR was also performed using primers to
amplify across the region of the K-ras gene containing
codons 12 and 13. PCR products were subsequently analyzed by DNA cycle
sequencing as described.23
Loss of heterozygosity (LOH) at
the BRCA2 locus was determined by the absence of the
wild-type nucleotide sequence at the site of the germline mutation.
 |
Results
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The number and morphology of PanIN lesions were reviewed by
obtaining all available hematoxylin-and-eosin-stained archival slides
of the pancreatic carcinoma resection specimens from the three patients
with known germline BRCA2 mutations. All three patients had
undergone a Whipple resection. There were no observable qualitative or
quantitative differences in the number or morphology of the PanINs
compared to that seen in patients without germline BRCA2
mutations.
Three PanINs were selected from case PX182, four PanIN from case PX66,
and seven PanIN from case PX101 for microdissection. The PX series
comprises unique patients whose carcinomas were expanded by
xenografting to allow genetic analysis. These 14 PanINs included 13
PanIN-1 and one PanIN-3. The germline mutations in the three cases were
6158insT, 6174delT, and 2481insT, respectively, as previously
described.4
Loss of the wild-type allele was evident in
xenografts of the pancreatic adenocarcinoma in two of the three cases
(6174delT and 2481insT).4
LOH at the BRCA2 gene
locus was present in the single PanIN-3 (from PX101), but in none of 13
low grade duct lesions (PanIN-1; Figures 1 and 2
).
LOH was not detected in three normal ducts, nor was it detected in
multiple microdissections of pancreatic acini containing ~10002000
cells.

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Figure 1. Sequence of the BRCA2 gene upstream of the 2481insT
germline mutation in two PanIN lesions from a patient with pancreas
cancer (case PX101). The
first duct lesion (lane pair 1,
PanIN-3) shows the mutant sequence with loss of
the wild-type BRCA2 sequence, whereas the second lesion
(lane pair 2, PanIN-1)
shows the sequence of both the wild-type and mutant alleles
(note the double bands for each nucleotide, best
seen on comparison of the T lanes). G and T
refer to deoxyguanosine and deoxythymidine termination reactions,
respectively.
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Figure 2. A: The PanIN-3 duct lesion that harbored LOH of the
wild-type BRCA2 gene. B: An example of
a PanIN-1 duct lesion from patient PX66 that did not have LOH at
BRCA2. Hematoxylin and eosin; original magnification,
x40.
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In contrast, distinct K-ras mutations were found by
sequencing in 7 of the 14 PanINs. All of the K-ras mutations
were at codon 12; six had GAT mutations, one was GTT. Results are
summarized in Table 1
. The high signal ratio of mutant to wild-type
alleles confirmed the adequacy of the microdissection to exclude
non-neoplastic cells. The variety of mutations confirmed the
expectations that these lesions would represent independent neoplasms,
each with an independent chance to suffer LOH during clonal
progression. Thus, the lack of LOH in PX182 (potentially due to a
subtle somatic mutation in the other allele) would not preclude the
evolution of independent mutations in the PanIN lesions.
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Discussion
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In this study we provide evidence for the biallelic inactivation
of the BRCA2 gene as a late event in the development of
adenocarcinoma of the pancreas in patients with germline
BRCA2 mutations. Biallelic inactivation of the
BRCA2 gene was found in only one high-grade PanIN. This low
second hit rate of LOH of BRCA2 in such lesions contrasts
with the relative frequency of K-ras mutation and
p16 inactivation.16,24-29
From this data we
conclude that BRCA2 inactivation is generally not the first
genetic alteration in PanINs from patients with germline
BRCA2 mutations.
There are a number of alternative conclusions that could be considered,
although we do not find them compelling. First, if the PanIN lesions we
studied were not clonal, then LOH would not be expected. Pancreatic
cancer is thought to occur through the clonal evolution of precursor
lesions. Yet the presence of K-ras mutations in
approximately half of the PanINs analyzed supports the clonal nature of
these lesions. The prior demonstration of the genetic inactivation of
p16 in a subset of PanINs also provided strong genetic
evidence that PanINs are the precursor lesions for pancreatic
adenocarcinoma and that they are clonal.28,29
Second, the
use of LOH as an indicator of biallelic inactivation does not take into
account alternative mechanisms of gene inactivation such as intragenic
mutation or promoter methylation. Indeed, some occasional cancers in
carriers of BRCA2 mutations lack LOH at BRCA2,
and one such cancer is included in the current study.4
Still, the available evidence points to LOH as the main mechanism of
biallelic inactivation of the BRCA2 gene in carcinomas that
arise in carriers of BRCA2 mutations.13
LOH
therefore should be present in the vast majority of PanIN if the early
biallelic inactivation of BRCA2 were critical to the
carcinogenic mechanism. Third, one might consider the low rate of
observed LOH in microdissected samples to reflect the contamination of
samples with non-neoplastic DNA. This DNA could come from two sources;
it could theoretically come from non-neoplastic tissues adjacent to the
microdissected foci or cells or DNA other than from the patient. The
first potential source of DNA contamination was controlled first by
ensuring optimal microdissection using a needle micromanipulator with
repeat analysis of all samples and then by confirmation of the expected
high rate of K-ras mutations at the expected high allele
ratio. The second potential source of DNA would be readily identified
on sequencing as the allele ratios would exhibit a predominance of the
wild-type allele of BRCA2, yet this was not observed.
Finally, an artifact could derive from the use of PEP, which, in the
case of low template copy numbers, could introduce a biased
amplification of one allele due to stochastic errors (as when the
estimated DNA template number is <50 copies). We confirmed that PEP
had amplified both alleles of the BRCA2 gene equally among a
large panel of samples; hence, an artifact of PEP is not a likely
explanation for the single LOH event that we identified in a PanIN
sample.
The rarity and apparently late onset of biallelic inactivation of
BRCA2 in PanIN contrasts with the timing of APC
genetic alterations in colorectal adenomas. Biallelic inactivation of
APC is almost certainly the first genetic event leading to
the development of adenomas in carriers of a mutant APC
gene.9-12
Another notable difference between the
APC gene and the BRCA2 gene is that patients with
familial adenomatous polyposis have a greatly increased number of
precursor neoplasms; this does not appear to be the case for precursor
neoplasms in patients with germline BRCA2
mutations.30
This suggests that unlike the APC
gene in the colon, the BRCA2 gene is not a gatekeeper for
neoplasia. An attractive explanation for the rarity of LOH of
BRCA2 is that other genetic alterations are required before
the biallelic inactivation of BRCA2 can experience favorable
selective advantages. In this regard, recent evidence in knockout mouse
models suggests that inactivation of the p53 pathway may be such a
prerequisite for the survival of embryos that have knockout of
BRCA2.31-34
Two alternate explanations might be entertained. First, epigenetic
changes might silence the remaining wild-type gene in some
circumstances, Second, BRCA2 may serve a caretaker rather
than a strictly suppressor role by participating in chromosome
maintenance functions.35
For example, inherited mutations
in caretakers such as the DNA mismatch repair genes produce few
precursor lesions and can manifest the tumor phenotype in the absence
of allelic deletions. Yet neither possibility is suggested by the
accumulated data regarding BRCA2 inactivation in carcinomas,
wherein LOH is the prevailing (although not universal) means of
inactivation of the remaining wild-type copy of BRCA2.
The relatively late onset of LOH at BRCA2 represents an
additional manifestation of the Knudson hypothesis. The fact that
biallelic inactivation of a hereditary susceptibility gene is not
always the first event in cancer progression may help explain the low
penetrance and late age of onset of pancreatic cancer in carriers of
BRCA2 mutations4,5
and the normal number of
precursor lesions. Indeed, this paradigm may be a common, yet
underappreciated, manifestation of the role of susceptibility genes in
cancer predisposition.
 |
Footnotes
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Address reprint requests to Scott Kern, M.D., Department of Oncology, 451 Cancer Research Building, The Johns Hopkins Medical Institutions, 1650 Orleans Street, Baltimore, MD 21205-2196. E-mail: sk{at}jhmi.edu
Supported by National Institutes of Health SPORE (Specialized Program of Research Excellence) in Gastrointestinal Cancer (CA62924).
Accepted for publication January 6, 2000.
 |
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L. E. Jones, M. J. Humphreys, F. Campbell, J. P. Neoptolemos, and M. T. Boyd
Comprehensive Analysis of Matrix Metalloproteinase and Tissue Inhibitor Expression in Pancreatic Cancer: Increased Expression of Matrix Metalloproteinase-7 Predicts Poor Survival
Clin. Cancer Res.,
April 15, 2004;
10(8):
2832 - 2845.
[Abstract]
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S. A. Hahn, B. Greenhalf, I. Ellis, M. Sina-Frey, H. Rieder, B. Korte, B. Gerdes, R. Kress, A. Ziegler, J. A. Raeburn, et al.
BRCA2 Germline Mutations in Familial Pancreatic Carcinoma
J Natl Cancer Inst,
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214 - 221.
[Abstract]
[Full Text]
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S. M. Ginolhac, S. Gad, M. Corbex, B. Bressac-de-Paillerets, A. Chompret, Y.-J. Bignon, J.-P. Peyrat, J. Fournier, C. Lasset, S. Giraud, et al.
BRCA1 Wild-Type Allele Modifies Risk of Ovarian Cancer in Carriers of BRCA1 Germ-Line Mutations
Cancer Epidemiol. Biomarkers Prev.,
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[Abstract]
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N. T. van Heek, A. K. Meeker, S. E. Kern, C. J. Yeo, K. D. Lillemoe, J. L. Cameron, G. J. A. Offerhaus, J. L. Hicks, R. E. Wilentz, M. G. Goggins, et al.
Telomere Shortening Is Nearly Universal in Pancreatic Intraepithelial Neoplasia
Am. J. Pathol.,
November 1, 2002;
161(5):
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[Abstract]
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K. M. Murphy, K. A. Brune, C. Griffin, J. E. Sollenberger, G. M. Petersen, R. Bansal, R. H. Hruban, and S. E. Kern
Evaluation of Candidate Genes MAP2K4, MADH4, ACVR1B, and BRCA2 in Familial Pancreatic Cancer: Deleterious BRCA2 Mutations in 17%
Cancer Res.,
July 1, 2002;
62(13):
3789 - 3793.
[Abstract]
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F X Real, N Malats, G Lesca, M Porta, S Chopin, G M Lenoir, and O Sinilnikova
Family history of cancer and germline BRCA2 mutations in sporadic exocrine pancreatic cancer
Gut,
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50(5):
653 - 657.
[Abstract]
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N. Fukushima, N. Sato, T. Ueki, C. Rosty, K. M. Walter, R. E. Wilentz, C. J. Yeo, R. H. Hruban, and M. Goggins
Aberrant Methylation of Preproenkephalin and p16 Genes in Pancreatic Intraepithelial Neoplasia and Pancreatic Ductal Adenocarcinoma
Am. J. Pathol.,
May 1, 2002;
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1573 - 1581.
[Abstract]
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A. V. Biankin, J. G. Kench, A. L. Morey, C.-S. Lee, S. A. Biankin, D. R. Head, T. B. Hugh, S. M. Henshall, and R. L. Sutherland
Overexpression of p21WAF1/CIP1 is an Early Event in the Development of Pancreatic Intraepithelial Neoplasia
Cancer Res.,
December 1, 2001;
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[Abstract]
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N. Sato, C. Rosty, M. Jansen, N. Fukushima, T. Ueki, C. J. Yeo, J. L. Cameron, C. A. Iacobuzio-Donahue, R. H. Hruban, and M. Goggins
STK11/LKB1 Peutz-Jeghers Gene Inactivation in Intraductal Papillary-Mucinous Neoplasms of the Pancreas
Am. J. Pathol.,
December 1, 2001;
159(6):
2017 - 2022.
[Abstract]
[Full Text]
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R. E. Wilentz, P. Argani, and R. H. Hruban
Loss of Heterozygosity or Intragenic Mutation, Which Comes First?
Am. J. Pathol.,
May 1, 2001;
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E EFTHIMIOU, T CRNOGORAC-JURCEVIC, N R LEMOINE, and T A BRENTNALL
Inherited predisposition to pancreatic cancer
Gut,
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[Full Text]
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E. Montgomery, M. Goggins, S. Zhou, P. Argani, R. E. Wilentz, M. Kaushal, S. Booker, K. Romans, P. Bhargava, R. H. Hruban, et al.
Nuclear Localization of Dpc4 (Madh4, Smad4) in Colorectal Carcinomas and Relation to Mismatch Repair/Transforming Growth Factor-{beta} Receptor Defects
Am. J. Pathol.,
February 1, 2001;
158(2):
537 - 542.
[Abstract]
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R. H. Hruban, M. Goggins, J. Parsons, and S. E. Kern
Progression Model for Pancreatic Cancer
Clin. Cancer Res.,
August 1, 2000;
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2969 - 2972.
[Full Text]
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R. H. Hruban, R. E. Wilentz, and S. E. Kern
Genetic Progression in the Pancreatic Ducts
Am. J. Pathol.,
June 1, 2000;
156(6):
1821 - 1825.
[Abstract]
[Full Text]
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R. E. Wilentz, M. Goggins, M. Redston, V. A. Marcus, N. V. Adsay, T. A. Sohn, S. S. Kadkol, C. J. Yeo, M. Choti, M. Zahurak, et al.
Genetic, Immunohistochemical, and Clinical Features of Medullary Carcinoma of the Pancreas : A Newly Described and Characterized Entity
Am. J. Pathol.,
May 1, 2000;
156(5):
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[Abstract]
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