
[Cancer Research 60, 6311-6313, November 15, 2000]
© 2000 American Association for Cancer Research
Somatic Mutations of LKB1 and ß-Catenin Genes in Gastrointestinal Polyps from Patients with Peutz-Jeghers Syndrome1
Michiko Miyaki2,
Takeru Iijima,
Keiko Hosono,
Reiko Ishii,
Masamichi Yasuno,
Takeo Mori,
Masakazu Toi,
Tsunekazu Hishima,
Nobuyuki Shitara,
Kazuo Tamura,
Joji Utsunomiya,
Naoyuki Kobayashi,
Toshio Kuroki and
Takeo Iwama
Hereditary Tumor Research Project [M. M., T. Ii., K. H., R. I.] and Departments of Surgery [M. Y., T. M., M. T.], Pathology [T. H.], and Neurosurgery [N. S.], Tokyo Metropolitan Komagome Hospital, Tokyo 113-8677; Hyogo College of Medicine, Hyogo 663-8501 [K. T., J. U.]; Keio University, Tokyo 160-8582 [N. K.]; Institute of Molecular Oncology, Showa University, Tokyo 142-8666 [M. M., T. Ii., T. K.]; and Kyoundo Hospital Sasaki Institute, Tokyo 101-0062 [T. Iw.], Japan
 |
ABSTRACT
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Peutz-Jeghers syndrome (PJS) is characterized by multiple
gastrointestinal hamartomatous polyps, mucocutaneous melanin
deposition, and increased risk of cancer, mainly in the
gastrointestinal tract. We examined mutations of the
LKB1, ß-catenin,
APC, K-ras, and p53 genes
in 27 gastrointestinal hamartomatous polyps from 10 patients in nine
PJS families. Of these hamartomatous polyps, one intestinal polyp had
an adenomatous lesion, and one gastric polyp contained adenomatous and
carcinomatous lesions. Germ-line mutations of the LKB1
gene were detected in six PJS families. Somatic mutations of the
LKB1 gene were found in 5 polyps, whereas loss of
heterozygosity (LOH) at the LKB1 locus at 19p was seen
in 14 other polyps. In adenomatous lesions microdissected from
hamartomatous polyps, both ß-catenin
mutation and 19p LOH were detected. Furthermore, a carcinomatous lesion
in a gastric hamartomatous polyp was found to contain a mutation of the
p53 gene and LOH at the p53 locus in
addition to LOH at the LKB1 locus and a
ß-catenin mutation.
K-ras mutations were detected in a few polyps, whereas
no APC mutation or 5q LOH was detected in hamartomatous
polyps. These results suggest that gastrointestinal hamartomatous
polyps in PJS patients develop through inactivation of the
LKB1 gene by germ-line mutation plus somatic mutation or
LOH of the unaffected LKB1 allele, and that additional
mutations of the ß-catenin gene and
p53 gene convert hamartomatous polyps into adenomatous
and carcinomatous lesions.
 |
Introduction
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PJS3
(1
, 2)
is an autosomal dominant disease that is
characterized by multiple gastrointestinal hamartomatous polyps and
melanin spots on the lips and buccal mucosa at a young age. Patients
are at increased risk of forming carcinomas in the gastrointestinal
tract, uterus, ovary, breast, and other extragastrointestinal organs
(3, 4, 5)
. The LKB1 (STK11) gene at
chromosome 19p13.3 was identified in 1998 (6
, 7)
as the
main causative gene for PJS, and germ-line mutations of this gene have
been detected in about 60 PJS families. However, mechanisms underlying
the development of hamartomas and carcinomas are still not fully
understood. Hamartomas are assumed to progress to carcinomas because
some hamartomas have been reported to contain adenomatous and/or
carcinomatous lesions (8, 9, 10)
, but it is not known whether
carcinomas that develop from hamartomas acquire genetic alterations
similar to those seen in sporadic colon carcinomas. Although LOH at 19p
near the LKB1 gene has been demonstrated in hamartomatous
polyps (11
, 12)
, and 17p LOH has been detected in colon
carcinomas (12)
, other somatic alterations in tumors from
PJS patients remain unclear. In the present study, we examined
alterations in LKB1 and other genes in gastrointestinal
polyps from PJS patients. We found somatic mutations of the
LKB1 gene and the ß-catenin gene,
which are probably causative alterations for hamartomas and/or
carcinomas in PJS.
 |
Materials and Methods
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Materials.
Twenty-seven gastrointestinal polyps and corresponding normal tissues
were obtained from 10 patients in nine Japanese PJS families after
obtaining informed consent. Polyp samples (555 mm in diameter)
included 14 freshly frozen specimens and 13 formalin-fixed,
paraffin-embedded specimens. These polyps were diagnosed
histopathologically as hamartomatous polyps. One intestinal polyp had
an adenomatous lesion, and one gastric polyp contained adenomatous and
intestinal-type carcinomatous lesions. Genomic DNA was extracted from
each specimen using proteinase K, SDS, and phenol-chloroform.
Mutation Analysis.
DNA samples were amplified for SSCP analysis of the LKB1,
APC, ß-catenin, K-ras,
and p53 genes using PCR. The primers used to analyze
germ-line and somatic mutation of LKB1 gene were the same as
those reported previously (7
, 13)
. Primers for somatic
mutations of the APC, ß-catenin, and
p53 genes were the same as those described in previously
published reports (14, 15, 16)
, and primers for the
K-ras mutation were the ras Gene Primer set
(Takara Biochemicals, Kyoto, Japan). Conditions for PCR were the same
as those described previously (14)
. When abnormal bands
were detected in the SSCP analysis, single-stranded DNA fragments were
extracted, amplified by asymmetrical PCR, and then subjected to direct
sequencing by a dideoxy chain termination reaction (14)
.
LOH Analysis.
LOH was analyzed in acrylamide gel electrophoresis after amplification
of DNA by PCR using microsatellite primers. Primers used for 19p LOH
were D19S886, D19S878, D19S894, and D19S565 (11
, 17)
;
primers D5S346 and (AC)10 were used for 5q LOH, and primer TP53 was
used for 17p LOH.
 |
Results
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Germ-line Mutation of the LKB1 Gene.
Germ-line mutations of the LKB1 gene were identified in six
patients in independent Japanese PJS families (Table 1)
. Five of the six patients had frameshift mutations resulting in a
truncated LKB1 protein, and one patient had a mutation at the 5' splice
site of exon 2. Of these frameshift mutations, 1 was a 25-bp insertion
that was a repeat of the sequence from the third base of codon 189 to
the third base of codon 197. All six of these mutations were novel
mutations.
Somatic Mutation and LOH of the LKB1 Gene in
Hamartomatous Polyps.
Somatic mutations of the LKB1 gene were found in 5 of 27
(19%) intestinal hamartomatous polyps (Fig. 1
; Table 2
). TTGT deletions at codons 263265 were detected in four polyps with
an identified germ-line mutation, and a C insertion at codons 216217
was detected in one polyp, all of which caused truncation of the LKB1
protein. LOH of the LKB1 locus was analyzed using
microsatellite markers of 19p near to the LKB1 gene.
Fourteen of 27 (52%) hamartomatous polyps without somatic mutation of
the LKB1 gene showed 19p LOH. In total, 19 of 27 (70%)
hamartomatous polyps showed possible inactivation of the
LKB1 gene. In PCR-SSCP analysis, one intestinal
hamartomatous polyp, PJR4 P1, showed a decreased intensity of the
normal bands (corresponding to the normal allele) compared with the
mutant bands (corresponding to germ-line mutation; data not shown),
indicating inactivation of the LKB1 gene through germ-line
mutation and loss of the normal allele. K-ras mutations of
GGC (Gly) to GAC (Asp) at codon 13 were detected in 3 of 27 polyps, but
no APC mutation or 5q LOH was detected in hamartomatous
polyps.

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Fig. 1. Nucleotide sequence of somatic mutation of the
LKB1 gene in DNA from an intestinal hamartomatous polyp
from a PJS patient. TTGT deletion at codons 263265 is present in
polyp P1 from PJS patient PLK76, with a germ-line mutation at codon 37
of this gene.
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Genetic Alterations in Adenomatous Legions in PJS Polyps.
In one hamartomatous polyp, PLK396P6 (35 mm in size), a somatic
ß-catenin gene mutation was present in a small
(3-mm) adenomatous legion that was dissected from a paraffin-embedded
specimen (Fig. 2)
. This lesion also had 19p LOH. Mutations of the
ß-catenin gene were also detected in five other
large (2550-mm) intestinal polyps (Table 3)
. In a specimen of gastric hamartomatous polyp PLK2763GaP1 (55 mm),
an adenomatous lesion was present between the hamartomatous and
carcinomatous areas. This adenomatous lesion was found to have
ß-catenin gene mutation and 19p LOH. Mutations
detected in adenomatous lesions of intestinal polyps were ACC (Thr) to
GCC (Ala) at codon 41, and a mutation in an adenomatous lesion of
gastric polyp was TCT (Ser) to CCT (Pro) at codon 37.

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Fig. 2. ß-Catenin gene mutation in
DNA from an adenomatous lesion within an intestinal hamartomatous polyp
from a PJS patient. The adenomatous area was dissected and analyzed for
ß-catenin mutation. Mutation from ACC
to GCC at codon 41 is present in DNA from the adenomatous lesion of
hamartomatous polyp PLK396P6.
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Genetic Alterations in a Carcinomatous Lesion in a PJS Polyp.
A carcinomatous lesion of gastric polyp PLK2763GaP1 had a
p53 gene mutation from CAT (His) to GAT (Asp) at codon 193
and LOH at TP53. The carcinomatous lesion exhibited the same
ß-catenin gene mutation as that in an
adenomatous lesion of this polyp. 19p LOH near the LKB1
locus was detected in this carcinoma as well as in the adenomatous and
hamartomatous areas of the same gastric polyp.
 |
Discussion
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In hamartomatous polyps from PJS patients, the present study
demonstrated somatic mutations as well as germ-line mutations of the
LKB1 gene, suggesting inactivation of the both alleles.
Somatic mutations of the LKB1 gene were present in 5 of 27
(19%) hamartomatous polyps, and LOH of 19p near the LKB1
locus was detected in 14 of 27 (52%) hamartomatous polyps. Loss of the
normal allele of LKB1 was confirmed in a hamartoma (PJR4P1)
by SSCP analysis. These results suggest that although a certain
percentage of hamartomatous polyps of PJS patients develop by
inactivation of the LKB1 gene through germ-line mutation
plus loss of the normal allele, additional mutation in the normal
allele facilitates development of this disease.
With respect to the further development of hamartomatous polyps,
adenomatous and carcinomatous lesions are often found within some
gastrointestinal hamartomatous polyps from PJS patients (8
, 9
, 10)
. Because gastrointestinal carcinomas of PJS are
adenocarcinomas, precursors of these carcinomas are assumed to be
adenomas. Histopathological observations indicated that adenomatous
epithelium evolves within preexisting hamartomatous polyps, and
carcinomatous changes were seen in the adenomatous area
(8)
. It has also been reported that PJS patients have such
adenomatous lesions in the colon, small intestine, pancreatic duct, and
stomach (8)
.
In the present study, we detected ß-catenin
gene mutations in six large intestinal polyps, with the position and
direction of these ß-catenin mutations being
similar to those observed frequently in sporadic colorectal adenomas
and adenocarcinomas. Moreover, we found that a small adenomatous lesion
within one large hamartomatous polyp had both
ß-catenin gene mutation and 19p LOH. This
result seems to be consistent with previous histopathological
observations. In five other intestinal polyps with
ß-catenin mutation, the intensity of mutant
bands of the ß-catenin gene in SSCP was rather
weak compared with that of the normal bands, which indicated that the
extent of cells having ß-catenin mutation
within hamartomatous polyps was small, although the adenomatous areas
could not be clearly identified in these five polyps. In the gastric
polyp, ß-catenin gene mutation and 19p LOH were
detected in an adenomatous lesion between the carcinomatous and the
hamartomatous area. The carcinomatous lesion of this polyp had
p53 gene mutation and 17p LOH, in addition to 19p LOH and
the same ß-catenin gene mutation as that seen
in the adenomatous lesion. These data suggest the following mechanisms:
inactivation of the LKB1 gene causes the formation of a
hamartomatous polyp, and during growth of the hamartoma,
ß-catenin gene mutation occurs, resulting in a
change of the hamartoma to an adenoma because the activating
ß-catenin mutation has been known to be
associated with adenoma formation (18
, 19)
. Such an
adenomatous lesion may progress to carcinoma through additional
genetic alterations, such as p53 mutation, as seen in
sporadic colorectal carcinogenesis (16)
. The contribution
of APC mutation to PJS carcinogenesis appears to be
small.
Considering the present and previous observations, we propose a
possible hamartoma-adenoma-carcinoma sequence for gastrointestinal
carcinogenesis in PJS patients, rather than the direct change of a
hamartoma into an adenocarcinoma. PJS patients develop not only
gastrointestinal carcinomas but also various extragastrointestinal
carcinomas, which are assumed to be formed by inactivation of the
LKB1 gene, as evidenced by breast carcinomas from our PJS
patients showing 19p LOH (data not shown). To clarify the mechanism of
increased risk of carcinogenesis in PJS patients, a more detailed
analysis of genetic changes in both gastrointestinal and
extragastrointestinal tumors is necessary.
 |
FOOTNOTES
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The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
1 Supported in part by the Project
"High-Technology Research Center" and a Grant from the Ministry of
Education, Science, Sports and Culture of Japan. 
2 To whom requests for reprints should be
addressed, at Hereditary Tumor Research Project, Tokyo Metropolitan
Komagome Hospital, 3-18-22 Honkomagome, Bunkyo-ku, Tokyo 113-8677,
Japan. Phone: 81-3-3823-2101, ext. 4425; Fax: 81-3-3823-5433;
E-mail: mmiyaki{at}opal.famille.ne.jp 
3 The abbreviations used are: PJS, Peutz-Jeghers
syndrome; LOH, loss of heterozygosity; SSCP, single-strand conformation
polymorphism. 
Received 7/ 5/00.
Accepted 9/26/00.
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