Gastric adenomas are considered to be precancerous lesions, but are clinically heterogeneous,
since some may progress to adenocarcinoma, whereas others persist unchanged for long
periods (Kamiya et al, 1982; Kolodziejczyk et al, 1994; Orlowska et al, 1995). Identification
of adenoma cases with a progressive nature is important since intervention (e.g. endoscopic
mucosal resection) is mandatory. Tumour size is a prognostic indicator, but exceptional
cases are frequently observed. Histological grading of adenomas as per the Vienna
classification (Schlemper et al, 2000) has been used to assess the potential for progression.
However, exceptional cases are frequent, since 80% of high-grade adenomas progress
to adenocarcinomas, whereas 15% of low-grade adenomas progress to high-grade adenomas
or adenocarcinomas (Lauwers and Riddell, 1999). Histological diagnosis of biopsy specimens
cannot definitively identify adenomas with aggressive potential because sampling errors
may contribute to the underestimation of tumour grade or depth of invasion. Thus,
an additional prognostic indicator that is independent of conventional clinicopathological
findings (e.g. molecular markers) is essential.
Recent comprehensive analyses of gene expression, such as a microarray analysis, identified
relevant genes whose expression profiles appeared to be linked to tumour stage, histological
grade, susceptibility to chemotherapy, clinical aggressiveness or prognosis (Golub
et al, 1999; Alizadeh et al, 2000; Perou et al, 2000; Dhanasekaran et al, 2001; Sorlie
et al, 2001; Shipp et al, 2002; van’t Veer et al, 2002). Studies on gastric adenocarcinomas
revealed several gene-expression profiles that are linked to lymph node metastasis
(Hasegawa et al, 2002; Hippo et al, 2002). Using a similar approach, it may be possible
to develop an improved classification scheme for gastric tumours that is capable of
distinguishing subgroups of adenomas with progressive natures. Such expression profiles
have not been applied to gastric adenomas. In the present study, suppression subtractive
hybridization (SSH) analysis (Diatchenko et al, 1996; von Stein et al, 1997) was used
to identify genes relevant to gastric adenomas. Their expression profiles were subsequently
assessed in order to identify different progressive potentials of gastric adenomas
in comparison to adenocarcinoma.
MATERIALS AND METHODS
Tissue sample
Tissue samples of low-grade (6 mm in diameter) and high-grade (15 mm in diameter)
gastric adenomas and adjacent gastric mucosa were obtained from 68- and 44-year-old
male subjects, respectively, by endoscopic mucosal resection for use in SSH analysis.
Additional paired tissue samples of gastric tumours and adjacent mucosa were obtained
by endoscopic biopsy or mucosal resection from 14 low-grade adenomas, nine high-grade
adenomas and nine adenocarcinomas for gene-expression profiles (Table 1
Table 1
Clinical backgrounds of patients
Adenoma
Low grade (n=14)
High grade (n=9)
Adenocarcinoma (n=9)a
Age (years) (mean±s.d.)
73.5±7.6
67.2±10.8
76.1±9.2
Sex (male/female)
13/1
7/2
7/2
Tumour size (mean±s.d.)b
10.1±4.0
19.8±8.3
30.8±12.8
a
Adenocarcinoma cases included six cases of T1, two cases of T2 and one case of T3
in TNM clinical classification. Histopathological grading was G1 in seven cases and
G2 in two cases.
b
P<0.005 for low- vs high-grade adenoma, low-grade adenoma vs adenocarcinoma and P<0.05
for high-grade adenoma vs adenocarcinoma.
). The nine adenocarcinomas consisted of six T1, two T2 and one T3 tumours based on
TNM clinical classification. Histopathological grading was G1 in seven cases and G2
in two cases. Tumour size was significantly different among the low- and high-grade
adenomas and the adenocarcinomas. Informed consent was obtained from each patient
before biopsy or mucosal resection. The study conformed to the ethical guidelines
of the 1975 Declaration of Helsinki and was approved by the IRB.
RNA extraction and SMART™ cDNA synthesis
Total RNA was extracted by the modified acid–guanidium–chloroform method (Chomczynski
and Sacchi, 1987) using ISOGEN™ (Nippon Gene, Toyama, Japan). Full-length cDNAs were
generated from the total RNA using the SMART™ (Switch Mechanism at 5′ end of RNA Template)
PCR cDNA synthesis kit (Clontech, Palo Alto, CA, USA) (Matz et al, 1999) following
the manufacturer’s instruction and used for SSH analysis. Quantitative analysis of
specific genes was performed on cDNAs generated from 1 μg of total RNA in 10 μl mixture
with 200 U of Superscript™ reverse transcriptase (Gibco, Madison, WI, USA) using random
hexamer primers.
Suppression SSH and sequencing
Subtractive hybridization was performed using a PCR-Select™ cDNA subtraction kit (CLONTECH,
Tokyo, Japan) according to the manufacturer’s instructions. Briefly, after two different
adaptors were ligated to RsaI-digested SMART™ cDNA from the gastric adenoma tissues
(tester), 2.5 ng of each adaptor-ligated SMART™ cDNA was hybridised with 1.5 μg of
RsaI-digested SMART™ cDNA from the adjacent gastric mucosa (driver). In this process,
cDNA sequences specific to the tester were enriched. A total of 10 ng of PCR products
were cloned into plasmids pGEM-T Easy Vector™ (Stratagene, Cedar Creek, TX, USA) and
transformed to competent Escherichia coli XL2-blue™ Ultracompetent cells (Gibco, Madison,
WI, USA). In all, 100 colonies were randomly picked and sequenced using the PRISM
dye termination kit™ (ABI, Chiba, Japan). BLAST Search 2.0 (www.ncbi.nlm.nih.gov/blast/blast.cgi)
was used to analyse sequence homologies in the gene database.
Quantitative analysis of identified genes
Overexpression of genes identified by SSH was verified in the original samples by
semiquantitative RT–PCR using gene-specific primer sets. The PCR products were obtained
during the exponential phase of amplification and the amounts of products were compared
by agarose-gel electrophoresis. Subsequently, the mRNA expression levels of these
genes were quantitated using real-time PCR (Light Cycler System™, Roche Diagnostics,
Manheim, Germany) (Wittwer et al, 1997). The expression level of the target gene was
standardised with that of the house-keeping beta-actin gene and the ratio of each
gene expression in paired samples (adenoma or adenocarcinoma/adjacent mucosa) was
calculated. The primers used in the quantitative PCR were as follows: acyl-CoA binding
protein (ACBP)-sense, 5′AgTTTgAgAAAgCTgCAgAggAgg3′; ACBP-antisense, 5′TCCCgAATTCCCACCATCCACggT3′;
eukaryotic elongation factor 1 gamma (EEF1G)-sense, 5′TATCgCTTCCCTgAAgAACTCACT3′;
EEF1G-antisense, 5′TCgCTgCCAggATCCAgTTTCCgC3′; peripheral-type benzodiazepine receptor
(BZRP)-sense, 5′gCgACCACACTCAACTACTgCgTA3′; BZRP-antisense, 5′gCATgCAGAAAgCACAggACACTg3′;
arginase II (ARG2)-sense, 5′gAgACAAAgACCAATCCATTTgA3′; ARG2-antisense, 5′gTgTATTTCCTCAgCAATATACAT3′;
histone H2A.Z (H2AFZ)-sense, 5′TggCAggAAATgCATCAAAAgACT3′; H2AFZ-antisense, 5′ggAAAgCTAATTAAACTTCCAACT3′;
GW112-sense, 5′gAATCTTCTACCTCATAACTTCCT3′; GW112-antisense, 5′gCAACAACTgATACACTCATAAgT3′;
pepsinogen C (PGC)-sense, 5′CAGCTTGACCTTCATCATCAATG3′; PGC-antisense, 5′CCAGAGTGGAAAGACAGATACAA3′;
defensin alpha 5 (DEFA5)-sense, 5′ATCCTTgCTgCCATTCTCCTggTg3′; DEFA5-antisense, 5′ACCTgAggTTCTAAgAgCAgAgA3′;
receptor for activated C-kinase (RACK1)-sense, 5′AACAgCAAgCAACCCTATCATCgT3′; RACK1-antisense,
5′gATAACTTCTTgCTTCAgTTCATC3′; LI-Cadherin (CDH17)-sense, 5′AACTTAACgATAgAggTgTCTgAC3′;
and CDH17-antisense 5′gCTTTgAACACAATgTTggAAACA3′.
When the expression levels of target genes were below the sensitivity of the assay,
the detection limits for each gene were substituted to calculate the ratio. The ratio
was not determined when the target gene could not be quantitated in both the paired
samples. For example, the expression level of GW112 was below the sensitivity of this
quantitation system in all samples and, therefore, this gene was not included in subsequent
analyses.
Analysis of gene-expression profiles
Unsupervised hierarchical clustering analysis was performed based on similarities
of gene expression using web-available software (Expression Profiler; European Bioinformatics
Institute; http://ep.ebi. ac.uk/). The ratio of each gene expression in paired samples
was log transformed and applied to the clustering algorithm.
Statistical analyses
Data were compared using the χ
2 test or Fisher’s exact test. Distributions of continuous data were analysed by the
Mann–Whitney U-test or Student’s t-test between two groups and by ANOVA with adjustment
for multiple comparison by Scheffe’s method among three groups using Statview 5.0
software (Abacus Concepts, Berkeley, CA, USA).
RESULTS
SMART™ RT–PCR and SSH
The cDNA generated by SMART™ RT–PCR exhibited a smear pattern representing amplification
of the whole mRNA species on agarose-gel electrophoresis, whereas after SSH it consisted
of several discrete bands derived from differentially expressed genes (Figure 1
Figure 1
Electrophoretic band patterns of SSH and gene overexpression in original tester tissue.
(A) The cDNA amplified by SMART™ RT–PCR and cDNA after subtraction by SSH were electrophoresed
on 2.0% agarose. The amplified cDNA derived from gastric adenoma and adjacent gastric
mucosa appears as a smear before SSH. After SSH, it exhibits several distinct bands.
(B) Semiquantitative RT–PCR using gene-specific primer sets for each identified gene
were performed. PCR products were analysed at the PCR cycle number in the exponential
phase of amplification (ACBP 36 cycles, RACK1 27 cycles, DEFA5 36 cycles, EEF1G 13
cycles, H2FAZ 30 cycles, ARG2 36 cycles, PGC 25 cycles, BZRP 24 cycles, CDH17 22 cycles,
GW112 40 cycles, beta-actin 20 cycles). The expression level of housekeeping gene
(beta-actin) was at the same level, but those of genes identified by SSH were clearly
more abundant in low- and high-grade gastric adenoma tissues compared to their corresponding
adjacent mucosa. T and N indicate gastric tumour and adjacent normal mucosa, respectively.
). Nucleotide sequencing was performed for 100 independent clones for each SSH experiment.
Six genes represented more than once in low-grade adenoma samples: (a) RACK1 (accession
number NM_001172), (b) ARG2 (NM_001172), (c) EEF1G (NM_001404), (d) ACBP (NM_02054),
(e) H2AFZ (NM_00210) and (f) DEFA5 (NM_021010). In addition, five genes represented
more than once in high-grade adenoma samples: (a) PGC (NM_002630), (b) BZRP (NM_007311),
(c) DEFA5, (d) GW112 (AF097021), and (e) CDH17 (NM_004063) (Table 2
Table 2
Genes identified by SSH analysis
Gene
Symbol
Accession no.
Low-grade adenoma derived
Receptor for activated C-kinase
RACK1
NM-006098
Arginase II
ARG2
NM-001172
Eukaryotic elongation factor 1 gamma
EEF1G
NM-001404
Acyl-CoA binding protein
ACBP
NM-020548
Histone H2A.Z
H2AFZ
NM-002106
Defensin alpha 5
DEFA5
NM-021010
High-grade adenoma derived
Pepsinogen C
PGC
NM-002630
Peripheral-type benzodiazepine receptor
BZRP
NM-007311
Defensin alpha 5
DEFA5
NM-021010
GW112
GW112
AF097021
LI-Cadherin
CDH17
NM-004063
). Defensin alpha 5 was detected in both the SSH samples. All these genes have been
previously implicated in carcinogenesis in different organs or cell proliferation.
These repetitively detected genes were used for further analysis and the miscellaneous
genes that were detected only once were excluded.
Confirmation of differential gene expression by semiquantitative RT–PCR
The overexpression of genes in SMART-cDNA samples was verified by semiquantitative
RT–PCR using gene-specific primer sets. PCR products isolated during the exponential
phase of amplification were analysed by agarose-gel electrophoresis in order to compare
the amount of specific products. The minimal number of PCR cycles required for visualisation
on agarose gels was selected for each gene. The amount of PCR product at the same
PCR cycle was similar for beta-actin, a representative housekeeping gene, but those
of genes identified by SSH were clearly more abundant in the gastric adenoma or adenocarcinoma
tissues (Figure 1 in comparison to their corresponding adjacent gastric mucosa tissues.
Quantification of identified genes
The expression levels of the identified genes were quantified by quantitative PCR
in 14 low-grade adenomas, nine high-grade adenomas and nine adenocarcinomas, including
the original samples used in SSH analyses (Figure 2
Figure 2
Expression levels of identified genes in 32 cases. The expression levels of nine genes
in 14 low-grade adenomas, nine high-grade adenomas and nine adenocarcinomas are shown
as the mean and 95% confidence interval. Data for ACBP in adenocarcinomas were not
available for analysis due to the small number of cases. RACK1, ACBP, CDH17 and EEF1G
were significantly overexpressed in low-grade adenoma; DEFA5 was significantly overexpressed
in high-grade adenoma and suppressed in adenocarcinoma.
). RACK1, ACBP, CDH17 and EEF1G were significantly overexpressed in low-grade adenomas,
whereas DEFA5 was significantly overexpressed in high-grade adenomas and suppressed
in adenocarcinomas. These findings suggest that these genes reflect the molecular
features of gastric tumours with different histological diagnoses, but that individual
analysis of these genes does not define the progressive potential of gastric tumours.
Analysis of gene-expression profiles using unsupervised hierarchical clustering
In order to determine if the analysed samples could be classified into groups on the
basis of their gene-expression profiles alone, hierarchical clustering analysis was
performed. The ratio of gene expression was first log transformed and then applied
to the clustering algorithm. The expression patterns of nine genes across 32 samples
are shown in Figure 3
Figure 3
Hierarchical clustering analysis. The expression patterns of nine genes across 32
samples are shown. Each column indicates a gene, and each row indicates a sample.
Red and green indicate the overexpression and underexpression, respectively, of genes
in adenomas or adenocarcinomas in comparison to the adjacent mucosa. Graduated colour
patterns correspond to the degrees of expression changes. Black colour indicates that
the expression was not detected in both the paired samples. The dendrogram of the
32 cases at the right of the matrix, in which the pattern and length of branches reflect
the relatedness of the samples, indicates that the samples are clustered into three
major branches based on the similarity of gene-expression profiles. The abbreviations
Low, High and Ca stand for low-grade adenoma, high-grade adenoma and adenocarcinoma,
respectively.
. The dendrogram of the 32 cases at the right of the matrix, in which the pattern
and length of branches reflect the relatedness of the samples, separated samples into
three major groups based on the similarities in gene-expression profiles.
Clinicopathological factors in relation to clustered groups
In order to clarify the clinical features associated with this clustering, various
clinicopathological factors, including age, gender, histological diagnosis and tumour
size, were analysed. The proportions of low- and high-grade adenomas or adenocarcinomas
were significantly different among the three groups (P<0.05) (Table 3
Table 3
Clinical backgrounds of all cases categorised into three groups by cluster analysis
Group-1 (n=9)
Group-2 (n=11)
Group-3 (n=12)
Age (years) (mean±s.d.)
69.4±10.7
68.9±7.1
78.0±8.1
Sex (male/female)
9/0
9/2
9/3
Histological diagnosisa
6/3/0
6/4/1
2/2/8
Tumor size (mean±s.d.)
b
9.3±4.3
16.4±7.6
27.7±13.1
Adenomac
9.3±4.3
16.2±8.0
18.3±8.9
Adenocarcinoma (adenocarcinoma cases only)
18.0
32.4±12.7
TNM clinical classification (T1/T2 or T3)
1/0
5/3
Histological differentiation (G1/G2)
1/0
6/2
a
Low-grade adenoma/high-grade adenoma/adenocarcinoma according to Vienna classification.
P<0.05.
b
P<0.001 for group-1 vs 3 and P<0.05 for group-1 vs 2, group-2 vs 3.
c
P<0.05 for group-1 vs 2 and for group 1 vs groups 2 and 3 collectively (16.8±8.0).
). The first group consisted of six low-grade and three high-grade adenomas; the second
group consisted of six low-grade and five high-grade adenomas and an adenocarcinoma;
the third group consisted of eight adenocarcinomas, two low-grade adenomas and two
high-grade adenomas. Three cases of adenocarcinoma in advanced tumour stage (T2 or
T3 in TNM classification) and two cases with moderately differentiated adenocarcinoma
(G2) clustered into the third group. When adenomas and adenocarcinomas were analysed
together, tumour size became significant in the order of groups 1–3. The tumour size
of adenomas was significantly small in group 1 in comparison to group 2 or to groups
2 and 3 collectively. When low- and high-grade adenomas were compared separately,
the tumour size of low-grade adenomas in group 1 was significantly smaller in comparison
to those in group 2 or in groups 2 and 3 collectively. High-grade adenomas in group
1 were smaller than those in group 2 or in groups 2 and 3 collectively, although the
difference was not statistically significant (Figure 4
Figure 4
Plot of the tumour size according to groups separated by clustering analysis. Open
circles denote low-grade adenomas, grey circles denote high-grade adenomas and closed
circles denote adenocarcinomas. Tumour size was significantly larger in group 3 in
comparison to groups 1 or 2 and significantly larger in group 2 vs group 1. When a
comparison was made independent of histological diagnosis, tumour size of low-grade
adenomas was significantly smaller in group 1 in comparison to group 2 or to groups
2 and 3 collectively (P<0.05). High-grade adenoma in group 1 also tended to be smaller
than those in group 2 or groups 2 and 3 in combination, although the differences did
not reach statistic significance (P=0.09 for group 1 vs 2 and P=0.10 for group 1 vs
groups 2 and 3 collectively).
).
Gene-expression profiles with respect to clustered groups
In order to investigate the gene-expression profiles responsible for this clustering,
the expression levels of each gene were compared between each of the three groups
by the Mann–Whitney U-test. The expression levels of ACBP, PGC and RACK1 were significantly
higher in group 1 in comparison to group 2 and/or group 3 (ACBP: P<0.05 for 1 vs 2;
PGC: P<0.001 for 1 vs 2, P<0.005 for 1 vs 3; RACK1: P<0.05 for 1 vs 2 and 1 vs 3).
In contrast, the expression levels of CDH17 and DEFA5 were significantly higher in
group 2 in comparison to groups 1 or 3 (CDH17: P<0.005 1 vs 2 and 2 vs 3; and DEFA5:
P<0.0001 for 2 vs 3 and P<0.0005 for 1 vs 3). Only ARG2 exhibited a high level of
expression in group 3 (P<0.05 for 2 vs 3, P<0.01 for 1 vs 2). The hierarchical clustering
analysis using these six genes resulted in clusters identical to that using nine genes
(data not shown). The plot of the log-transformed ratio of these genes is shown in
Figure 5
Figure 5
Plot of the gene-expression ratio according to groups separated by clustering analysis.
The gene-expression ratios were box plotted according to three groups identified in
clustering analysis. The expression levels of ACBP, PGC and RACK1 were significantly
high in group 1 compared to group 2 and/or group 3. In contrast, the expression levels
of CDH17 and DEFA5 were significantly high in group 2 compared to group 1 or group
3. Only ARG2 showed a high level of expression in group 3. *1 P<0.05, *2 P<0.01, and
*3 P<0.005.
.
DISCUSSION
In the present study, we identified nine genes specifically overexpressed in low-
and high-grade gastric adenomas. Although these genes have been implicated in carcinogenesis
in a variety of organs, the overexpression of these genes in gastric adenomas has
not been investigated previously. Unsupervised clustering analysis of expression profiles
using these gastric adenoma-related genes was performed in a total of 32 gastric adenomas
and adenocarcinomas, resulting in a classification with a close correlation to histological
stages. Moreover, the adenomas were further divided into two subgroups with different
tumour sizes according to their expression profiles. These results suggest that expression
profiles may be linked to different biological properties of gastric adenomas or adenocarcinomas.
Analysis of the nine adenoma-related genes in 32 cases of gastric tumours demonstrated
that a portion of the genes exhibited significantly increased expressions in adenomas,
whereas none of these genes was overexpressed in adenocarcinomas (Figure 2). This
suggests that these genes play a specific role in the development of adenomas, but
that their expression levels were variable in these tumours. This observation raises
the possibility that the molecular nature of gastric adenomas is heterogeneous and
separate analyses of individual genes are not informative. Accordingly, we tried to
classify gastric tumours using gene-expression profiling.
Three distinct groups of gastric tumours were identified by an unsupervised hierarchical
clustering analysis of expression profiles of nine adenoma-related genes. A search
for clinicopathological features linked to this classification revealed that these
three groups differed significantly in their constitutive proportions of low- and
high-grade adenomas or adenocarcinomas. One group consisted predominantly of adenocarcinomas
(group 3) into which all advanced clinical stage or histological grade adenocarcinomas
were classified, suggesting that expression profiles successfully distinguished gastric
adenocarcinomas from adenomas.
The other two groups (groups 1 and 2) consisted of mixtures of low- and high-grade
adenomas. Group 1 adenomas were significantly smaller than group 2 tumours, demonstrating
that the expression profiles differentiate gastric adenomas into two subgroups with
potentially different biological properties undetected by conventional histopathological
classification. We suggest that gene-expression profiles not only confirm major histologic
distinctions between gastric adenomas and adenocarcinomas but may also define subgroups
of gastric adenomas with different biological natures.
Group 1, consisting of small adenomas with a slightly increased proportion of low-grade
cases and no adenocarcinomas, exhibited expression profiles characterised by three
overexpressed genes, that is, RACK1, PGC and ACBP. RACK1, previously known as G protein
beta-subunit-like protein 12.3, is a signal molecule involved in the MAPK pathway
through binding to Src, integrin beta-subunit or interferon receptor (Chang et al,
1998; Liliental and Chang, 1998; Croze et al, 2000; Kiely et al, 2002). Pepsinogen
C is the precursor of pepsin C that is expressed in the normal gastric mucosa, and
is also involved in gastric epithelial cell growth during mucosal healing (Kishi et
al, 1997). Acyl-CoA binding protein is involved in steroid biosynthesis and in the
stimulation of cell proliferation (Papadopoulos, 1993). Although these genes are related
to cell proliferation and their overexpression has been reported in tumours of different
organs, their association with gastric adenocarcinomas has not been confirmed (Diez-Itza
et al, 1993; Miettinen et al, 1995; Vizoso et al, 1995; Konishi et al, 1999; Venturini
et al, 1999; Berns et al, 2000; Miyasaka et al, 2001; Saito et al, 2002). In the present
study, the expression of these genes did not increase in either group 2, the larger
adenomas or group 3, the adenocarcinomas. These genes may play a role in the pathogenesis
of gastric adenomas in their early stages or in more benign courses, that is, a limited
role in the progression to adenocarcinoma.
In comparison, group 2, a mixture of larger low- and high-grade adenomas, as well
as one adenocarcinoma, was characterised by the overexpression of two intestine-specific
genes, CDH17 and DEFA5. LI-Cadherin is usually expressed in normal intestinal mucosa
and ectopically in well-differentiated gastric adenocarcinomas (Grotzinger et al,
2001). Defensin alpha 5 consists of a family of antimicrobial peptides that are highly
expressed in small intestinal Paneth cells (Inada et al, 2001). The defensin family
has alternative functions, such as promotion of cell differentiation (Frye et al,
2001). It is also known to be overexpressed in cancers of the kidney and oral mucosa
(Muller et al, 2002). The intestine-specific transcription factor, CDX2, has recently
been implicated in the regulation of CDH17 and DEFA5 (Eda et al, 2002; Hinoi et al,
2002). In the normal small intestine, CDX2 controls the expression of genes that determine
the cellular lineage of the small intestinal epithelium. The ectopical expression
of CDX2 has been reported in intestinal-type gastric adenocarcinomas (Almeida et al,
2003), and this is consistent with microarray data suggesting that a group of intestine-specific
genes are upregulated in gastric adenocarcinomas (Hippo et al, 2002). Collectively,
the upregulation of two CDX2-dependent genes, CDH17 and DEFA5, found in group 2, represents
a characteristic of intestinal cellular lineage that is, in the stomach, implicated
in the pathogenesis of intestinal-type gastric adenocarcinoma.
In group 3, which consisted mainly of gastric adenocarcinomas, only ARG2 exhibited
a high level of expression. Arginase II has been reported to be overexpressed in cancerous
tissues in general (Harris et al, 1983; Leu and Wang, 1992; Suer Gokmen et al, 1999;
del Ara et al, 2002; Porembska et al, 2003) and it is well established that this gene
is overexpressed in gastric adenocarcinomas (Wu et al, 1996). Since ARG2 catalyses
the conversion of arginine to ornithine, a crucial metabolite in biosynthesis of glutamic
acid, proline and polyamines (Vockley et al, 1996), an increase in the level of arginase
may reflect accelerated metabolism due to cell proliferation or tumour growth. Therefore,
the overexpression of ARG2 in adenocarcinomas, as defined in the present study, is
reasonable.
Genes other than those listed above were sporadically overexpressed in a portion of
the adenomas or adenocarcinomas, although their expression levels were not significantly
different among the three groups. Eukaryotic elongation factor 1 gamma is a subunit
of EF1 and it is involved in RNA translation (Janssen et al, 1991). Histone H2A.Z
is a histone protein of the H2A family and it is involved in DNA replication (Hatch
and Bonner, 1990). Peripheral-type benzodiazepine receptor is involved in mitochondrial
cholesterol transport and proliferation, steroid biosynthesis, and the stimulation
of cell proliferation (Papadopoulos, 1993). The overexpression of these genes is sporadic
in cancers of a variety of tissues (Lew et al, 1992; Miettinen et al, 1995; Mimori
et al, 1995, 1996; Mathur et al, 1998; Hardwick et al, 1999; Venturini et al, 1999).
Hierarchical clustering analysis excluding EEF1G, H2AFZ and BZRP resulted in clusters
identical to that using the original nine genes, suggesting that these three genes
do not contribute to the molecular classification of three groups, but may be involved
in the common pathophysiology of gastric tumours probably reflecting accelerated cell
division or metabolism.
Recent studies using a microarray analysis defined gene-expression profiles of gastric
adenocarcinoma (Hasegawa et al, 2002; Hippo et al, 2002). Interestingly, the spectra
of genes that were overexpressed in carcinoma tissues in these studies differ significantly
from the present study. The possible reason for this discrepancy may be that these
microarray studies analysed advanced staged gastric carcinoma tissues. Since the samples
used for the extraction of relevant genes in the present study were low- and high-grade
adenomas, the detected genes may be overexpressed specifically in adenoma tissues
and not in adenocarcinoma tissues. Thus, it seems reasonable that advanced staged
adenocarcinoma possess different gene-expression profiles from those obtained in the
present study. To elucidate the stage-specific gene expressions, different stages
of gastric tumours should be analysed.
The results of the present study raise the possibility that the expression profiles
of specific genes may distinguish gastric adenomas from adenocarcinomas and, more
importantly, may define subgroups of gastric adenomas that are unresolved by conventional
histopathology. Many studies have shown that gene-expression profiles can be used
to identify tumour subclasses independent of histopathological diagnosis. Furthermore,
these tumour subclasses are frequently related to distinct cellular lineages and are
closely associated with prognosis or response to treatment as shown in malignant lymphomas
(Alizadeh et al, 2000; Shipp et al, 2002) or breast cancer (Sorlie et al, 2001), confirming
the usefulness of expression profiling in the clinical practice of cancer. Group 2,
a mixture of low-and high-grade adenomas cases with larger tumour sizes, exhibited
gene-expression profiles specific to the cellular lineage of intestinal epithelium
that has been implicated in an intestinal-type gastric adenocarcinoma (Hippo et al,
2002). Thus, adenomas classified as group 2 tumours may have a biological nature more
closely related to adenocarcinomas in comparison to the adenomas classified in group1.
On the one hand, the expression profiles of CDH17, DEFA5 and other CDX2 regulated
genes may constitute specific tumour markers for a distinct subgroup of gastric adenomas
with a progressive nature. On the other , gastric adenomas with expression profiles
similar to those of the smaller adenomas in group 1 may be nonprogressive. There are
no definite histological or clinical markers to identify the progressive subgroup
of adenomas. Therefore, future applications of expression profiling of these genes
in biopsied samples may contribute to clinical practice and may promote objective
criteria for intervention, such as endoscopic mucosal resection.
However, there are several limitations in the present study including that it is cross-sectional.
There is no follow-up of the adenoma cases and no data available on the prognoses
or the disease progression of the adenoma cases. Thus, the actual prognostic value
of this classification remains to be elucidated. A longitudinal study is necessary
to determine if adenomas classified into group 2 actually develop into progressive
diseases. These types of studies are particularly difficult because lesions diagnosed
histologically as high-grade adenomas are resected endoscopically without follow-up,
as recommended in the literature (Lauwers and Riddell, 1999). Another issue is that
expression profiling is not in complete accord with conventional histopathological
classification, (e.g. three high-grade adenomas classified into group 1 or two low-
and high-grade adenomas classified into group 3). Nevertheless, we believe that more
accurate discrimination will be achieved by increasing the number of predictive genes
involved in expression profiling by extracting them through more comprehensive investigations
of gene expression, such as a large-scale DNA microarray analysis. Alternatively,
the detailed molecular and pathological analyses of exceptional cases may provide
additional predictive information on the biological nature of gastric tumours. All
the three high-grade adenomas in group 1 were less than 20 mm in diameter (8, 15 and
17 mm) and an adenocarcinoma in group 2 is the smallest T1/G1 tumour, raising the
possibility that such exceptional cases have particular biological characteristics
below the sensitivity of conventional histopathological examination. Their discrimination
may be achieved by gene-expression profiling.
In conclusion, taking advantage of the expression profiles of a set of genes identified
in two cases of gastric adenoma, gastric adenoma and adenocarcinoma can be classified
into three groups with distinct gene-expression patterns. One group consists primarily
of invasive adenocarcinoma, whereas the other two groups consist of adenomas with
potentially different biological properties, as suggested by significantly different
tumour sizes. These findings add new insight into our understanding of the molecular
pathogenesis involved in the early stages of gastric carcinogenesis, in developing
specific tumour markers for clinical practice and in designing potentially novel therapeutic
targets.