Chromosomal instability (CIN) is commonly observed in human solid tumours, with the
apparent gain or loss of large parts or whole chromosomes, leading to DNA aneuploidy
(Lengauer et al, 1997; Duesberg et al, 1999). In previous studies, CIN has been associated
in some cases with alterations in the cell-cycle checkpoint that monitors the integrity
of the spindle apparatus, a structure critical for proper bipolar segregation of duplicated
sister chromatids at mitosis (Cahill et al, 1999). A small fraction of CIN cancers
are associated with dominant mutations in the human homologues of yeast spindle checkpoint
genes BUB1 (Cahill et al, 1998; Imai et al, 1999; Gemma et al, 2000) and MAD2 (Li
and Benezra, 1996; Cahill et al, 1999). However, BUB1 and MAD2 mutations are relatively
rare, and gastric cancers frequently exhibit DNA aneuploidy (Abad et al, 1998; Esteban
et al, 1999; Imai et al, 1999; Russo et al, 2000; Tanaka et al, 2001).
Recently, the Chfr (checkpoint with forkhead associated (FHA) and ring finger (RF))
gene, involved in the mitotic stress checkpoint, was cloned and located to chromosome
12q24.33. Its product, CHFR, mediates the delayed entry into metaphase characterised
microscopically by delayed chromosomal condensation (Scolnick and Halazonetis, 2000).
In addition, CHFR promotes cell survival in response to mitotic stress (Scolnick and
Halazonetis, 2000). CHFR possesses an N-terminal FHA domain, a central RF domain and
a C-terminal cysteine-rich (CR) region (Scolnick and Halazonetis, 2000). Based on
functional analysis of Chfr deletion mutants, both the FHA and CR regions are required
for its checkpoint function. CHFR also has ubiquitin ligase activity dependent on
the RF domain (Chaturvedi et al, 2002). Northern blot analysis of Chfr using RNA from
eight colon, osteosarcoma and neuroblastoma cancer cell lines revealed that Chfr expression
was absent in three cell lines (Scolnick and Halazonetis, 2000). Loss of Chfr expression
due to hypermethylation of a CpG island in the promoter region has been observed in
tumour cell lines and primary cancers of the lung, oesophagus and colon (Mizuno et
al, 2002; Shibata et al, 2002; Corn et al, 2003; Toyota et al, 2003). Thus, it is
possible that Chfr promoter hypermethylation is also involved in gastric carcinogenesis.
As promoter hypermethylation of tumour suppressor or tumour-related genes are not
always cancer specific, the significance of promoter methylation status can vary among
different genes (Waki et al, 2003a, 2003b). In the present study, we investigated
Chfr promoter methylation status in gastric cancer cell lines, primary gastric cancers
and corresponding non-neoplastic gastric epithelia, as well as in non-neoplastic gastric
epithelia of noncancer-bearing stomachs to clarify both the significance and cancer
specificity of Chfr promoter hypermethylation in gastric carcinogenesis.
MATERIALS AND METHODS
Gastric cancer cell lines
In all, 10 gastric cancer cell lines with variable histologies were used in our study
and were cultured under appropriate conditions in our laboratory: MKN1, an adenosquamous
cell carcinoma; MKN7, a well-differentiated adenocarcinoma; MKN28 and MKN74, moderately
differentiated adenocarcinomas; MKN45 and KWS-I, poorly differentiated adenocarcinomas;
KATO-III, a signet-ring cell carcinoma; TSG11, a hepatoid carcinoma; and ECC10 and
ECC12, endocrine cell carcinomas.
Primary gastric cancers
In all, 71 pairs of gastric cancers (40 differentiated and 31 undifferentiated carcinomas;
15 early stage and 56 advanced stage) and corresponding non-neoplastic gastric mucosa
were surgically obtained from 71 patients. Tissue samples were immediately frozen
and stored at −80°C until analysis. All patients received a median of 36.7 months
of follow-up care (range, 1–77 months).
Autopsy samples
Non-neoplastic gastric mucosa samples from noncancer-bearing stomachs were obtained
from 34 autopsies. The autopsies consisted of 21 males and 13 females, ranging in
age from 0.7 to 87 years (mean, 56 years). For most autopsies, tissue samples were
obtained from the upper, middle and lower portions of the stomach. A total of 91 specimens
were obtained, frozen and stored at −80°C until analysis.
DNA and RNA extraction
DNA was extracted from the 10 gastric carcinoma cell lines, 71 primary gastric cancers
and their corresponding non-neoplastic gastric mucosa, and 91 non-neoplastic gastric
mucosa from autopsies using SepaGene (Sanko-Junyaku, Tokyo, Japan). Total RNA was
isolated from the 10 gastric carcinoma cell lines using TRIZOL reagent (Gibco BRL,
Life Technologies, Gaithersburg, MD, USA).
Bisulfite modification and methylation-specific polymerase chain reaction (MSP)
Sodium bisulphite treatment of DNA converts all unmethylated cytosines to uracils,
but leaves methylated cytosines unaffected. Briefly, 2 μg aliquots of genomic DNA
were denatured with sodium hydroxide and modified by sodium bisulphite. Samples were
then purified using Wizard DNA purification resin (Promega, Madison, WI, USA), treated
with NaOH, recovered in ethanol and resuspended in 30 μl distilled water. Amplification
was carried out in a 20 μl reaction volume containing 2 μl GeneAmp PCR Gold Buffer
(PE Applied Biosystems, Foster City, CA, USA), 1.0 mM MgCl2, 1 μl each primer, 0.2 mM
dNTPs and 1 U Taq polymerase (AmpliTaq Gold DNA Polymerase, PE Applied Biosystems).
After heating at 94°C for 10 min, PCR was performed in a thermal cycler (GeneAmp 2400,
PE Applied Biosystems) for 35 cycles of denaturation at 94°C for 30 s, annealing at
54°C for 60 s and extension at 72°C for 60 s, followed by a final 7-min extension
at 72°C. A positive control (Sss-I methylase-treated DNA) and negative control (distilled
water without DNA) were included in each amplification. The PCR products were separated
on 6% nondenaturing polyacrylamide gels. The following primer sets were used: Chfr
M forward (5′-GTA ATG TTT TTT GAT AGC GGC-3′) and Chfr M reverse (5′-AAT CCC CCT TCG
CCG-3′) for methylated Chfr sequences; Chfr U forward (5′-GGT TGT AAT GTT TTT TGA
TAG TGG T-3′) and Chfr U reverse (5′-CAA ATC CCC CTT CAC CA-3′) for unmethylated Chfr
sequences (Corn et al, 2003).
Reverse transcription–PCR (RT–PCR)
Isolated RNA was reverse transcribed and amplified using the One-Step RT–PCR System
(Gibco BRL). Primer sequences used were: Chfr forward (5′-TGG AAC AGT GAT TAA CAA
GC-3′) and Chfr reverse (5′-AGG TAT CTT TGG TCC CAT GG-3′) for Chfr; and β-actin forward
(5′-AAA TCT GGC ACC ACA CCT T-3′) and β-actin reverse (5′-AGC ACT GTG TTG GCG TAC
AG-3′) for β-actin. RT–PCR products were separated on 3% agarose gels.
5-aza-2′-deoxycytidine (5-aza-dC) treatment
To examine the restoration of Chfr expression, two cell lines (MKN1 and KATO-III)
were incubated for 96 h with 0.2 or 1 μ
M 5-aza-dC (Sigma), and then harvested for RNA extraction and RT–PCR.
Preparation of MSP-positive control
Sss-I methylase (New England BioLabs, Inc., Beverly, MA, USA) was used to methylate
100 μg peripheral blood DNA, which was modified by sodium bisulphite as described
above.
Statistical analysis
Statistical comparisons were performed using Fisher's exact test, with P<0.05 considered
statistically significant. Survival analysis was performed using a Kaplan–Meier curve
with a log-rank test.
RESULTS
Hypermethylation and expression of Chfr in gastric cancer cell lines
Chfr promoter hypermethylation was observed in two (MKN1 and KATO-III) of the 10 cell
lines tested (Figure 1
Figure 1
Methylation-specific polymerase chain reaction (A and B), RT–PCR (C and D) and comparison
of Chfr mRNA expression before (−) and after (+) 5 aza-dC treatment (E) in gastric
cancer cell lines. (A) Chfr-methylated-sequence-specific PCR and (B) Chfr-unmethylated-sequence-specific
PCR. Methylated Chfr product is present in lanes 1 and 6 (A), while demethylated Chfr
product is present in all lanes except lanes 1 and 6 (B). (C) Chfr RT–PCR and (D)
β-actin RT–PCR (internal control). Chfr product is absent in lanes 1 and 6 (C). β-actin
mRNA is present in all lanes (D). Lanes: 1, MKN1; 2, MKN7; 3, MKN28; 4, MKN45; 5,
MKN74; 6, KATO-III; 7, KWS-I; 8, TSG11; 9, ECC10; 10, ECC12; P, positive control;
DW, distilled water; and SM, size marker. (E) Treatment with 5 aza-dC restores Chfr
mRNA expression in KATO-III, but does not affect Chfr expression levels in MKN45.
). The remaining cells lines (MKN7, MKN28, MKN45, MKN74, KWS-I, TSG11, ECC10 and ECC12)
contained unmethylated Chfr alleles and expressed abundant Chfr mRNA. MKN1 and KATO-III
exhibited loss of Chfr expression (Figure 1), which was restored after treatment with
5-aza-dC (Figure 1). Thus, promoter methylation status of Chfr directly correlated
with expression.
Hypermethylation of Chfr in primary gastric cancers, corresponding non-neoplastic
gastric mucosa and autopsy samples
Hypermethylation of Chfr was detected in 35% (25 of 71) of primary gastric cancers
but only in 5% (four of 71) of the corresponding non-neoplastic gastric mucosa (Figure
2
Figure 2
Methylation-specific polymerase chain reaction of primary gastric cancers (T) and
their corresponding non-neoplastic gastric mucosa (N). M, Chfr-methylated-sequence-specific
PCR; U, Chf- unmethylated-sequence-specific PCR; P, positive control; DW, distilled
water; and SM, size marker. Methylated Chfr is present in primary gastric cancers
(M123, M137, M145, M157, M245), whereas non-neoplastic gastric mucosa samples do not
exhibit methylated Chfr.
). Chfr hypermethylation was observed in only one (1%) of the 91 autopsy samples.
This single sample showing Chfr hypermethylation was obtained from the lower portion
of the stomach from an 82-year-old-male patient with Parkinson's disease.
Correlation between Chfr promoter hypermethylation and clinicopathological parameters
Chfr hypermethylation occurred at a similar frequency in early and advanced gastric
cancers, and no significant correlations between Chfr promoter methylation status
and clinicopathological factors were observed (Table 1
Table 1
Correlation between Chfr promoter methylation status and clinicopathological characteristics
in gastric cancer patients
Promoter methylation status
Methylated
Unmethylated
NS=not significant by Fisher's exact probability test. Chfr=checkpoint with forkhead
associated and ring finger.
). Methylation status did not significantly influence event-free survival rate, as
analysed by Kaplan–Meier curve with log-rank test and the Breslow–Gehan–Wilcoxon test
(data not shown).
DISCUSSION
Although CIN is one of the most frequently recognised phenomenon in gastric cancer
(Abad et al, 1998; Esteban et al, 1999; Russo et al, 2000), the mitotic checkpoint
genes hsMAD2 and hBUB1 are rarely mutated in gastric and other types of human malignancy
(Imai et al, 1999; Tanaka et al, 2001). Checkpoints upstream of the spindle checkpoint
that delays chromosome condensation in response to mitotic stress are regulated by
CHFR. Normal primary cells and cancer cell lines that express CHFR exhibit delayed
entry into metaphase after treatment with microtubule inhibitors (Scolnick and Halazonetis,
2000). In contrast, cancer cell lines that lack CHFR enter metaphase without delay,
with ectopic expression of CHFR necessary and sufficient to restore cell-cycle delay
(Scolnick and Halazonetis, 2000). Recent studies of human tumours have shown that
Chfr inactivation can be due to hypermethylation of CpGs in the promoter region (Mizuno
et al, 2002; Shibata et al, 2002). However, whether Chfr promoter hypermethylation
is involved in gastric cancer has not yet been determined.
In the present study, we showed that Chfr promoter hypermethylation was present in
two of 10 (20%) gastric cancer cell lines and in 25 of 71 (35%) primary gastric cancers.
As for non-neoplastic gastric epithelia, 5% (four of 71) of samples from cancer-bearing
and 1% (one of 91) from noncancer-bearing stomachs exhibited Chfr promoter hypermethylation.
We have shown that many tumour suppressor and tumour-related genes, such as APC, DAP-kinase,
DCC, E-cadherin, hMLH1, p16, RASSF1A and RUNX3, exhibit promoter hypermethylation
in both neoplastic and non-neoplastic gastric epithelia at variable frequencies (Tamura,
2004). While GSTP1 and PTEN promoters remained unmethylated in both neoplastic and
non-neoplastic gastric epithelia (Sato et al, 2002; Tamura, 2004), TSLC1 promoter
hypermethylation is highly cancer specific, but is observed at only a low frequency
in gastric cancer (Honda et al, 2002).
Methylation generally increases with age in tissue-specific manner for different genes
(Waki et al, 2003b). In the present study, the only sample of non-neoplastic gastric
mucosa that exhibited Chfr hypermethylation was obtained from the noncancer-bearing
stomach of an 82-year-old male patient. In contrast, Chfr hypermethylation was present
in cancer-bearing stomachs from patients from 66 years of age. Based on these observations,
it appears that age-related Chfr hypermethylation may constitute a general defect
where individuals may become predisposed to the development of gastric cancer. The
cancer specificity of hypermethylation of a particular promoter can depend on the
CpG site examined (Satoh et al, 2002). Our present study revealed that Chfr promoter
hypermethylation appears to be one of the most cancer-specific alterations among the
various examples of tumour suppressor and tumour-related gene hypermethylation reported
to date (Tamura, 2004).
While Chfr promoter hypermethylation is a relatively infrequent non-neoplastic gastric
epithelia, it occurs at similar frequencies in early and advanced gastric cancers.
This suggests that Chfr promoter hypermethylation may be an early event in gastric
carcinogenesis. DNA aneuploidy has been observed in 50–71% of gastric cancers and
correlates with poor prognosis (Abad et al, 1998; Esteban et al, 1999; Russo et al,
2000). In the present study, we failed to find a statistically significant correlation
between Chfr hypermethylation and gastric cancer patient survival. Nonetheless, our
results did display a tendency towards a worse prognosis in patients with tumours
that displayed Chfr hypermethylation. Owing to the lack of a significant correlation
between Chfr methylation status and prognosis, and the relatively low frequency of
Chfr hypermethylation compared to that of DNA aneuploidy, other gene(s) and/or mechanism(s)
are likely to also contribute to CIN in gastric cancer.
In conclusion, Chfr promoter hypermethylation frequently occurs as an early event
of gastric carcinogenesis. Owing to its cancer specificity, detection of Chfr promoter
methylation could be useful as a molecular diagnostic marker for gastric cancer.