Hypermethylation of the 5' CpG island of the p14ARF flanking exon 1β in human colorectal cancer displaying a restricted pattern of p53 overexpression concomitant with increased MDM2 expression.

BACKGROUND: It has been suggested that inactivation of p14ARF, a tumor suppressor central to regulating p53 protein stability through interaction with the MDM2 oncoprotein, abrogates p53 activity in human tumors retaining the wild-type TP53 gene. Differences in expression of tumor suppressor genes are frequently associated with cancer. We previously reported on a pattern of restricted p53 immunohistochemical overexpression significantly associated with microsatellite instability (MSI), low TP53 mutation frequency, and MDM2 overexpression in colorectal cancers (CRCs). In this study, we investigated whether p14ARF alterations could be a mechanism for disabling the p53 pathway in this subgroup of CRCs.
RESULTS: Detailed maps of the alterations in the p14ARF gene were determined in a cohort of 98 CRCs to detect both nucleotide and copy-number changes. Methylation-specific PCR combined with bisulfite sequencing was used to evaluate the prevalence and distribution of p14ARF methylation. p14ARF alterations were then correlated with MSI status, TP53 mutations, and immunohistochemical expression of p53 and MDM2. The frequency of p14ARF mutations was extremely low (1/98; 1%), whereas coexistence of methylated and unmethylated alleles in both tumors and normal colon mucosa was common (91/98; 93%). Only seven of ninety-eight tumors (7%) had a distinct pattern of methylation compared with normal colon mucosa. Evaluation of the prevalence and distribution of p14ARF promoter methylation in a region containing 27 CpG sites in 35 patients showed a range of methylated CpG sites in tumors (0 to 25 (95% CI 1 to 13) versus 0 to 17 (95% CI 0 to 2)) in adjacent colon mucosa (P = 0.004). Hypermethylation of the p14ARF promoter was significantly correlated with the restricted p53 overexpression pattern (P = 0.03), and MDM2 overexpression (P = 0.02), independently of MSI phenotype. Although no significant correlation between p14ARF methylation and TP53 mutational status was seen (P = 0.23), methylation involving the proximal CpG sites within the 5' CpG flanking exon 1β was present more frequently in tumors with restricted p53 overexpression than in those with diffuse p53 overexpression (range of methylated clones 17 to 36% (95% CI 24 to 36%) versus range 0 to 3% (95% CI 0 to 3%), P = 0. 0003).
CONCLUSION: p14ARF epigenetic silencing may represent an important deregulating mechanism of the p53-MDM2-p14ARF pathway in CRCs exhibiting a restricted p53 overexpression pattern.

Background

The correct functioning of the p53-MDM2-p14ARF pathway requires a delicate balance between the opposing effects of its different components [1-3]. Genetic and epigenetic alterations have been shown to distort this balance in various human malignancies, allowing tumor cells to over-ride the tumor suppressor activity of the p53 protein, thereby facilitating neoplastic conversion [4]. In the vast majority of human neoplasia, including colorectal cancer (CRC), deregulation of the p53 pathway usually occurs by direct inactivation of the TP53 gene itself; this occurs mainly via point mutations [5], which usually increase the stability of the mutant p53 protein, leading to its overexpression [6]. However, a significant proportion of CRCs, which include mainly microsatellite instability-high (MSI-H) CRCs, and a subset of microsatellite-stable (MSS) sporadic CRCs, display a particular immunohistochemical p53 expression pattern characterized by an accumulation of p53 protein restricted to a limited number of tumor cells, a profile that we previously termed ‘restricted p53 overexpression’ [7]. This CRC subgroup has an extremely low frequency of TP53 mutation, and displays overexpression of MDM2 and normal expression of p21, suggesting that deregulation of p53 pathway in this CRC subgroup may be due to other alternative mechanisms than TP53 mutation.

Inactivation of the p14 gene has been proposed as a mechanism that is functionally equivalent to an inactivating p53 mutation, in that it disrupts p53 activity in tumors retaining the wild-type TP53 gene [4], and more particularly in sporadic MSI-H CRC [8,9]. In this study, we examined whether p14ARF inactivation could be one of the mechanisms disturbing the p53 pathway in CRCs, particularly in tumors displaying a restricted p53 overexpression pattern. Therefore, we conducted detailed genetics and epigenetics analysis of the p14ARF gene in CRC tumors for which we had complete data on MSI status and DNA mismatch repair deficiency or sufficiency, and we investigated the relationships between p14ARF alterations and MSI phenotype, between p14ARF alterations and the p53 protein expression pattern and its mutational status, as well as with MDM2 protein expression.

Results

p14ARF gene alterations in colorectal cancer

In our sample, we found that p14 mutations were extremely rare; we detected only a previously reported point mutation in one sample (1/98; 1%). This somatic missense mutation was detected in exon 2 and corresponds to a C→T transition on a CpG dinucleotide site, affecting the codon 121 (p.Ala121Val) for the p14 gene, and the codon 107 (p.Arg107Cys) for the p16/CDKN2A gene. Of the ninety-six patients, five (5%) patients, including two of the five patients with Lynch syndrome (hereditary non-polyposis colorectal cancer (HNPCC); OMIM #120435) were carriers of a polymorphic variant corresponding to a substitution of G→A in codon 148 in exon 2 (p.Ala148Thr) affecting only the p16/CDKN2A open-reading frame. Gene dosage detected no copy-number changes in any of the 98 CRCs examined.

p14ARF promoter methylation in tumors and adjacent colon mucosa from patients with colorectal cancer

Overall, MSP analysis within the 5′ CpG island of p14/ARF flanking exon 1β identified coexistence of methylated and unmethylated alleles in tumors and matched adjacent normal-appearing colon mucosa in 91 of the 98 patients (Figure 1 A). By contrast, a distinct methylation profile indicating heavy methylation was seen in seven of the ninety-eight (7.1%) CRCs examined. In these tumors, MSP results identified only methylated alleles in tumors, whereas matched adjacent normal colon mucosa contained both methylated and unmethylated PCR products (Figure 1 B).

Figure 1

Methylation of the promoter in tumors and normal colon mucosa from patients with colorectal cancer (A) promoter methylation analysis by methylation-specific PCR revealed coexistence of both unmethylated (U) and methylated (M) PCR products in tumor (T) and adjacent colon mucosa (N). (B) Extensive methylation of p14ARF promoter in tumors. The methylated PCR product was predominantly detected in tumor, whereas the adjacent colon mucosa produced both the unmethylated and methylated PCR products. MW, standard molecular weight, control +, positive control for methylated allele, (bisulfite-modified genomic blood DNA pretreated with the CpG methylase (M.SssI)); H2O, negative control with water only.

Evaluation of density of p14ARF promoter methylation in tumors and normal colon mucosa from patients with colorectal cancer

Next, we evaluated the degree of p14ARF promoter methylation, limiting the analysis to tumors and corresponding adjacent colon mucosa from 35 randomly selected patients (Table 1), including one of the seven CRCs that was identified as having heavy methylation by MSP (sample T2, Table 1).

Table 1

Clinicopathological and molecular data for patients analyzed by bisulfite genomic sequencing

Patient’s number	Location	Type of differentiation	Stage	MMR IHC	MSI status	p53 IHC	TP53 mutation
1	Sigmoid	Moderate	IV	Positive	MSS	D	p.R248W
3	Sigmoid	Moderate	IIIB	Positive	MSS	D	No
11	Left	Moderate	IV	Positive1	MSI-H	D	No
12	Rectum	Moderate	IIA	Positive	MSS	D	p.R273C
15	Sigmoid	Well	IV	Positive	MSS	D	p.R248Q
24	Left	Moderate	IIA	Positive	MSS	D	No
26	Rectum	Well	I	Positive	MSS	D	p.R248W
28	Rectum	Moderate	IV	Positive	MSS	D	No
30	Sigmoid	Poor	IIIB	Positive	MSS	D	p.C135R
33	Rectum	Well	IIA	Positive	MSS	D	No
34	Left	Well	IIB	Positive	MSS	D	No
35	Right	Well	IIA	Positive	MSS	D	p.[R158H (+)R267Q]
17	Sigmoid	Well	I	Positive	MSS	D	p.R248Q
2	Right	Well	IIA	MLH1-/PMS2-†	MSI-H	R	No
4	Right	Mucinous	IIIC	MLH1-/PMS2-‡	MSI-H	R	No
5	Left	Mucinous	IIA	MLH1-/PMS2-‡	MSI-H	R	No
6	Right	Well	IIA	MSH2-/MSH6-†	MSI-H	R	No
7	Right	Poor	IIA	MLH-/PMS2-‡	MSI-H	R	No
8	Left	Poor	IV	Positive	MSS	R	No
9	Right	Mucinous	IIIB	Positive	MSS	R	No
10	Right	Mucinous	IIIB	MLH1-/PMS2-†	MSI-H	R	No
13	Left	Well	IIA	Positive	MSS	R	No
14	Left	Well	IV	Positive	MSS	R	No
16	Sigmoid	Mucinous	IIA	Positive	MSS	R	No
18	Rectum	Poor	IIIC	MLH1-/PMS2-‡	MSI-H	R	No
19	Left	Well	I	MLH1-/PMS2-†	MSI-H	R	No
22	Right	Mucinous	IIIB	MLH1-/PMS2-‡	MSI-H	R	No
23	Right	Poor	IIA	MLH1-/PMS2-1	MSI-H	R	No
25	Left	Mucinous	IIA	Positive	MSS	R	No
29	Left	Moderate	IIA	Positive	MSS	R	No
20	Rectum	Moderate	IIA	Positive	MSS	N	No
21	Rectum	Mucinous	IIB	Positive	MSS	N	p.[K291X(+) H297Y]
27	Rectum	Moderate	IIA	Positive	MSS	N	c.672 + 1 G→A
31	Right	Moderate	IIIC	Positive	MSS	N	p.Q165X
32	Rectum	Moderate	IIIC	Positive	MSS	N	No

Abbreviations: MMR, DNA mismatch repair system; MSI, microsatellite instability; MSI-H, microsatellite instability-high; MSS, microsatellite-stable; IHC, immunohistochemistry; D, diffuse pattern of p53 expression, R, restricted pattern of p53 expression, N, negative pattern of p53 expression.

*MMR deficiency with unknown origin.

†Lynch syndrome.

‡Sporadic MSI-H colorectal cancer with activating V600E BRAF somatic mutation, indicating MLH1 epigenetic silencing.

Using BGS, we analyzed methylation within the 5′ CpG island of p14ARF flanking exon 1β, targeting a region containing 27 individual CpG sites, including all the CpG sites analyzed by MSP in this region (see Additional file 1: Figure S1). BGS showed different p14ARF promoter methylation levels among the 35 tumors and adjacent colon mucosa tested, with the highest methylation levels recorded in tumor samples (Figure 2, Figure 3). Of the 35 tumors examined, the range of fully methylated CpG was 0 to 25 and the median was 9 (95% CI 1 to 13), whereas in paired normal colon mucosa the range was 0 to 17 and the median 0 (95% CI 0 to 2) (P = 0.004). Of the thirty-five tumors, eighteen (51%) were extensively methylated (>9/27 CpG sites methylated, median of fully methylated CpG sites), six (17%) were partially methylated (>3/27 CpG sites partially methylated, median of partially methylated CpG sites), and 11 (32%) were unmethylated (Table 2).

Figure 2

Heterogeneity of promoter methylation in colorectal tumors. The samples analyzed are represented on the horizontal line, and the 27 CpG sites on the vertical line. For each case, the methylation status of each individual CpG site is shown: an empty block indicates that the concerned CpG site is unmethylated, a black block indicates that the concerned CpG sites is fully methylated, and a gray block indicates that the concerned CpG site is partially methylated.

Figure 3

promoter methylation in adjacent colon mucosa.

Table 2

Relationships between promoter methylation and clinicopathological data, p53 and MDM2 protein expression, mutational status, and microsatellite instability phenotype

Clinicopathological and molecular parameters	Overall (%)	p14ARF promoter methylation profile
		Unmethylated	Dense or partial methylation	Pvalue
Age, years, mean ± SD		64 ± 12	69 ± 10	0.20571
Gender
Male	16 (46%)	5 (45.5%)	11 (45.8%)	0.9833
Female	19 (54%)	6 (54.5%)	13 (54.2%)
Type of tissue
Tumor	35	11 (31%)	24 (69%)	0.0019
Adjacent colon mucosa	35	24 (69%)	11 (31%)
Tumor location
Right side	10 (29%)	3 (27.3%)	7 (29.2%)	>0.05
Left side	25 (71%)	8 (72.7%)	17 (70.8%)
Differentiation
Well or moderate	22 (62.9%)	10 (90.9%)	12 (50%)	0.0270
Poor or mucinous	13 (37.1%)	1 (9.1%)	12 (50%)
Clinical stage
Stage I	3 (8.6%)	-	3 (12.5%)	0.46742
Stage II	17 (48.6%)	5 (45.4%)	12 (50%)
Stage III	9 (25.7%)	3 (27.3%)	6 (25%)
Stage IV	6 (17.1%)	3 (27.3%)	3 (12.5%)
p53 immunohistochemistry
Negative pattern	5 (14.3%)	3 (27.3%)	2 (8.3%)	0.02752
Diffuse pattern	13 (37.1%)	6 (54.5%)	7 (29.2%)
Restricted overexpression	17 (48.6%)	2 (18.2%)	15 (62.5%)
MDM2 immunohistochemistry
Negative	12 (34.3%)	7 (63.6%)	5 (20.8%)	0.0223
Overexpression	23 (65.7%)	4 (36.4%)	19 (79.2%)
p21 immunohistochemistry
Loss to mild	9 (25.7%)	6 (54.5)	3 (12.5%)	0.01462
Moderate to high	26 (74.3%)	5 (45.5)	21 (87.5%)
TP53 mutational status
Mutation present	10 (28.6%)	5 (45.5%)	5 (20.8%)	0.2266
No mutation detected	25 (71.4%)	6 (54.5%)	19 (79.2%)
MSI status
MSI-H	11 (31.4%)	1 (9.1%)	10 (41.7%)	0.0539
MSS	24 (68.6%)	10 (90.9%)	14 (58.3%)

Abbreviations: MSI, Microsatellite instability, MSI-H, microsatellite instability-high; MSS, microsatellite-stable.

2Two-sided two-sample t-test.

2Two-tailed Fisher’s exact test.

Although the majority of normal colon mucosa tested (69%) showed a significantly low frequency of methylation compared with matched tumor samples (P = 0.0019) (Table 2), extensive methylation was detected in the normal colon mucosa from six patients, including a patient with Lynch syndrome (N7; Figure 3) with a germline mutation in the MLH1 gene (Table 1) and five patients with sporadic CRC: two MSS tumors (N1, N29; Figure 3) and three MSI-H tumors (N4, N5, N22; Figure 3) with activating V600E BRAF somatic mutation associated with MLH1 epigenetic silencing (Table 1).

Correlation between p14ARF promoter methylation, clinicopathological features, p53 pathway alterations, and microsatellite instability status in colorectal cancer

Further, we compared p14ARF methylation data from the 35 randomly selected patients analyzed by BGS with their clinicopathological features and the molecular changes in their tumors. No significant association was seen between p14ARF methylation and either age or gender (Table 2). Although the majority of right-sided colon tumors (7/10) had increased p14ARF methylation, no significant association between p14ARF methylation and tumor location was seen (Table 2). Correlation analysis identified a significant association between p14ARF hypermethylation and poorly differentiated or mucinous tumors (P = 0.0270) (Table 2), but no significant association between p14ARF methylation and clinical stage was seen (Table 2). Compared with tumors exhibiting negative and diffuse patterns of p53 protein immunohistochemical expression, the tumors displaying a restricted p53 overexpression profile (15/17) showed a significant increase in p14ARF methylation (P = 0.0274) (Table 2). p14ARF methylation was also significantly associated with MDM2 overexpression (P = 0.0223) (Table 2). Most tumors exhibiting p14ARF hypermethylation showed an absence of TP53 mutation (19/24; 79%), but no significant association between p14ARF promoter methylation and TP53 mutational status was seen (Table 2). MSI-H CRCs were more frequently hypermethylated than MSI-low (MSI-L)/MSS CRCs (P = 0.0539) (Table 2). However, after stratification by p53 immunohistochemical expression pattern, the relationship between MSI status and p14ARF methylation was no longer significant (Figure 4).

Figure 4

Relationship between promoter methylation and microsatellite instability (MSI) status. The MSI-high (MSI-H) tumors had an overall higher frequency of p14ARF promoter methylation compared with MSS tumors, but after stratification by restricted p53 overexpression, the relationship between p14ARF methylation and MSI status was no longer significant.

Quantification and distribution of p14ARF promoter methylation in tumors and normal colon mucosa from patients with colorectal cancer

We evaluated the density and the distribution of methylation within the 5′ CpG island of the p14ARF promoter region and exon 1β. Using bisulfite genomic cloning and direct sequencing, we analyzed 200 clones obtained from 10 tumors and matched adjacent colon mucosa. For each clone, the methylation status of each individual CpG site was determined (Figure 5). For all 27 CpG sites evaluated, we found a significantly (P <0.0001) increased number of methylated clones in tumors (median 38%; 95% CI 25 to 41%; range 13 to 47%) compared with the adjacent normal colon mucosa (median 9%; 95% CI 5 to 13%; range 1 to 24%) (Table 3). Although most normal colon mucosa (7/10) showed only sparse methylation (Figure 5), densely methylated clones were seen in three of the ten normal colon mucosa tested (N1, N18 and N29; Figure 5). Bisulfite genomic cloning and direct sequencing also showed that methylation involving both CpG sites within the proximal and the distal region of the 5′ UTR CpG island of the p14ARF flanking exon 1β (nucleotide position −69 to position +4 relative to the translation codon ATG) is not a frequent event in CRC, but seems to occur more particularly in tumors displaying a restricted pattern of p53 overexpression, including MSI-H and MSS tumors (Figure 5). Overall, the 3′ region of exon 1β was more densely methylated (median 41%; 95% CI 38 to 43%; range 27 to 47%) than the promoter and 5′ region of exon 1β (median 22%; 95% CI 17 to 25%; range 13 to 25%) (P = 0.0001) (Table 3). However, the number of methylated clones on CpG sites within the proximal region of the 5′ CpG island of p14ARF was significantly higher in tumors displaying a restricted pattern of p53 overexpression (median 30%; 95% CI 24 to 36%; range 17 to 36%) than in tumors exhibiting a strong diffuse p53 expression pattern (median 0%; 95% CI 0 to 3%; range 0 to 3%) (P = 0.0003) (Table 3).

Figure 5

Density and distribution of methylated CpG within the 5′ CpG island of flanking exon 1β. Depicted is the distribution of methylated CpG in tumors (up) and corresponding adjacent colon mucosa (down) from 10 patients. For each case, 10 independent clones (represented on horizontal line (a) to (j) were examined. The circles on the vertical line represent the 27 CpG (CpG 1 to 27) sites analyzed for each individual clone. Note that the translation start site is located between CpG sites 8 and 9. An empty circle indicates that the concerned CpG site is unmethylated, a black circle indicates that the concerned CpG site is methylated. For each tumor, the microsatellite instability (MSI) status and p53 immunohistochemistry are indicated. MSI-H, microsatellite instability-high; MSS, microsatellite-stable; p53 D, diffuse pattern of p53 expression, p53 R, restricted pattern of p53 overexpression.

Table 3

Distribution and density of methylation in tumors and adjacent colon mucosa from patients with colorectal cancer

CpG site	Position relative to ATG	% of methylated clones within the 5′ CpG island flanking exon 1β
		Tumor samples	Normal colon mucosa	Tumors with diffuse p53 overexpression pattern	Tumors with restricted p53 overexpression pattern
1	−69	13	5	3	17
2	−43	17	1	0	24
3	−40	22	1	0	31
4	−35	25	4	0	36
5	−28	22	2	0	31
6	−22	19	2	0	27
7	−10	22	5	3	30
8	−7	21	3	0	30
9	+4	30	5	3	41
10	+25	41	8	37	43
11	+31	42	12	43	41
12	+34	45	12	50	43
13	+36	43	18	43	43
14	+38	34	12	33	34
15	+42	43	13	43	43
16	+47	41	15	53	36
17	+50	45	9	50	43
18	+52	43	12	57	37
19	+66	47	9	50	46
20	+80	44	9	50	41
21	+82	40	13	37	41
22	+90	38	13	27	43
23	+105	38	13	47	34
24	+114	41	19	50	37
25	+118	34	17	37	33
26	+121	31	24	37	29
27	+133	27	9	13	33

The percentage of methylated clones was calculated in all tumors (n = 10, ≥10 clones analyzed for each tumor) and adjacent normal colon mucosa (n = 10, ≥ 10 clones analyzed for each sample) for every CpG site. The percentage of methylated clones was higher in tumors median 38%, 95% CI 25-41%; range 13-47%), than in normal colon mucosa (median 9%, 95% CI 1 to 24; range 1 to 24%) (Wilcoxon rank sum test, P <0.0001). The percentage of methylated clones on proximal CpG sites was also higher in tumors with a restricted p53 overexpression pattern (median 30%, 95% CI 24 to 36%, range 17 to 36%) than in tumors with a diffuse p53 overexpression pattern (median 0%, 95% CI 0 to 3%, range 0 to 3%) (Wilcoxon rank sum test, P = 0.0003).

Discussion

The purpose of this study was to investigate whether alteration of p14ARF, a key regulator of p53-MDM2 interaction, plays a role in deregulating the p53 pathway in a subgroup of CRCs exhibiting a restricted pattern of p53 overexpression significantly associated with MSI-H phenotype, low TP53 mutation, and MDM2 overexpression, and inversely correlated with p21 expression loss [7].

Contrary to the usual situation in solid tumor types such as melanoma, pancreatic tumors and some lung tumors [10-12], the present study confirmed the extremely low frequency of intragenic mutations and allelic losses at the p14ARF locus in CRC [13]. Indeed, direct sequencing detected only one (previously reported) missense mutation affecting both the p14ARF and p16/CDKN2A genes [14]. Only 5% percent of the cases, including two patients with Lynch syndrome, were carriers of p.Ala148Thr, a variant considered a non-synonymous single nucleotide polymorphisms (nsSNP, rs3731249) [14,15]. Although the functional significance of this SNP has been controversial in studies of several cancer types [15-17], its role in CRC risk assessment warrants investigation because this variant occurred at an evolutionarily conserved amino acid with a low intolerance index, as predicted by the Sorting Intolerance from Tolerance (SIFT) program [18].

We evaluated epigenetic changes within the p14ARF promoter using two different methylation assays, MSP and BGS. We used MSP because it is widely recognized as a highly sensitive methylation assay, allowing detection of up to 0.01% of methylated alleles of a given CpG island [19]. However, this method provides only qualitative data, so for quantitative analysis, BGS complemented by cloning and direct sequencing, was used [20-22]. Although BGS is a high-resolution assay, this technique is less sensitive than MSP as detection requires at least 25% of the alleles to be methylated [22], suggesting a risk of disagreement between the two methods.

One of our main findings was the detection of p14ARF promoter silencing as a potential cause of deregulation of the p53-MDM2-p14ARF signaling axis in a specific subgroup of CRCs. Using BGS, we fully characterized 35 of the 98 CRCs analyzed. A significant increase in p14ARF promoter methylation was evident in 24 CRCs (69%), and interestingly, 63% of the cases (15/24) were tumors exhibiting the restricted pattern of p53 overexpression (Figure 2, Table 2).

The p14ARF promoter has been previously reported to be preferentially hypermethylated in CRCs retaining the wild-type TP53 gene [8,13,23], and has been particularly associated with sporadic MSI-H CRCs associated with MLH1 epigenetic silencing [8,9]. In addition to the relationship between the restricted p53 overexpression pattern and the MSI-H phenotype [7], we found that p14ARF promoter methylation was increased in CRCs with restricted p53 overexpression, irrespective of MSI status (Figure 4). This observation, along with our previous findings, shows that regardless of the MSI status, CRCs with the restricted p53 overexpression pattern exhibit a significant overlap in terms of their pathobiology, supporting the hypothesis of a common tumorigenic event [7]. In agreement with these observations, previous studies have shown that although CRCs have been reported to evolve either through the classic chromosomal instability pathway or through the alternative MSI pathway known to be significantly associated with the CpG island methylator phenotype, the mechanisms underlying these genomic instability pathways are not always independent [24,25], and a significant degree of overlap can therefore be expected in some tumors, regardless of the MSI status.

Even though a high frequency of p14ARF promoter methylation has been previously reported to occur in tumors without TP53 mutations [8,13,23,26], an inverse correlation between TP53 mutations and epigenetic inactivation of p14ARF in CRCs does not always hold true [27]. In the current study, we found that although the majority of heavily methylated tumors did not have a TP53 mutation, p14ARF promoter methylation was increased in almost half of tumors (5/10) carrying TP53 mutations (Table 2). Interestingly, the most exceptional feature of these tumors was the distribution of p14ARF methylation. Using bisulfite genomic cloning and direct sequencing, we found that extensive methylation involving both the proximal and the distal CpG sites within the 5′ CpG island of p14ARF flanking exon 1β was rare in CRC generally, but occurred more frequently in CRCs displaying a restricted pattern of p53 overexpression (Table 3). In tumors showing a strong diffuse p53 expression pattern associated with missense TP53 mutations, the majority of the methylated clones exhibited partial methylation involving CpG sites downstream from the translation start site and extending throughout exon 1β (Figure 5, Table 3). This pattern of methylation was also seen in some normal colon mucosa (Figure 3; Figure 5 (N29)). Our results support previous observations by Zheng et al., who showed that partial methylation is the most common pattern of p14ARF methylation in primary sporadic CRCs [28].

Owing to the limited availability of an efficient antibody raised against the p14ARF protein, we were unable to examine p14ARF expression by immunohistochemistry in our tumor samples. However, previous experiments, mainly performed in CRC cell lines, have shown that extensive methylation of CpG sites within the 5′ CpG island and exon 1β of p14ARF is associated with transcription silencing and correlates with extremely low levels of p14ARF mRNA, whereas partial methylation correlates with intermediate mRNA expression [28,29]. Based on these findings, we suggest that the extensive methylation seen in CRCs with restricted p53 overexpression may represent an important functional defect in the p14ARF gene, but additional studies are needed to verify this hypothesis.

Additionally, a significant relationship between MDM2 overexpression and increased p14ARF methylation was seen (79%; P = 0.0223). It is known that tumors with reduced p14ARF activity have higher MDM2 activity, which potentially leads to p53 inactivation [30]. Moreover, using immunohistochemistry, a strong inverse relationship between MDM2 and p14ARF inactivation has been previously found in different tumor types, including a subtype of human lung carcinoma displaying an abnormally stabilized p53 protein [31]. Therefore, it is conceivable that the increased MDM2 expression seen in CRCs with restricted p53 overexpression may reflect cellular functional consequences of p14ARF epigenetic inactivation. Interestingly, a previous study found an association between p14ARF epigenetic silencing and an abnormal cytoplasmic localization of MDM2 in primary CRC and tumor cell lines, mainly explained as a direct consequence of p14ARF loss of function [32]. In the current study, we did not find any MDM2 subcellular localization in our cohort of 98 CRCs. Functional interpretation of MDM2 immunostaining data are complicated by the existence of several isoforms, of which detection depends on the antibody used, and this may explain these discrepancies.

It is widely believed that CpG islands in autosomal genes are usually unmethylated, except when associated with certain imprinted genes and with genes that undergo X-chromosome inactivation in females [33,34]. Supporting this paradigm, initial studies indicated methylation of the 5′ CpG island of the p14ARF promoter exclusively in tumor cells [13,27]. However, this view was challenged by detection of p14ARF methylation in normal colon mucosa from patients with CRC and from healthy people without clinical evidence of colon cancer [8,35,36]. In the current study, using the MSP assay, we found coexistence of unmethylated and methylated alleles in the majority of tumors and in all adjacent normal colon mucosa. A clear difference in methylation pattern between tumor and adjacent normal colon mucosa was seen only in the seven tumors (7.1%) that showed heavy methylation. The sensitivity of our MSP assay was significantly high. However, given that we used the conventional MSP assay, which provides qualitative data, we were limited by this high sensitivity, and were unable to distinguish the p14ARF methylation occurring in a small proportion of cells from the high-level methylation associated with epigenetic inactivation. Using the BGS approach, we found that the level of p14ARF methylation in normal tissues was generally below the threshold detection of the BGS assay, and was significantly increased in tumors compared with normal colon mucosa. However, hypermethylation was still present in normal colon mucosa from some patients, and more frequently in those with DNA mismatch repair deficiency associated with MLH1 gene inactivation. Indeed, hypermethylation of the 5′ CpG islands of the p14ARF and MLH1 genes in normal-appearing mucosa surrounding colorectal neoplastic lesions has been described as a ‘field cancerization’ phenomenon, which may occur before genetic alterations in the early stages of carcinogenesis [37].

Conclusion

In summary, this study provides evidence that p14ARF promoter hypermethylation may represent an important cause of deregulation of the p53-MDM2-p14ARF signaling axis in a subgroup of CRCs displaying a restricted overexpression pattern of the p53 protein, associated with the wild-type TP53 gene, concomitant MDM2 overexpression, and normal p21 expression. Although this subgroup of CRCs includes the majority of MSI-H tumors (namely Lynch syndrome-related CRCs and sporadic MSI-H CRCs), methylation involving both proximal and distal CpG sites within the 5′ CpG island flanking exon 1β of p14ARF preferentially occurs in these tumors independently of MSI status. Further investigations are warranted to clarify the significance of this high-level methylation on the transcriptional activity of the p14ARF gene. The results from this work could have clinical implications, because therapeutic delivery of small p14ARF peptides has been reported to mimic the growth-inhibitory effects of full-length p14ARF expression and to restore p53 activity in cancers in which MDM2 is overexpressed or p14ARF is functionally inactivated [38]. Evaluation of the clinical relevance of such promising therapeutic measures would essentially provide a new set of more efficient treatment possibilities in patients with CRC who have tumors displaying the restricted pattern of p53 overexpression.

Methods

Ethics approval

Tissues collection and analyses were approved by the institutional ethics committee of the Catholic University of Louvain (Faculty of Medicine UCL), and all participants provided written informed consent.

Patients

We examined 98 surgical resected tumors and corresponding adjacent normal colon mucosa from the cohort of patients (48 men, 50 women, mean ± SD age 64 ± 14 years) with primary CRC we reported previously [7]. For the 98 CRCs, clinicopathologic data and evaluation of the DNA mismatch repair (MMR) system (using MSI analysis, immunohistochemistry (IHC) for MMR proteins, MMR germline mutation) and somatic BRAF mutation, had been performed previously [7], but only data from the 35 patients extensively studied by bisulfite genomic sequencing (BGS) are shown in Table 1. Immunohistochemical analysis for p53, MDM2, and p21 proteins and mutational analysis for TP53 were also previously performed. Three distinct patterns of p53 expression were seen, including a restricted p53 overexpression pattern clearly distinguishable from both the negative pattern and the strong diffuse pattern [7]. MDM2 immunohistochemical expression was semi-quantitatively evaluated based on the percentage of positive tumor cells. MDM2 overexpression was recorded if a positive staining was evidenced in more than 10% of tumor cells nuclei [7].

p14ARF mutation screening and gene dosage

Sequence-specific primers (according to GenBank accession number NM_058195) for exon 1β and exon 2 (common to both p16/CDKN2A and p14ARF), including the intronic flanking regions of the p14ARF gene, were designed using Primer3 software (http://frodo.wi.mit.edu/cgi-bin/primer3/primer3.cgi). PCR was carried out for each sample, and PCR products were then purified, sequenced, and run on an automated laser fluorescent DNA sequencer (3130XL; AB Applied Biosystems, Foster City, CA, USA). To detect large rearrangements (allelic imbalances) throughout the p14ARF locus, multiplex ligation-dependent probe amplification (MLPA) was performed using (Salsa PO24B 9p21 CDKN2A/2B region kit; MRC-Holland BV, Amsterdam, the Netherlands), in accordance with the manufacturer’s instructions. MLPA PCR products were separated by capillary electrophoresis using an automated laser fluorescent DNA sequencer (3130 XL; AB Applied Biosystems, Foster City, CA, USA). The relative quantities of the amplified probes in each sample were determined using Genotyper (Applied Biosystems, Foster City, CA, USA) and Excel (Microsoft Corp., Redmond, WA, USA) software (Gene Marker version 1.5; Softgenetics Inc, State College, PA, USA). The gene dosage quotient was generated using peak height rather than peak area as an indicator of DNA template amount [39,40]. For each sample, a gene dosage quotient score (peak height relative to control) was calculated and adjusted as follows: homozygous loss ≤ 0 to 0.19 ≤ hemizygous loss ≤ 0.7 to 0.75 ≤ wild-type ≤ 1 to 1.3 < duplication.

Methylation-specific PCR

Genomic DNA was extracted from frozen tumors and matched normal tissues using a standard phenol/chloroform method. Thereafter, bisulfite treatment of 300 ng of genomic DNA was performed (Applied Biosystems methylSEQr™ Bisulfite Conversion Kit) in accordance with the manufacturer’s instructions (Applied Biosystems, Foster City, CA, USA). Methylation-specific PCR (MSP) [19] was performed to examine p14ARF promoter methylation within a region located at least −60 base pairs relative to the translation codon, previously reported to be associated with p14ARF gene silencing in CRC [27]. Methylation in this region was evaluated using the primer sets previously described [27]. These primers pairs allowed assessment of the methylation status of six CpG dinucleotides specific for the 5′ CpG island of the p14ARF gene flanking exon 1β (see Additional file 2: Table S1). The MSP reactions were carried out in a total volume of 25 μl containing 2.5 μl of the manufacturer’s 10× PCR buffer (Roche Diagnostics, Basel, Switzerland), 1.5 μl of 25 mmol/l MgCl2, and 0.25 μl of 100 μmol/l dNTPs (dATPs, dTTPs, dCTPs and dGTPs), 1 μl of primer (10 pmol/μl for each), 1 to 1.25 U of DNA polymerase (FastStart; Roche Diagnostics, Basel, Switzerland), and 1 μl of bisulfite-modified genomic DNA. Normal human leukocyte DNA was methylated in vitro with a CpG methylase (M.SssI; New England BioLabs, Beverly, MA, USA) in accordance with the manufacturer’s instructions, and used as the MSP methylated-allele positive control. After amplification, 5 μl of PCR products were run in an 8% non-denaturing acrylamide gel with an appropriate size marker. Amplicons were visualized by ethidium bromide staining under UV illumination.

Bisulfite genomic sequencing

BGS primers designed to recognize both methylated and unmethylated alleles were generated based on the human contig sequence (GenBank accession number L41934) using MethPrimer software (http://www.urogene.org/methprimer/index1.html) [41]. The designed BGS primers were located within the 5′ CpG island of the p14ARF region flanking exon 1β, and were used to amplify a DNA sequence containing 27 CpG sites, including all the CpG sites targeted by the MSP primers within this region (see Additional file 1: Figure S1). Bisulfite-converted DNA samples from tumor tissue and corresponding adjacent normal tissues from 35 patients, randomly selected from our cohort of patients (Table 1), were subjected to PCR amplification using primer pair A and B (forward and reverse, respectively), followed by a nested PCR amplification with primer pair C and D (forward and reverse; Additional file 1: Figure S1). All the primer sequences used are summarized in (Additional file 2: Table S1). After PCR amplification, the BGS products were purified (Qiaquick PCR Purification Kit; Qiagen Inc., Valencia, CA, USA), and directly sequenced in both directions using primers C and D (forward and reverse; see Additional file 1: Figure S1) with a commercial kit (Big Dye Terminator Cycle Sequencing Ready Reaction Kit, version 1.3; Perkin Elmer/Applied Biosystems, Foster City, CA, USA) in accordance with the manufacturer’s instructions. Sequencing reaction products were purified on filter plates for high-throughput separation (Multiscreen™; Millipore Corp., Bedford, MA, USA) using dextran gel beads (SephadexTM G-50 Fine Beads; GE Healthcare Bio-Sciences AB, Uppsala, Sweden) in accordance with the manufacturer’s instructions. After purification, sequencing reaction products were run on an automated laser fluorescent DNA sequencer (3130XL; AB Applied Biosystems, Foster City, CA, USA). Results were analyzed using the sequencing analysis software for the sequencer (Version 1.5; AB Applied Biosystems, Foster City, CA, USA).

For all 35 patients examined, the bisulfite sequencing chromatogram was analyzed for each individual CpG site, and a specific pattern was assigned: 1) unmethylated, in which the CpG site was fully converted into thymidine, indicating that the concerned CpG site is unmethylated on both alleles (see Additional file 3: Figure S2), 2) partial, showing an overlap of both thymidine and cytosine peaks on a sequencing chromatogram, indicating the presence of both methylated and unmethylated alleles (see Additional file 3: Figure S2 A), 3) methylated, in which the CpG site was fully methylated, indicating that the concerned CpG site is extensively methylated on both alleles (see Additional file 3: Figure S2 B).

Cloning and sequencing

For 10 patients, the amplified bisulfite PCR products from tumor and corresponding normal colon tissues were purified (Qiaquick PCR Purification Kit; Qiagen) and ligated into a pTZ57R/T plasmid vector using a TA cloning and bacterial transformation system (Ins TAclone™ PCR Cloning Kit). The plasmid was inserted into Escherichia coli cells, which were cultured overnight, then recombinant plasmid DNA was isolated and purified (Rapid Miniprep Plasmid Purification System; Marligen Bioscience, Ijamsville, Maryland, USA). Purified plasmid recombinant DNA was subjected to direct PCR amplification in a 25 μl reaction mixture containing 2.5 μl of the manufacturer’s 10× PCR buffer (Roche Diagnostics, Basel, Switzerland), 1.5 μl of 25 mmol/l MgCl2, and 0.25 μl of 100 μmol/l dNTPs, 1 μl of M13 forward and reverse primer (10 pmol/μl for each), 1 U of DNA polymerase (FastStart; Roche Diagnostics, Basel, Switzerland), and 1 μl of purified recombinant plasmid DNA template. Direct sequencing was performed in both directions using M13 primers with a commercial kit (Big Dye Terminator Cycle Sequencing Ready Reaction Kit, version 1.3; Perkin Elmer/Applied Biosystems, Foster City, CA, USA) in accordance with the manufacturer’s instructions. Sequencing reaction products were purified on filter plates for high-throughput separations Multiscreen™; Millipore Corp., Bedford, MA01730 USA) using dextran gel beads (SephadexTM G-50 Fine Beads; GE Healthcare Bio-Sciences AB, Uppsala, Sweden) and were run on an automated laser fluorescent DNA sequencer (3130XL; AB Applied Biosystems, Foster City, CA, USA). For each sample, at least 10 clones were analyzed. For each clone, the bisulfite genomic sequence was analyzed, and for each individual CpG site a methylation status was assigned.

Statistical analysis

We used the Pearson χ2 test (when the minimum expected value was ≥ 5) or the two-tailed Fisher’s exact test (when the minimum expected value was <5) to compare the frequency of p14ARF promoter methylation in 35 patients with CRC in relation to various clinicopathologic parameters and characteristics, including immunohistochemical expression of p53, MDM2, and p21, p53 mutational status and MSI status. Comparison in distribution and density of methylation between tumors and adjacent normal-appearing colon mucosa, and of tumor groups were assessed using the Mann–Whitney or Wilcoxon rank sum test. All statistical analyses were performed using the NCSS 2007 statistical & Power analysis software. All reported P-values were two sided, and the test was significant when the P ≤ 0.05.

Abbreviations

CRC: Colorectal cancer; IHC: Immunohistochemistry; MLPA: Multiplex ligation-dependent probe amplification; MSI: Microsatellite instability: MSI-H: Microsatellite instability-high; MSI-L: Microsatellite instability-low: MSP: Methylation-specific PCR; MSS: Microsatellite-stable; nsSNP: non-synonymous single nucleotide polymorphism; PCR: polymerase chain reaction; UV: Ultraviolet.

Competing interests

The authors have no competing interests to disclose.

Authors’ contributions

CN participated in the design of the study, carried out the pathological and molecular genetic studies, and drafted the manuscript. CS participated in the design of the study and collection of pathological data, and revised the manuscript. RD and AK participated in collection of patients. KD conceived and coordinated the study, and drafted the manuscript. All authors have read and approved the final manuscript.

Additional file 1

Figure S1 promoter methylation analysis by bisulfite genomic sequencing (BGS). The genomic sequence of 5′CpG island of p14ARF region flanking exon 1β was analyzed. The highlighted and numbered CpG indicates the 27 potential CpG sites analyzed. Bold arrows indicate position of forward and reverse MSP primers for methylated (MSPMF/MSPMR) and for unmethylated (MSPUF/MSPUR) alleles. Simple arrows indicate bisulfite genomic sequencing primers specific for both unmethylated and methylated sequences. The putative transcription start site, translation start site (+1), and the end of exon 1β (*) are indicated.

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Additional file 2

Table S1. Primer sequences for methylation-specific PCR, bisulfite genomic sequencing, and PCR amplification of exon 1β and exon 2 of the p14 gene.

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Additional file 3

Figure S2. Bisulfite DNA sequencing chromatograms representing the three different methylation profiles for single CpG dinucleotide sites. (A) DNA sequences from tumor samples showing an overlap of both thymidine and cytosine peaks indicating a partial methylation on CpG site located at position −31 relative to the translation start site (top) compared with another sample showing an unmethylated profile at the same CpG site (bottom), (B) DNA sequences from tumor samples showing full methylation on CpG sites located at +31 and +42 relative to the translation start site (top) compared with another sample showing an unmethylated profile at the same CpG sites (bottom).

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