Age-related methylation in normal colon mucosa differs between the proximal and distal colon in patients who underwent colonoscopy

Abstract

Objectives

To examine the difference in the methylation status in normal colon mucosa between the proximal and distal colon, in relation to the correlation between the methylation status of normal mucosa and characteristics of neoplasia.

Design and methods

Paired biopsy specimens of normal mucosa from the proximal and distal colon of 82 patients who underwent colonoscopy were obtained. The methylation status of the promoter region of estrogen receptor 1 (ESR1) and myogenic differentiation 1 (MYOD1) was examined.

Results

Normal mucosa was more highly methylated in the distal than in the proximal colon in both ESR1 and MYOD1 loci (p < 0.0001 and p = 0.0009, respectively). Advanced characteristics of polyps in the distal colon were frequently observed in patients with lower methylation of ESR1 in the distal colon normal mucosa.

Conclusions

Methylation levels in normal mucosa differ between the proximal and distal colon, and lower methylation of ESR1 in the distal colon normal mucosa may correlate with advanced features of neoplasia in the distal colon.

Introduction

Recently, epigenetic changes including DNA methylation have been found to play an important role in carcinogenesis in addition to genetic changes. DNA methylation is a covalent chemical modification resulting in the addition of a methyl (CH₃) group at the C5-position of cytosine in the sequence context 5′-CG-3′. This epigenetic effect is particularly striking when DNA methylation affects promoter CpG islands. Methylation of cytosines within CpG islands is associated with the loss of protein expression by the repression of transcription. Therefore, gene hypermethylation is associated with the silencing of tumor suppressor genes in cancer [1]. On the other hand, global hypomethylation of genes has been linked to genomic instability and increased mutation rates [2], [3]. Thus, gene hypomethylation is thought to be correlated with other types of carcinogenesis. According to the frequency of CpG island methylation, the idea of a CpG island methylator phenotype (CIMP) in carcinogenesis of the colon has also been proposed [4].

Meanwhile, methylation also occurs in normal colon mucosa. In particular, age-related methylation loci such as estrogen receptor 1 (ESR1) and myogenic differentiation 1 (MYOD1) have been highlighted [5], [6], [7], [8], [9], [10]. These loci were methylated in normal colon mucosa in an age-dependent manner, and this type of methylation is considered to serve as a functional link between aging and cancer, maybe by deregulating the growth and differentiation of normal colonic epithelial cells and predisposing them to neoplastic transformation. In fact, Kawakami et al. reported that ESR1 was methylated in normal colonic mucosa of patients who had cancer with microsatellite instability (MSI) or with CIMP [10]. In addition, methylation of ESR1 in colon mucosa occurs more frequently in patients with ulcerative colitis who had neoplasia than in ulcerative colitis patients who had no neoplasia [11]. However, there are few reports on the correlation between age-related methylation and risk factors for neoplasia (e.g., genetic factors or environmental factors), or characteristics of neoplasia.

Recently, moreover, clinical and biological differences in colorectal neoplasia between the proximal and distal colon have been revealed [12], [13], [14], [15], [16]. In particular, several reports have referred to the differences in epigenetic alteration according to the location of neoplasia [9], [17], [18], [19], [20], [21], [22]. In general, colorectal cancer (CRC) with CIMP (CIMP+ or CIMP-high) is observed more frequently in the proximal colon than in the distal colon, while CRC with CIMP- (or CIMP-low) is likely to be located in the distal colon [17], [18]. In addition, sporadic MSI cancers, which are generally accompanied by hypermethylation of the promoter region of hMLH1, are dominant in the proximal colon [19]. Moreover, our previous report indicated that a laterally spreading type of colorectal adenomas in the proximal colon were likely to be categorized as CIMP-high [23]. Thus, the correlation between methylation of tumor cells and tumor location has been discussed. However, few reports have studied the difference in methylation status of normal mucosa between the proximal and distal colon, although such an inquiry seems interesting in light of the difference in the initiation of neoplasia according to colonic location.

Accordingly, in this study, we investigated the methylation status of age-related methylation loci in normal mucosa of patients who underwent endoscopic polypectomy, with special attention paid to the differences between the proximal and distal colon. In addition, clinical factors that affect the age-related methylation of normal mucosa were examined based on the locational difference. Moreover, the relationship between the age-related methylation status of normal mucosa and the polyp characteristics that the patients carried was also examined and discussed.