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Fig. 2.
Relation between isolated clones and known genes. Three of the genes whose genomic sequence is available from GenBank are shown. Short vertical lines, positions of CpG dinucleotides.Filled boxes, exons . For the CSXgene, cDNA is shown in arectangular box. Bars at the top, position of the MINT clones.
The chromosomal position of most of the unknown clones was determined using a somatic cell hybrid panel and a radiation hybrid panel (Table 1) ⇓ . Of note, MINT3 and MINT9 mapped to chromosome 1p35–36, MINT13 mapped to 7q31, MINT24 mapped to 3p25–26, MINT25 mapped to 22q11–ter, and MINT31 mapped to 17q21. All of these chromosomal segments are areas that are frequently deleted in various tumors (23) .
Silencing of the Versican Gene in Colorectal Cancer.
To determine whether some of these clones truly represented genes silenced by methylation, we examined the Versican gene in more detail. Versican is a secreted glycoprotein that appears to be regulated by the RB1 tumor suppressor gene (24) . MINT11 corresponds to part of exon 1 and part of intron 1 of the Versican gene (Fig. 2) ⇓. Hypermethylation of the two SmaI sites in exon 1 and intron 1 in colon cancer cell lines was confirmed by both Southern blot analysis and MCA (data not shown). We hypothesized that this methylation was representative of the entire CpG island, including the proximal promoter. To address this issue, we used PCR of bisulfite-treated DNA using primers designed to amplify the region around the transcription start site of this gene. The PCR product was then digested with restriction enzymes that distinguish methylated from unmethylated DNA (Fig. 3A) ⇓ . The Versican promoter was found to be completely methylated in the colon cancer cell lines DLD1, LOVO, SW48, and SW837 and partially methylated in HCT116 and HT29 (Fig. 3B) ⇓ . In primary colon tumors, Versican was hypermethylated in 17 of 25 cases (68%; Fig. 3B ⇓ ). Interestingly, some methylation of the Versican promoter was also found in normal tissues, albeit at lower levels when compared with tumors. The level of methylation in normal colon mucosa increased with age of the patient (Fig. 3, B and C) ⇓ , from an average of 6.9% in patients between 20 and 30 years of age, to an average of 28.9% in patients >80. A linear regression analysis revealed a significant association between age and Versican promoter methylation (r = 0.7, P < 0.000001). Using RT-PCR, we next examined the expression of Versican in normal colon mucosa and colorectal cancer cell lines. As shown in Fig. 3D ⇓ , Versicanis expressed in normal colon epithelium but is markedly down-regulated or absent in methylated colon cancer cell lines. Expression of Versican in all of these cell lines was easily restored after treatment with the demethylating agent, 5-aza-2′-deoxycytidine (Fig. 3D) ⇓ . These data suggest that Versican becomes methylated in normal colon in an age-dependent manner, and that this leads to hypermethylation and loss of expression in most colorectal tumors.
