Telomere erosion is independent of microsatellite instability but related to loss of heterozygosity in gastric cancer

Abstract

Aim: To correlate the length of the telomere to microsatellite instability (MSI) and loss of heterozygosity (LOH) of APC, MCC and DCC genes in gastric carcinomas.

Methods: Telomeric restriction fragment (TRF) length of gastric cancer was measured with Southern blot. LOH of APC, MCC and DCC genes, microsatellite instability (MSI) and frameshift mutation of hMSH6, TGF-betaRII and BAX genes were analyzed by PCR-based methods.

Results: Sixty-eight cases of sporadic gastric carcinoma were studied for MSI using five microsatellite markers. MSI in at least one locus was detected in 17 (25%) of 68 tumors analyzed. Frameshift mutations of hMSH6, TGF-betaRII and BAX were detected in 2,6 and 3 of gastric carcinomas respectively showing high MSI (> or = 2 loci, n = 8), but none was found in those showing low MSI (only one locus, n = 9) or MSS (tumor lacking MSI or stable, n = 51). Thirty-five cases, including all high MSI and low MSI, were studied for TRF. The mean TRF length was not correlated with clinicopathological parameters. No association was observed between TRF length and MSI or frameshift mutation. On the contrary, LOH at the DCC locus was related to telomere shortening (P<0.01). This tendency was also observed in APC and MCC genes, although there was no statistical significance.

Conclusion: The development of gastric cancer can arise through two different genetic pathways. In high MSI gastric cancers, defective mismatch repair allows mutations to accumulate and generate the high MSI phenotype. In gastric cancers showing either low MSI or MSS, multiple deletions may represent the LOH pathway. Telomere erosion is independent of high MSI phenotype but related to the LOH pathway in gastric cancer.

Figure 1

MSI in gastric cancer using 5 microsatellite loci (BAT-25, BAT-26, BAT40, D2S123, and D5S346). Arrows indicate variant conformers. (N: normal DNA pattern; T: tumor specimens containing variant conformers representing MIS)

Figure 2

Frameshift mutations of hMSH6 TGF-betaRII, and BAX genes in gastric cancers. Arrows indicate conformational variants associated with frameshift mutations. (N: normal DNA; T: tumor DNA)

Figure 3

Southern blot analysis of telomere repeat arrays in DNA samples of patients with gastric cancer. DNA was digested with-Hin fI and hybridized with α-³²P-APT end-labeled (TTAGGG)₄ probe.T = Tumour; N = Corresponding normal tissues.

Figure 4

Representative LOH analysis of APC, MCC and DCC genes. (APC: Rsa1 RFLP in APC exon 11, loss of 133 bp allele is seen in tumor DNA, MCC: A 14 bp insertion/deletion polymorphism in MCC exon 10 gives rise to a 93 or 79 allele. Loss of the 93 bp allele is seen in the tumor. DCC: Losses of 396 bp, 240 bp and 200 bp alleles are seen at M2, M3 and VNTR polymorphic sequences in the tumor DNA)