DNA methylation and expression of LINE-1 and HERV-K provirus sequences in urothelial and renal cell carcinomas

Abstract

Summary Since DNA methylation is considered an important mechanism for silencing of retroelements in the mammalian genome, hypomethylation in human tumours may lead to their reactivation. The methylation status of LINE-1 retroposons was determined in 73 samples of urinary bladder cancers, 34 specimens of renal cell carcinoma and in the corresponding normal tissues by Southern blot analysis. LINE-1 sequences were strongly methylated in normal tissues and were significantly hypomethylated in 69 (95%) urothelial carcinomas, but in none of the renal carcinomas. Hypomethylation in bladder cancers was independent of stage and tended to increase with grade. The methylation status of HERV-K proviral DNA in normal and transformed urothelial cells paralleled that of LINE-1 sequences (r 2 = 0.87). It was shown by ligation-mediated polymerase chain reaction that hypomethylation also extended to the LINE-1 promoter sequence located at the 5′-ends of full-length elements which is repressed by methylation in somatic tissues. Accordingly, full-length LINE-1 transcripts were detected by Northern blot analysis in two urothelial carcinoma cell lines. In contrast, transcripts from HERV-K proviruses were restricted to teratocarcinoma cell lines. Our data indicate that genome-wide DNA hypomethylation is an early change in urothelial carcinoma, but is absent from renal cell carcinoma. The coordinate changes of LINE-1 and HERV-K DNA methylation suggest that hypomethylation in urothelial cancer affects a variety of different retroelements to similar extents. We speculate that decreased methylation of LINE-1 retroelements, in particular, may contribute to genomic instability in specific human tumours such as urothelial carcinoma by rendering these normally repressed sequences competent for transcription and recombination.

DNA methylation, methyltransferases, and cancer

“…Experimentally-induced demethylation of the genome by homozygous knockout of the Dnmt1 gene also supports this notion by demonstrating that transposable elements become demethylated and re-express in Dnmt1 7/7 ES cells (Walsh et al, 1998). Similar observations have been made in primary human and rodent tumors samples (Flori et al, 1999;Grassi et al, 1999). Transcription from many strong promoters could globally alter transcription patterns by altering transcription factor levels or by negatively e ecting speci®c growth-regulatory genes in which the reactivated elements reside.…”

Hypomethylation of LINE-1 and Alu in well-differentiated neuroendocrine tumors (pancreatic endocrine tumors and carcinoid tumors)

“…For example, hypomethylation is common in colon, liver, stomach, esophagus, lung, breast, head and neck, as well as urothelial and metastatic prostate carcinomas, but uncommon in renal cell carcinomas, papillary carcinomas of thyroid, lymphomas and most hematological malignancies. [25][26][27][28][29] This is also corroborated in experimental models of global hypomethylation in mice. The DNA methyltransferase-1 hypomorphic mice had suppression of polyp formation and reduction in the frequency of CpG island methylation in both the normal mucosa and adenomas in Apc (Min/ þ ) mice, but developed T-cell lymphomas and liver tumors.…”

MicroRNA Responses to Cellular Stress

“…Low dietary folate levels lead to reduction of DNA stability, through misincorporation of uracil and subsequent DNA damage during the repair process, as well as to increased genomic hypomethylation (16,17). When hypomethylation occurs in the 5 ¶ regulatory region of genes or viral elements, it has been associated with increased gene expression (18,19) and is also thought to be related to the overall genomic instability that is observed in cancer (20). However, the timing, targeting, and relationship of these various alterations to cancer or developmental anomalies remain unclear.…”