Hypomethylation Of A Line-1 Promoter Activates A Truncated Met Transcript In Bladder Cancer

AACR Annual Meeting-- Apr 18-22, 2009; Denver, CO In addition to hypermethylation, global hypomethylation of repetitive elements such as long interspersed nuclear elements (LINE-1s or L1s) is also involved in tumorigenesis. Although there are nearly 500,000 L1s in the human genome only about 60 to 100 have potentially functional promoters, which are located within CpG islands and silenced by methylation in normal somatic cells. A full length L1 sequence has a sense promoter driving transcription of its two open reading frames and an antisense promoter driving transcription in the opposite direction that can act as an alternate promoter for surrounding genes. We show, for the first time, that the specific L1 in the MET proto-oncogene is silenced by DNA methylation, H3K9me3, and nucleosomal occupancy at the two transcriptional start sites, resulting in a tetranucleosomal structure. Upon hypomethylation the L1 antisense promoter becomes active allowing for transcription of an alternate form of MET, L1-MET. In addition to loss of DNA methylation, nucleosomes are evicted from the transcriptional start sites, resulting in a dinucleosomal structure, and those remaining contain the histone variant H2A.Z, and acetylated H3 and H3K4me3. The consequences of global hypomethylation at repetitive elements in cancer has long been the subject of speculation regarding the generation of genomic instability and potential activation of oncogenes. Ever since studies on viable yellow agouti (Avy) mice revealed that hypomethylation of a retrotransposon could induce ectopic expression of a gene and influence disease susceptibility it has been postulated that similar events may occur in humans. We show, for the first time, a direct relationship between hypomethylation of a retrotranspositional element and altered gene expression in humans and its occurance in a diseased state. Specifically, hypomethylation of a LINE-1 promoter induces an alternate transcript of the MET oncogene in bladder tumors and across the entire urothelium of tumor-bearing bladders. The robust expression of the truncated MET does not lead to the clonal expansion of cells harboring the defect, since the urothelium remains polyclonal. The widespread epigenetic alteration is more likely to be permissive for the growth of newly mutated cells rather than being directly responsible for the clonal evolution of the tumor. Finally, ROC curves based on the hypomethylation of the specific LINE-1 promoter within MET are extraordinarily specific and sensitive, providing a useful marker for the diagnosis and treatment of bladder cancer. Citation Information: In: Proc Am Assoc Cancer Res; 2009 Apr 18-22; Denver, CO. Philadelphia (PA): AACR; 2009. Abstract nr LB-172.