Multiple mechanisms downregulate CDKN1C in human bladder cancer

Expression of the imprinted CDKNIC gene at chromosome 11p15.5 encoding the cell cycle inhibitor p57(KIP2) is disturbed in Beckwith-Wiedemann syndrome and in several human cancers by different mechanisms. Many advanced urothelial cancers (TCC) display downregulation of CDKNIC expression. The responsible mechanisms were investigated in TCC cell lines, with cultured normal urothelial cells (UEC) as controls. CDKNIC mRNA expression was diminished in 12/15 TCC lines and p57(KIP2) protein was decreased accordingly. Because CDKNIC is expressed from the maternal allele only, LOH at 11p15.5 represents one mechanism of downregulation. In 3 cell lines, several polymorphic markers flanking CDKNIC were homozygous compatible with this mechanism. Hypermethylation of the CDKNIC promoter, a reported cause of downregulation in other cancers, was detected by bisulfite sequencing in several cell lines and appeared associated with downregulation in at least one cell line. The methylation inhibitor 5-aza-2'deoxycytidine induced CDKNIC expression in this cell line and others. A third reported mechanism involves a switch of both alleles toward a paternal imprinting pattern, indicated by hypomethylation of a differentially methylated region (DMR) in the imprinting center (IC2). This hypomethylation was detected in most TCC lines, and was associated with re-expression of the non-coding LIT1 RNA and with downregulation of CDKNIC in several. Thus, CDKNIC downregulation in TCC seems to occur by several different mechanisms. This finding and the ability of p57(KIP2) to induce senescence in urothelial cells make CDKNIC a good candidate for a tumor suppressor at 11p in TCC. (C) 2004 Wiley-Liss, Inc.