Truncated Isoforms of lncRNA ANRIL Are Overexpressed in Bladder Cancer, But Do Not Contribute to Repression of INK4 Tumor Suppressors

The INK4/ARF locus at chromosome 9p21 encoding p14(ARF), p15(INK4B) and p16(INK4A) is a major tumor suppressor locus, constituting an important barrier for tumor growth. It is frequently inactivated in cancers, especially in urothelial carcinoma (UC). In addition to deletions and DNA hypermethylation, further epigenetic mechanisms might underlie its repression. One candidate factor is the long noncoding RNA ANRIL, which recruits Polycomb proteins (PcG) to regulate expression of target genes in cis and trans. We observed ANRIL overexpression in many UC tissues and cell lines mainly resulting from upregulation of 3’-truncated isoforms. However, aberrant ANRIL expression was neither associated with repression of INK4/ARF genes nor with proliferation activity or senescence. We wondered whether truncated ANRIL isoforms exhibit altered properties resulting in loss of function in cis. We excluded delocalization and performed RNA immunoprecipitation demonstrating interaction between full length or truncated ANRIL and PcG protein CBX7, but not SUZ12 of PRC2. Our data indicate that ANRIL in UC cells may not interact with PRC2, which is central for initializing gene repression. Thus, tissue-specific binding activities between ANRIL and PcG proteins may determine the regulatory function of ANRIL. In conclusion, ANRIL does not play a major role in repression of the INK4/ARF locus in UC.