Epigenetic Silencing of the p16INK4a Tumor Suppressor Is Associated with Loss of CTCF Binding and a Chromatin Boundary

The p16(INK4a) tumor suppressor gene is a frequent target of epigenetic inactivation in human cancers, which is an early event in breast carcinogenesis. We describe the existence of a chromatin boundary upstream of the p16 gene that is lost when this gene is aberrantly silenced. We show that the multifunctional protein CTCF associates in the vicinity of this boundary and absence of binding strongly coincides with p16 silencing in multiple types of cancer cells. CTCF binding also correlates with RASSF1A and CDH1 gene activation, and CTCF interaction is absent when these genes are methylated and silenced. Interestingly, defective poly(ADP-ribosyl)ation of CTCF and dissociation from the molecular chaperone Nucleolin occur in p 16-silenced cells, abrogating its proper function. Thus, destabilization of specific chromosomal boundaries through aberrant crosstalk between CTCF, poly(ADP-ribosyl)ation, and DNA methylation may be a general mechanism to inactivate tumor suppressor genes and initiate tumorigenesis in numerous forms of human cancers.