The inhibitor of growth (ING) gene family: Potential role in cancer therapy

The discovery of ING1 gene paved the way to the identification of other ING members (ING2-5) and their isoforms associated with cell cycle, apoptosis and senescence. The ING family has been an emerging putative tumor suppressor gene (TSG) in which the major mechanism is through interaction with the determinants of chromatin function and gene-specific transcription factors. The regulatory mechanism highly involves the conserved plant homeodomain (PHD), which binds to histones in a methylation-sensitive manner, suggesting that ING proteins may contribute to the maintenance of the epigenetic code. Furthermore, ING family members contain nuclear localization signals and N-terminal sequences important in the interaction with histone acetyltransferase (HAT) and histone deacetyltransferase (HDAC) that regulate gene promoter activity within chromatin. Although ING proteins have the same PHD motif, the variation in the N-terminal dictates the differences in tumor the suppressive ability of ING in various tumors. Inactivation of the normal function is achieved through allelic loss of genomic regions containing the ING gene, alteration in the ING promoter region, variation of mRNA splicing efficacy or reduced mRNA stability. It is most probably the apparent combination of these aberrant mechanisms that resulted in reduced availability of functional ING protein. In cancer cells, ING transcript levels are often suppressed but the genes are rarely mutated. The mechanism of suppression of ING expression may have to do with the abnormally high methylation levels of the ING gene promoter, which have been correlated with low transcript levels. Emerging evidence on the function of ING and related regulatory mechanisms strongly points to ING as a candidate TSG and therefore a potential target in the molecular therapy of some types of tumor.