Genomic organization of a tumor growth inhibitor gene ING1

ING1, a supposed tumor suppressor gene, codes for a p33 protein involved in cell proliferation control and regulation of apoptosis. A GenBank search revealed two groups of expressed sequence tags corresponding to ING1 mRNA forms. The 3′ exon 2 is the same in both forms whereas the 5′ exons 1a and 1b differ. ING1-containing cosmids were found in the LA13NC05 library. Each ING1 exon and flanking introns were sequenced using the cosmid 80H9 template. In the genome, the exons are arranged as 1b-1a-2. RT-PCR showed that both mRNA forms are simultaneously present in cell lines. The deduced amino acid sequence for 1b-2 proved similar to those of human proteins ING1L (2e^-72) and ING1L-7 (6e-24) and several proteins of lower eukaryotes having the ING-specific N-terminal domain and the zinc-binding domain PHD. Hence the ING-like proteins can be regarded as a separate evolutionarily old family. A peculiarity of the ING1 structure is the CpG islands surrounding each of its three exons, suggesting regulation of its expression through de novo methylation. The data on the fine structure of ING1 and its mRNA forms permit mutation screening and assessment of its methylation status in human tumor specimens.