Purpose: Methylated CpG sites are readily mutated, yet are strongly conserved in tumor suppressor genes. Uniquely, the arginine-encoding CGA codon may undergo methylation-dependent point mutation to a stop codon (TGA), whereas the arginine-encoding AGA codon is stable. To clarify the relationship between germline codon mutability and somatic carcinogenesis, we compared patterns of codon usage in tumor suppressor gene subsets known to differ in function and evolutionary stability.
Procedures: Human tumor suppressor genes were classified as either pro-apoptotic (gatekeepers; high-essentiality, low Ka/Ks) or repair-type (caretakers; low-essentiality, high Ka/Ks). Reference normal and tumor gene sequence databases were analysed in terms of nucleotide composition (GC%), intragenic CpG sites, and CGA/AGA codon patterns. Phylogenetic analyses of relevant codon frequencies across 24 species from the UCSC database were performed as a control.
Results: Species-specific stop codon frequency varies with genomic GC content in a methylation-regulated manner, whereas CGA frequency is GC-independent. Whole-genome frequencies of CGA and TGA codons are positively correlated (p = 0.013). CGA mutation in tumors is asymmetric (TGA >> CAA; p < 0.001), with such mutations 3-fold more likely to be nonsense-type in gatekeepers than in caretakers. Non-random CGA clustering is demonstrable in canonical gatekeepers such as NF1, Rb and TP53 (p < 0.02).
Conclusion: Human pro-apoptotic suppressor genes are selectively enriched for the presence of nonsense-prone CpG-containing codons involved in somatic adult-onset tumor progression. We propose that this germline vulnerability is conserved as part of a methylation-dependent embryonic response to environmental teratogens which otherwise cause excessive apoptosis, somatic malformations, and reduced fitness.
Citation Information: Clin Cancer Res 2010;16(7 Suppl):B9