Theoretical mechanisms for synthesis of carcinogen-induced embryonic proteins: IX. V-type position effect

Theoretical mechanisms for synthesis of carcinogen-induced embryonic protein: XXI. Oncogenes interpreted as embryonic genes

If oncogenes are interpreted as embryonic genes then the mechanisms for induction of these embryonic genes can be generalized and theoretical considerations can be derived as follows. Agents that induce activity by derepressing embyronic genes can be placed into major groups: i) translocation inducers, ii) non-mutating carcinogenic agents, iii) intercalating DNA groove distorters, and iv) mutating agents. Translocation inducers work via long terminal repeat insertions, antibody promoter region associations, or V-type position effects. Non-mutational agents such as ethionine cause hyporibosylation of nucleosome core histones or hypomethylation of promoter regions of conformation inducer proteins. All of these agents cause deheterochromatizations of facultative heterochromatin. These processes (or by a classical mutation with mutating agents), cause DNA replication to acquire a new abnormal methylation pattern that is held constant by maintenance DNA methylases. The resultant active series of repressed reduntant type embryonic genes, such as subsets of rDNA genes and tRNA methylase genes, including oncogenes, e.g., protein phosphokinases among other required genes, and spurious irrelevant embryonic genes to the process of carcinogenesis. From the above theory is derived the concept that normal sets of redundant genes, e.g., rDNA are normally activated by planar intercalating agents (steroids) that would simply be the limiting subset of the mechanism used to create an anomalous state when extended to the set of embryonically repressed genes that become activated.

Theoretical mechanisms for synthesis of carcinogen-induced embryonic proteins: XVII. Heterochromatin mechanisms

A mechanism for induced embryonic gene expression via a process of deheterochromatization using a model carcinogen has been derived. First ethionine becomes activated to S-adenosyl-L-ethionine which inhibits the methylation of nicotinamide, a resulting product of polyADP-ribose polymerase. This causes hyporibosylated nucleosome core histones which normally would base pair by virtue of the adenine moieties with thymidine-rich regions of DNA, being the precursor of heterochromatin. Thus in the anomalous state a deheterochromatized condition of embryonic genes would be created. Possibly embryonic genes are dispersed in AT-rich regions potentially capable of becoming hyperspiralized by this process. Repressable embryonic genes would not be inactivated. It was also noted that hyomethylated non-histone chromatin proteins cause an extension of the nucleosome chanins which would also favor the above situation. This mechanism explains our experimental findings of the relatively rapid reversal of ethionine induced alpha-fetoprotein levels by methionine. The process of heterochromatization is hypothesized to be induced by short (pentanucleotides) moities of poly (ADP-ribose), formed on core histones, that hydrogen bond to thymidine rich inter Nu body DNA.

Theoretical mechanisms for synthesis of carcinogen-induced embryonic proteins: XII mutational and non-mutational mechanism as subsets of a more general mechanism. Part A--Ethionine

The ethionine-induced genic derepression mechanism is visualized as a secondary process that occurs after the S-adenosyl-L methionine pool concentrations are lowered to critical levels. Although DNA methylation has been shown to be correlated with genic activity, the times observed for inducement (3 days) of the alpha-fetoprotein gene and its reversibility (within 7 days) does not make it likely that alterations in the methylated status of DNA is involved. The specific mechanism is theorized to be as follows: the adenine moiety of S-adenosyl-L-methionine base-pairs with thymine of a specific structural area of the alpha-fetoprotein gene. The process is visualized as a frequent event during moments of structural relaxation of an otherwise hyperspiralized condition of the chromatin. This weak hydrogen bonding situation allows the methylation by protein methylases of a precursor chromatin protein that after methylation by the S-adenosyl-L-methionine which is base-paired to the specific DNA site, conformationally is set or locked into place and acts as a specific repressor for the alpha-fetoprotein gene. This subsequently disallows RNA polymerase activity of the region. During turnover of this chromatin protein the replacement of the methylated repressor is normally maintained. But if the S-adenosyl-L-methionine pool concentration is lowered to a level below that required for base-pairing by the adenine moiety, then the repressed conformational condition of the alpha-fetoprotein gene is altered allowing transcription. In this manner the correlation between low S-adenosyl-L-methionine and alpha-fetoprotein synthesis can be made.