Theoretical mechanisms for synthesis of carcinogen-induced embryonic proteins. V. The steroids

Biochemical formulations of embryonic gene control

A search for control mechanisms involving embryonic genes is explored reviewing a variety of subject areas including, a) the methylation status of DNA and the globin gene, b) ethionine and steroid effects on expression of embryonic genes, c) alpha-fetoprotein gene activity induced by carcinogens and in hepatomas. Also taken into account are transcriptional and cytogenetic aspects. Theories of heterochromatin dynamics are developed in connection with certain contributions from chromatin experimental findings, especially regarding the status of methylation. The potential importance to control theory of the inverse correlation between ATP:L-methionine S-adenosyltransferase activity and alpha-fetoprotein synthesis is emphasized. Several generalizations were derived during the study. It appears that the depression mechanisms may act only on genes that have been active in embryonic stages and have become repressed during differentiation. Another idea concerned heterochromatin. Any heterochromatic segment of DNA may represent at its associated ends a certain amount of euchromatin that would be in a quasi-heterochromatic state. Such pseudoheterochromatin is hypothesized to be induced by the true heterochromatin (eigenheterochromatin).

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: XIX. Embryonic genes

The methylation status of a conformation-inducer protein that would effect the status of DNA in relation to its ability to be in an active or inactive state is proposed to be central in the regulation of embryonic genes. Thus a distinction can be drawn between induceable "adult" genes such as glucocorticoid induced tyrosine aminotransferase and induceable "embryonic" genes such as ethionine induced alpha-fetoprotein. However, in the proposed mechanism the methylation of DNA is also important in that a hypomethylated state of a CCGG sequence of a promotor region for a conformation-induced protein gene is required to initiate the induction events.

Theoretical mechanisms for synthesis of carcinogen-induced embryonic proteins: XV. Preliminary generalizations

The term embryonic gene is discussed in which an operational definition is given, namely that it be restricted for those genes which are active during the embryonic state but repressed during differentiation. After generalizing a large variety of different types of carcinogenic agents and their action, which in principle are capable of activating embryonic genes, a preliminary notion of the carcinogenic process was derived. It appears that the bioalkylation pattern can be perturbed by a variety of agents from electromagnetic radiation to ethionine. Specific genes or their corresponding repressors such as an embryonic type phospho-protein kinase would become derepressed because of their methylation status (or by some other analogous alteration, e.g., via a specific mutation of a proto-oncogene that would create an embryonic type kinase, DNA intercalation by planar molecules, or, a hereditary process such as V-type position effect). This would cause competent stem type precursor cells containing easily derepressed or partially repressed arrays of embryonic genes to become activated, producing many features characteristic of a neoplastic cell.

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

By the use of a simplistic concept of the structure of heterochromatin, one can derive a formulation for the mechanism by which the phenomenon of V-type position effect can repress genic activity. This notion would explain how the Bar locus and other position effect examples cause genic repression. The mechanism, when extended, may also explain the process of cancer induction by hereditary means. In its most concise form the theory states that heterochromatin has a stabilized structure through special proteins, such as phosphorylated species that allows it the capacity to induce new domains of a "pseudo-heterochromatic" conformation to juxta-positional euchromatic segments for short distances by virtue of electrostatic and hydrogen bond forces.

Theoretical mechanisms for synthesis of carcinogen-induced embryonic proteins. VI. Radiation

The following is an attempt to devise a theory of specific induction processes required of the neoplastic transformation using the non-specific carcinogenic agent - radiation. A variety of biological considerations including comparative radiosensitivity, radiation effects on chromatin and enzymes, radiomimetic chemical induction of chromosomal anomalies, lethality and the tRNA function are also presented. These topics serve as background for elaborating a scheme of how a specific array of genes could become decontrolled. A concept derived involves the fixation of despiralized genic areas which are induced by hypomethylated DNA caused by anomalous DNA methylation. This process could be a potentially critical part of the irradiation-induced carcinogenic event. The concept of fixation of chromatin in an informational sense leads to a mechanism requiring a classic mutation, modified by a repair process that ultimately leads to an epigenetic event. More specifically it would appear that a critical target to induce the neoplastic response from a cell with irradiation could be the DNA responsible for the template sites (or at least a secondary area that indirectly could cause inactivation of this site) of the genes for DNA methylation. This would not necessarily be the only genic change required but one can derive gene derepressions from this type of molecular lesion. A similar scheme for gene repression has not been devised in this writing beyond the simplistic, although not unwarranted, viewpoint of direct DNA damage.