A major goal in molecular biology is to understand the molecular basis of ontogeny. This process of cell differentiation appears to involve two phases. In an initial phase, cells in different environments of the developing organism are subjected to different signals, which then induce expression of different genes. Cells subsequently undergo a change in state and can then maintain their different patterns of gene expression in the absence of these external signals. Thereafter, the different states of expression are stable and can be inherited through many cell divisions, independent of the external environment. Nevertheless, the DNA in the different cell types has the same nucleotide sequence, implying that the different states are not "genetically" inherited but rather are maintained by some "epigenetic" mechanism. These general observations suggest that normal differentiation might depend on two types of DNA elements: an element which is sensitive to external signals and initiates the active or silent state of expression, and an element which serves to maintain that state of expression.
The molecular analysis of epigenesis is necessarily difficult to study in whole animals and explanted normal cells, as the diverse interactions between cells of many different types prevent reliable discrimination between changes in the epigenetic state and changes in the external cellular milieu. To overcome this limitation we have developed a model system in which expression of the endogenous immunoglobulin heavy chain gene is subject to two levels of epigenetic control in cells growing in tissue culture. Our analysis has focused on the role of two previously defined elements: the intronic (Eµ) enhancer and matrix attachment regions (MARs). Our recent studies have revealed that in the absence of Eµ, expression of the µ gene can exist in either of two epigenetically maintained, metastable states, in which the immunoglobulin gene is either fully on or fully off. Cells can switch between these states, and our measurements of the rate of switching indicate that the rates of switching can be decreased ~1000 fold by the MARs. Our results thus suggest that the Eµ element might provide the mechanism for initiating expression, and that the MARs might serve to stabilize expression in either the on or the off state. We have undertaken experiments to test these interpretations.
Understanding epigenesis is important in diverse areas of research. As noted above, epigenetics is the basis of the still very mysterious process of ontogeny. Second, failures in epigenetic inheritance as in the case of genetic mutations are likely to contribute importantly to disease and aging. Third, proper epigenetic control is essential for long term applications of gene therapy and bioproduction, where highly stable gene expression will be required.
Ronai, D., Berru, M., and Shulman, M.J. (1999) Variegated expression of the endogenous immunoglobulin heavy-chain gene in the absence of the intronic locus control region. Mol. Cell. Biol. 19: 7031-7040