Study sheds light on events orchestrating the changes when mammalian eggs are fertilized and become embryos

A new study sheds light on events orchestrating the changes when mammalian eggs are fertilized and become embryos. Researchers have discovered that expression of genes in mouse eggs and very early embryos is activated in part by regions of DNA called retrotransposons, which may have originated from retroviruses.

These regions, found in DNA of human, mouse, and other mammals in hundreds of thousands of copies, are called retrotransposons because they have the ability to propagate and insert themselves into different positions within the genome. The research, published in the October issue of Developmental Cell, suggests that retrotransposons may not be just the "junk DNA" once thought, but rather appear to be a large repository of start sites for initiating gene expression. Therefore, more than one third of the mouse and human genomes, previously thought to be non-functional, may play some role in the regulation of gene expression and promotion of genetic diversity.

Dr. Barbara B. Knowles and colleagues from The Jackson Laboratory in Bar Harbor, Maine, found that distinct retrotransposon types are unexpectedly active in mouse eggs, and others are activated in early embryos. Surprisingly, by acting as alternative promoters, retrotransposon-derived controlling elements drive the coordinated expression of multiple mouse genes. "To our knowledge, this is the first report that such elements can initiate synchronous, developmentally regulated expression of multiple genes," says Dr. Knowles. "Also, random insertions of these elements can introduce variation in genes, potentially altering their function."

The researchers think that expression of retrotransposons during very early stages may contribute to the reprogramming of the mammalian embryonic genome, a prerequisite for normal development.

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