Transposon Silencing and Imprint Establishment in Mammalian Germ Cells (original) (raw)

1. T.H. BESTOR and
2. D. BOURC'HIS
3. Department of Genetics and Development, College of Physicians and Surgeons of Columbia University, New York, New York 10032

Excerpt

Activators and repressors cannot explain the regulationof all genes. Some mammalian genes are not expressedeven in the presence of all the required transcription factors, and the behavior of these genes also fails to conformto Mendelian laws of transmission genetics. Such genesare said to be under epigenetic control or to be subject togene silencing. Mammalian gene silencing is especiallyconspicuous at genes subject to X-chromosome inactivation, at imprinted genes, and at the large number of transposable elements that contain promoter sequences (Bestor 2003). Genes on the inactive X chromosome areclearly in the presence of all factors required for their expression, as shown by the activity of the homologous alleles on the active X, but they remain silent for very longperiods of time. The same is true of imprinted genes. Thestate of activity of a gene subject to X-inactivation or genomic imprinting can be predicted only if the history ofthe gene is known; in the case of X-inactivation, the critical event occurs in somatic cells soon after implantation,while the state of activity of imprinted genes is determined during spermatogenesis and oogenesis in the previous generation. The promoters of retroposons are silenced in premeiotic prospermatogonia in males andmeiotic dictyate oocytes in females. Once established, thesilent state can persist for the life of the organism, whichin humans can exceed 100 years, and can only be reset inthe next reproductive cycle...