Biohistory Jyournal, Autumn, 2003

Research: Index > Genome Imprinting - Time Implanted in a Generation

Time in Cells Genome Imprinting - Time Implanted in a Generation

Fumitoshi Ishino, Medical Research Institute, Epigenetics Section, Tokyo Medical and Dental University

For mammals, fertilization is indispensable in the flow of time from parent to child. With the creation of somatic cell clone animal typified by the sheep Dolly, however, we have discovered that the genome of cells that have differentiated once returns to the same state as during fertilization. This is called initialization. The initialized genome is again marked as differentiated, becomes a cell in the body, and creates an individual.Not all markers are eliminated during initialization, however. In fact, it is extremely interesting that some markers must not be eliminated.     Some genes have a parent marker that distinguishes whether they are from the father or the mother. This is called genome imprinting. The genes are imprinted from the time of fertilization and are maintained for the entire life of the organism. What happens with the imprinting for the germ cell that is a link to the next generation? Our results were both unexpected and understandable at the same time.

Somatic cells have one gene from father and one gene from mother each. This set is referred to as an allelic gene. There are ordinary allelic genes with a differentiation marker, and there are allelic genes with a genome imprinting marker. There are two types of allelic genes with genome imprinting markers: the PEG and the MEG. Only one of these is functioning.

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The repetition of genome imprinting occurring with generative cells

Let's start from the conclusion. Genome imprinting is maintained over the entire life of the somatic cell. The germ cells, which are the link to the next generation, are all eliminated during the fetal period, however, and will be imprinted again. The DNA of the generative cells in a mouse fetus is too small to be analyzed. Therefore, a clone is created and that body is investigated. This enables the fate of imprinted genes in germ cells to be traced. In males, the genome imprinting is preserved for 7.5 days during the fetal period. It gradually disappears over 10.5 to 13.5 days. After 15.5 days, the paternally imprinting occurs. Also, as the research of Prof. Kawano of the Tokyo University of Agriculture showed, the imprinting in females is identical to that of males until it disappears. The maternally reimprinting is completed during a period of several days to three weeks after birth, however.     Experiments were conducted with mice, but this likely occurs with all mammals, including human beings. The individual cell spends its entire life within the imprinting during the fetal period, and is imprinted anew when the link is made to the next generation. It is very interesting to consider all the implications of this.

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Fumitoshi Ishino Completed his Ph.D. in Biochemistry at the University of Tokyo's Graduate School of Science. He has served as an assistant at the Tokyo University's Applied Microbiological Research Center (currently, the Institute of Molecular and Cellular Biosciences) and as an assistant professor at the Tokyo Institute of Technology's Gene Research Center. He is currently a professor at the Medical Research Institute of the Tokyo Medical and Dental University.

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