Jaenisch , MD
Member, Whitehead Institute
Professor of Biology, MIT
Our long-range goals are to understand epigenetic regulation of gene expression in mammalian development and genetic disease. Faulty epigenetic reprogramming is the main problem in the development of mammals cloned by nuclear transfer. Understanding the molecular basis of epigenetic reprogramming is a major focus of our work. DNA methylation is a crucial component of the epigenetic control of gene activity through the regulation of chromatin state. A number of factors involved in this process, such as methyl-binding proteins and histone-deacetylases have been identified. We are deriving mice carrying targeted mutations in the different components of the epigenetic machinery in order to understand how stable expression states of the genome are established and maintained. In particular, we are focusing on the role of DNA methylation in cancer, genomic imprinting, and the function of the postnatal brain. For this, we are employing gene-targeting methods to generate embryonic stem cells and mice with lineage-specific and inducible gene deletions.
Nuclear cloning and the reprogramming of the genome Animal cloning is an inefficient process since the vast majority of nuclear transfer embryos die during development. Normal development depends upon a precise sequence of changes in the configuration of the chromatin and in the methylation state of genomic DNA. Successful cloning likely requires the reprogramming of the donor nucleus, converting it from an epigenetic state that is characteristic of a differentiated cell to one that is normally formed in the zygote or early embryo. To understand the molecular basis of poor clone survival, a major focus of the laboratory is to compare the epigenetic state of the genome and the gene expression patterns of normal and cloned animals. For this, we are using standard molecular techniques as well as gene chip-based expression profiling. A long-term aim is to define the factors present in the egg cytoplasm responsible for the reprogramming of the genome.
DNA methylation The importance of DNA methylation in vertebrate gene control has remained a controversial issue because of the indirect and correlative nature of many studies in the field. The targeted inactivation of the Dnmt1 and Dnmt3 genes results in embryonic lethality, establishing the essential role of DNA methylation in mammals. We are creating systems that allow the tissue-specific and inducible inactivation of the methyl-transferases and methyl-binding proteins to assess the role of methylation in normal development and cancer.
Genomic imprinting Imprinted genes are expressed either from the maternal or the paternal allele but not from both. We have shown that monoallelic expression of imprinted genes depends on DNA methylation. It has been suggested that faulty imprinting in cloned animals derived by nuclear transfer contributes to the abnormal phenotype ("Offspring Syndrome"). Indeed, analysis of cloned pups shows widespread dysregulation of imprinted genes. To directly test whether imprinting has an intrinsic role in development, we are generating "non-imprinted" mice from "non-imprinted" embryonic stem cells by nuclear transplantation.
Cancer Genomic hypomethylation is a widely observed and early step in human tumorigenesis. Using different mutant alleles of the Dnmt1 gene, we have shown that hypomethylation results in a substantial increase in genomic mutation rates caused by enhanced mitotic recombination. These results are significant as they may explain the selective advantage of hypomethylation in early stages of transformation: hypomethylation leading to genomic instability may provide the incipient tumor cell with a mechanism to efficiently delete tumor suppressor genes by LOH. Indeed, the great majority of mice carrying a hypomorphic Dnmt1 allele develop aggressive thymic tumors. Our results indicate that therapeutic inhibition of Dnmt1 may prevent some forms of cancer such as intestinal tumors, but may promote cancer in other tissues. Therefore, to assess the consequences of Dnmt inhibitors as potential cancer drugs, it will be crucial to define the role of hypomethylation in diverse tumor models, as Dnmt inhibitors may have opposite effects on cancer incidence when arising in different tissues.
Epigenetic control of brain function Both Dnmt1 and Dnmt3 are highly expressed in post-mitotic neurons, raising the possibility that methylation may have an important role in the physiology and disease of the postnatal brain. This possibility has been dramatically supported by the recent discovery that the mutation of the methyl binding protein, MECP2, leads to Rett syndrome, one of the most frequent causes of severe mental retardation in girls. We have generated mice carrying a conditional mutation of the Mecp2 gene and found that the phenotype of mutant animals resembles that of patients. We are using this mouse strain to investigate the pathology of neuronal dysfunction and to devise potential strategies for therapeutic intervention. Finally, we are deriving mice carrying a Dnmt1 or Dnmt3 deletion in specific regions of the brain to directly assess the potential role of methylation in the brain physiology of processes such as memory or aging.
Jackson-Grusby, L., Beard, C., Possemato, R., Fambrough, D., Csankovszki, G., Dausman, J., Lee, P., Wilson, C., Lander, E. and Jaenisch, R. (2001). Loss of genomic methylation causes p53-dependent apoptosis and epigenetic deregulation. Nature Genet. 27, 31-39.
Eggan, K., Akutsu, H., Hochedlinger, K., Rideout, W., Yanagimachi, R., and Jaenisch, R. (2000). X chromosome inactivation in cloned embryos. Science 290, 1578-1581.
Rideout, W.M., Eggan, K., and Jaenisch, R. (2001). Nuclear cloning and epigenetic reprogramming of the genome. Science 293, 1093-1098.
Hochedlinger, K. and Jaenisch, R. Generation of monoclonal mice by nuclear transfer from mature B and T donor cells. (2002). Nature 415, 2124-2135.
Rideout, W.M., Hochedlinger, K., Kyba, M., Daley, G.Q., and Jaenisch, R. (2002). Correction of a genetic defect by nuclear transplantation and combined cell and gene therapy. Cell 109, 17-27.
Updated February 20, 2003
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