International Mammalian Genome Society

logo18th International Mouse Genome Conference

17-22 October 2004, Seattle, USA



8.30am – 9.00am


Champ M1, Vickaryous N1, Hemley S1, Preis JI 1, Arkell R2, Whitelaw E1

1School of Molecular Microbial Biosciences, University of Sydney, NSW, Australia, 2Mammalian Genetics Unit, MRC Harwell, United Kingdom

Phenotype-driven mutagenesis is proving to be a useful way of discovering the function of genes in higher organisms. In the mouse, ENU causes point mutations in progenitor spermatocytes. We are using ENU mutagenesis to identify genes that modify epigenetic state. Transcriptional silencing associated with epigenetic modifications is known to be responsible for parental imprinting and X-inactivation in mammals and there is increasing support for the idea that it plays a critical role in differentiation. Epigenetic silencing can result in mosaic expression of genes within a tissue, termed variegation, and between isogenic individuals, termed variable expressivity. While some of the proteins involved in these processes are known, many remain to be identified.

We have treated male mice carrying an erythroid-specific GFP transgene that displays variegation. We have screened 610 F1 offspring for dominant mutations and have identified six; four are suppressors and two are enhancers of variegation. In all of the cases where we have determined the dominance of the mutant alleles, we have found the they are semi-dominant and homozygous lethal, indicating the obligate requirement for the genes that have been hit. The semi-dominance is consistent with the idea that transcription is determined by a dynamic equilibrium between complexes that silence and those that activate. 120 male F1 mice (without any dominant mutations) have been used in a recessive screen. So far, we have identified five recessive mutations. We have mapped five of the mutations to between 3 and 20 cM intervals. A number of the dominant mutations affect the variable expressivity seen at the endogenous agouti viable yellow allele. This locus is known to display variable expressivity in isogenic mice. Interestingly, in most cases, we see parent-of-origin and sex-specific effects. We are currently investigating whether any of the mutated genes play a role in X inactivation or parental imprinting.

This project has the potential to find novel genes involved in epigenetic phenomena, and to produce hypomorphs and hypermorphs of known genes that alter epigenetic state.

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