The Use of Mouse Models to Study Epigenetics

Downloaded from http://cshperspectives.cshlp.org/ on September 26, 2021 - Published by Cold Spring Harbor Laboratory Press The Use of Mouse Models to Study Epigenetics Marnie Blewitt1 and Emma Whitelaw2 1Walter and Eliza Hall Institute, Melbourne, 3052 Victoria, Australia; 2Queensland Institute of Medical Research, Brisbane, 4006 Queensland, Australia Correspondence: [email protected] SUMMARY Much of what we know about the role of epigenetics in the determination of phenotype has come from studies of inbred mice. Some unusual expression patterns arising from endogenous and transgenic murine alleles, such as the Agouti coat color alleles, have allowed the study of variegation, variable expressivity, transgenerational epigenetic inheritance, parent-of-origin effects, and position effects. These phenomena have taught us much about gene silencing and the probabilistic nature of epigenetic processes. Based on some of these alleles, large-scale mutagenesis screens have broadened our knowledge of epigenetic control by identifying and characterizing novel genes involved in these processes. Outline 1 Using mouse models to identify modifiers of 3 Summary and future directions epigenetic reprogramming References 2 Epigenetic phenomena in inbred mouse colonies Editors: C. David Allis, Marie-Laure Caparros, Thomas Jenuwein, and Danny Reinberg Additional Perspectives on Epigenetics available at www.cshperspectives.org Copyright # 2013 Cold Spring Harbor Laboratory Press; all rights reserved; doi: 10.1101/cshperspect.a017939 Cite this article as Cold Spring Harb Perspect Biol 2013;5:a017939 1 Downloaded from http://cshperspectives.cshlp.org/ on September 26, 2021 - Published by Cold Spring Harbor Laboratory Press M. Blewitt and E. Whitelaw OVERVIEW The role of epigenetics in determining phenotype has been regulators have been performed in lower organisms, using progressed through studies of inbred mice. Under laboratory variegating phenotypes such as eye color in flies and pigmen- conditions the genome and the environment of mice are tight- tation in maize. Mammalian screens have been important ly controlled such that variance in phenotype or patterns of because certain epigenetic processes are specific to higher gene expression without a change in the underlying DNA organisms: for example, the inactivation of the second X chro- sequence is, by definition, epigenetic. Interestingly, trans- mosome in females (see Brockdorff and Turner 2014) and genes in mice appear to be particularly sensitive to epigenetic genomic imprinting (see Barlow and Bartolomei 2014). Fur- silencing, and as such provide a valuable model to study the thermore, byperformingscreens in mice, oneimmediately has underlying molecular mechanisms of epigenetic control. In the mutant mouse strains to study the effects of disruption to fact, we have come to realize that there are some endogenous epigenetic processes on phenotypes relevant to humans. Two alleles, resulting from transposon insertions, which are simi- mousemutagenesisscreenshavebeenspecificallydesignedto larly susceptible to epigenetic silencing. These alleles, termed identify genes involved in epigenetic control: the Momme and metastable epialleles, display unusual expression and inheri- the X inactivation-choice screens. The Momme mutagenesis tance patterns: for example, variegated expression in a single screen has used a GFP transgenic line that displays variegated cell type, variable expressivity between individuals, and trans- expression equivalent to position-effect variegation (PEV) in generational epigenetic inheritance. The study of these phe- Drosophila. This screen has thus far revealed .30 modifiers nomena has revealed fundamental features of epigenetic of epigenetic regulation, some known but others entirely nov- control. In some cases, these recapitulate those found in other el. Importantly, the novel players appear to be involved in complex organisms, such as position-effect variegation (PEV) mammalian specific processes, and these newly identified in Drosophila (see Elgin and Reuter 2013) and paramutation molecules expand our understanding of epigenetic control in in plants (Pikaard and Mittelsten 2014), but in other cases, the the mammalian system. phenomena are unique to mammals. Interestingly, mutation in one of the novel genes identified In addition to demonstrating many of the general features in the Momme screen has now been reported to be the under- of epigenetic control, metastable epialleles and other reporter lying cause of a rare human disease. Studies in the mouse alleles have enabled random mutagenesis screens to be per- modelsgeneratedbytheMommescreenhavebeeninstrumen- formed to find genes that are important in setting and resetting tal in helping us understand the molecular mechanisms of this epigenetic marks at these loci. Similar screens for epigenetic disease. We anticipate the same will be true in other cases. 2 Cite this article as Cold Spring Harb Perspect Biol 2013;5:a017939 Downloaded from http://cshperspectives.cshlp.org/ on September 26, 2021 - Published by Cold Spring Harbor Laboratory Press The Use of Mouse Models to Study Epigenetics 1 USING MOUSE MODELS TO IDENTIFY as Dnmt1 (Gaudet et al. 2004), HP1-b (Festenstein et al. MODIFIERS OF EPIGENETIC REPROGRAMMING 1999), and the polycomb group protein Mel18 (Blewitt et al. 2006). Together, these findings showed that metastable Random mutagenesis screens performed in mice, which epialleles are particularly sensitive to alterations in genetic have isolated mutations in epigenetic regulators, are de- scribed in this section. First, we detail screens that were makeup, ideal for a mutagenesis screen. specifically designed for the purpose of identifying epige- A mouse line carrying a variegating green fluorescent netic regulators: the Momme and the X inactivation-choice protein (GFP) transgene directed to express in red blood screen (Sections 1.1 and 1.3). Next, we briefly explain other cells was chosen (Fig. 1) (Preis et al. 2003). The advantages of using this transgenic line are many. First, the transgene is screens that were primarily aimed at finding genes involved in embryonic development, hematopoiesis or immune reproducibly expressed in 55% of red blood cells in ho- function (Section 1.4), but have produced novel mutations mozygous animals; the reproducibility of expression be- in epigenetic regulators all the same. tween isogenic littermates makes for a clean phenotype for screening with few false positives. Second, the transgene was produced and has been maintained on an inbred (FVB/ 1.1 A Screen for Modifiers of Murine Metastable N) genetic background, which simplifies later mapping Epialleles (Mommes) of the ethylnitrosourea (ENU)-induced mutations. Third, The mutagenesis screens performed in yeast, plants, and the expression of GFP in red blood cells means that trans- flies used variegating or epigenetically controlled pheno- gene expression was able to be efficiently and sensitively types as readouts of epigenetic state: for example, mating determined at a single cell level by flow cytometry. Fourth, type switching in yeast (see Allshire and Ekwall 2014), PEV by directing expression to red blood cells, analysis could in flies (Elgin and Reuter 2013), and paramutation, RNAi, be performed relatively simply (and without killing the and RNA-directed DNA methylation in plants (Pikaard animal) using a drop of blood taken from the tail of a mouse and Mittelsten Scheid 2014). These screens have iden- at weaning. Finally, alterations in the expression of the tified epigenetic modifiers critical for the phenotype being transgene itself do not inherently alter viability of the screened, but have also been a very useful tool in unraveling offspring. key molecular features of these unusual epigenetic processes. A similar screen has been performed in mice using a 1.1.1 The Dominant Screen variegating metastable epiallele. A metastable epiallele has transcriptional activity that is less stable than expected and Males homozygous for the transgene were treated with the is associated with changes in epigenetic state (Rakyan et al. chemical mutagen N-ethyl-N-nitrosourea (ENU), which 2002). The screen was thus performed with the hope of produces point mutations throughout the genome (Rin- finding novel epigenetic modifiers, creating useful new al- chik 1991). Mature germ cells are killed by the treatment, leles of known modifiers, and helping us to understand but point mutations are produced in the spermatogonial more about the remarkable features of metastable epialleles. stem cells, so when treated males recover fertility, they can Briefly, the activity state of metastable epialleles varies be bred and their G1 offspring screened for dominant- among genetically identical individuals brought up in the acting mutations. In essence, the mice were screened for same environment called variable expressivity, and is par- alterations in transgene silencing, assuming that any such ticularly sensitive to the epigenetic state of the locus. They alterations would be attributable to mutations in genes also display variegation (i.e., different expression states whose products are important in establishing epigenetic within one tissue type). These phenomena are discussed marks (see Fig. 1 for overview of the screen). in more detail in Section 2. More than 4000 G1 offspring have been screened and 40 Several studies suggested that using a variegating meta- strains have been isolated with transgene expression more stable epiallele in a mutagenesis screen would be a good

Load more