The agouti mouse model: an epigenetic biosensor for nutritional and environmental alterations on the fetal epigenome.

The ability of environmental factors to shape health and disease involves epigenetic mechanisms that mediate gene-environment interactions. Epigenetic gene regulation comprises the heritable changes in gene expression that occur in the absence of changes to the DNA sequence itself. Epigenetic mechanisms include chromatin folding and attachment to the nuclear matrix, packaging of DNA around nucleosomes, covalent modifications of histone tails (e.g. acetylation, methylation, phosphorylation), and DNA methylation. The influence of regulatory small RNAs and micro RNAs on gene transcription is also increasingly recognized as a key mechanism of epigenetic gene regulation. Conventional gene-environment interaction studies strive to understand how individuals with different genotypes respond to various environmental factors and how these responses change over time. Such research efforts have highlighted the important contribution of both genetic and environmental variability in human diseases. However, it has been argued that a full understanding of gene-environment interactions requires that epigenetic mechanisms be taken into account. Therefore, the interdisciplinary field of environmental epigenomics emphasizes the potential for nutritional and environmental factors to influence fetal, adult, and transgenerational epigenetic gene regulation, resulting in numerous phenotypic consequences.1 The viable yellow agouti (Avy) mousemodel, in which coat color variation is correlated to epigenetic marks established early in development, has been used to investigate the impacts of nutritional and environmental influences on the fetal epigenome (Fig. 1A and B). The wild-type murineAgouti gene encodes a paracrine signaling molecule that produces either black eumelanin (a) or yellow phaeomelanin (A). Both A and a transcriptions are initiated from a developmentally regulated hair-cycle-specific promoter in exon 2 (Fig. 1A). Transient A expression in hair follicles during a specific stage of hair growth results in a sub-apical yellow band on each black hair shaft, causing the brown agouti coat color of wild-type mice.2 The Avy metastable epiallele resulted from the insertion of an intracisternal A particle (IAP) murine retrotransposon upstream of the transcription start site of the Agouti gene (Fig. 1A).2,3 A cryptic promoter in the proximal end of the Avy IAP promotes constitutive ectopic Agouti transcription not only in hair follicles, but throughout all cells, leading to yellow fur, as well as adult-onset obesity, diabetes, and tumorigenesis.4,5 Interestingly, CpG methylation in the Avy IAP correlates inversely with ectopic Agouti expression. The degree of methylation within the 5′ IAP long terminal repeat (LTR) varies dramatically among individual isogenic Avy/a mice, causing a wide variation in coat color ranging from yellow (unmethylated) to pseudoagouti (methylated) (Fig. 1B). The Avy allele is the most extensively studied murine metastable epiallele. Metastable epialleles are identical alleles that are variably expressed due to epigenetic modifications that are established very early in development.6 They are most often associated with retroelements and transgenesis. Three of the identified murine metastable epialleles (Avy, AxinFu, CabpIAP) are associated with contraoriented IAP insertions.2,7,8 The extent of DNAmethylation at each allele is stochastic and dependent upon