Molecular characterization of the hypervariable yellow(A(hvy)) allele at the murine agouti locus and investigation of factors influencing phenotypic variability

The murine agouti gene regulates the production of eumelanin and phaeomelanin by hair bulb melanocytes. We have identified a new agouti allele, hypervariable yellow (A\sp{hvy}), that is responsible for the largest variation in phenotype of all agouti alleles. Mice carrying the A\sp{hvy} allele can range in phenotype from obese with yellow coats, to normal weight with almost pure black coats. We show that the expression of agouti is correlated with the amount of yellow pigment in the coats of A\sp{hvy} mice as well as their weight. Sequencing of 5\sp\prime RACE and genomic PCR products revealed that A\sp{hvy} is the result of an intracisternal A-particle that inserted in an antisense orientation into the 5\sp\prime untranslated exon 1C of agouti. This retrovirus-like element is responsible for deregulating agouti expression in A\sp{hvy} mice; agouti expression is correlated with the methylation state of CpG residues in the IAP LTR as well as in host genomic DNA. We have identified strain-specific differences in the variable expressivity of the A\sp{hvy} phenotype, through crosses of A\sp{hvy} to various inbred strains. To identify genetic factors that can influence the phenotypic variability exhibited by A\sp{hvy} offspring, we performed quantitative trait loci analysis using an F\sb2 intercross and a cross of the BXH RI series. We detected QTLs associated with A\sp{hvy} on several chromosomes. Genomic regions on chromosomes 1, 4, 9, and 19 were identified in both crosses. In addition, we report the dependence of phenotypic variability of A\sp{hvy} mice on epigenetic effects originating from differences in the maternal environment. We observed that yellow A\sp{hvy} females tend to produce more yellow offspring than black A\sp{hvy} females of the identical genotype

Transgenerational inheritance of epigenetic states at the murine Axin(Fu) allele occurs after maternal and paternal transmission

Phenotypic variation that cannot be explained by genetic or environmental heterogeneity has intrigued geneticists for decades. The molecular basis of this phenomenon, however, is largely a mystery. Axin-fused (Axin(Fu)), first identified in 1937, is a classic example of a mammalian allele displaying extremely variable expression states. Here we demonstrate that the presence or absence of its characteristic phenotype, a kinked tail, correlates with differential DNA methylation at a retrotransposon within Axin(Fu) and identify mutant transcripts arising adjacent to the retrotransposon LTR that are likely to be causative of the phenotype. Furthermore, the epigenetic state at Axin(Fu) can be inherited transgenerationally after both maternal and paternal transmission. This is in contrast to epigenetic inheritance at the murine agouti-viable yellow (A(vy)) allele, which occurs through the female only. Unlike the egg, the sperm contributes very little (if any) cytoplasm to the zygote, and therefore paternal inheritance at Axin(Fu) argues against the possibility that the effects are due to cytoplasmic or metabolic influences. Consistent with the idea of transgenerational inheritance of epigenetic marks, we find that the methylation state of Axin(Fu) in mature sperm reflects the methylation state of the allele in the somatic tissue of the animal, suggesting that it does not undergo epigenetic reprogramming during gametogenesis. Finally, we show that epigenetic inheritance is influenced by strain background. These findings enable us to propose a model for transgenerational epigenetic inheritance in mammals