The results of this naturalistic experiment show that in a population with a genetic tendency for obesity, maternal obesity has an obesogenic effect on her offspring, causing transgenerational amplification of body weight. Previous animal models have employed early dietary manipulations18
or high-energy diets19
to test the hypothesis that maternal obesity promotes obesity in her offspring. Our current data, however, provide the first demonstration that spontaneous interindividual variation in maternal adiposity induces body weight variation in the subsequent generation. As such, these results provide definitive evidence of non-genetic transgenerational transmission of obesity risk at the population level.
Providing a hypermethylating dietary supplement prevented the effect of maternal obesity on offspring body weight, suggesting a role for epigenetic mechanisms in this phenomenon. Dietary methyl supplementation of female mice before and during pregnancy induces persistent hypermethylation at the Avy
As transgenerational epigenetic inheritance occurs at Avy
it may seem intuitive that methyl supplementation over successive generations caused cumulative Avy
hypermethylation, leading to progressive epigenetic silencing of the obesogenic Avy
allele. Detailed analysis of Avy
coat color variation in this study, however, showed no evidence of transgenerational epigenetic inheritance of diet-induced hypermethylation at Avy
Moreover, adult body weight of Avy/a
mice was independent of coat color (), demonstrating that the body weight effects reported here are not due to epigenetic variation at Avy
. We instead interpret our results as evidence that maternal obesity affects developmental establishment of epigenetic mechanisms at other genomic loci affecting body weight regulation (for example in offspring hypothalamus) and that methyl supplementation interacts with these processes.
Given that ectopic agouti
expression causes hyperphagic obesity, and coat color variation among Avy/a
mice correlates with ectopic agouti
it may seem surprising that we found no association between Avy/a
coat color and adult body weight (). However, Wolff and colleagues13
reported years ago that among Avy/a
mice, only pseudoagouti animals (hypermethylated at Avy
) are protected from obesity. This was recently verified by Dolinoy et al
who showed that only pseudoagouti Avy/a
mice remain lean as adults, and body weight does not differ among the other coat color classes. Therefore, as the current study included only one pseudoagouti mouse,15
the lack of an association between adult body weight and coat color is in fact consistent with previous studies.
As the Avy
mouse has been studied for decades,21
one may wonder why transgenerational increases in body weight among Avy/a
mice have not been reported previously. It should be noted, however, that Avy
colonies are customarily propagated by mating a/a
females with Avy/a
males. This approach maintains Avy
in forced heterozygosity with the a
allele and obviates the need for genotyping; offspring genotype (a/a
) can be inferred from coat color. Passing Avy
through the male rather than the female avoids both accumulation of epigenetic inheritance at Avy12
and the potential for maternal obesity to interfere with reproductive efficiency. Hence, although it has not been customary to pass Avy
through obese Avy/a
females, our data indicate that doing so provides a very useful model in which to study the effects of maternal obesity on her offspring.
Future studies will be required to determine exactly why Avy/a mice progressively become more obese as the Avy allele is passed through successive generations of obese Avy/a females. Measurements of food intake and energy expenditure, both of the pregnant dams and their growing offspring, would establish whether the transgenerational increases in adiposity are due to increases in energy intake, decreased energy expenditure or both. Distinguishing among these possibilities will be a critical first step toward determining the organ system(s) in which fetal and/or early postnatal development is affected by maternal obesity and methyl supplementation. Another limitation of this study is that only Avy/a mice were studied. Although our data indicate that epigenetic variation at Avy does not mediate the reported dietary and transgenerational effects on body weight, the complex epigenetic characteristics of the obesogenic Avy allele underscore the need to determine if our multigenerational approach yields similar cumulative effects on body weight in other rodent models of obesity. Also, due to practical considerations, we did not measure adult body weight of a/a mice. It will be interesting to know if transgenerational increases in body weight and effects of methyl supplementation also occur in mice without a genetic tendency for obesity.
Twenty years ago, Holliday22
proposed that just as genetic mutations can cause cancer, so too might epimutations such as aberrant DNA methylation. This prediction has proved correct; the role of epigenetic dysregulation in cancer is now firmly established.23
Analogously, just as genetic variation can contribute to human obesity,24
so too can interindividual epigenetic variation. Data from animal models and human developmental syndromes demonstrate that epigenetic dysregulation causes obesity.25
Remarkably, however, we know virtually nothing about the epigenetic regulation of genes that play a central role in food intake regulation. Given the escalating worldwide prevalence of obesity among women of childbearing age, it is of crucial importance to understand the biologic mechanisms by which maternal obesity affects development of mammalian body weight regulation.