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* Departments of Pediatrics and Molecular and Human Genetics, Baylor College of Medicine, USDA Children’s Nutrition Research Center, Houston, Texas, USA;
Department of Biology and Biochemistry, University of Houston, Houston, Texas, USA; and
Department of Ecology, Evolution and Behavior, University of Minnesota, St. Paul, Minnesota, USA
1Correspondence: Departments of Pediatrics and Molecular and Human Genetics, Baylor College of Medicine, USDA Children’s Nutrition Research Center, Houston, TX 77030, USA. E-mail: waterland{at}bcm.edu
We appreciate the explanation offered by Cropley et al. for what they perceive is a discrepancy between their results showing an effect of methyl supplementation on the germline epigenetic state of Avy (1)
and ours showing that diet-induced hypermethylation at Avy is not inherited transgenerationally (2)
. Demonstrating an effect on the germline, however, is not the same as showing transgenerational epigenetic inheritance (3)
. Cropley et al. supplemented F0 dams during mid-gestation and assessed effects on coat color in F2 Avy/a offspring. During the period of supplementation the F1 generation was undergoing gametogenesis, forming the gametes that will become the F2 generation. The effects Cropley et al. detected in the F2 generation did not need to be inherited because the F2 generation was, in fact, "present" during the diet exposure! Our study, conversely, was specifically designed to test for transgenerational inheritance of diet-induced epigenetic modifications at Avy (2)
. By assessing cumulative effects of methyl supplementation on three generations of Avy/a mice, we showed that the effect of diet on Avy epigenotype is not conveyed transgenerationally.
We disagree that we failed to detect inheritance of Avy hypermethylation because there was nothing to be inherited. Cropley et al. offer no support for their assertion that "yellow (Avy/a) mice have no silent Avy alleles." To the contrary, Blewitt et al. showed last year (4)
that in oocytes from clear yellow Avy/a females, approximately 20% of Avy alleles are extensively methylated; pseudoagouti females produce oocytes with an even larger proportion of hypermethylated alleles (
50%). Hence, in the slightly mottled, mottled, and heavily mottled females that comprised the bulk of our breeding population (2)
, we estimate that between 20–50% of oocytes contained extensively methylated Avy alleles.
Cropley et al. also imply that our study failed to detect transgenerational inheritance of induced Avy hypermethylation merely due to insufficient statistical power. Our lineage-based analysis, however, showed a recurrent transgenerational decline in coat color score in the supplemented group (2)
. There is no reason to suppose that this statistically significant effect, indicating transgenerational loss of epigenetic information, will be reversed simply by studying a larger number of animals. Further, Cropley et al. (1)
were unable to detect, as we did (2)
, an effect of methyl supplementation on offspring coat color when the Avy allele was transmitted from the mother, suggesting our study design was more—not less—powerful than theirs.
Finally, it may be time to revisit the notion that "epigenetic inheritance at Avy results from germline inheritance of the silent state." Experiments focused on transgenerational recapitulation of coat color and Avy DNA methylation (5)
led to the proposal that epigenetic inheritance at Avy occurs by incomplete erasure of DNA methylation in the early embryo (6)
. The recent study of Blewitt et al. (4)
, however, indicates that Avy DNA methylation is not the inherited mark. Hence, the molecular basis for transgenerational epigenetic inheritance at Avy remains unknown. It is equally likely that epigenetic marks correlated with the transcriptionally active state mediate epigenetic inheritance at Avy.
FOOTNOTES
The opinions expressed in editorials, essays, letters to the editor, and other articles comprising the Up Front section are those of the authors and do not necessarily reflect the opinions of FASEB or its constituent societies. The FASEB Journal welcomes all points of view and many voices. We look forward to hearing these in the form of op-ed pieces and/or letters from its readers addressed to journals@faseb.org.
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