Mammary Gland May Have Epigenetic Memory

NIH | JUNE 3, 2015

Several studies have shown that humans and other mammals produce more milk during second pregnancies than first pregnancies. This suggests that the first pregnancy causes a long-lasting change in the mammary gland.

The structure of the mammary gland is virtually identical in previously pregnant (or “parous”) and never-pregnant (non-parous) animals. One possibility for how this change might occur is that pregnancy alters the organ’s epigenome, the collection of chemical markers that affect how genes are turned on and off, or expressed.

The researchers found that the mammary glands of parous mice developed more quickly and showed signs of milk production earlier than those of non-parous mice.

They looked at how pregnancy affects a type of epigenetic mark called DNA methylation. In mice that had been pregnant or received pregnancy-related hormones, many genes in mammary gland cells had reduced DNA methylation. The changes in DNA methylation in cells involved in milk production were different from those in cells that don’t produce milk. These changes were still present several months later.

The researchers found that the decreased methylation increased the expression of many genes, but only when the mice were exposed to pregnancy-related hormones. This suggests that these changes in DNA methylation only affect gene expression when an animal is pregnant.

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