What isn’t transferred from breast milk?

We all know that breast milk contains vital nutrients and immunity for the newborn, but can other proteins and gene regulators, like microRNAs, also be passed along to newborns? If so, are these absorbed by the infant? Do these potentially alter gene expression in the child?

Dr. Alexandra Title and colleagues published an interesting paper last week in the Journal of Biological Chemistry that looks at these questions. Specifically, the scientists asked if microRNAs from the mother are absorbed by the newborn and do they change the expression of specific genes.

MicroRNAs (miRNAs) are small, noncoding RNA molecules that play an important part in gene expression. They bind to the mRNA that codes for a specific protein and cause it to degrade, thereby reducing the amount of that protein found in the cell. It turns out that breast milk from a mother has high amounts of miRNAs. This finding has led to debate over whether these maternal miRNAs are absorbed by the infant and if so, if they regulate gene expression in the infant.

Why is this important? If miRNAs are passed from mother to infant and they can down regulate gene expression, it will show another way that maternal health can impact child health. Importantly, this would be a case of environmental influences changing gene expression in an individual. It could also suggest that other sources (i.e. food, cows milk) may impact gene expression, a finding that could affect individuals at any age, not just infants receiving breast milk.

Dr. Title and her team used a mouse model to answer this question. By removing the gene for 2 miRNAs (miR-375 or miR200) either in the mother or the pup, they were able to follow the movement of a miRNA from the mother to pup. Their data clearly show that when expressed, miRNAs were found in high levels in the milk, but that these maternal miRNAs were not absorbed by the pup. Further, they show that these maternal miRNAs are digested in the stomach of the pup before they get to the first site of absorption, the small intestine. So, the miRNAs are not passed from mother to child and do not alter gene expression in the infant.

Why are there high levels of miRNAs in breast milk? This is still unclear, but it may be that since miRNAs are short stretches of RNA that are destroyed in the stomach of the pup, they serve as source of RNA building blocks for new RNA molecules in the infant. What does this all mean? This means that miRNAs from external sources (mom, plants, animals, etc…) do not alter gene expression in the developing child. So, if there is some abnormal level of miRNA or altered function of the miRNA in the mother, this will not alter gene expression levels in the child.

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