Psychological stress is correlated with and may even cause DNA damage, which contributes to the etiology of various diseases. Recent studies point to the role of micro-RNA (miRNA), small molecules that regulate gene expression, in health and disease. This study investigated the relationship between transient stress and two cancer-related miRNAs, and determined whether health-behaviour moderated these relationships. Using a pre-post design, 37 German students completed measures on health-behaviour and perceived stress, the latter after a study break (low stress) and after an exam (high stress). On both occasions, students underwent blood tests to determine the expression of let-7b and miR-21, two miRNAs recently found to be related to cancer. The students reported significantly higher stress after the exam than in the study break period. The levels of let-7b and miR-21 expression significantly declined from low- to high-stress periods. Importantly, baseline health-behaviour interacted with time in relation to miR-21, such that the expression of this marker decreased only in students with inadequate health-behaviour, while it did not change in students with adequate health-behaviour. This is the first study showing that brief academic stress can alter the expression of two cancer-related miRNA molecules, and that health-behaviour may moderate these effects for miR-21.

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