BACKGROUND: Dysregulated expression of microRNAs (miRNAs) has been previously 
observed in human cancer tissues and shown promise in defining tumor status. 
However, there is little information as to if or when expression changes of 
miRNAs occur in normal tissues after carcinogen exposure.
RESULTS: To explore the possible time-course changes of miRNA expression induced 
by a carcinogen, we treated mice with one dose of 120 mg/kg 
N-ethyl-N-nitrosourea (ENU), a model genotoxic carcinogen, and vehicle control. 
The miRNA expression profiles were assessed in the mouse livers in a time-course 
design. miRNAs were isolated from the livers at days 1, 3, 7, 15, 30 and 120 
after the treatment and their expression was determined using a miRNA PCR Array. 
Principal component analysis of the miRNA expression profiles showed that miRNA 
expression at post-treatment days (PTDs) 7 and 15 were different from those at 
the other time points and the control. The number of differentially expressed 
miRNAs (DEMs) changed over time (3, 5, 14, 32, 5 and 5 at PTDs 1, 3, 7, 15, 30 
and 120, respectively). The magnitude of the expression change varied with time 
with the highest changes at PTDs 7 or 15 for most of the DEMs. In silico 
functional analysis of the DEMs at PTDs 7 and 15 indicated that the major 
functions of these ENU-induced DEMs were associated with DNA damage, DNA repair, 
apoptosis and other processes related to carcinogenesis.
CONCLUSION: Our results showed that many miRNAs changed their expression to 
respond the exposure of the genotoxic carcinogen ENU and the number and 
magnitude of the changes were highest at PTDs 7 to 15. Thus, one to two weeks 
after the exposure is the best time for miRNA expression sampling.