Precise regulation of DNA damage response is crucial for cellular survival after DNA damage, and its abrogation often results in
genomic instability in
cancer. Phosphorylated
histone H2AX (γH2AX) forms nuclear foci at sites of DNA damage and facilitates DNA damage response and repair.
MicroRNAs (
miRNA) are short, nonprotein-encoding
RNA molecules, which posttranscriptionally regulate gene expression by repressing translation of and/or degrading
mRNA. How
miRNAs modulate DNA damage response is largely unknown. In this study, we developed a cell-based screening assay using ionizing radiation (IR)-induced γH2AX foci formation in a human
osteosarcoma cell line, U2OS, as the readout. By screening a library of human
miRNA mimics, we identified several
miRNAs that inhibited γH2AX foci formation. Among them, miR-138 directly targeted the
histone H2AX 3'-untranslated region, reduced
histone H2AX expression, and induced
chromosomal instability after DNA damage. Overexpression of miR-138 inhibited homologous recombination and enhanced cellular sensitivity to multiple
DNA-damaging agents (
cisplatin,
camptothecin, and IR). Reintroduction of
histone H2AX in miR-138 overexpressing cells attenuated miR-138-mediated sensitization to
cisplatin and
camptothecin. Our study suggests that miR-138 is an important regulator of
genomic stability and a potential therapeutic agent to improve the efficacy of
radiotherapy and
chemotherapy with
DNA-damaging agents.