Tandem repeats of the 5′ flanking region of human MUC5AC have a role as a novel enhancer in MUC5AC gene expression

MUC5AC is a well-known gastric differentiation marker, which has been frequently used for the classification of stomach cancer. However, the molecular mechanism of regulation of MUC5AC expression remains to be elucidated. In previous studies, we have shown that Gli regulated MUC5AC transcription through the Gli-binding motif in the 5′ region of MUC5AC. Gli played important roles, but independently was not sufficient for MUC5AC expression. In this study, we analyzed a 4010 bp fragment of the 5′-flanking promoter region of the human MUC5AC gene by luciferase assay, and found a novel distal enhancer region located between −1434 bp to −3000 bp upstream from the first ATG initiation codon. This region is composed of repetitive DNA sequences 5′-TCACTCAC-3′. The strength of enhancer activities depended on the length of the repetitive region. The tandem repeats are conserved among primates, but not in other mammals. The tandem repeat regions enhanced promoter activities not only of MUC5AC but also of other genes. The enhancer effect of the tandem repeat regions was maintained even when inverted. ChIP analysis revealed that H3K9me3 binds to the tandem repeat regions. Together, our results suggest that the tandem repeat region in the MUC5AC promoter has the potential to act as a strong enhancer, and H3K9me3 may contribute to histone modifications of this region.