The transglutaminase 2 gene (TGM2), a potential molecular marker for chemotherapeutic drug sensitivity, is epigenetically silenced in breast cancer

Tissue transglutaminase (TG2) is a ubiquitously expressed enzyme capable of catalyzing protein cross-links. TG2-dependent cross-links are important in extracellular matrix integrity and it has been proposed that this TG2 activity establishes a barrier to tumor spread. Furthermore, TG2 controls sensitivity to the chemotherapeutic drug doxorubicin. Both doxorubicin sensitivity and TG2 expression are highly variable in cultured human breast cancer cell lines and inspection of the human gene (termed TGM2) determined that a canonical CpG island exists within its 5' flank. These features, when combined with its potential tumor suppressor activity, make TG2 an attractive candidate for epigenetic silencing. Consistent with this, we observed that culturing breast tumor cells with the DNA demethylating agent 5-aza-2'-deoxycytidine (5-azadC) resulted in a robust increase in TG2 expression. Analysis of DNA harvested from cultured lines and primary breast tumor samples indicated that TGM2 often displays aberrant hypermethylation and that there is a statistically significant correlation between gene methylation and reduced expression. Finally, we observed that doxorubicin-resistant MCF-7/ADR cells do not show TGM2 silencing but that doxorubicin-sensitive MCF-7 cells do and that culturing MCF-7 cells on 5-azadC and subsequently restoring TG2 expression reduced sensitivity to doxorubicin. This work indicates that the TGM2 gene is a target for epigenetic silencing in breast cancer and suggests that this aberrant molecular event is a potential marker for chemotherapeutic drug sensitivity.