Expression of fibronectin isoforms in human breast tissue: production of extra domain A+/extra domain B+ by cancer cells and extra domain A+ by stromal cells.

The expression of fibronectin (FN) isoforms including extra domain A (EDA) and extra domain B (EDB) segments, was investigated in 36 invasive ductal carcinomas and 13 benign tumors of human breast tissues by in situ hybridization using probes specific to alternative splicing sites. Signals for the constant region of FN mRNA in cancer cells were found in 53% of the invasive ductal carcinomas. The EDA+ and EDB+ mRNA signals were found in 47% and 33%, respectively. Stromal cells expressing FN, EDA+ and EDB+ mRNA signals were present in 100%, 69% and 14% of cases, respectively. Expression of FN mRNAs by cancer cells was most frequent in intraductal lesions or large cancer nests, and that by stromal cells was associated with desmoplastic areas. In representative cases, proportions of FN mRNA-positive cancer cells expressing EDA and EDB segments were 45% and 39%, respectively, signals for both being frequently found in the same cells. EDA+ and EDB+ mRNA were labeled in 25% and 6% of the FN mRNA-positive stromal cells, a large proportion thus being EDA-/EDB- FN. In conclusion, the splicing pattern of FN pre-mRNA is dependent on the cell type and histology of breast cancer tissues. The observed lack of expression in fibroadenomas and other benign conditions suggests a link with tumor progression.