It has long been suspected that there is a tumour suppressor gene on chromosome 8p, and our array CGH data 1 suggest that it may be close to the WRN and NEUREGULIN1 (NRG1) genes. NRG1 encodes growth factors that function as ligands for the tyrosine kinases ErbB3 and ErbB4, and can both stimulate cell proliferation, differentiation and apoptosis. We previously showed that many breast carcinoma (that is, 39% of cancer cell lines and 6% of breast tumours) have chromosome breakpoints in NRG1, suggesting that the gene plays an important role in tumourigenesis 23, and our initial hypothesis was that the translocations activate expression.

Our current work shows that NRG1 expression is silenced in many breast cancer cell lines (17 out of 23 lines), as compared with normal breast cell lines. Western blotting experiments also indicate that NRG1 is downregulated at the protein level. To investigate whether NRG1 maybe repressed by epigenetic mechanisms, we examined DNA methylation at a CpG island present in the promoter and the first exon of the gene using bisulphite sequencing. This region is heavily methylated in 76.5% (13/17) of breast cancer cell lines that have no NRG1 expression. In contrast, the region is relatively unmethylated in normal breast lines, and in cancer cell lines expressing NRG1. Treatment of cancer cell lines with 5-aza-2-deoxycytidine, which abolished DNA methylation, activated the expression of NRG1 by 7–100 times.

These results suggest that DNA methylation is a key mechanism that silences NRG1 expression in breast cancer cells, and our current view is that NRG1 could be the long-sought tumour suppressor on 8p, with the translocations either inactivating the gene or producing aberrant transcripts.