Differential expression of alternatively spliced c-erbB-2 mRNA in primary tumors, lymph node metastases, and bone marrow micrometastases from breast cancer patients.

We established an RT-PCR method to measure the amount of a 2.3-kb alternatively spliced mRNA of the human c-erbB-2/HER-2 proto-oncogene relative to the 4.6-kb full-length transcript. For the first time, we demonstrated production of c-erbB-2 extracellular domains via alternative splicing in breast cancer tissues, lymph node and bone marrow micrometastases. In 15 c-erbB-2-positive primary breast tumor samples, we found two significantly distinct subgroups: 6/15 had a low level of the extracellular fragment, and 9/15 showed an average 4.4-fold higher amount of the alternatively spliced mRNA. Additionally, six lymph nodes and six bone marrow aspirates from metastatic breast cancer patients were analyzed: 5/6 lymph nodes and 6/6 bone marrow aspirates were found to produce elevated relative amounts of the truncated fragment. The results demonstrate that our method is suitable for sensitive detection of c-erbB-2-positive micrometastasis and strongly suggest that the alternatively spliced c-erbB-2 variant is involved in the development of micrometastasis in breast cancer.