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Mutations of HER2 gene in HER2-positive metastatic breast cancer.

13118 Background: HER2 status of Breast Cancer has been assessed by IHC and FISH and used for therapeutic decision with a high degree of success. However, there were numbers of HER2-positive MBC who finally failed the Trastuzumab treatment after initial good response. Mechanisms of intrinsic and acquired Trastuzumab resistance are not yet known. Our Objective is to identify factor or factors responsible for Trastuzumab resistance.


METHODS
DNA extraction and Sequencing of HER2 gene were performed on primary tumors of HER2-positive 14 MBC patients undergoing Trastuzumab therapy. Re-biopsy were done on new metstatic sites of those cases discovered to have Trastuzumab resistance.


RESULTS
Of 14 MBC cases whose tumors showing positive IHC and FISH, there were no mutation found in their HER2 gene, exons 18,19, 20 and 21. However, 3 of 14 cases of MBC undergoing continuous Trastuzumab therapy with excellent response for more than one year, developed the resistance. All three cases had new metstatic sites biopsied, and showed D880N and E837Y mutations in the exon 21 of their HER2 genes. All three cases showed no response to trastuzumab therapy.


CONCLUSIONS
1) HER2-positive MBC tumors did not have any HER2 gene mutations in them. 2) Mutations arised in their HER2 gene, exon 21 may be responsible for the intrinsic and acquired Trastuzumab resistance. Additional work in this area is needed to further substantiate our findings. No significant financial relationships to disclose.