Investigating novel mechanisms of metastasis in endocrine resistant breast cancer.

Breast cancer is the commonest solid tumour in European females. Treatment of breast cancer using anti-oestrogenic agents provides a template for the development of targeted therapies, both in breast cancer and other neoplastic diseases. Tamoxifen remains the treatment of choice for pre-menopausal breast cancer patients who express an oestrogen receptor, while a newer class of drug, aromatase inhibitors, are the first-line treatment for post-menopausal patients. In spite of these advances, up to 25% of breast cancer patients will eventually develop a recurrent tumour and metastatic disease.

The mechanism by which cancers can overcome this endocrine therapy is poorly understood. Cancer stem cell theory would suggest that there are sub-populations of cells within tumours that are highly tumourigenic and give rise to subsequent, resistant disease. However, this does not fully explain why some tumours may be initially steroid responsive, but can slowly adapt to overcome and even utilise these treatments to drive tumour growth.

One of the major players in these adaptive responses is steroid receptor co-activator 1 (SRC-1). Initially described as an ER co-activator, SRC-1 was subsequently shown to interact with a number of transcription factors, including those downstream of an activated MAP kinase pathway, to drive recurrence and metastasis. Furthermore, SRC-1 is central to the metastatic phenomenon.

Hypothesis

The aim of this work was to use next-generation sequencing techniques such as chromatin immunoprecipitation sequencing and ribonucleic acid seqencing to identify SRC-1 dependent and independent mechanisms by which breast cancer can adapt to and evade therapies such as tamoxifen.

Results

SRC-1 ChIP sequencing in a tamoxifen resistant cell line identified a disintegrin and metalloproteinase 22 (ADAM22) as a target of SRC-1. Using functional assays and patient data, a role for ADAM22 was established in metastatic, endocrine resistant breast cancer. Expression of ADAM22 may be driven by treatment with tamoxifen. Having established a functional role for ADAM22, a natural ligand (LGI1) which regulates ADAM22 was identified. This was used as a template to develop a peptide mimetic to inhibit the functional role of ADAM22. This was confirmed using in vitro models and has now progressed to in vivo testing.

It was noted that SRC-1 may also have a role in repressing gene expression. Analysis of functional assays and patient data showed that SRC-1 may suppress markers of luminal A disease, such as PAWR and CD24 in the resistant setting. This is the first time that SRC-1 has been identified as a co-repressor. Furthermore, it suggests that master-regulatory proteins such as SRC-1 may have a far wider role in both promoting metastasis and suppressing markers of good prognosis than previously realised.

Lastly, RNA sequencing on matched patient primary, nodal and distant metastatic tissue was performed to identify alterations in gene expression that may occur following endocrine treatment. It was noted that there was a significantly altered gene expression profile in the metastases as compared with the primary and nodal tumours. There was over-representation of extra-cellular matrix genes the metastases as compared with the primary tumours. In addition, there was a common signature of genes that were up-regulated in metastases as compared with the primary tumours, including the cytokeratin KRT19 and the developmental gene CELSR1. This was confirmed using an in vivo model and in matched patient samples.

Conclusion

SRC-1 is a master-regulator that can utilise continued treatment with endocrine therapies to promote tumour metastasis. Furthermore, it can actively suppress genes which are striving to maintain a normal cell. The interaction of cancer cells with the ECM is central to metastasis. Understanding this cellular plasticity may allow the development of new therapeutic and diagnostic technologies.