Protein phosphatase 2A (PP2A) is a family of serine/threonine phosphatases that are generally considered tumour suppressors. The regulatory subunit, PP2A-B55α (PPP2R2A) is commonly deleted in breast tumours, and low expression associates with poor prognosis. Some studies, including our own, show that molecular knockdown of PPP2R2A in human breast cancer cells enhances breast tumour growth and metastasis, however other reports suggest PPP2R2A can play a tumour promoting role. The functional role in de-novo breast tumourigenesis is unknown.
The aim of this study was to use our recently developed Ppp2r2a knockout mice to investigate the functional role of PP2A-B55α in breast tumourigenesis.
Constitutive Ppp2r2a knockout, generated using CRISPR/Cas9, was embryonic lethal, with severe late-gestational epidermal defects associated with aberrant cell-cell junctions. Knockout embryos further displayed neural defects such as excencephaly and spina bifida. Heterozygous mice however, were born at Mendelian ratios. Adult heterozygous mice displayed decreased mammary gland branching, and developed fewer spontaneous neoplastic lesions with age, indicating that PP2A-B55α may function as a tumour promoter in vivo. Mammary-specific homozygous Ppp2r2a deletion in a HER2-driven mouse model of breast cancer (MMTV-NIC) showed delayed tumour onset compared to wildtype and heterozygous counterparts, further suggesting a tumour promoting role for Ppp2r2a in mice. Interestingly however, we identified a dose-dependent increase in estrogen receptor phosphorylation at serine 118 (p-Ser118 ER) with heterozygous or homozygous Ppp2r2a deletion. Increased p-Ser118 ER associates with poor survival and tamoxifen resistance in ER+ breast cancer patients, suggesting these tumours may be more resistant to anti-estrogen therapies. In support of this, we found that low PPP2R2A gene expression associated with poor outcome in tamoxifen treated patients, and that molecular knockdown of PP2A-B55α in the human ER+ breast cancer cell line, MCF7, induced tamoxifen resistance associated with increased p-Ser118 ER. Thus PP2A-B55α regulation of estrogen signalling is functionally important in tamoxifen resistance.
We are the first to show Ppp2r2a is vital for mammalian development. Furthermore, we found that mammary-specific homozygous Ppp2r2a deletion delayed, rather than enhanced, tumourigenesis driven by oncogenic HER2, suggesting PP2A-B55α functions as a tumour promoter in this model of breast cancer. The changes in ER signalling however suggest that low PP2A-B55α may confer poor response to anti-estrogen therapies.