SNPs near the cysteine proteinase cathepsin O gene (CTSO) determine tamoxifen sensitivity in ERα-positive breast cancer through regulation of BRCA1

Abstract

Tamoxifen is one of the most commonly employed endocrine therapies for patients with estrogen receptor α (ERα)-positive breast cancer. Unfortunately the clinical benefit is limited due to intrinsic and acquired drug resistance. We previously reported a genome-wide association study that identified common SNPs near the CTSO gene and in ZNF423 associated with development of breast cancer during tamoxifen therapy in the NSABP P-1 and P-2 breast cancer prevention trials. Here, we have investigated their roles in ERα-positive breast cancer growth and tamoxifen response, focusing on the mechanism of CTSO. We performed in vitro studies including luciferase assays, cell proliferation, and mass spectrometry-based assays using ERα-positive breast cancer cells and a panel of genomic data-rich lymphoblastoid cell lines. We report that CTSO reduces the protein levels of BRCA1 and ZNF423 through cysteine proteinase-mediated degradation. We also have identified a series of transcription factors of BRCA1 that are regulated by CTSO at the protein level. Importantly, the variant CTSO SNP genotypes are associated with increased CTSO and decreased BRCA1 protein levels that confer resistance to tamoxifen. Characterization of the effect of both CTSO SNPs and ZNF423 SNPs on tamoxifen response revealed that cells with different combinations of CTSO and ZNF423 genotypes respond differently to Tamoxifen, PARP inhibitors or the combination of the two drugs due to SNP dependent differential regulation of BRCA1 levels. Therefore, these genotypes might be biomarkers for selection of individual drug to achieve the best efficacy.

MIR2052HG regulates ERα levels and aromatase inhibitor resistance through LMTK3 by recruiting EGR1

“…Based on our finding showing MIR2025 regulation of LMTK3, we then determined whether the expression of LMTK3 might be also MIR2025HG SNP-and AI-dependent using the human lymphoblastoid cell lines (LCLs) system. This cell line model system, consisting of 300 individual LCLs for which we have extensive genomic and transcriptomic data, has shown repetitively to make it possible for us to study the relationship between common genetic variant and cellular phenotypes [28, 39,40]. In the presence of androstenedione, LCLs with variant genotypes for both of the MIR2052HG SNPs, rs4476990 and rs3802201, showed dose-dependent increases in LMTK3 expression ( Fig 7A, B).…”

ZNF423: A New Player in Estrogen Receptor-Positive Breast Cancer

“…CTSO degrades specifically ZNF423 and BRCA1, consequently reducing dsDNA break repair and increasing proliferation. The response to SERMs treatment is ineffective as the variant SNP for CTSO interrupts an ERE site in the CTSO promoter ( 13 ). Cells with a deficit or low BRCA1 become being addicted to other DNA repair pathways and were more sensitive to PARP inhibitors.…”

“…In the absence of ZNF423-induced BRCA1 expression, enforced overexpression of BRCA1 could result in effective dsDNA repair. Instead, when CTSO was knocked down and BRCA1 is overexpressed, the degradation determined by CTSO was minimal and BRCA1 could act on DNA repair ( 12 , 13 ).…”

“…A recent functional study has been performed to understand the mechanism underlying the phenomena associated with the CTSO and ZNF423 SNPs ( 13 ). Of the SNPs identified for CTSO , one interrupted an ERE site, and another was in relative close proximity.…”

“…Mass spectrometry analysis of proteins ( 13 ) that coimmunoprecipitate with CTSO has identified four transcription factors: metadherin (MTDH), poly(A) binding protein cytoplasmic 4 like (PABPC4L), lamin A/C (LMNA) and eukaryotic translation elongation factor 1 alpha 1 (EEF1A1); when these factors were knocked-down, it resulted in a net decrease in BRCA1 mRNA expression. When CTSO was overexpressed, these four proteins were all decreased, presumably degraded by CTSO.…”