The Estrogen Receptor Cofactor SPEN Functions as a Tumor Suppressor and Candidate Biomarker of Drug Responsiveness in Hormone-Dependent Breast Cancers

Abstract

The treatment of breast cancer has benefitted tremendously from the generation of estrogen receptor-a (ERa)-targeted therapies, but disease relapse continues to pose a challenge due to intrinsic or acquired drug resistance. In an effort to delineate potential predictive biomarkers of therapy responsiveness, multiple groups have identified several uncharacterized cofactors and interacting partners of ERa, including Split Ends (SPEN), a transcriptional corepressor. Here, we demonstrate a role for SPEN in ERa-expressing breast cancers. SPEN nonsense mutations were detectable in the ERa-expressing breast cancer cell line T47D and corresponded to undetectable protein levels. Further analysis of 101 primary breast tumors revealed that 23% displayed loss of heterozygosity at the SPEN locus and that 3% to 4% harbored somatically acquired mutations. A combination of in vitro and in vivo functional assays with microarray-based pathway analyses showed that SPEN functions as a tumor suppressor to regulate cell proliferation, tumor growth, and survival. We also found that SPEN binds ERa in a ligand-independent manner and negatively regulates the transcription of ERa targets. Moreover, we demonstrate that SPEN overexpression sensitizes hormone receptor-positive breast cancer cells to the apoptotic effects of tamoxifen, but has no effect on responsiveness to fulvestrant. Consistent with these findings, two independent datasets revealed that high SPEN protein and RNA expression in ERa-positive breast tumors predicted favorable outcome in patients treated with tamoxifen alone. Together, our data suggest that SPEN is a novel tumor-suppressor gene that may be clinically useful as a predictive biomarker of tamoxifen response in ERa-positive breast cancers. Cancer Res; 75(20); 4351-63. Ó2015 AACR.

The Subclonal Architecture of Metastatic Breast Cancer: Results from a Prospective Community-Based Rapid Autopsy Program “CASCADE”

“…SPEN has recently been implicated as a novel tumour suppressor that regulates cell proliferation and inhibits oestrogen receptor downstream signalling, with a role in resistance to tamoxifen [53]. The truncal location of SPEN alterations in three different cases suggests this is a bona fide tumour suppressor and mediator of resistance to endocrine therapy.…”

An autonomous metabolic role for Spen

“…Importantly, the mammalian homologs of both Spen (SPEN/MINT/SHARP, hereafter mSpen) and Nito (Rbm15/OTT1, hereafter mNito) were recently found to be regulators of X chromosome inactivation via RRM-mediated interactions with the long, noncoding RNA (lncRNA) Xist [32–35]. In addition to activation or repression of transcription, Spen family proteins influence alternative splicing [30, 31, 36–39] and nuclear export of RNAs [36, 40, 41], and are commonly mutated in cancers [20, 42], but mechanistic details are lacking. Identification of spen hypomorphs in our unbiased screen for fat mutant larvae [16] represented the first evidence that Spen family proteins have a role in organismal adiposity.…”

SPEN protein expression and interactions with chromatin in mouse testicular cells

“…Numerous SPEN cofactors (which are known to be involved in regulation of gene expression) have been identified in different in vitro screens. These include SMRT (Silencing Mediator for Retinoid and Thyroid receptors) (Shi et al 2001), nuclear hormone receptor PPARδ (peroxisome proliferator activated receptor gamma, PPARG) (Shi et al 2002), ERα (estrogen receptor α) (Légaré et al 2015), RBPJ (recombining binding protein suppressor of hairless) (Oswald et al 2002, Kuroda et al 2003, Li et al 2005, EB2 (Epstein-Barr virus early protein 2) (Hiriart et al 2005), CRYBP1 (the zinc finger transcription factor aA-crystallin-binding protein 1) (Yang et al 2005) and the E2 ubiquitin-conjugating enzyme UbcH8 (Li et al 2006). Interactions of SPEN with its cofactors additionally depend on the binding of CtIP/CtBP (DNA endonuclease RBBP8 in complex with C-terminalbinding protein) (Oswald et al 2005) and ETO (eighttwenty-one, RUNX1T1, a runt-related transcription factor) (Salat et al 2008).…”