Estrogen receptor (ER)-positive
breast cancer recurrence is thought to be driven by tumor-initiating cells (
TIC).
TICs are enriched by endocrine
therapy through NOTCH signaling. Side effects have limited clinical trial testing of NOTCH-targeted
therapies. Death-associated factor 6 (DAXX) is a newly identified marker whose
RNA expression inversely correlates with NOTCH in human ER+
breast tumor samples. In this study, knockdown and overexpression approaches were used to investigate the role of DAXX on stem/pluripotent gene expression,
TIC survival in vitro, and
TIC frequency in vivo, and the mechanism by which DAXX suppresses
TICs in ER+
breast cancer. 17β-Estradiol (E2)-mediated ER activation stabilized the DAXX
protein, which was required for repressing stem/pluripotent genes (NOTCH4, SOX2, OCT4, NANOG, and ALDH1A1), and
TICs in vitro and in vivo. Conversely, endocrine
therapy promoted rapid
protein depletion due to increased
proteasome activity. DAXX was enriched at promoters of stem/pluripotent genes, which was lost with endocrine
therapy. Ectopic expression of DAXX decreased stem/pluripotent gene transcripts to levels similar to E2 treatment. DAXX-mediated repression of stem/pluripotent genes and suppression of
TICs was dependent on DNMT1. DAXX or DNMT1 was necessary to inhibit methylation of CpGs within the SOX2 promoter and moderately within the gene body of NOTCH4, NOTCH activation, and
TIC survival. E2-mediated stabilization of DAXX was necessary and sufficient to repress stem/pluripotent genes by recruiting DNMT1 to methylate some promoters and suppress
TICs. These findings suggest that a combination of endocrine
therapy and DAXX-
stabilizing agents may inhibit ER+
tumor recurrence. SIGNIFICANCE:
Estradiol-mediated stabilization of DAXX is necessary and sufficient to repress genes associated with stemness, suggesting that the combination of endocrine
therapy and DAXX-
stabilizing agents may inhibit
tumor recurrence in ER+
breast cancer.