Abstract Circadian rhythm (CR) orchestrates the physiological functions of nearly all organs in the human body. Disruption of circadian rhythm (CRD), caused by environmental factors such as shift work or by loss of core clock genes, is associated with heightened susceptibility to malignancies of the female reproductive system, particularly breast cancer (BC). However, whether deficiency of a specific clock gene directly confers breast cancer susceptibility and systemic immune suppression remains largely unresolved. Here, we investigated the role of the circadian regulator Period 2 (PER2) in breast tumorigenesis and immune evasion. Combined the data from TCGA datasets and a DMBA-induced mammary tumor model established in Per2def and Per2wt mice we analyzed the tumor immune profiles assessed by IHC and flow cytometry. The effects of PER2 loss on breast cancer malignancy were tested using proliferation, invasion, and clonogenic assays. Mechanistic studies (Western blot, qRT-PCR, and luciferase assays) defined the PER2-dependent pathway regulating tumor immune escape. PER2 expression was markedly reduced in BC and correlated with poor prognosis in female patients. Per2ᵈef mice exhibited significantly increased susceptibility to DMBA-induced mammary tumor formation, indicating an intrinsic tumor-protective role for PER2. Immune profiling revealed a profoundly immunosuppressed microenvironment in Per2ᵈef tumors, characterized by reduced cytotoxic CD8+ T cells and CD80+ M1 macrophages, accompanied by elevated pro-tumor FOXP3+ regulatory T cells and CD206+ M2 macrophages. In human breast cancer cells, PER2 knockdown induced aggressive and clonogenic phenotypes, whereas PER2 reconstitution reversed these effects. Loss of PER2 led to increased expression of HER2 and CD47, two oncogenic drivers of proliferation and immune evasion, respectively. Consistent with this, PER2 overexpression suppressed, whereas PER2 silencing enhanced, the transcriptional activities of HER2 and CD47 promoters. JASPAR predictions identified CLOCK-binding motifs within both promoters, and mutation of these motifs markedly diminished transcriptional activation in MCF7 and SKBR3 cells. Luciferase reporter assays further demonstrated that PER2 represses HER2 and CD47 expression by inhibiting CLOCK-mediated transactivation. Collectively, these findings reveal that PER2 deficiency triggers CLOCK-dependent upregulation of HER2 and CD47, promoting both tumor aggressiveness and immune escape. This study uncovers a dual mechanism by which clock gene loss enhances breast cancer susceptibility with systemic immunosuppression, highlighting PER2def-CLOCK-HER2/CD47 signaling as a potential integrated therapeutic axis to mitigate CRD-associated breast cancer risk and tumor aggressiveness. Citation Format: Yixin Duan, Helen Zhang, Hongyong Zhang, Jie Huang, Xiong Zhang, Aris Alexandro, Dake Hao, Jonathan I. Berg, Ming Fan, Yili Wang, Loning Fu, Hong-Wu Chen, Jian Jian Li. Lack of circadian regulator PER2 enhances breast cancer susceptibility and progression via immunosuppression abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts) ; 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86 (7 Suppl): Abstract nr 610.
Duan et al. (Fri,) studied this question.