Beyond the Double-Strand Breaks: The Role of DNA Repair Proteins in Cancer Stem-Cell Regulation.

Cancer stem cells (CSCs) are pluripotent and highly tumorigenic cells that can re-populate a tumor and cause relapses even after initially successful therapy. As with tissue stem cells, CSCs possess enhanced DNA repair mechanisms. An active DNA damage response alleviates the increased oxidative and replicative stress and leads to therapy resistance. On the other hand, mutations in DNA repair genes cause genomic instability, therefore driving tumor evolution and developing highly aggressive CSC phenotypes. However, the role of DNA repair proteins in CSCs extends beyond the level of DNA damage. In recent years, more and more studies have reported the unexpected role of DNA repair proteins in the regulation of transcription, CSC signaling pathways, intracellular levels of reactive oxygen species (ROS), and epithelial-mesenchymal transition (EMT). Moreover, DNA damage signaling plays an essential role in the immune response towards tumor cells. Due to its high importance for the CSC phenotype and treatment resistance, the DNA damage response is a promising target for individualized therapies. Furthermore, understanding the dependence of CSC on DNA repair pathways can be therapeutically exploited to induce synthetic lethality and sensitize CSCs to anti-cancer therapies. This review discusses the different roles of DNA repair proteins in CSC maintenance and their potential as therapeutic targets.

Oct4 confers stemness and radioresistance to head and neck squamous cell carcinoma by regulating the homologous recombination factors PSMC3IP and RAD54L.

Head and neck squamous cell carcinoma (HNSCC) is often being diagnosed at an advanced stage, conferring a poor prognosis. The probability of local tumor control after radiotherapy depends on the eradication of cancer stem cells (CSCs) with activated DNA repair. This study provides evidence that the CSC-related transcription factor Oct4 contributes to HNSCC radioresistance by regulating DNA damage response and the CSC phenotype. Knockdown of Oct4 A isoform reduced self-renewal capacity in HNSCC and led to partial tumor cell radiosensitization caused by transcriptional downregulation of the cell cycle checkpoint kinases CHK1 and WEE1 and homologous recombination (HR) repair genes PSMC3IP and RAD54L. Besides, PARP inhibition with Olaparib selectively radiosensitized Oct4 A knockout, but not wild-type HNSCC cells. This finding links Oct4 A to the HR-mediated DNA repair mechanisms. In turn, knockdown of PSMC3IP and RAD54L reduced the HNSCC self-renewal capacity and clonogenic cell survival after irradiation, suggesting the interplay between DNA repair and the CSC phenotype. Similar to the effect of Oct4 knockdown, overexpression of Oct4 also resulted in significant HNSCC radiosensitization and increased DNA damage, suggesting that Oct4-dependent regulation of DNA repair depends on its fine-tuned expression. In line with this observation, HNSCC patients with high and low nuclear Oct4 expression at the invasive tumor front exhibited better loco-regional tumor control after postoperative radio(chemo)therapy compared to the intermediate expression subgroup. Thus, we found that the Oct4-driven transcriptional program plays a critical role in regulating HNSCC radioresistance, and a combination of radiotherapy with PARP inhibitors may induce synthetic lethality in Oct4-deregulated tumors.

Recurrent HNSCC Harbor an Immunosuppressive Tumor Immune Microenvironment Suggesting Successful Tumor Immune Evasion.

PURPOSE: Recurrent tumors (RT) of head and neck squamous cell carcinoma (HNSCC) occur in up to 60%, with poor therapeutic response and detrimental prognosis. We hypothesized that HNSCC RTs successfully evade antitumor immune response and aimed to reveal tumor immune microenvironment (TIME) changes of primary tumors (PT) and corresponding RTs. EXPERIMENTAL DESIGN: Tumor-infiltrating leukocytes (TIL) of 300 PTs and 108 RTs from two large independent and clinically well-characterized HNSCC cohorts [discovery cohort (DC), validation cohort (VD)] were compared by IHC. mRNA expression analysis of 730 immune-related genes was performed for 18 PTs and RTs after adjuvant chemoradiotherapy (CRT). The effect of chemotherapy and radiation resistance was assessed with an in vitro spheroid/immunocyte coculture model. RESULTS: TIME analysis revealed overall decrease of TILs with significant loss of CD8+ T cells (DC P = 0.045/VC P < 0.0001) and B lymphocytes (DC P = 0.036/VC P < 0.0001) in RTs compared with PTs in both cohorts. Decrease predominantly occurred in RTs after CRT. Gene expression analysis confirmed loss of TILs (P = 0.0004) and B lymphocytes (P < 0.0001) and showed relative increase of neutrophils (P = 0.018), macrophages (P < 0.0001), dendritic cells (P = 0.0002), and mast cells (P = 0.0057) as well as lower overall expression of immune-related genes (P = 0.018) in RTs after CRT. Genes involved in B-lymphocyte functions and number of tertiary lymphoid structures showed the strongest decrease. SPP1 and MAPK1 were upregulated in vivo and in vitro, indicating their potential suitability as therapeutic targets in CRT resistance. CONCLUSIONS: HNSCC RTs have an immunosuppressive TIME, which is particularly apparent after adjuvant CRT and might substantially contribute to poor therapeutic response and prognosis.

Cancer Stem Cells in Head and Neck Squamous Cell Carcinoma: Identification, Characterization and Clinical Implications.

Head and neck squamous cell carcinoma (HNSCC) is the sixth most commonly diagnosed cancer worldwide. Despite advances in the treatment management, locally advanced disease has a poor prognosis, with a 5-year survival rate of approximately 50%. The growth of HNSCC is maintained by a population of cancer stem cells (CSCs) which possess unlimited self-renewal potential and induce tumor regrowth if not completely eliminated by therapy. The population of CSCs is not only a promising target for tumor treatment, but also an important biomarker to identify the patients at risk for therapeutic failure and disease progression. This review aims to provide an overview of the recent pre-clinical and clinical studies on the biology and potential therapeutic implications of HNSCC stem cells.