The chromatin-associated Sin3B protein is required for hematopoietic stem cell functions in mice.

Hematopoietic stem cells (HSCs) reside at the top of the hematopoietic hierarchy and are the origin of all blood cells produced throughout an individual's life. The balance between HSC self-renewal and differentiation is maintained by various intrinsic and extrinsic mechanisms. Among these, the molecular pathways that restrict cell cycle progression are critical to the maintenance of functional HSCs. Alterations in the regulation of cell cycle progression in HSCs invariably lead to the development of hematologic malignancies or bone marrow failure syndromes. Here we report that hematopoietic-specific genetic inactivation of Sin3B, an essential component of the mammalian Sin3-histone deacetylase corepressor complex, severely impairs the competitive repopulation capacity of HSCs. Sin3B-deleted HSCs accumulate and fail to properly differentiate following transplantation. Moreover, Sin3B inactivation impairs HSC quiescence and sensitizes mice to myelosuppressive therapy. Together, these results identify Sin3B as a novel and critical regulator of HSC functions.

Brief Report: Long-Term Follow-Up of Adjuvant Pembrolizumab After Locally Ablative Therapy for Oligometastatic NSCLC.

Introduction: Patients with oligometastatic NSCLC benefit from locally ablative therapies (LAT); the role of adjuvant systemic therapies, however, remains less clear. In a single-arm, phase II clinical trial, we found that patients with oligometastatic NSCLC treated with a year of pembrolizumab after LAT had superior progression-free survival (PFS) compared with a historical control cohort. Herein, we present long-term follow-up on PFS and overall survival (OS). Methods: From February 1, 2015, to September 30, 2017, 45 patients with synchronous or metachronous oligometastatic (≤4 metastatic sites) NSCLC treated with LAT to all sites received adjuvant pembrolizumab every 21 days for up to 16 cycles. The primary efficacy end point was PFS from the start of pembrolizumab. Secondary end points included OS and safety. Median duration of follow-up was 66 months, and data cutoff was December 1, 2022. Results: A total of 45 patients were enrolled and treated with pembrolizumab after LAT (median age, 64 y [range, 46-82]; 21 women [47%]; 31 with a solitary oligometastatic site [69%]). At the data cutoff, 32 patients had progressive disease, 19 patients had died, and 13 patients had no evidence of relapse. Median PFS was 19.7 months (95% confidence interval: 7.6-31.7 mo); median OS was not reached (95% confidence interval: 37.7 mo-not reached). OS at 5 years was 60.0% (SE, 7.4%). Metachronous oligometastatic disease was associated with improved OS and PFS through Cox proportional hazard models. Conclusions: Pembrolizumab after LAT for oligometastatic NSCLC results in promising PFS and OS with a tolerable safety profile.

Targeting KRAS-Mutated NSCLC: Novel TKIs and Beyond.

KRAS-mutated non-small cell lung cancer (NSCLC) is the most common genetically altered subtype of NSCLC, yet targeted therapies remain limited. Multiple studies have investigated combinations of MEK inhibitors with chemotherapy without success. Here we discuss these studies and novel approaches to targeting KRAS-mutated NSCLC. See related article by Gadgeel et al., p. 3641.

Impaired Expression of Rearranged Immunoglobulin Genes and Premature p53 Activation Block B Cell Development in BMI1 Null Mice.

B cell development is a highly regulated process that requires stepwise rearrangement of immunoglobulin genes to generate a functional B cell receptor (BCR). The polycomb group protein BMI1 is required for B cell development, but its function in developing B cells remains poorly defined. We demonstrate that BMI1 functions in a cell-autonomous manner at two stages during early B cell development. First, loss of BMI1 results in a differentiation block at the pro-B cell to pre-B cell transition due to the inability of BMI1-deficient cells to transcribe newly rearranged Igh genes. Accordingly, introduction of a pre-rearranged Igh allele partially restored B cell development in Bmi1-/- mice. In addition, BMI1 is required to prevent premature p53 signaling, and as a consequence, Bmi1-/- large pre-B cells fail to properly proliferate. Altogether, our results clarify the role of BMI1 in early B cell development and uncover an unexpected function of BMI1 during VDJ recombination.

The HDAC-Associated Sin3B Protein Represses DREAM Complex Targets and Cooperates with APC/C to Promote Quiescence.

The mammalian DREAM complex is responsible for the transcriptional repression of hundreds of cell-cycle-related genes in quiescence. How the DREAM complex recruits chromatin-modifying entities to aid in its repression remains unknown. Using unbiased proteomics analysis, we have uncovered a robust association between the chromatin-associated Sin3B protein and the DREAM complex. We have determined that genetic inactivation of Sin3B results in the de-repression of DREAM target genes during quiescence but is insufficient to allow quiescent cells to resume proliferation. However, inactivation of APC/CCDH1 was sufficient for Sin3B-/- cells, but not parental cells, to re-enter the cell cycle. These studies identify Sin3B as a transcriptional corepressor associated with the DREAM complex in quiescence and reveals a functional cooperation between E2F target repression and APC/CCDH1 in the negative regulation of cell-cycle progression.

The potential of targeting Sin3B and its associated complexes for cancer therapy.

INTRODUCTION: Sin3B serves as a scaffold for chromatin-modifying complexes that repress gene transcription to regulate distinct biological processes. Sin3B-containing complexes are critical for cell cycle withdrawal, and abrogation of Sin3B-dependent cell cycle exit impacts tumor progression. Areas covered: In this review, we discuss the biochemical characteristics of Sin3B-containing complexes and explore how these complexes regulate gene transcription. We focus on how Sin3B-containing complexes, through the association of the Rb family of proteins, repress the expression of E2F target genes during quiescence, differentiation, and senescence. Finally, we speculate on the potential benefits of the inhibition of Sin3B-containing complexes for the treatment of cancer. Expert opinion: Further identification and characterization of specific Sin3B-containing complexes provide a unique opportunity to prevent the pro-tumorigenic effects of the senescence-associated secretory phenotype, and to abrogate cancer stem cell quiescence and the associated resistance to therapy.

Chromatin-Associated Protein SIN3B Prevents Prostate Cancer Progression by Inducing Senescence.

Distinguishing between indolent and aggressive prostate adenocarcinoma remains a priority to accurately identify patients who need therapeutic intervention. SIN3B has been implicated in the initiation of senescence in vitro Here we show that in a mouse model of prostate cancer, SIN3B provides a barrier to malignant progression. SIN3B was required for PTEN-induced cellular senescence and prevented progression to invasive prostate adenocarcinoma. Furthermore, SIN3B was downregulated in human prostate adenocarcinoma correlating with upregulation of its target genes. Our results suggest a tumor suppressor function for SIN3B that limits prostate adenocarcinoma progression, with potential implications for the use of SIN3B and its target genes as candidate diagnostic markers to distinguish indolent from aggressive disease. Cancer Res; 77(19); 5339-48. ©2017 AACR.

Transcriptional repression of Sin3B by Bmi-1 prevents cellular senescence and is relieved by oncogene activation.

The Polycomb group protein Bmi-1 is an essential regulator of cellular senescence and is believed to function largely through the direct repression of the Ink4a/Arf locus. However, concurrent deletion of Ink4a/Arf does not fully rescue the defects detected in Bmi-1(-/-) mice, indicating that additional Bmi-1 targets remain to be identified. The expression of the chromatin-associated Sin3B protein is stimulated by oncogenic stress, and is required for oncogene-induced senescence. Here we demonstrate that oncogenic stress leads to the dissociation of Bmi-1 from the Sin3B locus, resulting in increased Sin3B expression and subsequent entry into cellular senescence. Furthermore, Sin3B is required for the senescent phenotype and elevated levels of reactive oxygen species elicited upon Bmi-1 depletion. Altogether, these results identify Sin3B as a novel direct target of Bmi-1, and establish Bmi-1-driven repression of Sin3B as an essential regulator of cellular senescence.

SIN3B, the SASP, and pancreatic cancer.

Cellular senescence is classically considered a tumor suppressive mechanism. In addition to having stably exited the cell cycle, senescent cells secrete inflammatory factors. We recently demonstrated that senescence correlates with accelerated cancer progression in a mouse model of pancreatic ductal adenocarcinoma. Here, we discuss the implications of this study.

Senescence-associated SIN3B promotes inflammation and pancreatic cancer progression.

Pancreatic ductal adenocarcinoma (PDAC) is strikingly resistant to conventional therapeutic approaches. We previously demonstrated that the histone deacetylase-associated protein SIN3B is essential for oncogene-induced senescence in cultured cells. Here, using a mouse model of pancreatic cancer, we have demonstrated that SIN3B is required for activated KRAS-induced senescence in vivo. Surprisingly, impaired senescence as the result of genetic inactivation of Sin3B was associated with delayed PDAC progression and correlated with an impaired inflammatory response. In murine and human pancreatic cells and tissues, levels of SIN3B correlated with KRAS-induced production of IL-1α. Furthermore, evaluation of human pancreatic tissue and cancer cells revealed that Sin3B was decreased in control and PDAC samples, compared with samples from patients with pancreatic inflammation. These results indicate that senescence-associated inflammation positively correlates with PDAC progression and suggest that SIN3B has potential as a therapeutic target for inhibiting inflammation-driven tumorigenesis.