Summary Circadian rhythms are 24-hour oscillations that control a variety of biological processes in living systems, including two hallmarks of cancer, cell division and metabolism. Circadian rhythm disruption by shift-work is associated with greater risk for cancer development and poor prognosis, suggesting a putative tumor suppressive role for circadian rhythm homeostasis. Using a genetically-engineered mouse model (GEMM) of lung adenocarcinoma, we have characterized the effects of circadian rhythm disruption on lung tumorigenesis. We demonstrate that both physiologic perturbation (jet lag) and genetic mutation of the central circadian clock components decreased survival and promoted lung tumor growth and progression. The core circadian genes Per2 and Bmal1 were shown to have cell-autonomous tumor suppressive roles in transformation and lung tumor progression. Loss of the central clock components led to increased c-Myc expression, enhanced proliferation and metabolic dysregulation. Our findings demonstrate that both systemic and somatic disruption of circadian rhythms contribute to cancer progression.

Developmental and lineage plasticity have been observed in numerous malignancies and have been correlated with tumor progression and drug resistance. However, little is known about the molecular mechanisms that enable such plasticity to occur. Here, we describe the function of the plant homeodomain finger protein 6 (PHF6) in leukemia and define its role in regulating chromatin accessibility to lineage-specific transcription factors. We show that loss of Phf6 in B-cell leukemia results in systematic changes in gene expression via alteration of the chromatin landscape at the transcriptional start sites of B-cell-and T-cell-specific factors. Additionally, Phf6KO cells show significant downregulation of genes involved in the development and function of normal B cells, show up-regulation of genes involved in T-cell signaling, and give rise to mixed-lineage lymphoma in vivo. Engagement of divergent transcriptional programs results in phenotypic plasticity that leads to altered disease presentation in vivo, tolerance of aberrant oncogenic signaling, and differential sensitivity to frontline and targeted therapies. These findings suggest that active maintenance of a precise chromatin landscape is essential for sustaining proper leukemia cell identity and that loss of a single factor (PHF6) can cause focal changes in chromatin accessibility and nucleosome positioning that render cells susceptible to lineage transition.

Loss of function mutations in the plant homeodomain factor 6 (PHF6) are responsible for the Börjeson–Forssman–Lehmann syndrome, a familial X-linked intellectual disability disorder, and are observed in approximately 20% of adult T-cell acute lymphoblastic leukemias (T-ALLs) and 3% of adult acute myeloid leukemias (AMLs). However, mutations in B-cell lineage malignancies are notably absent. Interestingly, our recent work has uncovered PHF6 as a positive growth regulator in B-cell acute lymphoblastic leukemia (B-ALL) through a genome-scale in vivo loss-of-function screen. To identify the molecular mechanism by which PHF6 acts to promote B-ALL growth in vivo, we utilized CRISPR-Cas9 to delete Phf6 in murine B-ALL cells. Transplantation of Phf6 knockout cells (Phf6KO) into immunocompetent syngeneic recipients significantly extends disease latency and survival, therefore validating PHF6 as a bona fide positive growth regulator of B-ALL in vivo. Strikingly, these mice develop lymphoma-like disease with complete penetrance, characterized by significantly enlarged lymph nodes, decreased disease burden in the spleen and increased expression of the canonical T-cell marker CD4, suggesting that Phf6KO B-ALL cells transdifferentiated to cells resembling those of the T-cell lineage. To dissect the mechanism by which PHF6 regulates this lineage decision, we carried out a combination of RNA sequencing and chromatin immunoprecipitation (ChIP) analyses in Phf6WT and Phf6KO cells. RNA sequencing analysis revealed many differentially expressed genes in Phf6KO B-ALL cells , including gene sets involved in pathways important for B-cell development. ChIP-sequencing analysis of PHF6 and several histone marks (H3K27Ac, H3K27me3, H3K4me3) in Phf6WT B-ALL cells revealed that PHF6 and H3K27Ac signals co-localize close to the transcription start site of a significant proportion of differentially expressed genes. Transcription factor binding motif analysis revealed significant enrichment for several well-described master regulators of B-cell development, including PU.1, EGR-1, EBF-1, NF-κB and TCF3/TCF12. Notably, a number of these predicted transcription factors co-immunoprecipitated with PHF6 in Phf6WT B-ALL cells. These findings reveal an essential role for PHF6 in the maintenance of B-cell identity in B-ALL by activating, directly or indirectly, genes that are crucial for B-cell lineage commitment. Collectively, these results indicate that loss-of-function of PHF6 in B-ALL leads to transdifferentiation to the T-cell lineage, potentially explaining the apparent absence of PHF6 mutations in human B cell-lineage malignancies. Citation Format: Yadira M. Soto-Feliciano, Jordan ME Bartlebaugh, Yunpeng Liu, Francisco J. Sánchez-Rivera, Abraham S. Weintraub, Arjun Bhutkar, Tyler E. Jacks, Richard A. Young, Michael T. Hemann. The role of PHF6 in maintaining pre-B cell commitment in B-cell acute lymphoblastic leukemia. [abstract]. In: Proceedings of the AACR Special Conference on Chromatin and Epigenetics in Cancer; Sep 24-27, 2015; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2016;76(2 Suppl):Abstract nr A44.