Research vision workshopping: Peer mentoring to support the transition to independence

The transition to independence requires shared enthusiasm for one's research goals from broad audiences. In this commentary, we describe the use of "research vision workshopping" within peer mentoring networks. We contend that this approach is broadly useful for the development and refinement of research visions for the academic job search.

Modeling diverse genetic subtypes of lung adenocarcinoma with a next-generation alveolar type 2 organoid platform

Lung cancer is the leading cause of cancer-related death worldwide. Lung adenocarcinoma (LUAD), the most common histological subtype, accounts for 40% of all cases. While existing genetically engineered mouse models (GEMMs) recapitulate the histological progression and transcriptional evolution of human LUAD, they are time-consuming and technically demanding. In contrast, cell line transplant models are fast and flexible, but these models fail to capture the full spectrum of disease progression. Organoid technologies provide a means to create next-generation cancer models that integrate the most advantageous features of autochthonous and transplant-based systems. However, robust and faithful LUAD organoid platforms are currently lacking. Here, we describe optimized conditions to continuously expand murine alveolar type 2 (AT2) cells, a prominent cell of origin for LUAD, in organoid culture. These organoids display canonical features of AT2 cells, including marker gene expression, the presence of lamellar bodies, and an ability to differentiate into the AT1 lineage. We used this system to develop flexible and versatile immunocompetent organoid-based models of KRAS, BRAF, and ALK mutant LUAD. Notably, organoid-based tumors display extensive burden and complete penetrance and are histopathologically indistinguishable from their autochthonous counterparts. Altogether, this organoid platform is a powerful, versatile new model system to study LUAD.

Single-Cell Epigenomics Reveals Mechanisms of Cancer Progression

Cancer initiation is driven by the cooperation between genetic and epigenetic aberrations that disrupt gene regulatory programs critical to maintaining specialized cellular functions. After initiation, cells acquire additional genetic and epigenetic alterations influenced by tumor-intrinsic and -extrinsic mechanisms, which increase intratumoral heterogeneity, reshape the cell's underlying gene regulatory networks and promote cancer evolution. Furthermore, environmental or therapeutic insults drive the selection of heterogeneous cell states, with implications for cancer initiation, maintenance, and drug resistance. The advancement of single-cell genomics has begun to uncover the full repertoire of chromatin and gene expression states (cell states) that exist within individual tumors. These single-cell analyses suggest that cells diversify in their regulatory states upon transformation by co-opting damage-induced and nonlineage regulatory programs that can lead to epigenomic plasticity. Here, we review these recent studies related to regulatory state changes in cancer progression and highlight the growing single-cell epigenomics toolkit poised to address unresolved questions in the field.

Spatial genomics enables multi-modal study of clonal heterogeneity in tissues

The state and behaviour of a cell can be influenced by both genetic and environmental factors. In particular, tumour progression is determined by underlying genetic aberrations^1-4 as well as the makeup of the tumour microenvironment^5,6. Quantifying the contributions of these factors requires new technologies that can accurately measure the spatial location of genomic sequence together with phenotypic readouts. Here we developed slide-DNA-seq, a method for capturing spatially resolved DNA sequences from intact tissue sections. We demonstrate that this method accurately preserves local tumour architecture and enables the de novo discovery of distinct tumour clones and their copy number alterations. We then apply slide-DNA-seq to a mouse model of metastasis and a primary human cancer, revealing that clonal populations are confined to distinct spatial regions. Moreover, through integration with spatial transcriptomics, we uncover distinct sets of genes that are associated with clone-specific genetic aberrations, the local tumour microenvironment, or both. Together, this multi-modal spatial genomics approach provides a versatile platform for quantifying how cell-intrinsic and cell-extrinsic factors contribute to gene expression, protein abundance and other cellular phenotypes.

SMARCA4 inactivation promotes lineage-specific transformation and early metastatic features in the lung

SMARCA4/BRG1 encodes for one of two mutually exclusive ATPases present in mammalian SWI/SNF chromatin remodeling complexes and is frequently mutated in human lung adenocarcinoma. However, the functional consequences of SMARCA4 mutation on tumor initiation, progression, and chromatin regulation in lung cancer remain poorly understood. Here, we demonstrate that loss of Smarca4 sensitizes CCSP+ cells within the lung in a cell-type dependent fashion to malignant transformation and tumor progression, resulting in highly advanced dedifferentiated tumors and increased metastatic incidence. Consistent with these phenotypes, Smarca4-deficient primary tumors lack lung lineage transcription factor activities and resemble a metastatic cell state. Mechanistically, we show that Smarca4 loss impairs the function of all three classes of SWI/SNF complexes, resulting in decreased chromatin accessibility at lung lineage motifs and ultimately accelerating tumor progression. Thus, we propose that the SWI/SNF complex - via Smarca4 - acts as a gatekeeper for lineage-specific cellular transformation and metastasis during lung cancer evolution.

Protocol for single-cell ATAC sequencing using combinatorial indexing in mouse lung adenocarcinoma

Single-cell ATAC sequencing using combinatorial indexing (sciATAC-seq) enables the identification of chromatin accessibility profiles at single-cell resolution with a dual-barcoding approach during transposition and library construction. Unlike commercial droplet-based approaches, sciATAC-seq is a cost-effective, extensible strategy that permits flexibility in the experimental scale via multiplexed barcoding across samples or perturbations. In contrast, droplet-based approaches have higher cell recovery and may be advantageous when cell input is limited. The step-by-step sciATAC-seq protocol described here is amenable to diverse cell types and fixed samples. For complete details on the use and execution of this protocol, please refer to LaFave et al. (2020).

Chromatin Potential Identified by Shared Single-Cell Profiling of RNA and Chromatin

Cell differentiation and function are regulated across multiple layers of gene regulation, including modulation of gene expression by changes in chromatin accessibility. However, differentiation is an asynchronous process precluding a temporal understanding of regulatory events leading to cell fate commitment. Here we developed simultaneous high-throughput ATAC and RNA expression with sequencing (SHARE-seq), a highly scalable approach for measurement of chromatin accessibility and gene expression in the same single cell, applicable to different tissues. Using 34,774 joint profiles from mouse skin, we develop a computational strategy to identify cis-regulatory interactions and define domains of regulatory chromatin (DORCs) that significantly overlap with super-enhancers. During lineage commitment, chromatin accessibility at DORCs precedes gene expression, suggesting that changes in chromatin accessibility may prime cells for lineage commitment. We computationally infer chromatin potential as a quantitative measure of chromatin lineage-priming and use it to predict cell fate outcomes. SHARE-seq is an extensible platform to study regulatory circuitry across diverse cells in tissues.

Epigenomic State Transitions Characterize Tumor Progression in Mouse Lung Adenocarcinoma

Regulatory networks that maintain functional, differentiated cell states are often dysregulated in tumor development. Here, we use single-cell epigenomics to profile chromatin state transitions in a mouse model of lung adenocarcinoma (LUAD). We identify an epigenomic continuum representing loss of cellular identity and progression toward a metastatic state. We define co-accessible regulatory programs and infer key activating and repressive chromatin regulators of these cell states. Among these co-accessibility programs, we identify a pre-metastatic transition, characterized by activation of RUNX transcription factors, which mediates extracellular matrix remodeling to promote metastasis and is predictive of survival across human LUAD patients. Together, these results demonstrate the power of single-cell epigenomics to identify regulatory programs to uncover mechanisms and key biomarkers of tumor progression.

Abstract PR08: Leveraging single-cell epigenomics to uncover regulatory programs in lung adenocarcinoma

A hallmark of cancer development is the loss of cellular identity due to the dysregulation of regulatory networks that maintain functional, differentiated states. Chromatin state has been linked to this control of cellular identity and developmental processes; however, the mechanisms by which these regulatory landscapes are disrupted during cancer progression are not well understood. To this end, we leveraged an optimized single-cell ATAC-sequencing (scATAC-seq) strategy using a combinatorial indexing-based technology to assess chromatin state changes that occur in a mouse model of lung adenocarcinoma (LUAD). We utilized a well-established LUAD mouse model in which mice conditionally express the KrasG12D mutation and lose p53 expression in alveolar type II cells following Cre-mediated recombination, termed the KP model. In this model, KP tumors progress from early-stage hyperplasias to metastatic, advanced adenocarcinomas without acquiring additional somatic mutations, suggesting that these phenotypic transitions may be driven in part by epigenetic mechanisms. Using scATAC-seq, we profiled 16,044 normal lung, primary KP tumor, and metastatic cells to assess chromatin state changes across LUAD progression. We found that LUAD tumor evolution was characterized by a heterogeneous epigenomic continuum that was associated with loss of alveolar identity and progression to metastasis. Interestingly, we identified a reproducible metastatic state that was homogeneous, suggesting that cancer cells funnel towards a stable epigenetic state. In addition, we utilized novel computational tools to assess combinatorial transcription factor (TF)-driven programs that delineate stages of cancer progression in the KP model. From these analyses, we identified a clear transition point that corresponded to progressive gain of RUNX2 transcription factor activity. Using CRISPR activation and knockout strategies, we found that RUNX2 regulated the chromatin accessibility of several regulatory programs involved in extracellular matrix remodeling, suggesting that RUNX2 signaling is a critical event in LUAD progression and metastasis. In addition, gene signatures derived from this RUNX-activated state were highly predictive of survival in human LUAD. Together, these results demonstrated the power of single-cell epigenomics to identify TF-driven regulatory programs as key biomarkers of tumor progression.

This abstract is also being presented as Poster A40.

Citation Format: Lindsay M. LaFave, Vinay Kartha, Sai Ma, Kevin Meli, Isabella Del Priore, Caleb Lareau, Venkat Sanker, Santiago Naranjo, Peter Westcott, Zachary Chiang, Alison Brack, Travis Law, Aviv Regev, Jason D Buenrostro, Tyler Jacks. Leveraging single-cell epigenomics to uncover regulatory programs in lung adenocarcinoma [abstract]. In: Proceedings of the AACR Special Conference on the Evolving Landscape of Cancer Modeling; 2020 Mar 2-5; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2020;80(11 Suppl):Abstract nr PR08.

Abstract A09: Coordinate regulation of chromatin state by JAK2 and ASXL1 mutations in myeloid malignancies

Myeloproliferative neoplasms (MPNs) are clonal myeloid malignancies which are initiated by somatic mutations in hematopoietic stem and progenitor cells. Of the classic MPNs, the disease myelofibrosis (MF) is characterized by the shortest median survival as well as progressive anemia, splenomegaly, and constitutional symptoms. Somatic JAK2V617F mutations are observed in approximately 60% of MF patients; however, murine studies have shown that JAK2V617F mutations are not sufficient to induce MF in the absence of additional mutations. Notably, the most common mutations that co-occur with JAK2V617F in MF are in the polycomb gene Additional Sex Combs Like-1 (ASXL1). Importantly, ASXL1 mutations are the strongest predictor of poor overall survival in MF. Given the role of ASXL1 mutations in MF pathogenesis and outcome, we hypothesized that loss of ASXL1 and JAK2V617F would collaborate to modulate epigenetic regulation and facilitate hematopoietic disease transformation. Previous studies have shown that JAK2V617F can translocate to the nucleus and phosphorylate histone H3 on tyrosine 41 (H3Y41), allowing for regulation of specific target genes including JAK2 itself. We used RNA interference to knock down ASXL1 in two JAK2V617F-mutant leukemic cell lines, SET2 and UKE1, which revealed that global and locus-specific H3Y41 phosphorylation levels increased after ASXL1 silencing leading to increased expression of JAK2/H3Y41 target genes. To evaluate whether H3Y41 levels were changed in vivo as well, we crossed a Jak2V617F knock-in mouse to our Asxl1 hematopoietic conditional knockout allele. Heterozygous expression of Jak2V617F and heterozygous deletion of Asxl1 driven by Vav-Cre hastened myeloid expansion and decreased overall survival in primary mice consistent with in vivo cooperativity between Jak2V617F and Asxl1. Multiparameter flow cytometric staining revealed that Vav+ Jak2V617F/+ Asxl1f/+ mice have an expansion of immature erythrocytes in their bone marrow as compared to Vav+ Jak2V617F/+ mice, suggestive of impaired hematopoietic differentiation, which was more severe in Vav+ Jak2V617 VF/+ Asxl1 f/f mice. We then used the Mx1-Cre transgene to induce Jak2V617F and delete Asxl1 in the adult hematopoietic compartment. Mx1-Cre+ Jak2V617F/+ Asxl1f/f mice developed severe, progressive anemia, erythroid precursor expansion in the peripheral blood, bone marrow and spleen, and increased disease burden in primary mice and serially transplanted mice. Importantly, mice expressing Jak2V617F/+ and concomitant Asxl1 loss/haploinsufficency have increased Jak2 protein expression, and increased elevated expression of H3Y41 target genes consistent with coordinate dysregulation of chromatin state by JAK2V617F and ASXL1 loss. These findings suggest ASXL1 loss contributes to myeloid transformation in part through modulation of H3Y41 phosphorylation levels, leading to amplified JAK2 expression, increased JAK-STAT signaling, and disease progression.

Citation Format: Lindsay M. LaFave, Young Rock Chung, Matt Keller, Maria Kleppe, Ann Mullally, Omar Abdel-Wahab, Ross L. Levine. Coordinate regulation of chromatin state by JAK2 and ASXL1 mutations in myeloid malignancies. [abstract]. In: Proceedings of the AACR Special Conference on Chromatin and Epigenetics in Cancer; Jun 19-22, 2013; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2013;73(13 Suppl):Abstract nr A09.

ASXL1 mutations promote myeloid transformation through loss of PRC2-mediated gene repression

Recurrent somatic ASXL1 mutations occur in patients with myelodysplastic syndrome, myeloproliferative neoplasms, and acute myeloid leukemia, and are associated with adverse outcome. Despite the genetic and clinical data implicating ASXL1 mutations in myeloid malignancies, the mechanisms of transformation by ASXL1 mutations are not understood. Here, we identify that ASXL1 mutations result in loss of polycomb repressive complex 2 (PRC2)-mediated histone H3 lysine 27 (H3K27) tri-methylation. Through integration of microarray data with genome-wide histone modification ChIP-Seq data, we identify targets of ASXL1 repression, including the posterior HOXA cluster that is known to contribute to myeloid transformation. We demonstrate that ASXL1 associates with the PRC2, and that loss of ASXL1 in vivo collaborates with NRASG12D to promote myeloid leukemogenesis.

Loss of the tumor suppressor BAP1 causes myeloid transformation

De-ubiquitinating enzyme BAP1 is mutated in a hereditary cancer syndrome with increased risk of mesothelioma and uveal melanoma. Somatic BAP1 mutations occur in various malignancies. We show that mouse Bap1 gene deletion is lethal during embryogenesis, but systemic or hematopoietic-restricted deletion in adults recapitulates features of human myelodysplastic syndrome (MDS). Knockin mice expressing BAP1 with a 3xFlag tag revealed that BAP1 interacts with host cell factor-1 (HCF-1), O-linked N-acetylglucosamine transferase (OGT), and the polycomb group proteins ASXL1 and ASXL2 in vivo. OGT and HCF-1 levels were decreased by Bap1 deletion, indicating a critical role for BAP1 in stabilizing these epigenetic regulators. Human ASXL1 is mutated frequently in chronic myelomonocytic leukemia (CMML) so an ASXL/BAP1 complex may suppress CMML. A BAP1 catalytic mutation found in a MDS patient implies that BAP1 loss of function has similar consequences in mice and humans.

An argument for choice. An alternative women's treatment program

Current substance abuse treatment strategies tend to be relatively narrow and not empirically based. This is a result of a general failure on the part of practitioners to keep abreast of current research, which would direct them toward more effective models. Substance abuse treatment, usually based on a "one size fits all" philosophy, is often not particularly effective. Substance abuse research is often limited to the measurement of a single outcome--abstinence--and research on alternative models is scarce. This article argues for more flexible models of treatment that are based on research. A substance abuse treatment program that uses an empirically based model is described. The program incorporates broader outcomes research, which is yielding encouraging results. The suppositions that directed the research are discussed.