Temporal chromatin accessibility changes define transcriptional states essential for osteosarcoma metastasis.

The metastasis-invasion cascade describes the series of steps required for a cancer cell to successfully spread from its primary tumor and ultimately grow within a secondary organ. Despite metastasis being a dynamic, multistep process, most omics studies to date have focused on comparing primary tumors to the metastatic deposits that define end-stage disease. This static approach means we lack information about the genomic and epigenomic changes that occur during the majority of tumor progression. One particularly understudied phase of tumor progression is metastatic colonization, during which cells must adapt to the new microenvironment of the secondary organ. Through temporal profiling of chromatin accessibility and gene expression in vivo, we identify dynamic changes in the epigenome that occur as osteosarcoma tumors form and grow within the lung microenvironment. Furthermore, we show through paired in vivo and in vitro CRISPR drop-out screens and pharmacological validation that the upstream transcription factors represent a class of metastasis-specific dependency genes. While current models depict lung colonization as a discrete step within the metastatic cascade, our study shows it is a defined trajectory through multiple epigenetic states, revealing new therapeutic opportunities undetectable with standard approaches.

Functional Enhancers Shape Extrachromosomal Oncogene Amplifications.

Non-coding regions amplified beyond oncogene borders have largely been ignored. Using a computational approach, we find signatures of significant co-amplification of non-coding DNA beyond the boundaries of amplified oncogenes across five cancer types. In glioblastoma, EGFR is preferentially co-amplified with its two endogenous enhancer elements active in the cell type of origin. These regulatory elements, their contacts, and their contribution to cell fitness are preserved on high-level circular extrachromosomal DNA amplifications. Interrogating the locus with a CRISPR interference screening approach reveals a diversity of additional elements that impact cell fitness. The pattern of fitness dependencies mirrors the rearrangement of regulatory elements and accompanying rewiring of the chromatin topology on the extrachromosomal amplicon. Our studies indicate that oncogene amplifications are shaped by regulatory dependencies in the non-coding genome.

Ex vivo screen identifies CDK12 as a metastatic vulnerability in osteosarcoma.

Despite progress in intensification of therapy, outcomes for patients with metastatic osteosarcoma (OS) have not improved in thirty years. We developed a system that enabled preclinical screening of compounds against metastatic OS cells in the context of the native lung microenvironment. Using this strategy to screen a library of epigenetically targeted compounds, we identified inhibitors of CDK12 to be most effective, reducing OS cell outgrowth in the lung by more than 90% at submicromolar doses. We found that knockout of CDK12 in an in vivo model of lung metastasis significantly decreased the ability of OS to colonize the lung. CDK12 inhibition led to defects in transcription elongation in a gene length- and expression-dependent manner. These effects were accompanied by defects in RNA processing and altered the expression of genes involved in transcription regulation and the DNA damage response. We further identified OS models that differ in their sensitivity to CDK12 inhibition in the lung and provided evidence that upregulated MYC levels may mediate these differences. Our studies provided a framework for rapid preclinical testing of compounds with antimetastatic activity and highlighted CDK12 as a potential therapeutic target in OS.

Mismatch repair-signature mutations activate gene enhancers across human colorectal cancer epigenomes.

Commonly-mutated genes have been found for many cancers, but less is known about mutations in cis-regulatory elements. We leverage gains in tumor-specific enhancer activity, coupled with allele-biased mutation detection from H3K27ac ChIP-seq data, to pinpoint potential enhancer-activating mutations in colorectal cancer (CRC). Analysis of a genetically-diverse cohort of CRC specimens revealed that microsatellite instable (MSI) samples have a high indel rate within active enhancers. Enhancers with indels show evidence of positive selection, increased target gene expression, and a subset is highly recurrent. The indels affect short homopolymer tracts of A/T and increase affinity for FOX transcription factors. We further demonstrate that signature mismatch-repair (MMR) mutations activate enhancers using a xenograft tumor metastasis model, where mutations are induced naturally via CRISPR/Cas9 inactivation of MLH1 prior to tumor cell injection. Our results suggest that MMR signature mutations activate enhancers in CRC tumor epigenomes to provide a selective advantage.

The genomic landscape of core-binding factor acute myeloid leukemias.

Acute myeloid leukemia (AML) comprises a heterogeneous group of leukemias frequently defined by recurrent cytogenetic abnormalities, including rearrangements involving the core-binding factor (CBF) transcriptional complex. To better understand the genomic landscape of CBF-AMLs, we analyzed both pediatric (n = 87) and adult (n = 78) samples, including cases with RUNX1-RUNX1T1 (n = 85) or CBFB-MYH11 (n = 80) rearrangements, by whole-genome or whole-exome sequencing. In addition to known mutations in the Ras pathway, we identified recurrent stabilizing mutations in CCND2, suggesting a previously unappreciated cooperating pathway in CBF-AML. Outside of signaling alterations, RUNX1-RUNX1T1 and CBFB-MYH11 AMLs demonstrated remarkably different spectra of cooperating mutations, as RUNX1-RUNX1T1 cases harbored recurrent mutations in DHX15 and ZBTB7A, as well as an enrichment of mutations in epigenetic regulators, including ASXL2 and the cohesin complex. This detailed analysis provides insights into the pathogenesis and development of CBF-AML, while highlighting dramatic differences in the landscapes of cooperating mutations for these related AML subtypes.

LINE-1 and Alu retrotransposition exhibit clonal variation.

BACKGROUND: The non-long terminal repeat (non-LTR) retrotransposons, long interspersed element-1 (LINE-1) and Alu are currently active retroelements in humans. We, and others, have observed that different populations of HeLa cells from different laboratories support retrotransposition of LINE-1 and Alu to varying degrees. We therefore tested whether individual cell clones of HeLa and HCT116 cell lines supported different levels of LINE-1 and Alu retrotransposition, and whether these variations were stable upon re-cloning. FINDINGS: Standard retrotransposition tissue culture assays were used to measure a cell's ability to support LINE-1 and Alu retrotransposition in clonal HeLa and HCT116 cell lines. We observed that both LINE-1 and Alu retrotransposition exhibited clonal variation in HeLa cells, with certain HeLa cell clones supporting high levels of LINE-1 and Alu retrotransposition and other cell clones being essentially retrotransposition-dead. This clonal variation was similarly observed in HCT116 cells, although possibly not to the same extent. These patterns of clonal variation are relatively consistent upon re-cloning. CONCLUSIONS: Observations of the variability of LINE-1 and Alu retrotransposition in different populations of the same cell line are supported by our results that indicate in some cell types, individual cell clones can have dramatically differing capacity for retrotransposition. The mixed populations of cells commonly used in laboratories have often been passaged for many generations and accumulated significant genetic and epigenetic diversity. Our results suggest that the clonal variability observed by our cloning experiments may lead to a homogenization of retrotransposition capacity, with the resulting mixed population of cells being composed of individual variants having either increased or decreased retrotransposition potential compared to the starting population.

HPV 5 and 8 E6 expression reduces ATM protein levels and attenuates LINE-1 retrotransposition.

The expression of the E6 protein from certain members of the HPV genus ß (ß HPV 5 and 8 E6) can disrupt p53 signaling by diminishing the steady state levels of two p53 modifying enzymes, ATR and p300. Here, we show that ß-HPV 5 and 8 E6 are also capable of reducing the steady state levels of another p53 modifying enzyme, ATM, and as a result restrict LINE-1 retrotransposition. Furthermore, we show that the reduction of both ATM and LINE-1 retrotransposition is dependent upon the ability of ß-HPV 8 E6 to bind and degrade p300. We use inhibitors and dominant negative mutants to confirm that ATM is needed for efficient LINE-1 retrotransposition. Furthermore, neither sensitivity to LINE-1 expression nor LINE-1 induced DSB formation is altered in an ATM deficient background. Together, these data illustrate the broad impact some ß-HPVs have on DNA damage signaling by promoting p300 degradation.