Faculty Bibliography

BACKGROUND:Zr]Zr-DFO-SC16.56 (composed of the anti-DLL3 antibody SC16.56 conjugated to p-SCN-Bn-deferoxamine [DFO] serving as a chelator for zirconium-89) in patients with neuroendocrine-derived cancer. METHODS:Zr]Zr-DFO-SC16.56 at the same activity and mass dose as in the initial cohort followed by a single PET-CT scan 3-6 days later. Retrospectively collected tumour biopsy samples were assessed for DLL3 by immunohistochemistry. The primary outcome of phase 2 of the study in cohort 2 was to determine the potential association between tumour uptake of the tracer and intratumoural DLL3 protein expression, as determined by immunohistochemistry. This study is ongoing and is registered with ClinicalTrials.gov, NCT04199741. FINDINGS/RESULTS:Zr]Zr-DFO-SC16.56 was congruent with DLL3 immunohistochemistry in 15 (94%) of 16 patients with evaluable tissue. Two patients with non-avid DLL3 SCLC and neuroendocrine prostate cancer by PET scan showed the lowest DLL3 expression by tumour immunohistochemistry. One (6%) of 18 patients had a grade 1 allergic reaction; no grade 2 or worse adverse events were noted in either cohort. INTERPRETATION/CONCLUSIONS:Zr]Zr-DFO-SC16.56 for non-invasive in-vivo detection of DLL3-expressing malignancies. FUNDING/BACKGROUND:National Institutes of Health, Prostate Cancer Foundation, and Scannell Foundation.

The coronavirus disease 2019 (COVID-19) pandemic has claimed the lives of over one million people worldwide. The causative agent, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), is a member of the Coronaviridae family of viruses that can cause respiratory infections of varying severity. The cellular host factors and pathways co-opted during SARS-CoV-2 and related coronavirus life cycles remain ill defined. To address this gap, we performed genome-scale CRISPR knockout screens during infection by SARS-CoV-2 and three seasonal coronaviruses (HCoV-OC43, HCoV-NL63, and HCoV-229E). These screens uncovered host factors and pathways with pan-coronavirus and virus-specific functional roles, including major dependency on glycosaminoglycan biosynthesis, sterol regulatory element-binding protein (SREBP) signaling, bone morphogenetic protein (BMP) signaling, and glycosylphosphatidylinositol biosynthesis, as well as a requirement for several poorly characterized proteins. We identified an absolute requirement for the VMP1, TMEM41, and TMEM64 (VTT) domain-containing protein transmembrane protein 41B (TMEM41B) for infection by SARS-CoV-2 and three seasonal coronaviruses. This human coronavirus host factor compendium represents a rich resource to develop new therapeutic strategies for acute COVID-19 and potential future coronavirus pandemics.

Patient-derived xenografts are high fidelity in vivo tumor models that accurately reflect many key aspects of human cancer. In contrast to either cancer cell lines or genetically engineered mouse models, the utility of PDXs has been limited by the inability to perform targeted genome editing of these tumors. To address this limitation, we have developed methods for CRISPR-Cas9 editing of PDXs using a tightly regulated, inducible Cas9 vector that does not require in vitro culture for selection of transduced cells. We demonstrate the utility of this platform in PDXs (1) to analyze genetic dependencies by targeted gene disruption and (2) to analyze mechanisms of acquired drug resistance by site-specific gene editing using templated homology-directed repair. This flexible system has broad application to other explant models and substantially augments the utility of PDXs as genetically programmable models of human cancer.

Small cell lung cancer is initially highly responsive to cisplatin and etoposide but in almost every case becomes rapidly chemoresistant, leading to death within 1 year. We modeled acquired chemoresistance in vivo using a series of patient-derived xenografts to generate paired chemosensitive and chemoresistant cancers. Multiple chemoresistant models demonstrated suppression of SLFN11, a factor implicated in DNA-damage repair deficiency. In vivo silencing of SLFN11 was associated with marked deposition of H3K27me3, a histone modification placed by EZH2, within the gene body of SLFN11, inducing local chromatin condensation and gene silencing. Inclusion of an EZH2 inhibitor with standard cytotoxic therapies prevented emergence of acquired resistance and augmented chemotherapeutic efficacy in both chemosensitive and chemoresistant models of small cell lung cancer.

Retrons are a retroelement class found in diverse prokaryotes that can be adapted to augment CRISPR-Cas9 genome engineering technology to efficiently rewrite short stretches of genetic information in bacteria and yeast. However, efficiency in human cells has been limited by unknown factors. We identified non-coding RNA (ncRNA) instability and impaired Cas9 activity due to 5' sgRNA extension as key contributors to low retron editor efficiency in human cells. We re-engineered the Eco1 ncRNA to incorporate an exoribonuclease-resistant RNA pseudoknot from the Zika virus 3' UTR and devised an RNA processing strategy using Csy4 ribonuclease to minimize 5' sgRNA extension. This strategy increased steady-state ncRNA levels and rescued sgRNA activity, leading to increased templated repair. This work reveals a previously unappreciated role for ncRNA stability in retron editor efficiency in human cells and presents an enhanced Eco1 retron editor capable of precise genome editing in human cells from a single integrated lentivirus and, in the context of the nCas9 H840A nickase, without creating double-strand breaks.

Small cell lung cancer (SCLC) is an exceptionally lethal malignancy for which more effective therapies are urgently needed. Several lines of evidence, from SCLC primary human tumours, patient-derived xenografts, cancer cell lines and genetically engineered mouse models, appear to be converging on a new model of SCLC subtypes defined by differential expression of four key transcription regulators: achaete-scute homologue 1 (ASCL1; also known as ASH1), neurogenic differentiation factor 1 (NeuroD1), yes-associated protein 1 (YAP1) and POU class 2 homeobox 3 (POU2F3). In this Perspectives article, we review and synthesize these recent lines of evidence and propose a working nomenclature for SCLC subtypes defined by relative expression of these four factors. Defining the unique therapeutic vulnerabilities of these subtypes of SCLC should help to focus and accelerate therapeutic research, leading to rationally targeted approaches that may ultimately improve clinical outcomes for patients with this disease.

Small cell lung cancer patient outcomes have not yet been significantly impacted by the revolution in precision oncology, primarily due to a paucity of genetic alterations in actionable driver oncogenes. Nevertheless, systemic therapies that include immunotherapy are beginning to show promise in the clinic. While these results are encouraging, many patients do not respond to or rapidly recur after current regimens, necessitating alternative or complementary therapeutic strategies. In this review, we discuss ongoing investigations into the pathobiology of this recalcitrant cancer and the therapeutic vulnerabilities that are exposed by the disease state. Included within this discussion is a snapshot of the current biomarker and clinical trial landscapes for small cell lung cancer. Finally, we identify key knowledge gaps that should be addressed in order to advance the field in pursuit of reduced small cell lung cancer mortality. This review largely summarizes work presented at the Third Biennial IASLC Small Cell Lung Cancer Meeting.

Schlafen family member 11 (SLFN11) expression sensitizes cells to a spectrum of DNA-damaging chemotherapies. Previous studies have shown that SLFN11 is recruited to stalled replication forks in response to replication stress; however, the role of SLFN11 at stressed replication forks remains unclear. Using single-molecule DNA fiber analysis and super-resolution microscopy to interrogate the dynamics of individual replication forks, we show that SLFN11 acts upon stalled replication forks to suppress efficient fork restart. In the absence of SLFN11 expression, fork restart proceeds through a pathway involving the ubiquitin ligase RFWD3 and the DNA primase-polymerase PRIMPOL to facilitate gapped DNA synthesis, thereby ensuring that cells do not accumulate replication-associated DNA damage. SLFN11 antagonizes this pathway by disrupting recruitment of RFWD3 and PRIMPOL to stalled forks in a manner dependent on a functional ATPase domain and persistent fork localization, but not on tRNA hydrolysis or ssDNA binding. Collectively, our results provide a mechanistic basis for how SLFN11 can counteract DNA damage tolerance by suppressing the RFWD3-PRIMPOL fork restart pathway.

Fourth-generation EGFR tyrosine kinase are in development to overcome common resistance mutations. We performed deep mutational scanning (DMS) of the EGFR kinase domain in the context of L858R by introducing a saturation library of ~17,000 variants into Ba/F3 cells. DMS library-expressing cells were exposed to osimertinib or BLU-945 to identify escape mutations. L718X mutations were enriched across all conditions. BLU-945 specific mutations included K714R, K716T, L718V, T725M, K728E, K754E/N, N771S/T, T783I, Q791L/K, G863S, S895N, K929I, and M971L. A secondary DMS screen combining osimertinib and BLU-945, exclusively enriched for L718X mutations. Clinically, L718X mutations emerged in two patients treated with BLU-945. One patient with baseline EGFR L858R and L718Q mutations experienced early progression. Another with baseline EGFR L858R, T790M, and C797S acquired an L718V mutation at progression. This study demonstrate how comprehensive resistance profiling of targeted therapies can predict clinically relevant mutations and guide rational combinations to delay or prevent resistance.

Interferons (IFNs) play a crucial role in the regulation and evolution of host-virus interactions. Here, we conducted a genome-wide arrayed CRISPR knockout screen in the presence and absence of IFN to identify human genes that influence Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) infection. We then performed an integrated analysis of genes interacting with SARS-CoV-2, drawing from a selection of 67 large-scale studies, including our own. We identified 28 genes of high relevance in both human genetic studies of Coronavirus Disease 2019 (COVID-19) patients and functional genetic screens in cell culture, with many related to the IFN pathway. Among these was the IFN-stimulated gene PLSCR1. PLSCR1 did not require IFN induction to restrict SARS-CoV-2 and did not contribute to IFN signaling. Instead, PLSCR1 specifically restricted spike-mediated SARS-CoV-2 entry. The PLSCR1-mediated restriction was alleviated by TMPRSS2 overexpression, suggesting that PLSCR1 primarily restricts the endocytic entry route. In addition, recent SARS-CoV-2 variants have adapted to circumvent the PLSCR1 barrier via currently undetermined mechanisms. Finally, we investigate the functional effects of PLSCR1 variants present in humans and discuss an association between PLSCR1 and severe COVID-19 reported recently.