SIRT6 Suppresses Pancreatic Cancer through Control of Lin28b.

Chromatin remodeling proteins are frequently dysregulated in human cancer, yet little is known about how they control tumorigenesis. Here, we uncover an epigenetic program mediated by the NAD(+)-dependent histone deacetylase Sirtuin 6 (SIRT6) that is critical for suppression of pancreatic ductal adenocarcinoma (PDAC), one of the most lethal malignancies. SIRT6 inactivation accelerates PDAC progression and metastasis via upregulation of Lin28b, a negative regulator of the let-7 microRNA. SIRT6 loss results in histone hyperacetylation at the Lin28b promoter, Myc recruitment, and pronounced induction of Lin28b and downstream let-7 target genes, HMGA2, IGF2BP1, and IGF2BP3. This epigenetic program defines a distinct subset with a poor prognosis, representing 30%-40% of human PDAC, characterized by reduced SIRT6 expression and an exquisite dependence on Lin28b for tumor growth. Thus, we identify SIRT6 as an important PDAC tumor suppressor and uncover the Lin28b pathway as a potential therapeutic target in a molecularly defined PDAC subset. PAPERCLIP.

Chromatin and beyond: the multitasking roles for SIRT6.

In recent years there has been a large expansion in our understanding of SIRT6 biology including its structure, regulation, biochemical activity, and biological roles. SIRT6 functions as an ADP-ribosylase and NAD(+)-dependent deacylase of both acetyl groups and long-chain fatty-acyl groups. Through these functions SIRT6 impacts upon cellular homeostasis by regulating DNA repair, telomere maintenance, and glucose and lipid metabolism, thus affecting diseases such diabetes, obesity, heart disease, and cancer. Such roles may contribute to the overall longevity and health of the organism. Until recently, the known functions of SIRT6 were largely restricted to the chromatin. In this article we seek to describe and expand this knowledge with recent advances in understanding the mechanisms of SIRT6 action and their implications for human biology and disease.

Structural determinants of human proton-coupled folate transporter oligomerization: role of GXXXG motifs and identification of oligomeric interfaces at transmembrane domains 3 and 6.

The human proton-coupled folate transporter (hPCFT) is expressed in solid tumours and is active at pHs characterizing the tumour microenvironment. Recent attention focused on exploiting hPCFT for targeting solid tumours with novel cytotoxic anti-folates. hPCFT has 12 transmembrane domains (TMDs) and forms homo-oligomers with functional significance. The hPCFT primary sequence includes GXXXG motifs in TMD2 (G(93)XXXG(97)) and TMD4 (G(155)XXXG(159)). To investigate roles of these motifs in hPCFT function, stability and surface expression, we mutated glycine to leucine to generate single or multiple substitution mutants. Only the G93L and G159L mutants preserved substantial [(3)H]methotrexate (Mtx) transport when expressed in hPCFT-null (R1-11) HeLa cells. Transport activity of the glycine-to-leucine mutants correlated with surface hPCFT by surface biotinylation and confocal microscopy with ECFP*-tagged hPCFTs, suggesting a role for GXXXG in hPCFT stability and intracellular trafficking. When co-expressed in R1-11 cells, haemagglutinin-tagged glycine-to-leucine mutants and His10-tagged wild-type (WT) hPCFT co-associated on nickel affinity columns, suggesting that the GXXXG motifs are not directly involved in hPCFT oligomerization. This was substantiated by in situ FRET experiments with co-expressed ECFP*- and YFP-tagged hPCFT. Molecular modelling of dimeric hPCFT structures showed juxtaposed TMDs 2, 3, 4 and 6 as potential structural interfaces between monomers. hPCFT cysteine insertion mutants in TMD3 (Q136C and L137C) and TMD6 (W213C, L214C, L224C, A227C, F228C, F230C and G231C) were expressed in R1-11 cells and cross-linked with 1,6-hexanediyl bismethanethiosulfonate, confirming TMD juxtapositions. Altogether, our results imply that TMDs 3 and 6 provide critical interfaces for formation of hPCFT oligomers, which might be facilitated by the GXXXG motifs in TMD2 and TMD4.

RNA Polymerase II hypertranscription in cancer FFPE samples.

Hypertranscription is common in human cancers and predicts poor prognosis. However detection of hypertranscription is indirect, relying on accurately quantifying mRNA levels and estimating cell numbers. Previously, we introduced FFPE-CUTAC, a genome-wide method for mapping RNA Polymerase II (RNAPII) in formalin-fixed paraffin-embedded (FFPE) sections. Here we use FFPE-CUTAC to demonstrate genome-wide hypertranscription both in transgene-driven mouse gliomas and in assorted human tumors at active regulatory elements and replication-coupled histone genes with reduced mitochondrial DNA abundance. FFPE-CUTAC identified RNAPII-bound regulatory elements shared among diverse cancers and readily categorized human tumors despite using very small samples and low sequencing depths. Remarkably, RNAPII FFPE-CUTAC identified de novo and precisely mapped HER2 amplifications punctuated by likely selective sweeps including genes encoding direct positive regulators of RNAPII itself. Our results demonstrate that FFPE-CUTAC measurements of hypertranscription and classifications of tumors using small sections provides an affordable and sensitive genome-wide strategy for personalized medicine.

The glutathione S-transferase Gstt1 drives survival and dissemination in metastases.

Identifying the adaptive mechanisms of metastatic cancer cells remains an elusive question in the treatment of metastatic disease, particularly in pancreatic cancer (pancreatic adenocarcinoma, PDA). A loss-of-function shRNA targeted screen in metastatic-derived cells identified Gstt1, a member of the glutathione S-transferase superfamily, as uniquely required for dissemination and metastasis, but dispensable for primary tumour growth. Gstt1 is expressed in latent disseminated tumour cells (DTCs), is retained within a subpopulation of slow-cycling cells within existing metastases, and its inhibition leads to complete regression of macrometastatic tumours. This distinct Gstt1high population is highly metastatic and retains slow-cycling phenotypes, epithelial-mesenchymal transition features and DTC characteristics compared to the Gstt1low population. Mechanistic studies indicate that in this subset of cancer cells, Gstt1 maintains metastases by binding and glutathione-modifying intracellular fibronectin, in turn promoting its secretion and deposition into the metastatic microenvironment. We identified Gstt1 as a mediator of metastasis, highlighting the importance of heterogeneity and its influence on the metastatic tumour microenvironment.

SRC inhibition enables formation of a growth suppressive MAGI1-PP2A complex in isocitrate dehydrogenase-mutant cholangiocarcinoma.

Intrahepatic cholangiocarcinoma (ICC) is an aggressive bile duct malignancy that frequently exhibits isocitrate dehydrogenase (IDH1/IDH2) mutations. Mutant IDH (IDHm) ICC is dependent on SRC kinase for growth and survival and is hypersensitive to inhibition by dasatinib, but the molecular mechanism underlying this sensitivity is unclear. We found that dasatinib reduced p70 S6 kinase (S6K) and ribosomal protein S6 (S6), leading to substantial reductions in cell size and de novo protein synthesis. Using an unbiased phosphoproteomic screen, we identified membrane-associated guanylate kinase, WW, and PDZ domain containing 1 (MAGI1) as an SRC substrate in IDHm ICC. Biochemical and functional assays further showed that SRC inhibits a latent tumor-suppressing function of the MAGI1-protein phosphatase 2A (PP2A) complex to activate S6K/S6 signaling in IDHm ICC. Inhibiting SRC led to activation and increased access of PP2A to dephosphorylate S6K, resulting in cell death. Evidence from patient tissue and cell line models revealed that both intrinsic and extrinsic resistance to dasatinib is due to increased phospho-S6 (pS6). To block pS6, we paired dasatinib with the S6K/AKT inhibitor M2698, which led to a marked reduction in pS6 in IDHm ICC cell lines and patient-derived organoids in vitro and substantial growth inhibition in ICC patient-derived xenografts in vivo. Together, these results elucidated the mechanism of action of dasatinib in IDHm ICC, revealed a signaling complex regulating S6K phosphorylation independent of mTOR, suggested markers for dasatinib sensitivity, and described a combination therapy for IDHm ICC that may be actionable in the clinic.

Epigenomic analysis of formalin-fixed paraffin-embedded samples by CUT&Tag.

For more than a century, formalin-fixed paraffin-embedded (FFPE) sample preparation has been the preferred method for long-term preservation of biological material. However, the use of FFPE samples for epigenomic studies has been difficult because of chromatin damage from long exposure to high concentrations of formaldehyde. Previously, we introduced Cleavage Under Targeted Accessible Chromatin (CUTAC), an antibody-targeted chromatin accessibility mapping protocol based on CUT&Tag. Here we show that simple modifications of our CUTAC protocol either in single tubes or directly on slides produce high-resolution maps of paused RNA Polymerase II at enhancers and promoters using FFPE samples. We find that transcriptional regulatory element differences produced by FFPE-CUTAC distinguish between mouse brain tumors and identify and map regulatory element markers with high confidence and precision, including microRNAs not detectable by RNA-seq. Our simple workflows make possible affordable epigenomic profiling of archived biological samples for biomarker identification, clinical applications and retrospective studies.

Oligonucleotide-directed proximity-interactome mapping (O-MAP): A unified method for discovering RNA-interacting proteins, transcripts and genomic loci in situ.

Throughout biology, RNA molecules form complex networks of molecular interactions that are central to their function, but remain challenging to investigate. Here, we introduce Oligonucleotide-mediated proximity-interactome MAPping (O-MAP), a straightforward method for elucidating the biomolecules near an RNA of interest, within its native cellular context. O-MAP uses programmable oligonucleotide probes to deliver proximity-biotinylating enzymes to a target RNA, enabling nearby molecules to be enriched by streptavidin pulldown. O-MAP induces exceptionally precise RNA-localized in situ biotinylation, and unlike alternative methods it enables straightforward optimization of its targeting accuracy. Using the 47S pre-ribosomal RNA and long noncoding RNA Xist as models, we develop O-MAP workflows for unbiased discovery of RNA-proximal proteins, transcripts, and genomic loci. This revealed unexpected co-compartmentalization of Xist and other chromatin-regulatory RNAs and enabled systematic characterization of nucleolar-chromatin interactions across multiple cell lines. O-MAP is portable to cultured cells, organoids, and tissues, and to RNAs of various lengths, abundances, and sequence composition. And, O-MAP requires no genetic manipulation and uses exclusively off-the-shelf parts. We therefore anticipate its application to a broad array of RNA phenomena.

Sirtuin 6 is required for the integrated stress response and resistance to inhibition of transcriptional cyclin-dependent kinases.

Pancreatic ductal adenocarcinoma (PDAC) is classified into two key subtypes, classical and basal, with basal PDAC predicting worse survival. Using in vitro drug assays, genetic manipulation experiments, and in vivo drug studies in human patient-derived xenografts (PDXs) of PDAC, we found that basal PDACs were uniquely sensitive to transcriptional inhibition by targeting cyclin-dependent kinase 7 (CDK7) and CDK9, and this sensitivity was recapitulated in the basal subtype of breast cancer. We showed in cell lines, PDXs, and publicly available patient datasets that basal PDAC was characterized by inactivation of the integrated stress response (ISR), which leads to a higher rate of global mRNA translation. Moreover, we identified the histone deacetylase sirtuin 6 (SIRT6) as a critical regulator of a constitutively active ISR. Using expression analysis, polysome sequencing, immunofluorescence, and cycloheximide chase experiments, we found that SIRT6 regulated protein stability by binding activating transcription factor 4 (ATF4) in nuclear speckles and protecting it from proteasomal degradation. In human PDAC cell lines and organoids as well as in murine PDAC genetically engineered mouse models where SIRT6 was deleted or down-regulated, we demonstrated that SIRT6 loss both defined the basal PDAC subtype and led to reduced ATF4 protein stability and a nonfunctional ISR, causing a marked vulnerability to CDK7 and CDK9 inhibitors. Thus, we have uncovered an important mechanism regulating a stress-induced transcriptional program that may be exploited with targeted therapies in particularly aggressive PDAC.

Positioning of nucleosomes containing γ-H2AX precedes active DNA demethylation and transcription initiation.

In addition to nucleosomes, chromatin contains non-histone chromatin-associated proteins, of which the high-mobility group proteins are the most abundant. Chromatin-mediated regulation of transcription involves DNA methylation and histone modifications. However, the order of events and the precise function of high-mobility group proteins during transcription initiation remain unclear. Here we show that high-mobility group AT-hook 2 protein (HMGA2) induces DNA nicks at the transcription start site, which are required by the histone chaperone FACT complex to incorporate nucleosomes containing the histone variant H2A.X. Further, phosphorylation of H2A.X at S139 (γ-H2AX) is required for repair-mediated DNA demethylation and transcription activation. The relevance of these findings is demonstrated within the context of TGFB1 signaling and idiopathic pulmonary fibrosis, suggesting therapies against this lethal disease. Our data support the concept that chromatin opening during transcriptional initiation involves intermediates with DNA breaks that subsequently require DNA repair mechanisms to ensure genome integrity.

Cholesterol Biosynthesis Influences Subtype Specificity and Plasticity in Pancreas Cancer.

Cellular plasticity contributes to intratumoral heterogeneity, metastatic spread, and treatment resistance of cancers. In this issue of Cancer Cell, Gabitova-Cornell et al. identify the potential to inadvertently develop an undifferentiated and more aggressive pancreas cancer with agents commonly prescribed to manage heart disease risk.

Identification of and Molecular Basis for SIRT6 Loss-of-Function Point Mutations in Cancer.

Chromatin factors have emerged as the most frequently dysregulated family of proteins in cancer. We have previously identified the histone deacetylase SIRT6 as a key tumor suppressor, yet whether point mutations are selected for in cancer remains unclear. In this manuscript, we characterized naturally occurring patient-derived SIRT6 mutations. Strikingly, all the mutations significantly affected either stability or catalytic activity of SIRT6, indicating that these mutations were selected for in these tumors. Further, the mutant proteins failed to rescue sirt6 knockout (SIRT6 KO) cells, as measured by the levels of histone acetylation at glycolytic genes and their inability to rescue the tumorigenic potential of these cells. Notably, the main activity affected in the mutants was histone deacetylation rather than demyristoylation, pointing to the former as the main tumor-suppressive function for SIRT6. Our results identified cancer-associated point mutations in SIRT6, cementing its function as a tumor suppressor in human cancer.

The histone deacetylase SIRT6 controls embryonic stem cell fate via TET-mediated production of 5-hydroxymethylcytosine.

How embryonic stem cells (ESCs) commit to specific cell lineages and yield all cell types of a fully formed organism remains a major question. ESC differentiation is accompanied by large-scale histone and DNA modifications, but the relations between these epigenetic categories are not understood. Here we demonstrate the interplay between the histone deacetylase sirtuin 6 (SIRT6) and the ten-eleven translocation enzymes (TETs). SIRT6 targets acetylated histone H3 at Lys 9 and 56 (H3K9ac and H3K56ac), while TETs convert 5-methylcytosine into 5-hydroxymethylcytosine (5hmC). ESCs derived from Sirt6 knockout (S6KO) mice are skewed towards neuroectoderm development. This phenotype involves derepression of OCT4, SOX2 and NANOG, which causes an upregulation of TET-dependent production of 5hmC. Genome-wide analysis revealed neural genes marked with 5hmC in S6KO ESCs, thereby implicating TET enzymes in the neuroectoderm-skewed differentiation phenotype. We demonstrate that SIRT6 functions as a chromatin regulator safeguarding the balance between pluripotency and differentiation through Tet-mediated production of 5hmC.