WNT Oncogenic Transcription Requires MYC Suppression of Lysosomal Activity and EPCAM Stabilization in Gastric Tumours.

BACKGROUND AND AIMS: WNT signaling is central to spatial tissue arrangement, regulating stem cell activity, and represents the hallmark of gastrointestinal cancers. While its role in driving intestinal tumors is well characterized, WNT's role in gastric tumorigenesis remains elusive. METHODS: We have developed mouse models to control the specific expression of an oncogenic form of B-CATENIN in combination with MYC activation in Lgr5+ cells of the gastric antrum. We used multi-omics approaches applied in vivo and in organoid models to characterize their cooperation in driving gastric tumorigenesis. RESULTS: We report that constitutive B-CATENIN stabilization in the stomach has negligible oncogenic effects and requires MYC activation to induce gastric tumour formation. While physiologically low MYC levels in gastric glands limit B-CATENIN transcriptional activity, increased MYC expression unleashes the WNT oncogenic transcriptional program, promoting B-CATENIN enhancer invasion without a direct transcriptional cooperation. MYC activation induces a metabolic rewiring that suppresses lysosomal biogenesis through mTOR and ERK activation and MiT/TFE inhibition. This prevents EPCAM degradation by macropinocytosis, promoting B-CATENIN chromatin accumulation and activation of WNT oncogenic transcription. CONCLUSION: Our results uncovered a new signaling framework with important implications for the control of gastric epithelial architecture and WNT-dependent oncogenic transformation.

PCGF6 controls murine Tuft cell differentiation via H3K9me2 modification independently of Polycomb repression.

Cell fate is determined by specific transcription programs that are essential for tissue homeostasis and regeneration. The E3-ligases RING1A and B represent the core activity of the Polycomb repressive complex 1 (PRC1) that deposits repressive histone H2AK119 mono-ubiquitination (H2AK119ub1), which is essential for mouse intestinal homeostasis by preserving stem cell functions. However, the specific role of different PRC1 forms, which are defined by the six distinct PCGF1-6 paralogs, remains largely unexplored in vivo. We report that PCGF6 regulates mouse intestinal Tuft cell differentiation independently of H2AK119ub1 deposition. We show that PCGF6 chromatin occupancy expands outside Polycomb repressive domains, associating with unique promoter and distal regulatory elements. This occurs in the absence of RING1A/B and involves MGA-mediated E-BOX recognition and specific H3K9me2 promoter deposition. PCGF6 inactivation induces an epithelial autonomous accumulation of Tuft cells that was not phenocopied by RING1A/B loss. This involves direct PCGF6 association with a Tuft cell differentiation program that identified Polycomb-independent properties of PCGF6 in adult tissues homeostasis.

Increased genomic instability and reshaping of tissue microenvironment underlie oncogenic properties of Arid1a mutations.

Oncogenic mutations accumulating in many chromatin-associated proteins have been identified in different tumor types. With a mutation rate from 10 to 57%, ARID1A has been widely considered a tumor suppressor gene. However, whether this role is mainly due to its transcriptional-related activities or its ability to preserve genome integrity is still a matter of intense debate. Here, we show that ARID1A is largely dispensable for preserving enhancer-dependent transcriptional regulation, being ARID1B sufficient and required to compensate for ARID1A loss. We provide in vivo evidence that ARID1A is mainly required to preserve genomic integrity in adult tissues. ARID1A loss primarily results in DNA damage accumulation, interferon type I response activation, and chronic inflammation leading to tumor formation. Our data suggest that in healthy tissues, the increased genomic instability that follows ARID1A mutations and the selective pressure imposed by the microenvironment might result in the emergence of aggressive, possibly immune-resistant, tumors.