Biology subjects

Hill, L. A.

Publications and source records attributed to Hill, L. A..

3 recordsLinked to original sources

bioRxiv2024

Spatial single cell transcriptomic analysis of a novel DICER1 Syndrome GEMM informs the cellular origin and developmental hierarchy of associated sarcomas

DICER1 syndrome predisposes children and young adults to tumor development across various organs. Many of these cancers are sarcomas, which uniquely express the RNase IIIb domain-deficient form of DICER1 and exhibit consistent histological and molecular similarities regardless of their anatomical origins. To uncover their cellular origin and developmental hierarchy, we established a lineage-traceable genetically engineered mouse model that allows for precise activation of Dicer1 mutations in Hic1+ mesenchymal stromal cells. This model resulted in the development of renal tumors closely mirroring human DICER1 sarcoma histologically and molecularly. Single-cell transcriptomics coupled with targeted spatial gene expression analysis revealed a Hic1+ progenitor population marked by Pdgfra, Dpt, and Mfap4, corresponding to universal fibroblasts of steady-state kidneys. These fibroblastic progenitors exhibit the capacity to undergo rhabdomyoblastic differentiation or transition to highly proliferative anaplastic sarcoma. Investigation of patient samples identified analogous cell states. This study uncovers a fibroblastic origin for DICER1 sarcoma and provides a faithful model for mechanistic investigation and therapeutic development for tumors within the rhabdomyosarcoma spectrum.

cancer biology↗

bioRxiv2024

Cell-of-origin epigenome underlies SS18::SSX-mediated transformation

Synovial sarcoma is an aggressive soft-tissue malignancy that is characterized by a pathognomonic t(X;18)(p11.2;q11.2) translocation, which produces the fusion oncogene named SS18::SSX. Despite recent advancements in our understanding of synovial sarcoma biology, the cell-of-origin remains undefined. A mesenchymal stromal cell (MSC) specific CreERT2 line was employed to express SS18::SSX in fibroblasts and related cell types, resulting in 100% penetrant synovial sarcoma development in mice, with a median latency period of 16.2 {+/-} 2.5 weeks. Murine tumours exhibited high concordance with human synovial sarcoma sub-types at the histological and molecular levels1. Genetic refinement of the cell-of-origin revealed that synovial sarcomas derive from a rare Hic1+ Pdgfra+ Lgr5+ fibroblastic population. Furthermore, longitudinal multi-omic profiling along the transformation continuum revealed the step-wise acquisition of a transformed phenotype initiated by the loss of a mature fibroblastic profile and subsequently, the gradual unmasking of an epigenetically embedded embryonic MSC program. Adult and embryonic MSCs exhibited overlapping H2AK119ub and H3K4me3/H3K27me3 (bivalent) histone marks, while SS18::SSX-mediated transformation culminated in the widespread loss of H3K27me3 at these genes and their consequent transcription. Collectively, these studies define a rare MSC context, conducive for SS18::SSX-mediated transformation, and demonstrate that SS tumorigenesis involves the induction and maintenance of an embryonic-like MSC phenotype.

cancer biology↗

bioRxiv2024

SS18::SSX redistributes BAF chromatin remodelers selectively to activate and repress transcription

Synovial sarcoma (SyS) is driven by the expression of a chromosomal translocation-generated SS18::SSX fusion oncoprotein1,2. This oncoprotein fuses the SSX carboxy terminal tail that binds ubiquitylated histone H2A (H2AK119ub)3-6 to the SS18 component of both canonical (CBAF) and non-canonical (GBAF, GLTSCR1-containing) BAF-family chromatin remodeling complexes7-11, wherein SS18::SSX reprograms the epigenetic landscape. Mice that express SS18::SSX develop tumors that faithfully recapitulate human SyS histopathologically and molecularly12-14, allowing dissection of transcriptional reprogramming in vivo15,16. Here, we show that SS18::SSX redistributes GBAF complexes broadly to promoters and distal enhancers decorated by H2AK119ub, which uncharacteristically lack the transcription-silencing trimethylation of H3K27 that otherwise accompanies H2AK119ub. Fusion-GBAF instead associates with H2AK119ub and transcription-enabling H3K4 trimethylation (promoters), monomethylation (distal enhancers), and H3K27 acetylation (both). CBAF with the fusion redistributes away from typical loci, avoids H2AK119ub-bearing regions, and instead narrowly flanks transcription start sites (TSSs), co-distributed with polybromo BAF (PBAF). Accelerated tumorigenesis in our SyS mouse model followed deletion of Smarcb1 (PBAF and CBAF), Pbrm1 (PBAF-specific), and Arid1a or Arid1b (CBAF-specific). While tumors lacking Arid1a or Arid1b retained SyS character, loss of Smarcb1 or Pbrm1 resulted in tumors lacking SyS features. These findings suggest that SyS transcriptional reprogramming includes both improper GBAF localization and gene activation at H2AK119ub-bearing regulatory regions and improper gene silencing through loss of CBAF.

cancer biology↗