Till SMARCA5 loss do nucleosomes part.

In this issue of Molecular Cell, Bomber et al. demonstrate that acute loss of SMARCA5 in human cells leads to eviction of CTCF and an increase in nucleosome repeat length without direct impact on transcriptional activity.

Srsf2P95H initiates myeloid bias and myelodysplastic/myeloproliferative syndrome from hemopoietic stem cells.

Mutations in SRSF2 occur in myelodysplastic syndromes (MDS) and MDS/myeloproliferative neoplasms (MPN). SRSF2 mutations cluster at proline 95, with the most frequent mutation being a histidine (P95H) substitution. They undergo positive selection, arise early in the course of disease, and have been identified in age-related clonal hemopoiesis. It is not clear how mutation of SRSF2 modifies hemopoiesis or contributes to the development of myeloid bias or MDS/MPN. Two prior mouse models of Srsf2P95H mutation have been reported; however, these models do not recapitulate many of the clinical features of SRSF2-mutant disease and relied on bone marrow (BM) transplantation stress to elicit the reported phenotypes. We describe a new conditional murine Srsf2P95H mutation model, where the P95H mutation is expressed physiologically and heterozygously from its endogenous locus after Cre activation. Using multiple Cre lines, we demonstrate that during native hemopoiesis (ie, no BM transplantation), the Srsf2P95H mutation needs to occur within the hemopoietic stem-cell-containing populations to promote myelomonocytic bias and expansion with corresponding transcriptional and RNA splicing changes. With age, nontransplanted Srsf2P95H animals developed a progressive, transplantable disease characterized by myeloid bias, morphological dysplasia, and monocytosis, hallmarks of MDS/MPN in humans. Analysis of cooccurring mutations within the BM demonstrated the acquisition of additional mutations that are recurrent in humans with SRSF2 mutations. The tractable Srsf2P95H/+ knock-in model we have generated is highly relevant to human disease and will serve to elucidate the effect of SRSF2 mutations on initiation and maintenance of MDS/MPN.

Chromatin organization in myelodysplastic syndrome.

Disordered chromatin organization has emerged as a new aspect of the pathogenesis of myelodysplastic syndrome (MDS). Characterized by lineage dysplasia and a high transformation rate to acute myeloid leukemia (AML), the genetic determinant of MDS is thought to be the main driver of the disease's progression. Among the recurrently mutated pathways, alterations in chromatin organization, such as the cohesin complex, have a profound impact on hematopoietic stem cell (HSC) function and lineage commitment. The cohesin complex is a ring-like structure comprised of structural maintenance of chromosomes (SMC), RAD21, and STAG proteins that involve three-dimensional (3D) genome organization via loop extrusion in mammalian cells. The partial loss of the functional cohesin ring leads to altered chromatin accessibility specific to key hematopoietic transcription factors, which is thought to be the molecular mechanism of cohesin dysfunction. Currently, there are no specific targeting agents for cohesin mutant MDS/AML. Potential therapeutic strategies have been proposed based on the current understanding of cohesin mutant leukemogenesis. Here, we will review the recent advances in investigation and targeting approaches against cohesin mutant MDS/AML.

Srsf2P95H/+ co-operates with loss of TET2 to promote myeloid bias and initiate a chronic myelomonocytic leukemia-like disease in mice.

Recurrent mutations in RNA splicing proteins and epigenetic regulators contribute to the development of myelodysplastic syndrome (MDS) and related myeloid neoplasms. In chronic myelomonocytic leukemia (CMML), SRSF2 mutations occur in ~50% of patients and TET2 mutations in ~60%. Clonal analysis indicates that either mutation can arise as the founder lesion. Based on human cancer genetics we crossed an inducible Srsf2P95H/+ mutant model with Tet2fl/fl mice to mutate both concomitantly in hematopoietic stem cells. At 20-24 weeks post mutation induction, we observed subtle differences in the Srsf2/Tet2 mutants compared to either single mutant. Under conditions of native hematopoiesis with aging, we see a distinct myeloid bias and monocytosis in the Srsf2/Tet2 mutants. A subset of the compound Srsf2/Tet2 mutants display an increased granulocytic and distinctive monocytic proliferation (myelomonocytic hyperplasia), with increased immature promonocytes and monoblasts and binucleate promonocytes. Exome analysis of progressed disease demonstrated mutations in genes and pathways similar to those reported in human CMML. Upon transplantation, recipients developed leukocytosis, monocytosis, and splenomegaly. We reproduce Srsf2/Tet2 co-operativity in vivo, yielding a disease with core characteristics of CMML, unlike single Srsf2 or Tet2 mutation. This model represents a significant step toward building high fidelity and genetically tractable models of CMML.

Genome-wide screening identifies cell-cycle control as a synthetic lethal pathway with SRSF2P95H mutation.

Current strategies to target RNA splicing mutant myeloid cancers proposes targeting the remaining splicing apparatus. This approach has only been modestly sensitizing and is also toxic to non-mutant-bearing wild-type cells. To explore potentially exploitable genetic interactions with spliceosome mutations, we combined data mining and functional screening for synthetic lethal interactions with an Srsf2P95H/+ mutation. Analysis of missplicing events in a series of both human and murine SRSF2P95H mutant samples across multiple myeloid diseases (acute myeloid leukemia, myelodysplastic syndromes, chronic myelomonocytic leukemia) was performed to identify conserved missplicing events. From this analysis, we identified that the cell-cycle and DNA repair pathways were overrepresented within the conserved misspliced transcript sets. In parallel, to functionally define pathways essential for survival and proliferation of Srsf2P95H/+ cells, we performed a genome-wide Clustered regularly interspaced short palindromic repeat loss-of-function screen using Hoxb8 immortalized R26-CreERki/+Srsf2P95H/+ and R26-CreERki/+Srsf2+/+ cell lines. We assessed loss of single guide RNA representation at 3 timepoints: immediately after Srsf2P95H/+ activation, and at 1 week and 2 weeks after Srsf2P95H/+ mutation. Pathway analysis demonstrated that the cell-cycle and DNA damage response pathways were among the top synthetic lethal pathways with Srsf2P95H/+ mutation. Based on the loss of guide RNAs targeting Cdk6, we identified that palbociclib, a CDK6 inhibitor, showed preferential sensitivity in Srsf2P95H/+ cell lines and in primary nonimmortalized lin-cKIT+Sca-1+ cells compared with wild-type controls. Our data strongly suggest that the cell-cycle and DNA damage response pathways are required for Srsf2P95H/+ cell survival, and that palbociclib could be an alternative therapeutic option for targeting SRSF2 mutant cancers.

Modeling human RNA spliceosome mutations in the mouse: not all mice were created equal.

Myelodysplastic syndromes (MDS) and related myelodysplastic/myeloproliferative neoplasms (MDS/MPNs) are clonal stem cell disorders, primarily affecting patients over 65 years of age. Mapping of the MDS and MDS/MPN genome identified recurrent heterozygous mutations in the RNA splicing machinery, with the SF3B1, SRSF2, and U2AF1 genes being frequently mutated. To better understand how spliceosomal mutations contribute to MDS pathogenesis in vivo, numerous groups have sought to establish conditional murine models of SF3B1, SRSF2, and U2AF1 mutations. The high degree of conservation of hematopoiesis between mice and human and the well-established phenotyping and genetic modification approaches make murine models an effective tool with which to study how a gene mutation contributes to disease pathogenesis. The murine models of spliceosomal mutations described to date recapitulate human MDS or MDS/MPN to varying extents. Reasons for the differences in phenotypes reported between alleles of the same mutation are varied, but the nature of the genetic modification itself and subsequent analysis methods are important to consider. In this review, we summarize recently reported murine models of SF3B1, SRSF2, and U2AF1 mutations, with a particular focus on the genetically engineered modifications underlying the models and the experimental approaches applied.