Cancer synthetic vulnerabilities to protein arginine methyltransferase inhibitors

Protein arginine methylation is an abundant post-translational modification involved in the modulation of essential cellular processes ranging from transcription, post-transcriptional RNA metabolism, and propagation of signaling cascades to the regulation of the DNA damage response. Excitingly for the field, in the past few years there have been remarkable advances in the development of molecular tools and clinical compounds able to selectively and potently inhibit protein arginine methyltransferase (PRMT) functions. In this review, we first discuss how the somatic mutations that confer advantages to cancer cells are often associated with vulnerabilities that can be exploited by PRMTs' inhibition. In a second part, we discuss strategies to uncover synthetic lethal combinations between existing therapies and PRMT inhibitors.

Cytokine receptor splice variants in hematologic diseases

Cytokine and cytokine receptors are important regulators of hematopoiesis. Hematopoietic stem cells (HSCs) and progenitors differentiate into the myeloid or lymphoid lineage in response to specific cytokines. Cell-type specific receptors are expressed on committed progenitors that bind to other late-acting cytokines that are involved in terminal differentiation of hematopoietic cells. In normal hematopoiesis, these receptors undergo alternative splicing and are developmentally regulated. Splicing changes can significantly affect the structure and function of the receptors resulting in alterations of either the extracellular ligand binding domain or the cytoplasmic signaling domain responsible for cellular growth and differentiation. Most alternatively spliced isoforms generally lose the ability to promote differentiation. Evidently, overexpression of naturally occurring cytokine receptor alternate isoforms are observed in multiple myeloid diseases such as myelodysplastic syndromes (MDS), acute myeloid leukemia (AML), and polycythemia vera (PV). The purpose of this review is to introduce the various isoforms of key cytokine receptors that play a crucial role in myeloid development and their potential role in myeloid diseases.

Therapeutic Targeting of RNA Splicing Catalysis through Inhibition of Protein Arginine Methylation

Cancer-associated mutations in genes encoding RNA splicing factors (SFs) commonly occur in leukemias, as well as in a variety of solid tumors, and confer dependence on wild-type splicing. These observations have led to clinical efforts to directly inhibit the spliceosome in patients with refractory leukemias. Here, we identify that inhibiting symmetric or asymmetric dimethylation of arginine, mediated by PRMT5 and type I protein arginine methyltransferases (PRMTs), respectively, reduces splicing fidelity and results in preferential killing of SF-mutant leukemias over wild-type counterparts. These data identify genetic subsets of cancer most likely to respond to PRMT inhibition, synergistic effects of combined PRMT5 and type I PRMT inhibition, and a mechanistic basis for the therapeutic efficacy of PRMT inhibition in cancer.

[Dyserythropoiesis in myelodysplastic syndrome]

Myelodysplastic syndrome (MDS) is characterized by ineffective hematopoiesis including dyserythropoiesis. Recently, several signaling pathways have been implicated in dyserythropoiesis in MDS, such as the p53-S100A8/9-TLR4 pathway, which is involved in ineffective erythropoiesis in 5q- syndrome. Somatic mutations that target SF3B1, which encodes a component of the mRNA splicing machinery, have been identified in approximately 85% of patients with MDS presenting with ring sideroblasts (MDS-RS). SF3B1 mutations confer a change-of-function and cause aberrant splicing of genes that may be involved in the pathogenesis of MDS-RS. Recurrent mutations have also been identified in epigenetic regulator genes in MDS, including polycomb repressive complex 2 (PRC2) genes, and the loss of Ezh2, an enzymatic component of PRC2, enhances ineffective hematopoiesis and induces impaired erythropoiesis. A better understanding of the molecular mechanisms underlying dyserythropoiesis in MDS may lead to innovative novel therapeutic modalities.