The X-linked helicase DDX3X is required for lymphoid differentiation and MYC-driven lymphomagenesis

Diverse mechanisms of DDX3Y suppression by DDX3X

The DEAD-box RNA helicase DDX3X has important roles in development and disease. Loss of DDX3X during developmental and pathological processes such as tumorigenesis can lead to compensatory upregulation of the close paralog DDX3Y in males, which may underlie the sexual dimorphism displayed by some DDX3X-associated diseases. However, how DDX3X cross-regulates DDX3Y remains largely unknown. Here, we investigated the regulation of DDX3Y by DDX3X in two male-derived human cancer cell lines, HCT116 and U87MG. Depletion of DDX3X in HCT116 cells results in moderately increased DDX3Y mRNA and protein, in part due to stabilization of DDX3Y transcripts. Conversely, reduction of DDX3X in U87MG cells markedly upregulates DDX3Y protein without affecting its mRNA, mainly by enhancing DDX3Y protein stability. We further show that DDX3X physically interacts with DDX3Y. DDX3Y is much less stable than DDX3X in U87MG cells, and substitution of two lysine residues in DDX3Y with the corresponding arginine in DDX3X stabilizes DDX3Y. Thus, the compensatory upregulation of DDX3Y following DDX3X loss can occur at either transcript or protein level, suggesting complex and cell type-specific cross-regulation between these X- and Y-linked paralogs to keep the total DDX3 dosage in check.

DDX3X Activates Chondrocyte Pyroptosis to Promote Osteoarthritis Progression

The RNA-binding protein DDX3X is associated with several biological processes including inflammation and immunity. However, the role of DDX3X in the pathology of inflammation-related osteoarthritis (OA) remains unclear. This study was to explore the action of DDX3X in the progression of OA as well as the underlying mechanisms by using RNA immunoprecipitation (RIP), Immunohistochemical (IHC) and DDX3X knockout mice, etc. We found that DDX3X expression was upregulated in cartilage tissue of OA patient. The in vitro study also showed upregulation of DDX3X in the inflammatory chondrocytes stimulated by LPS. DDX3X overexpression reduced cell viability by inducing pyroptosis in chondrocytes. Knockdown of DDX3X rescued LPS-induced chondrocytes pyroptosis through regulating NLRP3 signaling. In addition, DDX3X deletion attenuates osteoarthritis in vivo. In conclusion, DDX3X promotes OA progression by regulating chondrocytes pyroptosis via the activation of NLRP3 signaling.

RNA binding proteins in cardiovascular development and disease

Congenital heart disease (CHD) is the most common birth defect affecting>1.35 million newborn babies worldwide. CHD can lead to prenatal, neonatal, postnatal lethality or life-long cardiac complications. RNA binding protein (RBP) mutations or variants are emerging as contributors to CHDs. RBPs are wizards of gene regulation and are major contributors to mRNA and protein landscape. However, not much is known about RBPs in the developing heart and their contributions to CHD. In this chapter, we will discuss our current knowledge about specific RBPs implicated in CHDs. We are in an exciting era to study RBPs using the currently available and highly successful RNA-based therapies and methodologies. Understanding how RBPs shape the developing heart will unveil their contributions to CHD. Identifying their target RNAs in the embryonic heart will ultimately lead to RNA-based treatments for congenital heart disease.

The variant landscape and function of DDX3X in cancer and neurodevelopmental disorders

RNA molecules rely on proteins across their life cycle. DDX3X encodes an X-linked DEAD-box RNA helicase with a Y-linked paralog, DDX3Y. DDX3X is central to the RNA life cycle and is implicated in many conditions, including cancer and the neurodevelopmental disorder DDX3X syndrome. DDX3X-linked conditions often exhibit sex differences, possibly due to differences between expression or function of the X- and Y-linked paralogs DDX3X and DDX3Y. DDX3X-related diseases have different mutational landscapes, indicating different roles of DDX3X. Understanding the role of DDX3X in normal and disease states will inform the understanding of DDX3X in disease. We review the function of DDX3X and DDX3Y, discuss how mutation type and sex bias contribute to human diseases involving DDX3X, and review possible DDX3X-targeting treatments.

The RNA helicase DDX3 and its role in c-MYC driven germinal center-derived B-cell lymphoma

DDX3X is an RNA helicase with many functions in RNA metabolism such as mRNA translation, alternative pre-mRNA splicing and mRNA stability, but also plays a role as a regulator of transcription as well as in the Wnt/beta-catenin- and Nf-κB signaling pathways. The gene encoding DDX3X is located on the X-chromosome, but escapes X-inactivation. Hence females have two active copies and males only one. However, the Y chromosome contains the gene for the male DDX3 homologue, called DDX3Y, which has a very high sequence similarity and functional redundancy with DDX3X, but shows a more restricted protein expression pattern than DDX3X. High throughput sequencing of germinal center (GC)-derived B-cell malignancies such as Burkitt Lymphoma (BL) and Diffuse large B-cell lymphoma (DLBCL) samples showed a high frequency of loss-of-function (LOF) mutations in the DDX3X gene revealing several features that distinguish this gene from others. First, DDX3X mutations occur with high frequency particularly in those GC-derived B-cell lymphomas that also show translocations of the c-MYC proto-oncogene, which occurs in almost all BL and a subset of DLBCL. Second, DDX3X LOF mutations occur almost exclusively in males and is very rarely found in females. Third, mutations in the male homologue DDX3Y have never been found in any type of malignancy. Studies with human primary GC B cells from male donors showed that a loss of DDX3X function helps the initial process of B-cell lymphomagenesis by buffering the proteotoxic stress induced by c-MYC activation. However, full lymphomagenesis requires DDX3 activity since an upregulation of DDX3Y expression is invariably found in GC derived B-cell lymphoma with DDX3X LOF mutation. Other studies with male transgenic mice that lack Ddx3x, but constitutively express activated c-Myc transgenes in B cells and are therefore prone to develop B-cell malignancies, also showed upregulation of the DDX3Y protein expression during the process of lymphomagenesis. Since DDX3Y is not expressed in normal human cells, these data suggest that DDX3Y may represent a new cancer cell specific target to develop adjuvant therapies for male patients with BL and DLBCL and LOF mutations in the DDX3X gene.

Lessons from Using Genetically Engineered Mouse Models of MYC-Induced Lymphoma

Oncogenic overexpression of MYC leads to the fatal deregulation of signaling pathways, cellular metabolism, and cell growth. MYC rearrangements are found frequently among non-Hodgkin B-cell lymphomas enforcing MYC overexpression. Genetically engineered mouse models (GEMMs) were developed to understand MYC-induced B-cell lymphomagenesis. Here, we highlight the advantages of using Eµ-Myc transgenic mice. We thoroughly compiled the available literature to discuss common challenges when using such mouse models. Furthermore, we give an overview of pathways affected by MYC based on knowledge gained from the use of GEMMs. We identified top regulators of MYC-induced lymphomagenesis, including some candidates that are not pharmacologically targeted yet.