Exonuclease domain mutants of yeast DIS3 display genome instability

Myeloma and DNA damage

ABSTRACT

DNA-damaging agents have represented the first effective treatment for the blood cancer multiple myeloma, and after 65 years since their introduction to the clinic, they remain one of the mainstay therapies for this disease. Myeloma is a cancer of plasma cells. Despite exceedingly slow proliferation, myeloma cells present extended genomic rearrangements and intense genomic instability, starting at the premalignant stage of the disease. Where does such DNA damage stem from? A reliable model argues that the powerful oncogenes activated in myeloma as well the phenotypic peculiarities of cancer plasma cells, including the dependency on the proteasome for survival and the constant presence of oxidative stress, all converge on modulating DNA damage and repair. Beleaguered by these contraposing forces, myeloma cells survive in a precarious balance, in which the robust engagement of DNA repair mechanisms to guarantee cell survival is continuously challenged by rampant genomic instability, essential for cancer cells to withstand hostile selective pressures. Shattering this delicate equilibrium has been the goal of the extensive use of DNA-damaging agents since their introduction in the clinic, now enriched by novel approaches that leverage upon synthetic lethality paradigms. Exploiting the impairment of homologous recombination caused by myeloma genetic lesions or treatments, it is now possible to design therapeutic combinations that could target myeloma cells more effectively. Furthermore, DNA-damaging agents, as demonstrated in solid tumors, may sensitize cells to immune therapies. In all, targeting DNA damage and repair remains as central as ever in myeloma, even for the foreseeable future.

RNA exosome ribonuclease DIS3 degrades Pou6f1 to promote mouse pre-implantation cell differentiation

Mammalian development is precisely controlled by cell differentiation. Identifying new regulators and investigating their interactions provide insight into genetic networks defining pre-implantation development. We established a knockout mouse model of Dis3, an exosome associated ribonuclease. Homozygous Dis3 null embryos arrest at the morula stage of development. Using single-embryo RNA sequencing (RNA-seq), we observed persistence of Pou6f1 mRNA in homozygous null Dis3 embryos and that the cognate protein represses transcription of Nanog and Cdx2. The resultant defects in cell differentiation disrupt the morula-to-blastocyst transition and are embryonic lethal. Microinjection of Dis3 mRNA into zygotes rescues the phenotype. Point mutations of Dis3 ribonuclease in individual blastomeres prevents their incorporation into embryos. To overcome the paucity of embryos, we derived homozygous Dis3 null mouse embryonic stem cells to identify additional gene targets of POU6F1. Our findings delineate a regulatory pathway of DIS3-POU6F1 in pre-implantation mammalian embryogenesis.

DIS3: The Enigmatic Gene in Multiple Myeloma

Recent studies have revealed the genetic aberrations involved in the initiation and progression of various cancers, including multiple myeloma (MM), via next-generation sequencing analysis. Notably, DIS3 mutations have been identified in approximately 10% of patients with MM. Moreover, deletions of the long arm of chromosome 13, that includes DIS3, are present in approximately 40% of patients with MM. Regardless of the high incidence of DIS3 mutations and deletions, their contribution to the pathogenesis of MM has not yet been determined. Herein, we summarize the molecular and physiological functions of DIS3, focusing on hematopoiesis, and discuss the characteristics and potential roles of DIS3 mutations in MM. Recent findings highlight the essential roles of DIS3 in RNA homeostasis and normal hematopoiesis and suggest that the reduced activity of DIS3 may be involved in myelomagenesis by increasing genome instability.

Alternative splicing in multiple myeloma is associated with the non-homologous end joining pathway

Alternative splicing plays a pivotal role in tumorigenesis and proliferation. However, its pattern and pathogenic role has not been systematically analyzed in multiple myeloma or its subtypes. Alternative splicing profiles for 598 newly diagnosed myeloma patients with comprehensive genomic annotation identified primary translocations, 1q amplification, and DIS3 events to have more differentially spliced events than those without. Splicing levels were correlated with expression of splicing factors. Moreover, the non-homologous end joining pathway was an independent factor that was highly associated with splicing frequency as well as an increased number of structural variants. We therefore identify an axis of high-risk disease encompassing expression of the non-homologous end joining pathway, increase structural variants, and increased alternative splicing that are linked together. This indicates a joint pathogenic role for DNA damage response and alternative RNA processing in myeloma.

Nuclear RNA Exosome and Pervasive Transcription: Dual Sculptors of Genome Function

The genome is pervasively transcribed across various species, yielding numerous non-coding RNAs. As a counterbalance for pervasive transcription, various organisms have a nuclear RNA exosome complex, whose structure is well conserved between yeast and mammalian cells. The RNA exosome not only regulates the processing of stable RNA species, such as rRNAs, tRNAs, small nucleolar RNAs, and small nuclear RNAs, but also plays a central role in RNA surveillance by degrading many unstable RNAs and misprocessed pre-mRNAs. In addition, associated cofactors of RNA exosome direct the exosome to distinct classes of RNA substrates, suggesting divergent and/or multi-layer control of RNA quality in the cell. While the RNA exosome is essential for cell viability and influences various cellular processes, mutations and alterations in the RNA exosome components are linked to the collection of rare diseases and various diseases including cancer, respectively. The present review summarizes the relationships between pervasive transcription and RNA exosome, including evolutionary crosstalk, mechanisms of RNA exosome-mediated RNA surveillance, and physiopathological effects of perturbation of RNA exosome.

Landscape of functional interactions of human processive ribonucleases revealed by high-throughput siRNA screenings

Processive exoribonucleases are executors of RNA decay. In humans, their physical but not functional interactions were thoughtfully investigated. Here we have screened cells deficient in DIS3, XRN2, EXOSC10, DIS3L, and DIS3L2 with a custom siRNA library and determined their genetic interactions (GIs) with diverse pathways of RNA metabolism. We uncovered a complex network of positive interactions that buffer alterations in RNA degradation and reveal reciprocal cooperation with genes involved in transcription, RNA export, and splicing. Further, we evaluated the functional distinctness of nuclear DIS3 and cytoplasmic DIS3L using a library of all known genes associated with RNA metabolism. Our analysis revealed that DIS3 mutation suppresses RNA splicing deficiency, while DIS3L GIs disclose the interplay of cytoplasmic RNA degradation with nuclear RNA processing. Finally, genome-wide DIS3 GI map uncovered relations with genes not directly involved in RNA metabolism, like microtubule organization or regulation of telomerase activity.

Yeast RNA exosome activity is necessary for maintaining cell wall stability through proper protein glycosylation

Nuclear RNA exosome is the main 3'→5' RNA degradation and processing complex in eukaryotic cells and its dysregulation therefore impacts gene expression and viability. In this work we show that RNA exosome activity is necessary for maintaining cell wall stability in yeast Saccharomyces cerevisiae. While the essential RNA exosome catalytic subunit Dis3 provides exoribonuclease catalytic activity, the second catalytic subunit Rrp6 has a noncatalytic role in this process. RNA exosome cofactors Rrp47 and Air1/2 are also involved. RNA exosome mutants undergo osmoremedial cell lysis at high temperature or at physiological temperature upon treatment with cell wall stressors. Finally, we show that a defect in protein glycosylation is a major reason for cell wall instability of RNA exosome mutants. Genes encoding enzymes that act in the early steps of the protein glycosylation pathway are down-regulated at high temperature in cells lacking Rrp6 protein or Dis3 exoribonuclease activity and overexpression of the essential enzyme Psa1, that catalyzes synthesis of the mannosylation precursor, suppresses temperature sensitivity and aberrant morphology of these cells. Furthermore, this defect is connected to a temperature-dependent increase in accumulation of noncoding RNAs transcribed from loci of relevant glycosylation-related genes.

Genomic Instability in Multiple Myeloma

Genomic instability (GIN), an increased tendency to acquire genomic alterations, is a cancer hallmark. However, its frequency, underlying causes, and disease relevance vary across different cancers. Multiple myeloma (MM), a plasma cell malignancy, evolves through premalignant phases characterized by genomic abnormalities. Next-generation sequencing (NGS) methods are deconstructing the genomic landscape of MM across the continuum of its development, inextricably linking malignant transformation and disease progression with increasing acquisition of genomic alterations, and illuminating the mechanisms that generate these alterations. Although GIN drives disease evolution, it also creates vulnerabilities such as dependencies on 'superfluous' repair mechanisms and the induction of tumor-specific antigens that can be targeted. We review the mechanisms of GIN in MM, the associated vulnerabilities, and therapeutic targeting strategies.

Nuclear Argonaute Piwi Gene Mutation Affects rRNA by Inducing rRNA Fragment Accumulation, Antisense Expression, and Defective Processing in Drosophila Ovaries

Drosophila key nuclear piRNA silencing pathway protein Piwi of the Argonaute family has been classically studied as a factor controlling transposable elements and fertility. Piwi has been shown to concentrate in the nucleolus for reasons largely unknown. Ribosomal RNA is the main component of the nucleolus. In this work the effect of a piwi mutation on rRNA is described. This work led to three important conclusions: A mutation in piwi induces antisense 5S rRNA expression, a processing defect of 2S rRNA orthologous to the 3′-end of eukaryotic 5.8S rRNA, and accumulation of fragments of all five rRNAs in Drosophilamelanogaster ovaries. Hypotheses to explain these phenomena are proposed, possibly involving the interaction of the components of the piRNA pathway with the RNA surveillance machinery.