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Xu, Z.

Publications and source records attributed to Xu, Z..

At least 181 records · Page 10Linked to original sources

In-silico modeling and interpretation of RBP binding disentangle m6A-RBP interaction

RNA binding protein (RBP) binding and N6-methyladenosine (m6A) are both essential post-transcriptional regulatory layers for RNA fate decisions. However, the intricate mechanism underlying the interaction between m6A and RBP binding remains underexplored. Here, we develop TransRBP, an interpretable deep learning framework, to model the base-resolution binding of RBPs from RNA sequences and to subsequently investigate the interaction between m6A and RBPs. TransRBP achieves a median accuracy of 0.59 across 32 m6A-related RBPs, representing a 28% increase over the state-of-the-art model. Using gradient-based interpretation, we demonstrate that the binding motifs of the m6A-related RBPs strongly enrich for splicing consensus, laying a foundation for studying the RBP-dependent crosstalk between m6A and splicing. Moreover, we develop an in-silico mutagenesis assay to assess the impact of m6A on RBPs, and utilize the self-attention mechanism to elucidate the interplay between RBP binding and m6A. We further uncover 1,806 variant-RBP combinations with the in-silico mutagenesis, revealing variants that strongly alter RBP binding for genetic diseases including Parkinsons disease, autism, and cardiomyopathy. In particular, we identify m6A cis-acting variants that alter RBP binding in an m6A-proximal manner, including the binding of UPF1 that contributes to Alzheimers disease, and the DDX3X binding to cardiomyopathy and muscular dystrophy. Together, TransRBP accurately models the binding of RBP and its interaction with m6A, shedding light on the m6A-RBP dynamics and providing multi-layer mechanistic insights for genetic diseases.

bioinformatics↗

Microglia-Dependent and Independent Modulation of Brain Lipid Metabolism in Alzheimer's Disease Revealed by Pharmacological and Genetic Microglial Depletion

Abnormal lipid metabolism in Alzheimers disease (AD) was first documented by Alois Alzheimer in his early observations of patients with a then unrecognized brain disease, which he noted was characterized by a significant presence of "adipose inclusions" or "lipoid granules". Despite this early recognition, until recently the significance of abnormal lipid metabolism in AD has been largely overlooked by the scientific community for decades, highlighting a critical gap in our understanding of this complex disease. In the past decade, numerous loci and genes with genome-wide significant evidence of affecting AD risk have been reported. Notably, a significant portion of these AD risk genes are either preferentially or exclusively expressed by microglia in the brain and/or code for enzymes that directly or indirectly regulate lipid metabolism. This suggests a major, yet uncharacterized, role of microglia in modulating brain lipid metabolism under AD pathological conditions. In our study, we dissected microglia-dependent and independent regulation of lipid metabolism in an AD-like mouse model of amyloidosis, 5xFAD, taking advantage of pharmacological and genetic interventions to eliminate microglia. Using multidimensional mass spectrometry-based shotgun lipidomics (MDMS-SL), we identified overt changes in a number of AD-associated lipids (ADALs) in postmortem patient brains and mouse models of amyloidosis. This included bis(monoacylglycerol)phosphate (BMP), a lipid class enriched in endosomal/lysosomal compartments, and the two most abundant classes of lysophospholipids: lysophosphatidylcholine (LPC) and lysophosphatidylethanolamine (LPE), which are commonly associated with inflammation. Our findings revealed that microglial depletion prevented the accumulation of arachidonic acid-containing BMP species, which are associated with lysosomal activation induced by amyloidosis via a mechanism that involves progranulin, coded by AD risk gene GRN, as shown by targeted transcriptomics, immunoblotting, and immunofluorescence. Surprisingly, AD-associated LPC and LPE accumulation was not driven by microglia. Instead, LPC accumulation correlated with astrocytic activation, while LPE accumulation seems to be associated with oxidative stress. In summary, we uncovered novel microglia-dependent and independent mechanisms that drive lipid dysregulation in AD. These findings may be mechanistically linked with the early glial lipoid deposits described by Dr. Alzheimer. O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=133 SRC="FIGDIR/small/624173v1_ufig1.gif" ALT="Figure 1"> View larger version (41K): org.highwire.dtl.DTLVardef@6fe55aorg.highwire.dtl.DTLVardef@3a7d10org.highwire.dtl.DTLVardef@1e8c247org.highwire.dtl.DTLVardef@2ac75f_HPS_FORMAT_FIGEXP M_FIG O_FLOATNOGraphical Abstract.C_FLOATNO C_FIG

neuroscience↗

Functional Connectivity Alterations in Cocaine Use Disorder: Insights from the Triple Network Model and the Addictions Neuroclinical Assessment Framework

Cocaine use disorder (CUD) disrupts functional connectivity within key brain networks, specifically the default mode network (DMN), salience network (SN), and central executive network (CEN). While the triple network model has been proposed to explain various psychiatric disorders, its applicability to CUD requires further exploration. In the present study, we built machine learning classifiers based on different combinations of DMN/SN/CEN to distinguish cocaine-use disorder (CUD) subjects from healthy control (HC) subjects. Among them, the combination of the SN and the CEN results in a remarkably high accuracy of 73.4% (sensitivity/specificity: 69.6%/78.6%, AUC: 0.78), outperforming the model based on the full triple network. This supports the hypothesis that during the binge/intoxication stage of addiction, the SN and the CEN play a more critical role than the DMN, consistent with the Addictions Neuroclinical Assessment (ANA) framework. Functional connectivity analysis revealed decreased connectivity within the DMN and the SN and increased connectivity within the CEN in CUD patients, suggesting that alterations in these networks could serve as biomarkers for addiction severity.

neuroscience↗

Morphological brain alterations and morphological brain network disorganizations in heroin and methamphetamine abstinent patients

Heroin and methamphetamine are the two common types of drugs abused, which poses significant health risks. However, the neural mechanisms underlying the effects of drug addiction on human brain are unclear. In this study, we collected T1-weighted magnetic resonance imaging data from 26 heroin abstinent (HA) patients, 24 methamphetamine abstinent (MA) patients and 32 healthy controls. Four surface-based morphological features including cortical thickness (CT), fractal dimension (FD), gyrification index (GI), and sulcal depth (SD) were calculated, and further used to construct the morphological brain networks. We observed the common CT reductions of the right TE 1.0 and TE 1.2 and SD reductions of the right intermediate lateral area 20 for HA and MA patients, HA-specific CT reductions in the left area 2, and the MA-specific GI reductions in the left medial area 6 and right dorsomedial parietooccipital sulcus. For the morphological brain networks, HA patients exhibited the global disorganizations (higher shortest path length) in CT-based networks, whereas MA patients showed the disrupted nodal efficiency of the left medial area 38 in CT-based networks, the right caudal area 7 in GI-based networks, and the right inferior occipital gyrus in SD-based networks. Furthermore, the altered SD of HA patients and disrupted nodal efficiency of MA patients were associated with drug abuse-related clinical variables. Our findings suggest the morphological index-dependent effects of drug addiction on human brain morphology, and indicate the differential neural mechanism underlying heroin and methamphetamine abuses which attack the global and local information transfer of morphological brain networks, respectively.

neuroscience↗

AI Assisted Native Proteomics: Delineating Ribosomal Protein Conformations Pre- and Post-Assembly

The simultaneous identification of proteins as well as their conformations in a biological system would greatly enhance our understanding of cellular mechanisms and disease. As an emerging technique, native proteomics analyzes proteins in their native states, facilitating the acquisition of protein stoichiometry, post-translational modifications (PTMs), and interactions with ligands. However, revealing protein conformations at the proteome scale remains a significant challenge. In this study, we propose an AI-assisted native proteomics method that integrates a protein structure prediction (PSP) module with top-down proteomics (TDP) and native mass spectrometry (nMS) to acquire both proteome identity and conformations. First, protein sequences and PTMs are obtained using the TDP method, while protein solvent-accessible surface area (SASA) is measured in parallel by nMS. These data are then input into the PSP module to acquire the most probable conformation of the protein under nMS experimental conditions. We validate the feasibility and accuracy of this method through the analysis of both a globular protein and an intrinsically disordered protein. Additionally, we apply this approach to delineate the conformations of ribosomal proteins pre- and post-assembly. Interactions of ribosomal proteins with drug molecules are also explored. By enabling proteome identification and conformation characterization from relatively small amounts of endogenous proteins, this method would bridge the gap between structural biology and conventional proteomics technologies.

molecular biology↗

Structural basis of gap-filling DNA synthesis in the nucleosome by DNA Polymerase β

Single-strand breaks (SSBs) are one of the most prevalent forms of DNA damage found in the chromatinized genome and are repaired by direct single-strand break repair (SSBR) or base excision repair (BER). DNA polymerase beta (Pol {beta}) is the primary enzyme responsible for processing the 1-nt gap intermediate in chromatin during SSBR and BER. However, the mechanism used by Pol {beta} to process a 1-nt gap in the context of the nucleosome and chromatin remains poorly understood. Here, we use biochemical assays and cryogenic electron microscopy (cryo-EM) to determine the kinetic and structural basis of gap-filling DNA synthesis in the nucleosome by Pol {beta}. Kinetic analysis identified that gap-filling DNA synthesis in the nucleosome by Pol {beta} is position-dependent, where solvent exposed 1-nt gaps are processed more efficiently than histone-occluded 1-nt gaps. A series of cryo-EM structures of Pol {beta} bound to a solvent-exposed 1-nt gap in the nucleosome reveal a global DNA sculpting mechanism for 1-nt gap recognition, which is mediated by sequential engagement of the Pol {beta} lyase domain and polymerase domain. Finally, cryo-EM structures of Pol {beta} bound to 1-nt gaps at two additional positions in the nucleosomal DNA define the structural basis for position-dependent nucleotide insertion in the nucleosome. This work establishes the mechanism used by Pol {beta} for processing 1-nt gaps in the nucleosome during SSBR and BER, providing fundamental insight into DNA repair in chromatin.

biochemistry↗

Critical assay parameters facilitating confident detection of expression changes, fusions and short variants in RNA isolated from tissue

Molecular assays based on next-generation sequencing of RNA can provide clinically relevant information by measuring gene expression levels, identifying gene fusions, aberrant transcript isoforms and detecting small variants. Nevertheless, achieving good performance and reliable result interpretation present a significant challenge. In this work, we dissect the impact of various technical factors on the concurrent detection of several biomarker types using a hybridization enrichment-based targeted RNA sequencing applied to a cohort of more than one hundred samples derived from diverse solid tumors. We demonstrate that several critical parameters inferred from the sequencing data should be controlled to interpret the results accurately. These include molecular coverage of reference genes as a proxy of RNA conversion, DNA content measure, and the extent of target molecular coverage. We summarize our findings with recommendations allowing maximum reliable information recovery from targeted RNA sequencing data.

cancer biology↗

Embryonic Mast Cells Arise from Cpa3-expressing Precursors Independent of Granulocyte-Monocyte Progenitors

Most of the mast cells (MCs) in connective tissues, such as skin, are long-lived embryonic-derived immune cells that play important roles in host defense and various immunological diseases, including allergies. Their embryonic origin and ontogeny remain to be fully studied since several overlapping waves of embryonic hematopoiesis have been shown to give rise to these cells. Here, combining single-cell RNA sequencing and new genetic fate mapping models, we identified a Cpa3-expressing population sequentially appearing in the yolk sac, fetal liver, and peripheral tissues which gives rise to dermal MCs during embryonic days 11.5 to 14.5. Using in vitro differentiation and in vivo transplant methods, we identified a Ter119-F4/80- CD45+CD117+CD16/32+CD135-CD115-Ly6C-CD34lo population as potential fetal liver MC precursors. Fate mapping with Cpa3CreERT2and Zbtb16CreERT2 models, as well as the granulocyte-monocyte progenitor (GMP) models Ms4a3Cre and ElaneCre, demonstrated that MCs arise from Cpa3+ progenitors rather than Ms4a3+ or Elane+ GMPs. A corresponding population with a similar developmental trajectory was also identified in human early yolk sac and fetal liver, suggesting a conserved MC developmental program across species. These findings delineate the detailed developmental path of MCs in embryos, permitting future functional studies in immunity and development. HighlightsO_LICpa3-expressing cells in the yolk sac and fetal liver contain mast cell precursors C_LIO_LICpa3CreERT2 labels embryonic mast cells and their precursors C_LIO_LIEmbryonic mast cells arise through Cpa3+ mast cell precursors, but not Elane+/Ms4a3+ GMPs C_LIO_LIThe seeding of MCPs to embryonic skin slows down after E14.5 C_LI

immunology↗

Mapping intratumoral myeloid-T cell interactomes at single-cell resolution reveals targets for overcoming checkpoint inhibitor resistance

Effective cancer immunotherapies restore anti-tumor immunity by rewiring cell-cell communication. Treatment-induced changes in communication can be inferred from single-cell RNA-sequencing (scRNA-seq) data, but current methods do not effectively manage heterogeneity within cell types. Here we developed a computational approach to efficiently analyze scRNA-seq-derived, single-cell-resolved cell-cell interactomes, which we applied to determine how agonistic CD40 (CD40ag) alters immune cell crosstalk alone, across tumor models, and in combination with immune checkpoint blockade (ICB). Our analyses suggested that CD40ag improves responses to ICB by targeting both immuno-stimulatory and immunosuppressive macrophage subsets communicating with T cells, and we experimentally validated a spatial basis for these subsets with immunofluorescence and spatial transcriptomics. Moreover, treatment with CD40ag and ICB established coordinated myeloid-T cell interaction hubs that are critical for reestablishing antitumor immunity. Our work advances the biological significance of hypotheses generated from scRNA-seq-derived cell-cell interactomes and supports the clinical translation of myeloid-targeted therapies for ICB-resistant tumors.

systems biology↗

Cryo-electron microscopy reveals a single domain antibody with a unique binding epitope on fibroblast activation protein alpha

Fibroblast activation protein alpha (FAP) is a serine protease that is expressed at basal levels in benign tissues but is overexpressed in a variety of pathologies, including cancer. Despite this unique expression profile, designing effective diagnostic and therapeutic agents that effectively target this biomarker remain elusive. Here we report the structural characterization of the interaction between a novel single domain antibody (sdAbs), I3, and FAP using cryo-electron microscopy. The reconstructions were determined to a resolution of 2.7 [A] and contained two distinct populations; one I3 bound and two I3 molecules bound to the FAP dimer. In both cases, the sdAbs bound a unique epitope that was distinct from the active site of the enzyme. Furthermore, this report describes the rational mutation of specific residues within the complementarity determining region 3 (CDR3) loop to enhance affinity and selectivity of the I3 molecule for FAP. This report represents the first sdAb-FAP structure to be described in the literature.

biochemistry↗

DEMINERS enables clinical metagenomics and comparative transcriptomic analysis by increasing throughput and accuracy of nanopore direct RNA sequencing

Nanopore direct RNA sequencing (DRS) advances RNA biology but is limited by relatively low basecalling accuracy, low throughput, yet high RNA input and costs. Here we introduce a novel DRS toolkit, DEMINERS, which integrates an RNA multiplexing experimental workflow, a machine-learning barcode classifier based on Random Forest and a novel basecaller built on an optimized convolutional neural network providing an additional species-specific training module. With the increased accuracy in barcode classification and basecalling, DEMINERS can demultiplex up to 24 samples and the required RNA input and running time are both substantially reduced. We demonstrated the applications of DEMINERS in clinical metagenomics, cancer transcriptomics and parallel comparison of transcriptomic features in different biological conditions, revealing altered airway microbial diversity in COVID-19 and a potential role of m6A in increasing transcriptomic diversity in glioma and the mature blood-stage of malaria parasites. Overall, DEMINERS is a simple, robust, high-throughput DRS method for accurately estimating transcript levels, poly(A) lengths, and mutation and RNA modification heterogeneity at single-read level, with minimal sequencing biases.

bioinformatics↗

RNAi epimutations conferring antifungal drug resistance are inheritable

Epimutations modify gene expression and lead to phenotypic variation while the encoding DNA sequence remains unchanged. Epimutations mediated by RNA interference (RNAi) and/or chromatin modifications can confer antifungal drug resistance and may impact virulence traits in fungi. However, whether these epigenetic modifications can be transmitted across generations following sexual reproduction was unclear. This study demonstrates that RNAi epimutations conferring antifungal drug resistance are transgenerationally inherited in the human fungal pathogen Mucor circinelloides. Our research revealed that RNAi-based antifungal resistance follows a DNA sequence-independent, non-Mendelian inheritance pattern. Small RNAs (sRNAs) are the exclusive determinants of inheritance, transmitting drug resistance independently of other known repressive epigenetic modifications. Unique sRNA signature patterns can be traced through inheritance from parent to progeny, further supporting RNA as an alternative molecule for transmitting information across generations. Understanding how epimutations occur, propagate, and confer resistance may enable their detection in other eukaryotic pathogens, provide solutions for challenges posed by rising antimicrobial drug resistance (AMR), and advance research on phenotypic adaptability and its evolutionary implications.

microbiology↗

Whole-Process 3D ECM-Encapsulated Organoid-Based Automated High-Throughput Screening Platform Accelerates Drug Discovery for Rare Diseases

The use of organoids, especially patient-derived organoids, for high-throughput screening (HTS) is widely accepted due to their ability to mimic the three-dimensional (3D) structure, function, and drug responses of in vivo tissues. However, the complexity of handling extracellular matrix (ECM) components with traditional HTS devices leads to the utilization of suspension cultures in matrix-free or matrix-low conditions during HTS, which can alter their transcriptomic landscape and drug responses. Here, we develop a whole-process 3D ECM-encapsulated organoid-based automated HTS (wp3D-OAHTS) platform, which enables the rapid and accurate generation of uniformly distributed 3D cell-matrix mixture domes at the center of each well in 96-well plates. This approach replicates the process of manual organoid culture but with superior stability and reproducibility. Utilizing this platform, we screened 2,802 compounds on neuroendocrine cervical cancer organoids, a rare malignancy with significant unmet clinical needs. We identified 7 top hits that display strong anti-tumor effects with remarkably low half-maximal inhibitory concentration (IC50) and validated the in vivo efficacy of Quisinostat 2HCl. Additionally, we demonstrated that employing 3D ECM-encapsulated organoid cultures for HTS, rather than suspended cultures, provides optimal conditions for drug discovery. Our wp3D-OAHTS platform significantly improves the rapidity and efficiency of new drug discovery for rare diseases.

bioengineering↗

Heterogeneity in chromatin structure drives core regulatory pathways in B-cell Acute Lymphoblastic Leukemia

B-cell acute lymphoblastic leukemia (B-ALL) is the most common pediatric malignancy. Based on gene expression profiling, B-ALL can be classified into distinct transcriptional subtypes with differing disease outcomes. Many of these transcriptional subtypes are defined by mutations in transcription factors and chromatin-modifying enzymes, but how such diverse mutations lead to distinct transcriptional subtypes remains unclear. To illuminate the chromatin regulatory landscape in B-ALL, we analyzed 3D genome organization, open chromatin, and gene expression in 53 primary patient samples. At the level of 3D genome organization, we identified chromatin interactions that vary across transcriptional subtypes. These sites of variable 3D chromatin interactions correlate with local gene expression changes and are enriched for core drivers of B-ALL observed in genome-wide CRISPR knock-out screens. Sites of variable 3D genome interactions are frequently shared across multiple transcriptional subtypes and are enriched for open chromatin sites found in normal B-cell development but repressed in mature B-cells. Within an individual patient sample, the chromatin landscape can resemble progenitor chromatin states at some loci and mature B-cell chromatin at others, suggesting that the chromatin in B-ALL patient tumor cells is in a partially arrested immature state. By analyzing transcriptomic data from large cohorts of B-ALL patients, we identify gene expression programs that are shared across transcriptional subtypes, associated with B-cell developmental stages, and predictive of patient survival. In combination, these results show that the 3D genome organization of B-ALL reflects B-cell developmental stages and helps illustrate how B-cell developmental arrest interacts with transcriptional subtypes to drive B-ALL.

cancer biology↗

A STAG2-PAXIP1/PAGR1 axis suppresses lung tumorigenesis

The cohesin complex is a critical regulator of gene expression. STAG2 is the most frequently mutated cohesin subunit across several cancer types and is a key tumor suppressor in lung cancer. Here, we coupled somatic CRISPR-Cas9 genome editing and tumor barcoding with an autochthonous oncogenic KRAS-driven lung cancer model and show that STAG2 is uniquely tumor suppressive among all core and auxiliary cohesin components. The heterodimeric complex components PAXIP1 and PAGR1 have highly correlated effects with STAG2 in human lung cancer cell lines, are tumor suppressors in vivo, and are epistatic to STAG2 in oncogenic KRAS-driven lung tumorigenesis in vivo. STAG2 inactivation elicits changes in gene expression, chromatin accessibility and 3D genome conformation that impact cancer cell state. Gene expression and chromatin accessibility similarities between STAG2- and PAXIP1-deficient neoplastic cells further relates STAG2-cohesin to PAXIP1/PAGR1. These findings reveal a STAG2-PAXIP1/PAGR1 tumor-suppressive axis and uncover novel PAXIP1-dependent and PAXIP1-independent STAG2-cohesin mediated mechanisms of lung tumor suppression. SUMMARYSTAG2 is a frequently mutated cohesin subunit across several cancers and one of the most important functional suppressors of lung adenocarcinoma. Our findings underscore important roles of STAG2 in suppressing lung tumorigenesis and highlight a STAG2-PAXIP1/PAGR1 tumor-suppressive program that may transcend cancer type.

cancer biology↗

Modulation of blood-tumor barrier transcriptional programs improves intra-tumoral drug delivery and potentiates chemotherapy in GBM

Glioblastoma (GBM) is the most common malignant primary brain tumor. GBM has an extremely poor prognosis and new treatments are badly needed. Efficient drug delivery to GBM is a major obstacle as the blood-brain barrier (BBB) prevents passage of the majority of cancer drugs into the brain. It is also recognized that the blood-brain tumor barrier (BTB) in the growing tumor represents a challenge. The BTB is heterogeneous and poorly characterized, but similar to the BBB it can prevent therapeutics from reaching effective intra-tumoral doses, dramatically hindering their potential. Here, we identified a 12-gene signature associated with the BTB, with functions related to vasculature development, morphogenesis and cell migration. We identified CDH5 as a core molecule in this set and confirmed its over-expression in GBM vasculature using spatial transcriptomics of GBM patient specimens. We found that the indirubin-derivative, 6-bromoindirubin acetoxime (BIA), could downregulate CDH5 and other BTB signature genes, causing endothelial barrier disruption in endothelial monolayers and BBB 3D spheroids in vitro. Treatment of tumor-bearing mice with BIA enabled increased intra-tumoral accumulation of the BBB non-penetrant chemotherapeutic drug cisplatin and potentiated cisplatin-mediated DNA damage by targeting DNA repair pathways. Finally, using an injectable BIA nanoparticle formulation, PPRX-1701, we significantly improved the efficacy of cisplatin in patient-derived GBM xenografts and prolonged their survival. Overall, our work reveals potential targets at the BTB for improved chemotherapy delivery and the bifunctional properties of BIA as a BTB modulator and potentiator of chemotherapy, supporting its further development.

cancer biology↗

High-throughput Image-based Clustering of CAR-T/Tumor Cocultures for Rapid and Facile Hit Identification

AO_SCPLOWBSTRACTC_SCPLOWChimeric antigen receptor T cell is important because of its potential to treat various diseases. As deep learning continues to advance, using unsupervised methods to classify medical images has become a significant focus because collecting high-quality labeled data for medical images is labor-intensive and time-consuming. Beyond the need for accurate labeling, there is a desire to explore the underlying characteristics of the data, even when labels may be ambiguous or uncertain. To address these challenges, we present a novel approach that combines image clustering with an insightful explanation of how these clusters are formed. Our method employs a U-net combined with a clustering algorithm to segment the dataset into different groups. After clustering, we use various techniques to interpret and elucidate the results. Moreover, our paper introduces a unique dataset focused on cell data, specifically highlighting the developmental patterns of cancer cells and T cells under various experimental conditions. This dataset offers a rich source of information and presents a complex challenge for image classification due to the diversity of conditions and cell behaviors involved. Our study thoroughly compares different architectural models on this new dataset, demonstrating the superior performance of our proposed architecture. Through experimental analysis and ablation studies, we provide substantial evidence of the benefits offered by our architecture, not only in terms of accuracy but also in its ability to reveal deeper insights into the data. This work advances the field of image classification and opens new possibilities for understanding complex biological processes through computer vision.

cell biology↗

Distinct evolutionary trajectories following loss of RNA interference in Cryptococcus neoformans

While increased mutation rates typically have negative consequences in multicellular organisms, hypermutation can be advantageous for microbes adapting to the environment. Previously, we identified two hypermutator Cryptococcus neoformans clinical isolates that rapidly develop drug resistance due to transposition of a retrotransposon, Cnl1. Cnl1-mediated hypermutation is caused by a nonsense mutation in the gene encoding a novel RNAi component, Znf3, combined with a tremendous transposon burden. To elucidate adaptative mechanisms following RNAi loss, two bioinformatic pipelines were developed to identify RNAi loss-of-function mutations in a collection of 387 sequenced C. neoformans isolates. Remarkably, several RNAi-loss isolates were identified that are not hypermutators and have not accumulated transposons. To test if these RNAi loss-of-function mutations can cause hypermutation, the mutations were introduced into a non-hypermutator strain with a high transposon burden, which resulted in a hypermutator phenotype. To further investigate if RNAi-loss isolates can become hypermutators, in vitro passaging was performed. Although no hypermutators were found in two C. neoformans RNAi-loss strains after short-term passage, hypermutation was observed in a passaged C. deneoformans strain with increased transposon burden. Consistent with a two-step evolution, when an RNAi-loss isolate was crossed with an isolate containing a high Cnl1 burden, F1 hypermutator progeny inheriting a high transposon burden were identified. In addition to Cnl1 transpositions, insertions of a novel gigantic DNA transposon KDZ1 ([~]11 kb), contributed to hypermutation in the progeny. Our results suggest that RNAi loss is relatively common (7/387, [~]1.8%) and enables distinct evolutionary trajectories: hypermutation following transposon accumulation or survival without hypermutation. Significance StatementThere is a dearth of antifungal drugs available to treat Cryptococcus neoformans, a human fungal pathogen of global impact. We previously identified natural hypermutators with a loss-of-function mutation in the RNAi machinery and transposon expansion. Here, we identified several novel natural isolates with RNAi defects, none of which are hypermutators or have undergone transposon expansion. Furthermore, we demonstrate that these isolates can lie on a pathway to hypermutation following introduction of a transposon burden. In addition, a novel DNA transposon class was discovered that contributes to antifungal drug resistance. These findings highlight the importance of transposons in driving rapid adaptation in the absence of RNAi and reveal distinct evolutionary trajectories following RNAi loss, a relatively common event in C. neoformans.

genetics↗