The phenotypic impact of nonsense variants is determined by nonsense-mediated mRNA decay (NMD), which degrades transcripts with premature termination codons (PTCs). Despite the clinical importance of nonsense variants, transcript-specific and context-dependent variations in NMD activity remain poorly understood. Here, we show that the amino acid preceding the PTC strongly influences NMD activity. Glycine codons promote robust NMD efficiency and show striking enrichment before PTCs but are depleted before normal termination codons. Glycine-PTC enrichment is particularly pronounced in genes tolerant to loss-of-function variants, suggesting efficient elimination of truncated proteins from nonessential genes. We further demonstrate that the peptide release rate during translation termination is an important determinant of NMD activity. We propose a "window of opportunity" model where translation termination kinetics modulate NMD activity. By revealing how sequence context shapes NMD activity through translation termination dynamics, our findings provide a mechanistic framework for improved clinical interpretation of nonsense variants.

Invasive lobular carcinoma of the breast (ILC) is typically estrogen receptor α (ER)-positive and presents with biomarkers of anti-estrogen sensitive disease, yet patients with ILC face particularly poor long-term outcomes with increased recurrence risk, suggesting endocrine response and ER function are unique in ILC. ER is co-regulated by the DNA repair protein Mediator of DNA Damage Checkpoint 1 (MDC1) specifically in ILC cells, driving distinct ER activity. However, this novel MDC1 activity is associated with dysfunctional canonical DNA repair activity by MDC1, but without typical features of DNA repair deficiency. To understand reciprocal activities of MDC1, we profiled the MDC1 interactome and found MDC1-associated proteins in ILC cells mirror a "BRCA-like" state lacking key homologous recombination (HR) proteins, consistent with HR dysfunction but distinct from classic "BRCAness". HR dysfunction in ILC cells is supported by single-cell transcriptome and DNA repair activity analyses, with DNA repair signaling and functional data, showing dysfunctional induction and resolution of HR. In parallel, ILC tumor data are consistent with a distinct form of HR dysfunction via impaired HR resolution, lacking BRCA-like genomic scarring but showing elevated signatures of PARP inhibitor sensitivity. We demonstrate this HR dysfunction can be exploited using PARP inhibition, and found that talazoparib treatment produced a durable growth suppression both in vitro and in multiple ILC xenografts in vivo. ILC-specific ER:MDC1 activity creates a new context for ER and MDC1 function in ILC, at the cost of a DNA repair dysfunction, which may be therapeutically targetable.

BACKGROUND:Pediatric high-grade gliomas (PHGG) are aggressive brain tumors with 5-year survival rates ranging from <2% to 20% depending upon subtype. PHGG presents differently from patient to patient and is intratumorally heterogeneous, posing challenges in designing therapies. We hypothesized that heterogeneity occurs because PHGG comprises multiple distinct tumor and immune cell types in varying proportions, each of which may influence tumor characteristics. METHODS:We obtained 19 PHGG samples from our institution's pediatric brain tumor bank. We constructed a comprehensive transcriptomic dataset at the single-cell level using single-cell RNA-Seq (scRNA-Seq), identified known glial and immune cell types, and performed differential gene expression and gene set enrichment analysis. We conducted multi-channel immunofluorescence (IF) staining to confirm the transcriptomic results. RESULTS:Our PHGG samples included 3 principal predicted tumor cell types: astrocytes, oligodendrocyte progenitors (OPCs), and mesenchymal-like cells (Mes). These cell types differed in their gene expression profiles, pathway enrichment, and mesenchymal character. We identified a macrophage population enriched in mesenchymal and inflammatory gene expression as a possible source of mesenchymal tumor characteristics. We found evidence of T-cell exhaustion and suppression. CONCLUSIONS:PHGG comprises multiple distinct proliferating tumor cell types. Microglia-derived macrophages may drive mesenchymal gene expression in PHGG. The predicted Mes tumor cell population likely derives from OPCs. The variable tumor cell populations rely on different oncogenic pathways and are thus likely to vary in their responses to therapy.

Early-life stress has been linked to multiple neurodevelopmental and neuropsychiatric deficits. Our previous studies have linked maternal presence/absence from the nest in developing rat pups to changes in prefrontal cortex (PFC) activity. Furthermore, we have shown that these changes are modulated by serotonergic signaling. Here we test whether changes in PFC activity during early life affect the developing cortex leading to behavioral alterations in the adult. We show that inhibiting the PFC of mouse pups leads to cognitive deficits in the adult comparable to those seen following maternal separation. Moreover, we show that activating the PFC during maternal separation can prevent these behavioral deficits. To test how maternal separation affects the transcriptional profile of the PFC we performed single-nucleus RNA-sequencing. Maternal separation led to differential gene expression almost exclusively in inhibitory neurons. Among others, we found changes in GABAergic and serotonergic pathways in these interneurons. Interestingly, both maternal separation and early-life PFC inhibition led to changes in physiological responses in prefrontal activity to GABAergic and serotonergic antagonists that were similar to the responses of more immature brains. Prefrontal activation during maternal separation prevented these changes. These data point to a crucial role of PFC activity during early life in behavioral expression in adulthood.

Pediatric low-grade gliomas (pLGG) comprise 35% of all brain tumors. Despite favorable survival, patients experience significant morbidity from disease and treatments. A deeper understanding of pLGG biology is essential to identify novel, more effective, and less toxic therapies. We utilized single cell RNA sequencing (scRNA-seq), spatial transcriptomics, and cytokine analyses to characterize and understand tumor and immune cell heterogeneity across pLGG. scRNA-seq revealed tumor and immune cells within the tumor microenvironment (TME). Tumor cell subsets revealed a developmental hierarchy with progenitor and mature cell populations. Immune cells included myeloid and lymphocytic cells. There was a significant difference between the prevalence of two major myeloid subclusters between pilocytic astrocytoma (PA) and ganglioglioma (GG). Bulk and single-cell cytokine analyses evaluated the immune cell signaling cascade with distinct immune phenotypes among tumor samples. KIAA1549-BRAF tumors appeared more immunogenic, secreting higher levels of immune cell activators and chemokines, compared to BRAF V600E tumors. Spatial transcriptomics revealed the differential gene expression of these chemokines and their location within the TME. A multi-pronged analysis of pLGG demonstrated the complexity of the pLGG TME and differences between genetic drivers that may influence their response to immunotherapy. Further investigation of immune cell infiltration and tumor-immune interactions is warranted. Key points There is a developmental hierarchy in neoplastic population comprising of both progenitor-like and mature cell types in both PA and GG. A more immunogenic, immune activating myeloid population is present in PA compared to GG. Functional analysis and spatial transcriptomics show higher levels of immune mobilizing chemokines in KIAA1549-BRAF fusion PA tumor samples compared to BRAF V600E GG samples. Importance of the Study While scRNA seq provides information on cellular heterogeneity within the tumor microenvironment (TME), it does not provide a complete picture of how these cells are interacting or where they are located. To expand on this, we used a three-pronged approach to better understand the biology of pediatric low-grade glioma (pLGG). By analyzing scRNA-seq, secreted cytokines and spatial orientation of cells within the TME, we strove to gain a more complete picture of the complex interplay between tumor and immune cells within pLGG. Our data revealed a complex heterogeneity in tumor and immune populations and identified an interesting difference in the immune phenotype among different subtypes.

Plexiform neurofibroma (PN) is a leading cause of morbidity in children with the genetic condition Neurofibromatosis Type 1 (NF1), often disfiguring or threatening vital structures. During formation of PN, a complex tumor microenvironment (TME) develops, with recruitment of neoplastic and non-neoplastic cell types being critical for growth and progression. Due to the cohesive cellularity of PN, single-cell RNA-sequencing is difficult and may result in a loss of detection of critical cellular subpopulations. To bypass this barrier, we performed single-nuclei RNA-sequencing (snRNA-seq) on 8 frozen PN samples, and integrated this with spatial transcriptomics (ST) in 4 PN samples and immunohistochemistry to provide morphological context to transcriptomic data. SnRNA-seq analysis definitively charted the heterogeneous cellular subpopulations in the PN TME, with the predominant fraction being fibroblast subtypes. PN showed a remarkable amount of inter-sample homogeneity regarding cellular subpopulation proportions despite being resected from a variety of anatomical locations. ST analysis identified distinct cellular subpopulations which were annotated using snRNA-seq data and correlated with histological features. Schwann cell/fibroblast interactions were identified by receptor/ligand interaction analysis demonstrating a high probability of Neurexin 1/Neuroligin 1 (NRXN1/NLGN1) receptor-ligand cross-talk predicted between fibroblasts and non-myelinated Schwann cells (NM-SC) and subtypes, respectively. We observed aberrant expression of NRXN1 and NLGN1 in our PN snRNA-seq data compared to a normal mouse sciatic nerve single-cell RNA-seq dataset. This pathway has never been described in PN and may indicate a clear and direct communication pathway between putative NM-SC cells of origin and surrounding fibroblasts, potentially driving disease progression. SnRNA-seq integrated with spatial transcriptomics advances our understanding of the complex cellular heterogeneity of PN TME and identify potential novel communication pathways that may drive disease progression, a finding that could provide translational therapy options for patients with these devastating tumors of childhood and early adulthood.

The human adrenal gland consists of concentrically organized, functionally distinct regions responsible for hormone production. Dysregulation of adrenocortical cell differentiation alters the proportion and organization of the functional zones of the adrenal cortex leading to disease. Current models of adrenocortical cell differentiation are based on mouse studies, but there are known organizational and functional differences between human and mouse adrenal glands. This study aimed to investigate the centripetal differentiation model in the human adrenal cortex and characterize aldosterone-producing micronodules (APMs) to better understand adrenal diseases such as primary aldosteronism. We applied spatially resolved in situ transcriptomics to human adrenal tissue sections from 2 individuals and identified distinct cell populations and their positional relationships. The results supported the centripetal differentiation model in humans, with cells progressing from the outer capsule to the zona glomerulosa, zona fasciculata, and zona reticularis. Additionally, we characterized 2 APMs in a 72-year-old woman. Comparison with earlier APM transcriptomes indicated a subset of core genes, but also heterogeneity between APMs. The findings contribute to our understanding of normal and pathological cellular differentiation in the human adrenal cortex.

Abstract BACKGROUND Pediatric low-grade gliomas (pLGGs) are the most common brain tumors in children. Although pLGGs represent a significant number of patients with slow-growing, often curable tumors, they are also affected by neurologic injuries, multiple relapses and requiring repeated courses of therapy, including surgery, chemotherapy, and radiotherapy. Novel, more effective therapies are urgently needed. METHODS We used a multipronged approach to better understand the complexity of the tumor microenvironment (TME) within pLGG. We utilized scRNA sequencing, spatial transcriptomics, and cytokine analyses on 23 patient-derived pLGG samples including different tumor types (pilocytic astrocytoma (PA), ganglioglioma (GG), and undefined low-grade glioma (LGG)) and mutation profiles (BRAF V600E, BRAF fusions, and BRAF wild-type) to identify cell types, their spatial arrangement within a tissue, and their underlying functional status. RESULTS scRNA seq revealed both tumor and immune cells within the TME. Analyses of tumor cell subsets revealed a developmental hierarchy identifying progenitor and mature cell populations. Immune cells comprised both myeloid and T lineage cells, with myeloid cells comprising > 50% of the entire population. Interestingly, there was a significant difference between the prevalence of two major myeloid subclusters between two pLGG subtypes (PA and GG). To better understand the underlying difference, we performed bulk and single-cell cytokine analyses to identify immune signals via which these cells communicate. Cytokine analyses revealed distinct immune phenotypes among tumor samples. Notably, BRAF fusion tumors appeared more immunogenic, especially by secreting higher levels of immune cell activators, chemokines (including CCL3 and CCL4), compared to BRAF V600E tumors. Additionally, spatial transcriptomics revealed the differential expression of these chemokines between PA and GG tumors. CONCLUSIONS Characterization of pLGG demonstrated the complexity of tumor microenvironment among different pLGG subtypes. Further investigation of the immune cells and tumor-immune cell interactions has the potential to unravel novel therapeutic targets against pLGG.

BACKGROUND:The diverse cellular constituents of childhood brain tumor ependymoma, recently revealed by single cell RNA-sequencing, may underly therapeutic resistance. Here we use spatial transcriptomics to further advance our understanding of the tumor microenvironment, mapping cellular subpopulations to the tumor architecture of ependymoma posterior fossa subgroup A (PFA), the commonest and most deadly childhood ependymoma variant.METHODS:Spatial transcriptomics data from intact PFA sections was deconvoluted to resolve the histological arrangement of neoplastic and non-neoplastic cell types. Key findings were validated using immunohistochemistry, in vitro functional assays and outcome analysis in clinically-annotated PFA bulk transcriptomic data.RESULTS:PFA are comprised of epithelial and mesenchymal histological zones containing a diversity of cellular states, each zone including co-existing and spatially distinct undifferentiated progenitor-like cells; a quiescent mesenchymal zone population, and a second highly mitotic progenitor population that is restricted to hypercellular epithelial zones and that is more abundant in progressive tumors. We show that myeloid cell interaction is the leading cause of mesenchymal transition in PFA, occurring in zones spatially distinct from hypoxia-induced mesenchymal transition, and these distinct EMT-initiating processes were replicated using in vitro models of PFA.CONCLUSIONS:These insights demonstrate the utility of spatial transcriptomics to advance our understanding of ependymoma biology, revealing a clearer picture of the cellular constituents of PFA, their interactions and influence on tumor progression.

Supplementary Tables S1-S6 describing patient characteristics, gene lists, LSC signatures, and reagents.

Abstract Disclosure: F. Rui: None. K. Walters: None. K. Koc: None. A. Baldwin: Stock Owner; Self; Tempus Labs. K. Kiseljak-Vassiliades: Advisory Board Member; Self; HRA Pharma. L.M. Fishbein: Consulting Fee; Self; Lantheus/Progenics Azedra. N. Mukherjee: None. The human adrenal gland is a vital endocrine organ controlling a multitude of functions through the secretion of cortical steroid and medullary catecholamine hormones. Several outstanding questions remain about the adrenal spatial organization and related differential developmental signaling pathways which have functional consequences within and between the cortex and medulla. The current model of the functional zonation of the cortex posits a centripetal differentiation process in which the cortical cells arise in the capsule and migrate inwards changing their gene expression patterns as they progress to the three different functional layers of the cortex. The evidence for this model is based on mouse lineage tracing experiments, and it is unclear to what extent it applies to human adrenal glands. To address this, we performed spatially resolved transcriptomics on four adult adrenal sections from two different human specimens to identify cell populations and their spatial relationship in this highly organized and functionally diverse organ. We identified 14 distinct cell populations; 79% of these spots represented the cortex or medulla and exhibited the concentric expression pattern reflecting the known spatial organization of the human adrenal gland. Trajectory inference analysis of the cortical cells showed continuous expression transitions from cells of the capsule to zona glomerulosa to zona fasciculata to zona reticularis. These data demonstrate for the first time that the centripetal differentiation model for functional zonation in mice is consistent for adult human adrenal tissue. We also validated the spatial expression pattern of a subset of WNT-related genes specifically expressed with distinct patterns in the capsule and zona glomerulosa cells. These genes represent novel regulatory factors that may be crucial in the differentiation of adult human adrenal cortex. Finally, we observed two CYP11B2 positive aldosterone producing cell clusters (APCC) in the adrenal tissue section from a 72-year-old woman. APCCs are thought to play a role in the pathological progression of primary aldosteronism; however, it is unclear if there are unifying expression signatures other than CYP11B2 protein expression. Our analysis found both similarities and differences to previously published data sets of APCC signatures. It is unclear to what extent these expression differences are technical (i.e. different platforms) or due to biological heterogeneity (between individuals or APCCs). The subgroup of similar gene expression in APCCs across all studies thus far may represent a core APCC signature which should be validated on a larger cohort. Altogether, our study uses spatial transcriptomics of the normal human adrenal and the results have important implications for the differentiation and functional zonation,as well as, for the early dysregulation leading to primary aldosteronism. Presentation: Friday, June 16, 2023

Abstract BACKGROUND Pediatric high-grade gliomas (PHGG) are aggressive, undifferentiated central nervous system (CNS) tumors. PHGG comprises subtypes that differ phenotypically, histologically and by cellular composition. Despite intensive research, overall survival rates have remained for decades around 2% for diffuse midline glioma (DMG) and 20% for non-DMG PHGG. We hypothesized that PHGG’s heterogeneity, a key difficulty in devising therapies, results from functional and lineage differences in tumor cell types. METHODS Using 19 PHGG samples from our institution’s pediatric brain tumor bank, we constructed a single-cell RNA-Seq (scRNA-Seq) dataset that included tumor and immune cells, generated predicted cell types by mapping the scRNA-Seq data onto known cell types, and performed differential gene expression and geneset enrichment analyses. We acquired bulk RNA-Seq and DNA methylation data to characterize overall tumor properties. RESULTS Samples consisted of several principal tumor cell types, including cells with astrocyte characteristics, oligodendrocyte progenitor cell (OPC) characteristics, and cells that expressed OPC markers but had slight enrichment in mesenchymal and inflammatory gene expression (OPC-like/Mes cells). We also found a potential stemlike population that expressed neural stem cell markers. The predicted astrocytes, OPCs and OPC-like/Mes cells differed in their gene expression profiles, pathway enrichment, and tendency to proliferate as assessed from expression of cell cycle genes. A combined microglia/macrophage population, strongly enriched in mesenchymal and inflammatory gene expression, may serve to induce inflammatory gene expression in tumor cells. Key conclusions from the scRNA-Seq analysis were validated using multi-channel immunofluorescence staining. CONCLUSIONS PHGG comprises a small stemlike population and varying proportions of proliferating glial lineage tumor cells. CNS-resident microglia/macrophages may trigger mesenchymal gene expression in PHGG. We will use our results to investigate the roles of PHGG’s diverse tumor cell populations in resistance to radiotherapy and to develop treatments to target individual cell types comprising PHGG.

Supplementary Figures S1-S5 supporting Main Figures 1-5.

Nonsense-mediated RNA decay (NMD) degrades transcripts carrying premature termination codons. NMD is thought to prevent the synthesis of toxic truncated proteins. However, whether loss of NMD results in wide-spread production of truncated proteins is unclear. A human genetic disease, facioscapulohumeral muscular dystrophy (FSHD), features acute inhibition of NMD upon expression of the disease-causing transcription factor, DUX4. Using a cell-based model of FSHD, we show production of truncated proteins from physiolog-ical NMD targets and find that RNA-binding proteins are enriched for aberrant truncations. The NMD isoform of one RNA-binding protein, SRSF3, is translated to produce a stable truncated protein, which is detected in FSHD patient-derived myotubes. Ectopic expression of truncated SRSF3 confers toxicity, and its downregu-lation is cytoprotective. Our results delineate the genome-scale impact of NMD loss. This widespread production of potentially deleterious truncated proteins has implications for FSHD biology as well as other genetic diseases where NMD is therapeutically modulated.

Abstract Gramma-glutamyl-hydrolase (GGH) has been implicated in antitumor resistance and unfavourable prognosis across various tumour types. However, the potential role and mechanism of GGH in gliomas remain poorly understood. We undertook the bioinformatics analysis to systematically explore the correlation between GGH expression and prognosis, clinical features, immune cell infiltration, tumor microenvironment and biological functions in the TGGA database. Subsequently, we performed knockdown and overexpression experiments on two gliomas cell lines (U251 and A299) and verified their expression stability using Western blotting assay. Cell function assays, including EdU immunofluorescence, colony formation, transwell assay and wound-healing assay, were conducted to investigate the impact of GGH expression on proliferation, invasion and migration of glioma cells in vitro. Additionally, a subcutaneous xenograft experiment was performed to evaluate the effect of GGH expression on tumor growth in vivo. GGH was highly expressed in gliomas and correlated with poor prognosis, clinic features and the microenvironment of gliomas. Cell function assays manifested that overexpression of GGH promoted the proliferation, migration, and invasion of glioma cells, while knockdown of GGH had the opposite effect. In vivo xenograft experiments showed that GGH knockdown suppressed the growth rate of gliomas. We identified GGH as a relevant biomarker for the malignant behaviors of gliomas and it holds great promise in therapeutic strategy and prognostic evaluation.

Abstract Ependymoma (EPN) posterior fossa group A (PFA) has the highest rate of recurrence and the worst prognosis of all EPN types. At relapse, it is typically incurable even with re-resection and re-irradiation. The biology of recurrent PFA EPN remains largely unknown, which hinders clinical advances. In this longitudinal large multicenter study, we examined matched samples of primary and recurrent disease from PFA EPN patients (n=95) to investigate the biology of recurrence. DNA methylomic data was used to measure copy number variants (CNVs), revealing progressive large scale chromosome gains and losses in successive recurrences. These CNV changes were dominated by chromosome 1q gain and/or 6q loss (1q+/6q-), both previously identified as high-risk factors in PFA EPN, which were present in ~20% at presentation but increased to ~60% at 1st recurrence. Because 6q testing is not routinely performed, this very high incidence at recurrence has not been previously reported. Evolution of chromosomal aberrations was further explored using CNV analysis of single-nuclei RNAseq on approximately 46,000 PFA EPN cells from 6 matched pairs of primary and 1st relapse tumors that harbored CNV changes at recurrence. No evidence of rare subclones in primary tumors was observed, suggesting that chromosomal rearrangement events occur after initial presentation. Cellular and molecular characteristics associated with CNVs were examined by single-nuclei RNAseq, bulk transcriptomic analysis and immunohistochemistry, revealing that 1q+/6q- PFA have a significantly higher mitotic index, increased proportions of proliferative epithelial progenitors and decreased differentiated neoplastic subpopulations. Multivariate survival analyses showed that cases with 1q gain or 6q loss at 1st recurrence were significantly more likely to recur than cases with no 1q or 6q change. The high prevalence of 1q+/6q- at recurrence and the associated shortened survival, suggest that both these abnormalities should be routinely tested for, and used for trial stratification.

Hibernation is a natural model of extreme physiology in a mammal. Throughout winter, small hibernators repeatedly undergo rapid, dramatic swings in body temperature, perfusion, and oxygen delivery. To gain insight into the molecular mechanisms that support homeostasis despite the numerous challenges posed by this dynamic physiology, we collected 13-lined ground squirrel adrenal glands from at least five individuals representing six key timepoints across the year using body temperature telemetry. Differentially expressed genes were identified using RNA-seq, revealing both strong seasonal and torpor-arousal cycle effects on gene expression. Two novel findings emerge from this study. First, transcripts encoding multiple genes involved in steroidogenesis decreased seasonally. Taken together with morphometric analyses, the data are consistent with preservation of mineralocorticoids but suppression of glucocorticoid and androgen output throughout winter hibernation. Second, a temporally orchestrated, serial gene expression program unfolds across the brief arousal periods. This program initiates during early rewarming with the transient activation of a set of immediate early response (IER) genes, comprised of both transcription factors and the RNA degradation proteins that assure their rapid turnover. This pulse in turn activates a cellular stress response program to restore proteostasis comprised of protein turnover, synthesis, and folding machinery. These and other data support a general model for gene expression across the torpor-arousal cycle that is facilitated in synchrony with whole body temperature shifts; induction of the immediate early response upon rewarming activates a proteostasis program followed by a restored tissue-specific gene expression profile enabling renewal, repair, and survival of the torpid state.NEW & NOTEWORTHY This pioneer study of adrenal gland gene expression dynamics in hibernating ground squirrels leverages the power of RNA-seq on multiple precisely timed samples to demonstrate: 1) steroidogenesis is seasonally reorganized to preserve aldosterone at the expense of glucocorticoids and androgens throughout winter hibernation; 2) a serial gene expression program unfolds during each short arousal whereby immediate early response genes induce the gene expression machinery that restores proteostasis and the cell-specific expression profile before torpor reentry.

Ependymoma (EPN) posterior fossa group A (PFA) has the highest rate of recurrence and the worst prognosis of all EPN types. At relapse, it is typically incurable even with re-resection and re-irradiation. The biology of recurrent PFA EPN remains largely unknown, which hinders clinical advances. In this longitudinal large multicenter study, we examined matched samples of primary and recurrent disease from PFA EPN patients (n=95) to investigate the biology of recurrence. DNA methylomic data was used to measure copy number variants (CNVs), revealing progressive large scale chromosome gains and losses in successive recurrences. These CNV changes were dominated by chromosome 1q gain and/or 6q loss (1q+/6q-), both previously identified as high-risk factors in PFA EPN, which were present in ~20% at presentation but increased to ~60% at 1st recurrence. Because 6q testing is not routinely performed, this very high incidence at recurrence has not been previously reported. Evolution of chromosomal aberrations was further explored using CNV analysis of single-nuclei RNAseq on approximately 46,000 PFA EPN cells from 6 matched pairs of primary and 1st relapse tumors that harbored CNV changes at recurrence. No evidence of rare subclones in primary tumors was observed, suggesting that chromosomal rearrangement events occur after initial presentation. Cellular and molecular characteristics associated with CNVs were examined by single-nuclei RNAseq, bulk transcriptomic analysis and immunohistochemistry, revealing that 1q+/6q- PFA have a significantly higher mitotic index, increased proportions of proliferative epithelial progenitors and decreased differentiated neoplastic subpopulations. Multivariate survival analyses showed that cases with 1q gain or 6q loss at 1st recurrence were significantly more likely to recur than cases with no 1q or 6q change. The high prevalence of 1q+/6q- at recurrence and the associated shortened survival, suggest that both these abnormalities should be routinely tested for, and used for trial stratification.

BACKGROUND:Ependymoma (EPN) posterior fossa group A (PFA) has the highest rate of recurrence and the worst prognosis of all EPN molecular groups. At relapse, it is typically incurable even with re-resection and re-irradiation. The biology of recurrent PFA remains largely unknown; however, the increasing use of surgery at first recurrence has now provided access to clinical samples to facilitate a better understanding of this.METHODS:In this large longitudinal international multicenter study, we examined matched samples of primary and recurrent disease from PFA patients to investigate the biology of recurrence.RESULTS:DNA methylome derived copy number variants (CNVs) revealed large-scale chromosome gains and losses at recurrence in PFA. CNV changes were dominated by chromosome 1q gain and/or 6q loss, both previously identified as high-risk factors in PFA, which were present in 23% at presentation but increased to 61% at first recurrence. Multivariate survival analyses of this cohort showed that cases with 1q gain or 6q loss at first recurrence were significantly more likely to recur again. Predisposition to 1q+/6q- CNV changes at recurrence correlated with hypomethylation of heterochromatin-associated DNA at presentation. Cellular and molecular analyses revealed that 1q+/6q- PFA had significantly higher proportions of proliferative neuroepithelial undifferentiated progenitors and decreased differentiated neoplastic subpopulations.CONCLUSIONS:This study provides clinically and preclinically actionable insights into the biology of PFA recurrence. The hypomethylation predisposition signature in PFA is a potential risk-classifier for trial stratification. We show that the cellular heterogeneity of PFAs evolves largely because of genetic evolution of neoplastic cells.

ABSTRACTNonsense-mediated RNA decay (NMD) is a surveillance mechanism that degrades both canonical and aberrant transcripts carrying premature translation termination codons. NMD is thought to have evolved to prevent the synthesis of toxic truncated proteins. However, whether global inhibition of NMD results in widespread production of truncated proteins is unknown. A human genetic disease, facioscapulohumeral muscular dystrophy (FSHD) features acute inhibition of NMD upon expression of the disease-causing transcription factor, DUX4. Here, using a cell-based model of FSHD, we show the production of hundreds of truncated proteins from physiological NMD targets. Using ribosome profiling, we map the precise C-terminal end of these aberrant truncated proteins and find that RNA-binding proteins are especially enriched for aberrant truncations. The stabilized NMD isoform of one RNA-binding protein, SRSF3, is robustly translated to produce a stable truncated protein, which can also be detected in FSHD patient-derived myotubes. Notably, ectopic expression of truncated SRSF3 alone confers toxicity and its downregulation is cytoprotective. Our results demonstrate the genome-scale impact of NMD inhibition. This widespread production of potentially deleterious truncated proteins has implications for FSHD biology as well as other genetic diseases where NMD is therapeutically modulated.