Cross-species genetic screens identify transglutaminase 5 as a regulator of polyglutamine-expanded ataxin-1.

Many neurodegenerative disorders are caused by abnormal accumulation of misfolded proteins. In spinocerebellar ataxia type 1 (SCA1), accumulation of polyglutamine-expanded (polyQ-expanded) ataxin-1 (ATXN1) causes neuronal toxicity. Lowering total ATXN1, especially the polyQ-expanded form, alleviates disease phenotypes in mice, but the molecular mechanism by which the mutant ATXN1 is specifically modulated is not understood. Here, we identified 22 mutant ATXN1 regulators by performing a cross-species screen of 7787 and 2144 genes in human cells and Drosophila eyes, respectively. Among them, transglutaminase 5 (TG5) preferentially regulated mutant ATXN1 over the WT protein. TG enzymes catalyzed cross-linking of ATXN1 in a polyQ-length-dependent manner, thereby preferentially modulating mutant ATXN1 stability and oligomerization. Perturbing Tg in Drosophila SCA1 models modulated mutant ATXN1 toxicity. Moreover, TG5 was enriched in the nuclei of SCA1-affected neurons and colocalized with nuclear ATXN1 inclusions in brain tissue from patients with SCA1. Our work provides a molecular insight into SCA1 pathogenesis and an opportunity for allele-specific targeting for neurodegenerative disorders.

Dual targeting of brain region-specific kinases potentiates neurological rescue in Spinocerebellar ataxia type 1.

A critical question in neurodegeneration is why the accumulation of disease-driving proteins causes selective neuronal loss despite their brain-wide expression. In Spinocerebellar ataxia type 1 (SCA1), accumulation of polyglutamine-expanded Ataxin-1 (ATXN1) causes selective degeneration of cerebellar and brainstem neurons. Previous studies revealed that inhibiting Msk1 reduces phosphorylation of ATXN1 at S776 as well as its levels leading to improved cerebellar function. However, there are no regulators that modulate ATXN1 in the brainstem-the brain region whose pathology is most closely linked to premature death. To identify new regulators of ATXN1, we performed genetic screens and identified a transcription factor-kinase axis (ZBTB7B-RSK3) that regulates ATXN1 levels. Unlike MSK1, RSK3 is highly expressed in the human and mouse brainstems where it regulates Atxn1 by phosphorylating S776. Reducing Rsk3 rescues brainstem-associated pathologies and deficits, and lowering Rsk3 and Msk1 together improves cerebellar and brainstem function in an SCA1 mouse model. Our results demonstrate that selective vulnerability of brain regions in SCA1 is governed by region-specific regulators of ATXN1, and targeting multiple regulators could rescue multiple degenerating brain areas.

Modulation of ATXN1 S776 phosphorylation reveals the importance of allele-specific targeting in SCA1.

Spinocerebellar ataxia type 1 (SCA1) is an adult-onset neurodegenerative disorder characterized by motor incoordination, mild cognitive decline, respiratory dysfunction, and early lethality. It is caused by the expansion of the polyglutamine (polyQ) tract in Ataxin-1 (ATXN1), which stabilizes the protein, leading to its toxic accumulation in neurons. Previously, we showed that serine 776 (S776) phosphorylation is critical for ATXN1 stability and contributes to its toxicity in cerebellar Purkinje cells. Still, the therapeutic potential of disrupting S776 phosphorylation on noncerebellar SCA1 phenotypes remains unstudied. Here, we report that abolishing S776 phosphorylation specifically on the polyQ-expanded ATXN1 of SCA1-knockin mice reduces ATXN1 throughout the brain and not only rescues the cerebellar motor incoordination but also improves respiratory function and extends survival while not affecting the hippocampal learning and memory deficits. As therapeutic approaches are likely to decrease S776 phosphorylation on polyQ-expanded and WT ATXN1, we further disrupted S776 phosphorylation on both alleles and observed an attenuated rescue, demonstrating a potential protective role of WT allele. This study not only highlights the role of S776 phosphorylation to regulate ATXN1 levels throughout the brain but also suggests distinct brain region-specific disease mechanisms and demonstrates the importance of developing allele-specific therapies for maximal benefits in SCA1.

miR760 regulates ATXN1 levels via interaction with its 5' untranslated region.

Identifying modifiers of dosage-sensitive genes involved in neurodegenerative disorders is imperative to discover novel genetic risk factors and potential therapeutic entry points. In this study, we focus on Ataxin-1 (ATXN1), a dosage-sensitive gene involved in the neurodegenerative disease spinocerebellar ataxia type 1 (SCA1). While the precise maintenance of ATXN1 levels is essential to prevent disease, the mechanisms that regulate ATXN1 expression remain largely unknown. We demonstrate that ATXN1's unusually long 5' untranslated region (5' UTR) negatively regulates its expression via posttranscriptional mechanisms. Based on recent reports that microRNAs (miRNAs) can interact with both 3' and 5' UTRs to regulate their target genes, we identify miR760 as a negative regulator that binds to a conserved site in ATXN1's 5' UTR to induce RNA degradation and translational inhibition. We found that delivery of Adeno-associated virus (AAV)-expressing miR760 in the cerebellum reduces ATXN1 levels in vivo and mitigates motor coordination deficits in a mouse model of SCA1. These findings provide new insights into the regulation of ATXN1 levels, present additional evidence for miRNA-mediated gene regulation via 5' UTR binding, and raise the possibility that noncoding mutations in the ATXN1 locus may act as risk factors for yet to be discovered progressive ataxias.

PolyA-miner: accurate assessment of differential alternative poly-adenylation from 3'Seq data using vector projections and non-negative matrix factorization.

Almost 70% of human genes undergo alternative polyadenylation (APA) and generate mRNA transcripts with varying lengths, typically of the 3' untranslated regions (UTR). APA plays an important role in development and cellular differentiation, and its dysregulation can cause neuropsychiatric diseases and increase cancer severity. Increasing awareness of APA's role in human health and disease has propelled the development of several 3' sequencing (3'Seq) techniques that allow for precise identification of APA sites. However, despite the recent data explosion, there are no robust computational tools that are precisely designed to analyze 3'Seq data. Analytical approaches that have been used to analyze these data predominantly use proximal to distal usage. With about 50% of human genes having more than two APA isoforms, current methods fail to capture the entirety of APA changes and do not account for non-proximal to non-distal changes. Addressing these key challenges, this study demonstrates PolyA-miner, an algorithm to accurately detect and assess differential alternative polyadenylation specifically from 3'Seq data. Genes are abstracted as APA matrices, and differential APA usage is inferred using iterative consensus non-negative matrix factorization (NMF) based clustering. PolyA-miner accounts for all non-proximal to non-distal APA switches using vector projections and reflects precise gene-level 3'UTR changes. It can also effectively identify novel APA sites that are otherwise undetected when using reference-based approaches. Evaluation on multiple datasets-first-generation MicroArray Quality Control (MAQC) brain and Universal Human Reference (UHR) PolyA-seq data, recent glioblastoma cell line NUDT21 knockdown Poly(A)-ClickSeq (PAC-seq) data, and our own mouse hippocampal and human stem cell-derived neuron PAC-seq data-strongly supports the value and protocol-independent applicability of PolyA-miner. Strikingly, in the glioblastoma cell line data, PolyA-miner identified more than twice the number of genes with APA changes than initially reported. With the emerging importance of APA in human development and disease, PolyA-miner can significantly improve data analysis and help decode the underlying APA dynamics.

A kinome-wide RNAi screen identifies ERK2 as a druggable regulator of Shank3 stability.

Neurons are sensitive to changes in the dosage of many genes, especially those regulating synaptic functions. Haploinsufficiency of SHANK3 causes Phelan-McDermid syndrome and autism, whereas duplication of the same gene leads to SHANK3 duplication syndrome, a disorder characterized by neuropsychiatric phenotypes including hyperactivity and bipolar disorder as well as epilepsy. We recently demonstrated the functional modularity of Shank3, which suggests that normalizing levels of Shank3 itself might be more fruitful than correcting pathways that function downstream of it for treatment of disorders caused by alterations in SHANK3 dosage. To identify upstream regulators of Shank3 abundance, we performed a kinome-wide siRNA screen and identified multiple kinases that potentially regulate Shank3 protein stability. Interestingly, we discovered that several kinases in the MEK/ERK2 pathway destabilize Shank3 and that genetic deletion and pharmacological inhibition of ERK2 increases Shank3 abundance in vivo. Mechanistically, we show that ERK2 binds Shank3 and phosphorylates it at three residues to promote its poly-ubiquitination-dependent degradation. Altogether, our findings uncover a druggable pathway as a potential therapeutic target for disorders with reduced SHANK3 dosage, provide a rich resource for studying Shank3 regulation, and demonstrate the feasibility of this approach for identifying regulators of dosage-sensitive genes.

Molecular profiling predicts meningioma recurrence and reveals loss of DREAM complex repression in aggressive tumors.

Meningiomas account for one-third of all primary brain tumors. Although typically benign, about 20% of meningiomas are aggressive, and despite the rigor of the current histopathological classification system there remains considerable uncertainty in predicting tumor behavior. Here, we analyzed 160 tumors from all 3 World Health Organization (WHO) grades (I through III) using clinical, gene expression, and sequencing data. Unsupervised clustering analysis identified 3 molecular types (A, B, and C) that reliably predicted recurrence. These groups did not directly correlate with the WHO grading system, which classifies more than half of the tumors in the most aggressive molecular type as benign. Transcriptional and biochemical analyses revealed that aggressive meningiomas involve loss of the repressor function of the DREAM complex, which results in cell-cycle activation; only tumors in this category tend to recur after full resection. These findings should improve our ability to predict recurrence and develop targeted treatments for these clinically challenging tumors.

Loss of Ataxin-1 Potentiates Alzheimer's Pathogenesis by Elevating Cerebral BACE1 Transcription.

Expansion of CAG trinucleotide repeats in ATXN1 causes spinocerebellar ataxia type 1 (SCA1), a neurodegenerative disease that impairs coordination and cognition. While ATXN1 is associated with increased Alzheimer's disease (AD) risk, CAG repeat number in AD patients is not changed. Here, we investigated the consequences of ataxin-1 loss of function and discovered that knockout of Atxn1 reduced CIC-ETV4/5-mediated inhibition of Bace1 transcription, leading to increased BACE1 levels and enhanced amyloidogenic cleavage of APP, selectively in AD-vulnerable brain regions. Elevated BACE1 expression exacerbated Aß deposition and gliosis in AD mouse models and impaired hippocampal neurogenesis and olfactory axonal targeting. In SCA1 mice, polyglutamine-expanded mutant ataxin-1 led to the increase of BACE1 post-transcriptionally, both in cerebrum and cerebellum, and caused axonal-targeting deficit and neurodegeneration in the hippocampal CA2 region. These findings suggest that loss of ataxin-1 elevates BACE1 expression and Aß pathology, rendering it a potential contributor to AD risk and pathogenesis.

Cross-species genetic screens to identify kinase targets for APP reduction in Alzheimer's disease.

An early hallmark of Alzheimer's disease is the accumulation of amyloid-ß (Aß), inspiring numerous therapeutic strategies targeting this peptide. An alternative approach is to destabilize the amyloid beta precursor protein (APP) from which Aß is derived. We interrogated innate pathways governing APP stability using a siRNA screen for modifiers whose own reduction diminished APP in human cell lines and transgenic Drosophila. As proof of principle, we validated PKCß-a known modifier identified by the screen-in an APP transgenic mouse model. PKCß was genetically targeted using a novel adeno-associated virus shuttle vector to deliver microRNA-adapted shRNA via intracranial injection. In vivo reduction of PKCß initially diminished APP and delayed plaque formation. Despite persistent PKCß suppression, the effect on APP and amyloid diminished over time. Our study advances this approach for mining druggable modifiers of disease-associated proteins, while cautioning that prolonged in vivo validation may be needed to reveal emergent limitations on efficacy.

PAK1 regulates ATXN1 levels providing an opportunity to modify its toxicity in spinocerebellar ataxia type 1.

Spinocerebellar ataxia type 1 (SCA1) is caused by the expansion of a trinucleotide repeat that encodes a polyglutamine tract in ataxin-1 (ATXN1). The expanded polyglutamine in ATXN1 increases the protein's stability and results in its accumulation and toxicity. Previous studies have demonstrated that decreasing ATXN1 levels ameliorates SCA1 phenotypes and pathology in mouse models. We rationalized that reducing ATXN1 levels through pharmacological inhibition of its modulators could provide a therapeutic avenue for SCA1. Here, through a forward genetic screen in Drosophila we identified, p21-activated kinase 3 (Pak3) as a modulator of ATXN1 levels. Loss-of-function of fly Pak3 or Pak1, whose mammalian homologs belong to Group I of PAK proteins, reduces ATXN1 levels, and accordingly, improves disease pathology in a Drosophila model of SCA1. Knockdown of PAK1 potently reduces ATXN1 levels in mammalian cells independent of the well-characterized S776 phosphorylation site (known to stabilize ATXN1) thus revealing a novel molecular pathway that regulates ATXN1 levels. Furthermore, pharmacological inhibition of PAKs decreases ATXN1 levels in a mouse model of SCA1. To explore the potential of using PAK inhibitors in combination therapy, we combined the pharmacological inhibition of PAK with MSK1, a previously identified modulator of ATXN1, and examined their effects on ATXN1 levels. We found that inhibition of both pathways results in an additive decrease in ATXN1 levels. Together, this study identifies PAK signaling as a distinct molecular pathway that regulates ATXN1 levels and presents a promising opportunity to pursue for developing potential therapeutics for SCA1.

Reduction of protein kinase A-mediated phosphorylation of ATXN1-S776 in Purkinje cells delays onset of Ataxia in a SCA1 mouse model.

Spinocerebellar ataxia type 1 (SCA1) is a polyglutamine (polyQ) repeat neurodegenerative disease in which a primary site of pathogenesis are cerebellar Purkinje cells. In addition to polyQ expansion of ataxin-1 protein (ATXN1), phosphorylation of ATXN1 at the serine 776 residue (ATXN1-pS776) plays a significant role in protein toxicity. Utilizing a biochemical approach, pharmacological agents and cell-based assays, including SCA1 patient iPSC-derived neurons, we examine the role of Protein Kinase A (PKA) as an effector of ATXN1-S776 phosphorylation. We further examine the implications of PKA-mediated phosphorylation at ATXN1-S776 on SCA1 through genetic manipulation of the PKA catalytic subunit Cα in Pcp2-ATXN1[82Q] mice. Here we show that pharmacologic inhibition of S776 phosphorylation in transfected cells and SCA1 patient iPSC-derived neuronal cells lead to a decrease in ATXN1. In vivo, reduction of PKA-mediated ATXN1-pS776 results in enhanced degradation of ATXN1 and improved cerebellar-dependent motor performance. These results provide evidence that PKA is a biologically important kinase for ATXN1-pS776 in cerebellar Purkinje cells.

Loss of Capicua alters early T cell development and predisposes mice to T cell lymphoblastic leukemia/lymphoma.

Capicua (CIC) regulates a transcriptional network downstream of the RAS/MAPK signaling cascade. In Drosophila, CIC is important for many developmental processes, including embryonic patterning and specification of wing veins. In humans, CIC has been implicated in neurological diseases, including spinocerebellar ataxia type 1 (SCA1) and a neurodevelopmental syndrome. Additionally, we and others have reported mutations in CIC in several cancers. However, whether CIC is a tumor suppressor remains to be formally tested. In this study, we found that deletion of Cic in adult mice causes T cell acute lymphoblastic leukemia/lymphoma (T-ALL). Using hematopoietic-specific deletion and bone marrow transplantation studies, we show that loss of Cic from hematopoietic cells is sufficient to drive T-ALL. Cic-null tumors show up-regulation of the KRAS pathway as well as activation of the NOTCH1 and MYC transcriptional programs. In sum, we demonstrate that loss of CIC causes T-ALL, establishing it as a tumor suppressor for lymphoid malignancies. Moreover, we show that mouse models lacking CIC in the hematopoietic system are robust models for studying the role of RAS signaling as well as NOTCH1 and MYC transcriptional programs in T-ALL.

Jak2-mediated phosphorylation of Atoh1 is critical for medulloblastoma growth.

Treatment for medulloblastoma, the most common malignant brain tumor in children, remains limited to surgical resection, radiation, and traditional chemotherapy; with long-term survival as low as 50-60% for Sonic Hedgehog (Shh)-type medulloblastoma. We have shown that the transcription factor Atonal homologue 1 (Atoh1) is required for Shh-type medulloblastoma development in mice. To determine whether reducing either Atoh1 levels or activity in tumors after their development is beneficial, we studied Atoh1 dosage and modifications in Shh-type medulloblastoma. Heterozygosity of Atoh1 reduced tumor occurrence and prolonged survival. We discovered tyrosine 78 of Atoh1 is phosphorylated by a Jak2-mediated pathway only in tumor-initiating cells and in human SHH-type medulloblastoma. Phosphorylation of tyrosine 78 stabilizes Atoh1, increases Atoh1's transcriptional activity, and is independent of canonical Jak2 signaling. Importantly, inhibition of Jak2 impairs tyrosine 78 phosphorylation and tumor growth in vivo. Taken together, inhibiting Jak2-mediated tyrosine 78 phosphorylation could provide a viable therapy for medulloblastoma.

Reduction of Nuak1 Decreases Tau and Reverses Phenotypes in a Tauopathy Mouse Model.

Many neurodegenerative proteinopathies share a common pathogenic mechanism: the abnormal accumulation of disease-related proteins. As growing evidence indicates that reducing the steady-state levels of disease-causing proteins mitigates neurodegeneration in animal models, we developed a strategy to screen for genes that decrease the levels of tau, whose accumulation contributes to the pathology of both Alzheimer disease (AD) and progressive supranuclear palsy (PSP). Integrating parallel cell-based and Drosophila genetic screens, we discovered that tau levels are regulated by Nuak1, an AMPK-related kinase. Nuak1 stabilizes tau by phosphorylation specifically at Ser356. Inhibition of Nuak1 in fruit flies suppressed neurodegeneration in tau-expressing Drosophila, and Nuak1 haploinsufficiency rescued the phenotypes of a tauopathy mouse model. These results demonstrate that decreasing total tau levels is a valid strategy for mitigating tau-related neurodegeneration and reveal Nuak1 to be a novel therapeutic entry point for tauopathies.

Cerebellar Transcriptome Profiles of ATXN1 Transgenic Mice Reveal SCA1 Disease Progression and Protection Pathways.

SCA1, a fatal neurodegenerative disorder, is caused by a CAG expansion encoding a polyglutamine stretch in the protein ATXN1. We used RNA sequencing to profile cerebellar gene expression in Pcp2-ATXN1[82Q] mice with ataxia and progressive pathology and Pcp2-ATXN1[30Q]D776 animals having ataxia in absence of Purkinje cell progressive pathology. Weighted Gene Coexpression Network Analysis of the cerebellar expression data revealed two gene networks that significantly correlated with disease and have an expression profile correlating with disease progression in ATXN1[82Q] Purkinje cells. The Magenta Module provides a signature of suppressed transcriptional programs reflecting disease progression in Purkinje cells, while the Lt Yellow Module reflects transcriptional programs activated in response to disease in Purkinje cells as well as other cerebellar cell types. Furthermore, we found that upregulation of cholecystokinin (Cck) and subsequent interaction with the Cck1 receptor likely underlies the lack of progressive Purkinje cell pathology in Pcp2-ATXN1[30Q]D776 mice.

A native interactor scaffolds and stabilizes toxic ATAXIN-1 oligomers in SCA1.

Recent studies indicate that soluble oligomers drive pathogenesis in several neurodegenerative proteinopathies, including Alzheimer and Parkinson disease. Curiously, the same conformational antibody recognizes different disease-related oligomers, despite the variations in clinical presentation and brain regions affected, suggesting that the oligomer structure might be responsible for toxicity. We investigated whether polyglutamine-expanded ATAXIN-1, the protein that underlies spinocerebellar ataxia type 1, forms toxic oligomers and, if so, what underlies their toxicity. We found that mutant ATXN1 does form oligomers and that oligomer levels correlate with disease progression in the Atxn1(154Q/+) mice. Moreover, oligomeric toxicity, stabilization and seeding require interaction with Capicua, which is expressed at greater ratios with respect to ATXN1 in the cerebellum than in less vulnerable brain regions. Thus, specific interactors, not merely oligomeric structure, drive pathogenesis and contribute to regional vulnerability. Identifying interactors that stabilize toxic oligomeric complexes could answer longstanding questions about the pathogenesis of other proteinopathies.

Deficiency of Capicua disrupts bile acid homeostasis.

Capicua (CIC) has been implicated in pathogenesis of spinocerebellar ataxia type 1 and cancer in mammals; however, the in vivo physiological functions of CIC remain largely unknown. Here we show that Cic hypomorphic (Cic-L(-/-)) mice have impaired bile acid (BA) homeostasis associated with induction of proinflammatory cytokines. We discovered that several drug metabolism and BA transporter genes were down-regulated in Cic-L(-/-) liver, and that BA was increased in the liver and serum whereas bile was decreased within the gallbladder of Cic-L(-/-) mice. We also found that levels of proinflammatory cytokine genes were up-regulated in Cic-L(-/-) liver. Consistent with this finding, levels of hepatic transcriptional regulators, such as hepatic nuclear factor 1 alpha (HNF1α), CCAAT/enhancer-binding protein beta (C/EBPß), forkhead box protein A2 (FOXA2), and retinoid X receptor alpha (RXRα), were markedly decreased in Cic-L(-/-) mice. Moreover, induction of tumor necrosis factor alpha (Tnfα) expression and decrease in the levels of FOXA2, C/EBPß, and RXRα were found in Cic-L(-/-) liver before BA was accumulated, suggesting that inflammation might be the cause for the cholestasis in Cic-L(-/-) mice. Our findings indicate that CIC is a critical regulator of BA homeostasis, and that its dysfunction might be associated with chronic liver disease and metabolic disorders.

RAS-MAPK-MSK1 pathway modulates ataxin 1 protein levels and toxicity in SCA1.

Many neurodegenerative disorders, such as Alzheimer's, Parkinson's and polyglutamine diseases, share a common pathogenic mechanism: the abnormal accumulation of disease-causing proteins, due to either the mutant protein's resistance to degradation or overexpression of the wild-type protein. We have developed a strategy to identify therapeutic entry points for such neurodegenerative disorders by screening for genetic networks that influence the levels of disease-driving proteins. We applied this approach, which integrates parallel cell-based and Drosophila genetic screens, to spinocerebellar ataxia type 1 (SCA1), a disease caused by expansion of a polyglutamine tract in ataxin 1 (ATXN1). Our approach revealed that downregulation of several components of the RAS-MAPK-MSK1 pathway decreases ATXN1 levels and suppresses neurodegeneration in Drosophila and mice. Importantly, pharmacological inhibitors of components of this pathway also decrease ATXN1 levels, suggesting that these components represent new therapeutic targets in mitigating SCA1. Collectively, these data reveal new therapeutic entry points for SCA1 and provide a proof-of-principle for tackling other classes of intractable neurodegenerative diseases.

Polyglutamine disease toxicity is regulated by Nemo-like kinase in spinocerebellar ataxia type 1.

Polyglutamine diseases are dominantly inherited neurodegenerative diseases caused by an expansion of a CAG trinucleotide repeat encoding a glutamine tract in the respective disease-causing proteins. Extensive studies have been performed to unravel disease pathogenesis and to develop therapeutics. Here, we report on several lines of evidence demonstrating that Nemo-like kinase (NLK) is a key molecule modulating disease toxicity in spinocerebellar ataxia type 1 (SCA1), a disease caused by a polyglutamine expansion in the protein ATAXIN1 (ATXN1). Specifically, we show that NLK, a serine/threonine kinase that interacts with ATXN1, modulates disease phenotypes of polyglutamine-expanded ATXN1 in a Drosophila model of SCA1. Importantly, the effect of NLK on SCA1 pathology is dependent upon NLK's enzymatic activity. Consistent with this, reduced Nlk expression suppresses the behavioral and neuropathological phenotypes in SCA1 knock-in mice. These data clearly indicate that either reducing NLK enzymatic activity or decreasing NLK expression levels can have beneficial effects against the toxicity induced by polyglutamine-expanded ATXN1.

Ataxin1L is a regulator of HSC function highlighting the utility of cross-tissue comparisons for gene discovery.

Hematopoietic stem cells (HSCs) are rare quiescent cells that continuously replenish the cellular components of the peripheral blood. Observing that the ataxia-associated gene Ataxin-1-like (Atxn1L) was highly expressed in HSCs, we examined its role in HSC function through in vitro and in vivo assays. Mice lacking Atxn1L had greater numbers of HSCs that regenerated the blood more quickly than their wild-type counterparts. Molecular analyses indicated Atxn1L null HSCs had gene expression changes that regulate a program consistent with their higher level of proliferation, suggesting that Atxn1L is a novel regulator of HSC quiescence. To determine if additional brain-associated genes were candidates for hematologic regulation, we examined genes encoding proteins from autism- and ataxia-associated protein-protein interaction networks for their representation in hematopoietic cell populations. The interactomes were found to be highly enriched for proteins encoded by genes specifically expressed in HSCs relative to their differentiated progeny. Our data suggest a heretofore unappreciated similarity between regulatory modules in the brain and HSCs, offering a new strategy for novel gene discovery in both systems.