Distal enhancers upstream of the Charcot-Marie-Tooth type 1A disease gene PMP22

Transcriptional Control of Peripheral Nerve Regeneration

Transcription factors are master regulators of various cellular processes under diverse physiological and pathological conditions. Many transcription factors that are differentially expressed after injury to peripheral nerves play important roles in nerve regeneration. Considering that rapid and timely regrowth of injured axons is a prerequisite for successful target reinnervation, here, we compile transcription factors that mediates axon elongation, including axon growth suppressor Klf4 and axon growth promoters c-Myc, Sox11, STAT3, Atf3, c-Jun, Smad1, C/EBPδ, and p53. Besides neuronal changes, Schwann cell phenotype modulation is also critical for nerve regeneration. The activation of Schwann cells at early time points post injury provides a permissive microenvironment whereas the re-differentiation of Schwann cells at later time points supports myelin sheath formation. Hence, c-Jun and Sox2, two critical drivers for Schwann cell reprogramming, as well as Krox-20 and Sox10, two essential regulators of Schwann cell myelination, are reviewed. These transcription factors may serve as promising targets for promoting the functional recovery of injured peripheral nerves.

Disease Modeling of Rare Neurological Disorders in Zebrafish

Rare diseases are those which affect a small number of people compared to the general population. However, many patients with a rare disease remain undiagnosed, and a large majority of rare diseases still have no form of viable treatment. Approximately 40% of rare diseases include neurologic and neurodevelopmental disorders. In order to understand the characteristics of rare neurological disorders and identify causative genes, various model organisms have been utilized extensively. In this review, the characteristics of model organisms, such as roundworms, fruit flies, and zebrafish, are examined, with an emphasis on zebrafish disease modeling in rare neurological disorders.

Mechanisms and Treatments in Demyelinating CMT

Demyelinating forms of Charcot-Marie-Tooth disease (CMT) are genetically and phenotypically heterogeneous and result from highly diverse biological mechanisms including gain of function (including dominant negative effects) and loss of function. While no definitive treatment is currently available, rapid advances in defining the pathomechanisms of demyelinating CMT have led to promising pre-clinical studies, as well as emerging clinical trials. Especially promising are the recently completed pre-clinical genetic therapy studies in PMP-22, GJB1, and SH3TC2-associated neuropathies, particularly given the success of similar approaches in humans with spinal muscular atrophy and transthyretin familial polyneuropathy. This article focuses on neuropathies related to mutations in PMP-22, MPZ, and GJB1, which together comprise the most common forms of demyelinating CMT, as well as on select rarer forms for which promising treatment targets have been identified. Clinical characteristics and pathomechanisms are reviewed in detail, with emphasis on therapeutically targetable biological pathways. Also discussed are the challenges facing the CMT research community in its efforts to advance the rapidly evolving biological insights to effective clinical trials. These considerations include the limitations of currently available animal models, the need for personalized medicine approaches/allele-specific interventions for select forms of demyelinating CMT, and the increasing demand for optimal clinical outcome assessments and objective biomarkers.

Insights Into the Role and Potential of Schwann Cells for Peripheral Nerve Repair From Studies of Development and Injury

Peripheral nerve injuries arising from trauma or disease can lead to sensory and motor deficits and neuropathic pain. Despite the purported ability of the peripheral nerve to self-repair, lifelong disability is common. New molecular and cellular insights have begun to reveal why the peripheral nerve has limited repair capacity. The peripheral nerve is primarily comprised of axons and Schwann cells, the supporting glial cells that produce myelin to facilitate the rapid conduction of electrical impulses. Schwann cells are required for successful nerve regeneration; they partially “de-differentiate” in response to injury, re-initiating the expression of developmental genes that support nerve repair. However, Schwann cell dysfunction, which occurs in chronic nerve injury, disease, and aging, limits their capacity to support endogenous repair, worsening patient outcomes. Cell replacement-based therapeutic approaches using exogenous Schwann cells could be curative, but not all Schwann cells have a “repair” phenotype, defined as the ability to promote axonal growth, maintain a proliferative phenotype, and remyelinate axons. Two cell replacement strategies are being championed for peripheral nerve repair: prospective isolation of “repair” Schwann cells for autologous cell transplants, which is hampered by supply challenges, and directed differentiation of pluripotent stem cells or lineage conversion of accessible somatic cells to induced Schwann cells, with the potential of “unlimited” supply. All approaches require a solid understanding of the molecular mechanisms guiding Schwann cell development and the repair phenotype, which we review herein. Together these studies provide essential context for current efforts to design glial cell-based therapies for peripheral nerve regeneration.

Genetic mechanisms of peripheral nerve disease

Peripheral neuropathies of genetic etiology are a very diverse group of disorders manifesting either as non-syndromic inherited neuropathies without significant manifestations outside the peripheral nervous system, or as part of a systemic or syndromic genetic disorder. The former and most frequent group is collectively known as Charcot-Marie-Tooth disease (CMT), with prevalence as high as 1:2,500 world-wide, and has proven to be genetically highly heterogeneous. More than 100 different genes have been identified so far to cause various CMT forms, following all possible inheritance patterns. CMT causative genes belong to several common functional pathways that are essential for the integrity of the peripheral nerve. Their discovery has provided insights into the normal biology of axons and myelinating cells, and has highlighted the molecular mechanisms including both loss of function and gain of function effects, leading to peripheral nerve degeneration. Demyelinating neuropathies result from dysfunction of genes primarily affecting myelinating Schwann cells, while axonal neuropathies are caused by genes affecting mostly neurons and their long axons. Furthermore, mutation in genes expressed outside the nervous system, as in the case of inherited amyloid neuropathies, may cause peripheral neuropathy resulting from accumulation of β-structured amyloid fibrils in peripheral nerves in addition to various organs. Increasing insights into the molecular-genetic mechanisms have revealed potential therapeutic targets. These will enable the development of novel therapeutics for genetic neuropathies that remain, in their majority, without effective treatment.

Pmp22 super-enhancer deletion causes tomacula formation and conduction block in peripheral nerves

Copy number variation of the peripheral nerve myelin gene Peripheral Myelin Protein 22 (PMP22) causes multiple forms of inherited peripheral neuropathy. The duplication of a 1.4 Mb segment surrounding this gene in chromosome 17p12 (c17p12) causes the most common form of Charcot-Marie-Tooth disease type 1A, whereas the reciprocal deletion of this gene causes a separate neuropathy termed hereditary neuropathy with liability to pressure palsies (HNPP). PMP22 is robustly induced in Schwann cells in early postnatal development, and several transcription factors and their cognate regulatory elements have been implicated in coordinating the gene's proper expression. We previously found that a distal super-enhancer domain was important for Pmp22 expression in vitro, with particular impact on a Schwann cell-specific alternative promoter. Here, we investigate the consequences of deleting this super-enhancer in vivo. We find that loss of the super-enhancer in mice reduces Pmp22 expression throughout development and into adulthood, with greater impact on the Schwann cell-specific promoter. Additionally, these mice display tomacula formed by excessive myelin folding, a pathological hallmark of HNPP, as have been previously observed in heterozygous Pmp22 mice as well as sural biopsies from patients with HNPP. Our findings demonstrate a mechanism by which smaller copy number variations, not including the Pmp22 gene, are sufficient to reduce gene expression and phenocopy a peripheral neuropathy caused by the HNPP-associated deletion encompassing PMP22.

A de novo EGR2 variant, c.1232A > G p.Asp411Gly, causes severe early-onset Charcot-Marie-Tooth Neuropathy Type 3 (Dejerine-Sottas Neuropathy)

EGR2 (early growth response 2) is a crucial transcription factor for the myelination of the peripheral nervous system. Mutations in EGR2 are reported to cause a heterogenous spectrum of peripheral neuropathy with wide variation in both severity and age of onset, including demyelinating and axonal forms of Charcot-Marie Tooth (CMT) neuropathy, Dejerine-Sottas neuropathy (DSN/CMT3), and congenital hypomyelinating neuropathy (CHN/CMT4E). Here we report a sporadic de novo EGR2 variant, c.1232A > G (NM_000399.5), causing a missense p.Asp411Gly substitution and discovered through whole-exome sequencing (WES) of the proband. The resultant phenotype is severe demyelinating DSN with onset at two years of age, confirmed through nerve biopsy and electrophysiological examination. In silico analyses showed that the Asp411 residue is evolutionarily conserved, and the p.Asp411Gly variant was predicted to be deleterious by multiple in silico analyses. A luciferase-based reporter assay confirmed the reduced ability of p.Asp411Gly EGR2 to activate a PMP22 (peripheral myelin protein 22) enhancer element compared to wild-type EGR2. This study adds further support to the heterogeneity of EGR2-related peripheral neuropathies and provides strong functional evidence for the pathogenicity of the p.Asp411Gly EGR2 variant.

Regulating PMP22 expression as a dosage sensitive neuropathy gene

Structural variation in the human genome has emerged as a major cause of disease as genomic data have accumulated. One of the most common structural variants associated with human disease causes the heritable neuropathy known as Charcot-Marie-Tooth (CMT) disease type 1A. This 1.4 Mb duplication causes nearly half of the CMT cases that are genetically diagnosed. The PMP22 gene is highly induced in Schwann cells during development, although its precise role in myelin formation and homeostasis is still under active investigation. The PMP22 gene can be considered as a nucleoprotein complex with enzymatic activity to produce the PMP22 transcript, and the complex is allosterically regulated by transcription factors that respond to intracellular signals and epigenomic modifications. The control of PMP22 transcript levels has been one of the major therapeutic targets of therapy development, and this review summarizes those approaches as well as efforts to characterize the regulation of the PMP22 gene.

Regulation of the neuropathy-associated Pmp22 gene by a distal super-enhancer

Peripheral nerve myelination is adversely affected in the most common form of the hereditary peripheral neuropathy called Charcot-Marie-Tooth Disease. This form, classified as CMT1A, is caused by a 1.4 Mb duplication on chromosome 17, which includes the abundantly expressed Schwann cell myelin gene, Peripheral Myelin Protein 22 (PMP22). This is one of the most common copy number variants causing neurological disease. Overexpression of Pmp22 in rodent models recapitulates several aspects of neuropathy, and reduction of Pmp22 in such models results in amelioration of the neuropathy phenotype. Recently we identified a potential super-enhancer approximately 90-130 kb upstream of the Pmp22 transcription start sites. This super-enhancer encompasses a cluster of individual enhancers that have the acetylated histone H3K27 active enhancer mark, and coincides with smaller duplications identified in patients with milder CMT1A-like symptoms, where the PMP22 coding region itself was not part of the duplication. In this study, we have utilized genome editing to create a deletion of this super-enhancer to determine its role in Pmp22 regulation. Our data show a significant decrease in Pmp22 transcript expression using allele-specific internal controls. Moreover, the P2 promoter of the Pmp22 gene, which is used in other cell types, is affected, but we find that the Schwann cell-specific P1 promoter is disproportionately more sensitive to loss of the super-enhancer. These data show for the first time the requirement of these upstream enhancers for full Pmp22 expression.

Waardenburg syndrome: a rare cause of inherited neuropathy due to SOX10 mutation

Waardenburg syndrome (WS) is a rare disorder comprising sensorineural deafness and pigmentation abnormalities. Four distinct subtypes are defined based on the presence or absence of additional symptoms. Mutations in six genes have been described in WS. SOX10 mutations are usually associated with a more severe phenotype of WS with peripheral demyelinating neuropathy, central dysmyelinating leukodystrophy, and Hirschsprung disease. Here we report a 32-year-old man with a novel heterozygous missense variant in SOX10 gene, who presented with congenital deafness, Hirschsprung disease, iris heterochromia, foot deformity, and intermediate conduction velocity length-dependent sensorimotor neuropathy. This case highlights that the presence of other non-neuropathic features in a patient with presumed hereditary neuropathy should alert the clinician to possible atypical rare causes.

Structural variations causing inherited peripheral neuropathies: A paradigm for understanding genomic organization, chromatin interactions, and gene dysregulation

Inherited peripheral neuropathies (IPNs) are a clinically and genetically heterogeneous group of diseases affecting the motor and sensory peripheral nerves. IPNs have benefited from gene discovery and genetic diagnosis using next-generation sequencing with over 80 causative genes available for testing. Despite this success, up to 50% of cases remain genetically unsolved. In the absence of protein coding mutations, noncoding DNA or structural variation (SV) mutations are a possible explanation. The most common IPN, Charcot-Marie-Tooth neuropathy type 1A (CMT1A), is caused by a 1.5 Mb duplication causing trisomy of the dosage sensitive gene PMP22. Using genome sequencing, we recently identified two large genomic rearrangements causing IPN subtypes X-linked CMT (CMTX3) and distal hereditary motor neuropathy (DHMN1), thereby expanding the spectrum of SV mutations causing IPN. Understanding how newly discovered SVs can cause IPN may serve as a useful paradigm to examine the role of topologically associated domains (TADs), chromatin interactions, and gene dysregulation in disease. This review will describe the growing role of SV in the pathogenesis of IPN and the importance of considering this type of mutation in Mendelian diseases where protein coding mutations cannot be identified.

Epigenetic Control of Schwann Cells

The journey of Schwann cells from their origin in the neural crest to their ensheathment and myelination of peripheral nerves is a remarkable one. Their apparent static function in enabling saltatory conduction of mature nerve is not only vital for long-term health of peripheral nerve but also belies an innate capacity of terminally differentiated Schwann cells to radically alter their differentiation status in the face of nerve injury. The transition from migrating neural crest cells to nerve ensheathment, and then myelination of large diameter axons has been characterized extensively and several of the transcriptional networks have been identified. However, transcription factors must also modify chromatin structure during Schwann cell maturation and this review will focus on chromatin modification machinery that is involved in promoting the transition to, and maintenance of, myelinating Schwann cells. In addition, Schwann cells are known to play important regenerative roles after peripheral nerve injury, and information on epigenomic reprogramming of the Schwann cell genome has emerged. Characterization of epigenomic requirements for myelin maintenance and Schwann cell responses to injury will be vital in understanding how the various Schwann cell functions can be optimized to maintain and repair peripheral nerve function.

Genome-Edited Cell Lines for High-Throughput Screening

Measurement of gene expression for high-throughput screening is an increasingly used technique that has been developed for not only gene dosage disorders resulting from disease-associated copy number variations, but also for induction/repression of genes modulating the severity of a disease phenotype. Traditional methods have employed transient or stable transfection of reporter constructs in which a single reporter is driven by selected regulatory elements from the candidate gene. However, individual regulatory elements are inherently unable to capture the integrated regulation of multiple enhancers at the endogenous locus, and random reporter insertion can result in neighborhood effects that impact the physiological responsiveness of the reporter. Therefore, we outline a general method of employing genome editing to insert reporters into the 3' UTR of a candidate gene, which has been used successfully in our studies of the Pmp22 gene associated with Charcot-Marie-Tooth disease. The method employs genome editing to insert two nonhomologous reporters that maximize the efficiency of identification of biologically active molecules through concordant responses in small molecule screening. We include a number of aspects of the design and construction of these reporter assays that will be applicable to creation of similar assays in a variety of cell types.

Dual specificity phosphatase 15 regulates Erk activation in Schwann cells

Schwann cells and oligodendrocytes are the myelinating cells of the peripheral and central nervous system, respectively. Despite having different myelin components and different transcription factors driving their terminal differentiation there are shared molecular mechanisms between the two. Sox10 is one common transcription factor required for several steps in development of myelinating glia. However, other factors are divergent as Schwann cells need the transcription factor early growth response 2/Krox20 and oligodendrocytes require Myrf. Likewise, some signaling pathways, like the Erk1/2 kinases, are necessary in both cell types for proper myelination. Nonetheless, the molecular mechanisms that control this shared signaling pathway in myelinating cells remain only partially characterized. The hypothesis of this study is that signaling pathways that are similarly regulated in both Schwann cells and oligodendrocytes play central roles in coordinating the differentiation of myelinating glia. To address this hypothesis, we have used genome-wide binding data to identify a relatively small set of genes that are similarly regulated by Sox10 in myelinating glia. We chose one such gene encoding Dual specificity phosphatase 15 (Dusp15) for further analysis in Schwann cell signaling. RNA interference and gene deletion by genome editing in cultured RT4 and primary Schwann cells showed Dusp15 is necessary for full activation of Erk1/2 phosphorylation. In addition, we show that Dusp15 represses expression of several myelin genes, including myelin basic protein. The data shown here support a mechanism by which early growth response 2 activates myelin genes, but also induces a negative feedback loop through Dusp15 to limit over-expression of myelin genes.

Stringent comparative sequence analysis reveals SOX10 as a putative inhibitor of glial cell differentiation

Background

The transcription factor SOX10 is essential for all stages of Schwann cell development including myelination. SOX10 cooperates with other transcription factors to activate the expression of key myelin genes in Schwann cells and is therefore a context-dependent, pro-myelination transcription factor. As such, the identification of genes regulated by SOX10 will provide insight into Schwann cell biology and related diseases. While genome-wide studies have successfully revealed SOX10 target genes, these efforts mainly focused on myelinating stages of Schwann cell development. We propose that less-biased approaches will reveal novel functions of SOX10 outside of myelination.

Results

We developed a stringent, computational-based screen for genome-wide identification of SOX10 response elements. Experimental validation of a pilot set of predicted binding sites in multiple systems revealed that SOX10 directly regulates a previously unreported alternative promoter at SOX6, which encodes a transcription factor that inhibits glial cell differentiation. We further explored the utility of our computational approach by combining it with DNase-seq analysis in cultured Schwann cells and previously published SOX10 ChIP-seq data from rat sciatic nerve. Remarkably, this analysis enriched for genomic segments that map to loci involved in the negative regulation of gliogenesis including SOX5, SOX6, NOTCH1, HMGA2, HES1, MYCN, ID4, and ID2. Functional studies in Schwann cells revealed that: (1) all eight loci are expressed prior to myelination and down-regulated subsequent to myelination; (2) seven of the eight loci harbor validated SOX10 binding sites; and (3) seven of the eight loci are down-regulated upon repressing SOX10 function.

Conclusions

Our computational strategy revealed a putative novel function for SOX10 in Schwann cells, which suggests a model where SOX10 activates the expression of genes that inhibit myelination during non-myelinating stages of Schwann cell development. Importantly, the computational and functional datasets we present here will be valuable for the study of transcriptional regulation, SOX protein function, and glial cell biology.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-3167-3) contains supplementary material, which is available to authorized users.

Whole Genome Sequencing Identifies a 78 kb Insertion from Chromosome 8 as the Cause of Charcot-Marie-Tooth Neuropathy CMTX3

With the advent of whole exome sequencing, cases where no pathogenic coding mutations can be found are increasingly being observed in many diseases. In two large, distantly-related families that mapped to the Charcot-Marie-Tooth neuropathy CMTX3 locus at chromosome Xq26.3-q27.3, all coding mutations were excluded. Using whole genome sequencing we found a large DNA interchromosomal insertion within the CMTX3 locus. The 78 kb insertion originates from chromosome 8q24.3, segregates fully with the disease in the two families, and is absent from the general population as well as 627 neurologically normal chromosomes from in-house controls. Large insertions into chromosome Xq27.1 are known to cause a range of diseases and this is the first neuropathy phenotype caused by an interchromosomal insertion at this locus. The CMTX3 insertion represents an understudied pathogenic structural variation mechanism for inherited peripheral neuropathies. Our finding highlights the importance of considering all structural variation types when studying unsolved inherited peripheral neuropathy cases with no pathogenic coding mutations.

Tead1 regulates the expression of Peripheral Myelin Protein 22 during Schwann cell development

Schwann cells are myelinating glia in the peripheral nervous system that form the myelin sheath. A major cause of peripheral neuropathy is a copy number variant involving the Peripheral Myelin Protein 22 (PMP22) gene, which is located within a 1.4-Mb duplication on chromosome 17 associated with the most common form of Charcot-Marie-Tooth Disease (CMT1A). Rodent models of CMT1A have been used to show that reducing Pmp22 overexpression mitigates several aspects of a CMT1A-related phenotype. Mechanistic studies of Pmp22 regulation identified enhancers regulated by the Sox10 (SRY sex determining region Y-box 10) and Egr2/Krox20 (Early growth response protein 2) transcription factors in myelinated nerves. However, relatively little is known regarding how other transcription factors induce Pmp22 expression during Schwann cell development and myelination. Here, we examined Pmp22 enhancers as a function of cell type-specificity, nerve injury and development. While Pmp22 enhancers marked by active histone modifications were lost or remodeled after injury, we found that these enhancers were permissive in early development prior to Pmp22 upregulation. Pmp22 enhancers contain binding motifs for TEA domain (Tead) transcription factors of the Hippo signaling pathway. We discovered that Tead1 and co-activators Yap and Taz are required for Pmp22 expression, as well as for the expression of Egr2 Tead1 directly binds Pmp22 and Egr2 enhancers early in development and Tead1 binding is induced during myelination, correlating with Pmp22 expression. The data identify Tead1 as a novel regulator of Pmp22 expression during development in concert with Sox10 and Egr2.

YAP and TAZ control peripheral myelination and the expression of laminin receptors in Schwann cells

Myelination is essential for nervous system function. Schwann cells interact with neurons and the basal lamina to myelinate axons, using known receptors, signals and transcription factors. In contrast, the transcriptional control of axonal sorting and the role of mechanotransduction in myelination are largely unknown. Yap and Taz are effectors of the Hippo pathway that integrate chemical and mechanical signals in cells. We describe a previously unknown role for the Hippo pathway in myelination. Using conditional mutagenesis in mice we show that Taz is required in Schwann cells for radial sorting and myelination, and that Yap is redundant with Taz. Yap/Taz are activated in Schwann cells by mechanical stimuli, and regulate Schwann cell proliferation and transcription of basal lamina receptor genes, both necessary for proper radial sorting of axons and subsequent myelination. These data link transcriptional effectors of the Hippo pathway and of mechanotransduction to myelin formation in Schwann cells.

Differential Sox10 genomic occupancy in myelinating glia

Myelin is formed by specialized myelinating glia: oligodendrocytes and Schwann cells in the central and peripheral nervous systems, respectively. While there are distinct developmental aspects and regulatory pathways in these two cell types, myelination in both systems requires the transcriptional activator Sox10. Sox10 interacts with cell type-specific transcription factors at some loci to induce myelin gene expression, but it is largely unknown how Sox10 transcriptional networks globally compare between oligodendrocytes and Schwann cells. We used in vivo ChIP-Seq analysis of spinal cord and peripheral nerve (sciatic nerve) to identify unique and shared Sox10 binding sites and assess their correlation with active enhancers and transcriptional profiles in oligodendrocytes and Schwann cells. Sox10 binding sites overlap with active enhancers and critical cell type-specific regulators of myelination, such as Olig2 and Myrf in oligodendrocytes, and Egr2/Krox20 in Schwann cells. Sox10 sites also associate with genes critical for myelination in both oligodendrocytes and Schwann cells and are found within super-enhancers previously defined in brain. In Schwann cells, Sox10 sites contain binding motifs of putative partners in the Sp/Klf, Tead, and nuclear receptor protein families. Specifically, siRNA analysis of nuclear receptors Nr2f1 and Nr2f2 revealed downregulation of myelin genes Mbp and Ndrg1 in primary Schwann cells. Our analysis highlights different mechanisms that establish cell type-specific genomic occupancy of Sox10, which reflects the unique characteristics of oligodendrocyte and Schwann cell differentiation. GLIA 2015;63:1897-1914.

Dynamic regulation of Schwann cell enhancers after peripheral nerve injury

Myelination of the peripheral nervous system is required for axonal function and long term stability. After peripheral nerve injury, Schwann cells transition from axon myelination to a demyelinated state that supports neuronal survival and ultimately remyelination of axons. Reprogramming of gene expression patterns during development and injury responses is shaped by the actions of distal regulatory elements that integrate the actions of multiple transcription factors. We used ChIP-seq to measure changes in histone H3K27 acetylation, a mark of active enhancers, to identify enhancers in myelinating rat peripheral nerve and their dynamics after demyelinating nerve injury. Analysis of injury-induced enhancers identified enriched motifs for c-Jun, a transcription factor required for Schwann cells to support nerve regeneration. We identify a c-Jun-bound enhancer in the gene for Runx2, a transcription factor induced after nerve injury, and we show that Runx2 is required for activation of other induced genes. In contrast, enhancers that lose H3K27ac after nerve injury are enriched for binding sites of the Sox10 and early growth response 2 (Egr2/Krox20) transcription factors, which are critical determinants of Schwann cell differentiation. Egr2 expression is lost after nerve injury, and many Egr2-binding sites lose H3K27ac after nerve injury. However, the majority of Egr2-bound enhancers retain H3K27ac, indicating that other transcription factors maintain active enhancer status after nerve injury. The global epigenomic changes in H3K27ac deposition pinpoint dynamic changes in enhancers that mediate the effects of transcription factors that control Schwann cell myelination and peripheral nervous system responses to nerve injury.

Genome editing-enabled HTS assays expand drug target pathways for Charcot-Marie-tooth disease

Copy number variation resulting in excess PMP22 protein causes the peripheral neuropathy Charcot–Marie–Tooth disease, type 1A. To broadly interrogate chemically sensitive transcriptional pathways controlling PMP22 protein levels, we used the targeting precision of TALEN-mediated genome editing to embed reporters within the genetic locus harboring the Peripheral Myelin Protein 22 (Pmp22) gene. Using a Schwann cell line with constitutively high endogenous levels of Pmp22, we obtained allelic insertion of secreted bioluminescent reporters with sufficient signal to enable a 1536-well assay. Our findings from the quantitative high-throughput screening (qHTS) of several thousand drugs and clinically investigated compounds using this assay design both overlapped and expanded results from a previous assay using a randomly inserted reporter gene controlled by a single regulatory element of the Pmp22 gene. A key difference was the identification of a kinase-controlled inhibitory pathway of Pmp22 transcription revealed by the activity of the Protein kinase C (PKC)-modulator bryostatin.

MicroRNA and transcriptional crosstalk in myelinating glia

Several recent studies have addressed the important role of microRNA in regulation of differentiation of myelinating glia. While Schwann cells and oligodendrocytes in the peripheral and central nervous systems, respectively, exhibit significant morphological and regulatory differences, some aspects of transcriptional and microRNA regulation are shared between these two cell types. This review focuses on the intersection of microRNAs with transcriptional regulation in Schwann cell and oligodendrocyte differentiation. In particular, several microRNAs have been shown to modulate expression of critical transcription factors, and in turn, the regulation of microRNA expression is enmeshed within transcriptional networks that coordinate both coding gene and noncoding RNA profiles of myelinating cells. These hubs of regulation control both myelin gene expression as well as the cell cycle transitions of Schwann cells and oligodendrocytes as they terminally differentiate. In addition, some studies have begin to highlight the combinatorial effects of different microRNAs that establish the narrow range of gene regulation required for efficient and stable myelin formation. Overall, the integration of microRNA and transcriptional aspects will help elucidate mechanistic control of the myelination process.

Haplotype-specific modulation of a SOX10/CREB response element at the Charcot-Marie-Tooth disease type 4C locus SH3TC2

Loss-of-function mutations in the Src homology 3 (SH3) domain and tetratricopeptide repeats 2 (SH3TC2) gene cause autosomal recessive demyelinating Charcot-Marie-Tooth neuropathy. The SH3TC2 protein has been implicated in promyelination signaling through axonal neuregulin-1 and the ERBB2 Schwann cell receptor. However, little is known about the transcriptional regulation of the SH3TC2 gene. We performed computational and functional analyses that revealed two cis-acting regulatory elements at SH3TC2-one at the promoter and one ∼150 kb downstream of the transcription start site. Both elements direct reporter gene expression in Schwann cells and are responsive to the transcription factor SOX10, which is essential for peripheral nervous system myelination. The downstream enhancer harbors a single-nucleotide polymorphism (SNP) that causes an ∼80% reduction in enhancer activity. The SNP resides directly within a predicted binding site for the transcription factor cAMP response element binding protein (CREB), and we demonstrate that this regulatory element binds to CREB and is activated by CREB expression. Finally, forskolin induces Sh3tc2 expression in rat primary Schwann cells, indicating that SH3TC2 is a CREB target gene. These findings prompted us to determine if SNP genotypes at SH3TC2 are associated with differential phenotypes in the most common demyelinating peripheral neuropathy, CMT1A. Interestingly, this revealed several associations between SNP alleles and disease severity. In summary, our data indicate that SH3TC2 is regulated by the transcription factors CREB and SOX10, define a regulatory SNP at this disease-associated locus and reveal SH3TC2 as a candidate modifier locus of CMT disease phenotypes.

Parental binge alcohol abuse alters F1 generation hypothalamic gene expression in the absence of direct fetal alcohol exposure

Adolescent binge alcohol exposure has long-lasting effects on the expression of hypothalamic genes that regulate the stress response, even in the absence of subsequent adult alcohol exposure. This suggests that alcohol can induce permanent gene expression changes, potentially through epigenetic modifications to specific genes. Epigenetic modifications can be transmitted to future generations therefore, and in these studies we investigated the effects of adolescent binge alcohol exposure on hypothalamic gene expression patterns in the F1 generation offspring. It has been well documented that maternal alcohol exposure during fetal development can have devastating neurological consequences. However, less is known about the consequences of maternal and/or paternal alcohol exposure outside of the gestational time frame. Here, we exposed adolescent male and female rats to a repeated binge EtOH exposure paradigm and then mated them in adulthood. Hypothalamic samples were taken from the offspring of these animals at postnatal day (PND) 7 and subjected to a genome-wide microarray analysis followed by qRT-PCR for selected genes. Importantly, the parents were not intoxicated at the time of mating and were not exposed to EtOH at any time during gestation therefore the offspring were never directly exposed to EtOH. Our results showed that the offspring of alcohol-exposed parents had significant differences compared to offspring from alcohol-naïve parents. Specifically, major differences were observed in the expression of genes that mediate neurogenesis and synaptic plasticity during neurodevelopment, genes important for directing chromatin remodeling, posttranslational modifications or transcription regulation, as well as genes involved in regulation of obesity and reproductive function. These data demonstrate that repeated binge alcohol exposure during pubertal development can potentially have detrimental effects on future offspring even in the absence of direct fetal alcohol exposure.

The PMP22 gene and its related diseases

Peripheral myelin protein-22 (PMP22) is primarily expressed in the compact myelin of the peripheral nervous system. Levels of PMP22 have to be tightly regulated since alterations of PMP22 levels by mutations of the PMP22 gene are responsible for >50 % of all patients with inherited peripheral neuropathies, including Charcot-Marie-Tooth type-1A (CMT1A) with trisomy of PMP22, hereditary neuropathy with liability to pressure palsies (HNPP) with heterozygous deletion of PMP22, and CMT1E with point mutations of PMP22. While overexpression and point-mutations of the PMP22 gene may produce gain-of-function phenotypes, deletion of PMP22 results in a loss-of-function phenotype that reveals the normal physiological functions of the PMP22 protein. In this article, we will review the basic genetics, biochemistry and molecular structure of PMP22, followed by discussion of the current understanding of pathogenic mechanisms involving in the inherited neuropathies with mutations in PMP22 gene.

Genome-wide analysis of EGR2/SOX10 binding in myelinating peripheral nerve

Myelin is essential for the rapidity of saltatory nerve conduction, and also provides trophic support for axons to prevent axonal degeneration. Two critical determinants of myelination are SOX10 and EGR2/KROX20. SOX10 is required for specification of Schwann cells from neural crest, and is required at every stage of Schwann cell development. Egr2/Krox20 expression is activated by axonal signals in myelinating Schwann cells, and is required for cell cycle arrest and myelin formation. To elucidate the integrated function of these two transcription factors during peripheral nerve myelination, we performed in vivo ChIP-Seq analysis of myelinating peripheral nerve. Integration of these binding data with loss-of-function array data identified a range of genes regulated by these factors. In addition, although SOX10 itself regulates Egr2/Krox20 expression, leading to coordinate activation of several major myelin genes by the two factors, there is a large subset of genes that are activated independent of EGR2. Finally, the results identify a set of SOX10-dependent genes that are expressed in early Schwann cell development, but become subsequently repressed by EGR2/KROX20.

Murine therapeutic models for Charcot-Marie-Tooth (CMT) disease

INTRODUCTION OR BACKGROUND

Charcot-Marie-Tooth (CMT) disease represents a broad group of inherited motor and sensory neuropathies which can originate from various genetic aberrations, e.g. mutations, deletions and duplications.

SOURCES OF DATA

We performed a literature review on murine animal models of CMT disease with regard to experimental therapeutic approaches. Hereby, we focussed on the demyelinating subforms of CMT (CMT1). PubMed items were CMT, animal model, demyelination and therapy.

AREAS OF AGREEMENT

Patients affected by CMT suffer from slowly progressive, distally pronounced muscle atrophy caused by an axonal loss. The disease severity is highly variable and impairments may result in wheelchair boundness. No therapy is available yet.

AREAS OF CONTROVERSY

Numerous rodent models for the various CMT subtypes are available today. The selection of the correct animal model for the specific CMT subtype provides an important prerequisite for the successful translation of experimental findings in patients.

GROWING POINTS

Despite more than 20 years of remarkable progress in CMT research, the disease is still left untreatable. There is a growing number of experimental therapeutic strategies that may be translated into future clinical trials in patients with CMT.

AREAS TIMELY FOR DEVELOPING RESEARCH

The slow disease progression and insensitive outcome measures hamper clinical therapy trials in CMT. Biomarkers may provide powerful tools to monitor therapeutic efficacy. Recently, we have shown that transcriptional profiling can be utilized to assess and predict the disease severity in a transgenic rat model and in affected humans.