Long-term outcomes of hospitalized patients with SARS-CoV-2/COVID-19 with and without neurological involvement: 3-year follow-up assessment.

BACKGROUND: Acute neurological manifestation is a common complication of acute Coronavirus Disease 2019 (COVID-19) disease. This retrospective cohort study investigated the 3-year outcomes of patients with and without significant neurological manifestations during initial COVID-19 hospitalization. METHODS AND FINDINGS: Patients hospitalized for Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) infection between 03/01/2020 and 4/16/2020 in the Montefiore Health System in the Bronx, an epicenter of the early pandemic, were included. Follow-up data was captured up to 01/23/2023 (3 years post-COVID-19). This cohort consisted of 414 patients with COVID-19 with significant neurological manifestations and 1,199 propensity-matched patients (for age and COVID-19 severity score) with COVID-19 without neurological manifestations. Neurological involvement during the acute phase included acute stroke, new or recrudescent seizures, anatomic brain lesions, presence of altered mentation with evidence for impaired cognition or arousal, and neuro-COVID-19 complex (headache, anosmia, ageusia, chemesthesis, vertigo, presyncope, paresthesias, cranial nerve abnormalities, ataxia, dysautonomia, and skeletal muscle injury with normal orientation and arousal signs). There were no significant group differences in female sex composition (44.93% versus 48.21%, p = 0.249), ICU and IMV status, white, not Hispanic (6.52% versus 7.84%, p = 0.380), and Hispanic (33.57% versus 38.20%, p = 0.093), except black non-Hispanic (42.51% versus 36.03%, p = 0.019). Primary outcomes were mortality, stroke, heart attack, major adverse cardiovascular events (MACE), reinfection, and hospital readmission post-discharge. Secondary outcomes were neuroimaging findings (hemorrhage, active and prior stroke, mass effect, microhemorrhages, white matter changes, microvascular disease (MVD), and volume loss). More patients in the neurological cohort were discharged to acute rehabilitation (10.39% versus 3.34%, p < 0.001) or skilled nursing facilities (35.75% versus 25.35%, p < 0.001) and fewer to home (50.24% versus 66.64%, p < 0.001) than matched controls. Incidence of readmission for any reason (65.70% versus 60.72%, p = 0.036), stroke (6.28% versus 2.34%, p < 0.001), and MACE (20.53% versus 16.51%, p = 0.032) was higher in the neurological cohort post-discharge. Per Kaplan-Meier univariate survival curve analysis, such patients in the neurological cohort were more likely to die post-discharge compared to controls (hazard ratio: 2.346, (95% confidence interval (CI) [1.586, 3.470]; p < 0.001)). Across both cohorts, the major causes of death post-discharge were heart disease (13.79% neurological, 15.38% control), sepsis (8.63%, 17.58%), influenza and pneumonia (13.79%, 9.89%), COVID-19 (10.34%, 7.69%), and acute respiratory distress syndrome (ARDS) (10.34%, 6.59%). Factors associated with mortality after leaving the hospital involved the neurological cohort (odds ratio (OR): 1.802 (95% CI [1.237, 2.608]; p = 0.002)), discharge disposition (OR: 1.508 (95% CI [1.276, 1.775]; p < 0.001)), congestive heart failure (OR: 2.281 (95% CI [1.429, 3.593]; p < 0.001)), higher COVID-19 severity score (OR: 1.177 (95% CI [1.062, 1.304]; p = 0.002)), and older age (OR: 1.027 (95% CI [1.010, 1.044]; p = 0.002)). There were no group differences in radiological findings, except that the neurological cohort showed significantly more age-adjusted brain volume loss (p = 0.045) than controls. The study's patient cohort was limited to patients infected with COVID-19 during the first wave of the pandemic, when hospitals were overburdened, vaccines were not yet available, and treatments were limited. Patient profiles might differ when interrogating subsequent waves. CONCLUSIONS: Patients with COVID-19 with neurological manifestations had worse long-term outcomes compared to matched controls. These findings raise awareness and the need for closer monitoring and timely interventions for patients with COVID-19 with neurological manifestations, as their disease course involving initial neurological manifestations is associated with enhanced morbidity and mortality.

Microglial reduction of colony stimulating factor-1 receptor expression is sufficient to confer adult onset leukodystrophy.

Adult onset leukoencephalopathy with axonal spheroids and pigmented glia (ALSP) is a dementia resulting from dominantly inherited CSF1R inactivating mutations. The Csf1r+/- mouse mimics ALSP symptoms and pathology. Csf1r is mainly expressed in microglia, but also in cortical layer V neurons that are gradually lost in Csf1r+/- mice with age. We therefore examined whether microglial or neuronal Csf1r loss caused neurodegeneration in Csf1r+/- mice. The behavioral deficits, pathologies and elevation of Csf2 expression contributing to disease, previously described in the Csf1r+/- ALSP mouse, were reproduced by microglial deletion (MCsf1rhet mice), but not by neural deletion. Furthermore, increased Csf2 expression by callosal astrocytes, oligodendrocytes, and microglia was observed in Csf1r+/- mice and, in MCsf1rhet mice, the densities of these three cell types were increased in supraventricular patches displaying activated microglia, an early site of disease pathology. These data confirm that ALSP is a primary microgliopathy and inform future therapeutic and experimental approaches.

Increased localization of APP-C99 in mitochondria-associated ER membranes causes mitochondrial dysfunction in Alzheimer disease.

In the amyloidogenic pathway associated with Alzheimer disease (AD), the amyloid precursor protein (APP) is cleaved by ß-secretase to generate a 99-aa C-terminal fragment (C99) that is then cleaved by γ-secretase to generate the ß-amyloid (Aß) found in senile plaques. In previous reports, we and others have shown that γ-secretase activity is enriched in mitochondria-associated endoplasmic reticulum (ER) membranes (MAM) and that ER-mitochondrial connectivity and MAM function are upregulated in AD We now show that C99, in addition to its localization in endosomes, can also be found in MAM, where it is normally processed rapidly by γ-secretase. In cell models of AD, however, the concentration of unprocessed C99 increases in MAM regions, resulting in elevated sphingolipid turnover and an altered lipid composition of both MAM and mitochondrial membranes. In turn, this change in mitochondrial membrane composition interferes with the proper assembly and activity of mitochondrial respiratory supercomplexes, thereby likely contributing to the bioenergetic defects characteristic of AD.

Loss-of-Huntingtin in Medial and Lateral Ganglionic Lineages Differentially Disrupts Regional Interneuron and Projection Neuron Subtypes and Promotes Huntington's Disease-Associated Behavioral, Cellular, and Pathological Hallmarks.

Emerging studies are providing compelling evidence that the pathogenesis of Huntington's disease (HD), a neurodegenerative disorder with frequent midlife onset, encompasses developmental components. Moreover, our previous studies using a hypomorphic model targeting huntingtin during the neurodevelopmental period indicated that loss-of-function mechanisms account for this pathogenic developmental component (Arteaga-Bracho et al., 2016). In the present study, we specifically ascertained the roles of subpallial lineage species in eliciting the previously observed HD-like phenotypes. Accordingly, we used the Cre-loxP system to conditionally ablate the murine huntingtin gene (Httflx) in cells expressing the subpallial patterning markers Gsx2 (Gsx2-Cre) or Nkx2.1 (Nkx2.1-Cre) in Httflx mice of both sexes. These genetic manipulations elicited anxiety-like behaviors, hyperkinetic locomotion, age-dependent motor deficits, and weight loss in both Httflx;Gsx2-Cre and Httflx;Nkx2.1-Cre mice. In addition, these strains displayed unique but complementary spatial patterns of basal ganglia degeneration that are strikingly reminiscent of those seen in human cases of HD. Furthermore, we observed early deficits of somatostatin-positive and Reelin-positive interneurons in both Htt subpallial null strains, as well as early increases of cholinergic interneurons, Foxp2+ arkypallidal neurons, and incipient deficits with age-dependent loss of parvalbumin-positive neurons in Httflx;Nkx2.1-Cre mice. Overall, our findings indicate that selective loss-of-huntingtin function in subpallial lineages differentially disrupts the number, complement, and survival of forebrain interneurons and globus pallidus GABAergic neurons, thereby leading to the development of key neurological hallmarks of HD during adult life. Our findings have important implications for the establishment and deployment of neural circuitries and the integrity of network reserve in health and disease.SIGNIFICANCE STATEMENT Huntington's disease (HD) is a progressive degenerative disorder caused by aberrant trinucleotide expansion in the huntingtin gene. Mechanistically, this mutation involves both loss- and gain-of-function mechanisms affecting a broad array of cellular and molecular processes. Although huntingtin is widely expressed during adult life, the mutant protein only causes the demise of selective neuronal subtypes. The mechanisms accounting for this differential vulnerability remain elusive. In this study, we have demonstrated that loss-of-huntingtin function in subpallial lineages not only differentially disrupts distinct interneuron species early in life, but also leads to a pattern of neurological deficits that are reminiscent of HD. This work suggests that early disruption of selective neuronal subtypes may account for the profiles of enhanced regional cellular vulnerability to death in HD.

Selective expression of mutant huntingtin during development recapitulates characteristic features of Huntington's disease.

Recent studies have identified impairments in neural induction and in striatal and cortical neurogenesis in Huntington's disease (HD) knock-in mouse models and associated embryonic stem cell lines. However, the potential role of these developmental alterations for HD pathogenesis and progression is currently unknown. To address this issue, we used BACHD:CAG-Cre(ERT2) mice, which carry mutant huntingtin (mHtt) modified to harbor a floxed exon 1 containing the pathogenic polyglutamine expansion (Q97). Upon tamoxifen administration at postnatal day 21, the floxed mHtt-exon1 was removed and mHtt expression was terminated (Q97(CRE)). These conditional mice displayed similar profiles of impairments to those mice expressing mHtt throughout life: (i) striatal neurodegeneration, (ii) early vulnerability to NMDA-mediated excitotoxicity, (iii) impairments in motor coordination, (iv) temporally distinct abnormalities in striatal electrophysiological activity, and (v) altered corticostriatal functional connectivity and plasticity. These findings strongly suggest that developmental aberrations may play important roles in HD pathogenesis and progression.

Emerging roles of non-coding RNAs in brain evolution, development, plasticity and disease.

Novel classes of small and long non-coding RNAs (ncRNAs) are being characterized at a rapid pace, driven by recent paradigm shifts in our understanding of genomic architecture, regulation and transcriptional output, as well as by innovations in sequencing technologies and computational and systems biology. These ncRNAs can interact with DNA, RNA and protein molecules; engage in diverse structural, functional and regulatory activities; and have roles in nuclear organization and transcriptional, post-transcriptional and epigenetic processes. This expanding inventory of ncRNAs is implicated in mediating a broad spectrum of processes including brain evolution, development, synaptic plasticity and disease pathogenesis.

Postnatal and adult consequences of loss of huntingtin during development: Implications for Huntington's disease.

The mutation in huntingtin (mHtt) leads to a spectrum of impairments in the developing forebrain of Huntington's disease (HD) mouse models. Whether these developmental alterations are due to loss- or gain-of-function mechanisms and contribute to HD pathogenesis is unknown. We examined the role of selective loss of huntingtin (Htt) function during development on postnatal vulnerability to cell death. We employed mice expressing very low levels of Htt throughout embryonic life to postnatal day 21 (Hdhd•hyp). We demonstrated that Hdhd•hyp mice exhibit: (1) late-life striatal and cortical neuronal degeneration; (2) neurological and skeletal muscle alterations; and (3) white matter tract impairments and axonal degeneration. Hdhd•hyp embryos also exhibited subpallial heterotopias, aberrant striatal maturation and deregulation of gliogenesis. These results indicate that developmental deficits associated with Htt functions render cells present at discrete neural foci increasingly susceptible to cell death, thus implying the potential existence of a loss-of-function developmental component to HD pathogenesis.

Phenotypic characterization of a Csf1r haploinsufficient mouse model of adult-onset leukodystrophy with axonal spheroids and pigmented glia (ALSP).

Mutations in the colony stimulating factor-1 receptor (CSF1R) that abrogate the expression of the affected allele or lead to the expression of mutant receptor chains devoid of kinase activity have been identified in both familial and sporadic cases of ALSP. To determine the validity of the Csf1r heterozygous mouse as a model of adult-onset leukoencephalopathy with axonal spheroids and pigmented glia (ALSP) we performed behavioral, radiologic, histopathologic, ultrastructural and cytokine expression studies of young and old Csf1r+/- and control Csf1r+/+ mice. Six to 8-month old Csf1r+/- mice exhibit cognitive deficits, and by 9-11 months develop sensorimotor deficits and in male mice, depression and anxiety-like behavior. MRIs of one year-old Csf1r+/- mice reveal lateral ventricle enlargement and thinning of the corpus callosum. Ultrastructural analysis of the corpus callosum uncovers dysmyelinated axons as well as neurodegeneration, evidenced by the presence of axonal spheroids. Histopathological examination of 11-week-old mice reveals increased axonal and myelin staining in the cortex, increase of neuronal cell density in layer V and increase of microglial cell densities throughout the brain, suggesting that early developmental changes contribute to disease. By 10-months of age, the neuronal cell density normalizes, oligodendrocyte precursor cells increase in layers II-III and V and microglial densities remain elevated without an increase in astrocytes. Also, the age-dependent increase in CSF-1R+ neurons in cortical layer V is reduced. Moreover, the expression of Csf2, Csf3, Il27 and Il6 family cytokines is increased, consistent with microglia-mediated inflammation. These results demonstrate that the inactivation of one Csf1r allele is sufficient to cause an ALSP-like disease in mice. The Csf1r+/- mouse is a model of ALSP that will allow the critical events for disease development to be determined and permit rapid evaluation of therapeutic approaches. Furthermore, our results suggest that aberrant activation of microglia in Csf1r+/- mice may play a central role in ALSP pathology.

Receptor-type protein-tyrosine phosphatase ζ is a functional receptor for interleukin-34.

Interleukin-34 (IL-34) is highly expressed in brain. IL-34 signaling via its cognate receptor, colony-stimulating factor-1 receptor (CSF-1R), is required for the development of microglia. However, the differential expression of IL-34 and the CSF-1R in brain suggests that IL-34 may signal via an alternate receptor. By IL-34 affinity chromatography of solubilized mouse brain membrane followed by mass spectrometric analysis, we identified receptor-type protein-tyrosine phosphatase ζ (PTP-ζ), a cell surface chondroitin sulfate (CS) proteoglycan, as a novel IL-34 receptor. PTP-ζ is primarily expressed on neural progenitors and glial cells and is highly expressed in human glioblastomas. IL-34 selectively bound PTP-ζ in CSF-1R-deficient U251 human glioblastoma cell lysates and inhibited the proliferation, clonogenicity, and motility of U251 cells in a PTP-ζ-dependent manner. These effects were correlated with an increase in tyrosine phosphorylation of the previously identified PTP-ζ downstream effectors focal adhesion kinase and paxillin. IL-34 binding to U251 cells was abrogated by chondroitinase ABC treatment, and CS competed with IL-34 for binding to the extracellular domain of PTP-ζ and to the cells, indicating a dependence of binding on PTP-ζ CS moieties. This study identifies an alternate receptor for IL-34 that may mediate its action on novel cellular targets.

Sex, epilepsy, and epigenetics.

Epilepsy refers to a heterogeneous group of disorders that are associated with a wide range of pathogenic mechanisms, seizure manifestations, comorbidity profiles, and therapeutic responses. These characteristics are all influenced quite significantly by sex. As with other conditions exhibiting such patterns, sex differences in epilepsy are thought to arise-at the most fundamental level-from the "organizational" and "activational" effects of sex hormones as well as from the direct actions of the sex chromosomes. However, our understanding of the specific molecular, cellular, and network level processes responsible for mediating sex differences in epilepsy remains limited. Because increasing evidence suggests that epigenetic mechanisms are involved both in epilepsy and in brain sexual dimorphism, we make the case here that analyzing epigenetic regulation will provide novel insights into the basis for sex differences in epilepsy.

Epigenetics of sleep and chronobiology.

The circadian clock choreographs fundamental biological rhythms. This system is comprised of the master circadian pacemaker in the suprachiasmatic nucleus and associated pacemakers in other tissues that coordinate complex physiological processes and behaviors, such as sleep, feeding, and metabolism. The molecular circuitry that underlies these clocks and orchestrates circadian gene expression has been the focus of intensive investigation, and it is becoming clear that epigenetic factors are highly integrated into these networks. In this review, we draw attention to the fundamental roles played by epigenetic mechanisms in transcriptional and post-transcriptional regulation within the circadian clock system. We also highlight how alterations in epigenetic factors and mechanisms are being linked with sleep-wake disorders. These observations provide important insights into the pathogenesis and potential treatment of these disorders and implicate epigenetic deregulation in the significant but poorly understood interconnections now emerging between circadian processes and neurodegeneration, metabolic diseases, cancer, and aging.

The CSF-1 receptor ligands IL-34 and CSF-1 exhibit distinct developmental brain expression patterns and regulate neural progenitor cell maintenance and maturation.

The CSF-1 receptor (CSF-1R) regulates CNS microglial development. However, the localization and developmental roles of this receptor and its ligands, IL-34 and CSF-1, in the brain are poorly understood. Here we show that compared to wild type mice, CSF-1R-deficient (Csf1r-/-) mice have smaller brains of greater mass. They further exhibit an expansion of lateral ventricle size, an atrophy of the olfactory bulb and a failure of midline crossing of callosal axons. In brain, IL-34 exhibited a broader regional expression than CSF-1, mostly without overlap. Expression of IL-34, CSF-1 and the CSF-1R were maximal during early postnatal development. However, in contrast to the expression of its ligands, CSF-1R expression was very low in adult brain. Postnatal neocortical expression showed that CSF-1 was expressed in layer VI, whereas IL-34 was expressed in the meninges and layers II-V. The broader expression of IL-34 is consistent with its previously implicated role in microglial development. The differential expression of CSF-1R ligands, with respect to CSF-1R expression, could reflect their CSF-1R-independent signaling. Csf1r-/- mice displayed increased proliferation and apoptosis of neocortical progenitors and reduced differentiation of specific excitatory neuronal subtypes. Indeed, addition of CSF-1 or IL-34 to microglia-free, CSF-1R-expressing dorsal forebrain clonal cultures, suppressed progenitor self-renewal and enhanced neuronal differentiation. Consistent with a neural developmental role for the CSF-1R, ablation of the Csf1r gene in Nestin-positive neural progenitors led to a smaller brain size, an expanded neural progenitor pool and elevated cellular apoptosis in cortical forebrain. Thus our results also indicate novel roles for the CSF-1R in the regulation of corticogenesis.

Corepressor for element-1-silencing transcription factor preferentially mediates gene networks underlying neural stem cell fate decisions.

The repressor element-1 (RE1) silencing transcription factor/neuron-restrictive silencer factor (REST/NRSF) silences neuronal genes in neural stem cells (NSCs) and nonneuronal cells through its role as a dynamic modular platform for recruitment of transcriptional and epigenetic regulatory cofactors to RE1-containing promoters. In embryonic stem cells, the REST regulatory network is highly integrated with the transcriptional circuitry governing self-renewal and pluripotency, although its exact functional role is unclear. The C-terminal cofactor for REST, CoREST, also acts as a modular scaffold, but its cell type-specific roles have not been elucidated. We used chromatin immunoprecipitation-on-chip to examine CoREST and REST binding sites in NSCs and their proximate progenitor species. In NSCs, we identified a larger number of CoREST (1,820) compared with REST (322) target genes. The majority of these CoREST targets do not contain known RE1 motifs. Notably, these CoREST target genes do play important roles in pluripotency networks, in modulating NSC identity and fate decisions and in epigenetic processes previously associated with both REST and CoREST. Moreover, we found that NSC-mediated developmental transitions were associated primarily with liberation of CoREST from promoters with transcriptional repression favored in less lineage-restricted radial glia and transcriptional activation favored in more lineage-restricted neuronal-oligodendrocyte precursors. Clonal NSC REST and CoREST gene manipulation paradigms further revealed that CoREST has largely independent and previously uncharacterized roles in promoting NSC multilineage potential and modulating early neural fate decisions.

Brain MRI findings in severe COVID-19 patients: a meta-analysis.

Introduction: Neurocognitive symptoms and dysfunction of various severities have become increasingly recognized as potential consequences of SARS-CoV-2 infection. Although there are numerous observational and subjective survey-reporting studies of neurological symptoms, by contrast, those studies describing imaging abnormalities are fewer in number. Methods: This study conducted a metanalysis of 32 studies to determine the incidence of the common neurological abnormalities using magnetic resonance imaging (MRI) in patients with COVID-19. Results: We also present the common clinical findings associated with MRI abnormalities. We report the incidence of any MRI abnormality to be 55% in COVID-19 patients with perfusion abnormalities (53%) and SWI abnormalities (44%) being the most commonly reported injuries. Cognitive impairment, ICU admission and/or mechanical ventilation status, older age, and hospitalization or longer length of hospital stay were the most common clinical findings associated with brain injury in COVID-19 patients. Discussion: Overall, the presentation of brain injury in this study was diverse with no substantial pattern of injury emerging, yet most injuries appear to be of vascular origin. Moreover, analysis of the association between MRI abnormalities and clinical findings suggests that there are likely many mechanisms, both direct and indirect, by which brain injury occurs in COVID-19 patients.

Impairment of developmental stem cell-mediated striatal neurogenesis and pluripotency genes in a knock-in model of Huntington's disease.

The pathogenesis of Huntington's disease (HD) remains elusive. The identification of increasingly early pathophysiological abnormalities in HD suggests the possibility that impairments of striatal medium spiny neuron (MSN) specification and maturation may underlie the etiology of HD. In fact, we demonstrate that HD knock-in (Hdh-Q111) mice exhibited delayed acquisition of early striatal cytoarchitecture with aberrant expression of progressive markers of MSN neurogenesis (Islet1, DARPP-32, mGluR1, and NeuN). Hdh-Q111 striatal progenitors also displayed delayed cell cycle exit between E13.5-15.5 (BrdU birth-dating) and an enhanced fraction of abnormal cycling cells in association with expansion of the pool of intermediate progenitors and over expression of the core pluripotency (PP) factor, Sox2. Clonal analysis further revealed that Hdh-Q111 neural stem cells (NSCs) displayed: impaired lineage restriction, reduced proliferative potential, enhanced late-stage self-renewal, and deregulated MSN subtype specification. Further, our analysis revealed that in addition to Sox2, the core PP factor, Nanog is expressed within the striatal generative and mantle regions, and in Hdh-Q111 embryos the fraction of Nanog-expressing MSN precursors was substantially increased. Moreover, compared to Hdh-Q18 embryos, the Hdh-Q111 striatal anlagen exhibited significantly higher levels of the essential PP cofactor, Stat3. These findings suggest that Sox2 and Nanog may play roles during a selective window of embryonic brain maturation, and alterations of these factors may, in part, be responsible for mediating the aberrant program of Hdh-Q111 striatal MSN specification and maturation. We propose that these HD-associated developmental abnormalities might compromise neuronal homeostasis and subsequently render MSNs more vulnerable to late life stressors.

Neurogenetic disorders across the lifespan: from aberrant development to degeneration.

Intellectual disability and autism spectrum disorder (ASD) are common, and genetic testing is increasingly performed in individuals with these diagnoses to inform prognosis, refine management and provide information about recurrence risk in the family. For neurogenetic conditions associated with intellectual disability and ASD, data on natural history in adults are scarce; however, as older adults with these disorders are identified, it is becoming clear that some conditions are associated with both neurodevelopmental problems and neurodegeneration. Moreover, emerging evidence indicates that some neurogenetic conditions associated primarily with neurodegeneration also affect neurodevelopment. In this Perspective, we discuss examples of diseases that have developmental and degenerative overlap. We propose that neurogenetic disorders should be studied continually across the lifespan to understand the roles of the affected genes in brain development and maintenance, and to inform strategies for treatment.

Advances in epigenetics and epigenomics for neurodegenerative diseases.

In the post-genomic era, epigenetic factors-literally those that are "over" or "above" genetic ones and responsible for controlling the expression and function of genes-have emerged as important mediators of development and aging; gene-gene and gene-environmental interactions; and the pathophysiology of complex disease states. Here, we provide a brief overview of the major epigenetic mechanisms (ie, DNA methylation, histone modifications and chromatin remodeling, and non-coding RNA regulation). We highlight the nearly ubiquitous profiles of epigenetic dysregulation that have been found in Alzheimer's and other neurodegenerative diseases. We also review innovative methods and technologies that enable the characterization of individual epigenetic modifications and more widespread epigenomic states at high resolution. We conclude that, together with complementary genetic, genomic, and related approaches, interrogating epigenetic and epigenomic profiles in neurodegenerative diseases represent important and increasingly practical strategies for advancing our understanding of and the diagnosis and treatment of these disorders.

RNA regulation of epigenetic processes.

There is increasing evidence that dynamic changes to chromatin, chromosomes and nuclear architecture are regulated by RNA signalling. Although the precise molecular mechanisms are not well understood, they appear to involve the differential recruitment of a hierarchy of generic chromatin modifying complexes and DNA methyltransferases to specific loci by RNAs during differentiation and development. A significant fraction of the genome-wide transcription of non-protein coding RNAs may be involved in this process, comprising a previously hidden layer of intermediary genetic information that underpins developmental ontogeny and the differences between species, ecotypes and individuals. It is also evident that RNA editing is a primary means by which hardwired genetic information in animals can be altered by environmental signals, especially in the brain, indicating a dynamic RNA-mediated interplay between the transcriptome, the environment and the epigenome. Moreover, RNA-directed regulatory processes may also transfer epigenetic information not only within cells but also between cells and organ systems, as well as across generations.