Nav1.7 gain-of-function mutation I228M triggers age-dependent nociceptive insensitivity and C-LTMR dysregulation

Gain-of-function mutations in Scn9a, which encodes the peripheral sensory neuron-enriched voltage-gated sodium channel Na_v1.7, cause paroxysmal extreme pain disorder (PEPD), inherited erythromelalgia (IEM), and small fiber neuropathy (SFN). Conversely, loss-of-function mutations in the gene are linked to congenital insensitivity to pain (CIP). These mutations are evidence for a link between altered sodium conductance and neuronal excitability leading to somatosensory aberrations, pain, or its loss. Our previous work in young adult mice with the Na_v1.7 gain-of-function mutation, I228M, showed the expected DRG neuron hyperexcitability, but unexpectedly the mice had normal mechanical and thermal behavioral sensitivity. We now show that with aging both male and female mice with this mutation unexpectedly develop a profound insensitivity to noxious heat and cold, as well skin lesions that span the body. Electrophysiology demonstrates that, in contrast to young mice, aged I228M mouse DRGs have a profound loss of sodium conductance and changes in activation and slow inactivation dynamics, representing a loss-of-function. Through RNA sequencing we explored how these age-related changes may produce the phenotypic changes and found a striking and specific decrease in C-low threshold mechanoreceptor- (cLTMR) associated gene expression, suggesting a potential contribution of this DRG neuron subtype to Na_v1.7 dysfunction phenotypes. A GOF mutation in a voltage-gated channel can therefore produce over a prolonged time, highly complex and unexpected alterations in the nervous system beyond excitability changes.

Understanding the biological basis of psychiatric disease: What's next?

Psychiatric disease is one of the greatest health challenges of our time. The pipeline for conceptually novel therapeutics remains low, in part because uncovering the biological mechanisms of psychiatric disease has been difficult. We asked experts researching different aspects of psychiatric disease: what do you see as the major urgent questions that need to be addressed? Where are the next frontiers, and what are the current hurdles to understanding the biological basis of psychiatric disease?

Progranulin mutations as risk factors for Alzheimer disease

IMPORTANCE

Mutations in the progranulin gene are known to cause diverse clinical syndromes, all attributed to frontotemporal lobar degeneration. We describe 2 patients with progranulin gene mutations and evidence of Alzheimer disease (AD) pathology. We also conducted a literature review.

OBSERVATIONS

This study focused on case reports of 2 unrelated patients with progranulin mutations at the University of California, San Francisco, Memory and Aging Center. One patient presented at age 65 years with a clinical syndrome suggestive of AD and showed evidence of amyloid aggregation on positron emission tomography. Another patient presented at age 54 years with logopenic progressive aphasia and, at autopsy, showed both frontotemporal lobar degeneration with TDP-43 inclusions and AD.

CONCLUSIONS AND RELEVANCE

In addition to autosomal-dominant frontotemporal lobar degeneration, mutations in the progranulin gene may be a risk factor for AD clinical phenotypes and neuropathology.

Divergent whole-genome methylation maps of human and chimpanzee brains reveal epigenetic basis of human regulatory evolution

DNA methylation is a pervasive epigenetic DNA modification that strongly affects chromatin regulation and gene expression. To date, it remains largely unknown how patterns of DNA methylation differ between closely related species and whether such differences contribute to species-specific phenotypes. To investigate these questions, we generated nucleotide-resolution whole-genome methylation maps of the prefrontal cortex of multiple humans and chimpanzees. Levels and patterns of DNA methylation vary across individuals within species according to the age and the sex of the individuals. We also found extensive species-level divergence in patterns of DNA methylation and that hundreds of genes exhibit significantly lower levels of promoter methylation in the human brain than in the chimpanzee brain. Furthermore, we investigated the functional consequences of methylation differences in humans and chimpanzees by integrating data on gene expression generated with next-generation sequencing methods, and we found a strong relationship between differential methylation and gene expression. Finally, we found that differentially methylated genes are strikingly enriched with loci associated with neurological disorders, psychological disorders, and cancers. Our results demonstrate that differential DNA methylation might be an important molecular mechanism driving gene-expression divergence between human and chimpanzee brains and might potentially contribute to the evolution of disease vulnerabilities. Thus, comparative studies of humans and chimpanzees stand to identify key epigenomic modifications underlying the evolution of human-specific traits.

Conditioning lesions before or after spinal cord injury recruit broad genetic mechanisms that sustain axonal regeneration: superiority to camp-mediated effects

Previous studies indicate that peripheral nerve conditioning lesions significantly enhance central axonal regeneration via modulation of cAMP-mediated mechanisms. To gain insight into the nature and temporal dependence of neural mechanisms underlying conditioning lesion effects on central axonal regeneration, we compared the efficacy of peripheral sciatic nerve crush lesions to cAMP elevations (in lumbar dorsal root ganglia) on central sensory axonal regeneration when administered either before or after cervical spinal cord lesions. We found significantly greater effects of conditioning lesions compared to cAMP elevations on central axonal regeneration when combined with cellular grafts at the lesion site and viral neurotrophin delivery; further, these effects persisted whether conditioning lesions were applied prior to or shortly after spinal cord injury. Indeed, conditioning lesions recruited extensively greater sets of genetic mechanisms of possible relevance to axonal regeneration compared to cAMP administration, and sustained these changes for significantly greater time periods through the post-lesion period. We conclude that cAMP-mediated mechanisms account for only a portion of the potency of conditioning lesions on central axonal regeneration, and that recruitment of broader genetic mechanisms can extend the effect and duration of cellular events that support axonal growth.

Accelerating axonal growth promotes motor recovery after peripheral nerve injury in mice

Although peripheral nerves can regenerate after injury, proximal nerve injury in humans results in minimal restoration of motor function. One possible explanation for this is that injury-induced axonal growth is too slow. Heat shock protein 27 (Hsp27) is a regeneration-associated protein that accelerates axonal growth in vitro. Here, we have shown that it can also do this in mice after peripheral nerve injury. While rapid motor and sensory recovery occurred in mice after a sciatic nerve crush injury, there was little return of motor function after sciatic nerve transection, because of the delay in motor axons reaching their target. This was not due to a failure of axonal growth, because injured motor axons eventually fully re-extended into muscles and sensory function returned; rather, it resulted from a lack of motor end plate reinnervation. Tg mice expressing high levels of Hsp27 demonstrated enhanced restoration of motor function after nerve transection/resuture by enabling motor synapse reinnervation, but only within 5 weeks of injury. In humans with peripheral nerve injuries, shorter wait times to decompression surgery led to improved functional recovery, and, while a return of sensation occurred in all patients, motor recovery was limited. Thus, absence of motor recovery after nerve damage may result from a failure of synapse reformation after prolonged denervation rather than a failure of axonal growth.

Combined intrinsic and extrinsic neuronal mechanisms facilitate bridging axonal regeneration one year after spinal cord injury

Despite advances in promoting axonal regeneration after acute spinal cord injury (SCI), elicitation of bridging axon regeneration after chronic SCI remains a formidable challenge. We report that combinatorial therapies administered 6 weeks, and as long as 15 months, after SCI promote axonal regeneration into and beyond a midcervical lesion site. Provision of peripheral nerve conditioning lesions, grafts of marrow stromal cells, and establishment of NT-3 gradients supports bridging regeneration. Controls receiving partial components of the full combination fail to exhibit bridging. Notably, intraneuronal molecular mechanisms recruited by delayed therapies mirror those of acute injury, including activation of transcriptional activators and regeneration-associated genes. Collectively, these findings provide evidence that regeneration is achievable at unprecedented postinjury time points.

Altered iron metabolism is part of the choroid plexus response to peripheral inflammation

Iron is essential for normal cellular homeostasis but in excess promotes free radical formation and is detrimental. Therefore, iron metabolism is tightly regulated. Here, we show that mechanisms regulating systemic iron metabolism may also control iron release into the brain at the blood-choroid plexus-cerebrospinal fluid (CSF) barrier. Intraperitoneal administration of lipopolysaccharide (LPS) in mice triggers a transient transcription of the gene encoding for hepcidin, a key regulator of iron homeostasis, in the choroid plexus, which correlated with increased detection of pro-hepcidin in the CSF. Similarly, the expression of several other iron-related genes is influenced in the choroid plexus by the inflammatory stimulus. Using primary cultures of rat choroid plexus epithelial cells, we show that this response is triggered not only directly by LPS but also by molecules whose expression increases in the blood in response to inflammation, such as IL-6. Intracellular conveyors of these signaling molecules include signal transducer and activator of transcription 3, which becomes phosphorylated, and SMAD family member 4, whose mRNA levels increase soon after LPS administration. This novel role for the choroid plexus-CSF barrier in regulating iron metabolism may be particularly relevant to restrict iron availability for microorganism growth, and in neurodegenerative diseases in which an inflammatory underlying component has been reported.

Methyl-CpG-binding protein 2 polymorphisms and vulnerability to autism

The methyl-binding protein gene, MECP2, is a candidate for involvement in autism through its implication as a major causative factor in Rett syndrome that has similarities to autism. Rare mutations in MECP2 have also been identified in autistic individuals. We have examined the possible broader involvement of MECP2 as a predisposing factor in the disorder. Analysis of polymorphic markers spanning the gene and comprising both microsatellites and single nucleotide polymorphisms (SNPs) by the transmission disequilibrium test in two collections of families (219 in total), one in the USA and one in the UK, has provided evidence for significant association (P = 0.009) for a three-marker SNP haplotype of MECP2 with autism/autism spectrum disorders. This association is supported by association of both Single Sequence Repeat (SSR) and SNP single markers located at the 3' end of the MECP2 locus and flanking sequence, the most significant being that of an indel marker located in intron 2 (P = 0.001 - Bonferroni corrected P = 0.006). This suggests that one or more functional variants of MECP2 existing at significant frequencies in the population may confer increased risk of autism/autism spectrum disorders and warrants further investigation in additional independent samples.

Klinefelter syndrome: Expanding the phenotype and identifying new research directions

PURPOSE

The purpose of this study is to summarize new data on etiology and clinical features of Klinefelter syndrome in order to derive research priorities.

METHODS

This study was conducted using critical reviews of selective topics, emphasizing less well-recognized clinical findings.

RESULTS AND CONCLUSIONS

The phenotype of the prototypic 47,XXY case is well recognized: seminiferous tubule dysgenesis and androgen deficiency. Less well appreciated is the varied expressivity of 47,XXY Klinefelter syndrome, in particular neurological/cognitive perturbations like language and behavioral problems. Effective therapies are available. Reproductive technologies allow 47,XXY men to sire offspring through intracytoplasmic sperm injection (ICSI); however, genetic counseling is complex and success is low. Behavioral and expressive language difficulties are amenable to treatment by androgen therapy and psychological help. Early treatment may be imperative for optimal outcome.

Human ARX gene: genomic characterization and expression

Arx is a homeobox-containing gene with a high degree of sequence similarity between mouse and zebrafish. Arx is expressed in the forebrain and floor plate of the developing central nervous systems of these vertebrates and in the presumptive cortex of fetal mice. Our goal was to identify genes in Xp22.1-p21.3 involved in human neuronal development. Our in silico search for candidate genes noted that annotation of a human Xp22 PAC (RPCI1-258N20) sequence (GenBank Accession No. AC002504) identified putative exons consistent with an Arx homologue in Xp22. Northern blot analysis showed that a 3.3kb human ARX transcript was expressed at high levels in fetal brain. A 5.9kb transcript was expressed in adult heart, skeletal muscle, and liver with very faint expression in other adult tissues, including brain. In situ hybridization of ARX in human fetal brain sections at various developmental stages showed the highest expression in neuronal precursors in the germinal matrix of the ganglionic eminence and in the ventricular zone of the telencephalon. Expression was also observed in the hippocampus, cingulate, subventricular zone, cortical plate, caudate nucleus, and putamen. The expression pattern suggests that ARX is involved in the differentiation and maintenance of specific neuronal cell types in the human central nervous system. We also mapped the murine Arx gene to the mouse genome using a mouse/hamster radiation hybrid panel and showed that Arx and ARX are orthologues. Therefore, investigations in model vertebrates may provide insight into the role of ARX in development. The recent identification of ARX mutations in patients with various forms of mental retardation make such studies in model organisms even more compelling.

The use of representational difference analysis and cDNA microarrays in neural repair research

Genetic subtraction studies may be useful tools for neural repair research by identifying genes expressed under one condition, but not under another. However, these studies suffer from some limitations, including a lack of heterogeneity of subtracted cDNA pools and the difficulty of screening out false positives in the subtracted pools. Our strategy to overcome these difficulties was to combine one subtractive method - representational difference analysis - with screening of the subtracted products using a custom CDNA microarray. Using the neurosphere culture system, we have used this stepwise approach in order to identify genes that are selectively expressed by CNS progenitor cells, but not by more differentiated cells. Following microarray screening, we confirmed the localization of putatively differentially expressed clones by in situ hybridization analysis. These genes, both novel and previously identified, now become candidate therapeutic targets for CNS repair strategies

Oculomotor phenotypes in autosomal dominant ataxias

OBJECTIVE

To quantify the oculomotor features of the common spinocerebellar ataxia (SCA) syndromes.

SETTING

University ataxia clinic.

PATIENTS

Twenty probands with documented SCA mutations.

METHODS

Electro-oculographic recordings of saccadic, smooth pursuit, optokinetic, vestibular, and visual-vestibular eye movements.

RESULTS

Distinct phenotype and genotype patterns were identified with modest overlap between patterns. Slowing of saccade peak velocities occurred only in SCA1 and SCA2, being present in 100% of patients with SCA2. Impaired vestibulo-ocular reflex gain occurred with SCA3 only. Patients with SCA6 had prominent deficits in smooth tracking but normal saccade velocities and vestibuloocular reflex gain.

CONCLUSIONS

The oculomotor findings are consistent with pure cerebellar involvement in SCA6, pontine involvement in SCA1 and SCA2, and vestibular nerve or nuclei involvement in SCA3. These phenotypes can be useful for clinical diagnosis and for investigating the mechanism of system specificity with the SCA syndromes.

The apolipoprotein E epsilon4 allele is not a significant risk factor for frontotemporal dementia

Frontotemporal dementia (FTD) is the most common early-onset non-Alzheimer's dementia (non-AD). Although the role of the epsilon4 allele of apolipoprotein E (ApoE) has been well established in AD, studies of ApoE allele distribution in patients with FTD have produced variable results. We studied 33 rigorously diagnosed FTD patients, including several who were pathologically confirmed, and compared the frequency of the epsilon4 allele in patients with FTD with the frequency in those with early-onset AD (EOAD), in those with late-onset AD (LOAD), and in non-demented elderly controls. The frequency of ApoE epsilon4 was 21% in patients with FTD, significantly less than the ApoE epsilon4 frequency in those patients with EOAD (38%) and those with LOAD (40%), but not significantly different from the ApoE epsilon4 frequency in elderly controls (13%).