A flanking gene problem leads to the discovery of a Gprc5b splice variant predominantly expressed in C57Bl/6J mouse brain and in maturing neurons

Megalencephalic leukoencephalopathy with subcortical cysts: a variant update and review of the literature

The leukodystrophy megalencephalic leukoencephalopathy with subcortical cysts (MLC) is characterized by infantile-onset macrocephaly and chronic edema of the brain white matter. With delayed onset, patients typically experience motor problems, epilepsy and slow cognitive decline. No treatment is available. Classic MLC is caused by bi-allelic recessive pathogenic variants in MLC1 or GLIALCAM (also called HEPACAM). Heterozygous dominant pathogenic variants in GLIALCAM lead to remitting MLC, where patients show a similar phenotype in early life, followed by normalization of white matter edema and no clinical regression. Rare patients with heterozygous dominant variants in GPRC5B and classic MLC were recently described. In addition, two siblings with bi-allelic recessive variants in AQP4 and remitting MLC have been identified. The last systematic overview of variants linked to MLC dates back to 2006. We provide an updated overview of published and novel variants. We report on genetic variants from 508 patients with MLC as confirmed by MRI diagnosis (258 from our database and 250 extracted from 64 published reports). We describe 151 unique MLC1 variants, 29 GLIALCAM variants, 2 GPRC5B variants and 1 AQP4 variant observed in these MLC patients. We include experiments confirming pathogenicity for some variants, discuss particularly notable variants, and provide an overview of recent scientific and clinical insight in the pathophysiology of MLC.

Comprehensive Spatial Profile of the Orphan G Protein Coupled Receptor GPRC5B Expression in Mouse Brain

Orphan G Protein Coupled Receptors (GPCRs) are GPCRs whose endogenous ligands are unknown or still debated. Due to the lack of pharmacological modulators, the physiological function of orphan GPCRs is understudied. However, relevant physiological roles associated with orphan GPCRs have been revealed by analysis of animal models and genome wide association studies illuminating an untapped potential for drug discovery. G Protein Coupled Receptor class C Group 5 Member B (GPRC5B) is among the most expressed GPCRs in the central nervous system. Thus, the expression profiling of GPRC5B is an essential step toward understanding GPRC5B function in health and disease. In this study, we generated new GPRC5B polyclonal antibodies and investigated the expression levels of GPRC5B across different organs and brain regions. We identified high levels of GPRC5B glycosylation both in transfected cells and in mouse brain. Moreover, in situ hybridization imaging analysis indicated that Gprc5b was expressed at the highest level in olfactory bulb, hippocampus, cerebellum, and pons. To dissect expression within various neuronal populations, we conducted a comprehensive spatial profiling of Gprc5b across excitatory and inhibitory neuronal types in medial prefrontal cortex, motor cortex, hippocampal regions, hypothalamus, and cerebellum. Overall, we discovered that GABAergic neurons displayed higher Gprc5b expression levels than glutamatergic neurons in most of the analyzed regions with the important exception of the hippocampal dentate gyrus. Overall, the expression analysis of GPRC5B in mouse brain will guide functional studies ultimately positioning GPRC5B in pathophysiological mechanisms and drug discovery.

Atlas of RNA editing events affecting protein expression in aged and Alzheimer's disease human brain tissue

RNA editing is a feature of RNA maturation resulting in the formation of transcripts whose sequence differs from the genome template. Brain RNA editing may be altered in Alzheimer's disease (AD). Here, we analyzed data from 1,865 brain samples covering 9 brain regions from 1,074 unrelated subjects on a transcriptome-wide scale to identify inter-regional differences in RNA editing. We expand the list of known brain editing events by identifying 58,761 previously unreported events. We note that only a small proportion of these editing events are found at the protein level in our proteome-wide validation effort. We also identified the occurrence of editing events associated with AD dementia, neuropathological measures and longitudinal cognitive decline in: SYT11, MCUR1, SOD2, ORAI2, HSDL2, PFKP, and GPRC5B. Thus, we present an extended reference set of brain RNA editing events, identify a subset that are found to be expressed at the protein level, and extend the narrative of transcriptomic perturbation in AD to RNA editing.

Genetic meta-analysis of diagnosed Alzheimer's disease identifies new risk loci and implicates Aβ, tau, immunity and lipid processing

Risk for late-onset Alzheimer's disease (LOAD), the most prevalent dementia, is partially driven by genetics. To identify LOAD risk loci, we performed a large genome-wide association meta-analysis of clinically diagnosed LOAD (94,437 individuals). We confirm 20 previous LOAD risk loci and identify five new genome-wide loci (IQCK, ACE, ADAM10, ADAMTS1, and WWOX), two of which (ADAM10, ACE) were identified in a recent genome-wide association (GWAS)-by-familial-proxy of Alzheimer's or dementia. Fine-mapping of the human leukocyte antigen (HLA) region confirms the neurological and immune-mediated disease haplotype HLA-DR15 as a risk factor for LOAD. Pathway analysis implicates immunity, lipid metabolism, tau binding proteins, and amyloid precursor protein (APP) metabolism, showing that genetic variants affecting APP and Aβ processing are associated not only with early-onset autosomal dominant Alzheimer's disease but also with LOAD. Analyses of risk genes and pathways show enrichment for rare variants (P = 1.32 × 10-7), indicating that additional rare variants remain to be identified. We also identify important genetic correlations between LOAD and traits such as family history of dementia and education.

Integrated Systems Approach Reveals Sphingolipid Metabolism Pathway Dysregulation in Association with Late-Onset Alzheimer's Disease

Late-onset Alzheimer’s disease (LOAD) and age are significantly correlated such that one-third of Americans beyond 85 years of age are afflicted. We have designed and implemented a pilot study that combines systems biology approaches with traditional next-generation sequencing (NGS) analysis techniques to identify relevant regulatory pathways, infer functional relationships and confirm the dysregulation of these biological pathways in LOAD. Our study design is a most comprehensive systems approach combining co-expression network modeling derived from RNA-seq data, rigorous quality control (QC) standards, functional ontology, and expression quantitative trait loci (eQTL) derived from whole exome (WES) single nucleotide variant (SNV) genotype data. Our initial results reveal several statistically significant, biologically relevant genes involved in sphingolipid metabolism. To validate these findings, we performed a gene set enrichment analysis (GSEA). The GSEA revealed the sphingolipid metabolism pathway and regulation of autophagy in association with LOAD cases. In the execution of this study, we have successfully tested an integrative approach to identify both novel and known LOAD drivers in order to develop a broader and more detailed picture of the highly complex transcriptional and regulatory landscape of age-related dementia.

Strictly co-isogenic C57BL/6J-Prnp-/- mice: A rigorous resource for prion science

Although its involvement in prion replication and neurotoxicity during transmissible spongiform encephalopathies is undisputed, the physiological role of the cellular prion protein (PrP(C)) remains enigmatic. A plethora of functions have been ascribed to PrP(C) based on phenotypes of Prnp(-/-) mice. However, all currently available Prnp(-/-) lines were generated in embryonic stem cells from the 129 strain of the laboratory mouse and mostly crossed to non-129 strains. Therefore, Prnp-linked loci polymorphic between 129 and the backcrossing strain resulted in systematic genetic confounders and led to erroneous conclusions. We used TALEN-mediated genome editing in fertilized mouse oocytes to create the Zurich-3 (ZH3) Prnp-ablated allele on a pure C57BL/6J genetic background. Genomic, transcriptional, and phenotypic characterization of Prnp(ZH3/ZH3) mice failed to identify phenotypes previously described in non-co-isogenic Prnp(-/-) mice. However, aged Prnp(ZH3/ZH3) mice developed a chronic demyelinating peripheral neuropathy, confirming the crucial involvement of PrP(C) in peripheral myelin maintenance. This new line represents a rigorous genetic resource for studying the role of PrP(C) in physiology and disease.

Comparative expression study of the endo-G protein coupled receptor (GPCR) repertoire in human glioblastoma cancer stem-like cells, U87-MG cells and non malignant cells of neural origin unveils new potential therapeutic targets

Glioblastomas (GBMs) are highly aggressive, invasive brain tumors with bad prognosis and unmet medical need. These tumors are heterogeneous being constituted by a variety of cells in different states of differentiation. Among these, cells endowed with stem properties, tumor initiating/propagating properties and particularly resistant to chemo- and radiotherapies are designed as the real culprits for tumor maintenance and relapse after treatment. These cells, termed cancer stem-like cells, have been designed as prominent targets for new and more efficient cancer therapies. G-protein coupled receptors (GPCRs), a family of membrane receptors, play a prominent role in cell signaling, cell communication and crosstalk with the microenvironment. Their role in cancer has been highlighted but remains largely unexplored. Here, we report a descriptive study of the differential expression of the endo-GPCR repertoire in human glioblastoma cancer stem-like cells (GSCs), U-87 MG cells, human astrocytes and fetal neural stem cells (f-NSCs). The endo-GPCR transcriptome has been studied using Taqman Low Density Arrays. Of the 356 GPCRs investigated, 138 were retained for comparative studies between the different cell types. At the transcriptomic level, eight GPCRs were specifically expressed/overexpressed in GSCs. Seventeen GPCRs appeared specifically expressed in cells with stem properties (GSCs and f-NSCs). Results of GPCR expression at the protein level using mass spectrometry and proteomic analysis are also presented. The comparative GPCR expression study presented here gives clues for new pathways specifically used by GSCs and unveils novel potential therapeutic targets.

The G protein-coupled receptor GPRC5B contributes to neurogenesis in the developing mouse neocortex

Neural progenitor cells in the developing brain give rise to neurons and glia. Multiple extrinsic signalling molecules and their cognate membrane receptors have been identified to control neural progenitor fate. However, a role for G protein-coupled receptors in cell fate decisions in the brain remains largely putative. Here we show that GPRC5B, which encodes an orphan G protein-coupled receptor, is present in the ventricular surface of cortical progenitors in the mouse developing neocortex and is required for their neuronal differentiation. GPRC5B-depleted progenitors fail to adopt a neuronal fate and ultimately become astrocytes. Furthermore, GPRC5B-mediated signalling is associated with the proper regulation of β-catenin signalling, a pathway crucial for progenitor fate decision. Our study uncovers G protein-coupled receptor signalling in the neuronal fate determination of cortical progenitors.

Spontaneous metastasis in congenic mice with transgenic breast cancer is unaffected by plasminogen gene ablation

Plasminogen (Plg) plays a central role in tissue remodeling during ontogeny, development, and in pathological tissue remodeling following physical injury, inflammation and cancer. Plg/plasmin is, however, not critical for these processes, as they all occur to a varying extent in its absence, suggesting that there is a functional redundancy with other proteases. To explore this functional overlap in the transgenic MMTV-PyMT breast cancer metastasis model, we have combined Plg deficiency and a pharmacological metalloprotease inhibitor, which is known to reduce metastasis in this model, and has been shown to synergistically inhibit other tissue remodeling events in Plg-deficient mice. While metalloprotease inhibition dramatically reduced metastasis, we found no effect of Plg deficiency on metastasis, either independently or in combination with metalloprotease inhibition. We further show that Plg gene deficiency is of no significant consequence in this metastasis model, when analyzed in two different congenic strains: the FVB strain, and a F1 hybrid of the FVB and C57BL/6J strains. We suggest that the extensive backcrossing performed prior to our studies has eliminated the confounding effect of a known polymorphic metastasis modifier gene region located adjacent to the Plg gene.

The gene or not the gene--that is the question: understanding the genetically engineered mouse phenotype

Embryonic stem cells have had a significant impact on understanding gene function and gene interactions through the use of genetically engineered mice. However, the genetic context (ie, mouse strain) in which these modifications in alleles are made may have a considerable effect on the phenotypic changes identified in these mice. In addition, tissue- and time-specific gene expression systems may generate unanticipated outcomes. This article discusses the history of embryonic stem cells, reviews how mouse strain can affect phenotype (using specific examples), and examines some of the caveats of conditional gene expression systems.

Stems to GEMs: impact of stem cell technology on engineered animal models

Collectively, these presentations introduced the audience to the roles of ES cells in generating phenotypes of transgenic animals,and they provided examples where the GEMs were used to define molecular mechanisms of disease or where ES cells were used as a therapeutic modality. Points of discussion among audience members reinforced the importance of strain-associated background lesions in animal models, technological advances in imaging functional biology, opportunities for stem cell therapies, and ubiquitination in regulation of cell proliferation. The 2012 American College of Veterinary Pathologists symposium ‘‘Evolutionary Aspects of Animal Models’’ will focus on the proper selection of a relevant animal model in biomedical research as critical to investigative success. Recent work characterizing rapid evolutionary changes and differences in physiology between species questions the validity of some comparative models. Dr. Robert Hamlin will be speaking on cardiovascular disease in ‘‘Animals as Models of Human Cardiovascular Disease: Or the Search to Overcome Outdated Evolutionary Homeostatic Mechanisms.’’ Dr. Stefan Niewiesk will discuss evolutionary factors that affect modeling the human immune system in ‘‘Of Mice and Men: Evolutionarily, What Are the Best Rodent Models of the Human Immune System for Infectious Disease Research?’’ Dr. Steven Austad will consider evolution in ‘‘Evolutionary Aspects of Animal Models of Aging.’’Finally, Dr. Elizabeth Uhl will conclude the session with ‘‘Modeling Disease Phenotypes: How an Evolutionary Perspective Enhances the Questions.’’