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Equine sarcoids are common non-metastasising skin tumours in horses, associated with bovine papillomavirus (BPV) infection. Six subtypes are recognised (occult, verrucose, nodular, fibroblastic, mixed and malevolent lesions), with variable clinical behaviour. The pathophysiology underlying varying tumour phenotype is poorly understood, and previous data on associations with viral load have been conflicting. To better understand this clinical variation, we investigated associations between tumour subtype and viral load, viral early protein gene expression, and expression of 10 host genes by quantitative polymerase chain reaction in 27 sarcoids and 5 normal skin samples. Viral DNA copy number did not differ between subtypes but was significantly higher in animals with fewer tumours. Expression of BPV E2 and E6 was higher in occult lesions compared to fibroblastic or nodular lesions, while E5 expression was higher in previously-treated lesions. Of the host genes, only IL6 and IL1B differed between subtypes, with higher expression in fibroblastic lesions, while IL10 and CCL5 were elevated compared to skin in all lesion types, and elevations in TNF and TGFB1 were significant for occult lesions only. Expression of TLR9, ATR, VEGFA and PTGS2 in sarcoids was not significantly different from normal skin, suggesting differences between BPV and human papillomavirus tumorigenesis. Results for BPV viral load and gene expression differed from previous reports and are insufficient to explain the spectrum of tumour phenotypes. Activation of both pro-inflammatory and anti-inflammatory immune pathways in sarcoids could influence tumour growth and effective immune responses, and the contribution of specific infiltrating immune cells requires further investigation., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 The Authors. Published by Elsevier B.V. All rights reserved.)

Background: Fractures of the paracondylar process of the occipital bone may cause headshaking, neck pain and neurologic deficits. The condition is being recognised more frequently with increasing availability of computed tomography. However, to date only limited information is available as to presentation, treatment, surgical approach and outcome., Objectives: To describe the clinical signs, imaging findings, treatment, surgical approach and outcome in three horses diagnosed with paracondylar process fracture., Study Design: Retrospective case series., Methods: Clinical records and diagnostic images of affected cases were reviewed., Results: Two cases had ventral nonunion fractures-one of these presented with neck pain, headshaking and behavioural changes, while in the other the fracture was a suspected incidental finding in a case of poor performance. A third case with a more dorsal fracture presented with acute facial nerve paralysis. Diagnosis was by computed tomography in all cases, although imaging of ventral fractures by radiography was found to be feasible. Where clinical signs could be associated confidently with the fracture, conservative management resulted in improvement but not complete resolution. Repeated recurrence of clinical signs after prolonged periods of remission necessitated surgical removal in one case, which was readily accomplished with the aid of ultrasound guidance, and led to rapid resolution of clinical signs without significant post-operative complications. The surgical approach is described., Main Limitations: Limited follow-up was available., Conclusions: Paracondylar process fracture should be considered as a differential diagnosis for headshaking, neck pain, poor performance and facial paresis, and is a justification for performing computed tomography in such cases. A multi-disciplinary approach is beneficial due to the potential for orthopaedic, neurologic, ophthalmologic and behavioural clinical signs, with additional need for expertise in diagnostic imaging and pain management. Surgical fragment removal should be considered for ventral fractures., (© 2024 The Author(s). Equine Veterinary Journal published by John Wiley & Sons Ltd on behalf of EVJ Ltd.)

Objective: Myalgic Encephalomyelitis (ME; sometimes referred to as Chronic Fatigue Syndrome) is a chronic disease without laboratory test, detailed aetiological understanding or effective therapy. Its symptoms are diverse, but it is distinguished from other fatiguing illnesses by the experience of post-exertional malaise, the worsening of symptoms even after minor physical or mental exertion. Its frequent onset after infection suggests autoimmune involvement or that it arises from abnormal T-cell activation., Results: To test this hypothesis, we sequenced the genomic loci of α/δ, β and γ T-cell receptors (TCR) from 40 human blood samples from each of four groups: severely affected people with ME; mildly or moderately affected people with ME; people diagnosed with Multiple Sclerosis, as disease controls; and, healthy controls. Seeking to automatically classify these individuals' samples by their TCR repertoires, we applied P-SVM, a machine learning method. However, despite working well on a simulated data set, this approach did not allow statistically significant partitioning of samples into the four subgroups. Our findings do not support the hypothesis that blood samples from people with ME frequently contain altered T-cell receptor diversity., (© 2024. The Author(s).)

Hypoxemia is a defining feature of acute respiratory distress syndrome (ARDS), an often-fatal complication of pulmonary or systemic inflammation, yet the resulting tissue hypoxia, and its impact on immune responses, is often neglected. In the present study, we have shown that ARDS patients were hypoxemic and monocytopenic within the first 48 h of ventilation. Monocytopenia was also observed in mouse models of hypoxic acute lung injury, in which hypoxemia drove the suppression of type I interferon signaling in the bone marrow. This impaired monopoiesis resulted in reduced accumulation of monocyte-derived macrophages and enhanced neutrophil-mediated inflammation in the lung. Administration of colony-stimulating factor 1 in mice with hypoxic lung injury rescued the monocytopenia, altered the phenotype of circulating monocytes, increased monocyte-derived macrophages in the lung and limited injury. Thus, tissue hypoxia altered the dynamics of the immune response to the detriment of the host and interventions to address the aberrant response offer new therapeutic strategies for ARDS., (© 2022. The Author(s).)

Herpesviruses are large DNA viruses, which encode up to 300 different proteins including enzymes enabling efficient replication. Nevertheless, they depend on a multitude of host cell proteins for successful propagation. To uncover cellular host factors important for replication of pseudorabies virus (PrV), an alphaherpesvirus of swine, we performed an unbiased genome-wide CRISPR/Cas9 forward screen. To this end, a porcine CRISPR-knockout sgRNA library (SsCRISPRko.v1) targeting 20,598 genes was generated and used to transduce porcine kidney cells. Cells were then infected with either wildtype PrV (PrV-Ka) or a PrV mutant (PrV-gD - Pass) lacking the receptor-binding protein gD, which regained infectivity after serial passaging in cell culture. While no cells survived infection with PrV-Ka, resistant cell colonies were observed after infection with PrV-gD - Pass. In these cells, sphingomyelin synthase 1 (SMS1) was identified as the top hit candidate. Infection efficiency was reduced by up to 90% for PrV-gD - Pass in rabbit RK13-sgms1 KO cells compared to wildtype cells accompanied by lower viral progeny titers. Exogenous expression of SMS1 partly reverted the entry defect of PrV-gD - Pass. In contrast, infectivity of PrV-Ka was reduced by 50% on the knockout cells, which could not be restored by exogenous expression of SMS1. These data suggest that SMS1 plays a pivotal role for PrV infection, when the gD-mediated entry pathway is blocked.

Host dependency factors that are required for influenza A virus infection may serve as therapeutic targets as the virus is less likely to bypass them under drug-mediated selection pressure. Previous attempts to identify host factors have produced largely divergent results, with few overlapping hits across different studies. Here, we perform a genome-wide CRISPR/Cas9 screen and devise a new approach, meta-analysis by information content (MAIC) to systematically combine our results with prior evidence for influenza host factors. MAIC out-performs other meta-analysis methods when using our CRISPR screen as validation data. We validate the host factors, WDR7, CCDC115 and TMEM199, demonstrating that these genes are essential for viral entry and regulation of V-type ATPase assembly. We also find that CMTR1, a human mRNA cap methyltransferase, is required for efficient viral cap snatching and regulation of a cell autonomous immune response, and provides synergistic protection with the influenza endonuclease inhibitor Xofluza.

Background: Venous air embolism is a potentially life-threatening complication of IV catheter use in horses. Despite widespread anecdotal reports of their occurrence, few cases have been reported in the literature and the prognosis is currently unknown., Hypothesis/objectives: Our objective was to describe the surrounding circumstances, clinical signs, treatment, progression, and outcome of venous air embolism in hospitalized horses., Animals: Thirty-two horses with acute onset of compatible clinical signs associated with IV catheter disconnection or damage., Methods: Multicenter retrospective study. Data extracted from clinical records included signalment, presenting complaint, catheter details, clinical signs, treatments, and outcome., Results: Most cases resulted from extension set disconnection occurring within approximately 24 hours after catheter placement. In fewer horses, extension set damage was cited as a cause. Common clinical signs included tachycardia, tachypnea, recumbency, muscle fasciculations and agitation, with abnormal behavior including kicking and flank biting. Less commonly, pathological arrhythmias or more severe neurologic signs, including blindness and seizures, were noted. Progression was unpredictable, with some affected horses developing delayed-onset neurologic signs. Mortality was 6/32 (19%), including 2 cases of sudden death and other horses euthanized because of persistent neurologic deficits. Negative outcomes were more common in horses with recorded blindness, sweating or recumbency, but blindness resolved in 5/8 affected horses., Conclusions and Clinical Importance: The prognosis for resolution of clinical signs after air embolism is fair, but permanent neurologic deficits or pathologic cardiac arrhythmias can arise. Unpredictable progression warrants close monitoring. Systematic clinic-based surveillance could provide additional useful information to aid prevention., (Copyright © 2018 The Authors. Journal of Veterinary Internal Medicine published by Wiley Periodicals, Inc. on behalf of the American College of Veterinary Internal Medicine.)

The innate immune response to lipopolysaccharide contributes substantially to the morbidity and mortality of gram-negative sepsis. Horses and humans share an exquisite sensitivity to lipopolysaccharide and thus the horse may provide valuable comparative insights into this aspect of the inflammatory response. MicroRNAs, small non-coding RNA molecules acting as post-transcriptional regulators of gene expression, have key roles in toll-like receptor signaling regulation but have not been studied in this context in horses. The central hypothesis of this study was that lipopolysaccharide induces differential microRNA expression in equine peripheral blood mononuclear cells in a manner comparable to humans. Illumina Next Generation Sequencing was used to characterize the basal microRNA transcriptome in isolated peripheral blood mononuclear cells from healthy adult horses, and to evaluate LPS-induced changes in microRNA expression in cells cultured for up to four hours. Selected expression changes were validated using quantitative reverse-transcriptase PCR. Only miR-155 was significantly upregulated by LPS, changing in parallel with supernatant tumor necrosis factor-α concentration. Eight additional microRNAs, including miR-146a and miR-146b, showed significant expression change with time in culture without a clear LPS effect. Target predictions indicated a number of potential immunity-associated targets for miR-155 in the horse, including SOCS1, TAB2 and elements of the PI3K signaling pathway, suggesting that it is likely to influence the acute inflammatory response to LPS. Gene alignment showed extensive conservation of the miR-155 precursor gene and associated promoter regions between horses and humans. The basal and LPS-stimulated microRNA expression pattern characterized here were similar to those described in human leukocytes. As well as providing a resource for further research into the roles of microRNAs in immune responses in horses, this will facilitate inter-species comparative study of the role of microRNAs in the inflammatory cascade during endotoxemia and sepsis.

V(D)J genomic recombination joins single gene segments to encode an extensive repertoire of antigen receptor specificities in T and B lymphocytes. This process initiates with double-stranded breaks adjacent to conserved recombination signal sequences that contain either 12- or 23-nucleotide spacer regions. Only recombination between signal sequences with unequal spacers results in productive coding genes, a phenomenon known as the "12/23 rule." Here we present two novel genomic tools that allow the capture and analysis of immune locus rearrangements from whole thymic and splenic tissues using second-generation sequencing. Further, we provide strong evidence that the 12/23 rule of genomic recombination is frequently violated under physiological conditions, resulting in unanticipated hybrid recombinations in ∼10% of Tcra excision circles. Hence, we demonstrate that strict adherence to the 12/23 rule is intrinsic neither to recombination signal sequences nor to the catalytic process of recombination and propose that nonclassical excision circles are liberated during the formation of antigen receptor diversity., (© 2015 Parkinson et al.; Published by Cold Spring Harbor Laboratory Press.)

Reasons for Performing the Study: Severe hypertriglyceridaemia in horses and ponies with endocrine disorders has been reported anecdotally but has not been documented in the literature., Objectives: To describe historical and clinicopathological findings as well as progression and outcome in horses and ponies with severe hypertriglyceridaemia (serum triglyceride concentration >5.65 mmol/l) secondary to an endocrine disorder that were otherwise apparently healthy., Methods: Cases from 6 participating institutions were identified and case details extracted from the medical records., Results: Case details of 3 horses and 4 ponies were available. Presenting complaints included weight loss despite good appetite in 4 animals, while in 3 hypertriglyceridaemia was identified incidentally. All animals were bright and alert and showed a normal or increased appetite. Serum triglyceride concentrations ranged from 10.5 to 60.3 mmol/l. Other abnormalities included hyperglycaemia in 6 animals, suspected insulin resistance and mild to severe increases in hepatic enzyme activities. In 2 animals, moderate hepatic lipidosis was confirmed histologically. Three horses and 3 ponies were diagnosed with pituitary pars intermedia dysfunction based on clinical signs and basal adrenocorticotropic hormone (ACTH) concentrations or dexamethasone suppression test results. In 5 of these, type 2 diabetes mellitus was also confirmed, while one pony suffered from type 2 diabetes mellitus without concurrent pituitary pars intermedia dysfunction. Laboratory abnormalities improved in 4 animals with treatment (pergolide and/or insulin), in one horse specific treatment was not attempted, and in 2 ponies treatment was impaired by the owner or only partly effective. In one of the latter cases, biochemical abnormalities persisted for 7 years without apparent ill effects., Conclusions and Potential Relevance: Horses and ponies may develop severe hypertriglyceridaemia secondary to endocrine disorders that are associated with insulin resistance. Hypertriglyceridaemia can resolve with treatment of the endocrinopathy. Although biochemical evidence of hepatic compromise was present, clinical abnormalities were not noted in these animals., (© 2013 EVJ Ltd.)

New sequencing technologies can address diverse biomedical questions but are limited by a minimum required DNA input of typically 1 μg. We describe how sequencing libraries can be reproducibly created from 20 pg of input DNA using a modified transpososome-mediated fragmentation technique. Resulting libraries incorporate in-line bar-coding, which facilitates sample multiplexes that can be sequenced using Illumina platforms with the manufacturer's sequencing primer. We demonstrate this technique by providing deep coverage sequence of the Escherichia coli K-12 genome that shows equivalent target coverage to a 1-μg input library prepared using standard Illumina methods. Reducing template quantity does, however, increase the proportion of duplicate reads and enriches coverage in low-GC regions. This finding was confirmed with exhaustive resequencing of a mouse library constructed from 20 pg of gDNA input (about seven haploid genomes) resulting in ∼0.4-fold statistical coverage of uniquely mapped fragments. This implies that a near-complete coverage of the mouse genome is obtainable with this approach using 20 genomes as input. Application of this new method now allows genomic studies from low mass samples and routine preparation of sequencing libraries from enrichment procedures.

The sequence of the Streptococcus equi subspecies equi (S equi) M-like protein (SeM) gene was determined for 105 isolates of S equi from strangles outbreaks in the UK during 2010 and compared with previous data from 2007 to 2008. Twenty-three distinct alleles were identified, including 11 novel alleles. One allele giving rise to a putative truncated M protein was identified from the guttural pouch of an asymptomatic carrier. Allele 9 was the most prevalent, comprising 57.7 per cent of isolates, followed by allele 6 (10.3 per cent). Significant changes in allele prevalence were found between 2007, 2008 and 2010, with an increasing prevalence in SeM-9-related alleles and a corresponding decreasing prevalence in SeM-6-related alleles observed over the period (P<0.001). Geographical proximity of outbreaks caused by some uncommon alleles was apparent between 2007, 2008 and 2010.

Spinal muscular atrophy is a severe motor neuron disease caused by inactivating mutations in the SMN1 gene leading to reduced levels of full-length functional SMN protein. SMN is a critical mediator of spliceosomal protein assembly, and complete loss or drastic reduction in protein leads to loss of cell viability. However, the reason for selective motor neuron degeneration when SMN is reduced to levels which are tolerated by all other cell types is not currently understood. Widespread splicing abnormalities have recently been reported at end-stage in a mouse model of SMA, leading to the proposition that disruption of efficient splicing is the primary mechanism of motor neuron death. However, it remains unclear whether splicing abnormalities are present during early stages of the disease, which would be a requirement for a direct role in disease pathogenesis. We performed exon-array analysis of RNA from SMN deficient mouse spinal cord at 3 time points, pre-symptomatic (P1), early symptomatic (P7), and late-symptomatic (P13). Compared to littermate control mice, SMA mice showed a time-dependent increase in the number of exons showing differential expression, with minimal differences between genotypes at P1 and P7, but substantial variation in late-symptomatic (P13) mice. Gene ontology analysis revealed differences in pathways associated with neuronal development as well as cellular injury. Validation of selected targets by RT-PCR confirmed the array findings and was in keeping with a shift between physiologically occurring mRNA isoforms. We conclude that the majority of splicing changes occur late in SMA and may represent a secondary effect of cell injury, though we cannot rule out significant early changes in a small number of transcripts crucial to motor neuron survival., Competing Interests: The authors have declared that no competing interests exist.

Deafness is the most common sensory disorder in humans and the aetiology of genetic deafness is complex. Mouse mutants have been crucial in identifying genes involved in hearing. However, many deafness genes remain unidentified. Using N-ethyl N-nitrosourea (ENU) mutagenesis to generate new mouse models of deafness, we identified a novel semi-dominant mouse mutant, Cloth-ears (Clth). Cloth-ears mice show reduced acoustic startle response and mild hearing loss from approximately 30 days old. Auditory-evoked brainstem response (ABR) and distortion product otoacoustic emission (DPOAE) analyses indicate that the peripheral neural auditory pathway is impaired in Cloth-ears mice, but that cochlear function is normal. In addition, both Clth/Clth and Clth/+ mice display paroxysmal tremor episodes with behavioural arrest. Clth/Clth mice also show a milder continuous tremor during movement and rest. Longitudinal phenotypic analysis showed that Clth/+ and Clth/Clth mice also have complex defects in behaviour, growth, neurological and motor function. Positional cloning of Cloth-ears identified a point mutation in the neuronal voltage-gated sodium channel alpha-subunit gene, Scn8a, causing an aspartic acid to valine (D981V) change six amino acids downstream of the sixth transmembrane segment of the second domain (D2S6). Complementation testing with a known Scn8a mouse mutant confirmed that this mutation is responsible for the Cloth-ears phenotype. Our findings suggest a novel role for Scn8a in peripheral neural hearing loss and paroxysmal motor dysfunction.

Mutations in the ubiquitously expressed survival motor neuron 1 (SMN1) and superoxide dismutase 1 (SOD1) genes are selectively lethal to motor neurons in spinal muscular atrophy (SMA) and familial amyotrophic lateral sclerosis (ALS), respectively. Genetic association studies provide compelling evidence that SMN1 and SMN2 genotypes encoding lower SMN protein levels are implicated in sporadic ALS, suggesting that SMN expression is a potential determinant of ALS severity. We therefore sought genetic evidence of SMN involvement in ALS by generating transgenic mutant SOD1 mice on an Smn deficient background. Partial genetic disruption of Smn significantly worsened motor performance and survival in transgenic SOD1(G93A) mice. Furthermore, ALS-linked mutant SOD1 expression severely reduced SMN protein levels, but not transcript, in neuronal culture and mouse models from early presymptomatic disease. SMN protein depletion was linked to the nuclear compartment and a physical interaction between SMN and mutant SOD1 was confirmed in mouse spinal cord. Treatment with the environmental toxin paraquat also depleted SMN protein, implicating oxidative stress in the mechanism underlying SMN deficiency in familial ALS and potentially sporadic disease. In contrast, transgenic SOD1(WT) overexpression in SMA type I mice was incapable of modulating SMN protein levels or disease progression. These data establish that SMN deficiency accelerates phenotypic severity in transgenic familial ALS mice, consistent with an enhancing genetic modifier role. We therefore propose that SMN replacement and upregulation strategies considered for SMA therapy may have protective potential for ALS.

Background: Redistribution of nuclear TAR DNA binding protein 43 (TDP-43) to the cytoplasm and ubiquitinated inclusions of spinal motor neurons and glial cells is characteristic of amyotrophic lateral sclerosis (ALS) pathology. Recent evidence suggests that TDP-43 pathology is common to sporadic ALS and familial ALS without SOD1 mutation, but not SOD1-related fALS cases. Furthermore, it remains unclear whether TDP-43 abnormalities occur in non-ALS forms of motor neuron disease. Here, we characterise TDP-43 localisation, expression levels and post-translational modifications in mouse models of ALS and spinal muscular atrophy (SMA)., Results: TDP-43 mislocalisation to ubiquitinated inclusions or cytoplasm was notably lacking in anterior horn cells from transgenic mutant SOD1G93A mice. In addition, abnormally phosphorylated or truncated TDP-43 species were not detected in fractionated ALS mouse spinal cord or brain. Despite partial colocalisation of TDP-43 with SMN, depletion of SMN- and coilin-positive Cajal bodies in motor neurons of affected SMA mice did not alter nuclear TDP-43 distribution, expression or biochemistry in spinal cords., Conclusion: These results emphasise that TDP-43 pathology characteristic of human sporadic ALS is not a core component of the neurodegenerative mechanisms caused by SOD1 mutation or SMN deficiency in mouse models of ALS and SMA, respectively.

Spinal muscular atrophy (SMA) is a devastating neuromuscular disease characterised by progressive loss of spinal motor neurons. Mutations in the genes underlying spontaneous bovine and feline models of SMA have recently been described. The clinical and pathological features of these disorders are similar to human forms of SMA making both genes excellent candidates in patients with motor neuron loss of no known aetiology. Here we report that a screen for mutations in coding regions and splice sites of the LIX1 and FVT1 genes in a cohort of 96 non-5q SMA patients and 119 familial and sporadic Amyotrophic Lateral Sclerosis patients identified no obvious pathogenic changes. This study indicates that mutations in these genes do not contribute significantly to the cause of motor neuron diseases in the human population.

We have previously reported a large Danish pedigree with autosomal dominant frontotemporal dementia (FTD) linked to chromosome 3 (FTD3). Here we identify a mutation in CHMP2B, encoding a component of the endosomal ESCRTIII complex, and show that it results in aberrant mRNA splicing in tissue samples from affected members of this family. We also describe an additional missense mutation in an unrelated individual with FTD. Aberration in the endosomal ESCRTIII complex may result in FTD and neurodegenerative disease.

The autosomal recessive mouse mutation quivering (qv), which arose spontaneously in 1953, produces progressive ataxia with hind limb paralysis, deafness and tremor. Six additional spontaneous alleles, qvJ, qv2J, qv3J, qv4J, qvlnd and qvlnd2J, have been identified. Ear twitch responses (Preyer's reflex) to sound are absent in homozygous qv/qv mice, although cochlear morphology seems normal and cochlear potentials recorded at the round window are no different from those of control mice. However, responses from brainstem auditory nuclei show abnormal transmission of auditory information, indicating that, in contrast to the many known mutations causing deafness originating in the cochlea, deafness in qv is central in origin. Here we report that quivering mice carry loss-of-function mutations in the mouse beta-spectrin 4 gene (Spnb4) that cause alterations in ion channel localization in myelinated nerves; this provides a rationale for the auditory and motor neuropathies of these mice.

To understand the cellular and molecular mechanisms that underlie generalized absence seizures sufficiently well to design rational, efficacious new therapies for patients, it is necessary to turn to animal models to gain insights into these mechanisms. The lethargic (lh/lh) mutant mouse expresses spontaneous absence seizures that share behavioral, electrographic, and anticonvulsant profiles with absence seizures in patients. This validates its use to study the mechanisms that underlie absence seizures. This chapter discusses two scientific approaches that involve the use of lh/lh mice. The first part of the chapter discusses neurobiologic approaches used to investigate critical mechanisms that regulate the synchronized burst firing within the thalamocortical network that generates absence seizures. Two of these critical mechanisms have been studied in detail with lh/lh mice. The first critical mechanism involves the required activation of gamma-aminobutyric acid B (GABAB) receptors to generate absence seizures. Because the numbers of GABAB receptors are increased in thalamocortical populations among lh/lh mice compared with littermates without epilepsy, these receptors appear to play a pathophysiologic role in the expression of absence seizures among lh/lh mice. Moreover, there may be a role for GABAB receptors in the generation of absence seizures among humans, because administration of compounds that activate GABAB receptors can produce absence seizures among humans. These findings suggest that GABAB receptor antagonists may represent a new class of antiabsence compounds that will be efficacious against absence seizures among patients. A second critical mechanism that regulates generation of absence seizures involves GABAA receptors in the nucleus reticularis thalami (NRT), a nucleus that sends GABA-ergic afferents to thalamic relay nuclei. Activation of GABAA receptors in the NRT appears to suppress the generation of absence seizures among lh/lh mice and in other models. Moreover, clonazepam may exert its antiabsence actions through this mechanism. Together, these findings suggest that compounds that selectively activate GABAA receptor isoforms expressed in NRT may represent a class of antiabsence drugs that could have fewer side effects than compounds currently used to treat patients. The second part of the chapter discusses a molecular genetic approach to delineation of the mechanisms that underlie absence seizures. Absence seizures among lh/lh mice are caused by a single-gene defect on chromosome 2. If positional cloning and gene isolation techniques are successful, it will be possible to identify the lh disease gene. Subsequent studies of the lh gene product should greatly increase not only our understanding of the pathophysiologic basis for absence seizures among lh/lh mice but also our ability to seek similar mutations in homologous genes in human families that express absence seizures. Accordingly, strategies and progress in cloning and identifying the lh disease gene are presented.