Molecular epidemiology of strangles outbreaks in the UK during 2010

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.

Analysis of the mouse mutant Cloth-ears shows a role for the voltage-gated sodium channel Scn8a in peripheral neural hearing loss

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 ∼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 α-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.

Mutant beta-spectrin 4 causes auditory and motor neuropathies in quivering mice

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.

Studies of the lethargic (lh/lh) mouse model of absence seizures: regulatory mechanisms and identification of the lh gene

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.