Biomedical subjects

Simon D Shorvon

Publications and source records attributed to Simon D Shorvon.

13 recordsLinked to original sources

PubMed2006

Genetic association studies in epilepsy pharmacogenomics: lessons learnt and potential applications.

Although epilepsy is one of the most common neurological disorders and genetic factors are well known to play a role in response to antiepileptic drug (AED) treatment, the study of the pharmacogenetics of epilepsy has received relatively little attention and has not resulted in clinical applications to date. Our improved understanding of the pathogenesis of epilepsy and the mechanism of action of AEDs, together with recent advances in genetics and decreasing genotyping costs, have now paved the way for a more systematic application of pharmacogenetics in the field of epilepsy. It is hoped that the resulting knowledge will lead to a more rational treatment of epilepsy, development of more efficacious AEDs, and facilitation of clinical trials of new AEDs. However, there are formidable practical, methodological and theoretical hurdles to overcome before pharmacogenomic information will have any major utility in the clinical setting. Here, we discuss the evidence for a genetic contribution to AED response, review current knowledge in epilepsy pharmacogenetics and discuss potential future avenues with their implications, both for the clinical treatment of epilepsy and new AED development.

Animals↗

PubMed2005

Genetic predictors of the maximum doses patients receive during clinical use of the anti-epileptic drugs carbamazepine and phenytoin.

Phenytoin and carbamazepine are effective and inexpensive anti-epileptic drugs (AEDs). As with many AEDs, a broad range of doses is used, with the final "maintenance" dose normally determined by trial and error. Although many genes could influence response to these medicines, there are obvious candidates. Both drugs target the alpha-subunit of the sodium channel, encoded by the SCN family of genes. Phenytoin is principally metabolized by CYP2C9, and both are probable substrates of the drug transporter P-glycoprotein. We therefore assessed whether variation in these genes associates with the clinical use of carbamazepine and phenytoin in cohorts of 425 and 281 patients, respectively. We report that a known functional polymorphism in CYP2C9 is highly associated with the maximum dose of phenytoin (P = 0.0066). We also show that an intronic polymorphism in the SCN1A gene shows significant association with maximum doses in regular usage of both carbamazepine and phenytoin (P = 0.0051 and P = 0.014, respectively). This polymorphism disrupts the consensus sequence of the 5' splice donor site of a highly conserved alternative exon (5N), and it significantly affects the proportions of the alternative transcripts in individuals with a history of epilepsy. These results provide evidence of a drug target polymorphism associated with the clinical use of AEDs and set the stage for a prospective evaluation of how pharmacogenetic diagnostics can be used to improve dosing decisions in the use of phenytoin and carbamazepine. Although the case made here is compelling, our results cannot be considered definitive or ready for clinical application until they are confirmed by independent replication.

Adolescent↗

PubMed2005

Cerebral damage in epilepsy: a population-based longitudinal quantitative MRI study.

PURPOSE: Whether cerebral damage results from epileptic seizures remains a contentious issue. We report on the first longitudinal community-based quantitative magnetic resonance imaging (MRI) study to investigate the effect of seizures on the hippocampus, cerebellum, and neocortex. METHODS: One hundred seventy-nine patients with epilepsy (66 temporal lobe epilepsy, 51 extratemporal partial epilepsy, and 62 generalized epilepsy) and 90 control subjects underwent two MRI brain scans 3.5 years apart. Automated and manual measurement techniques identified changes in global and regional brain volumes and hippocampal T2 relaxation times. RESULTS: Baseline hippocampal volumes were significantly reduced in patients with temporal lobe epilepsy and could be attributed to an antecedent neurologic insult. Rates of hippocampal, cerebral, and cerebellar atrophy were not syndrome specific and were similar in control and patient groups. Global and regional brain atrophy was determined primarily by age. A prior neurologic insult was associated with reduced hippocampal and cerebellar volumes and an increased rate of cerebellar atrophy. Significant atrophy of the hippocampus, neocortex, or cerebellum occurred in 17% of patients compared with 6.7% of control subjects. Patients with and without significant volume reduction were comparable in terms of seizure frequency, antiepileptic drug (AED) use, and epilepsy duration, with no identifiable risk factors for the development of atrophy. CONCLUSIONS: Overt structural cerebral damage is not an inevitable consequence of epileptic seizures. In general, brain volume reduction in epilepsy is the cumulative effect of an initial precipitating injury and age-related cerebral atrophy. Significant atrophy developed in individual patients, particularly those with temporal lobe and generalized epilepsy. Longer periods of observation may detect more subtle effects of seizures.

Adolescent↗

PubMed2004

Life expectancy in people with newly diagnosed epilepsy.

Epilepsy carries a risk of premature mortality, but little is known about life expectancy in people with the condition. The UK National General Practice Study of Epilepsy is a prospective, population-based study of people with newly diagnosed epilepsy. A cohort of 564 patients with definite epilepsy has been followed for nearly 15 years and there have been 177 deaths. These data have been used to estimate life expectancy of people in this cohort by employing a parametric survival model based on the Weibull distribution. Life expectancy in people with epilepsy was estimated as a function of age at, and time from, diagnosis according to two broad aetiological groups. These estimates were then compared with life expectancy in people of the same age and sex in the general population. Reduction in life expectancy can be up to 2 years for people with a diagnosis of idiopathic/cryptogenic epilepsy, and the reduction can be up to 10 years in people with symptomatic epilepsy. Reductions in life expectancy are highest at the time of diagnosis and diminish with time. Our model provides broad estimates, but it appears that the higher mortality rates in people with newly diagnosed epilepsy translate into decreased life expectancy.

Adolescent↗

PubMed2003

Selection and evaluation of tagging SNPs in the neuronal-sodium-channel gene SCN1A: implications for linkage-disequilibrium gene mapping.

Association studies are widely seen as the most promising approach for finding polymorphisms that influence genetically complex traits, such as common diseases and responses to their treatment. Considerable interest has therefore recently focused on the development of methods that efficiently screen genomic regions or whole genomes for gene variants associated with complex phenotypes. One key element in this search is the use of linkage disequilibrium to gain maximal information from typing a selected subset of highly informative single-nucleotide polymorphism (SNP) markers, now often called "tagging SNPs" (tSNPs). Probably the most common approach to linkage-disequilibrium gene mapping involves a three-step program: (1) characterization of the haplotype structure in candidate genes or genomic regions of interest, (2) identification of tSNPs sufficient to represent the most common haplotypes, and (3) typing of tSNPs in clinical material. Early definitions of tSNPs focused on the amount of haplotype diversity that they explained. To select tSNPs that would have maximal power in a genetic association study, however, we have developed optimization criteria based on the r2 measure of association and have compared these with other criteria based on the haplotype diversity. To evaluate the full program and to assess how well the selected tags are likely to perform, we have determined the haplotype structure and have assessed tSNPs in the SCN1A gene, an important candidate gene for sporadic epilepsy. We find that as few as four tSNPs are predicted to maintain a consistently high r2 value with all other common SNPs in the gene, indicating that the tags could be used in an association study with only a modest reduction in power relative to direct assays of all common SNPs. This implies that very large case-control studies can be screened for variation in hundreds of candidate genes with manageable experimental effort, once tSNPs are identified. However, our results also show that tSNPs identified in one population may not necessarily perform well in another, indicating that the preliminary study to identify tSNPs and the later case-control study should be performed in the same population. Our results also indicate that tSNPs will not easily identify discrepant SNPs, which lie on importantly discriminating but apparently short genealogical branches. This could significantly complicate tagging approaches for phenotypes influenced by variants that have experienced positive selection.

Adult↗

PubMed2003

Progressive neocortical damage in epilepsy.

Our objective was to determine the pattern and extent of generalized and focal neocortical atrophy that develops in patients with epilepsy and the factors associated with such changes. As part of a prospective, longitudinal follow-up study of 122 patients with chronic epilepsy, 68 newly diagnosed patients, and 90 controls, serial magnetic resonance imaging scans were obtained 3.5 years apart. Image subtraction was used to identify diffuse and focal neocortical change that was quantified with a regional brain atlas and a fully automated segmentation algorithm. New focal or generalized neocortical volume losses were identified in 54% of patients with chronic epilepsy, 39% of newly diagnosed patients and 24% of controls. Patients with chronic epilepsy were significantly more likely to develop neocortical atrophy than control subjects. The increased risk of cerebral atrophy in epilepsy was not related to a history of documented seizures. Risk factors for neocortical atrophy were age and multiple antiepileptic drug exposure. Focal and generalized neocortical atrophy commonly develops in chronic epilepsy. Neocortical changes seen in a quarter of our control group over 3.5 years were likely to reflect physiological changes. Our results show that ongoing cerebral atrophy may be widespread and remote from the putative epileptic focus, possibly reflecting extensive networks and interconnections between cortical regions.

Adolescent↗

PubMed2003

Polymicrogyria and absence of pineal gland due to PAX6 mutation.

Identification of genes involved in human cerebral development is important for our understanding of disorders with potential neurodevelopmental causes such as epilepsy and learning disability. Murine models suggest that PAX6 plays a key role in human brain development. With magnetic resonance imaging in 24 humans heterozygous for defined PAX6 mutations, we demonstrated widespread structural abnormalities including absence of the pineal gland and unilateral polymicrogyria.

Abnormalities, Multiple↗

PubMed2003

Quantitative MR image analysis in subjects with defects in the PAX6 gene.

Cerebral development is governed by genetic and environmental factors. Animal models provide much of our knowledge about genetic influences in the brain but it is not clear how similar or relevant these are to the human brain. The investigation of human subjects with mutations in genes known to be expressed in the developing brain is an alternative approach to improving our understanding of the role of genetic factors in brain development. We investigated 24 subjects with known mutations in the PAX6 gene (including representatives of all the known mutations of the gene) which are associated with characteristic ocular abnormalities in humans and animals. We have quantified MRI data using several techniques to assess qualities of cerebral structure which are difficult to interpret visually. Abnormalities were identified using voxel-based morphometry, statistical morphometrics, and measurement of corpus callosum cross-sectional area when comparing data from subjects with PAX6 abnormality and 72 age-and sex-matched control subjects. These abnormalities include grey matter changes in the cerebellum and occipital poles and white matter loss in the corpus callosum, alteration of sulcal orientation in the occipital lobe, and alteration to overall neuronal connectivity. These abnormalities complement and exceed the changes seen in the mouse models, and those seen on visual inspection alone of the human MRI data. Structural differences were also identified between the two largest genotype mutation subgroups.

Adult↗

PubMed2003

A prospective study of the requirement for and the provision of epilepsy surgery in the United Kingdom.

PURPOSE: Of the 30,000 persons in whom epilepsy develops annually in the United Kingdom, in approximately 6000 (20%), intractability develops. Some of these patients will be appropriate for epilepsy surgery. We aimed to estimate the number of patients who should be considered surgical candidates, by extrapolation from a population-based study of prognosis and the number who are receiving epilepsy surgery, by a survey of U.K. neurosurgeons. METHODS: We identified the number of patients who may eventually require surgery from a prospective cohort of patients with newly diagnosed epilepsy. We identified all U.K. neurosurgeons who had performed any epilepsy surgery in the past year. Each identified surgeon prospectively recorded the number and types of operations carried out for 6 months. RESULTS: Of newly diagnosed patients each year, 450 (1.5%) may eventually require surgery. Thirty-two respondents (22% of all U.K. neurosurgeons) reported that they performed epilepsy surgery. The 211 operations were carried out in the 6 months surveyed (422 operations annually or 13 per surgeon per year). Temporal lobe resection (77%) was the most common procedure. CONCLUSIONS: Based on a prevalence of 5/1,000 persons with epilepsy, < or =4,500 patients in the U.K. require epilepsy surgery. Every year, 450 patients with newly diagnosed epilepsy who may eventually require surgery are added to this "surgical pool." At the current annual rate of operations, a large number of refractory patients remain untreated. This is probably partly because many patients are not referred for specialist care and therefore remain underinvestigated.

Adolescent↗

PubMed2003

Clues to the genetic influences of drug responsiveness in epilepsy.

Despite the introduction of newer antiepileptic drugs (AEDs), nonresponsiveness to AEDs remains a common problem in epilepsy clinics. There may be important genetic determinants for responsiveness, and this discussion focused on some potential areas: drug transporters; drug-metabolizing enzymes, and ion channels. We review the literature and speculate the contribution of each of these factors in management of patients with epilepsy in the future.

Anticonvulsants↗

PubMed2003

Reliable callosal measurement: population normative data confirm sex-related differences.

BACKGROUND AND PURPOSE: Corpus callosal cross-sectional area (CCA) may be a clinical indicator of disease progression, but factors influencing callosal morphology in healthy subjects must be determined before comparisons can be made in patients. We sought to define a reliable and easily repeatable method for CCA measurement and to examine the effects of sex, age, handedness, and cerebral volume. METHODS: Neurologically healthy volunteers (age range, 14-68 years; mean age, 32.6 years +/- 12.3 [SD]; 44 men, 56 women; 87 right handed) underwent conventional MR imaging. Data were reoriented in the image space to account for intersubject variations in head position before the midsagittal plane was defined by using midpoints of the anterior commissure (AC), posterior commissure (PC), and interhemispheric fissure (IF). Midsagittal CCA and total cerebral volume were measured and correlated with sex, age, and handedness. RESULTS: The mean CCA was 6.27 cm(2) +/- 0.90. Women had a larger CCA proportional to cerebral volume (6.16 x 10(-3) cm(-1) vs 5.78 x 10(-3) cm(-1) in men; P =.02). The percentage difference for the CCA-cerebral volume from the group mean was +2.6% in women and -3.6% in men. Only a small linear relationship of CCA with cerebral volume was noted (r(2) = 0.15), and CCA was not significantly correlated with age or handedness. CONCLUSION: To our knowledge, this is the largest study of callosal area in a community-based sample of control subjects; such subjects provide controls for future studies. Our findings provide anatomic evidence of sex differences in interhemispheric connectivity. Much CCA variability is independent of cerebral volume.

Adolescent↗

PubMed2002

The structural consequences of newly diagnosed seizures.

Intractable epilepsy may be associated with widespread structural cerebral damage. We determined whether structural damage occurs to the hippocampus, cerebellum and neocortex in the first few years following a diagnosis of seizures. Sixty-eight patients over the age of 14 years with newly diagnosed seizures and 90 matched controls underwent serial magnetic resonance imaging (MRI) brain scans 3.5 years apart. Using quantitative analysis of serial scans, we determined changes in hippocampal volume, hippocampal T2 relaxometry and total and regional brain volumes. Thirty-four (50%) patients had recurrent unprovoked seizures between baseline and follow-up scans. One patient with pre-existing hippocampal sclerosis (HS) did not develop progressive hippocampal damage. Group analyses found no difference in change in cerebral measures between patients and controls or between patients with and without recurrent seizures. Significant quantitative changes in individuals were largely attributable to pre-existing cerebral lesions or alcohol abuse. Subtle changes detected in individuals over 3.5 years but were not related to a history of overt seizures. Our results show patients with newly diagnosed seizures are not generally at increased risk of seizure-induced structural cerebral damage as detected with MRI. Cerebral damage may occur before the onset of seizures or develop insidiously over a more prolonged period.

Adolescent↗

PubMed2002

Mitochondrial dysfunction associated with neuronal death following status epilepticus in rat.

Status epilepticus (SE) in humans and animal models results in significant cerebral damage and an increased risk of subsequent seizures, associated with a characteristic pattern of neuronal loss particularly affecting the hippocampus. Seizure related cell death is considered to be excitotoxic, but studies have been limited, concentrating on terminal events rather than initial mechanisms. We have studied the biochemical events in the first few days following SE. Self-sustaining limbic SE was induced in adult rats using perforant path stimulation, and animals were allowed to recover. Biochemical studies were performed at 16, 44 h and 8 days following SE, using spectrophotometric enzyme assays and HPLC on regional brain homogenates compared with those from sham-operated controls. Haematoxylin and eosin histology was also undertaken at each time point. Brain aconitase and alpha-ketoglutarate dehydrogenase (alphaKDH) activity were both significantly (P<0.05) reduced by approximately 20% in the first 16-44 h following status, but had returned to normal by 8 days. These enzymes are part of the tri-carboxylic acid (Krebbs) cycle in the mitochondrial matrix, and are known to be sensitive to free radical, especially peroxynitrite damage. There was a similar decrease in reduced glutathione levels. Histological studies confirmed evidence of acute neuronal damage up to 44 h, and neuronal loss by 8 days. This is the first in vivo demonstration of this pattern of mitochondrial dysfunction and loss of brain glutathione following SE. The pattern of abnormalities is consistent with reversible mechanisms being involved in excitotoxic cell damage. This, together with the timing of changes, suggests new avenues for therapeutic intervention.

Animals↗