Dr Martin Turner
Research Registrar

His special interest is in using a brain scanning technique called positron emission tomography (PET scanning) to improve our understanding of the degenerative process in MND. This technique may prove sensitive enough to detect the earliest changes even before symptoms arise.

The image on the right shows a PET scan which highlights changes to the brain.

MAIN FEATURES OF THE DISEASE

Motor neurone disease (MND) is a progressive fatal disorder of the central nervous system characterised by degeneration of motor nerve cells in the brain (upper motor neurones) and spinal cord (lower motor neurones). There are about 1000 new cases in the UK each year. Most patients die within 3-4 years of diagnosis. In MND, motor nerve cells in the cerebral cortex, the base of the brain (brain stem) and in the spinal cord degenerate, resulting in muscular wasting and weakness, with death due to involvement of the muscles controlling breathing. Currently there is no cure, although we (in conjunction with colleagues in Paris) have been instrumental in developing the first drug (Riluzole; trade name Rilutek) shown to slow the progression of the disease. A number of other drugs are under development and the Kings College School of Medicine and Dentistry (KCSMD)/Institute of Psychiatry MND Care and Research Centre has taken a leading role in these.

CAUSE

The cause of MND is unknown. However, in rare familial cases, several gene defects have been identified, providing important new clues that could be relevant to the 95% of cases that are not familial. Professor Leigh and the MND research team at the Institute of Psychiatry and (KCSMD), are applying this new knowledge of understanding why nerve cells die. Of particular interest are free radicals, which are now firmly implicated in this disease. A team of molecular biologists and biochemists are working to identify the role of free radical damage in MND. This will have important implications for treatment in the future.

In addition, Professor Leigh's team is working on the hypothesis that certain amino acids may be toxic to motor neurones. Chief of these is glutamate. Professor Leigh is a collaborator in an international study testing the effects of a growth factor known as insulin-like growth factor 1 (IGF-1).

CURRENT RESEARCH

In order to study the biochemical and molecular causes of motor nerve cell degeneration, the team is developing tissue culture systems so that they can investigate the effect of growth factors, free radicals and gene mutations.

Magnetic resonance spectroscopy and functional magnetic imaging provide new approaches to understanding both disease mechanisms. In the Neuroimaging Research Centre at the Institute of Psychiatry we have developed new approaches to examining the upper motor neurone component of the motor system in health and disease. In essence, we can accurately locate the motor cortex by performing a simple activation task, and at the same time examine its chemistry.

In the research we have done over the last five years we have identified characteristic abnormalities in activation of the motor cortex in motor neurone disease using a different approach, known as positron emission tomography. But this uses trace elements of radioactivity. But the advantage of functional magnetic imaging is that it uses no radioactivity, and thus can be safely repeated over time.

Using brain mapping in the motor cortex we will examine the concentration of a chemical known as N-acetylaspartate, which is found only in nerve cells. The concentration of this chemical decreases when nerve cells die, and therefore provides a marker for neurodegeneration. We will therefore be able to correlate the extent of neuronal damage with the abnormalities of activation of the motor cortex, specifically what we have identified in our previous PET studies as the "boundary effect". By this we mean an abnormal expansion of the area of motor cortex activated by the motor task. We have suggested that this could be due to loss of intrinsic inhibitory mechanisms, a hypothesis in keeping with the notion that glutamate could be causing abnormal excitation of motor neurone nerves leading to their degeneration.

Professor Nigel Leigh

What the Institute is doing, in brief:

Demonstrating that Riluzole (a glutamate release antagonist) improves survival in people with motor neurone disease, the first time that any drug has convincingly been shown to alter survival in any neurodegenerative disease.

Demonstrating for the first time that non-invasive ventilation is associated with improved quality of life in people with motor neurone disease.

Identifying new apsects of neuronal damage in MND using positron emission tomography (PET) scanning and neuropsychological tests.

Current Studies:

THE TIM PERKINS MND RESEARCH FUND (PRT)

The PRT administers a fund established in the memory of a MND patient: Mr Tim Perkins. The Perkins family has raised large sums for research and the fund has enabled us to "pump-prime" projects of four clinical research fellows who have gone on to win MRC or Wellcome Trust Research Training Fellowships.

Molecular Genetics

Among the research that the PRT has contributed to is in the molecular genetics of MND. Research has focused on basic mechanisms of cell damage in MND. This includes:

Studies on the molecular genetics of SOD1. We have identified a number of novel mutations and characterised their frequency in familial and sporadic MND patients. Studies on the recessive D90A ("Scandinavian") SOD1 mutation have shown a unique haplotype at the SOD1 locus and suggested that a protective factor for SOD-1 linked MND may exist. Further work led by Dr Chris Shaw and in collaboration with Dr Peter Andersen (Umea, Sweden) and others in Europe and North America, is now underway to determine the nature of any protective factor (with Wellcome Trust and MRC support. We have also identified a unique pattern of single nucleotide polymorphisms (SNPs) close to the SOD1 locus and are pursuing the functional significance of this finding in relationship to the regulation of SOD1 gene expression.

Studies on the neurofilament heavy chain (NFH) gene. With Dr John Powell, Nigel Leigh and an international collaborative group we have identified new mutations in the NFH gene, including the first mutations identified in familial MND. Investigations into the role of these and other potential mutations in intermediate filaments and related genes continue and links with work on the functional importance of neurofilaments in neuronal function (see below).

Linkage analysis on a large MND kindred (led by Dr Chris Shaw, in collaboration with Professors Frank Baas and Vianney de Jong (Academic Medical Centre, Amsterdam). Dr Deborah Ruddy (MRC Training Fellow) is nearing the completion of a genome-wide screen to hopefully identify another causative gene for MND.

Other molecular genetic projects include those by Dr Jo Flowers, an MRC Training Fellow (supervised by Dr Chris Shaw and Dr John Powell). She screened the persyn gene, (related to a-synuclein, mutations in which cause Parkinson's) but found no mutations in MND or PD. She has gone on to quantify mRNA transcripts of the glial excitatory amino acid transporter (EEAT2) in post-mortem brain from MND subjects and controls. She has clearly shown that the aberrant transcripts reported by the group led by Jeffrey Rothstein are not disease related (Flowers e al., Ann Neurol, in press) and that levels of EEAT2 protein are not decreased in affected brain regions.

Positron Emission Tomography

The Tim Perkins MND Research Fund has also supported Dr Martin Turner (see picture above), a very able young clinical neurologist who is working with functional brain imaging. His plan is to use new positron emission tomography (PET) tracers to identify and measure damage to the motor system in motor neurone disease. This is exciting and ground-breaking research.

Dr Turner is hoping to collaborate with colleagues in Sweden to study a unique genetically-determined form of motor neurone disease alongside the more typical patients with sporadic disease

In addition he will be applying new neurophysiological techniques to probe the balance between excitatory and inhibitory influences in the motor cortex.

Dr Martin R. Turner
Professor Nigel Leigh

Amyotrophic Lateral Sclerosis (ALS, or Motor Neuron Disease), and Parkinson's Disease (PD) are two devastating neurodegenerative diseases both characterised by degeneration of relatively selective neuronal populations – spinal anterior horn cells and corticospinal tract glutamatergic motor neurons in ALS, and nigro-striatal dopaminergic neurons in PD. In the case of ALS this leads to a rapid loss of limb and/or bulbar function with respiratory muscle compromise leading to death within an average of three and a half years. PD is more slowly progressive but the combination of tremor, rigidity and bradykinesia can result in severe disability. There is no cure for either disease. Several disease-modifying pharmacological options are available for PD but only one, riluzole, in ALS. The basis of the neuronal selectivity in these diseases represents a fundamental key to unraveling pathogenesis, and the development of future treatments.

There are at least two well-described 'syndromes' where the clinical features of ALS and PD coincide. The first is the extensively studied ALS-parkinsonism-dementia complex (ALS-PDC), also known as Lytico-Bodig, found in natives of the Pacific island of Guam . A toxic aetiology for this occurrence has been suggested. The second is an autosomal dominant syndrome known as the disinhibition, dementia, parkinsonism and amyotrophy complex (DDPAC) . This seems to be associated with cytoskeletal abnormalities in the microtubule associated protein tau.

Other authors have noted that the combination of the clinical features of ALS and PD is not simply confined to a handful of Pacific islanders. Similarity has been noted between the Guamanian disease and a post-encephalitic parkinsonism/ALS disease, with the possibility of a common viral cause being suggested (although work on a viral theory for sporadic ALS has so far proved inconclusive ). In a follow-up report, common pathological features were identified in these ‘cross-over’ cases which would not normally be found in ALS, notably atrophy of basal ganglia structures including the substantia nigra - features thought to be solely in the realm of PD.

The explosive advance in imaging techniques over the last two decades has revolutionised research possibilities in neurology. In particular it has enhanced the ability to delineate complex syndromes previously classified in vivo by purely clinical methods, and so subject to the limitations of purely external human examination. In particular, the advent of positron emission tomography (PET) has enabled the visualisation of the brain at a chemical level. Undoubtedly one of the greatest impacts of PET has been in the study of basal ganglia function in primary parkinsonian syndromes, but it has also permitted further insights in to the extent of parkinsonian pathology in ALS cases. For example, PET studies in Guamanians with Lytico-Bodig revealed a nigrostriatal dopaminergic lesion similar to that found in idiopathic parkinsonism. Later studies provided weight to the idea of a dopaminergic deficit in sporadic ALS and so the possibility of common themes in the pathogenesis of the two conditions.

PET ligands are now permitting neuropathological studies in vivo. [11C]-PK11195 is a ligand for peripheral benzodiazepine receptors and a sensitive marker of microglial cell activation. The ligand has been shown to identify activated microglia in experimental models of neurological disease , and in human stroke. Furthermore it has also been shown to be useful in monitoring disease activity in multiple sclerosis. Ongoing work indicates that this ligand is useful in the sometimes difficult clinical task of delineating patients with multiple system atrophy from other forms of parkinsonism (Gerhard et al., unpublished work). Activated microglia are a prominent feature of degeneration in ALS, and [11C]-PK11195 is now being evaluated by the author in pilot studies using ALS patients.

One of the theories of pathogenesis in ALS, known as ‘excitotoxicity’ relates to an alteration in homeostasis of the predominant cerebral excitatory neurotransmitter, glutamate. Excess stimulation, or reduced local inhibitory circuits, contributes to neuronal death. This forms the basis of the only proven disease-modifying treatment in ALS, the glutamate antagonist riluzole. Work using transcranial magnetic stimulation (TMS) in ALS has allowed the study of excitatory and inhibitory mechanisms within the brains of ALS patients. Results from paired stimulation studies (where a conditioning test stimulus allows excitability to be probed by the second stimulus), has shown an overall reduction in the level of cortical inhibition . This is postulated to relate, at least in part, to reduced action of local GABA-ergic inhibitory circuits. Indeed, the relative retention of such circuits is suggested to underlie the prolonged survival of a Scandinavian familial group of ALS patients, with the D90A mutation of the superoxide dismutase (SOD1) gene, when compared to other ALS patients.

The PET ligand [11C]-flumazenil binds to GABAA receptors, and represents a potential marker of cerebral local inhibitory circuits. Recently published work using this ligand has revealed that pathology is not confined simply to motor areas of the brain in ALS. Research proposed by our group will combine PET studies using this ligand with neurophysiological studies using TMS, to characterise further the role of excitation/inhibition in the pathogenesis of ALS. Such excitotoxic mechanisms are not necessarily confined to ALS, and it is thought they may operate in a variety of other neurodegenerative conditions including PD. Indeed clinical trials of the glutamate antagonist riluzole are underway or planned for patients with other neurodegenerative conditions, including patients with idiopathic PD and multiple system atrophy.

The results of our proposed research using ALS patients may therefore have direct relevance to several other neurodegenerative conditions, including PD. Excitotoxic mechanisms may represent one of the major common pathways in neurodegeneration that previous studies have suggested exist, and so warrants further investigation. The finding of common themes in the pathogenesis of ALS, PD and other neurodegenerative conditions offers improved hope of future treatments. There is an urgent need for these particularly in ALS, where survival time from diagnosis is so short.