Genes, mechanisms, models and treatments for hereditary myasthenia

Signalling between nerves is achieved largely by the opening and closing of ion channels, in their cell surface, at specialised sites called synapses. We believe an understanding of how synapses malfunction will help in finding the causes of many neurological disorders and in devising new therapies. We study inherited diseases, called congenital myasthenic syndromes, that cause severe disability in children and adults and affect ion channels, called acetylcholine receptors (AChR), located at the connection between nerves and muscles. These inherited mutations cause disease by affecting AChR in different ways. Understanding how the mutations cause disease enables us to give patients correct treatments and genetic counselling, and provides the knowledge base required for exploring new therapies. Several different genes have been shown to cause these disorders but in around 40% of cases the underlying defect is not known. Recently we identified mutations in a new gene, called DOK7. Standard treatments used in congenital myasthenia often make patients with DOK7 mutations worse. It is thought the faulty gene underlies a defect in the process that places the receptor in the correct position opposite the nerve ends. Here, in studies of this process we will learn why the signal from the nerve is not received by the muscle. Knowledge gained will help identify new genes involved in our disease, and help us understand why these patients do not respond to standard treatments. At present many patients with these mutations go undiagnosed or are misdiagnosed. Success in this project should prevent misdiagnosis, and enable us to provide better treatment for our patients.

Congenital myasthenic syndromes (CMS) stem from genetic defects that affect transmission of information from nerves to muscles at the neuromuscular junction (NMJ) and result in fatiguable muscle weakness. They comprise a disabling and sometimes potentially lethal set of disorders with subtly different clinical features that arise from different underlying molecular mechanisms. Understanding the different disease mechanisms enables appropriate treatments to be given to patients, facilitates investigations of more common CNS disorders, and provides prototypic examples of synaptic dysfunction that may be used to evaluate new treatment strategies.
Acetylcholine receptors (AChR) are the ion channels that receive the signal for muscle contraction. Many mutations in CMS patients are in the AChR, but it is now apparent that an equal number will be identified in proteins that are responsible for the localisation and clustering of the AChR and also in maintaining the synaptic structure. We have detected a large number of mutations both in the AChR-clustering protein rapsyn and the newly identified muscle protein, Dok-7. Dok-7 has been identified as an essential component of the AChR clustering pathway.

The objective of this proposal is threefold: i) to investigate the molecular mechanisms of pathogenic mutations that impair AChR clustering, ii) to identify novel CMS-associated genes that are expressed on the postsynaptic side of the neuromuscular junction, and iii) to study current and novel therapies for congenital myasthenic syndromes in cellular and mouse models of disease.

In studies of rapsyn and Dok-7 mutations we have developed and are acquiring an exceptional set of experimental tools, including EGFP-tagged functional AChR, a series of pathogenic mutants, ?knock out? muscle cell lines and transgenic disease models, including a model of DOK7 CMS. We shall use these in a combination of microscopic, biochemical and genetic techniques to investigate the defective signalling from MuSK to the AChR that results in reduced and less stable rapsyn-associated clustering. We will use a multifaceted approach, including linkage analysis, the study of fetal akinesis, and knowledge of the MuSK signalling pathway to identify new candidate CMS genes. Therapies for CMS will be given to our transgenic mouse disease models and their response will be analysed in vivo by electromyography and ex vivo by electrophysiology, microscopy, and biochemical analysis