Genetic Dissection of Neuromuscular Disorders

Peripheral neuropathy is common, affecting approximately 2.5% of the UK population. When inherited in a family the condition is usually called Charcot-Marie-Tooth disease (CMT) and is one of the commonest inherited neurological disorders affecting 1 in 2500 people.

CMT usually starts in childhood or teenage years with progressive distal limb weakness, muscle wasting and glove and stocking sensory loss. Walking problems and foot deformities are common requiring ankle or leg supports, sticks and later wheelchair use. Many patients develop complications such as pain, degeneration of the joints, scoliosis and limb ulcers. There is no effective treatment available. CMT can essentially be divided into type 1 (CMT1) that affects the nerve myelin sheath or type 2 (CMT2) that affects the nerve axon. The motor form of CMT is called distal hereditary motor neuropathy (HMN) and the sensory form hereditary sensory and autonomic neuropathy (HSAN). There a number of genes that when defective, can cause CMT. The majority have been identified in CMT1, few genes in only a small proportion of cases have been found in the other types of CMT. Around 40% of CMT patients remain genetically unknown.

Next generation sequencing technology has transformed our ability to identify disease genes. Mutations in the protein encoding exons of genes account for most of human genetic diseases. We can now use revolutionary enrichment methods to select all the exons to sequence in an individual (exome) using this technology. In our laboratory we have established an effective exome sequencing pipeline which is perfectly suited to CMT gene identification, in small families or groups of patients with very similar phenotypes.

We propose an unprecedented gene discovery effort to identify and characterise a large proportion of the unknown CMT genes. We will focus on exome sequencing 100 CMT families, divided between CMT1, CMT2, distal HMN and HSAN. We expect to identify a number of disease genes, prove these defects and examine our entire series of CMT patients. Many genetic defects will require studies on other patient material such as fresh blood to prove the mutation mechanism. To further analyse the function of the CMT genes identified we have established a number of fruitful collaborations. We expect genes to cluster into pathways that are important for peripheral nerve homeostasis and when defective lead to nerve degeneration. Through the identification of these processes we hope to reveal treatment targets that will be promising candidates for therapeutic drug trials.

Peripheral neuropathy is common, affecting approximately 2.5% of the UK population. Inherited neuropathy is usually called Charcot-Marie-Tooth disease (CMT) and is one of the commonest inherited neurological disorders affecting 1 in 2500 people.

CMT usually starts in childhood or teenage years with progressive distal limb weakness, muscle wasting and glove and stocking sensory loss. Walking problems and foot deformities are common requiring orthoses, sticks and later wheelchair use. CMT is genetically heterogeneous with over 30 genes identified that have revealing important biological insights into the pathophysiology of the peripheral nervous system. The majority of genes have been identified in the demyelinating form of CMT (type 1), few genes in only a small proportion of cases have been found in the other types of CMT. After excluding the known genes, around 40% of CMT patients remain genetically undefined.

Here we propose an unprecedented gene discovery effort to identify and characterise a large proportion of the unknown CMT genes. In the past we have been limited by the sequencing technology and the size of families, however, we can now perform whole exome sequencing of all protein encoding regions (exome) and flanking intron boundaries from enriched patient DNA samples. In our laboratory we have established next generation sequencing developing an effective exome sequencing pipeline that has so far, identified one novel disease gene. This method is perfectly suited to CMT gene identification, in small families or groups of patients with very similar phenotypes.

Whole exome sequencing will be performed in 100 CMT families. Half the families will be dominant and half recessive, divided between CMT1, CMT2, distal HMN and HSAN. In dominant families two individuals will be sequenced and in the recessives one. After thorough filtering and bioinformatic analysis we expect to identify a number of disease genes, prove these defects in families and then examine our extensive CMT database of cases to assess frequency, mutation spectrum and phenotype correlations. Many genetic defects will require protein and/or RNA studies from patient material to prove the mutation mechanism. True functional analysis is beyond the scope of this proposal but we have established a number of fruitful collaborations that have allowed the successful investigation of CMT genes. We expect these genes to cluster into pathways that are important for peripheral nerve homeostasis and when defective lead to nerve degeneration. Through the identification of these processes the hope must be that treatment targets will become apparent.