Identification of disease causing mutations in early-onset neuropathies by exome sequencing

Charcot-Marie-Tooth (CMT) neuropathy is one of the most common forms of inherited peripheral neuropathies with the prevalence of one in 2500 individuals. CMT is phenotypically and genetically a very heterogeneous disease. It can be inherited as an autosomal recessive, dominant or X-linked trait. CMT is characterized by distal muscle weakness, atrophy and deformity of the feet as well as clumsiness of gait. The onset of CMT varies and also the symptoms of the disease can vary even among the members of a single family. So far more than 40 genes have been identified for CMT and the list is estimated to grow by 30-50 genes. Whole exome sequencing (WES) is a next generation sequencing technique, which targets the protein coding area of the genome. Through WES analysis it is possible to search for disease causing mutations with all kinds of inheritance patterns. Patients suffering from CMT are good candidates for WES analysis because of the genetic heterogeneity of their disease. WES can be used for diagnosing Mendelian disorders with atypical symptoms as well as diseases, which are difficult to confirm using clinical criteria alone and which require costly evaluation, e.g. CMT. In this master study new disease causing mutations for early-onset neuropathies are identified by whole exome sequencing. The aims of this study include using WES for the molecular diagnosis of four patients suffering from early-onset axonal neuropathies, the functional analysis of possible causative variants and improving and developing the process of analyzing variants from whole exome sequencing data, especially the analyzing steps of insertion and deletion variants. Finding causative variants among the insertion and deletion variants has previously been often left out from the WES analysis because of the lack of systematic analysis technique. As a result of the WES data analysis a new candidate disease gene, tripartite motif containing 2 (TRIM2) was identified. A missense mutation c.761T>A (p.E254V) and a deletion c.1779delA (p.K594Rfs7X) were found in patient 2, who suffers from severe CMT type 2. The carrier frequency was analysed to see whether the variants are present in the general population or not. The functional analysis of TRIM2 was started by preparing constructs carrying the missense mutation and the deletion and by setting up conditions for western blotting