Chromosome 17<unk>  is caused by genetic mutations that cause defects in neuronal proteins. Nerve signals are conducted by an axon with a myelin sheath wrapped around it. Most mutations in CMT affect the myelin sheath, but some affect the axon. Classification CMT is a heterogeneous<unk>  and the mutations linked to it may occur in a number of different genes. Based on the affected gene, CMT is categorized into several types and subtypes. Chromosome 17 The most common cause of CMT (70–80% of the cases) is the duplication of a large region on the short arm of chromosome 17 that includes the gene PMP22. Some mutations affect the gene MFN2, on chromosome 1, which codes for a mitochondrial protein. Mutated MFN2 causes the mitochondria to form large clusters, or clots, which are unable to travel down the axon towards the synapses. This prevents the synapses from functioning. X-linked CMT and Schwann cells CMT can also be produced by X-linked mutations, and is named X-linked CMT (CMTX). In CMTX, mutated connexons create non-functional gap junctions that interrupt molecular exchange and signal transport. The mutation can appear in GJB1 coding for connexin 32, a gap junction protein expressed in Schwann cells. Because this protein is also present in oligodendrocytes, demyelination can appear in the CNS. Schwann cells create the myelin sheath, by wrapping its plasma membrane around the axon. Neurons, Schwann cells, and fibroblasts work together to create a functional nerve. Schwann cells and neurons exchange molecular signals by gap junctions that regulate survival and differentiation. Demyelinating Schwann cells causes abnormal axon structure and function. They may cause axon degeneration, or they may simply cause axons to malfunction. The myelin sheath allows nerve cells to conduct signals faster. When the myelin sheath is damaged, nerve signals are slower, and this can be measured by a common neurological test, electromyography. When the axon is damaged, though, this results in a reduced compound muscle action potential.
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