'A breakdown in fast axonal transport would selectively kill neurons because neurons are especially dependent on the transport system,' Brady said. Kennedy disease is also known as spinal and bulbar muscular atrophy, or SBMA. Like the better-known ALS and Huntington's, it is a rare but devastating disease, affecting one in 40,000 people, usually between the ages of 30 and 50. Huntington's strikes about four times as many. Neurodegenerative diseases like SBMA are caused by the lengthening of part of a gene that encodes repetitions of the amino acid glutamine in the protein. Although different genes are affected, all of the polyglutamine-expansion or "polyQ" diseases are characterized by symptoms that begin in middle age and by the loss of certain types of neurons through a pattern in which the neuron's terminals die before the cell body.
PolyQ genes are expressed in many types of cells, but only neurons are affected. Earlier studies had linked specific neurodegenerative diseases to mutations in proteins involved in intracellular transport. This led researchers to wonder if the deranged polyQ proteins inhibit fast axonal transport in several diseases, including SBMA, in which a mutation in the receptor for testosterone leads to the loss of motor neurons. In the new study, Brady and his co-workers were able to show that polyQ-AR, the mutated protein in SBMA, caused inhibition of fast axon transport by activating an enzyme called JNK that can inhibit these transport proteins. Brady said this is the first proposed mechanism for polyQ diseases that explains why only nerve cells die and why the terminals die before the cell body. The link to the activation of the JNK enzyme suggests a new therapeutic target that might limit, delay or perhaps prevent progressive neurodegeneration, the researchers conclude.