Written by Dr. Colleen A. Stoyas Edited by Dr. Monica Banez
Duke University researchers have found that altered cellular metabolism and mitochondrial dysfunction play a central role in spinocerebellar ataxia type 7 (SCA7), a result that has therapeutic implications for this disease.
Spinocerebellar ataxia type 7 (SCA7) is a dominantly-inherited ataxia characterized by retinal degeneration and cerebellar atrophy. As retinal degeneration advances, patients experience progressive central vision loss. Atrophy (i.e., cell loss) in the cerebellum causes a progressive loss of balance, as the cerebellum is the region of the brain that controls coordinated movement and motor learning. SCA7 patients also experience difficulty speaking and swallowing in later stages of the disease. Symptoms can manifest at any age, though the disease is particularly aggressive when found in infants and children. SCA7 is caused by an expansion mutation in the Ataxin-7 (ATXN7) gene, which produces a protein containing extra repeats of the amino acid glutamine. These additional glutamines make the protein fold in an incorrect shape. Much like an umbrella turned inside-out, this protein, once it loses its shape, does not work in the way it’s meant to. Dr. Albert La Spada has previously shown that the ataxin-7 protein is necessary for the expression of genes that are central to the normal function of the eye – particularly, the retina. Now, his group has provided evidence that abnormal cellular metabolism underlies the brain changes observed in SCA7.
Mice whose brains carry the SCA7 mutation model the juvenile forms of this disease. Using this mouse model, the La Spada group observed changes in the network and physical size of the brain’s mitochondria. Mitochondria are the cell’s “power plants,” and are responsible for the chemical reactions (known as cellular metabolism) that generate the energy our cells need to function. Cellular metabolism is assessed by measuring metabolites, which are the products of these chemical reactions. The La Spada group’s researchers identified dysfunction in the mitochondria in SCA7 due to an underlying decrease in one specific metabolite: NAD+.
Short for nicotinamide adenine dinucleotide, NAD+ is necessary for proper mitochondrial function. A general reduction of NAD+ occurs as humans age, as well as in a host of other neurodegenerative disorders (many of which exhibit mitochondrial dysfunction). This recent recent by Dr. La Spada and his team has shown that NAD+ is also reduced in mitochondria in SCA7.