In patients with progressive external ophthalmoplegia (PEO), the muscles that move the eyes gradually deteriorate until the only way patients can follow an object is by turning their heads. Last year, Christine Van Broeckhoven and colleagues at the University of Antwerp, Belgium, linked PEO in a Belgian family to a point mutation in the gene for polymerase gamma, the DNA polymerase responsible for replicating the 16.5-kilobase mitochondrial chromosome.1 Now, William Copeland's lab at the National Institute of Environmental Health Sciences in Research Triangle Park, NC, has determined what the point mutation does: It decreases the fidelity of DNA replication, an effect seen previously only in vitro.2

Polymerase gamma becomes a mutator polymerase, accelerating the accumulation of point mutations, frameshifts, and deletions in mitochondrial DNA. The steady accumulation of mutations is what accounts for the late onset and progressive nature of PEO. As a result, researchers now know...

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