We present the case of a 19-year-old female with a history of Down syndrome (DS) who was referred to our neuro-ophthalmology clinic for evaluation of Leber's hereditary optic neuropathy (LHON). The patient's family history was significant for a known G11778A mutation in a maternal relative, consistent with LHON. The patient was also positive for the G11778A mutation; however, the genotype demonstrated low penetrance in the pedigree, with only 1 out of 10 adult male offspring showing signs or symptoms of the disease. Mitochondrial mutations implicated in LHON have been shown to impair complex I of the electron transport chain and thereby reducing the effective generation of adenosine triphosphate and increasing the production of toxic reactive oxygen species. Although the partial or complete triplicate of chromosome 21 constitutes the etiology of DS, some of the pleiotropic phenotypes of the syndrome have been attributed to oxidative stress and mitochondrial dysfunction. Given the low penetrance of the mutation and the patient's sex, this case illustrates the possibility that the mitochondrial mutation demonstrated increased penetrance due to pre-existing mitochondrial dysfunction related to DS.
