A novel frameshift mutation of the mtDNA COIII gene leads to impaired assembly of cytochrome c oxidase in a patient affected by Leigh-like syndrome

We report on a novel frameshift mutation in the mtDNA gene encoding cytochr ome c oxidase (COX) subunit Ill, The proband is an Ii-year-old girl with a negative family history and an apparently healthy younger brother, Since 4 years of age, she has developed a progressive spastic paraparesis associate d with ophthalmoparesis and moderate mental retardation. The presence of se vere lactic acidosis and Leigh-like lesions of putamina prompted us to perf orm muscle and skin biopsies, In both, a profound, isolated defect of COX w as found by histochemical and biochemical assays. Sequence analysis of musc le mtDNA resulted in the identification of a virtually homoplasmic frameshi ft mutation in the COIII gene, due to the insertion of an extra C at nucleo tide position 9537 of mtDNA, Although the 9537C(ins) does not impair transc ription of COIII, no full-length COX III protein was detected in mtDNA tran slation assays in vivo. Western blot analysis of two-dimensional blue-nativ e electrophoresis showed a reduction of specific crossreacting material and the accumulation of early-assembly intermediates of COX, whereas the fully assembled complex was absent, One of these intermediates had an electropho retic mobility different from those seen in controls, suggesting the presen ce of a qualitative abnormality of COX assembly. Immunostaining with specif ic antibodies failed to detect the presence of several smaller subunits in the complex lacking COX III, in spite of the demonstration that these subun its were present in the crude mitochondrial fraction of patient's cultured fibroblasts. Taken together, the data indicate a role for COX III in the in corporation and maintenance of smaller COX subunits within the complex.