MITOCHONDRIAL DISEASES (Posters): P.191Single muscle fiber analysis of extraocular and skeletal muscles in a CPEO patient harboring a pathogenic point mutation in the MT-TN gene

Abstract

Chronic progressive external ophthalmoplegia (CPEO) is a frequent feature of mitochondrial disorders and usually associated with mitochondrial DNA (mtDNA) mutations. Skeletal muscle (SKM) tissue is most frequently used for biochemical analyses and mitochondrial genome testing. However, extraocular muscle (EOM) is the clinically most affected tissue but usually not available for routine work-up. Consequently, systematic data on EOM is limited and the reason for preferential clinical affection remains unclear. We addressed this unsolved question by histochemical and genetic analyses of EOM and SKM single muscle fibers in a patient with isolated CPEO caused by a heteroplasmic point mutation in the MT-TN gene. The histochemical analysis showed higher absolute numbers of cytochrome c oxidase (COX)-deficient EOM fibers compared to COX-deficient SKM fibers. However, genetic analyses by restriction fragment length polymorphism revealed no significant difference in the mutation loads between COX-negative single muscle fibers in EOM compared to SKM. Quantitative single fiber real-time PCR revealed higher mtDNA copy numbers in single muscle fibers of EOM compared to SKM. COX-negative single muscle fibers of EOM and SKM showed significantly higher mtDNA copy numbers compared to COX-positive fibers suggestive of a compensatory mtDNA proliferation. We show that high loads of the MT-TN mutation correlate with a biochemical loss of COX activity in single muscle fibers of EOM and SKM at a similar threshold. The higher absolute numbers of COX-negative fibers in EOM compared to SKM might be caused by facilitated segregation of the mutation into the EOM providing thereby a possible explanation of the preferential ocular manifestation in CPEO.