To investigate the pathogenesis of chronic progressive external ophthalmoplegia, a study in Investigative Ophthalmology & Visual Science looked at the susceptibility of the fiber types found in 5 extraocular muscles (EOMs) to pathogenic mitochondrial DNA (mtDNA) deletions. 

Chronic progressive external ophthalmoplegia (CPEO) typically leads to bilateral ptosis and limited eye movements. It’s caused by the slow, progressive paralysis of EOMs, which are structurally, functionally, and biochemically distinct from other skeletal muscles in the body. In particular, EOMs are loosely packed with unusually high amounts of connective tissue, generally have a higher mitochondrial content, are more vascularized, and have higher calcium-handling capacities. In addition to having fibers that express the typical slow I, fast IIA, and fast IIB myosin heavy chain (MyHC) isoforms, adult EOMs also have fibers that express embryonic MyHC (MyHC-emb), neonatal (MyHC-neo), slow-tonic MyHC, and EOM-specific isoforms (MyHC-eoms). Further, individual EOM fibers can co-express several MyHC isoforms along their length. While these unique properties allow EOMs to produce a wide range of sophisticated eye movements at varying speeds, it makes them vulnerable to neuromuscular diseases, including mitochondrial myopathies like CPEO.

In a previous study of patients with CPEO, EOMs were found to have a lower threshold for cytochrome C oxidase (COX) deficiency induced by mtDNA deletions and, consequently, an up to threefold-higher proportion of COX-deficient (COX) fibers compared with quadriceps muscle. EOMs also have been found to have a higher mtDNA deletion load and proportion of COX fibers compared with limb muscles during normal human aging. These studies demonstrated that EOMs have a higher susceptibility of accumulating mtDNA deletions and that these unique muscles suffer more from the associated respiratory chain defects.

However, whether this increased vulnerability to mtDNA deletions was uniform across all types of EOM fibers was not clear. To research the effect on different fiber types, the investigators of the current study expressed a dominant-negative mutant of the mitochondrial replicative helicase, K320E-Twinkle, using mice expressing Cre recombinase under the control of the myosin light chain 1f (Mlc1f)-promoter, thereby inducing accelerated accumulation of mtDNA deletions in all skeletal muscles. They found that different EOM fiber types do exhibit differential vulnerability to COX deficiency induced by mtDNA deletions, with fiber type IIB being almost exclusively affected and MyHC-emb-expressing fibers nearly completely unaffected.

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To conduct the study, consecutive pairs of EOM sections were analyzed by cytochrome C oxidase (COX)/succinate dehydrogenase (SDH) assay and fiber type-specific immunohistochemistry (type I, IIA, IIB, embryonic, and EOM-specific staining). The mean average of COX− in the recti muscles of mutant mice was 1.04% (± 0.52%) at 12 months and increased with age (7.01 % ± 1.53% at 24 months). A significant proportion of these COX fibers were of the fast-twitch, glycolytic type IIB (> 50% and > 35% total COX fibers at 12 and 24 months, respectively), whereas embryonic myosin heavy chain-expressing fibers were almost completely spared. Additionally, the proportion of COX fibers in the type IIB-rich retractor bulbi muscle was more than 2-fold higher compared with the M. recti at both 12 (2.6% ± 0.78%) and 24 months (20.85% ± 2.69%).

The results demonstrate a selective vulnerability of type IIB fibers to mitochondrial DNA (mtDNA) deletions in EOMs and retractor bulbi muscle. The EOMs of mutant mice also displayed histopathological abnormalities such as altered fiber type composition, increased fibrosis, ragged red fibers, and infiltration of mononucleated non-muscle cells.


RR Oexner, D Pla-Martín, T Paß, et al. Extraocular muscle reveals selective vulnerability of type iib fibers to respiratory chain defects induced by mitochondrial dna alterations. Invest Ophthalmol Vis Sci. 2020;61(12):14. doi: