Spontaneous Kearns-Sayre/chronic external ophthalmoplegia plus syndrome associated with a mitochondrial DNA deletion: a slip-replication model and metabolic therapy.

Abstract

The muscle mitochondria of a patient with Kearns-Sayre/chronic external ophthalmoplegia plus syndrome were found to be completely deficient in respiratory complex I activity and partially deficient in complex IV and V activities. Treatment of the patient with coenzyme Q10 and succinate resulted in clinical improvement of respiratory function, consistent with the respiratory deficiencies. Restriction enzyme analysis of the muscle mtDNA revealed a 4.9-kilobase deletion in 50% of the mtDNA molecules. Polymerase chain reaction analysis demonstrated that the deletion was present in the patient's muscle but not in her lymphocytes or platelets. Furthermore, the deletion was not present in the muscle or platelets of two sisters. Hence, the mutation probably occurred in the patient's somatic cells. Direct sequencing of polymerase chain reaction-amplified DNA revealed a 4977-base-pair deletion removing four genes for subunits ofcomplex I, one gene for complex IV, two genes for complex V, and five genes for tRNAs, which paralleled the respiratory enzymes affected in the disease. A 13-base-pair direct repeat was observed upstream from both breakpoints. Relative to the direction of heavy-strand replication, the first repeat was retained and the second repeat was deleted, suggesting a slip-replication mechanism. Sequence analysis of the human mtDNA revealed many direct repeats of 10 base pairs or greater, indicating that this mechanism could account for other reported deletions. We postulate that the prevalence of direct repeats in the mtDNA is a consequence of the guanine-cytosine bias of the heavy and light strands.

Mitochondrial involvement in neurodegenerative diseases

“…Current hypotheses are based on properties of previously observed deleted mtDNA molecules. The fact that deletions have often been observed in the major arc between the two replication origins of the mitochondrial genome lead to the assumption that slipped-strand replication is one of the main sources of mtDNA deletion generation (68). Replication can, however, also play a role in deletion formation through inducing double-strand breaks (DSBs).…”

Photoaging of human skin

“…These studies found several large-scale deletions of mtDNA in photoaged skin. To explain the generation of these largescale deletions in mtDNA, a modified slip-replication mechanism and a central role of ROS have been postulated (39)(40)(41). Recent work has been able to provide a possible link for the involvement of ROS in the generation of the most frequent mtDNA deletion, the so-called common deletion (42).…”

The 4977bp deletion of mitochondrial DNA in human skeletal muscle, heart and different areas of the brain: A useful biomarker or more?

“…These modified bases are especially prone to mispairing of repetitive elements and are correlated with deletions (Lezza et al, 1999). This observation has led to the proposal that deletions are induced via mispairing and slippage during replication (Shoffner et al, 1989). Nowadays it is quite clear that large-scale deletions may be attributed to polymerase slippage, homologous recombination, double strand breaks, inefficient repair mechanisms, or mutations in the helicase twinkle (Zeviani et al, 2003, Phadnis et al, 2005Srivastava and Moraes, 2005).…”