Objective: To characterize the deletions of mitochondrial DNA (mtDNA) in Chinese patients with Kearns-Sayre syndrome (KSS) and chronic progressive external ophthalmolegia (CPEO) and identify deletion mutations of mtDNA be the etiology of these diseases.

Methods: Patients and preparation of total DNA: Two patients with KSS and two with CPEO and ten with other mitochondrial myopathies or encephalomyopathies were determined by histological and biochemical assays. Total DNA was isolated from 100 mg to 150 mg of frozen muscle obtained by biopsy using the methods described by Zeviani et al. (1988). Preparation of mtDNA probes: MtDNA were isolated and purified according to Palva's method (1985). The mtDNA was linearized by digestion with the restriction endonuclease Pvu II and separated by electrophoresis through low melting agarose gel. The 16.5 kb DNA band was recovered from the gel and labeled with alpha-p32 dCTP by random primer labeling. Southern blot analysis: Portions of five micrograms total DNA obtained from the patients and controls were digested with Pvu II, EcoR I, Hind III And Xba I respectively. The digested DNA was separated by agarose gel electrophoresis and transferred to nitrocellulose membrane and then hybridized with P32 labeled mtDNA as previously described (Zhang, 1991).

Results: A large-scale deletions of mtDNA were identified in two patients with KSS and two patients with CEPO. The deletions ranged in size from 2.4 kb to 5.5 kb. The proportion of mutated mtDNA in each patients ranged from 54.6% to 84.6% of total mtDNA. No detectable deletions were found in mtDNA of ten patients with other mitochondrial myopathies or encepholomyopathies and normal controls.

Conclusions: Deletions of mtDNA were found in all KSS and CEPO patients detected. These results supported The view point of the deletions are important causes of physical defect in KSS and CEPO.