We report the case of a patient with severe exercise intolerance caused by a 2-bp deletion in the ND2 gene of mtDNA. A striking finding was that the mutation occurred on a paternal mtDNA background. Because the patient had an isolated myopathy due to a mutation found only in skeletal muscle, and because family members were unaffected and did not carry the mutation in blood or muscle, we conclude that the 2-bp deletion arose spontaneously in early embryogenesis or in the paternal germ line. However, we cannot rule out the possibility that the father harbored this mutation at a low level in other tissues. The origin of the mutation could be similar to that of sporadic, single, large-scale deletions, which so far have been thought to arise spontaneously in maternal mtDNA, either in the germ line or in early embryogenesis.16 Mutations of mtDNA cause symptoms only when high levels of mutant mtDNA are present: typically, 50 to 60 percent for single, large-scale deletions and 80 to 90 percent for point mutations.17,18 For a high percentage of mutant mtDNA to be present, a replicative advantage for the mutated mitochondria is probably required. However, not only single, large-scale deletions, but also mtDNA point mutations, can proliferate with time and outnumber wild-type mtDNA.19 It has been suggested that the replicative advantage of the large-scale deletions is due to a faster completion of replication of the smaller mtDNA molecules, but the experimental evidence for this hypothesis is controversial.2

Complex I of the respiratory chain consists of 41 subunits, 7 of which are encoded by mtDNA. The stop codon introduced by the 2-bp deletion affects translation of the C-terminal portion of the gene product. It therefore inactivates the ND2 subunit and the catalytic function of complex I. In support of this concept, complex I activity was severely impaired in muscle from the patient we describe.

The patient also harbored a novel variant (1303G→A) in the 12S rRNA gene. This variant could theoretically influence the patient's phenotype. However, the variant is situated in a part of the 12S rRNA gene that is not conserved in evolution. Mouse and rat 12S rRNA genes have an adenine, and rabbit and elephant genes have a thymine in this position (Table 1), which suggests that 1303G→A is a harmless variant. Furthermore, the homoplasmic occurrence of 1303G→A in the healthy father and uncle also indicates the nonpathogenic nature of this variant.

Until now, pathogenic mtDNA has been assumed to be maternally inherited or to have arisen spontaneously on a maternal mtDNA background. However, paternal mtDNA inheritance may go unrecognized in cases with sporadic, single, large-scale deletions, because mitochondrial haplotypes are rarely investigated in diagnostic analyses. The same may be true of the rare cases of sporadic point mutations in mtDNA that give rise to a clinical picture similar to that in the present patient. Mutations in mtDNA that cannot be detected in the mother, are found in skeletal muscle only, and have no effects other than exercise intolerance have been reported for the respiratory-chain subunits ND4,20 cytochrome b, 21 cytochrome c oxidase subunit I,22 and cytochrome c oxidase subunit III.23 Among these, mutations of the cytochrome b gene have been reported in more than one patient.21 However, sporadic cytochrome b mutations may also affect tissues other than muscle.24

The present case could be the result of the survival of one or a few sperm mitochondria that probably would have been diluted out and never have been recognized had the pathogenic mutation not conferred a selective proliferative advantage on the mitochondria. There is now strong evidence that highly effective processes exist for eliminating healthy sperm mitochondria in early mammalian embryogenesis.5-7 When a sperm, including the midpiece, which is rich in mitochondria, is injected directly into an oocyte, as in assisted reproductive techniques to treat human infertility, paternal mtDNA can be detected in the four-to-eight-cell stage of some abnormal embryos.25 However, paternal mtDNA has not been detected in infants born after intracytoplasmic sperm injection.26

The underlying mechanism responsible for the elimination of sperm mtDNA in normal embryos is not well understood. We speculate that the process in some cases may be defective, allowing sperm mitochondria to survive and giving those with a selective advantage the possibility of prevailing in certain tissues. The present findings suggest that investigation of paternal mtDNA inheritance may be warranted in cases with sporadic mitochondrial mutations.