Advances in the genetic mechanisms of mitochondrial disease.

During the past 16 years since the delineation of the human mitochondrial genome, substantial advances have been made in identifying pathogenic mutations causing mitochondrial disorders. However, just as we have come to accept the unexpected in the nontraditional aspects of Mendelian inheritance with the discovery of trinucleotide expansions, imprinting and uniparental disomy, unusual characteristics of mitochondrial inheritance also have been found that defy existing laws. For example, we now know that the nuclear genetic background of an individual might influence the expression and tissue specificity of mitochondrial mutations. Pathogenic mitochondrial DNA mutations contribute to the generation of new mutations by compromising mitochondrial function and increasing free radical production. Evidence for recombination raises new questions about repair mechanisms of mitochondrial DNA. It appears that the more we learn about the bases of mitochondrial disease, the more complex diagnosis, treatment, and genetic counseling become.