Mitochondrial Genetic Mutations

Mitochondrial genetic mutations are changes in the sequence of the mitochondrial genome, ranging from single base substitutions to large deletions and rearrangements. Because mtDNA encodes core components of the energy-producing respiratory chain, such mutations can impair oxidative phosphorylation and, when present above a threshold, cause disease affecting energy-hungry tissues.

Definition

Mitochondrial genetic mutations are inherited or acquired sequence alterations of mitochondrial DNA, including point mutations and large-scale deletions, that can disrupt the genome's coding or regulatory functions and thereby impair mitochondrial energy metabolism.

Scope

This topic surveys the kinds of variants found in mtDNA, point mutations in protein-coding, tRNA, and rRNA genes, and single or multiple large-scale deletions, together with whether they are inherited or arise somatically. It explains how mutation type, location, and heteroplasmy level shape consequences. Detailed clinical syndromes are covered under clinical-genetics entries; this topic stays at the level of mutation biology.

Core questions

• What types of mutations occur in mitochondrial DNA?
• How do point mutations differ in effect from large-scale deletions?
• Why are tRNA-gene mutations particularly disruptive?
• How do inherited mtDNA mutations differ from somatic ones?
• How do mutation type, location, and heteroplasmy together determine the consequence?

Key concepts

• Point mutations in protein-coding genes
• tRNA and rRNA gene mutations affecting mitochondrial translation
• Single and multiple large-scale deletions
• Inherited (germline) versus somatic (acquired) mutations
• Dependence of effect on heteroplasmy and threshold
• Higher mutation rate of mtDNA than nuclear DNA
• Accumulation of somatic mtDNA mutations with age

Mechanisms

Mutations affect mtDNA in two broad ways. Point mutations may fall in one of the 13 protein-coding genes, altering a respiratory-chain subunit, or in a tRNA or rRNA gene, where they impair the organelle's own protein-synthesis machinery and so can affect many proteins at once; the 1988 identification of a point mutation associated with Leber hereditary optic neuropathy (Wallace and colleagues) was the first such disease-causing mtDNA point mutation described. Large-scale deletions remove a contiguous block of genes; Holt and colleagues first reported deletions of muscle mtDNA in mitochondrial myopathy in 1988. Whether a mutation produces disease depends on its type and position and, critically, on the heteroplasmy level, since a defect usually emerges only above a threshold. Some mutations are inherited maternally through the germline, while others are somatic, arising in tissues during life; mtDNA mutates more readily than nuclear DNA, and somatic mtDNA mutations accumulate with age.

Clinical relevance

The spectrum of mtDNA mutations underlies a recognized group of disorders affecting muscle, brain, eye, heart, and other tissues, and the interplay of mutation type with heteroplasmy explains much of their variability. This entry describes mutation biology for educational orientation and does not provide diagnostic criteria, prognosis, or treatment recommendations for any individual.

Epidemiology

Reviews estimate that pathogenic mtDNA mutations, together with nuclear-gene defects affecting mitochondria, make mitochondrial disorders one of the more common groups of inherited metabolic disease, on the order of about 1 in 5,000 people, though estimates vary with population and ascertainment method.

History

The link between mtDNA mutations and human disease was established in 1988 by two landmark reports: Holt and colleagues described large-scale deletions of muscle mtDNA in patients with mitochondrial myopathy, and Wallace and colleagues identified a point mutation associated with Leber hereditary optic neuropathy. These findings opened the field of mitochondrial medicine, and the catalogue of pathogenic point mutations and deletions has grown steadily since.

Key figures

• Douglas C. Wallace
• Ian J. Holt
• Anita E. Harding
• Salvatore DiMauro

Seminal works

• holt-1988
• wallace-1988-lhon

Frequently asked questions

What kinds of mutations affect mitochondrial DNA?

They range from single base point mutations, which can hit protein-coding, tRNA, or rRNA genes, to large-scale deletions that remove a block of genes; mutations may be inherited from the mother or arise somatically during life.

Why can the same mutation cause different effects in different people?

Because mtDNA is multicopy, the consequence depends not only on the mutation itself but on the proportion of molecules that carry it (heteroplasmy) and on which tissues exceed the threshold for dysfunction.