Heteroplasmy

Heteroplasmy is the presence of more than one mitochondrial DNA sequence within a single cell, tissue, or individual, typically a mixture of normal (wild-type) and variant molecules. Because each cell carries many mtDNA copies, a pathogenic variant rarely affects all of them; the proportion that is mutant, the heteroplasmy level, is a central determinant of whether and how mitochondrial dysfunction appears.

Definition

Heteroplasmy is the coexistence of two or more distinct mitochondrial DNA genotypes within a cell, tissue, or individual; the fraction of molecules carrying a given variant is the heteroplasmy level, in contrast to homoplasmy where all copies share one sequence.

Scope

This topic defines heteroplasmy and its counterpart homoplasmy, explains how mutant proportions shift between cells and generations through random partitioning and the genetic bottleneck, and introduces the threshold effect by which a phenotype emerges only above a critical mutant load. It treats heteroplasmy as a mechanistic concept; clinical descriptions of specific syndromes belong to clinical-genetics topics.

Core questions

• What does it mean for a cell to be heteroplasmic rather than homoplasmic?
• How does the proportion of mutant mtDNA change between cells, tissues, and generations?
• What is the threshold effect and why does it matter?
• How does the genetic bottleneck influence heteroplasmy in offspring?
• Why can heteroplasmy levels differ between tissues in the same person?

Key concepts

• Heteroplasmy versus homoplasmy
• Heteroplasmy level (mutant fraction)
• Threshold effect for biochemical and clinical expression
• Random partitioning of mtDNA at cell division (replicative segregation)
• Mitochondrial genetic bottleneck
• Tissue-specific heteroplasmy distribution
• Selection for or against variants in the germline

Mechanisms

Because a cell contains many mtDNA molecules, a new variant arises against a background of wild-type copies, creating heteroplasmy. When the cell divides, mtDNA copies are partitioned more or less at random between daughter cells (replicative segregation), so mutant proportions can drift upward in some lineages and downward in others, producing tissue-to-tissue variation. A biochemical defect, and any resulting phenotype, typically appears only when the mutant fraction exceeds a threshold, often a high proportion for many pathogenic point mutations and deletions, so that remaining wild-type genomes can compensate below that level (Wallace, 1999). Between generations, a developmental bottleneck during oogenesis, in which only a subset of mtDNA molecules effectively populates the next generation, can shift offspring heteroplasmy sharply away from the mother's level (Wai and colleagues, 2008), and germline selection can act against the most damaging variants (Fan and colleagues, 2008). Sensitive sequencing has shown that low-level heteroplasmy is widespread even in healthy tissues (He and colleagues, 2010).

Clinical relevance

Heteroplasmy and the threshold effect help explain why the same mtDNA variant can cause severe disease in one person and remain silent in another, and why severity can differ between organs and change with age. This understanding underlies how variability and recurrence are interpreted in mitochondrial conditions; the entry is educational and does not provide individualized prognostic or treatment guidance.

History

As pathogenic mtDNA mutations were identified in the late 1980s, it became clear that affected individuals often carried mixtures of mutant and normal genomes, and the threshold effect was invoked to explain why phenotype depended on mutant proportion. Work in the 2000s clarified the quantitative basis of the inter-generational bottleneck and demonstrated germline selection, while deep sequencing later revealed that low-level heteroplasmy is a common feature of normal tissues.

Debates

How is the inter-generational bottleneck generated?

Whether the rapid shifts in heteroplasmy between mother and offspring arise mainly from a reduction in mtDNA copy number, from replication of only a subpopulation of genomes, or from physical partitioning has been investigated and debated, with evidence pointing to replication of a subset of molecules.

Key figures

• Douglas C. Wallace
• Eric A. Shoubridge
• Salvatore DiMauro

Seminal works

• wallace-1999
• wai-2008

Frequently asked questions

What is the difference between heteroplasmy and homoplasmy?

Homoplasmy means all of a cell's mitochondrial DNA copies share the same sequence; heteroplasmy means the cell carries a mixture of two or more sequences, usually normal and variant molecules together.

Why does heteroplasmy level matter for disease?

Many pathogenic mtDNA variants cause a biochemical defect only when the mutant fraction passes a threshold; below that level the remaining normal genomes can compensate, which is why higher heteroplasmy tends to associate with more severe effects.