A recent publication in The New England Journal of Medicine has reported observations of reduced heteroplasmy in T cells – a finding consistent with purifying selection within this lineage.
Heteroplasmy of mtDNA
Many complex mitochondrial diseases arise due to defects in the mitochondrial genome (mtDNA). These diseases often manifest with significant clinical heterogeneity, which can be attributed to the presence of both nonmutant and mutant mtDNA. This phenomenon is known as heteroplasmy. Heteroplasmy differs dramatically across family members, tissues and time. The molecular mechanism that underpins mtDNA segregation is still poorly understood but is suggested to be due to the presence of tissue-specific genetic influences.
The most common heteroplasmic pathogenic mtDNA mutation is the A3243G mutation in the MT-TL1 gene. This mutation is associated with a mitochondrial cytopathy – mitochondrial encephalomyopathy with lactic acidosis and stroke-like episodes (MELAS). The phenotypic spectrum of A3243G mutations is broad and can be associated with different symptoms, including diabetes, deafness and epilepsy. The severity of symptoms can also differ. Due to the wide variation of A3243G heteroplasmy among siblings and across tissue, clinical management and genetic counselling remain challenging.
Investigating heteroplasmy and segregation of pathogenic mtDNA mutations has been difficult using bulk analysis. This is due to tissues consisting of different cell types with distinct developmental origins and proportions. The development of single-cell genomic technologies holds promise for the elucidation of mechanisms regulating heteroplasmy. However, current studies are limited to a few dozen cells, primarily germline.
Single-cell ATAC sequencing
In this study, researchers used mtDNA single-cell ATAC sequencing to determine mtDNA heteroplasmy and also cell types from peripheral-blood mononuclear cells (PBMCs). PBMCs were obtained from three unrelated patients with MELAS.
The researchers observed a broad range of heteroplasmy across all cell types. Levels of heteroplasmy were lower within lymphocytes compared to cells within myeloid lineages. In particular, T cells were found to show consistently low levels of heteroplasmy. The team note that purifying selection against the pathogenic A3243G mutation is the most parsimonious explanation for this observation. They highlight that the A3243G mutation is known to cause deficiency in the activity of complex I of the electron transport train, which impairs T cell development in mice. Therefore, they propose that there may be some T-cell-specific process that selects against high heteroplasmy.
Exploring the dynamics of heteroplasmy within blood lineages has important clinical implications. For example, the researchers suggest that the analysis of heteroplasmy in defined blood lineages could potentially have greater diagnostic and prognostic value than analysis of the whole blood. They also propose that gaining further insight into reduced T cell heteroplasmy may inspire new therapeutic approaches.
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