B3881 - Associations between mitochondrial and nuclear DNA variants in human populations - 28/09/2021
Present in nearly all types of human cells, mitochondria generate the majority of our cellular energy and are thus often referred to as the ‘powerhouse of the cell’. In addition, mitochondria play important roles in signalling between cells and cell death (known as apoptosis). Although most of our DNA is within the nucleus (the ‘nuclear genome’ or nDNA), mitochondria contain their own DNA (the ‘mitochondrial genome’ or mtDNA), and human health is dependent upon the coordination of the products of these two genomes.
Genetic variants within mtDNA can cause disease (1) and are also linked to a growing number of age-related complex diseases (2), particularly neurodegenerative diseases including Parkinson’s disease, Alzheimer’s disease, schizophrenia and multiple sclerosis (3). In addition, there are examples where the progression or severity of mtDNA disease is modulated by nDNA variants (e.g. LHON (4, 5)). Conversely, there is evidence in complex diseases where nuclear susceptibility factors are implicated, that common, inherited, mtDNA variation can influence traits such the age of onset (e.g. Alzheimer’s disease (6) and cardiomyopathy (7)).
More recently, the development of mitochondrial replacement therapy, a technique designed to avoid transmission of defective mitochondria from parents to offspring, sparked discussions on mitochondrial nuclear incompatibilities that would require matching donor and recipient mitochondrial and nuclear backgrounds, and more generally on whether the genetic makeup of healthy individuals reflects such incompatibilities (8).
This raises the question whether specific variant combinations of nuclear and mitochondrial alleles are depleted or enriched in the general population. This can be assessed by comparing their frequencies with the frequencies expected from the frequencies of the corresponding nuclear and the mitochondrial alleles assuming independent segregation. Such associations between variants can reflect mixing of different populations or selection against or in favour of particular variant combinations. Associations between nuclear and mitochondrial variants have been repeatedly reported (9, 10). However, it is unclear to what extent the observations reflect population stratification. This is of particular interest because mitochondrial genetic variants have been extensively used to track migration of populations and ancestry.
The aim of this study is to use the ALSPAC cohort genotyping data to: 1) identify combinations of mitochondrial and nuclear variants that are over or underrepresented in a well-characterised population, 2) ascertain whether under or over representation can be explained by heterogeneity within the population and 3) assess the effects of factors such as age or sex. The results will also provide a reference for studying the role of such combinations in human disease.