High-throughput single-cell analysis reveals progressive mitochondrial DNA mosaicism throughout life

Diagnosis of Primary Mitochondrial Diseases

Primary mitochondrial diseases are clinically heterogeneous and present diagnostic challenges due to the highly variable genotype-phenotype correlation. Clinical symptoms can range from non-specific fatigue, exercise intolerance, and weakness to syndromic phenotypes. Though multiple testing modalities exist to identify mitochondrial diseases, most of these tests are nonspecific, or results are associated with other diseases. Molecular testing can provide an efficient path toward diagnosis, as molecular detection techniques have improved and become less costly. A "genetics first" approach can reduce diagnostic delay and improve management, where the diagnostic pathway can be an invasive or noninvasive combination of targeted or comprehensive molecular testing. Prior to ordering these tests, clinicians must consider the ambiguities and nuances of various testing modalities during the work-up for mitochondrial diseases. Therefore, due to the diagnostic challenges associated with primary mitochondrial diseases, diagnosis should be made in the context of clinical and molecular data, potentially supplemented with histochemical and biochemical evidence. Confirmation of a diagnosis leads to improvements in the management of the disease, decreases unnecessary testing, informs reproductive planning, and improves research pipelines.

Profiling mitochondrial DNA variant segregation during human preimplantation development: a prerequisite to preimplantation genetic testing for mitochondrial DNA-related disorders

STUDY QUESTION

Is preimplantation genetic testing for mitochondrial DNA (mtDNA) disorders (PGT-mt) feasible at early compaction and blastocyst stages?

SUMMARY ANSWER

Pathogenic mtDNA variants segregate evenly among cell types and various lineages of a given embryo during preimplantation development, supporting the relevance of genetic analyses performed on Day 4 blastomere and on Day 5 or 6 trophectoderm (TE) samples.

WHAT IS KNOWN ALREADY

PGT-mt is validated at cleavage stage (Day 3 of development). However, its feasibility at later stages is questionable, as little is known regarding the segregation of pathogenic mtDNA variants during preimplantation development. Since mtDNA replication is silenced until the blastocyst stage (Day 5 or 6), uneven mtDNA segregation between preimplantation embryo cellular lineages known as a 'bottleneck' effect, cannot be excluded, posing a challenge for PGT-mt.

STUDY DESIGN, SIZE, DURATION

We analyzed 112 'mito' embryos carrying pathogenic mtDNA variants and 28 control embryos with mtDNA polymorphism. Heteroplasmy levels were assessed in single cells of the TE, in different parts of blastocysts (inner cell mass and TE), and at three time points of development, namely cleavage (Day 3), early compaction (Day 4), and blastocyst stages (Day 5 or 6).

PARTICIPANTS/MATERIALS, SETTING, METHODS

As part of clinical PGT, a blastomere biopsy was performed at cleavage or early compaction stages (Day 3 or 4) on 112 'mito' and 21/28 control embryos. Further analysis was carried out at Day 5 or 6 on 51 embryos deemed unsuitable for uterine transfer and donated to research. Heteroplasmy levels were determined by semi-quantitative PCR amplification of (i) the mtDNA pathogenic variants with additional enzymatic digestion or (ii) the mtDNA polymorphic hypervariable region 2.

MAIN RESULTS AND THE ROLE OF CHANCE

Here, we first show that mtDNA variants segregate evenly among blastomeres during early compaction (Day 4), supporting the feasibility of PGT-mt at this stage. We also found that mtDNA ratios remain stable between cleavage and blastocyst stages. Yet, the substantial variation of heteroplasmy levels occurring among single TE cells in 1/8 embryos suggests that PGT is only feasible when at least 5-10 cells are collected by standard TE biopsy.

LIMITATIONS, REASONS FOR CAUTION

This study sheds light on mtDNA segregation in human preimplantation embryo development. Its limitation lies in the scarcity of the material and the small number of embryos carrying a specific pathogenic mtDNA variant. Furthermore, the study of single cells from TE was performed on control embryos only.

WIDER IMPLICATIONS OF THE FINDINGS

By supporting the relevance of blastocyst biopsy in the context of PGT for pathogenic mtDNA variants, this study contributes to the general trend of postponing the biopsy to later stages of embryonic development. However, particular attention should be paid to the number of TE cells tested. Due to the potential variation of mutant load during in utero development, a control amniocentesis for evolutive pregnancies following the transfer of heteroplasmic embryos is still recommended.

STUDY FUNDING/COMPETING INTEREST(S)

This work was funded by 'Association Française contre les Myopathies/AFM Téléthon' (22112, 24317, 28525); and EUR G.E.N.E. (No. ANR-17-EURE-0013). The authors have no competing interests to declare.

TRIAL REGISTRATION NUMBER

N/A.

Genetic and reproductive strategies to prevent mitochondrial diseases

Mitochondrial DNA (mtDNA) diseases pose unique challenges for genetic counselling and require tailored approaches to address recurrence risks and reproductive options. The intricate dynamics of mtDNA segregation and heteroplasmy shift significantly impact the chances of having affected children. In addition to natural pregnancy, oocyte donation, and adoption, IVF-based approaches can reduce the risk of disease transmission. Prenatal diagnosis (PND) and preimplantation genetic testing (PGT) remain the standard methods for women carrying pathogenic mtDNA mutations; nevertheless, they are not suitable for every patient. Germline nuclear transfer (NT) has emerged as a novel therapeutic strategy, while mitochondrial gene editing has increasingly become a promising research area in the field. However, challenges and safety concerns associated with all these techniques remain, highlighting the need for long-term follow-up studies, an improved understanding of disease mechanisms, and personalized approaches to diagnosis and treatment. Given the inherent risks of adverse maternal and child outcomes, careful consideration of the balance between potential benefits and drawbacks is also warranted. This review will provide critical insights, identify knowledge gaps, and underscore the importance of advancing mitochondrial disease research in reproductive health.

Sequencing and characterizing human mitochondrial genomes in the biobank-based genomic research paradigm

Human mitochondrial DNA (mtDNA) harbors essential mutations linked to aging, neurodegenerative diseases, and complex muscle disorders. Due to its uniparental and haploid inheritance, mtDNA captures matrilineal evolutionary trajectories, playing a crucial role in population and medical genetics. However, critical questions about the genomic diversity patterns, inheritance models, and evolutionary and medical functions of mtDNA remain unresolved or underexplored, particularly in the transition from traditional genotyping to large-scale genomic analyses. This review summarizes recent advancements in data-driven genomic research and technological innovations that address these questions and clarify the biological impact of nuclear-mitochondrial segments (NUMTs) and mtDNA variants on human health, disease, and evolution. We propose a streamlined pipeline to comprehensively identify mtDNA and NUMT genomic diversity using advanced sequencing and computational technologies. Haplotype-resolved mtDNA sequencing and assembly can distinguish authentic mtDNA variants from NUMTs, reduce diagnostic inaccuracies, and provide clearer insights into heteroplasmy patterns and the authenticity of paternal inheritance. This review emphasizes the need for integrative multi-omics approaches and emerging long-read sequencing technologies to gain new insights into mutation mechanisms, the influence of heteroplasmy and paternal inheritance on mtDNA diversity and disease susceptibility, and the detailed functions of NUMTs.

Fit for purpose: Selecting the best mitochondrial DNA for the job

The factors determining levels of pathogenic mitochondrial DNA in cells and tissues are critical to disease pathology but remain poorly understood and contentious. In Nature, Kotrys et al. published a single-cell-based analysis casting fresh light on this thorny problem and introduced a powerful new investigative tool.

Mitochondrial genetics through the lens of single-cell multi-omics

Mitochondria carry their own genetic information encoding for a subset of protein-coding genes and translational machinery essential for cellular respiration and metabolism. Despite its small size, the mitochondrial genome, its natural genetic variation and molecular phenotypes have been challenging to study using bulk sequencing approaches, due to its variation in cellular copy number, non-Mendelian modes of inheritance and propensity for mutations. Here we highlight emerging strategies designed to capture mitochondrial genetic variation across individual cells for lineage tracing and studying mitochondrial genetics in primary human cells and clinical specimens. We review recent advances surrounding single-cell mitochondrial genome sequencing and its integration with functional genomic readouts, including leveraging somatic mitochondrial DNA mutations as clonal markers that can resolve cellular population dynamics in complex human tissues. Finally, we discuss how single-cell whole mitochondrial genome sequencing approaches can be utilized to investigate mitochondrial genetics and its contribution to cellular heterogeneity and disease.

Origins of tissue and cell-type specificity in mitochondrial DNA (mtDNA) disease

Mutations of mitochondrial (mt)DNA are a major cause of morbidity and mortality in humans, accounting for approximately two thirds of diagnosed mitochondrial disease. However, despite significant advances in technology since the discovery of the first disease-causing mtDNA mutations in 1988, the comprehensive diagnosis and treatment of mtDNA disease remains challenging. This is partly due to the highly variable clinical presentation linked to tissue-specific vulnerability that determines which organs are affected. Organ involvement can vary between different mtDNA mutations, and also between patients carrying the same disease-causing variant. The clinical features frequently overlap with other non-mitochondrial diseases, both rare and common, adding to the diagnostic challenge. Building on previous findings, recent technological advances have cast further light on the mechanisms which underpin the organ vulnerability in mtDNA diseases, but our understanding is far from complete. In this review we explore the origins, current knowledge, and future directions of research in this area.

Stochasticity in genetics and gene regulation

Development from fertilized egg to functioning multi-cellular organism requires precision. There is no precision, and often no survival, without plasticity. Plasticity is conferred partly by stochastic variation, present inherently in all biological systems. Gene expression levels fluctuate ubiquitously through transcription, alternative splicing, translation and turnover. Small differences in gene expression are exploited to trigger early differentiation, conferring distinct function on selected individual cells and setting in motion regulatory interactions. Non-selected cells then acquire new functions along the spatio-temporal developmental trajectory. The differentiation process has many stochastic components. Meiotic segregation, mitochondrial partitioning, X-inactivation and the dynamic DNA binding of transcription factor assemblies-all exhibit randomness. Non-random X-inactivation generally signals deleterious X-linked mutations. Correct neural wiring, such as retina to brain, arises through repeated confirmatory activity of connections made randomly. In immune system development, both B-cell antibody generation and the emergence of balanced T-cell categories begin through stochastic trial and error followed by functional selection. Aberrant selection processes lead to immune dysfunction. DNA sequence variants also arise through stochastic events: some involving environmental fluctuation (radiation or presence of pollutants), or genetic repair system malfunction. The phenotypic outcome of mutations is also fluid. Mutations may be advantageous in some circumstances, deleterious in others. This article is part of a discussion meeting issue 'Causes and consequences of stochastic processes in development and disease'.