The study, led by corresponding author Riddhiman Dhar, PhD, of the Indian Institute of Technology Kharagpur, took a multi-cancer approach, examining the relationship between mtDNA copy number and key features of tumor biology across different cancer types. The central question was deceptively simple: does the quantity of mitochondrial DNA in a cancer cell correlate with the behavior and progression of the disease? The answer, according to the research, appears to be yes. The team found a clear correlation between mtDNA copy number and the extent of genetic mutations within cancer cells, a measure known as mutational load, which is itself closely tied to how aggressive and advanced a cancer can be.

To understand why this finding matters, it helps to appreciate the unusual nature of mitochondrial DNA. Unlike the vast majority of our genetic material, which is packaged into chromosomes inside the cell nucleus, mtDNA resides within the mitochondria themselves. Humans typically inherit mtDNA exclusively from their mothers, and each mitochondrion can contain multiple copies of the mitochondrial genome, with a single cell housing hundreds to thousands of copies in total. This copy number is not fixed. It can rise or fall depending on the energy demands of the cell, environmental stresses, and disease states. In cancer, where cells divide rapidly and metabolism is dramatically rewired, the regulation of mtDNA copy number becomes a potentially critical variable.

One of the most striking observations in the new study concerns compensation. When mitochondrial function is impaired, the researchers found that an increase in mtDNA copy number showed a signature of compensation for that impaired function. In other words, cancer cells appear to respond to malfunctioning mitochondria by producing more copies of the mitochondrial genome, perhaps in an attempt to maintain adequate energy production despite underlying defects. This kind of compensatory mechanism has been observed in mitochondrial diseases before, but seeing it clearly reflected in cancer cells adds a new dimension to our understanding of tumor metabolism. It suggests that mtDNA copy number is not merely a passive byproduct of cellular state but an actively regulated feature that cancer cells may exploit to survive under stress.

The implications extend beyond energy production. The study also found that variation in mtDNA copy number was associated with differential expression of genes linked to chemotherapy response. This is a particularly significant result because chemotherapy remains a mainstay of cancer treatment worldwide, and predicting which patients will respond well to a given drug is one of the great challenges of modern oncology. If the amount of mitochondrial DNA in tumor cells influences the expression of genes involved in drug response, then mtDNA copy number could eventually serve as a biomarker, helping clinicians tailor treatment decisions to the specific biology of a patient’s tumor.

Furthermore, the research connected mtDNA copy number to various other aspects of cancer biology. Notably, high mtDNA copy number was linked with the expression of genes that support tumor growth and survival. This finding hints at a possible mechanism by which abundant mitochondrial DNA could actively promote cancer progression: by bolstering the cellular machinery that tumors need to proliferate and evade cell death. Cancer cells are known for their metabolic flexibility, often shifting their energy production away from normal oxidative phosphorylation toward glycolysis even in the presence of oxygen, a phenomenon called the Warburg effect. Yet mitochondria remain indispensable even in highly glycolytic tumors, contributing to biosynthesis, signaling, and cell survival pathways. The new results suggest that the mitochondrial genome’s abundance may be woven into these processes more intimately than scientists had realized.

The correlation between mtDNA copy number and mutational load is especially intriguing from an evolutionary perspective. Tumors evolve over time, accumulating mutations in their nuclear DNA that drive uncontrolled growth, immune evasion, and metastasis. A higher mutational load generally reflects a more evolved, and often more aggressive, malignancy. The finding that mtDNA copy number tracks with this mutational burden raises the possibility that mitochondrial genetic status and nuclear genome instability are interconnected processes in cancer evolution. Whether changes in mtDNA copy number contribute directly to genomic instability, or whether both are shaped by a common underlying stress, remains an open question that the researchers say warrants deeper investigation.

Speaking about the significance of the work, Dhar emphasized its potential to reshape how scientists think about mitochondria in cancer. The results, he noted, reveal a close association of mtDNA copy number with cancer progression and therapy response, and could pave the way for deeper investigations into the role of mitochondria in cancer that will enable new cancer management strategies. Such strategies could range from improved prognostic assessments, in which mtDNA copy number helps gauge how advanced or aggressive a tumor is likely to be, to therapeutic interventions that target mitochondrial function or the pathways that regulate mitochondrial genome maintenance.

It is worth stressing that the study establishes correlations rather than definitive proof of causation. A correlation between mtDNA copy number and cancer severity does not by itself demonstrate that altering mtDNA copy number will change the course of the disease. Future experimental work, including studies in cell and animal models, will be needed to determine whether manipulating mitochondrial DNA abundance can directly influence tumor behavior or treatment sensitivity. Nevertheless, the breadth of the associations uncovered in this multi-cancer analysis makes a compelling case that mtDNA copy number deserves attention as a key determinant of cancer biology, one that has often been overshadowed by the focus on nuclear mutations.

The research, published under the title Multi-cancer analysis reveals mtDNA copy number as a key determinant of mutational load and cancer progression, arrives at a moment when mitochondrial biology is enjoying renewed attention across medicine. Mitochondrial dysfunction has been implicated in neurodegenerative diseases, aging, metabolic disorders, and now increasingly in cancer. As sequencing technologies and computational analyses grow more powerful, researchers are able to probe the mitochondrial genome and its copy number with unprecedented precision across large collections of tumor samples. Studies like this one, which integrate mitochondrial genetics with mutation data and gene expression profiles across multiple cancer types, exemplify a growing trend toward viewing the cancer cell as an integrated system in which nuclear and mitochondrial genomes interact dynamically. If subsequent research confirms and extends these findings, the humble mitochondrial genome, long a footnote in cancer genetics, could move to the center of the stage, offering new biomarkers for prognosis, new predictors of chemotherapy response, and potentially new targets for the management of one of humanity’s most formidable diseases.

Subject of Research: The relationship between mitochondrial DNA copy number and cancer progression and therapy response

Article Title: Does mitochondrial DNA affect cancer progression?

Article References: Does mitochondrial DNA affect cancer progression?. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: mitochondrial DNA, mtDNA copy number, cancer progression, mutational load, tumor metabolism, chemotherapy response, gene expression, mitochondria, The FEBS Journal, cancer biomarkers, tumor growth, compensatory mechanisms

Down syndrome, caused in about 95 percent of cases by a full extra copy of chromosome 21, is the most common chromosomal disorder associated with intellectual disability. Its effects ripple across the neurological, musculoskeletal, and cardiovascular systems, and roughly half of all individuals with the condition are born with congenital heart defects. Over the past decade, researchers have documented widespread epigenetic alterations, changes to chemical marks on DNA that regulate gene activity without altering the underlying sequence, in the nuclear DNA of people with trisomy 21. These changes have been linked to features ranging from accelerated aging to hematological and cardiovascular complications. But the mitochondrial genome has remained largely terra incognita for epigeneticists, even as evidence accumulates that mitochondrial dysfunction is a genuine feature of the syndrome.

The reason mitochondria matter so much in Down syndrome comes down to gene dosage. The extra chromosome carries genes involved in oxidative stress pathways, and cells from people with the syndrome show impaired energy metabolism, chronic pro-oxidative states, and deficits in the molecular machinery that produces ATP, the cell’s energy currency. Studies in fibroblasts and brain tissue have repeatedly detected these functional disturbances. Yet measuring mitochondrial function directly is difficult, and one of the most accessible molecular proxies, the number of mitochondrial DNA copies per cell, has yielded only slight and statistically unconvincing increases in prior studies of Down syndrome tissues.

The new study focused on a very specific stretch of the mitochondrial genome: the D-loop, a non-coding regulatory region that contains both the origin of DNA replication and the main transcription start site for the mitochondrial chromosome. The D-loop is where DNA methyltransferases, the enzymes that attach methyl groups to cytosine bases, appear to concentrate their activity on mitochondrial DNA. Although methylation levels across the mitochondrial genome are typically very low, around 0.5 to 1 percent, the D-loop can reach peaks of 10 to 15 percent. Crucially, methylation in this region modulates the binding of TFAM, mitochondrial transcription factor A, a master regulator of both mitochondrial DNA transcription and replication. In diseases from Alzheimer’s to amyotrophic lateral sclerosis to cardiovascular conditions, D-loop methylation has been found altered and sometimes correlated with disease severity and mitochondrial DNA copy number, making it a plausible suspect in Down syndrome as well.

The suspicion was sharpened by a single earlier study from 2011, in which researchers examined immortalized lymphoblastoid cells from six children with Down syndrome and six matched controls. That work identified a breakdown in one-carbon metabolism, the cellular circuit that generates the methyl-donor molecule S-adenosylmethionine, and found reduced levels of this methylating agent inside the mitochondria of Down syndrome cells, along with global hypomethylation of mitochondrial DNA. If mitochondrial methylation capacity was genuinely compromised, it stood to reason that measurable differences should appear in patient blood samples.

To find out, the team led by Andrea Stoccoro and Fabio Coppedè of the University of Pisa, working with colleagues at the University of Rijeka and the Juraj Dobrila University of Pula, recruited 118 participants at the Clinical Hospital Centre Rijeka in collaboration with Down syndrome associations across Croatia. The cohort ranged in age from newborns to 55 years, with every Down syndrome case confirmed by karyotyping and every participant matched by age and sex to a healthy control. Among the newborns, ten had Down syndrome with congenital heart defects, eight had Down syndrome without such defects, and eighteen were healthy controls. The heart defects identified by ultrasound included five atrial septal defects, two atrioventricular septal defects, one ventricular septal defect, one patent ductus arteriosus, and one case of tetralogy of Fallot, and newborn blood samples were drawn within seventy-two hours of birth, before any surgical or medical correction of cardiac malformations could confound the measurements.

Methodologically, the study leaned on methylation-sensitive high-resolution melting, a technique that amplifies a bisulfite-converted segment of DNA and then reads out its methylation level from the melting behavior of the resulting PCR product. The team had previously developed and validated this protocol against pyrosequencing, and in the present study it targeted a 222-base-pair segment of the D-loop containing ten CpG sites. Calibration standards spanning 0 to 100 percent methylation were run in each assay, and an interpolation method developed in the same laboratory converted melting curves into precise single-value methylation percentages. Bisulfite conversion efficiency averaged 99 percent, and equal numbers of patient and control samples were processed together to suppress batch effects. Mitochondrial DNA copy number was measured independently by quantitative PCR comparing a mitochondrial target against the nuclear hemoglobin beta gene, with the caveat that limited DNA availability allowed copy number analysis in only 103 of the 118 participants.

The results were striking in their uniformity. Across individuals, D-loop methylation ranged from 0 to about 8 percent, and mitochondrial DNA copy number ranged from 33 to 900 copies, reflecting enormous person-to-person variability. But the distributions overlapped almost perfectly between groups. Median D-loop methylation was 1.3 percent in controls versus 4.0 percent in the Down syndrome group, a difference that fell short of statistical significance at p equal to 0.28. Mitochondrial DNA copy number medians were similarly indistinguishable, at roughly 128 copies in controls and 149 in the syndrome group, with p equal to 0.22. Age showed no correlation with either biomarker across the entire sample, spanning newborns to mid-adulthood. Sex made no significant difference either, although the authors noted a slight, non-significant tendency toward higher copy number in males. Most importantly for the congenital heart question, comparisons among the three newborn groups, those with Down syndrome and heart defects, those with Down syndrome and healthy hearts, and controls, revealed no differences in either methylation or copy number, with p values of 0.85 and 0.60 respectively.

The authors are careful to frame the work as a pilot study and to spell out its limits. The analysis covered only a small portion of the mitochondrial genome, so alterations elsewhere in the mitochondrial chromosome cannot be excluded. The newborn subgroup, particularly those without heart defects, was small, and too small to permit sex-stratified analysis even though previous genome-wide studies of nuclear DNA in Down syndrome newborns have shown that many methylation differences tied to congenital heart defects are sex-specific. Blood methylation may also simply fail to mirror what happens in fetal cardiac tissue, the site where the relevant biology would actually unfold. Larger cohorts and studies of cardiac tissue itself will be needed before mitochondrial methylation can be definitively ruled out in the syndrome’s cardiovascular complications.

Even so, the null result carries a useful message. It suggests that the mitochondrial dysfunction documented in Down syndrome, including the more pronounced oxidative stress seen in fetuses with congenital heart defects, does not register in the two most accessible mitochondrial molecular readouts available from a routine blood sample. Unlike nuclear DNA methylation, where robust signatures of trisomy 21 have been detected in blood, placenta, buccal cells, brain, and myocardial tissue, the mitochondrial D-loop appears to hold steady. For researchers hunting biomarkers that could distinguish Down syndrome newborns with heart defects from those without, the message is that the mitochondrial D-loop is unlikely to provide one, at least not in peripheral blood. For the broader field of mitochondrial epigenetics, the study adds a well-controlled data point to a growing literature showing that D-loop methylation shifts in some neurological and cardiovascular diseases but not in every condition marked by mitochondrial stress, a reminder that in biology, the absence of a signal can be as informative as its presence.

Subject of Research: Mitochondrial D-loop region methylation and mitochondrial DNA copy number in individuals with Down syndrome

Article Title: Analysis of mitochondrial D-loop region methylation and copy number in peripheral blood DNA of Down syndrome individuals including newborns with and without congenital heart defects

Article References: Analysis of mitochondrial D-loop region methylation and copy number in peripheral blood DNA of Down syndrome individuals including newborns with and without congenital heart defects. (n.d.). https://doi.org/10.1186/s43682-025-00039-x

Image Credits: AI Generated

DOI: 10.1186/s43682-025-00039-x

Keywords: Down syndrome, mitochondrial DNA, D-loop methylation, mtDNA copy number, epigenetics, congenital heart defects, trisomy 21, bisulfite conversion, high-resolution melting, mitochondrial dysfunction, newborns, biomarkers