Sperm mitochondrial DNA copy number: a marker of male fertility and reproductive success

Abstract BACKGROUND Sperm mitochondrial DNA copy number (mtDNAcn) has emerged as a promising biomarker of sperm health, providing molecular insight beyond what is captured by standard semen analysis. Elevated sperm mtDNAcn has been consistently associated with lower sperm motility, concentration, and morphology, as well as prolonged time-to-pregnancy in natural conception and reduced fertilization potential in ART. OBJECTIVE AND RATIONALE This review summarizes current evidence on the biological underpinnings of sperm mtDNAcn, including its regulation during spermatogenesis, the role of nuclear-encoded mitochondrial proteins such as TFAM (mitochondrial transcription factor A), and its potential epigenetic modulation through sperm DNA methylation. We evaluate general population and clinic-based studies linking sperm mtDNAcn to semen quality, couple fecundity, and early embryo development, while highlighting methodological considerations such as quantification techniques and somatic cell contamination. SEARCH METHODS A literature search was conducted to identify human studies evaluating sperm mtDNAcn in relation to male fertility, semen quality, reproductive outcomes, and embryology outcomes, as well as experimental models investigating the underlying biological mechanisms of mtDNA regulation during spermatogenesis via TFAM up to 1 April 2026. Searches were performed in PubMed, Web of Science, and Scopus using combinations of keywords and Medical Subject Headings (MeSH), including sperm mitochondrial DNA copy number, mtDNAcn, male infertility, pregnancy outcomes, ART outcomes, semen quality, sperm epigenetics, TFAM, mitochondrial transcription factor A, and sperm mtDNA regulation. Reference lists of relevant reviews and primary articles were manually screened to identify additional studies. Eligible studies included observational epidemiologic studies, clinical cohort studies, and experimental investigations that quantified sperm mtDNAcn and examined associations with semen parameters, fertility outcomes, or sperm epigenetics. No restrictions were placed on geographic location, while only articles published in English were considered. OUTCOMES Across 21 epidemiologic, experimental, and clinical studies, elevated sperm mtDNAcn has been consistently associated with reduced sperm quality and inconsistently associated with diminished couple-level reproductive potential. Higher mtDNAcn is associated with lower sperm concentration, total sperm count, motility, and normal morphology, as well as higher sperm DNA fragmentation and chromatin abnormalities. It has also been linked to reduced likelihood of pregnancy and poorer embryo quality. Emerging evidence indicates associations between sperm mtDNAcn and altered nuclear DNA methylation patterns, supporting a role for mitochondrial–nuclear crosstalk. Collectively, these findings position sperm mtDNAcn as a biologically informative and clinically relevant indicator of male reproductive health that may complement, or potentially enhance, traditional semen analysis in both research and clinical settings. WIDER IMPLICATIONS Sperm mtDNAcn holds promise as a biomarker for male fertility assessment, yet its full clinical potential has not been yet to be realized. Establishing standardized measurement protocols across sperm fractions and laboratory platforms will be essential for enabling cross-study comparability and facilitating clinical translation. Large prospective studies are needed to define clinically meaningful thresholds and better characterize the relationship between mtDNAcn alterations and spermatogenic impairment. Intervention strategies targeting mtDNA biogenesis and depletion, including antioxidant strategies and mitochondria-directed pharmacotherapy, warrant further investigation in the context of male fertility and fecundity. Collectively, sperm mtDNAcn may serve as an adjunctive marker in the assessment of male reproductive health and may inform future precision medicine approaches. REGISTRATION NUMBER N/A.