A method for measuring mitochondrial DNA copy number in pediatric populations.

The mitochondrion is a multifunctional organelle that modulates multiple systems critical for homeostasis during pathophysiological stress. Variation in mitochondrial DNA (mtDNA) copy number (mtDNAcn), a key mitochondrial change associated with chronic stress, is an emerging biomarker for disease pathology and progression. mtDNAcn can be quantified from whole blood samples using qPCR to determine the ratio of nuclear DNA to mtDNA. However, the collection of blood samples in pediatric populations, particularly in infants and young children, can be technically challenging, yield much smaller volume samples, and can be distressing for the patients and their caregivers. Therefore, we have validated a mtDNAcn assay utilizing DNA from simple buccal swabs (Isohelix SK-2S) and report here its performance in specimens from infants (age = <12 months). Utilizing qPCR to amplify ~200bp regions from two mitochondrial (ND1, ND6) and two nuclear (BECN1, NEB) genes, we demonstrated absolute (100%) concordance with results from low-pass whole genome sequencing (lpWGS). We believe that this method overcomes key obstacles to measuring mtDNAcn in pediatric populations and creates the possibility for development of clinical assays to measure mitochondrial change during pathophysiological stress. ### Competing Interest Statement The authors have declared no competing interest. ### Funding Statement This research project was supported by the California Initiative to Advance Precision Medicine ACES Program (OPR20139) and The JPB Research Network on Toxic Stress: A project of the Center on the Developing Child at Harvard University. ### Author Declarations I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained. Yes The details of the IRB/oversight body that provided approval or exemption for the research described are given below: IRB of Children's Hospital Los Angeles gave ethical approval for this work. Specifically, the IRB approved the following study cohorts presented in this work include Adverse Childhood Experiences (ACEs) program (Scalable Measurement and Clinical Deployment of Mitochondrial Biomarkers of Toxic Stress IRB CHLA2100174) and the Your Baby: Healthy Development and Resiliency (IRB CHLA1800547). Participants' or their legal representatives' provided written consent to take part in the aforementioned studies. I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals. Yes I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance). Yes I have followed all appropriate research reporting guidelines, such as any relevant EQUATOR Network research reporting checklist(s) and other pertinent material, if applicable. Yes All data produced in the present study are available upon reasonable request to the authors