Study design and participants
This study was based on a large population cohort from the Kunshan county, Jiangsu Province, China, between May 2019 and August 2021, while they took the annual health examinations which were offered free to the local older adults. The DNA for genomic assay were extracted from the residual blood samples after health examination.
Our inclusion criteria for participants are qualified mtDNA-CN detection, no kidney cancer, and having eGFR information. Finally, this study included 14,467 participants aged over 65 with health examination information obtained at the same time as DNA blood samples testing, for the cross-sectional study of mtDNA-CN and CKD prevalence. Of these participants, 7,500 did not have CKD and underwent their next health examination within the following 2 years, resulting for the prospective study of mtDNA-CN and incident CKD. Details of the protocol for the current study was approved by the institutional review board of the First People’s Hospital of Kunshan (IEC-C-007-A07-V3.0). The study was performed according to the guidelines of the Helsinki Declaration.
Definition of diseases
1). CKD and kidney function progression: CKD was defined according to the Kidney Disease Improving Global Outcomes (KDIGO) [18] and eGFR was calculated using the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) formula [19]. A diagnosis of CKD was defined either estimated glomerular filtration rate (eGFR) was < 60 mL/min/1.73 m2 or based on diagnosis record data using International Classification of Diseases-tenth version (ICD-10), as shown in Table S1 [20]. Kidney function progression defined by worsening of eGFR categories according to KDIGO guidelines: stage 1 (eGFR ≥ 90) stage 2 (eGFR 60–89), stage 3a (eGFR 45–59), stage 3b (eGFR 30–44), stage 4 (eGFR 15–29) and stage 5 (eGFR < 15), with stage 5 representing the most severe form.
2). Comorbidities and Clinical Covariates
Diagnoses of comorbidities (diabetes, hypertension, cardiovascular and cerebrovascular diseases) were obtained from health care data using ICD-10. Metabolic syndrome was defined based on five physical examination indicators, with a positive diagnosis requiring at least three of the five components [21].
Measurement of mtDNA Copy Number
Mitochondrial DNA copy number (mtDNA-CN) were estimated from the whole genome genotyping Array data. Detailed data processing has been described elsewhere [22]. Furthermore, we applied a linear regression model to control for the potential confounding effects of age, sex, white blood cell counts, platelet counts, blood collection date, and batch variability. This enabled us to obtain standardized mtDNA-CN estimates, which were used for subsequent association analysis.
Statistical Analyses
Clinical characteristics were presented as mean (SD), median (IQR) and number (%) for continuous, abnormal and categorical variables respectively. ANOVA, Wilcoxon and chi-square tests were applied to evaluate differences between phenotypic variables in CKD/non-CKD group. Spearman correlation testing was applied to evaluate the correlation between phenotypic variables and mtDNA-CN. We investigated the association between mtDNA-CN and the risk of CKD prevalence using multivariate logistic regression, adjusting all statistically significant relevant phenotypes for age, sex, waist, systolic blood pressure, diastolic blood pressure, serum glutamic pyruvic transaminase, serum glutamic-oxaloacetic transaminase, total cholesterol, triglyceride, high-density lipoprotein cholesterol, low-density lipoprotein, fasting glucose and comorbidities. Then, we utilized Restricted cubic spline analysis in logistic regression model was used to derive the shape of relationship between mtDNA-CN and the risk of CKD prevalence. The model had nodes at the 5th, 35th, 65th, and 95th percentiles of the distribution of mtDNA-CN. Sensitivity analyses were conducted for two diagnosed sources of CKD (CKD from ICD-10 or health examined kidney function of eGFR < 60 mL/min/1.73 m2). We also performed sensitivity analyses to explore the association of mtDNA-CN with CKD in different comorbidities. We used the same covariates in the cross-sectional analysis and in the prospective analysis.
Multinomial logistic regression was applied to analysis the association between mtDNA-CN and six stages of kidney function progression. Additionally, mtDNA-CN was categorized into quartiles among 7,500 non-CKD participants, and Cox proportional hazards models were used to estimate hazard ratios for the associations of mtDNA-CN quartiles with CKD incidence. Follow-up time was used as the time scale, with censoring at the time of loss to follow-up or end of follow-up period (August 2021). Statistical analyses were performed using R version 4.1, with significance defined as P < 0.05.