Heat exposure during susceptible windows of spermatogenesis and sperm epigenetic age

STUDY QUESTION: Does preconception exposure to outdoor heat stress during spermatogenesis alter sperm epigenetic age among men from an infertility treatment population? SUMMARY ANSWER: Exposure to outdoor heat stress during the developmental stages of mitosis and meiosis during spermatogenesis was associated with accelerated sperm epigenetic age. WHAT IS KNOWN ALREADY: Spermatogenesis is uniquely susceptible to redox stress. Age-related disruption of the blood–testes barrier is associated with changes in sperm DNA methylation linked to reduced fecundity and pregnancy complications. Preconception heat stress may cause similar disruptions in the sperm epigenome, providing a pathway through which high temperatures may impair men’s reproductive health. STUDY DESIGN, SIZE, DURATION: We evaluated exposure to high ambient temperatures and sperm epigenetic age among 1,220 men residing along the Wasatch Front in Utah in an ancillary prospective cohort study set within the Folic Acid and Zinc Supplementation Trial (FAZST; 2013–2018). PARTICIPANTS/MATERIALS, SETTING, METHODS: Sperm epigenetic age (SEA), the acceleration or deceleration of age-related changes in sperm DNA methylation, was calculated in semen samples collected 6 months after enrollment. Utilizing local hourly temperature data, average temperature and hours per day exceeding the 98th, 95th, 90th, and 75th percentile thresholds for dry bulb temperature (ambient air) and wet bulb temperature (relative temperature with 100% humidity) were calculated across spermatogenesis and susceptible windows of mitosis, meiosis I + II, spermiogenesis, and spermiation. Temperature was modeled using three approaches: (i) Generalized linear models (GLMs) for average temperature by warm vs. cold season, (ii) GLMs incorporating natural cubic splines for average temperature, and (iii) GLMs for temperature thresholds. MAIN RESULTS AND THE ROLE OF CHANCE: Across spermatogenesis, each additional 10% increase in proportion of time exposed to wet bulb temperatures ≥90th (16.1°C), ≥95th (17.2°C), and ≥98th (17.8°C) percentiles was associated with 0.063 (95% CI −0.001, 0.128), 0.121 (95% CI 0.022, 0.220), and 0.173 (95% CI 0.030, 0.316) years accelerated sperm epigenetic age, respectively. For spermatogenic-specific developmental windows, associations were strongest during meiosis I + II (e.g. 0.146 [95% CI 0.028, 0.264] years for ≥98th percentile) and, in spline models, both warmer and colder wet bulb temperatures during meiosis I + II were associated with accelerated sperm epigenetic age (P = 0.028). Associations for dry bulb temperatures were similar, although less precise, and no clear associations were observed when modeling average temperature by season. LIMITATIONS, REASONS FOR CAUTION: Due to reliance on outdoor temperatures rather than personal exposure, misclassification of exposure to temperature is likely. Findings should be generalized with caution to groups with different levels of exposure to and susceptibility to outdoor heat. While we observed modest effect sizes up to 0.173 years for accelerated SEA per 10% increased time exposed to heat conditions, these changes at the population-level may be impactful for downstream impacts on fertility and pregnancy health, and future work replicating and extending findings is an important next step. WIDER IMPLICATIONS OF THE FINDINGS: Associations between high wet bulb temperatures, which capture impaired efficiency of sweating for cooling body temperature, and accelerated epigenetic aging add evidence that heat-related disruption of sperm DNA methylation may adversely impact men’s reproductive health. STUDY FUNDING/COMPETING INTEREST(S): This study was supported by K01ES034005 from the National Institute of Environmental Health Sciences. The parent trial was supported by funding from the Intramural Research Program of the Eunice Kennedy Shriver National Institute of Child Health and Human Development (HHSN275201200007C and HHSN275201300026I). The authors have no conflicts of interest to declare. TRIAL REGISTRATION NUMBER: NCT01857310.