Gauri G. Shastri ^1,2 Gustavo Sudre ¹ Kwangmi Ahn ¹ Benjamin Jung ¹ Bhaskar Kolachana ³ Pavan K. Auluck ⁴ Laura Elnitski ⁵ Philip Shaw ^1,6*

• ¹ Social and Behavioral Research Branch, National Human Genome Research Institute (NIH), Bethesda, Maryland, United States
• ² Department of Psychiatry, Zucker Hillside Hospital, Northwell Health, New York, United States
• ³ Human Brain Collection Core, National Institute of Mental Health (NIH), Bethesda, Maryland, United States
• ⁴ National Institute of Mental Health (NIH), Bethesda, Maryland, United States
• ⁵ Translational and Functional Genomics Branch, National Human Genome Research Institute (NIH), Bethesda, Maryland, United States
• ⁶ Pears Maudsley Center for Children and Young People, Institute of Psychiatry, Psychology and Neuroscience, King's College London, London, England, United Kingdom

Mathematical algorithms known as "epigenetic clocks" use methylation values at a set of CpG sites to estimate the biological age of an individual in a tissue-specific manner. These clocks have demonstrated both acceleration and delays in epigenetic aging in multiple neuropsychiatric conditions, including schizophrenia and neurodevelopmental disorders such as autism spectrum disorder. However, no study to date has examined epigenetic aging in ADHD despite its status as one of the most prevalent neurodevelopmental conditions, with 1 in 9 children having ever received an ADHD diagnosis in the US. Only a handful of studies have examined epigenetic age in brain tissue from neurodevelopmental conditions, with none focused on ADHD, despite the obvious relevance to pathogenesis. Thus, here we asked if post-mortem brain tissue in those with lifetime histories of ADHD would show accelerated or delayed epigenetic age, as has been found for other neurodevelopmental conditions.We applied four different epigenetic clocks to estimate epigenetic age in individuals with ADHD and unaffected controls from cortical (anterior cingulate cortex, N=55) and striatal (caudate, N=56) postmortem brain tissue, as well as peripheral blood (N= 84) and saliva (N = 112). After determining which epigenetic clock performed best in each tissue, we asked if ADHD was associated with altered biological aging in corticostriatal brain and peripheral tissues.We found that a range of epigenetic clocks accurately predicted chronological age in all tissues. We also found that a diagnosis of ADHD was not significantly associated with differential epigenetic aging, neither for the postmortem ACC or caudate, nor for peripheral tissues. These findings held when accounting for comorbid psychiatric diagnoses, substance use, and stimulant medication.Thus, in this study of epigenetic clocks in ADHD, we find no evidence of altered epigenetic aging in corticostriatal brain regions nor in peripheral tissue. We consider reasons for this unexpected finding, including the limited sampling of brain regions, the age range of individuals studied, and the possibility that processes that accelerate epigenetic age may be counteracted by the developmental delay posited in some models of ADHD.