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BACKGROUND: The pathophysiology of PNAFLD is important to understand in order to improve outcomes. Integration of critical components including metabolic stress, inflammation, organelle function, and DNA damage and repair should be particularly critical for this purpose. To emphasize the contribution of DNA damage response and cell survival mechanisms on PNAFLD outcomes during the period of active liver growth, we focused on the ATM pathway. In toxic liver injury settings, replenishment of ATM overcomes restrictions in liver homeostasis and regeneration. METHODS and RESULTS: We undertook a study of PNAFLD in a cohort of children under 12 years of age to represent the period of active liver growth (N=47). Demographic details were noted including age, gender, ethnic origin and duration of obesity as defined by BMI greater than 85th percentile. B iopsies were examined for NAFLD activity score (NAS) components steatosis, ballooning and inflammation as well as fibrosis. Mitochondrial DNA content was quantitated by qPCR for the mitochondrial gene, TL1, vs the nuclear gene, RNAseP. Ploidy class distributions was evaluated by cytofluorimetric analysis of nuclei stained with Hoechst dye. Cell growth-arrest was examined by immunostaining of p21. ATM expression was quantitated by qPCR for ATM relative to GAPDH. Mitochondrial homeostasis in mice with PNAFLD was evaluated by electron microscopy and Western Blot of fission and fusion proteins. Of the 47 children in the cohort, 74% were male, 26% female; 85% hispanic, 11% white, black or asian, and 4% unspecified. Mean BMI of the cohort was 94±7th percentile. The duration of obesity until time of liver biopsy was 4±2 years. 61% displayed simple steatosis or fatty liver (FL) with NAS less than 5, and 39% had steatohepatitis (NASH), with NAS equal to or greater than 5. Significantly, 50% of children in the cohort had fibrosis at the time of liver biopsy. Disease progression was associated with dyregulated ATM expression as evidenced by less ATM mRNA expression in FL compared to healthy controls (obtained from NIDDK supported biorepository LTPADS from the University of Minnesota) which further declined with NASH. This was associated with greater degree of mitochondrial DNA depletion in NASH vs FL, compared to healthy controls, which is dependent on ATM. Consequences of mitochondrial depletion extended to increased p21 mediated cell growth arrest and pathological polyploidy. In modelling of PNAFLD in mice, exposure of weanling mice to high fat diet plus low dose CCl4 plus ATM kinase antagonism for 4 weeks reproduced hepatic steatosis, inflammation ballooning and fibrogenesis. Moreover, there was disruption of mitochondrial homeostatis including fission and fusion. CONCLUSION: Disruption of ATM is particularly critical for PNAFLD in early liver growth stage by increasing susceptibility to nuclear and mitochondrial injury. These insights should advance translational and clinical studies in this area. 1K08DK125881-01A1 This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.