Sex differences in DNA damage response

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Abstract: The sex of an individual has been associated with the frequency of various health conditions, responses to disease treatment, incidence of drug-associated toxicities, and lifespan. The current literature remains unclear and conflicting with regard to sex-dependent differences in DNA damage response, especially as it relates to DNA damaging agents, such as radiation therapy (RT) which is an indicated treatment in more than half of newly diagnosed cancers. Thus, pharmacodynamic (PD) biomarkers based on assessment of DNA damage endpoints may prove valuable not only in translational and clinical cancer research but in standard RT practices. Presently, there is a lack of robust validated PD biomarkers that can quantify cellular responses to DNA damage. Ataxia telangiectasia mutated kinase (ATM) is activated by DNA double-strand breaks (DSBs) through intermolecular autophosphorylation on serine-1981. ATM serine-1981 phosphorylation (p-ATM) is increased >50% in cells exposed to as little as 5 cGy γ-rays. This sensitivity/stoichiometry suggest that p-ATM may be an appropriate and reliable PD biomarker of radiation-induced DNA damage. Thus, we developed a fit-for-purpose quantitative multiplexed assay to analyze p-ATM and pan ATM protein. This assay documented the first reported induction of p-ATM in patient PBMCs following radiation therapy or chemotherapy. To address sex differences in ATM activation, we obtained PBMCs from male and female volunteers and measured the normal basal levels of ATM as well as the ATM activation following ex vivo irradiation. PBMCs isolated from women have a 2.6-fold greater induction of p-S1981-ATM expression than men following exposure to 2 Gy IR. It is therefore likely that RT-induced p-ATM will differ between male and female patients and that this difference may manifest as different responses and/or toxicities in the clinical setting. We hypothesize that ATM phosphorylation at serine-1981 is a sensitive PD biomarker of DNA damage response capable of defining sex differences and RT toxicities. To address this hypothesis, we propose two specific aims in this application. Aim 1 will quantify DNA damage signaling and repair in PBMCs from male and female cancer patients receiving RT. Aim 2 will evaluate and quantify DNA damage signaling in sorted PBMCs obtained from normal subjects following ex vivo DNA damage to identify specific immune cell populations most induced by radiation. Completion of these Aims will validate p-ATM as a biomarker of DNA damage and repair in patients receiving RT and advance p-ATM as a biomarker of radiation toxicity that may allow dose reduction such that patients experiencing toxicity can complete their therapy.