This application entitled, "Estrogen deprivation and Aromatase Inhibitor associated Arthralgia," seeks to apply pharmacogenetic epidemiology, appropriate biomarkers, and validated patient-reported outcomes to define the role of estrogen deprivation in arthralgia (joint pain) occurrence, severity, and functional interference among postmenopausal women receiving aromatase inhibitors (AIs) as adjuvant therapy for early stage breast cancer. AIs are a class of medications that block the conversion of androgens to estrogens inhibiting aromatase enzyme, thereby resulting in significant estrogen depletion. Nearly 50 percent of breast cancer patients taking AIs report AI-associated arthralgia (AIAA). AIAA impairs functional status and quality of life, leading to premature medication discontinuation in 10-20percent of patients. The mechanisms underlying AIAA are not well understood, making it difficult to identify individuals at risk for developing such symptoms and hampering efforts to devise effective interventions. The etiology of AIAA is likely to be complex. Based upon basic science literature, clinical experience, and our preliminary results, we hypothesize that estrogen withdrawal induced by AIs may result in an increase in pain sensitivity, leading to the subjective experience of AIAA. Recently, we have identified polymorphisms in CYP19A1 (encoding the aromatase enzyme) that were associated with patient-reported AIAA occurrence, demonstrating that genetic variations in the genes encoding certain enzymes of the estrogen pathways may predict the risk of AIAA. To extend this work, we will bring together a multidisciplinary team of researchers and clinicians in pain and symptom management, genetic epidemiology and biostatistics, reproductive endocrinology, breast oncology, and nursing to address the following aims: Specific Aim 1: To determine the associations between genetic variants related to estrogen pathways and patient-reported AIAA occurrence. We will conduct a cross-sectional study of 1,000 stage I-III breast cancer survivors currently receiving AIs to determine relationships between specific genetic variants and patient-reported outcomes of arthralgia. Specific Aim 2: To determine whether estrogen levels mediate the association between CYP19A1 genetic polymorphism and AIAA. To answer this aim, we will conduct a prospective cohort study among 450 breast cancer patients who are due to initiate AIs; we will follow them before AIs (Baseline), and at one, three, and six months after initiation of AI therapy. Exploratory Aim: To explore the role of AI-related estrogen deprivation in pain sensitivity. To answer this aim, we will perform multimodal sensory testing in a subset of eligible and consenting participants from the above prospective cohort (N=100). The proposed study will increase our understanding of the mechanisms underlying AIAA. This increased understanding will facilitate the development and translation of new therapeutics and preventative strategies for this important problem affecting hundreds of thousands of breast cancer patients.