Studies On Argininosaccinate Synthase1as A Novel Biomarker And A Therapeutic Target For Breast Cancer Personalized Medicine

Breast cancer is the most common malignant tumor and the leading cause of death in female. In clinics, breast cancer is classified into four subtypes including Luminal A, Luminal B, Triple negative and Her-2positive according to the expression status of ER, PR, Ki-67and Her-2protein in tumor tissues. This classification is useful for making treatment plans for patients, but it is obviously defective due to the high heterogeneity of breast cancer. Abundant evidence has shown that tumor tissues have unique metabolic features different from normal tissues, which has provided potential tumor-specific therapeutic targets for the treatment of cancer. For instance, arginine is a non-essential amino acid and Argininosaccinate Synthase1(ASS1) is the key rate-limiting enzyme in arginine synthesis. While normal tissues commonly express high level of ASS1, some tumors such as hepatic carcinoma and melanoma have arginine auxotrophy due to deficiency of ASS1. As a result, arginine deprivation through pegylated arginine deiminase (ADI-PEG20) which converting arginine to itrulline has been proven to be effective for the treatment of tumors with arginine auxotrophy. However, no related reports are available for breast cancer. In this thesis, using breast cancer multiple tissue array assay, we found that63.75%(95/149) of breast tumors enrolled in this study were ASS1low-expressing. Subsequent Kaplan-Meier analysis demonstrated that low expression level of ASS1was associated with poor overall survival and poor disease-free survival. Further COX multivariate analysis indicated that ASS1was an independant indicator for overall survival and disease-free survival, and the predictive role of ASS1was independent of other known biomarkers such as ER, PR, Ki-67and Her-2, suggesting that ASS1may serve as a new biomarker for breast cancer molecular classification.Moreover, we investigated whether arginine deprivation was efficacious for the treatment of breast cancer. The obtained data demonstrated that ADI-PEG20significantly inhibited the growth of ASS1low-expressing bresat cancer cells in vitro and in vivo, while had little impact on ASS1high-expressing breast cancer cells, indicating that ASS1might be a useful therapeutic target for breast cancer personalized medicine. Further molecular studies demonstrated that arginine deprivatin had little influence on the induction of apoptosis, but dramatically inhibited ATP production and mitochondrial oxidative phosphorylation (OXPHOS). Subsequently, ROS (especially mitochondria ROS) production was significantly increased and the mitochondria membrane potential (MMP) was lowered. Due to these massive mitochondria dysfunctions, arginine deprivation ultimately caused lethal mitophagy in order to selectively remove damaged mitochondria and maintain mitochondrial homeostasis.In summary, for the first time, this study demonstrated that ASS1holds the promise to become an independent factor for prognosis prediction and a candidate novel biomarker for classification in breast cancer. Arginine deprivation may be adopted for personerlized medicine of breast cancer, e.g. for the treatment of ASS1low-expression breast tumors. We also demonstrated that arginine deprivation could induce lethal mitophagy, which may play critical role in arginine deprivation-mediated breast cancer cell death.