The Role of Gene Expression Variation in the Development of Mammalian Drug Resistance

Simran Kaur ‘20

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Fig 1. Chemotherapy resistance in cancer cells is a pertinent issue that can be attributed to the presence of gene variation and consequent evolution. 

Designing drugs is often challenging because identical cells within a specific network will exhibit varying genetic expression (noise), resulting in drug resistance. The source of this variation is most often stochastic, accumulations of random fluctuations occurring during transcription, translation, and post-translational regulation. Gene expression noise currently poses as the greatest barrier in finding a cure for cancer and Human Immunodeficiency Virus (HIV). Researchers in this study sought to determine the role of genetic noise in the evolution of mammalian drug resistance, by using two identical cell lines in isogenic Chinese Hamster Ovary (CHO) cells. 

Two synthetic gene circuits, low-noise positive-feedback (mNF) and high-noise positive-feedback (mPF), were integrated into the CHO cells. The circuits were engineered to control the expression of Puromycin N-acetyltransferase (pac), the gene responsible for antibiotic Puromycin resistance. Puromycin concentrations of 0, 10, 22.5, 35, and 50 μg/mL were administered to both cell lines and population curves were created at these respective concentrations. Using parallel flow cytometry and microscopy, cell growth was observed and quantified. Significant observations were immediate growth with no adaptation for the low Puromycin dose and rapid growth that increased concurrently with stress for the high dose. Researchers identified, using these results, three general steps in the process of adaptation development: growth suppression, rapid regeneration, and saturation at confluency. With increasing Puromycin concentrations, the duration of the growth suppression phase became increasingly variable.  

Gene expression noise plays an important role in cancer, because an increased amount of noise enables cancer cells to develop resistance against chemotherapy, and more broadly microbes against antibiotics. This study indicates the significance of variation, more specifically cell heterogeneity, in the development of adaptations to drugs. A notable finding of this study is under high stress, mPF networks harbor evolution, but inhibit it under low stress. Nevertheless, future studies are necessary and beneficial to understanding the enigma of chemotherapy resistance in mammalian cancer cells. 



  1. K.S Farquhar, et al. Role of network-mediated stochasticity in mammalian drug resistance. Nature Communications, (2019).
  2. Image retrieved from:!d

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