Overview of Cytosine Deaminase
Cytosine deaminase (CD) is an enzyme that catalyzes the chemical reaction between cytosine and water (H2O) to produce uracil and ammonia (NH3). Cytosine deaminases are found in bacteria and fungi, but not in mammalian cells. Cytosine deaminase expression can be induced in response to nutritional stress. Cytosine deaminase can not only deaminate cytosine, but also deaminate the antifungal drug 5-fluorocytosine (5-FC), and convert it into the anti-tumor fluoropyrimidine drug 5-fluorouracil (5-FU). 5-FU undergoes a series of metabolism in cells, and finally inhibits the activity of thymine synthase, blocks the synthesis of DNA and RNA, and leads to cell death.
Cytosine deaminase is very important in disease treatment because it can be used as a suicide gene for gene therapy. Suicide genes refer to genes that can convert non-toxic or low-toxic prodrugs into highly toxic active drugs. By transfecting the cytosine deaminase gene into tumor cells and then administering 5-FC, tumor cells can produce a large amount of 5-FU and die, while normal cells are not affected. This method has the advantages of selectivity, safety and efficacy, and has been verified in animal models and clinical trials.
Application of Cytosine Deaminase in Gene Therapy
The principle of the cytosine deaminase/5-FC system is to use cytosine deaminase to convert the non-toxic prodrug 5-fluorocytosine (5-FC) into the toxic drug 5-fluorouracil (5-FU), which can inhibit thymine Synthetase and DNA synthesis, leading to tumor cell death. The cytosine deaminase/5-FC system has several advantages. First, it can reduce the toxic side effects of normal cells because there is no cytosine deaminase in mammalian cells, and 5-FC itself has low toxicity. Second, it can enhance the sensitivity of tumor cells because cells expressing cytosine deaminase are 100–10,000 times more sensitive to 5-FC than wild-type cells. Third, it can overcome the drug resistance of tumor cells because 5-FU is not S-phase specific and can work synergistically with other treatments. Finally, it can improve drug bioavailability and stability since 5-FC is an approved drug with good absorption and a known clinical profile.
Tumor cells of different types and sources can be treated by using the cytosine deaminase/5-FC system. Many studies have observed the effects of this system on tumor growth, metastasis, angiogenesis, and the immune response and explored the molecular and cellular mechanisms behind them. Some studies have shown that the cytosine deaminase/5-FC system can effectively inhibit a variety of tumor models, such as colon cancer, liver cancer, pancreatic cancer, gastric cancer, breast cancer, lung cancer, osteosarcoma, melanoma, and so on. There are several possible mechanisms. For example, this system directly kills tumor cells expressing cytosine deaminase, producing cytotoxic and apoptotic effects. For another example, the system transfers 5-FU to adjacent non-cytosine deaminase-expressing tumor cells through gap junctions or lysosome release, resulting in a bystander effect. Also, the cytosine deaminase/5-FC system inhibits tumor angiogenesis and metastasis by inhibiting angiogenesis factors or promoting apoptosis of endothelial cells. In addition, by activating natural killer cells, dendritic cells, and T lymphocytes, the cytosine deaminase/5-FC system induces tumor-specific immune responses and immune memory.
The cytosine deaminase/5-FC system can be used in synergistic treatment with other methods that can enhance its anti-tumor effect or reduce its toxic side effects. Some studies have shown that the cytosine deaminase/5-FC system can be combined with various methods. (1) Other suicide gene systems, such as HSV-TK/GCV, NTR/CB1954, can produce synergistic cytotoxicity and a bystander effect. (2) Other drugs, such as radionuclides, chemotherapy drugs, and targeted drugs, can enhance the sensitivity of tumor cells or inhibit their drug resistance. (3) Other gene therapy methods, such as immune gene therapy and angiogenesis inhibitory gene therapy, can induce tumor-specific immune responses or inhibit tumor angiogenesis. However, this approach faces some difficulties and limitations. First, the interaction and coordination between the cytosine deaminase/5-FC system and other methods need to be optimized. Second, the safety and efficacy of the cytosine deaminase/5-FC system and other methods need to be evaluated. Third, the dose, route, timing, and frequency between the cytosine deaminase/5-FC system and other methods need to be determined. Fourth, the adaptability and selectivity between the cytosine deaminase/5-FC system and other methods need to be verified.
The Role of Cytosine Deaminase in Chemotherapy
Cytosine drugs are a class of anti-tumor drugs that can interfere with DNA synthesis and repair, thereby inhibiting the proliferation and survival of tumor cells. Cytosine drugs include a variety of different compounds, such as gemcitabine, cytarabine, azacytidine, and decitabine, all of which are based on cytosine and have different substitutions or modifications to enhance their activity or stability. Cytosine drugs usually exist in the form of prodrugs, which need to undergo multi-step enzymatic reactions to convert into the active form, that is, the triphosphate form (such as gemcitabine triphosphate), which is then incorporated into the newly synthesized DNA strand by DNA polymerase, resulting in DNA strand breaks or terminations. Cytosine deaminase can deaminate the prodrugs of cytosine drugs into inactive uracil compounds, thereby reducing the effectiveness and toxicity of cytosine drugs. Some studies have shown that the expression level of cytosine deaminase is negatively correlated with the metabolic rate and sensitivity of cytosine drugs, that is, cells or individuals with high expression of cytosine deaminase have a fast metabolic rate and low sensitivity to cytosine drugs, and vice versa. Therefore, cytosine deaminase can be used as a prognostic or individualized treatment indicator for cytosine drug therapy, and can also be used as a target or synergistic factor for cytosine drug therapy.
Cytosine deaminase can affect the response and tolerance of tumor cells to cytosine drugs, thereby affecting the efficacy and safety of chemotherapy. The expression level of cytosine deaminase is negatively correlated with the metabolic rate of cytosine drugs, that is, cells or individuals with high expression of cytosine deaminase can metabolize cytosine drugs quickly, the half-life of drugs in the body is short, and the effective concentration is low, which leads to tumor cells. Increased resistance to cytosine-based drugs. The expression level of cytosine deaminase is positively correlated with the toxicity of cytosine drugs, that is, cells or individuals with high cytosine deaminase expression have increased toxicity to cytosine drugs, mainly because cytosine deaminase can convert cytosine drugs into uracil compounds. The latter can interfere with the activity of uracil nucleotide synthase (UMPS), resulting in a deficiency of uracil nucleotide (UTP) and uridine triphosphate (UTP), thereby affecting RNA synthesis and cellular function. The expression level of cytosine deaminase can be used as a prognostic or individualized treatment indicator for cytosine drug therapy, that is, cells or individuals with low expression of cytosine deaminase are more suitable for cytosine drug treatment because they are highly sensitive to cytosine drugs and have low toxicity. low, and vice versa. In addition, the expression level of cytosine deaminase can be used as a target or synergistic factor for cytosine drug therapy. By inhibiting or enhancing the activity or expression of cytosine deaminase, cytosine deaminase can change the metabolism and sensitivity of tumor cells to cytosine drugs, thereby improving the efficacy and safety of chemotherapy.
The expression level of cytosine deaminase can be used as a prognostic indicator for tumor chemotherapy, that is, tumor cells or individuals with low expression of cytosine deaminase are more likely to have an effective anti-tumor effect on cytosine drugs, and vice versa. This is because the expression level of cytosine deaminase is negatively correlated with the metabolic rate and effective concentration of cytosine drugs, that is, tumor cells or individuals with high expression of cytosine deaminase have a fast metabolic rate and a low effective concentration of cytosine drugs, resulting in tumor cells Increased resistance to cytosine drugs. Therefore, the sensitivity and prognosis of tumor cells to cytosine drugs can be predicted by detecting the expression level of cytosine deaminase in tumor tissue or blood. Moreover, the expression level of cytosine deaminase can be used as an indicator for individualized treatment of tumor chemotherapy, that is, the dose, route, time, and frequency of cytosine drugs can be adjusted according to the expression level of cytosine deaminase to improve the effect and safety of chemotherapy. However, the possibility of cytosine deaminase expression level as an indicator of tumor chemotherapy prognosis or individualized treatment is affected by some factors. For example, the catalytic efficiency, stability, immunogenicity, and targeting of cytosine deaminase itself need to be improved to increase its activity and specificity in vivo. Moreover, the interaction and coordination between cytosine deaminase and other enzymes or factors need to be optimized to increase its activity and specificity in vivo. In addition, the safety and effectiveness of cytosine deaminase and other enzymes or factors need to be evaluated in order to reduce its toxic side effects on normal cells or tissues.
Expression and Activity Regulation of Cytosine Deaminase
In different cells and conditions, cytosine deaminase's gene transcription, protein translation, enzyme catalysis, protein interaction, and other processes are affected by many factors, thus changing its level and function in vivo. The gene transcription of cytosine deaminase is regulated by transcription factors, signal pathways, environmental pressure, and other factors. Also, protein translation of cytosine deaminase is regulated by factors such as RNA splicing, RNA editing, and RNA binding proteins. In addition, the enzymatic catalysis of cytosine deaminase is regulated by factors such as substrate specificity, substrate concentration, enzyme inhibitors, and enzyme activators. Besides, the protein interaction of cytosine deaminase is regulated by factors such as protein composition, protein modification, and protein distribution.
