An Overview of Patient-derived Tumor Xenograft (PDX) Model
|30.7.2020||Posted by tactical33 under Advertising & Marketing|
The patient-derived tumor xenograft model (PDTX) established by transplanting fresh tumor tissue of the patient into immune-deficient mice can better maintain the genetic characteristics and heterogeneity of the primary tumor. Its advantages are: the specimens used for transplantation are directly derived from human tumor tissue, which better retains the tumor stromal and stem cell components, so that the microenvironment of tumor growth is closer to the primary tumor; different PDX models reflect difference between the samples from patients. It is closer to the actual situation of the patient, and is suitable for the study of anti-tumor drugs and biomarkers; it can also provide a large number of specimens for the preservation and passage of tumor samples. Cell line-derived xenograft (CDX), after long-term in vitro cultivation, tumor cells lose their heterogeneity in order to adapt to the external environment, and the obtained experimental results tend to be homogenized. The PDX model has not been cultured in vitro, and has stably retained the genetic characteristics, histology and phenotype characteristics of the primary tumor, and the experimental results have better clinical predictability.
Basic characteristics of the PDX model
Inoculate human tumor specimens into mice, and the growing tumor tissues are continuously inoculated into the mice. The third-generation tumors can stably grow and more than 80% form tumors, indicating the PDX model is successfully established. The tumor formation time in vivo is no more than 12 weeks; the tumor model tissue is frozen in liquid nitrogen and transplanted into mice after resuscitation to grow stably. In order to maintain the morphology of the primary tumor, the number of passages in vivo should not exceed 10 generations.
Commonly used PDX model graft methods include: (1) Subcutaneous transplantation, tumors are generally limited to the growth of subcutaneous mass, rarely metastasis and spread, the success rate of transplantation is low, only about 25%. (2) Due to the abundant blood supply and good tissue compatibility, renal capsule is a commonly used transplantation method. Its success rate can reach more than 70%, and it can reach more than 95% in the transplantation of bladder cancer, colon cancer, and lung cancer. (3) Orthotopic transplantation is the transplantation of tumor tissue into the homologous tissue in mice, which has many influencing factors, low success rate, and difficulty in modeling. Especially for some digestive tract tumors, because they are in the digestive tract, the success rate of its transplantation is further reduced.
It is very important to confirm the tissue similarity between the PDX model and the primary tumor. The traceability methods include: (1) Histomorphological comparison, analysis of the morphology of the transplanted tumor and the primary tumor by methods such as histological type and Lauren classification, and judgment– Consistency. Also pay attention to whether the PDX model retains the microenvironment characteristics of the primary tumor. (2)Tumor specific marker analysis, human specific marker detection can determine the origin of the transplanted tumor, and then compare the consistency of the two. The expression of specific markers is closely related to the occurrence of a certain type of tumor, and it is also a potential molecular target to guide clinical diagnosis and treatment strategies, such as alpha fetoprotein (AFP), carcinoembryonic antigen (CEA), Prostate-specific antigen (PSA) and carbohydrate antigen 19-9 (CA19-9), etc. (3) Short tandem repeat (STR). STR sequences are widely present in mammalian genomes and have a high degree of polymorphism, usually consisting of 2~6 bases to form a core sequence, determined by the change in the number of repeats Polymorphism. For a specific individual, the repeating sequence and the number of repeating at a certain position of chromosome are fixed. Through testing, you can clearly distinguish individuals and determine kinship. (4) Sequencing analysis, through multiple gene exons and RNA sequencing, you can determine whether the genetic characteristics of the primary tumor have changed during the allogeneic growth and passage of the PDX model; you can also analyze the base mutation characteristics and alleles of the PDX model by sequencing Whether gene frequency and RNA expression level are related to the corresponding primary tumor.
Problems in the research of PDX model
- Transplant success rate.
To improve the success rate of the PDX model, it is necessary to focus on the following factors: (1) tumor: compared with other tumors, breast cancer and prostate cancer PDX model modeling is more difficult. High malignant degree, low degree of differentiation tumor transplantation have a higher success rate; the higher the number of transplanted tumor cells, the higher the success rate. (2) Host: knocking out some immune-related genes can obtain severe combined immunodeficiency mice, suitable for human-derived cell transplantation, especially NOD-Prdec*dI1rgo mice, which are the most immunodeficient mice to date, ensuring high success rate. (3)Technical factors: the freshness of the specimen and the time from the specimen to the transplantation are important factors affecting cell viability, and require the close cooperation of surgeons, histologists and researchers; the feeding environment and the degree of contamination of the specimen will also affect transplant success rate. Therefore, each sample needs to be transplanted with a large number of recipient mice, and it needs to be observed for more than 3 months to determine whether the model is successful.
Humans and rats are always two different species and cannot completely eliminate the differences between species. During transplantation, the mouse matrix will gradually replace the primary tumor matrix. During the passage of tumor tissue in mice,
It may also lead to individual gene changes that cannot accurately reflect the patient’s condition; due to the use of immunodeficiency animals in the PDX model, it is impossible to screen immunomodulatory drugs or drugs activated by immune function.
- Occurrence of lymphoma
In the PDX models of different types of tumors, there are individual cases of EBV Epstein-Barr virus (EBV), the highest occurrence in the PDX model of gastric cancer, and all models used were NOD/SCID or NOD mice. The reason for the analysis may be that EBV-infected B cells are in a latent state in the human body, but when transplanted into severely immunodeficient mice, due to the lack of functional immune cells, they are easily activated to form lymphomas. Because Helicobacter pylori-related gastritis is highly correlated with the occurrence of gastric cancer, the occurrence of basal inflammation in gastric cancer cases is significantly higher than that of other tumors, so the incidence of lymphoma in the PDX model of gastric cancer is significantly higher than other tumors.
- Selection of treatment strategies in drug evaluation
Because the PDX model represents the status of different patients, in the screening process, the dosage of chemotherapy drugs and the frequency of treatment need to be individualized and adjusted through pre-experiment; at the same time, the grouping, treatment timing, and treatment cycle of the PDX model before treatment, after drug withdrawal Observation time and other factors will affect the evaluation effect of drugs.
- Correctly evaluate and apply the PDX model
At present, the PDX model is widely used in the evaluation of new drugs and the screening of new tumor targets, reflecting good application prospects.
The establishment and application of the PDX model provide a good experimental platform for oncology research, especially the individual application of drugs. But it should be noted that while emphasizing the individualization of the tumor PDX model, It is necessary to carry out targeted therapy based on the genomic characteristics of the PDX model, improve the application efficiency of the model, and obtain more tumor subtypes and more efficient targeting options. At the same time, different departments and institutions should be encouraged to strengthen collaboration, establish a database of tumor xenograft models, and share information and research results of PDX models to benefit more patients.