Era of the 4D animal model

Revealing the entire dynamics of pathogenesis is critical for understanding, preventing and treating human disease but is limited by systematic clinical sampling. This drawback can be overcome with animal model studies. Recent advances in phenotyping, omics and bioinformatics technologies promote the development of the 4D animal model to simulate and digitally display the spatiotemporal landscapes of phenotypes and molecular dynamics in human diseases and reveal novel targets for diagnosis and therapy. In this commentary, the origin, supporting technologies, content, function and application, and advantages of 4D animal models over clinical studies and traditional animal models, as well as their limitations, are presented.


| LIMITATI ON OF CLINI C AL S TUD IE S
In clinical studies, the comparison of genomes from different patients with those of population controls is used to elucidate the genetic mechanisms of disease, and transcriptome-wide association analysis and colocalization are applied to infer the effect of gene expression on disease occurrence and severity. 1 Additionally, proteomic techniques targeting the expression of proteins can be used to investigate the cellular composition and spatial landscape of tissue pathology at single-cell resolution. 2 Moreover, advances in imaging technology have enabled the integration of time-resolution data and phenotypic 3-dimensional image information to provide a 4-dimensional (4D) image atlas at high resolution. 3 Well-established databases of human diseases, such as MalaCards, provide an integrated compendium of annotated disease data for disease representation and scrutiny. 4 Nevertheless, comprehensive sampling of internal organs from a sufficient number of patients following the time dimension of disease progression is usually not feasible. In addition, mapping the spatiotemporal landscape of disease in the clinic needs to exclude the interference of variance in genetic background, age, host microbiome and fundamental disease characteristics of the patient population, which will frequently affect the calculation of corresponding values as well as the continuity of the data curve along the timeline of disease progression. 5

| ADVANTAG E S OF A 4D ANIMAL MODEL
Assuming that the above functional aim is achieved, the application prospects and advantages of a 4D animal model are expected to be  important and extensive, especially in the following areas. First, we can systematically understand the nature of human disease based on the whole spatiotemporal landscape of molecular dynamics and phenotypes. 11 Second, we can screen a series of molecular targets related to early warning, pathogenesis, host immunity and repair during disease progression, which will provide comprehensive information on target selection for the development of diagnostic reagents, target drugs and drug combinations against multiple targets. 7 Third, comprehensive and standardized data will contribute to guiding the development and precise application of animal models to research on pathogenesis and vaccines as well as drug evaluation. 14 Fourth, repeated use of animal models for a specific human disease can fulfill the demands of a tremendous number of projects that overlap to a great extent in research content, which will prevent the unnecessary use of animals and improve animal welfare. 13 Finally, based on a 4D animal model, multiple data streams related to, for example, drug treatment, gene regulation and other interventions regarding phenotypes and molecular dynamics, will be integrated gradually, and then a 4D+ digital and artificial intelligent animal model can be generated via deep learning and applied to predict the effect of drug candidates. 15

| FAC TOR S INFLUEN CING THE DE S I G N AND DE VELOPMENT OF A 4D ANIMAL MODEL
Providing a 4D animal model is critical for future advances in medicine; however, its development requires significant funding, including funds for animal's purchase, phenotyping, omics analysis, bioinformatics calculations and website establishment. Therefore, each 4D animal model for a specific human disease should be carefully designed. Most importantly, it is essential to consider the similarity between the selected animal model and the corresponding human disease. Due to the genetic differences between humans and animals, as well as the diversity in microbiota, nutrition and other elements, animal models usually fail to simulate the natural state or full clinical manifestation of disease. 14,16,17 Misuse of animal models that are inconsistent with the key clinical factors related to pathogenesis or phenotypes associated with research and drug targets will lead to lower reproducibility of animal experimental results in clinical trials. 15,18,19 In addition to the simulation issues, it is equally important that 4D animal models are shared to avoid repeated construction and allow availability of complete phenotypic information. To achieve complete phenotyping of animal models, it is essential that To achieve this goal, several 4D animal models for human diseases should be developed as first attempts, and then any disadvantages of these models should be determined and the models should subsequently be optimized.

CO N FLI C T O F I NTE R E S T
The authors have no competing interests to declare.

E TH I C S S TATEM ENT
None.