When conducting medical research into organ-specific diseases, scientists often have to either retrospectively or prospectively recruit patients with the relevant medical conditions. However, all modern clinical trials must be approved by an ethics board, and some trials are simply deemed too risky to be conducted on human subjects.
In addition, any tests done on human subjects (and indeed, animal models too) can result in unexpected side effects that have little connection to the main area of study. This is because humans respond to any iatrogenic stimuli systemically; one cannot, for example, conduct experiments on the human liver in isolation while definitively preventing other organs, such as the heart and the gut, from responding in some fashion as well.
What if there was a way of growing in the lab a product that resembles a human organ and behaves like one, thus eliminating the need to conduct risky experiments on human subjects? Well, scientists have successfully developed organoids, which function just as described.
Prior and colleagues, in their study on the therapeutic applications of liver organoids, defined an organoid as “an in vitro 3D cellular cluster derived from tissue-resident stem/progenitor cells, embryonic stem cells or induced pluripotent stem cells capable of self-renewal and self-organization that recapitulates the functionality of the tissue of origin.”
Adding to Our Understanding
Caiazza and colleagues, in their study on the uses of liver organoids in disease modeling, echoed Prior et al on the role of organoids today: “In the last decades, several groups established 3D structures, defined organoids, obtained by culturing stem cells with an extracellular matrix (ECM), allowing a self-organization in a structure resembling structural and functional parameters of the tissue of interest.”
Liver organoids, for example, have extensive uses in helping us understand liver diseases. Let’s take ALGS, for example. Scientists have been able to generate liver organoids with an ALGS-like phenotype through the introduction of a disease-causing mutation in JAG1. These liver organoids are able to mirror the pathological features of ALGS, such as bile duct paucity and chronic cholestasis.
“Liver organoids derived from biopsies from patients with ALGS, or generated by differentiation from induced pluripotent stem cells from patients with ALGS, mirrored the in vivo biliary defects that characterize the disease,” Prior et al wrote.
Read more about ALGS etiology
The more liver organoids behave like the ALGS liver, the more valuable they become for scientists who want to conduct research into ALGS-related pathophysiology and treatment. Liver organoids derived from a mouse model with ALGS demonstrated a delay in their ability to differentiate into mature cholangiocytes and an inability to form and maintain bile ducts, which “are proof-of-principle that liver organoids recapitulated key features of the disease in vitro and enable further understanding of the disease processes,” wrote Prior and colleagues.
Liver organoids have also been used by scientists to conduct important research into primary liver cancers. Among the most prominent primary liver cancers are CCAs, hepatocellular carcinomas (HCCs), and a combination of both.
Before the availability of liver organoids, scientists largely conducted studies into primary liver cancers using 2D culture and mouse models. However, these methodologies are not without their drawbacks.
“2D cultures did not mirror the high grade of heterogeneity found in primary liver cancers as the clones with the most beneficial mutations prevail,” Caiazza et al wrote. “Patient-derived xenograft (PDX), in which cells from patients are transplanted in immunocompromised mice, are more suitable for primary liver cancer studies as they recapitulate the features of the original tumors.”
Read more about CCA etiology
Prior et al acknowledged the benefits of PDXs, while noting that some limitations remain. ”The PDX approach has been used with great success to study resistance mechanisms and test different therapeutics,” they wrote. ”However, while PDXs show great translational potential to direct treatment in a patient-tailored manner, this strategy has several drawbacks: PDXs are not amenable to large-scale drug screens, are costly, and can take a considerable amount of time to establish.”
An In Vivo Approach
A possible solution is to develop primary liver tumor organoids in vivo, which are known as tumoroids. There are certain conditions that need to be met for tumoroids to be successfully developed. Caiazza and colleagues explained that “the establishment of cancer organoids was successful only when the tumor was poorly differentiated and the proliferative index was high, excluding the possibility of deriving tumoroids from patients at early stages of the disease.”
However, once they have been successfully developed, tumoroids can be immensely helpful in allowing scientists to observe the behavior of liver cancer. Prior and colleagues reported, “Importantly, the tumoroids maintained tissue-of-origin features over time; global exome sequencing showed that over 90% of the genetic alterations in the patient’s tumor tissue were maintained in the respective tumoroids when in culture for less than 2 months and over 80% after 4 months.”
“Furthermore, tumoroids recapitulated the metastatic potential of the original tumor when transplanted into immunocompromised mice,” they added
Both Caiazza et al and Prior et al wrote about their optimism concerning the potential use of organoids in regenerative medicine in the future. One day, liver organoids could potentially be used in lieu of human liver transplants. Although much more research needs to be conducted before this becomes a routine reality, the possibilities that medical advancements like organoids open up are indeed endless.
Caiazza C, Parisi S, Caiazzo M. Liver organoids: updates on disease modeling and biomedical applications. Biology (Basel). Published online August 27, 2021. doi:10.3390/biology10090835
Prior N, Inacio P, Huch M. Liver organoids: from basic research to therapeutic applications. Gut. Published online July 12, 2019. doi:10.1136/gutjnl-2019-319256