Can you imagine a world where doctors have the ability to replace disease-causing cells with healthy lab-created cells? This idea may seem like something from a sci-fi novel, but today it is possible with cellular reprogramming.
For a long time, the idea of correcting our cells and turning one cell into another was unthinkable. But science has progressed, and research has shown that this future is possible. We can now reprogram an adult cell into a different type of cell capable of performing new functions.
A breakthrough in the field occurred in 2007 when Yamanaka and Takahashi described a combination of genes that convert adult fibroblast cells into induced pluripotent stem cells (iPSCs). This discovery heralded a new paradigm in cellular reprogramming that has significant implications for disease therapies and regenerative medicine.
Since this seminal study, the field of cellular reprogramming has advanced significantly, with improvements to emerging strategies. In this article, we summarize the current state of the art and discuss potential clinical applications for cellular reprogramming.
Giving cells new identities:
Cellular conversions (reprogramming of one cell type into another) can be classified into major categories: dedifferentiation and transdifferentiation.
The first refers to the reversion of a committed or differentiated cell into a cell with greater developmental potential, such as a stem cell. This is the first generation of cell-reprogramming technology, which focused on reprogramming fibroblasts into iPSCs to generate a novel experimental tool for generating patient-specific cells for developmental studies. Dedifferentiation involves an intermediate step, the pluripotent stem cell, but clinical applications require subsequent redifferentiation of iPSCs into the desired cell type, which is inefficient and risky because transplanted undifferentiated iPSCs are tumorigenic.
Transdifferentiation, by contrast, refers to the conversion of one differentiated cell type into another, either within or across germ layers. The forced expression of lineage-specific transcription factors (TFs) creates a second-generation reprogramming paradigm, wherein fibroblasts are directly converted into somatic cells of the type needed for the patient. Recent studies have shown that it is possible to directly convert fibroblasts into many somatic cells, including neurons, neural precursors, cardiomyocytes, and hematopoietic cells. No intermediate step is required in direct cell reprogramming, thus greatly reducing the risk of cancer.
Using the plasticity of adult cells for cell reprogramming:
In most developed tissues, cellular identity is stable. In fact, maintenance of cellular identity is critical for tissue function. Imagine the chaos that would ensue if changes in cell differentiation status resulted in, for example, cardiomyocytes no longer contracting. This stability of cell identity is achieved by epigenetic regulation (i.e., modifications of chromatin or DNA), which leads to heritable patterns of expression of tissue-specific genes.
Nonetheless, cell identity can still be changed. An interesting early study described the transdifferentiation of the newt iris into a lens after surgical removal of the lens. The potential of fully mature adult cells to radically change their identity was discovered in the 1950s and 60s, when John Gurdon demonstrated that differentiated cells could be transformed into cells with the characteristics of a fertilized egg under extreme conditions of nuclear transplantation. Since these groundbreaking experiments, the magic of cellular plasticity has emerged and today enjoys great popularity as an approach among various studies seeking to develop new treatments.
Cell identity is fluid, and cells and tissues are more plastic. The idea of this fluidity means that cells change their identity (phenotype) and function to adapt to environmental changes. This process occurs under physiological conditions in response to stress or injury. The identity of each cell is defined by a specific gene expression program. This plasticity is interesting because it can express genes belonging to other cell identities at low levels. This “background noise” allows them to change their identity at a given time, and what was once “background noise” becomes the dominant genetic program.
Cell plasticity is critical for reprogramming strategies. Cells can adopt intermediate stages of identity as they undergo dedifferentiation or transdifferentiation. And all of these changes can be reversible. Moreover, plasticity can be driven by factors that induce a new identity or even by the loss of inhibitory factors that maintain the old identity.
Cellular reprogramming for understanding and treating human diseases:
The goal of cell reprogramming is to restore the structure and functionality of damaged organs and tissues. In terms of key early research, a 2008 study examined direct reprogramming in vivo by introducing TFs into mouse pancreatic exocrine cells to generate pancreatic β cells. These functional pancreatic β cells produced insulin and treated diabetes in a mouse model.
The cell reprogramming approach was used to convert scar-forming cardiac fibroblasts into iCMs in a mouse model of myocardial infarction, resulting in a reduction in scar size and improvement in cardiac function after cardiac injury. A similar strategy was used with endogenous astrocytes that were converted into neuroblasts as well as hepatocytes derived from hepatic myofibroblasts, which attenuated liver fibrosis.
Clinical trials are currently underway for cell therapies to restore important functional cell types lost in human disease, such as neurons in Parkinson’s disease and retinal pigment epithelium in age-related macular degeneration. However, before widespread clinical applications are possible, a detailed understanding of human cell types is required.
This problem has attracted significant research interest, with multiple international consortia founded to address it. In 2000, scientists at RIKEN in Japan spearheaded efforts to establish an atlas of human promoters and enhancers in disease regulation, cell fate, and pathology as part of the FANTOM project. In 2010, the US National Institutes of Health founded the genotype-tissue expression (GTEx) project, with the goal of establishing a comprehensive public resource to study tissue-specific gene expression and regulation. However, these early efforts used bulk data, thus missing key information on cellular heterogeneity.
More recently, numerous single-cell genomics consortia have emerged. For example, the Human Cell Atlas (HCA) was founded in 2016 by international scientists to create comprehensive reference maps of all human cells with the ultimate goal of diagnosing, monitoring, and treating human disease.
In tandem with these research incentives, startups have begun working on approaches to translate concepts of cell reprogramming into clinical practice.
For example, in 2011, Mogrify was formed. This startup company has developed its own set of platform technologies that use a systematic Big Data approach to direct cellular reprogramming. They generate functional cell types needed to transform therapies for various diseases in the areas of ophthalmology, immunotherapy for cancer, and pulmonary and metabolic diseases.
The ophthalmology therapy is already in proof-of-concept in vivo and awaiting a partner to conduct clinical trials, while the other approaches (immunotherapy, lung and metabolic diseases) are still undergoing in vitro studies. This pipeline is supported by two platforms. The first (Mogrify) focuses on predicting transcriptomics switches of any target cell type from any source of cell types, while the second (epiMogrify) identifies the epigenetically predicted factors required to control and maintain cell identity.
Asgard Therapeutics is another startup that focuses on the approach of cell reprogramming. Research into direct cell reprogramming to achieve an immune response based on dendritic cells (DCs) was started in 2015 at the University of Coimbra (Portugal), and in 2017, these scientists moved to Lund University (Sweden) and created Asgard Therapeutics to explore this application. They have filed a provisional patent application to protect their technology, TrojanDC. This technology is focused on direct cell reprogramming to discover novel approaches for cancer immunotherapy.
DCs are important to activate the immune system in the context of cancer; by using a DC-based strategy, scientists can force cancer cells to become immunogenic, unleashing the immune system against them. The idea is to use direct cell reprogramming as a strategy to generate immune cells and help our immune system to fight against cancer. Last year, Asgard raised €6 million in industry and VC-backed seed financing to continue developing its TrojanDC technology.
Barriers to cellular reprogramming: Are we close to clinical practice?
Despite the positive aspects of cell reprogramming, the technology still has a hurdle to overcome. Today, the best approach to reprogramming cells is to use a virus that executes the TFs necessary for reprogramming. This virus works like a Trojan horse, as the virus and TFs may increase the risk of cancer development, especially in iPS reprogramming.
Direct reprogramming can be used for this problem, which significantly reduces tumoral potency. However, the use of viruses may still pose a problem due to the possibility that the viral vector genome may integrate into the host genome and affect differentiation. A recent study has shown the potential of electroporation-based transfection for the transfer of TFs. This bulk electroporation creates pores under the influence of an electric field that allow TFs to enter the cell.
Another strategy under development is the so-called Tissue Nano Transfection, developed by researchers at Ohio State University. This process is based on direct cytosolic delivery of reprogramming factors into the outer membranes of cells through temporary channels. They use a chip that is loaded with the desired reprogramming factors and placed on the skin. A strong electric field is applied and the channels are opened. Then the desired genes are injected. The technique has already been successfully tested in two in vivo experiments.
Is this the last frontier to truly personalized medicine?
Reprogrammed cells provide patient-specific models for complex diseases. Although a constellation of factors (e.g., genetic background, environmental exposure, infectious history) influences disease onset and progression in complex diseases, cell reprogramming provides a radical new approach to understanding these conditions. By expanding the range of each donor’s genetic background, reprogrammed cells alter the ability to model disease at the cellular level, making it possible to study disease based on the cells that might be involved.
If we could reprogram our cells, we could develop a specific and personalized therapy for each person. But why is this so important? Just as each patient is unique, the same is true for each disease. We are not all healthy or all sick, as our immune system is always working to maintain homeostasis in our regular body functions. Our environment, our food, our water, our beliefs, and our lifestyles have a huge impact on disease and how our immune system fights it. Given this personalized story, cell reprogramming can be an excellent approach to personalized medicine.
What makes cellular reprogramming so important for personalized medicine is that it can be used both as an approach to understanding disease at the cellular level and as a therapy. As a diagnostic technology, it provides unparalleled insights into which cells might be affected in each person. Starting with fibroblasts from patients, we can reprogram them into the cells of interest and perform an individualized analysis. As a therapy, delivering reprogrammed cells to the site of disease can induce a more accurate and protective immune response in the patient. And all this is based on appreciating the individuality of a complex disease like cancer — making cellular reprogramming a therapy that is personalized both for each patient and each disease.
