The discovery of Yamanaka and his colleagues in 2006 revealed that there are a group of genes expressed in embryonic stem cells. He investigated that four transcription factors named Oct4, Sox2, Klf4 and c-Myc are enough to reprogram the fibroblasts of mouse into pluripotent stem cells which are called as induced pluripotent stem cells (iPSCs). Thompson and his group also worked on the reprogramming of cells into iPSCs with another set of factors called Oct4, Sox2, Lin28 and Nanog. The pluripotent cells have the ability to undergo a major change and differentiate into three germ layers. These cells have the capacity to develop into iPSC mouse by tetraploid complementation which specifies the totipotency of iPSCs. This review concentrates on the challenges of iPSC usage in clinical therapy rather than discussing on reprogramming mechanisms.
The iPSCs that are patient specific are obtained and differentiated into cell types with the similar genetic background as that of the donor. This helps the researchers to study about the pathogenesis In Vitro. It is a great challenge to study the pathogenesis of neurological disease because of the difficulty of the neuronal system and neuron culturing. The pathology of a neurological disease called spinal muscular atrophy was studied with human iPSCs. The iPSCs collected from the patient were used for generating the motor neurons possessing the same genotype, which is associated with certain deficiencies. These iPSCs are used for modeling the human disease.
There was another study where iPSCs were derived from patients with Rett syndrome and were not able to recapitulate the defects associated with the concerned disease. The iPSCs were used for testing the drug effects on the diseased model and in solving the defects at synapses. Huntington-Gilford progeria syndrome patients were used for deriving iPSCs from their body. These HGPS iPSCs were used for deriving smooth muscles which could recapitulate the premature senescence In Vitro. This study on HGPS iPSCs reveals the molecular mechanisms associated with HGPS.
The LRRK2 G2019-Smutation was found to be associated with Parkinson's disease (PD) and the nuclear envelope defects might also be involved in PD pathology. These aspects are used for the diagnosis and treatment of Parkinson's disease. The iPSCs collected from the long-QT syndrome patients were differentiated into functional cardiac myocytes that have recapitulated the electrophysiological defects that are characteristic of this disorder.
Two interesting studies revealed that 3-Dimensional liver and intestine can be generated from iPSCs. This indicates that iPSCs can be used for making functional organs In Vitro, in the regenerative medicine.
To make use of iPSC technology for therapy, extensive preclinical experiments are necessary to evaluate the safety and efficacy of the new type of therapy. The iPSCs were initially obtained by expressing the transcription factors with the help of retro virus or lenti virus. Later, reprogramming of mouse fibroblasts was performed by episomal or adenoviral vector transfection. A few studies insisted on direct production of iPSCs from mouse and human beings by inducing reprogramming factors into the target cells without making any changes in DNA.
Some of the methods for reprogramming the somatic cells into iPSCs can be done without any host genome integration. Recently, iPSCs were prepared from mouse cells by supplementing seven molecules into the cell culture indicating that human iPSCs can be generated for clinical therapy without any genetic changes. The integration free reprogramming technology helps in reducing the risks of cancer due to reprogramming factor integration. However, it is not established clearly that tumorigenesis is caused by reprogramming. Oct4, Sox2, Klf4 and c-Myc are the reprogramming factors found to enhance the malignant potential of ESC derived cells when they are over expressed. Some studies have revealed that Nanog expression can promote tumorigenesis and metastasis during reprogramming.
Though two studies have said about lack of immunogenicity or very low immunogenicity of iPSC derivatives, the researchers of these studies have accepted that a few tissue types differentiated from iPSCs are immunogenic. The cardiomyocytes differentiated from iPSCs can trigger immune rejection response. Another study by Guha reported that variation in immunogenicity was observed between ESC and iPSC derivatives.
Some important points have to be remembered about iPSC immunogenicity. They are
• Immune rejection response is stimulated by only a few and not by all the tissues derived from iPSCs.
• The immune rejection triggered by ESC derived allografts vary from the rejection response showed by autografts of iPSC origin. This is because MHC-I components are expressed in all the ESC derivatives that are allogenic while iPSC derivatives that are syngenic express few antigens.
• The autologous cell type if derived from iPSCs is considered as immunogenic, then it can trigger antigen induced rejection of the cells.
Array comparative genomic hybridization could identify subkaryotypic abnormalities in various iPSC lines. The genomic amplifications and deletions are revealed by comparative genomic hybridization analysis of iPSCs which indicates that DNA replication stress during reprogramming is triggered by oncogene.
Sub chromosomal copy number variations are observed in higher number in human iPSCs than in somatic cells as revealed by high resolution single nucleotide polymorphism analysis. Another study revealed that early passage iPSCs had more copy number variations than in fibroblasts, human iPSCs or human ESCs.
Xiao Lu, Tongbiao Zhao. Clinical therapy using iPSCs: Hopes and Challenges. Genomics Proteomics Bioinformatics 11 (2013), 294-298.
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