br Modeling of genetic and complex
Modeling of genetic and complex liver diseases iPSCs can be generated directly from patients with specific diseases and their differentiation program allows to recapitulate in vitro the progression of the disease, thus making them an ideal tool to model liver diseases, better if genetic. Several groups have applied iPSCs technology to model liver diseases and have provided proof of principle of its efficacy in understanding the pathophysiology of a broad range of disorders. An example is the work from Rashid et al. in which hepatocytes are derived from iPSC of patients with different inherited metabolic disorders (i.e A1AT deficiency, familial hypercholesterolemia and Glycogen Storage Disease Type 1a (GSD1a)). In this work, the authors were able to reproduce in vitro key cellular aspects of the three different diseases such as the retention of polymers of A1AT in the ER, the impaired ability to incorporate LDL, and the accumulation of intracellular glycogen . In a different study, Zhang and colleagues demonstrate that iPSC-derived hepatocytes from a patient with Wilson's disease, present with altered cytoplasmic localization of the liver transporter protein ATP7B and defects in copper export, accurately reproducing the disease phenotype . As for the other epithelial component of the liver, three studies show that iPSC- derived cholangiocytes can model in vitro different genetic cholangiopathies. Sampaziotis et al. used iPSC-derived organoids from a patient with polycystic liver disease (PLD) to validate in vitro the effect of Ocreotide, a drug already used in clinic to reduce cyst size in PLD patients . Cystic fibrosis (CF) cholangiopathy is an autosomic recessive disease caused by mutation of the gene coding for Ko 143 transmembrane conductance regulator (CFTR), a chloride channel that regulates fluid secretion in cholangiocytes. Both Sampaziotis and Ogawa have shown that cholangiocytes derived from iPSC of a patient carrying the common CFTR mutation ΔF508 reproduce in vitro the secretory defect that can be partially corrected by the use of clinically approved small corrector molecules. More interestingly, in our recent study, using iPSC-derived polarized cholangiocyte monolayers to model human CF liver disease (ΔF508 mutation), we have identified aberrant innate immune pathways that in addition to the secretory defect, are crucial in the development of liver disease in CF . CF human cholangiocytes show an increase NF-kB activation in response to LPS followed by an aberrant production of inflammatory mediators. We have identified the persistent activation of the Src family of tyrosine kinases (SFK), as a pathogenetic event in the inflammatory response of CF cholangiocytes. Interestingly, inhibition of Src in combination with the FDA approved CFTR potentiators/correctors mentioned above, was able to recover several cholangiocyte funtions (i.e secretion, barrier function, changes in cytoskeletal structure) . This study is a proof that disease modeling by using iPSC represents the next generation tool to investigate pathogenetic mechanism of human diseases. More recently, Guan and colleagues have used 3D organoids composed of hepatocytes and cholangiocyte-like cells organized as a layer of epithelia surrounding the lumen of bile duct-like structures, to study the effect of Jagged1 gene mutations responsible for Alagille syndrome (ALGS) . ALGS cultures fail to develop tubular structures and mainly consist of vesicles lined by hepatocytes, recapitulating the disease phenotype. Moreover, also mRNA expression of NOTCH2 and its target genes Hey1 and HES1 is reduced while JAG1 mRNA decreases significantly during the different developmental stages . All the genetic diseases modeled so far are monogenic diseases. Complex liver disorders also represent a major challenge in the field, since not only the genetic component is involved but also environmental factors and multiple cell types can influence the disease phenotype. Primary biliary cirrhosis (PBC) and Primary Sclerosing Cholangitis (PSC) represent an example of complex liver disorders whose etiology and pathophysiology remain still unclear [41,52]. iPSCs-based systems could represent an optimal in vitro tool to study the complex cellular interactions in co-culture systems with different cells derived from the same patient (i.e immune and mesenchimal cells). In addition, the comparison with other cell culture systems (i.e 3D organoids) could help discriminate the impact of the genetic component versus its environmental counterpart on the pathogenesis of these diseases. Studies on PBC and PSC are eagerly awaited.