Before human induced pluripotent stem (iPS) cells can be used to treat genetically inherited human disease, it will be necessary to develop methods of correcting disease-causing mutations that are compatible with clinical applications, combining efficiency with efficacy and leaving no residual sequences in the targeted genome. Yusa et al. present a proof-of-principle experiment demonstrating the complete genetic correction of a disease-causing mutation in patient-specific iPS cells. They use zinc finger nucleases and piggyBac technology to correction a point mutation in the α1-antitrypsin gene, which is responsible for α1-antitrypsin deficiency (A1ATD). The corrected iPS cells could efficiently differentiate to form hepatocyte-like cells and engraft into an animal model for liver injury without tumour formation. Human induced pluripotent stem cells (iPSCs) represent a unique opportunity for regenerative medicine because they offer the prospect of generating unlimited quantities of cells for autologous transplantation, with potential application in treatments for a broad range of disorders1,2,3,4. However, the use of human iPSCs in the context of genetically inherited human disease will require the correction of disease-causing mutations in a manner that is fully compatible with clinical applications3,5. The methods currently available, such as homologous recombination, lack the necessary efficiency and also leave residual sequences in the targeted genome6. Therefore, the development of new approaches to edit the mammalian genome is a prerequisite to delivering the clinical promise of human iPSCs. Here we show that a combination of zinc finger nucleases (ZFNs)7 and piggyBac8,9 technology in human iPSCs can achieve biallelic correction of a point mutation (Glu342Lys) in the α1-antitrypsin (A1AT, also known as SERPINA1) gene that is responsible for α1-antitrypsin deficiency. Genetic correction of human iPSCs restored the structure and function of A1AT in subsequently derived liver cells in vitro and in vivo. This approach is significantly more efficient than any other gene-targeting technology that is currently available and crucially prevents contamination of the host genome with residual non-human sequences. Our results provide the first proof of principle, to our knowledge, for the potential of combining human iPSCs with genetic correction to generate clinically relevant cells for autologous cell-based therapies.

Human induced pluripotent stem (iPS) cells hold great promise for advancements in developmental biology, cell-based therapy, and modeling of human disease. Here, we examined the use of human iPS cells for modeling inherited metabolic disorders of the liver. Dermal fibroblasts from patients with various inherited metabolic diseases of the liver were used to generate a library of patient-specific human iPS cell lines. Each line was differentiated into hepatocytes using what we believe to be a novel 3-step differentiation protocol in chemically defined conditions. The resulting cells exhibited properties of mature hepatocytes, such as albumin secretion and cytochrome P450 metabolism. Moreover, cells generated from patients with 3 of the inherited metabolic conditions studied in further detail (α₁-antitrypsin deficiency, familial hypercholesterolemia, and glycogen storage disease type 1a) were found to recapitulate key pathological features of the diseases affecting the patients from which they were derived, such as aggregation of misfolded α₁-antitrypsin in the endoplasmic reticulum, deficient LDL receptor–mediated cholesterol uptake, and elevated lipid and glycogen accumulation. Therefore, we report a simple and effective platform for hepatocyte generation from patient-specific human iPS cells. These patient-derived hepatocytes demonstrate that it is possible to model diseases whose phenotypes are caused by pathological dysregulation of key processes within adult cells.

Endoplasmic reticulum (ER) dysfunction might have an important part to play in a range of neurological disorders, including cerebral ischaemia, sleep apnoea, Alzheimer's disease, multiple sclerosis, amyotrophic lateral sclerosis, the prion diseases, and familial encephalopathy with neuroserpin inclusion bodies. Protein misfolding in the ER initiates the well studied unfolded protein response in energy-starved neurons during stroke, which is relevant to the toxic effects of reperfusion. The toxic peptide amyloid β induces ER stress in Alzheimer's disease, which leads to activation of similar pathways, whereas the accumulation of polymeric neuroserpin in the neuronal ER triggers a poorly understood ER-overload response. In other neurological disorders, such as Parkinson's and Huntington's diseases, ER dysfunction is well recognised but the mechanisms by which it contributes to pathogenesis remain unclear. By targeting components of these signalling responses, amelioration of their toxic effects and so the treatment of a range of neurodegenerative disorders might become possible.

Alpha1-antitrypsin is the most abundant circulating protease inhibitor. The severe Z deficiency allele (Glu342Lys) causes the protein to undergo a conformational transition and form ordered polymers that are retained within hepatocytes. This causes neonatal hepatitis, cirrhosis, and hepatocellular carcinoma. We have developed a conformation-specific monoclonal antibody (2C1) that recognizes the pathological polymers formed by α1-antitrypsin. This antibody was used to characterize the Z variant and a novel shutter domain mutant (His334Asp; α1-antitrypsin King's) identified in a 6-week-old boy who presented with prolonged jaundice. His334Asp α1-antitrypsin rapidly forms polymers that accumulate within the endoplasmic reticulum and show delayed secretion when compared to the wild-type M α1-antitrypsin. The 2C1 antibody recognizes polymers formed by Z and His334Asp α1-antitrypsin despite the mutations directing their effects on different parts of the protein. This antibody also recognized polymers formed by the Siiyama (Ser53Phe) and Brescia (Gly225Arg) mutants, which also mediate their effects on the shutter region of α1-antitrypsin. Conclusion: Z and shutter domain mutants of α1-antitrypsin form polymers with a shared epitope and so are likely to have a similar structure.

Point mutants of alpha1-antitrypsin (α1AT) form ordered polymers that are retained as inclusions within the endoplasmic reticulum (ER) of hepatocytes in association with neonatal hepatitis, cirrhosis, and hepatocellular carcinoma. These inclusions cause cell damage and predispose to ER stress in the absence of the classical unfolded protein response (UPR). The pathophysiology underlying this ER stress was explored by generating cell models that conditionally express wild-type (WT) α1AT, two mutants that cause polymer-mediated inclusions and liver disease (E342K [the Z allele] and H334D) and a truncated mutant (Null Hong Kong; NHK) that induces classical ER stress and is removed by ER-associated degradation. Expression of the polymeric mutants resulted in gross changes in the ER luminal environment that recapitulated the changes observed in liver sections from individuals with PI*ZZ α1AT deficiency. In contrast, expression of NHK α1AT caused electron lucent dilatation and expansion of the ER throughout the cell. Photobleaching microscopy in live cells demonstrated a decrease in the mobility of soluble luminal proteins in cells that express E342K and H334D α1AT, when compared to those that express WT and NHK α1AT (0.34 ± 0.05, 0.22 ± 0.03, 2.83 ± 0.30, and 2.84 ± 0.55 μm2/s, respectively). There was no effect on protein mobility within ER membranes, indicating that cisternal connectivity was not disrupted. Polymer expression alone was insufficient to induce the UPR, but the resulting protein overload rendered cells hypersensitive to ER stress induced by either tunicamycin or glucose depletion. Conclusion : Changes in protein diffusion provide an explanation for the cellular consequences of ER protein overload in mutants that cause inclusion body formation and α1AT deficiency.