Hematopoietic stem cell gene therapy for the multisystemic lysosomal storage disorder cystinosis

Current Status and Prospects of Viral Vector-Based Gene Therapy to Treat Kidney Diseases

Inherited kidney diseases are among the leading causes of chronic kidney disease, reducing the quality of life and resulting in substantial socioeconomic impact. The advent of early genetic testing and the growing understanding of the molecular basis and pathophysiology of these disorders have opened avenues for novel treatment strategies. Viral vector-based gene therapies have evolved from experimental treatments for rare diseases to potent platforms that carry the intrinsic potential to provide a cure with a single application. Several gene therapy products have reached the market, and the numbers are only expected to increase. Still, none target inherited kidney diseases. Gene transfer to the kidney has lagged when compared to other tissue-directed therapies such as hepatic, neuromuscular, and ocular tissues. Systemic delivery of genetic information to tackle kidney disease is challenging. The pharma industry is taking steps to take on kidney disease and to translate the current research into the therapeutic arena. In this review, we provide an overview of the current viral vector-based approaches and their potential. We discuss advances in platforms and injection routes that have been explored to enhance gene delivery toward kidney cells in animal models, and how these can fuel the development of viable gene therapy products for humans.

Increasing β-hexosaminidase A activity using genetically modified mesenchymal stem cells

GM2 gangliosidoses are a group of autosomal-recessive lysosomal storage disorders. These diseases result from a deficiency of lysosomal enzyme β-hexosaminidase A (HexA), which is responsible for GM2 ganglioside degradation. HexA deficiency causes the accumulation of GM2-gangliosides mainly in the nervous system cells, leading to severe progressive neurodegeneration and neuroinflammation. To date, there is no treatment for these diseases. Cell-mediated gene therapy is considered a promising treatment for GM2 gangliosidoses. This study aimed to evaluate the ability of genetically modified mesenchymal stem cells (MSCs-HEXA-HEXB) to restore HexA deficiency in Tay-Sachs disease patient cells, as well as to analyze the functionality and biodistribution of MSCs in vivo. The effectiveness of HexA deficiency cross-correction was shown in mutant MSCs upon interaction with MSCs-HEXA-HEXB. The results also showed that the MSCs-HEXA-HEXB express the functionally active HexA enzyme, detectable in vivo, and intravenous injection of the cells does not cause an immune response in animals. These data suggest that genetically modified mesenchymal stem cells have the potentials to treat GM2 gangliosidoses.

Evaluation of the efficacy of cystinosin supplementation through CTNS mRNA delivery in experimental models for cystinosis

Messenger RNA (mRNA) therapies are emerging in different disease areas, but have not yet reached the kidney field. Our aim was to study the feasibility to treat the genetic defect in cystinosis using synthetic mRNA in cell models and ctns-/- zebrafish embryos. Cystinosis is a prototype lysosomal storage disorder caused by mutations in the CTNS gene, encoding the lysosomal cystine-H+ symporter cystinosin, and leading to cystine accumulation in all cells of the body. The kidneys are the first and the most severely affected organs, presenting glomerular and proximal tubular dysfunction, progressing to end-stage kidney failure. The current therapeutic standard cysteamine, reduces cystine levels, but has many side effects and does not restore kidney function. Here, we show that synthetic mRNA can restore lysosomal cystinosin expression following lipofection into CTNS-/- kidney cells and injection into ctns-/- zebrafish. A single CTNS mRNA administration decreases cellular cystine accumulation for up to 14 days in vitro. In the ctns-/- zebrafish, CTNS mRNA therapy improves proximal tubular reabsorption, reduces proteinuria, and restores brush border expression of the multi-ligand receptor megalin. Therefore, this proof-of-principle study takes the first steps in establishing an mRNA-based therapy to restore cystinosin expression, resulting in cystine reduction in vitro and in the ctns-/- larvae, and restoration of the zebrafish pronephros function.

Rescue of Alzheimer's disease phenotype in a mouse model by transplantation of wild-type hematopoietic stem and progenitor cells

Alzheimer's disease (AD) is the most prevalent cause of dementia; microglia have been implicated in AD pathogenesis, but their role is still matter of debate. Our study showed that single systemic wild-type (WT) hematopoietic stem and progenitor cell (HSPC) transplantation rescued the AD phenotype in 5xFAD mice and that transplantation may prevent microglia activation. Indeed, complete prevention of memory loss and neurocognitive impairment and decrease of β-amyloid plaques in the hippocampus and cortex were observed in the WT HSPC-transplanted 5xFAD mice compared with untreated 5xFAD mice and with mice transplanted with 5xFAD HSPCs. Neuroinflammation was also significantly reduced. Transcriptomic analysis revealed a significant decrease in gene expression related to "disease-associated microglia" in the cortex and "neurodegeneration-associated endothelial cells" in the hippocampus of the WT HSPC-transplanted 5xFAD mice compared with diseased controls. This work shows that HSPC transplant has the potential to prevent AD-associated complications and represents a promising therapeutic avenue for this disease.

The gene therapy for corneal pathology with novel nonsense cystinosis mouse lines created by CRISPR Gene Editing

PURPOSE

Cystinosis is an autosomal recessive lysosomal storage disease (LSDs) caused by mutations in the gene encoding cystinosin (CTNS) that leads to cystine crystal accumulation in the lysosome that compromises cellular functions resulting in tissue damage and organ failure, especially in kidneys and eyes. However, the underlying molecular mechanism of its pathogenesis remains elusive. Two novel mice lines created via CRISPR are used to examine the pathogenesis of cystinosis in the kidney and cornea and the treatment efficacy of corneal pathology using self-complimentary Adeno-associated viral (scAAV-CTNS) vector.

METHODS

The CRISPR technique generated two novel cystinotic mouse lines, Ctnsis1 (an insertional mutation) and Ctnsis2 (a nonsense mutation). Immune histochemistry, renal functions test and HRT2 in vivo confocal microscopy were used to evaluate the age-related renal pathogenesis and treatment efficacy of the scAAV-CTNS virus in corneal pathology.

RESULTS

Both mutations lead to the production of truncated Ctns proteins. Ctnsis1 and Ctnsis 2 mice exhibit the characteristic of cystinotic corneal crystal phenotype at four-week-old. Treatment with the scAAV-CTNS viral vector decreased the corneal crystals in the treated mice cornea. Ctnsis 1 show renal abnormalities manifested by increased urine volume, reduced urine osmolality, and the loss of response to Desmopressin (dDAVP) at 22-month-old but Ctnsis2 don't manifest renal pathology up to 2 years of age.

CONCLUSIONS

Both Ctnsis1 and Ctnsis2 mice exhibit phenotypes resembling human intermediate nephropathic and ocular cystinosis, respectively. scAAV-CTNS viral vectors reduce the corneal cystine crystals and have a great potential as a therapeutic strategy for treating patients suffering from cystinosis.

Cell and gene therapy for kidney disease

Kidney disease is a leading cause of morbidity and mortality across the globe. Current interventions for kidney disease include dialysis and renal transplantation, which have limited efficacy or availability and are often associated with complications such as cardiovascular disease and immunosuppression. There is therefore a pressing need for novel therapies for kidney disease. Notably, as many as 30% of kidney disease cases are caused by monogenic disease and are thus potentially amenable to genetic medicine, such as cell and gene therapy. Systemic disease that affects the kidney, such as diabetes and hypertension, might also be targetable by cell and gene therapy. However, although there are now several approved gene and cell therapies for inherited diseases that affect other organs, none targets the kidney. Promising recent advances in cell and gene therapy have been made, including in the kidney research field, suggesting that this form of therapy might represent a potential solution for kidney disease in the future. In this Review, we describe the potential for cell and gene therapy in treating kidney disease, focusing on recent genetic studies, key advances and emerging technologies, and we describe several crucial considerations for renal genetic and cell therapies.

Ex vivo gene therapy for lysosomal storage disorders: future perspectives

INTRODUCTION

Lysosomal storage disorders (LSD) are a group of monogenic rare diseases caused by pathogenic variants in genes that encode proteins related to lysosomal function. These disorders are good candidates for gene therapy for different reasons: they are monogenic, most of lysosomal proteins are enzymes that can be secreted and cross-correct neighboring cells, and small quantities of these proteins are able to produce clinical benefits in many cases. Ex vivo gene therapy allows for autologous transplant of modified cells from different sources, including stem cells and hematopoietic precursors.

AREAS COVERED

Here, we summarize the main gene therapy and genome editing strategies that are currently being used as ex vivo gene therapy approaches for lysosomal disorders, highlighting important characteristics, such as vectors used, strategies, types of cells that are modified and main results in different disorders.

EXPERT OPINION

Clinical trials are already ongoing, and soon approved therapies for LSD based on ex vivo gene therapy approaches should reach the market.

Genistein improves renal disease in a mouse model of nephropathic cystinosis: a comparison study with cysteamine

Cysteamine is currently the only therapy for nephropathic cystinosis. It significantly improves life expectancy and delays progression to end-stage kidney disease; however, it cannot prevent it. Unfortunately, compliance to therapy is often weak, particularly during adolescence. Therefore, finding better treatments is a priority in the field of cystinosis. Previously, we found that genistein, an isoflavone particularly enriched in soy, can revert part of the cystinotic cellular phenotype that is not sensitive to cysteamine in vitro. To test the effects of genistein in vivo, we fed 2-month-old wild-type and Ctns-/- female mice with either a control diet, a genistein-containing diet or a cysteamine-containing diet for 14 months. Genistein (160 mg/kg/day) did not affect the growth of the mice or hepatic functionality. Compared with untreated mice at 16 months, Ctns-/- mice fed with genistein had lower cystine concentrations in their kidneys, reduced formation of cystine crystals, a smaller number of LAMP1-positive structures and an overall better-preserved parenchymal architecture. Cysteamine (400 mg/kg/day) was efficient in reverting the lysosomal phenotype and in preventing the development of renal lesions. These preclinical data indicate that genistein ameliorates kidney injury resulting from cystinosis with no side effects. Genistein therapy represents a potential treatment to improve the outcome for patients with cystinosis.

[Cystinosis: From the gene identification to the first gene therapy clinical trial]

Cystinosis is an autosomal recessive metabolic disease characterized by lysosomal accumulation of cystine in all the cells of the body. Infantile cystinosis begins in infancy by a renal Fanconi syndrome and eventually leads to multi-organ failure, including the kidney, eye, thyroid, muscle, and pancreas, eventually causing premature death in early adulthood. The current treatment is the drug cysteamine that only delays the progression of the disease. We identified the gene involved, CTNS, and showed that the encoded protein, cystinosin, is a proton-driven cystine transporter. We generated a mouse model of cystinosis, the Ctns-/- mice, that recapitulates the main disease complications. The goal was next to develop a gene therapy approach for cystinosis. We used bone marrow stem cells as a vehicle to bring the healthy CTNS gene to tissues, and we showed that wild-type hematopoietic stem and progenitor cell (HSPC) transplantation led to abundant tissue integration of bone marrow-derived cells, significant decrease of tissue cystine accumulation and long-term kidney, eye and thyroid preservation. We then developed an autologous transplantation approach of HSPCs modified ex vivo using a lentiviral vector to introduce a functional CTNS cDNA, and showed its efficacy in Ctns-/- mice. We conducted the pharmacology/toxicology studies, developed the manufacturing process using human CD34+ cells, and design the clinical trial. We received Food and Drug Administration (FDA)-clearance to start a phase 1/2 clinical trial for cystinosis in December 2018. Six patients have been treated so far. In this review, we describe the path to go from the gene to a gene therapy approach for cystinosis.

Gene therapy for lysosomal storage diseases: Current clinical trial prospects

Lysosomal storage diseases (LSDs) are a group of metabolic inborn errors caused by defective enzymes in the lysosome, resulting in the accumulation of undegraded substrates. LSDs are progressive diseases that exhibit variable rates of progression depending on the disease and the patient. The availability of effective treatment options, including substrate reduction therapy, pharmacological chaperone therapy, enzyme replacement therapy, and bone marrow transplantation, has increased survival time and improved the quality of life in many patients with LSDs. However, these therapies are not sufficiently effective, especially against central nerve system abnormalities and corresponding neurological and psychiatric symptoms because of the blood-brain barrier that prevents the entry of drugs into the brain or limiting features of specific treatments. Gene therapy is a promising tool for the treatment of neurological pathologies associated with LSDs. Here, we review the current state of gene therapy for several LSDs for which clinical trials have been conducted or are planned. Several clinical trials using gene therapy for LSDs are underway as phase 1/2 studies; no adverse events have not been reported in most of these studies. The administration of viral vectors has achieved good therapeutic outcomes in animal models of LSDs, and subsequent human clinical trials are expected to promote the practical application of gene therapy for LSDs.

Hematopoietic stem and progenitors cells gene editing: Beyond blood disorders

Lessons learned from decades-long practice in the transplantation of hematopoietic stem and progenitor cells (HSPCs) to treat severe inherited disorders or cancer, have set the stage for the current ex vivo gene therapies using autologous gene-modified hematopoietic stem and progenitor cells that have treated so far, hundreds of patients with monogenic disorders. With increased knowledge of hematopoietic stem and progenitor cell biology, improved modalities for patient conditioning and with the emergence of new gene editing technologies, a new era of hematopoietic stem and progenitor cell-based gene therapies is poised to emerge. Gene editing has the potential to restore physiological expression of a mutated gene, or to insert a functional gene in a precise locus with reduced off-target activity and toxicity. Advances in patient conditioning has reduced treatment toxicities and may improve the engraftment of gene-modified cells and specific progeny. Thanks to these improvements, new potential treatments of various blood- or immune disorders as well as other inherited diseases will continue to emerge. In the present review, the most recent advances in hematopoietic stem and progenitor cell gene editing will be reported, with a focus on how this approach could be a promising solution to treat non-blood-related inherited disorders and the mechanisms behind the therapeutic actions discussed.

The chitinases as biomarkers in immune-mediate diseases

The role of chitinases has been focused as potential biomarkers in a wide number of inflammatory diseases, in monitoring active disease state, and predicting prognosis and response to therapies. The main chitinases, CHIT1 and YKL-40, are derived from 18 glycosyl hydrolases macrophage activation and play important roles in defense against chitin-containing pathogens and in food processing. Moreover, chitinases may have organ- as well as cell-specific effects in the context of infectious diseases and inflammatory disorders and able to induce tissue remodelling. The CHIT1 measurement is an easy, reproducible, reliable, and cost-effective affordable assay. The clinical use of CHIT1 for the screening of lysosomal storage disorders is quite practical, when proper cut-off values are determined for each laboratory. The potential of CHIT1 and chitinases has not been fully explored yet and future studies will produce many surprising discoveries in the immunology and allergology fields of research. However, since the presence of a null CHIT1 gene in a subpopulation would be responsible of false-negative values, the assay should be completed with the other markers such ACE and, if necessary, by genetic analysis when CHIT1 is unexpected low.

Hematopoietic Stem Cell Transplantation for Neurological Disorders: A Focus on Inborn Errors of Metabolism

Hematopoietic stem cells have been investigated and applied for the treatment of certain neurological disorders for a long time. Currently, their therapeutic potential is harnessed in autologous and allogeneic hematopoietic stem cell transplantation (HSCT). Autologous HSCT is helpful in immune-mediated neurological diseases such as Multiple Sclerosis. However, clinical benefits derive more from the immunosuppressive conditioning regimen than the interaction between stem cells and the nervous system. Mainly used for hematologic malignancies, allogeneic HSCT explores the therapeutic potential of donor-derived hematopoietic stem cells. In the neurological setting, it has proven to be most valuable in Inborn Errors of Metabolism, a large spectrum of multisystem disorders characterized by congenital deficiencies in enzymes involved in metabolic pathways. Inborn Errors of Metabolism such as X-linked Adrenoleukodystrophy present with brain accumulation of enzymatic substrates that result in progressive inflammatory demyelination. Allogeneic HSCT can halt ongoing inflammatory neural destruction by replacing hematopoietic-originated microglia with donor-derived myeloid precursors. Microglia, the only neural cells successfully transplanted thus far, are the most valuable source of central nervous system metabolic correction and play a significant role in the crosstalk between the brain and hematopoietic stem cells. After transplantation, engrafted donor-derived myeloid cells modulate the neural microenvironment by recapitulating microglial functions and enhancing repair mechanisms such as remyelination. In some disorders, additional benefits result from the donor hematopoietic stem cell secretome that cross-corrects neighboring neural cells via mannose-6-phosphatase paracrine pathways. The limitations of allogeneic HSCT in this setting relate to the slow turnover of microglia and complications such as graft-vs.-host disease. These restraints have accelerated the development of hematopoietic stem cell gene therapy, where autologous hematopoietic stem cells are collected, manipulated ex vivo to overexpress the missing enzyme, and infused back into the patient. With this cellular drug vehicle strategy, the brain is populated by improved cells and exposed to supraphysiological levels of the flawed protein, resulting in metabolic correction. This review focuses on the mechanisms of brain repair resulting from HSCT and gene therapy in Inborn Errors of Metabolism. A brief mention will also be made on immune-mediated nervous system diseases that are treated with this approach.

Promoter considerations in the design of lentiviral vectors for use in treating lysosomal storage diseases

More than 50 lysosomal storage diseases (LSDs) are associated with lysosomal dysfunctions with the frequency of 1:5,000 live births. As a result of missing enzyme activity, the lysosome dysfunction accumulates undegraded or partially degraded molecules, affecting the entire body. Most of them are life-threatening diseases where patients could die within the first or second decade of life. Approximately 20 LSDs have the approved treatments, which do not provide the cure for the disorder. Therefore, the delivery of missing genes through gene therapy is a promising approach for LSDs. Over the years, ex vivo lentiviral-mediated gene therapy for LSDs has been approached using different strategies. Several clinical trials for LSDs are under investigation.Ex vivo lentiviral-mediated gene therapy needs optimization in dose, time of delivery, and promoter-driven expression. Choosing suitable promoters seems to be one of the important factors for the effective expression of the dysfunctional enzyme. This review summarizes the research on therapy for LSDs that has used different lentiviral vectors, emphasizing gene promoters.

Development of a fibrin-mediated gene delivery system for the treatment of cystinosis via design of experiment

Cystinosis is a rare disease, caused by a mutation in the gene cystinosin and characterised by the accumulation of cystine crystals. Advantages of biomaterial-mediated gene delivery include reduced safety concerns and the possibility to cure organs that are difficult to treat using systemic gene transfer methods. This study developed novel fibrin hydrogels for controlled, localised gene delivery, for the treatment of cystinosis. In the first part, fabrication parameters (i.e., DNA, thrombin, and aprotinin concentrations) were optimised, using a Design of Experiment (DOE) methodology. DOE is a statistical engineering approach to process optimisation, which increases experimental efficiency, reduces the number of experiments, takes into consideration interactions between different parameters, and allows the creation of predictive models. This study demonstrated the utility of DOE to the development of gene delivery constructs. In the second part of the study, primary fibroblasts from a patient with cystinosis were seeded on the biomaterials. Seeded cells expressed the recombinant CTNS and showed a decrease in cystine content. Furthermore, conditioned media contained functional copies of the recombinant CTNS. These were taken up by monolayer cultures of non-transfected cells. This study described a methodology to develop gene delivery constructs by using a DOE approach and ultimately provided new insights into the treatment of cystinosis.

Case Report: Cystinosis in a Chinese Child With a Novel CTNS Pathogenic Variant

OBJECTIVE

To report a rare case of cystinosis with a novel CTNS pathogenic variant in the Chinese population.

METHODS

Retrospective analysis of the clinical manifestations, laboratory results, and gene detection data of a child with cystinosis.

RESULTS

A Chinese Zang ethnic girl could not stand or walk until 3 years old, with additional symptoms including a loss of appetite. Since then, the girl gradually exhibited "X" leg, double wrist joints, a bilateral ankle deformity, and rickets. At the age of 9 years, the girl was hospitalized. Laboratory testing showed that her blood phosphorus, blood calcium and blood potassium levels were significantly decreased. At the same time, the girl's urine glucose and urine protein were positive, although her fasting blood glucose, glycosylated hemoglobin, and 75 g glucose tolerance were not significantly abnormal. Further, blood gas analysis showed metabolic acidosis. These symptoms corresponded to Fanconi syndrome. Gene analysis showed that there was a homozygous pathogenic variant c.140 ≤ 5G > A (p.?) in the CTNS gene, which was a small variation in the intron region. To our knowledge, this is the first report of the rare variant.

CONCLUSION

Attention should be paid to the differential diagnosis of cystinosis by gene analysis in children whose clinical manifestations include exercise dysplasia, renal damage, or multiple organ damage (including bone, thyroid, etc) and who cannot be firmly diagnosed for the time being.

OUP accepted manuscript

Recessive mutations in the CTNS gene encoding the lysosomal transporter cystinosin cause cystinosis, a lysosomal storage disease leading to kidney failure and multisystem manifestations. A Ctns knockout mouse model recapitulates features of cystinosis, but the delayed onset of kidney manifestations, phenotype variability and strain effects limit its use for mechanistic and drug development studies. To provide a better model for cystinosis, we generated a Ctns knockout rat model using CRISPR/Cas9 technology. The Ctns^−/− rats display progressive cystine accumulation and crystal formation in multiple tissues including kidney, liver and thyroid. They show an early onset and progressive loss of urinary solutes, indicating generalized proximal tubule dysfunction, with development of typical swan-neck lesions, tubulointerstitial fibrosis and kidney failure, and decreased survival. The Ctns^−/− rats also present crystals in the cornea, and bone and liver defects, as observed in patients. Mechanistically, the loss of cystinosin induces a phenotype switch associating abnormal proliferation and dedifferentiation, loss of apical receptors and transporters, and defective lysosomal activity and autophagy in the cells. Primary cultures of proximal tubule cells derived from the Ctns^−/− rat kidneys confirmed the key changes caused by cystine overload, including reduced endocytic uptake, increased proliferation and defective lysosomal dynamics and autophagy. The novel Ctns^−/− rat model and derived proximal tubule cell system provide invaluable tools to investigate the pathogenesis of cystinosis and to accelerate drug discovery.

Functionality of a bicistronic construction containing HEXA and HEXB genes encoding β-hexosaminidase A for cell-mediated therapy of GM2 gangliosidoses

Tay-Sachs disease and Sandhoff disease are severe hereditary neurodegenerative disorders caused by a deficiency of β-hexosaminidase A (HexA) enzyme, which results in the accumulation of GM2 gangliosides in the nervous system cells. In this work, we analyzed the efficacy and safety of cell-mediated gene therapy for Sandhoff disease and Sandhoff disease using a bicistronic lentiviral vector encoding cDNA of HexA α- and β-subunit genes separated by the nucleotide sequence of a P2A peptide (HEXA-HEXB). The functionality of the bicistronic construct containing the HEXA-HEXB genetic cassette was analyzed in a culture of HEK293T cells and human umbilical cord blood mononuclear cells (hUCBMCs). Our results showed that the enzymatic activity of HexA in the conditioned medium harvested from genetically modified HEK293T-HEXA-HEXB and hUCBMCs-HEXA-HEXB was increased by 23 and 8 times, respectively, compared with the conditioned medium of native cells. Western blot analysis showed that hUCBMCs-HEXA-HEXB secreted both completely separated HEXA and HEXB proteins, and an uncleaved protein containing HEXA + HEXB linked by the P2A peptide. Intravenous injection of genetically modified hUCBMCs-HEXA-HEXB to laboratory Wistar rats was carried out, and the HexA enzymatic activity in the blood plasma of experimental animals, as well as the number of live cells of immune system organs (spleen, thymus, bone marrow, lymph nodes) were determined. A significant increase in the enzymatic activity of HexA in the blood plasma of laboratory rats on days 6 and 9 (by 2.5 and 3 times, respectively) after the administration of hUCBMCs-HEXA-HEXB was shown. At the same time, the number of live cells in the studied organs remained unchanged. Thus, the functionality of the bicistronic genetic construct encoding cDNA of the HEXA and HEXB genes separated by the nucleotide sequence of the P2A peptide was shown in vitro and in vivo. We hypothesize that due to the natural ability of hUCBMCs to overcome biological barriers, such a strategy can restore the activity of the missing enzyme in the central nervous system of patients with GM2 gangliosidoses. Based on the obtained data, it can be concluded that intravenous administration of hUCBMCs with HexA overexpression is a promising method of the therapy for GM2 gangliosidoses. The animal protocol was approved by the Animal Ethics Committee of the Kazan Federal University (No. 23) on June 30, 2020.

In Vitro and In Vivo Models to Study Nephropathic Cystinosis

The development over the past 50 years of a variety of cell lines and animal models has provided valuable tools to understand the pathophysiology of nephropathic cystinosis. Primary cultures from patient biopsies have been instrumental in determining the primary cause of cystine accumulation in the lysosomes. Immortalised cell lines have been established using different gene constructs and have revealed a wealth of knowledge concerning the molecular mechanisms that underlie cystinosis. More recently, the generation of induced pluripotent stem cells, kidney organoids and tubuloids have helped bridge the gap between in vitro and in vivo model systems. The development of genetically modified mice and rats have made it possible to explore the cystinotic phenotype in an in vivo setting. All of these models have helped shape our understanding of cystinosis and have led to the conclusion that cystine accumulation is not the only pathology that needs targeting in this multisystemic disease. This review provides an overview of the in vitro and in vivo models available to study cystinosis, how well they recapitulate the disease phenotype, and their limitations.

Deficiency of the sedoheptulose kinase (Shpk) does not alter the ability of hematopoietic stem cells to rescue cystinosis in the mouse model

Cystinosis is an autosomal recessive lysosomal storage disorder caused by mutations in the CTNS gene encoding the lysosomal cystine transporter, cystinosin, and leading to multi-organ degeneration including kidney failure. A clinical trial for cystinosis is ongoing to test the safety and efficacy of transplantation of autologous hematopoietic stem and progenitor cells (HSPCs) ex vivo gene-modified to introduce functional CTNS cDNA. Preclinical studies in Ctns-/- mice previously showed that a single HSPC transplantation led to significant tissue cystine decrease and long-term tissue preservation. The main mechanism of action involves the differentiation of the transplanted HSPCs into macrophages within tissues and transfer of cystinosin-bearing lysosomes to the diseased cells via tunneling nanotubes. However, a major concern was that the most common cystinosis-causing mutation in humans is a 57-kb deletion that eliminates not only CTNS but also the adjacent sedopheptulose kinase SHPK/CARKL gene encoding a metabolic enzyme that influences macrophage polarization. Here, we investigated if absence of Shpk could negatively impact the efficiency of transplanted HSPCs to differentiate into macrophages within tissues and then to prevent cystinosis rescue. We generated Shpk knockout mouse models and detected a phenotype consisting of perturbations in the pentose phosphate pathway (PPP), the metabolic shunt regulated by SHPK. Shpk-/- mice also recapitulated the urinary excretion of sedoheptulose and erythritol found in cystinosis patients homozygous for the 57-kb deletion. Transplantation of Shpk-/--HSPCs into Ctns-/- mice resulted in significant reduction in tissue cystine load and restoration of Ctns expression, as well as improved kidney architecture comparable to WT-HSPC recipients. Altogether, these data demonstrate that absence of SHPK does not alter the ability of HSPCs to rescue cystinosis, and then patients homozygous for the 57-kb deletion should benefit from ex vivo gene therapy and can be enrolled in the ongoing clinical trial. However, because of the limits inherent to animal models, outcomes of this patient population will be carefully compared to the other enrolled subjects.

Hematopoietic Stem Cell Gene Therapy for Cystinosis: From Bench-to-Bedside

Cystinosis is an autosomal recessive metabolic disease that belongs to the family of lysosomal storage disorders. The gene involved is the CTNS gene that encodes cystinosin, a seven-transmembrane domain lysosomal protein, which is a proton-driven cystine transporter. Cystinosis is characterized by the lysosomal accumulation of cystine, a dimer of cysteine, in all the cells of the body leading to multi-organ failure, including the failure of the kidney, eye, thyroid, muscle, and pancreas, and eventually causing premature death in early adulthood. The current treatment is the drug cysteamine, which is onerous and expensive, and only delays the progression of the disease. Employing the mouse model of cystinosis, using Ctns-/- mice, we first showed that the transplantation of syngeneic wild-type murine hematopoietic stem and progenitor cells (HSPCs) led to abundant tissue integration of bone marrow-derived cells, a significant decrease in tissue cystine accumulation, and long-term kidney, eye and thyroid preservation. To translate this result to a potential human therapeutic treatment, given the risks of mortality and morbidity associated with allogeneic HSPC transplantation, we developed an autologous transplantation approach of HSPCs modified ex vivo using a self-inactivated lentiviral vector to introduce a functional version of the CTNS cDNA, pCCL-CTNS, and showed its efficacy in Ctns-/- mice. Based on these promising results, we held a pre-IND meeting with the Food and Drug Administration (FDA) to carry out the FDA agreed-upon pharmacological and toxicological studies for our therapeutic candidate, manufacturing development, production of the GMP lentiviral vector, design Phase 1/2 of the clinical trial, and filing of an IND application. Our IND was cleared by the FDA on 19 December 2018, to proceed to the clinical trial using CD34+ HSPCs from the G-CSF/plerixafor-mobilized peripheral blood stem cells of patients with cystinosis, modified by ex vivo transduction using the pCCL-CTNS vector (investigational product name: CTNS-RD-04). The clinical trial evaluated the safety and efficacy of CTNS-RD-04 and takes place at the University of California, San Diego (UCSD) and will include up to six patients affected with cystinosis. Following leukapheresis and cell manufacturing, the subjects undergo myeloablation before HSPC infusion. Patients also undergo comprehensive assessments before and after treatment to evaluate the impact of CTNS-RD-04 on the clinical outcomes and cystine and cystine crystal levels in the blood and tissues for 2 years. If successful, this treatment could be a one-time therapy that may eliminate or reduce renal deterioration as well as the long-term complications associated with cystinosis. In this review, we will describe the long path from bench-to-bedside for autologous HSPC gene therapy used to treat cystinosis.

Clinical Development of Cell Therapies to Halt Lysosomal Storage Diseases: Results and Lessons Learned

Although cross-correction was discovered more than 50 years ago, and held the promise of drastically improving disease management, still no cure exists for lysosomal storage diseases (LSDs). Cell therapies hold the potential to halt disease progression: either a subset of autologous cells can be ex vivo/ in vivo transfected with the functional gene or allogenic wild type stem cells can be transplanted. However, majority of cell-based attempts have been ineffective, due to the difficulties in reversing neuronal symptomatology, in finding appropriate gene transfection approaches, in inducing immune tolerance, reducing the risk of graft versus host disease (GVHD) when allogenic cells are used and that of immune response when engineered viruses are administered, coupled with a limited secretion and uptake of some enzymes. In the last decade, due to advances in our understanding of lysosomal biology and mechanisms of cross-correction, coupled with progresses in gene therapy, ongoing pre-clinical and clinical investigations have remarkably increased. Even gene editing approaches are currently under clinical experimentation. This review proposes to critically discuss and compare trends and advances in cell-based and gene therapy for LSDs. Systemic gene delivery and transplantation of allogenic stem cells will be initially discussed, whereas proposed brain targeting methods will be then critically outlined.

Improving Molecular Therapy in the Kidney

Mutations in approximately 80 genes have been implicated as the cause of various genetic kidney diseases. However, gene delivery to kidney cells from the blood is inefficient because of the natural filtering functions of the glomerulus, and research into and development of gene therapy directed toward kidney disease has lagged behind as compared with hepatic, neuromuscular, and ocular gene therapy. This lack of progress is in spite of numerous genetic mouse models of human disease available to the research community and many vectors in existence that can theoretically deliver genes to kidney cells with high efficiency. In the past decade, several groups have begun to develop novel injection techniques in mice, such as retrograde ureter, renal vein, and direct subcapsular injections to help resolve the issue of gene delivery to the kidney through the blood. In addition, the ability to retarget vectors specifically toward kidney cells has been underutilized but shows promise. This review discusses how recent advances in gene delivery to the kidney and the field of gene therapy can leverage the wealth of knowledge of kidney genetics to work toward developing gene therapy products for patients with kidney disease.

Molecular Mechanisms and Treatment Options of Nephropathic Cystinosis

Nephropathic cystinosis is a severe, monogenic systemic disorder that presents early in life and leads to progressive organ damage, particularly affecting the kidneys. It is caused by mutations in the CTNS gene, which encodes the lysosomal transporter cystinosin, resulting in intralysosomal accumulation of cystine. Recent studies demonstrated that the loss of cystinosin is associated with disrupted autophagy dynamics, accumulation of distorted mitochondria, and increased oxidative stress, leading to abnormal proliferation and dysfunction of kidney cells. We discuss these molecular mechanisms driving nephropathic cystinosis. Further, we consider how unravelling molecular mechanisms supports the identification and development of new strategies for cystinosis by the use of small molecules, biologicals, and genetic rescue of the disease in vitro and in vivo.

Free sialic acid storage disorder: Progress and promise

Lysosomal free sialic acid storage disorder (FSASD) is an extremely rare, autosomal recessive, neurodegenerative, multisystemic disorder caused by defects in the lysosomal sialic acid membrane exporter SLC17A5 (sialin). SLC17A5 defects cause free sialic acid and some other acidic hexoses to accumulate in lysosomes, resulting in enlarged lysosomes in some cell types and 10-100-fold increased urinary excretion of free sialic acid. Clinical features of FSASD include coarse facial features, organomegaly, and progressive neurodegenerative symptoms with cognitive impairment, cerebellar ataxia and muscular hypotonia. Central hypomyelination with cerebellar atrophy and thinning of the corpus callosum are also prominent disease features. Around 200 FSASD cases are reported worldwide, with the clinical spectrum ranging from a severe infantile onset form, often lethal in early childhood, to a mild, less severe form with subjects living into adulthood, also called Salla disease. The pathobiology of FSASD remains poorly understood and FSASD is likely underdiagnosed. Known patients have experienced a diagnostic delay due to the rarity of the disorder, absence of routine urine sialic acid testing, and non-specific clinical symptoms, including developmental delay, ataxia and infantile hypomyelination. There is no approved therapy for FSASD. We initiated a multidisciplinary collaborative effort involving worldwide academic clinical and scientific FSASD experts, the National Institutes of Health (USA), and the FSASD patient advocacy group (Salla Treatment and Research [S.T.A.R.] Foundation) to overcome the scientific, clinical and financial challenges facing the development of new treatments for FSASD. We aim to collect data that incentivize industry to further develop, obtain approval for, and commercialize FSASD treatments. This review summarizes current aspects of FSASD diagnosis, prevalence, etiology, and disease models, as well as challenges on the path to therapeutic approaches for FSASD.

Gene Therapy for Lysosomal Storage Disorders: Ongoing Studies and Clinical Development

Rare monogenic disorders such as lysosomal diseases have been at the forefront in the development of novel treatments where therapeutic options are either limited or unavailable. The increasing number of successful pre-clinical and clinical studies in the last decade demonstrates that gene therapy represents a feasible option to address the unmet medical need of these patients. This article provides a comprehensive overview of the current state of the field, reviewing the most used viral gene delivery vectors in the context of lysosomal storage disorders, a selection of relevant pre-clinical studies and ongoing clinical trials within recent years.

Amino Acid Transport Defects in Human Inherited Metabolic Disorders

Amino acid transporters play very important roles in nutrient uptake, neurotransmitter recycling, protein synthesis, gene expression, cell redox balance, cell signaling, and regulation of cell volume. With regard to transporters that are closely connected to metabolism, amino acid transporter-associated diseases are linked to metabolic disorders, particularly when they involve different organs, cell types, or cell compartments. To date, 65 different human solute carrier (SLC) families and more than 400 transporter genes have been identified, including 11 that are known to include amino acid transporters. This review intends to summarize and update all the conditions in which a strong association has been found between an amino acid transporter and an inherited metabolic disorder. Many of these inherited disorders have been identified in recent years. In this work, the physiological functions of amino acid transporters will be described by the inherited diseases that arise from transporter impairment. The pathogenesis, clinical phenotype, laboratory findings, diagnosis, genetics, and treatment of these disorders are also briefly described. Appropriate clinical and diagnostic characterization of the underlying molecular defect may give patients the opportunity to avail themselves of appropriate therapeutic options in the future.

Macrophage polarization impacts tunneling nanotube formation and intercellular organelle trafficking

Tunneling nanotubes (TNTs) are cellular extensions enabling cytosol-to-cytosol intercellular interaction between numerous cell types including macrophages. Previous studies of hematopoietic stem and progenitor cell (HSPC) transplantation for the lysosomal storage disorder cystinosis have shown that HSPC-derived macrophages form TNTs to deliver cystinosin-bearing lysosomes to cystinotic cells, leading to tissue preservation. Here, we explored if macrophage polarization to either proinflammatory M1-like M(LPS/IFNγ) or anti-inflammatory M2-like M(IL-4/IL-10) affected TNT-like protrusion formation, intercellular transport and, ultimately, the efficacy of cystinosis prevention. We designed new automated image processing algorithms used to demonstrate that LPS/IFNγ polarization decreased bone marrow-derived macrophages (BMDMs) formation of protrusions, some of which displayed characteristics of TNTs, including cytoskeletal structure, 3D morphology and size. In contrast, co-culture of macrophages with cystinotic fibroblasts yielded more frequent and larger protrusions, as well as increased lysosomal and mitochondrial intercellular trafficking to the diseased fibroblasts. Unexpectedly, we observed normal protrusion formation and therapeutic efficacy following disruption of anti-inflammatory IL-4/IL-10 polarization in vivo by transplantation of HSPCs isolated from the Rac2-/- mouse model. Altogether, we developed unbiased image quantification systems that probe mechanistic aspects of TNT formation and function in vitro, while HSPC transplantation into cystinotic mice provides a complex in vivo disease model. While the differences between polarization cell culture and mouse models exemplify the oversimplicity of in vitro cytokine treatment, they simultaneously demonstrate the utility of our co-culture model which recapitulates the in vivo phenomenon of diseased cystinotic cells stimulating thicker TNT formation and intercellular trafficking from macrophages. Ultimately, we can use both approaches to expand the utility of TNT-like protrusions as a delivery system for regenerative medicine.

Gene therapy research for kidney diseases

A resurgence in the development of newer gene therapy systems has led to recent successes in the treatment of B cell cancers, retinal degeneration and neuromuscular atrophy. Gene therapy offers the ability to treat the patient at the root cause of their malady by restoring normal gene function and arresting the pathological progression of their genetic disease. The current standard of care for most genetic diseases is based upon the symptomatic treatment with polypharmacy while minimizing any potential adverse effects attributed to the off-target and drug-drug interactions on the target or other organs. In the kidney, however, the development of gene therapy modifications to specific renal cells has lagged far behind those in other organ systems. Some positive strides in the past few years provide continued enthusiasm to invest the time and effort in the development of new gene therapy vectors for medical intervention to treat kidney diseases. This mini-review will systematically describe the pros and cons of the most commonly tested gene therapy vector systems derived from adenovirus, retrovirus, and adeno-associated virus and provide insight about their potential utility as a therapy for various types of genetic diseases in the kidney.

Potential use of stem cells as a therapy for cystinosis

Cystinosis is an autosomal recessive metabolic disease that belongs to the family of lysosomal storage disorders (LSDs). Initial symptoms of cystinosis correspond to the renal Fanconi syndrome. Patients then develop chronic kidney disease and multi-organ failure due to accumulation of cystine in all tissue compartments. LSDs are commonly characterized by a defective activity of lysosomal enzymes. Hematopoietic stem and progenitor cell (HSPC) transplantation is a treatment option for several LSDs based on the premise that their progeny will integrate in the affected tissues and secrete the functional enzyme, which will be recaptured by the surrounding deficient cells and restore physiological activity. However, in the case of cystinosis, the defective protein is a transmembrane lysosomal protein, cystinosin. Thus, cystinosin cannot be secreted, and yet, we showed that HSPC transplantation can rescue disease phenotype in the mouse model of cystinosis. In this review, we are describing a different mechanism by which HSPC-derived cells provide cystinosin to diseased cells within tissues, and how HSPC transplantation could be an effective one-time treatment to treat cystinosis but also other LSDs associated with a lysosomal transmembrane protein dysfunction.

Are cell-based therapies for kidney disease safe? A systematic review of preclinical evidence

The number of individuals affected by acute kidney injury (AKI) and chronic kidney disease (CKD) is constantly rising. In light of the limited availability of treatment options and their relative inefficacy, cell based therapeutic modalities have been studied. However, not many efforts are put into safety evaluation of such applications. The aim of this study was to review the existing published literature on adverse events reported in studies with genetically modified cells for treatment of kidney disease. A systematic review was conducted by searching PubMed and EMBASE for relevant articles published until June 2018. The search results were screened and relevant articles selected using pre-defined criteria, by two researchers independently. After initial screening of 6894 abstracts, a total number of 97 preclinical studies was finally included for full assessment. Of these, 61 (63%) presented an inappropriate study design for the evaluation of safety parameters. Only 4 studies (4%) had the optimal study design, while 32 (33%) showed sub-optimal study design with either direct or indirect evidence of adverse events. The high heterogeneity of studies included regarding cell type and number, genetic modification, administration route, and kidney disease model applied, combined with the consistent lack of appropriate control groups, makes a reliable safety evaluation of kidney cell-based therapies impossible. Only a limited number of relevant studies included looked into essential safety-related outcomes, such as inflammatory (48%), tumorigenic and teratogenic potential (12%), cell biodistribution (82%), microbiological safety with respect to microorganism contamination and latent viruses' reactivation (1%), as well as overall well-being and animal survival (19%). In conclusion, for benign cell-based therapies, well-designed pre-clinical studies, including all control groups required and good manufacturing processes securing safety, need to be done early in development. Preferably, this should be performed side by side with efficacy evaluation and according to the official guidelines of leading health organizations.

Nephropathic Cystinosis: Symptoms, Treatment, and Perspectives of a Systemic Disease

Cystinosis is a rare autosomal recessive lysosomal storage disorder caused by mutations in the CTNS gene. Main dysfunction is a defective clearance of cystine from lysosomes that leads to accumulation of cystine crystals in every tissue of the body. There are three different forms: infantile nephropathic cystinosis, which is the most common form, juvenile nephropatic, and non-nephropathic cystinosis. Mostly, first symptom in infantile nephropathic cystinosis is renal Fanconi syndrome that occurs within the first year of life. Another prominent symptom is photophobia due to corneal crystal deposition. Cystine depletion therapy with cysteamine delays end-stage renal failure but does not stop progression of the disease. A new cysteamine formulation with delayed-release simplifies the administration schedule but still does not cure cystinosis. Even long-term depletion treatment resulting in bypassing the defective lysosomal transporter cannot reverse Fanconi syndrome. A future perspective offering a curative therapy may be transplantation of CTNS-carrying stem cells that has successfully been performed in mice.

Transplantation of wild-type mouse hematopoietic stem and progenitor cells ameliorates deficits in a mouse model of Friedreich's ataxia

Friedreich's ataxia (FRDA) is an incurable autosomal recessive neurodegenerative disease caused by reduced expression of the mitochondrial protein frataxin due to an intronic GAA-repeat expansion in the FXN gene. We report the therapeutic efficacy of transplanting wild-type mouse hematopoietic stem and progenitor cells (HSPCs) into the YG8R mouse model of FRDA. In the HSPC-transplanted YG8R mice, development of muscle weakness and locomotor deficits was abrogated as was degeneration of large sensory neurons in the dorsal root ganglia (DRGs) and mitochondrial capacity was improved in brain, skeletal muscle, and heart. Transplanted HSPCs engrafted and then differentiated into microglia in the brain and spinal cord and into macrophages in the DRGs, heart, and muscle of YG8R FRDA mice. We observed the transfer of wild-type frataxin and Cox8 mitochondrial proteins from HSPC-derived microglia/macrophages to FRDA mouse neurons and muscle myocytes in vivo. Our results show the HSPC-mediated phenotypic rescue of FRDA in YG8R mice and suggest that this approach should be investigated further as a strategy for treating FRDA.

Lysosomal enzyme replacement therapies: Historical development, clinical outcomes, and future perspectives

Lysosomes and lysosomal enzymes play a central role in numerous cellular processes, including cellular nutrition, recycling, signaling, defense, and cell death. Genetic deficiencies of lysosomal components, most commonly enzymes, are known as "lysosomal storage disorders" or "lysosomal diseases" (LDs) and lead to lysosomal dysfunction. LDs broadly affect peripheral organs and the central nervous system (CNS), debilitating patients and frequently causing fatality. Among other approaches, enzyme replacement therapy (ERT) has advanced to the clinic and represents a beneficial strategy for 8 out of the 50-60 known LDs. However, despite its value, current ERT suffers from several shortcomings, including various side effects, development of "resistance", and suboptimal delivery throughout the body, particularly to the CNS, lowering the therapeutic outcome and precluding the use of this strategy for a majority of LDs. This review offers an overview of the biomedical causes of LDs, their socio-medical relevance, treatment modalities and caveats, experimental alternatives, and future treatment perspectives.

Worldwide view of nephropathic cystinosis: results from a survey from 30 countries

BACKGROUND

Nephropathic cystinosis is a rare inherited metabolic disorder leading to progressive renal failure and extra-renal comorbidity. The prognosis relies on early adherence to cysteamine treatment and symptomatic therapies. Developing nations [DiN] experience many challenges for management of cystinosis. The aim of this study was to assess the management characteristics in DiN compared with developed nations [DeN].

METHODS

A questionnaire was sent between April 2010 and May 2011 to 87 members of the International Pediatric Nephrology Association, in 50 countries.

RESULTS

A total of 213 patients were included from 41 centres in 30 nations (109 from 17 DiN and 104 from 13 DeN). 7% of DiN patients died at a median age of 5 years whereas no death was observed in DeN. DiN patients were older at the time of diagnosis. In DiN, leukocyte cystine measurement was only available in selected cases for diagnosis but never for continuous monitoring. More patients had reached end-stage renal disease in DiN (53.2 vs. 37.9%, p = 0.03), within a shorter time of evolution (8 vs. 10 yrs., p = 0.0008). The earlier the cysteamine treatment, the better the renal outcome, since the median renal survival increased up to 16.1 [12.5-/] yrs. in patients from DeN treated before the age of 2.5 years of age (p = 0.0001). However, the renal survival was not statistically different between DeN and DiN when patients initiated cysteamine after 2.5 years of age. The number of transplantations and the time from onset of ESRD to transplantation were not different in DeN and DiN. More patients were kept under maintenance dialysis in DiN (26% vs.19%, p = 0.02); 79% of patients from DiN vs. 45% in DeN underwent peritoneal dialysis.

CONCLUSIONS

Major discrepancies between DiN and DeN in the management of nephropathic cystinosis remain a current concern for many patients living in countries with limited financial resources.

Cystinosin, the small GTPase Rab11, and the Rab7 effector RILP regulate intracellular trafficking of the chaperone-mediated autophagy receptor LAMP2A

The lysosomal storage disease cystinosis, caused by cystinosin deficiency, is characterized by cell malfunction, tissue failure, and progressive renal injury despite cystine-depletion therapies. Cystinosis is associated with defects in chaperone-mediated autophagy (CMA), but the molecular mechanisms are incompletely understood. Here, we show CMA substrate accumulation in cystinotic kidney proximal tubule cells. We also found mislocalization of the CMA lysosomal receptor LAMP2A and impaired substrate translocation into the lysosome caused by defective CMA in cystinosis. The impaired LAMP2A trafficking and localization were rescued either by the expression of wild-type cystinosin or by the disease-associated point mutant CTNS-K280R, which has no cystine transporter activity. Defective LAMP2A trafficking in cystinosis was found to associate with decreased expression of the small GTPase Rab11 and the Rab7 effector RILP. Defective Rab11 trafficking in cystinosis was rescued by treatment with small-molecule CMA activators. RILP expression was restored by up-regulation of the transcription factor EB (TFEB), which was down-regulated in cystinosis. Although LAMP2A expression is independent of TFEB, TFEB up-regulation corrected lysosome distribution and lysosomal LAMP2A localization in Ctns^-/- cells but not Rab11 defects. The up-regulation of Rab11, Rab7, or RILP, but not its truncated form RILP-C33, rescued LAMP2A-defective trafficking in cystinosis, whereas dominant-negative Rab11 or Rab7 impaired LAMP2A trafficking. Treatment of cystinotic cells with a CMA activator increased LAMP2A localization at the lysosome and increased cell survival. Altogether, we show that LAMP2A trafficking is regulated by cystinosin, Rab11, and RILP and that CMA up-regulation is a potential clinically relevant mechanism to increase cell survival in cystinosis.

Hematopoietic Stem Cell Gene Therapy for Storage Disease: Current and New Indications

Lysosomal storage disorders (LSDs) are a broad class of monogenic diseases with an overall incidence of 1:7,000 newborns, due to the defective activity of one or more lysosomal hydrolases or related proteins resulting in storage of un-degraded substrates in the lysosomes. The over 40 different known LSDs share a life-threatening nature, but they are present with extremely variable clinical manifestations, determined by the characteristics and tissue distribution of the material accumulating due to the lysosomal dysfunction. The majority of LSDs lack a curative treatment. This is particularly true for LSDs severely affecting the CNS. Based on current preclinical and clinical evidences, among other treatment modalities, hematopoietic stem cell gene therapy could potentially result in robust therapeutic benefit for LSD patients, with particular indication for those characterized by severe brain damage. Optimization of current approaches and technology, as well as implementation of clinical trials for novel indications, and prolonged and more extensive follow-up of the already treated patients will allow translating this promise into new medicinal products.

Nephropathic cystinosis: an update

PURPOSE OF REVIEW

Over the past few decades, cystinosis, a rare lysosomal storage disorder, has evolved into a treatable metabolic disease. The increasing understanding of its pathophysiology has made cystinosis a prototype disease, delivering new insights into several fundamental biochemical and cellular processes.

RECENT FINDINGS

In this review, we aim to provide an overview of the latest advances in the pathogenetic, clinical, and therapeutic aspects of cystinosis.

SUMMARY

The development of alternative therapeutic monitoring strategies and new systemic and ocular cysteamine formulations might improve outcome of cystinosis patients in the near future. With the dawn of stem cell based therapy and new emerging gene-editing technologies, novel tools have become available in the search for a cure for cystinosis.

Bone marrow transplantation improves proximal tubule dysfunction in a mouse model of Dent disease

Dent disease is a rare X-linked tubulopathy caused by mutations in the endosomal chloride-proton exchanger (ClC-5) resulting in defective receptor-mediated endocytosis and severe proximal tubule dysfunction. Bone marrow transplantation has recently been shown to preserve kidney function in cystinosis, a lysosomal storage disease causing proximal tubule dysfunction. Here we test the effects of bone marrow transplantation in Clcn5^Y/- mice, a faithful model for Dent disease. Transplantation of wild-type bone marrow in Clcn5^Y/- mice significantly improved proximal tubule dysfunction, with decreased low-molecular-weight proteinuria, glycosuria, calciuria, and polyuria four months after transplantation, compared to Clcn5^Y/- mice transplanted with ClC-5 knockout bone marrow. Bone marrow-derived cells engrafted in the interstitium, surrounding proximal tubule cells, which showed a rescue of the apical expression of ClC-5 and megalin receptors. The improvement of proximal tubule dysfunction correlated with Clcn5 gene expression in kidneys of mice transplanted with wild-type bone marrow cells. Coculture of Clcn5^Y/- proximal tubule cells with bone marrow-derived cells confirmed rescue of ClC-5 and megalin, resulting in improved endocytosis. Nanotubular extensions between the engrafted bone marrow-derived cells and proximal tubule cells were observed in vivo and in vitro. No rescue was found when the formation of the tunneling nanotubes was prevented by actin depolymerization or when cells were physically separated by transwell inserts. Thus, bone marrow transplantation may rescue the epithelial phenotype due to an inherited endosomal defect. Direct contacts between bone marrow-derived cells and diseased tubular cells play a key role in the rescue mechanism.

The renal Fanconi syndrome in cystinosis: pathogenic insights and therapeutic perspectives

Cystinosis is an autosomal recessive metabolic disease that belongs to the family of lysosomal storage disorders. It is caused by a defect in the lysosomal cystine transporter, cystinosin, which results in an accumulation of cystine in all organs. Despite the ubiquitous expression of cystinosin, a renal Fanconi syndrome is often the first manifestation of cystinosis, usually presenting within the first year of life and characterized by the early and severe dysfunction of proximal tubule cells, highlighting the unique vulnerability of this cell type. The current therapy for cystinosis, cysteamine, facilitates lysosomal cystine clearance and greatly delays progression to kidney failure but is unable to correct the Fanconi syndrome. This Review summarizes decades of studies that have fostered a better understanding of the pathogenesis of the renal Fanconi syndrome associated with cystinosis. These studies have unraveled some of the early molecular changes that occur before the onset of tubular atrophy and identified a role for cystinosin beyond cystine transport, in endolysosomal trafficking and proteolysis, lysosomal clearance, autophagy and the regulation of energy balance. These studies have also led to the identification of new potential therapeutic targets and here, we outline the potential role of stem cell therapy for cystinosis and provide insights into the mechanism of haematopoietic stem cell-mediated kidney protection.

Cystinosis: a review

Cystinosis is the most common hereditary cause of renal Fanconi syndrome in children. It is an autosomal recessive lysosomal storage disorder caused by mutations in the CTNS gene encoding for the carrier protein cystinosin, transporting cystine out of the lysosomal compartment. Defective cystinosin function leads to intra-lysosomal cystine accumulation in all body cells and organs. The kidneys are initially affected during the first year of life through proximal tubular damage followed by progressive glomerular damage and end stage renal failure during mid-childhood if not treated. Other affected organs include eyes, thyroid, pancreas, gonads, muscles and CNS. Leucocyte cystine assay is the cornerstone for both diagnosis and therapeutic monitoring of the disease. Several lines of treatment are available for cystinosis including the cystine depleting agent cysteamine, renal replacement therapy, hormonal therapy and others; however, no curative treatment is yet available. In the current review we will discuss the most important clinical features of the disease, advantages and disadvantages of the current diagnostic and therapeutic options and the main topics of future research in cystinosis.

Hematopoietic Stem Cells Transplantation Can Normalize Thyroid Function in a Cystinosis Mouse Model

Hypothyroidism is the most frequent and earliest endocrine complication in cystinosis, a multisystemic lysosomal storage disease caused by defective transmembrane cystine transporter, cystinosin (CTNS gene). We recently demonstrated in Ctns(-/-) mice that altered thyroglobulin biosynthesis associated with endoplasmic reticulum stress, combined with defective lysosomal processing, caused hypothyroidism. In Ctns(-/-) kidney, hematopoietic stem cell (HSC) transplantation provides long-term functional and structural protection. Tissue repair involves transfer of cystinosin-bearing lysosomes from HSCs differentiated as F4/80 macrophages into deficient kidney tubular cells, via tunneling nanotubes that cross basement laminae. Here we evaluated the benefit of HSC transplantation for cystinotic thyroid and investigated the underlying mechanisms. HSC engraftment in Ctns(-/-) thyroid drastically decreased cystine accumulation, normalized the TSH level, and corrected the structure of a large fraction of thyrocytes. In the thyroid microenvironment, HSCs differentiated into a distinct, mixed macrophage/dendritic cell lineage expressing CD45 and major histocompatibility complex II but low CD11b and F4/80. Grafted HSCs closely apposed to follicles and produced tunneling nanotube-like extensions that crossed follicular basement laminae. HSCs themselves further squeezed into follicles, allowing extensive contact with thyrocytes, but did not transdifferentiate into Nkx2.1-expressing cells. Our observations revealed significant differences of basement lamina porosity between the thyroid and kidney and/or intrinsic macrophage invasive properties once in the thyroid microenvironment. The contrast between extensive thyrocyte protection and low HSC abundance at steady state suggests multiple sequential encounters and/or remanent impact. This is the first report demonstrating the potential of HSC transplantation to correct thyroid disease and supports a major multisystemic benefit of stem cell therapy for cystinosis.

Treatment of Inherited Eye Defects by Systemic Hematopoietic Stem Cell Transplantation

PURPOSE

Cystinosis is caused by a deficiency in the lysosomal cystine transporter, cystinosin (CTNS gene), resulting in cystine crystal accumulation in tissues. In eyes, crystals accumulate in the cornea causing photophobia and eventually blindness. Hematopoietic stem progenitor cells (HSPCs) rescue the kidney in a mouse model of cystinosis. We investigated the potential for HSPC transplantation to treat corneal defects in cystinosis.

METHODS

We isolated HSPCs from transgenic DsRed mice and systemically transplanted irradiated Ctns-/- mice. A year posttransplantation, we investigated the fate and function of HSPCs by in vivo confocal and fluorescence microscopy (IVCM), quantitative RT-PCR (RT-qPCR), mass spectrometry, histology, and by measuring the IOP. To determine the mechanism by which HSPCs may rescue disease cells, we transplanted Ctns-/- mice with Ctns-/- DsRed HSPCs virally transduced to express functional CTNS-eGFP fusion protein.

RESULTS

We found that a single systemic transplantation of wild-type HSPCs prevented ocular pathology in the Ctns-/- mice. Engraftment-derived HSPCs were detected within the cornea, and also in the sclera, ciliary body, retina, choroid, and lens. Transplantation of HSPC led to substantial decreases in corneal cystine crystals, restoration of normal corneal thickness, and lowered IOP in mice with high levels of donor-derived cell engraftment. Finally, we found that HSPC-derived progeny differentiated into macrophages, which displayed tunneling nanotubes capable of transferring cystinosin-bearing lysosomes to diseased cells.

CONCLUSIONS

To our knowledge, this is the first demonstration that HSPCs can rescue hereditary corneal defects, and supports a new potential therapeutic strategy for treating ocular pathologies.

Gene therapy for lysosomal storage disorders: a good start

Lysosomal storage disorders (LSDs) are a heterogeneous group of inherited diseases with a collective frequency of ∼1 in 7000 births, resulting from the deficiency in one or more enzymes or transporters that normally reside within the lysosomes. Pathology results from the progressive accumulation of uncleaved lipids, glycoproteins and/or glycosaminoglycans in the lysosomes and secondary damages that affect the brain, viscera, bones and connective tissues. Most treatment modalities developed for LSD, including gene therapy (GT), are based on the lysosome-specific cross-correction mechanism, by which close proximity of normal cells leads to the correction of the biochemical consequences of enzymatic deficiency within the neighboring cells. Here, GT efforts addressing these disorders are reviewed with an up-to-date discussion of their impact on the LSD disease phenotype in animal models and patients.

Brief reports: Lysosomal cross-correction by hematopoietic stem cell-derived macrophages via tunneling nanotubes

Despite controversies on the potential of hematopoietic stem cells (HSCs) to promote tissue repair, we previously showed that HSC transplantation could correct cystinosis, a multisystemic lysosomal storage disease, caused by a defective lysosomal membrane cystine transporter, cystinosin (CTNS gene). Addressing the cellular mechanisms, we here report vesicular cross-correction after HSC differentiation into macrophages. Upon coculture with cystinotic fibroblasts, macrophages produced tunneling nanotubes (TNTs) allowing transfer of cystinosin-bearing lysosomes into Ctns-deficient cells, which exploited the same route to retrogradely transfer cystine-loaded lysosomes to macrophages, providing a bidirectional correction mechanism. TNT formation was enhanced by contact with diseased cells. In vivo, HSCs grafted to cystinotic kidneys also generated nanotubular extensions resembling invadopodia that crossed the dense basement membranes and delivered cystinosin into diseased proximal tubular cells. This is the first report of correction of a genetic lysosomal defect by bidirectional vesicular exchange via TNTs and suggests broader potential for HSC transplantation for other disorders due to defective vesicular proteins.