Oh cystinosin: let me count the ways!

Extrarenal complications of cystinosis

Cystinosis is a rare autosomal recessive disease with an incidence 1 per 100,000-200,000 live births. It is caused by pathogenic variants of the cystinosin (CTNS) gene that lead to impaired cystine transport from lysosomes to cystosol, resulting in cystine accumulation in lysosomes and subsequent cellular dysfunction. The initial manifestation, cystine accumulation in proximal tubular cells (PTCs), causes renal Fanconi syndrome, which presents with proximal renal tubular acidosis and generalized dysfunction of the proximal tubule, including the presence of polyuria, glycosuria, phosphaturia, aminoaciduria, tubular proteinuria, growth retardation, and rickets. Eventually, glomerular involvement, glomerular proteinuria, focal segmental glomerulosclerosis (FSGS), and progression to kidney failure occur. Although the kidneys are the first organs affected, and play a key role in morbidity and mortality, extrarenal multiorgan involvement can occur in patients with cystinosis, which is seen not only in adults but in early ages in untreated patients, patients with insufficient treatment, and in those that don't comply with treatment. The treatment of cystinosis consists of supportive treatment for Fanconi syndrome, and specific lifelong cystine-depleting therapy using oral cysteamine. There is strong evidence that as early as possible, initiation and ongoing appropriate therapy with cysteamine are essential for delaying the progression to kidney failure, end-organ damage, and extrarenal involvement. The present review aimed to evaluate the extra renal complications of cystinosis.

Muscle and Bone Impairment in Infantile Nephropathic Cystinosis: New Concepts

Cystinosis Metabolic Bone Disease (CMBD) has emerged during the last decade as a well-recognized, long-term complication in patients suffering from infantile nephropathic cystinosis (INC), resulting in significant morbidity and impaired quality of life in teenagers and adults with INC. Its underlying pathophysiology is complex and multifactorial, associating complementary, albeit distinct entities, in addition to ordinary mineral and bone disorders observed in other types of chronic kidney disease. Amongst these long-term consequences are renal Fanconi syndrome, hypophosphatemic rickets, malnutrition, hormonal abnormalities, muscular impairment, and intrinsic cellular bone defects in bone cells, due to CTNS mutations. Recent research data in the field have demonstrated abnormal mineral regulation, intrinsic bone defects, cysteamine toxicity, muscle wasting and, likely interleukin-1-driven inflammation in the setting of CMBD. Here we summarize these new pathophysiological deregulations and discuss the crucial interplay between bone and muscle in INC. In future, vitamin D and/or biotherapies targeting the IL1β pathway may improve muscle wasting and subsequently CMBD, but this remains to be proven.

Cysteamine bitartrate delayed-release capsules control leukocyte cystine levels and promote statural growth and kidney health in an open-label study of treatment-naïve patients <6 years of age with nephropathic cystinosis

Nephropathic cystinosis is a rare autosomal recessive lysosomal storage disease that is characterized by accumulation of cysteine and formation of crystals within cells of different organs and tissues causing systemic manifestations in childhood that include poor linear growth, ocular involvement, hypothyroidism, and progressive kidney disease. This study was a long-term, prospective open-label evaluation of twice-daily delayed release (DR) cysteamine capsules in cystinosis patients <6 years of age who were naïve to any form of cysteamine treatment. Fifteen treatment-naïve patients <6 years old (mean age 2.2 ± 1.0 years, 53% male, 73% White) were enrolled and treated with DR-cysteamine capsules for up to 18 months. Patients had clinically meaningful decreases in WBC cysteine concentration during treatment (3.2 ± 3.0 nmol ½ cystine/mg protein at Day 1 to 0.8 ± 0.8 nmol ½ cystine/mg protein at study exit), and anthropometric data improvements were consistently observed in height, weight and body surface area. Additionally, estimated glomerular filtration rate increased from 55.93 ± 22.43 ml/min/1.73 m2 at baseline to 63.79 ± 21.44 ml/min/1.73 m2 at study exit. Pharmacokinetic/Pharmacodynamic results support the use of the same starting, escalation, and maintenance doses according to body surface for children aged <6 years that are currently recommended in adults and older children. All patients experienced ≥1 adverse event(s) with vomiting (80%) and upper respiratory tract infection (53%) most frequently reported. Based on our study, patients <6 years of age with nephropathic cystinosis without prior treatment can safely and effectively initiate treatment with DR-cysteamine, a delayed-release form of cysteamine bitartrate that can be given every 12 h.

Galectin-3 and Risk of Late Graft Failure in Kidney Transplant Recipients: A 10-year Prospective Cohort Study

BACKGROUND

Galectin-3 may play a causal role in kidney inflammation and fibrosis, which may also be involved in the development of kidney graft failure. With novel galectin-3-targeted pharmacological therapies increasingly coming available, we aimed to investigate whether galectin-3 is associated with risk of late graft failure in kidney transplant recipients (KTR).

METHODS

We studied adult KTR who participated in TransplantLines Insulin Resistance and Inflammation Biobank and Cohort Study, recruited in a university setting (2001-2003). Follow-up was performed for a median of 9.5 (interquartile range, 6.2-10.2) years. Overall and stratified (Pinteraction < 0.05) multivariable-adjusted Cox proportional-hazards regression analyses were performed to study the association of galectin-3 with risk of graft failure (restart of dialysis or retransplantation).

RESULTS

Among 561 KTR (age 52 ± 12 y; 54% males), baseline median galectin-3 was 21.1 (interquartile range, 17.0-27.2) ng/mL. During follow-up, 72 KTR developed graft failure (13, 18, and 44 events over increasing tertiles of galectin-3). Independent of adjustment for donor, recipient, and transplant characteristics, galectin-3-associated with increased risk of graft failure (hazard ratios [HR] per 1 SD change, 2.12; 95% confidence interval [CI], 1.63-2.75; P < 0.001), particularly among KTR with systolic blood pressure ≥140 mmHg (HR, 2.29; 95% CI, 1.80-2.92; P < 0.001; Pinteraction = 0.01) or smoking history (HR, 2.56; 95% CI, 1.95-3.37; P < 0.001; Pinteraction = 0.03). Similarly, patients in the highest tertile of galectin-3 were consistently at increased risk of graft failure.

CONCLUSIONS

Serum galectin-3 levels are elevated in KTR, and independently associated with increased risk of late graft failure. Whether galectin-3-targeted therapies may represent novel opportunities to decrease the long-standing high burden of late graft failure in stable KTR warrants further studies.

Adult complications of nephropathic cystinosis: a systematic review

While nephropathic cystinosis is classically thought of as a childhood disease, with improved treatments, patients are more commonly living into adulthood. We performed a systematic review of the literature available on what complications this population faces as it ages. Nearly every organ system is affected in cystinosis, either from the disease itself or from sequelae of kidney transplantation. While cysteamine is known to delay the onset of end-stage kidney disease, its effects on other complications of cystinosis are less well determined. More common adult-onset complications include myopathy, diabetes, and hypothyroidism. Some less common complications, such as neurologic dysfunction, can still have a profound impact on those with cystinosis. Areas for further research in this area include additional study of the impact of cysteamine on the nonrenal manifestations of cystinosis, as well as possible avenues for new and novel treatments.

Lysosome function in glomerular health and disease

The lysosome represents an important regulatory platform within numerous vesicle trafficking pathways including the endocytic, phagocytic, and autophagic pathways. Its ability to fuse with endosomes, phagosomes, and autophagosomes enables the lysosome to break down a wide range of both endogenous and exogenous cargo, including macromolecules, certain pathogens, and old or damaged organelles. Due to its center position in an intricate network of trafficking events, the lysosome has emerged as a central signaling node for sensing and orchestrating the cells metabolism and immune response, for inter-organelle and inter-cellular signaling and in membrane repair. This review highlights the current knowledge of general lysosome function and discusses these findings in their implication for renal glomerular cell types in health and disease including the involvement of glomerular cells in lysosomal storage diseases and the role of lysosomes in nongenetic glomerular injuries.

Distinct Modes of Balancing Glomerular Cell Proteostasis in Mucolipidosis Type II and III Prevent Proteinuria

BACKGROUND

The mechanisms balancing proteostasis in glomerular cells are unknown. Mucolipidosis (ML) II and III are rare lysosomal storage disorders associated with mutations of the Golgi-resident GlcNAc-1-phosphotransferase, which generates mannose 6-phosphate residues on lysosomal enzymes. Without this modification, lysosomal enzymes are missorted to the extracellular space, which results in lysosomal dysfunction of many cell types. Patients with MLII present with severe skeletal abnormalities, multisystemic symptoms, and early death; the clinical course in MLIII is less progressive. Despite dysfunction of a major degradative pathway, renal and glomerular involvement is rarely reported, suggesting organ-specific compensatory mechanisms.

METHODS

MLII mice were generated and compared with an established MLIII model to investigate the balance of protein synthesis and degradation, which reflects glomerular integrity. Proteinuria was assessed in patients. High-resolution confocal microscopy and functional assays identified proteins to deduce compensatory modes of balancing proteostasis.

RESULTS

Patients with MLII but not MLIII exhibited microalbuminuria. MLII mice showed lysosomal enzyme missorting and several skeletal alterations, indicating that they are a useful model. In glomeruli, both MLII and MLIII mice exhibited reduced levels of lysosomal enzymes and enlarged lysosomes with abnormal storage material. Nevertheless, neither model had detectable morphologic or functional glomerular alterations. The models rebalance proteostasis in two ways: MLII mice downregulate protein translation and increase the integrated stress response, whereas MLIII mice upregulate the proteasome system in their glomeruli. Both MLII and MLIII downregulate the protein complex mTORC1 (mammalian target of rapamycin complex 1) signaling, which decreases protein synthesis.

CONCLUSIONS

Severe lysosomal dysfunction leads to microalbuminuria in some patients with mucolipidosis. Mouse models indicate distinct compensatory pathways that balance proteostasis in MLII and MLIII.

Bone Disease in Nephropathic Cystinosis: Beyond Renal Osteodystrophy

Patients with chronic kidney disease (CKD) display significant mineral and bone disorders (CKD-MBD) that induce significant cardiovascular, growth and bone comorbidities. Nephropathic cystinosis is an inherited metabolic disorder caused by the lysosomal accumulation of cystine due to mutations in the CTNS gene encoding cystinosin, and leads to end-stage renal disease within the second decade. The cornerstone of management relies on cysteamine therapy to decrease lysosomal cystine accumulation in target organs. However, despite cysteamine therapy, patients display severe bone symptoms, and the concept of “cystinosis metabolic bone disease” is currently emerging. Even though its exact pathophysiology remains unclear, at least five distinct but complementary entities can explain bone impairment in addition to CKD-MBD: long-term consequences of renal Fanconi syndrome, malnutrition and copper deficiency, hormonal disturbances, myopathy, and intrinsic/iatrogenic bone defects. Direct effects of both CTNS mutation and cysteamine on osteoblasts and osteoclasts are described. Thus, the main objective of this manuscript is not only to provide a clinical update on bone disease in cystinosis, but also to summarize the current experimental evidence demonstrating a functional impairment of bone cells in this disease and to discuss new working hypotheses that deserve future research in the field.