Testicular function in males with infantile nephropathic cystinosis

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.

Cystinosis

Cystinosis is a very rare autosomal recessive lysosomal storage disorder with an incidence of 1 : 150,000 - 1 : 200,000, and is caused by mutations in the CTNS gene encoding the lysosomal membrane protein cystinosin, which transports cystine out of the lysosome into the cytoplasm. As a result, accumulation of cystine occurs in almost all cells and tissues, especially in the kidneys, leading to multiple organ involvement. Introduction of drug therapy with cysteamine in the mid 1980s, along with the availability of renal replacement therapy in childhood, have dramatically improved patient outcome. Whereas patients used to die without therapy with end-stage renal failure during the first decade of life, nowadays most patients live well into adulthood without renal replacement therapy, and several reach 40 years. There is robust evidence that early initiation and sustained lifelong therapy with cysteamine are both essential for morbidity and mortality. The rarity of the disease and the multi-organ involvement present an enormous challenge for those affected and the providers of care for this patient group.

Patients With Infantile Nephropathic Cystinosis in Germany and Austria: A Retrospective Cohort Study

Background

Infantile nephropathic cystinosis (INC) is a rare lysosomal storage disorder resulting in progressive chronic kidney disease (CKD) and a variety of extrarenal manifestations. This orphan disease remains a challenge for patients, their families and health care providers. There is currently no comprehensive study on patients' clinical course in Germany and Austria.

Methods

A retrospective cohort study including 74 patients at eleven centers of care was conducted. Data on time of diagnosis, CKD stage, leukocyte cystine levels (LCL), extrarenal manifestations, and treatment was collected from medical charts and subsequently analyzed using explorative statistics. Age at initiation of kidney replacement therapy (KRT) was evaluated by Kaplan-Meier analyses for different groups of patients.

Results

Patients were diagnosed at a median age of 15 months (IQR: 10-29, range: 0-110), more recent year of birth was not associated with earlier diagnosis. Oral cystine-depleting therapy (i.e., cysteamine) was prescribed at a median dose of 1.26 g/m² per day (IQR: 1.03-1.48, range: 0.22-1.99). 69.2% of all 198 LCL measurements of 67 patients were within the desired target range (≤ 1 nmol cystine/mg protein). Median time-averaged LCLs per patient (n = 65) amounted to 0.57 nmol cystine/mg protein (IQR: 0.33-0.98, range: 0.07-3.13) when considering only values at least 1 year after initiation of therapy. The overall median height of 242 measurements of 68 patients was at the 7^th percentile (IQR: 1-25, range: 1-99). 40.5% of the values were ≤ the 3^rd percentile. Patient sex and year of birth were not associated with age at initiation of KRT, but patients diagnosed before the age of 18 months required KRT significantly later than those patients diagnosed at the age of ≥ 18 months (p = 0.033): median renal survival was 21 years (95% CI: 16, -) vs. 13 years (95% CI, 10, -), respectively.

Conclusion

Early diagnosis and initiation of cystine depleting therapy is important for renal survival in children with INC. Cysteamine doses and LCL showed that treatment in this cohort met international standards although there is great interindividual variety. Patient growth and other aspects of the disease should be managed more effectively in the future.

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.

Fertility in Cystinosis

Cystinosis is a rare inheritable lysosomal storage disorder characterized by cystine accumulation throughout the body, chronic kidney disease necessitating renal replacement therapy mostly during adolescence, and multiple extra-renal complications. The majority of male cystinosis patients are infertile due to azoospermia, in contrast to female patients who are fertile. Over recent decades, the fertility status of male patients has evolved from a primary hypogonadism in the era before the systematic treatment with cysteamine to azoospermia in the majority of cysteamine-treated infantile cystinosis patients. In this review, we provide a state-of-the-art overview on the available clinical, histopathological, animal, and in vitro data. We summarize current insights on both cystinosis males and females, and their clinical implications including the potential effect of cysteamine on fertility. In addition, we identify the remaining challenges and areas for future research.

Human and animal fertility studies in cystinosis reveal signs of obstructive azoospermia, an altered blood-testis barrier and a subtherapeutic effect of cysteamine in testis

Cystinosis is an inherited metabolic disorder caused by autosomal recessive mutations in the CTNS gene leading to lysosomal cystine accumulation. The disease primarily affects the kidneys followed by extra-renal organ involvement later in life. Azoospermia is one of the unclarified complications which are not improved by cysteamine, which is the only available disease-modifying treatment. We aimed at unraveling the origin of azoospermia in cysteamine-treated cystinosis by confirming or excluding an obstructive factor, and investigating the effect of cysteamine on fertility in the Ctns^-/- mouse model compared with wild type. Azoospermia was present in the vast majority of infantile type cystinosis patients. While spermatogenesis was intact, an enlarged caput epididymis and reduced levels of seminal markers for obstruction neutral α-glucosidase (NAG) and extracellular matrix protein 1 (ECM1) pointed towards an epididymal obstruction. Histopathological examination in human and mouse testis revealed a disturbed blood-testis barrier characterized by an altered zonula occludens-1 (ZO-1) protein expression. Animal studies ruled out a negative effect of cysteamine on fertility, but showed that cystine accumulation in the testis is irresponsive to regular cysteamine treatment. We conclude that the azoospermia in infantile cystinosis is due to an obstruction related to epididymal dysfunction, irrespective of the severity of an evolving primary hypogonadism. Regular cysteamine treatment does not affect fertility but has subtherapeutic effects on cystine accumulation in testis.

Renal and Extra Renal Manifestations in Adult Zebrafish Model of Cystinosis

Cystinosis is a rare, incurable, autosomal recessive disease caused by mutations in the CTNS gene. This gene encodes the lysosomal cystine transporter cystinosin, leading to lysosomal cystine accumulation in all cells of the body, with kidneys being the first affected organs. The current treatment with cysteamine decreases cystine accumulation, but does not reverse the proximal tubular dysfunction, glomerular injury or loss of renal function. In our previous study, we have developed a zebrafish model of cystinosis through a nonsense mutation in the CTNS gene and have shown that zebrafish larvae recapitulate the kidney phenotype described in humans. In the current study, we characterized the adult cystinosis zebrafish model and evaluated the long-term effects of the disease on kidney and extra renal organs through biochemical, histological, fertility and locomotor activity studies. We found that the adult cystinosis zebrafish presents cystine accumulation in various organs, altered kidney morphology, impaired skin pigmentation, decreased fertility, altered locomotor activity and ocular anomalies. Overall, our data indicate that the adult cystinosis zebrafish model reproduces several human phenotypes of cystinosis and may be useful for studying pathophysiology and long-term effects of novel therapies.