AbstractSTUDY QUESTIONDo males with the rare lysosomal storage disease infantile nephropathic cystinosis (INC) have a chance of biological fatherhood?SUMMARY ANSWERCryostorage of semen could be an option for approximately 20% of young males with INC, with surgical sperm retrieval from the centre of the testes providing additional opportunities for fatherhood.WHAT IS KNOWN ALREADYBiallelic mutations in the cystinosin (CTNS) gene in INC cause dysfunction in cystine transport across lysosomal membranes and cystine accumulation throughout the body. Spontaneous paternity in cystinosis has not been described, despite the availability of cysteamine treatment. Azoospermia has been diagnosed in small case series of males with INC. ART using ICSI requires few spermatozoa, either from semen or extracted surgically from the testes of azoospermic men. However, there is limited evidence to suggest this could be successful in INC.STUDY DESIGN, SIZE, DURATIONIn this prospective cohort study performed between 2018 and 2019, we performed a cross-sectional investigation of 18 male patients with INC to delineate endocrine and spermatogenic testicular function.PARTICIPANTS/MATERIALS, SETTING, METHODSSerum hormone levels, semen samples (according to World Health Organization 2010 standards), and testicular ultrasound images were analysed in 18 male patients aged 15.4–40.5 years. Surgical sperm extraction was performed in two, and their testicular biopsies were investigated by light and electron microscopy. Past adherence to cysteamine treatment was assessed from medical record information, using a composite scoring system.MAIN RESULTS AND THE ROLE OF CHANCEAdherence to cysteamine treatment was high in most patients. Testicular volumes and testosterone levels were in the normal ranges, with the exception of two and three older patients, respectively. Serum LH levels were above the normal range in all subjects aged ≥20 years. FSH levels were elevated in all but four males: three with spermatozoa in semen and one adolescent. Inhibin B levels were shown to be lower in older men. Testicular ultrasound revealed signs of obstruction in 67% of patients. Reduced fructose and zinc seminal markers were found in 33%, including two patients with azoospermia who underwent successful surgical sperm retrieval. Histology identified fully preserved spermatogenesis in the centre of their testes, but also tubular atrophy and lysosomal overload in Sertoli and Leydig cells of the testicular periphery.LIMITATIONS, REASONS FOR CAUTIONLimitations of this study are the small number of assessed patients and the heterogeneity of their dysfunction in cystine transport across lysosomal membranes.WIDER IMPLICATIONS OF THE FINDINGSThis study suggests that testicular degeneration in cystinosis results from the lysosomal overload of Sertoli and Leydig cells of the testicular periphery, and that this can possibly be delayed, but not prevented, by good adherence to cysteamine treatment. Endocrine testicular function in INC may remain compensated until the fourth decade of life; however, azoospermia may occur during adolescence. Cryostorage of semen could be an option for approximately 20% of young males with INC, with surgical sperm retrieval providing additional opportunities for biological fatherhood.STUDY FUNDING/COMPETING INTEREST(S)This work was supported by the Cystinosis Foundation Germany. The authors have no competing interests to declare.TRIAL REGISTRATION NUMBERn/a.