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Cystinosis is an autosomal recessive lysosomal storage disorder, caused by mutations in the CTNS gene, resulting in an absent or altered cystinosin (CTNS) protein. Cystinosin exports cystine out of the lysosome, with a malfunction resulting in cystine accumulation and a defect in other cystinosin-mediated pathways. Cystinosis is a systemic disease, but the kidneys are the first and most severely affected organs. In the kidney, the disease initially manifests as a generalized dysfunction in the proximal tubules (also called renal Fanconi syndrome). MFSD12 is a lysosomal cysteine importer that directly affects the cystine levels in melanoma cells, HEK293T cells, and cystinosis patient-derived fibroblasts. In this study, we aimed to evaluate MFSD12 mRNA levels in cystinosis patient-derived proximal tubular epithelial cells (ciPTECs) and to study the effect of MFSD12 knockout on cystine levels. We showed similar MFSD12 mRNA expression in patient-derived ciPTECs in comparison with the control cells. CRISPR MFSD12 knockout in a patient-derived ciPTEC (CTNSΔ57kb) resulted in significantly reduced cystine levels. Furthermore, we evaluated proximal tubular reabsorption after injection of mfsd12a translation-blocking morpholino (TB MO) in a ctns−/− zebrafish model. This resulted in decreased cystine levels but caused a concentration-dependent increase in embryo dysmorphism. Furthermore, the mfsd12a TB MO injection did not improve proximal tubular reabsorption or megalin expression. In conclusion, MFSD12 mRNA depletion reduced cystine levels in both tested models without improvement of the proximal tubular function in the ctns−/− zebrafish embryo. In addition, the apparent toxicity of higher mfsd12a TB MO concentrations on the zebrafish development warrants further evaluation. NEW & NOTEWORTHY: In this study, we show that MFSD12 depletion with either CRISPR/Cas9-mediated gene editing or a translation-blocking morpholino significantly reduced cystine levels in cystinosis ciPTECs and ctns−/− zebrafish embryos, respectively. However, we observed no improvement in the proximal tubular reabsorption of dextran in the ctns−/− zebrafish embryos injected with mfsd12a translation-blocking morpholino. Furthermore, a negative effect of the mfsd12a morpholino on the zebrafish development warrants further investigation. [ABSTRACT FROM AUTHOR]

Renal Fanconi syndrome (RFS) is a generalised disorder of the proximal convoluted tubule. Many genes have been associated with RFS including those that cause systemic disorders such as cystinosis, as well as isolated RFS. We discuss the case of a 10‐year‐old female who presented with leg pain and raised creatinine on a screening blood test. Her mother has RFS and required a kidney transplant in her thirties. Further investigations confirmed RFS in the daughter. Exome sequencing was performed on the affected mother, child, and unaffected father. We identified a novel variant in GATM; c.965G>C p.(Arg322Pro) segregating dominantly in the mother and daughter. We validated our finding with molecular dynamics simulations and demonstrated a dynamic signature that differentiates our variant and two previously identified pathogenic variants in GATM from wildtype. Genetic testing has uncovered a novel pathogenic variant that predicts progression to end stage kidney failure and has important implications for family planning and cascade testing. We recommend that GATM is screened for in children presenting with RFS, in addition to adults, particularly with kidney failure, who may have had previous negative gene testing. [ABSTRACT FROM AUTHOR]

Renal Fanconi syndrome (RFS) is a proximal tubular dysfunction characterized by abnormalities in the absorption of electrolytes and substances by the proximal tubule. It can manifest as proteinuria, glycosuria, aminoaciduria, phosphaturia, and proximal renal tubular acidosis (RTA) type 2. Heavy metal-induced nephrotoxicity from heavy metals such as iron, cadmium, mercury, and lead is a common risk factor for type 2 RTA. Environmental contamination or prolonged contact with pesticides, such as lead arsenate, Bordeaux mixture, and copper sulfate, may also contribute to heavy metal exposure. A 25-year-old female patient with RFS was diagnosed, and the bicarbonate loading test showed typical features of generalized proximal tubular dysfunction and proximal RTA. A metabolic disorder in the bone was the predominant feature, and renal wasting of phosphorus and calcium was linked to osteoporosis and increased intact parathyroid hormone (PTH) levels. Due to reports of zoledronate-induced RFS, calcitriol was administered instead of bisphosphates. Drug-induced nephrotoxicity is the most probable cause of adult-onset RFS. Acute tubulointerstitial nephritis is a plausible hypothesis, and treatment was based on observations and assessments that improved clinical outcomes. Ayurvedic medicine, popular in India due to its traditional roots and cultural aversion to artificial drugs, has high levels of heavy metal content., Competing Interests: Human subjects: Consent was obtained or waived by all participants in this study. Conflicts of interest: In compliance with the ICMJE uniform disclosure form, all authors declare the following: Payment/services info: All authors have declared that no financial support was received from any organization for the submitted work. Financial relationships: All authors have declared that they have no financial relationships at present or within the previous three years with any organizations that might have an interest in the submitted work. Other relationships: All authors have declared that there are no other relationships or activities that could appear to have influenced the submitted work., (Copyright © 2024, Senthilnathan et al.)

Evolution moves in mysterious ways. Excretion of waste products by glomerular filtration made perfect sense when life evolved in the ocean. Yet, the associated loss of water and solutes became a problem when life moved onto land: a serious design change was needed and this occurred in the form of ever more powerful tubules that attached to the glomerulus. By reabsorbing typically more than 99% of the glomerular filtrate, the tubules not only minimise urinary losses, but, crucially, also maintain homeostasis: tubular reabsorption and secretion are adjusted so as to maintain an overall balance, in which urine volume and composition matches intake and environmental stressors. A whole orchestra of highly specialised tubular transport proteins is involved in this process and dysfunction of one or more of these results in the so-called kidney tubulopathies, characterised by specific patterns of clinical and biochemical abnormalities. In turn, recognition of these patterns helps establish a specific diagnosis and pinpoints the defective transport pathway. In this review, we will discuss these clinical and biochemical "fingerprints" of tubular disorders of salt-handling and how sodium handling affects volume homeostasis but also handling of other solutes. [ABSTRACT FROM AUTHOR]

Nephropathic cystinosis is a severe monogenic kidney disorder caused by mutations in CTNS, encoding the lysosomal transporter cystinosin, resulting in lysosomal cystine accumulation. The sole treatment, cysteamine, slows down the disease progression, but does not correct the established renal proximal tubulopathy. Here, we developed a new therapeutic strategy by applying omics to expand our knowledge on the complexity of the disease and prioritize drug targets in cystinosis. We identified alpha‐ketoglutarate as a potential metabolite to bridge cystinosin loss to autophagy, apoptosis and kidney proximal tubule impairment in cystinosis. This insight combined with a drug screen revealed a bicalutamide–cysteamine combination treatment as a novel dual‐target pharmacological approach for the phenotypical correction of cystinotic kidney proximal tubule cells, patient‐derived kidney tubuloids and cystinotic zebrafish. SYNOPSIS: Nephropathic cystinosis is a severe genetic disorder caused by mutations in the lysosomal cystine transporter, cystinosin. Although several cellular defects have been associated with cystinosis, the mechanism linking cystinosin loss, and epithelial dysfunction remains largely unknown. CRISPR‐generated isogenic cystinotic renal proximal tubule cells recapitulate the disease pathology and allow direct evaluation of the effect of cystinosin loss, independent of chronic exposure to disease‐related changes in the body.Alpha‐ketoglutarate (αKG), an important intermediate of the tricarboxylic acid cycle, is a part of a unifying mechanism linking cystinosin loss, lysosomal autophagy disruption and proximal tubule impairment in cystinotic renal proximal tubule cells.Cysteamine‐bicalutamide combination therapy restores proximal tubule function in advanced in vitro models and corrects cystinosis related defects in cystinotic zebrafish. Our findings demonstrate that the combination therapy is a novel potential treatment for patients with nephropathic cystinosis. [ABSTRACT FROM AUTHOR]

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. Nephropathic cystinosis is a severe, monogenic systemic disorder caused by mutations in the lysosomal cystine/proton cotransporter cystinosin and the leading cause of inherited renal Fanconi syndrome. Cysteamine efficiently depletes lysosomal cystine and improves clinical outcomes; however, it does not reverse established kidney failure. A multifaceted impact of cystinosin loss of function is observed in cystinosis pathology, involving increased oxidative stress, apoptosis, and impaired autophagy and energy metabolism. Several small molecules and biologics correcting non-transport functions of cystinosin are emerging and have been shown to be effective either alone or in combination with cysteamine. Hematopoietic stem cell transplantation and translational read-through drugs pose promising new treatment options. [ABSTRACT FROM AUTHOR]

Phosphate is freely filtered by the glomerulus and reabsorbed exclusively in the proximal tubule by two key transporters, NaPiIIA and NaPiIIC, encoded by SLC34A1 and SLC34A3 , respectively. Regulation of these transporters occurs primarily through the hormone FGF23 and, to a lesser degree, PTH. Consequently, inherited non-FGF23 mediated phosphaturic disorders are due to generalised proximal tubular dysfunction, loss-of-function variants in SLC34A1 or SLC34A3 or excess PTH signalling. The corresponding disorders are Renal Fanconi Syndrome, Infantile Hypercalcaemia type 2, Hereditary Hypophosphataemic Rickets with Hypercalciuria and Familial Hyperparathyroidism. Several inherited forms of Fanconi renotubular syndrome (FRTS) have also been described with the underlying genes encoding for GATM, EHHADH, HNF4A and NDUFAF6. Here, we will review their pathophysiology, clinical manifestations and the implications for treatment from a kidney-centric perspective, focussing on those disorders caused by dysfunction of renal phosphate transporters. Moreover, we will highlight specific genetic aspects, as the availability of large population genetic databases has raised doubts about some of the originally proposed gene-disease associations concerning phosphate transporters or their associated proteins. [ABSTRACT FROM AUTHOR]

Cystinosis is caused by mutations in the CTNS gene (17p13.2), which encodes for a lysosomal cystine/proton symporter termed cystinosin. It is the most common cause of inherited renal Fanconi syndrome in young children. Because of its rarity, the diagnosis and specific treatment of cystinosis are frequently delayed, which has a significant impact on the overall prognosis. In this document, we have summarized expert opinions on several aspects of the disease to improve knowledge and provide guidance for diagnosis and treatment.

Kidneys have a high energy demand to facilitate the reabsorption of the glomerular filtrate. For this reason, renal cells have a high density of mitochondria. Mitochondrial cytopathies can be the result of a mutation in both mitochondrial and nuclear DNA. Mitochondrial dysfunction can lead to a variety of renal manifestations. Examples of tubular manifestations are renal Fanconi Syndrome, which is often found in patients diagnosed with Kearns-Sayre and Pearson's marrow-pancreas syndrome, and distal tubulopathies, which result in electrolyte disturbances such as hypomagnesemia. Nephrotic syndrome can be a glomerular manifestation of mitochondrial dysfunction and is typically associated with focal segmental glomerular sclerosis on histology. Tubulointerstitial nephritis can also be seen in mitochondrial cytopathies and may lead to end-stage renal disease. The underlying mechanisms of these cytopathies remain incompletely understood; therefore, current therapies focus mainly on symptom relief. A better understanding of the molecular disease mechanisms is critical in order to improve treatments. [ABSTRACT FROM AUTHOR]

Background: Tenofovir disoproxil fumarate (TDF) is relatively safe, although renal toxicity has been reported. In Nigeria, there is insufficient data on renal toxicity among patients on TDF. This study assesses TDF-associated tubular dysfunction among human immunodeficiency virus (HIV) patients at a hospital in Nigeria. Methods: In this cohort study, 104 adult HIV patients were recruited with a simple random technique from the outpatient clinic. Biochemical indices of renal function were estimated from serum and urine at the 16th and 24th week after an initial assessment at baseline. Results: There were no significant differences in baseline proteinuria or glycosuria between TDF and non-TDF groups. Mean baseline urine and serum parameters did not differ significantly between the two groups (P⟩ 0.05). In the TDF group, all urine parameters differed significantly between baseline and 24th week values (P⟨ 0.001). After 16 weeks, mean urine phosphate and urine uric acid increased significantly (P⟨ 0.05) by 2.97 mg/dL and 50.9 mg/dL, respectively, in the TDF group. The rise in mean urine glucose from baseline to the 24th week was more marked in the TDF than the non-TDF group (0.25 vs. 0.07 mmol/L). Higher mean differences in urine albumin were also recorded in the TDF group from baseline to the 24th week. Conclusion: Indicators of tubular dysfunction were markedly higher among patients on the TDF-based treatment regimen. Biomarkers of tubular dysfunction could be useful for detecting pre-symptomatic nephrotoxicity before marked reduction of glomerular filtration rate in HIV patients on TDF.

Abstract Background An increasing number of case reports suggest that acquired renal Fanconi syndrome may be associated with prolonged use of adefovir against hepatitis B virus. Renal Fanconi syndrome is an uncommon disease, and its complication with nephrolithiasis is quite rare. Herein, we report a rare coexistence of nephrolithiasis and acquired renal Fanconi syndrome in a chronic hepatitis B-positive patient with prolonged adefovir therapy. Case presentation The patient presented with osteomalacia and nephrolithiasis. Consequently, extracorporeal shock-wave lithotripsy and left double-J ureteral stent insertion were considered for obstructive nephropathy, which was caused by nephrolithiasis. However, osteomalacia had been misdiagnosed as osteoporosis before admission to our hospital. On admission, a complexity of multiple fractures, hypophosphataemia, glycosuria without hyperglycaemia and non–anion-gap metabolic acidosis indicated a diagnosis of acquired renal Fanconi syndrome induced by adefovir. After switching from adefovir to entecavir, the patient’s symptoms and laboratory findings improved significantly. Conclusions The mechanism responsible for nephrolithiasis in renal Fanconi syndrome is still unclear. We recommend regularly monitoring renal function and serum calcium and serum phosphate to prevent renal Fanconi syndrome during the prolonged use of adefovir for hepatitis B virus.

Abstract Background Rare diseases may elude diagnosis due to unfamiliarity of the treating physicians with the specific disorder. Yet, advances in genetics have tremendously enhanced our ability to establish specific and sometimes surprising diagnoses. Case presentation We report a case of renal Fanconi syndrome associated with intermittent hypoglycemic episodes, the specific cause for which remained elusive for over 30 years, despite numerous investigations, including three kidney and one liver biopsy. The most recent kidney biopsy showed dysmorphic mitochondria, suggesting a mitochondrial disorder. When her son presented with hypoglycemia in the neonatal period, he underwent routine genetic testing for hyperinsulinemic hypoglycemia, which revealed a specific mutation in HNF4A. Subsequent testing of the mother confirmed the diagnosis also in her. Conclusion Modern sequencing technologies that test multiple genes simultaneously enable specific diagnoses, even if the underlying disorder was not clinically suspected. The finding of mitochondrial dysmorphology provides a potential clue for the mechanism, by which the identified mutation causes renal Fanconi syndrome.

Cystinosis is a rare autosomal-recessive lysosomal storage disease with high morbidity and mortality. It is caused by mutations in the CTNS gene that encodes the cystine transporter, cystinosin, which leads to lysosomal cystine accumulation. Patients with infantile nephropathic cystinosis, the most common and most severe clinical form of cystinosis, commonly present with renal Fanconi syndrome by 6-12 months of age, and without specific treatment, almost all will develop end-stage renal disease (ESRD) by 10-12 years of age. Early corneal cystine crystal deposition is a hallmark of the disease. Cystinosis also presents with gastrointestinal symptoms (e.g., vomiting, decreased appetite, and feeding difficulties) and severe growth retardation and may affect several other organs over time, including the eye, thyroid gland, gonads, pancreas, muscles, bone marrow, liver, nervous system, lungs, and bones. Cystine-depleting therapy with cysteamine orally is the only specific targeted therapy available for managing cystinosis and needs to be combined with cysteamine eye drops for corneal disease involvement. In patients with early treatment initiation and good compliance to therapy, long-term cysteamine treatment delays progression to ESRD, significantly improves growth, decreases the frequency and severity of extrarenal complications, and is associated with extended life expectancy. Therefore, early diagnosis of cystinosis and adequate life-long treatment with cysteamine are essential for preventing end-organ damage and improving the overall prognosis in these patients. [ABSTRACT FROM AUTHOR]

Refeeding syndrome is diagnosed based on the onset of multiple laboratory abnormalities (most commonly hypophosphatemia) and clinical signs in the setting of nutrition rehabilitation of malnourished patients. Because definitions are not uniform, a broad differential diagnosis should always include renal tubular dysfunction. Our report details a 3 year-old child with undiagnosed renal tubular dysfunction who presented with the clinical picture of refeeding syndrome with refractory electrolyte abnormalities. A diagnosis of renal Fanconi syndrome was made after urinalysis that revealed glucosuria and urine electrolyte losses. Thus, urinalysis can aid in making a positive diagnosis of refeeding syndrome. [ABSTRACT FROM AUTHOR]

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.

Background: Tenofovir disoproxil fumarate (TDF) is relatively safe, although renal toxicity has been reported. In Nigeria, there is insufficient data on renal toxicity among patients on TDF. This study assesses TDF-associated tubular dysfunction among human immunodeficiency virus (HIV) patients at a hospital in Nigeria. Methods: In this cohort study, 104 adult HIV patients were recruited with a simple random technique from the outpatient clinic. Biochemical indices of renal function were estimated from serum and urine at the 16th and 24th week after an initial assessment at baseline. Results: There were no significant differences in baseline proteinuria or glycosuria between TDF and non-TDF groups. Mean baseline urine and serum parameters did not differ significantly between the two groups (P > 0.05). In the TDF group, all urine parameters differed significantly between baseline and 24th week values (P < 0.001). After 16 weeks, mean urine phosphate and urine uric acid increased significantly (P < 0.05) by 2.97 mg/dL and 50.9 mg/dL, respectively, in the TDF group. The rise in mean urine glucose from baseline to the 24th week was more marked in the TDF than the non-TDF group (0.25 vs. 0.07 mmol/L). Higher mean differences in urine albumin were also recorded in the TDF group from baseline to the 24th week. Conclusion: Indicators of tubular dysfunction were markedly higher among patients on the TDF-based treatment regimen. Biomarkers of tubular dysfunction could be useful for detecting pre-symptomatic nephrotoxicity before marked reduction of glomerular filtration rate in HIV patients on TDF. [ABSTRACT FROM AUTHOR]

Background: Rare diseases may elude diagnosis due to unfamiliarity of the treating physicians with the specific disorder. Yet, advances in genetics have tremendously enhanced our ability to establish specific and sometimes surprising diagnoses.Case Presentation: We report a case of renal Fanconi syndrome associated with intermittent hypoglycemic episodes, the specific cause for which remained elusive for over 30 years, despite numerous investigations, including three kidney and one liver biopsy. The most recent kidney biopsy showed dysmorphic mitochondria, suggesting a mitochondrial disorder. When her son presented with hypoglycemia in the neonatal period, he underwent routine genetic testing for hyperinsulinemic hypoglycemia, which revealed a specific mutation in HNF4A. Subsequent testing of the mother confirmed the diagnosis also in her.Conclusion: Modern sequencing technologies that test multiple genes simultaneously enable specific diagnoses, even if the underlying disorder was not clinically suspected. The finding of mitochondrial dysmorphology provides a potential clue for the mechanism, by which the identified mutation causes renal Fanconi syndrome. [ABSTRACT FROM AUTHOR]

The oculocerebrorenal syndrome of Lowe is a rare X-linked multisystemic disorder characterized by the triad of congenital cataracts, intellectual disability, and proximal renal tubular dysfunction. Whereas the ocular manifestations and severe muscular hypotonia are the typical first diagnostic clues apparent at birth, the manifestations of incomplete renal Fanconi syndrome are often recognized only later in life. Other characteristic features are progressive severe growth retardation and behavioral problems, with tantrums. Many patients develop a debilitating arthropathy. Treatment is symptomatic, and the life span rarely exceeds 40 years. The causative oculocerebrorenal syndrome of Lowe gene ( OCRL) encodes the inositol polyphosphate 5-phosphatase OCRL-1. OCRL variants have not only been found in classic Lowe syndrome, but also in patients with a predominantly renal phenotype classified as Dent disease type 2 (Dent-2). Recent data indicate that there is a phenotypic continuum between Dent-2 disease and Lowe syndrome, suggesting that there are individual differences in the ability to compensate for the loss of enzyme function. Extensive research has demonstrated that OCRL-1 is involved in multiple intracellular processes involving endocytic trafficking and actin skeleton dynamics. This explains the multi-organ manifestations of the disease. Still, the mechanisms underlying the wide phenotypic spectrum are poorly understood, and we are far from a causative therapy. In this review, we provide an update on clinical and molecular genetic findings in Lowe syndrome and the cellular and physiological functions of OCRL-1. [ABSTRACT FROM AUTHOR]

Background: The p.R63W mutation in the hepatocyte nuclear factor-4 alpha (HNF4A) results in macrosomia and atypical Fanconi syndrome, in addition to hyperinsulinaemic hypoglycaemia (HI). We describe 2 infants carrying this mutation, presenting with additional features. Cases Series: Patient 1, a male born with a birth weight of 1.7 SDS, was diagnosed with HI on day 2 of life. He responded to 3-10 mg/kg/day of diazoxide. Raised serum creatinine led to the investigation of renal tubular function, showing leaking of electrolytes and protein. The patient also had conjugated hyperbilirubinaemia with liver steatosis. Patient 2 was a male born with a weight of 0.36 SDS. His mother had renal Fanconi syndrome. He received parenteral nutrition and presented with HI at 1 month of age, while establishing enteral feeds. Biochemistry workup showed renal tubular leaking of calcium, sodium, and phosphate. A hypoglycaemia screen documented HI, and the patient was commenced on 2 mg/kg/day of diazoxide. Continuous glucose monitoring was performed in his mother, revealing overnight hypoglycaemia. Conclusion: Renal Fanconi syndrome represents the only HNF4A feature showing complete penetrance. Our cases suggest that the p.R63W HNF4A mutation must be considered in subjects with a normal birth weight and postulate the possibility of liver involvement as a part of this condition. [ABSTRACT FROM AUTHOR]

Renal Fanconi syndrome is diagnosed by its cardinal features of glycosuria without diabetes, aminoaciduria, phosphaturia, and renal tubular acidosis. It is often associated with hypokalaemia, hypophosphatemia and rickets. We report a seven-year-old boy with nephropathic cystinosis who presented with all the cardinal features of renal Fanconi syndrome associated with rickets, pathological fractures, stage IV chronic kidney disease (CKD) and hypothyroidism. Slit-lamp examination of the cornea confirmed the diagnosis. However glycosuria was conspicuously absent. Whenever there are features of rickets with failure to thrive and recurrent vomiting renal rickets should be ruled out. Cystinosis is one such disorder and we report this case due its rarity and interesting clinical presentation.

Genetic deficiency of the SLC26A1 anion exchanger in mice is known to be associated with hyposulfatemia and hyperoxaluria with nephrolithiasis, but many aspects of human SLC26A1 function remain to be explored. We report here the functional characterization of human SLC26A1, a 4,4′-diisothiocyanato-2,2′-stilbenedisulfonic acid (DIDS)-sensitive, electroneutral sodium-independent anion exchanger transporting sulfate, oxalate, bicarbonate, thiosulfate, and (with divergent properties) chloride. Human SLC26A1-mediated anion exchange differs from that of its rodent orthologs in its stimulation by alkaline pH and inhibition by acidic pH but not pH and in its failure to transport glyoxylate. SLC26A1-mediated transport of sulfate and oxalate is highly dependent on allosteric activation by extracellular chloride or non-substrate anions. Extracellular chloride stimulates apparent V of human SLC26A1-mediated sulfate uptake by conferring a 2-log decrease in sensitivity to inhibition by extracellular protons, without changing transporter affinity for extracellular sulfate. In contrast to SLC26A1-mediated sulfate transport, SLC26A1-associated chloride transport is activated by acid pH, shows reduced sensitivity to DIDS, and exhibits cation dependence of its DIDS-insensitive component. Human SLC26A1 resembles SLC26 paralogs in its inhibition by phorbol ester activation of protein kinase C (PKC), which differs in its undiminished polypeptide abundance at or near the oocyte surface. Mutation of SLC26A1 residues corresponding to candidate anion binding site-associated residues in avian SLC26A5/prestin altered anion transport in patterns resembling those of prestin. However, rare SLC26A1 polymorphic variants from a patient with renal Fanconi Syndrome and from a patient with nephrolithiasis/calcinosis exhibited no loss-of-function phenotypes consistent with disease pathogenesis. [ABSTRACT FROM AUTHOR]

Renal Fanconi syndrome (RFS) refers to the generalized dysfunction of the proximal tubule (PT) (Kleta R. Fanconi or not Fanconi? Lowe syndrome revisited. Clin J Am Soc Nephrol 2008; 3: 1244-1245). In its isolated form, RFS only affects the PT, but not the other nephron segments. The study of isolated RFS can thus provide specific insights into the function of the PT. In a recent paper, Klootwijk et al. investigated one such form of isolated RFS and revealed the underlying molecular basis (Klootwijk ED, Reichold M, Helip-Wooley A et al. Mistargeting of peroxisomal EHHADH and inherited renal Fanconi's syndrome. N Engl J Med 2014; 370: 129-138). The affected family had been described previously, demonstrating the typical features of RFS, such as low-molecular weight proteinuria, aminoaciduria, glycosuria and phosphaturia with consequent rickets; yet, importantly, patients had no evidence of impaired glomerular filtration (Tolaymat A, Sakarcan A, Neiberger R. Idiopathic Fanconi syndrome in a family. Part I. Clinical aspects. J Am Soc Nephrol 1992; 2: 1310-1317). Inheritance was consistent with an autosomal dominant mode. Klootwijk et al. discovered a surprising explanation: a heterozygous missense mutation causing partial mistargeting of the peroxisomal enzyme EHHADH to the mitochondria. Notably, disease causing was not the absence of the enzyme in the peroxisome, but its interference with mitochondrial function. The discovery of this novel disease mechanism not only confirmed the importance of mitochondrial function for PT transport, but also demonstrated the critical dependence of PT on fatty acid metabolism for energy generation. [ABSTRACT FROM AUTHOR]

Background: Cystinosis is a rare, inherited autosomal recessive disease caused by the accumulation of free cystine in lysosomes. It is treated by the administration of cysteamine, which should be monitored by trough white blood cell (WBC) cystine measurements to ensure effective treatment. Case-Diagnosis/Treatment: The index case had an older brother who had previously been diagnosed with cystinosis, allowing early diagnosis of the index case at the age of 5 months. Cysteamine therapy was started at the age of 3 years; however, monitoring of WBC cystine levels did not occur on a regular basis during most of his life. Growth retardation improved after correction of electrolyte disturbances, the initiation of cysteamine therapy and treatment with recombinant human growth hormone. Renal replacement therapy was started at the age of 11 years, and renal transplantation was performed at the age of 12 years. Extra-renal cystine accumulation caused multiple endocrinopathies (including adrenal insufficiency, hypothyroidism and primary hypogonadism), neurological symptoms, pancytopenia owing to splenomegaly and portal hypertension due to nodular regenerative hyperplasia, aggravated by splenic vein thrombosis and partial portal vein thrombosis. The patient died of diffuse intra-abdominal bleeding caused by severe portal hypertension. Conclusion: Cysteamine treatment should be started as early as possible, and dosage should be monitored and adapted based on trough WBC cystine levels. Relevant international guideline: Emma F et al. (2014) Nephropathic cystinosis: an international consensus document. Nephrol Dial Transplant 29:iv87-iv94. [ABSTRACT FROM AUTHOR]

What is known and Objective Adefovir dipivoxil (ADV) is one of the commonly used antiviral agents in the treatment of chronic hepatitis B (CHB) infection. Safety of a daily dose of 10 mg ADV is advocated by the registration trials. We report a case of severe hypophosphatemic osteomalacia and renal Fanconi syndrome induced by low-dose ADV in a CHB-related cirrhosis patient, and discuss the case through a thorough review of other cases reported in the literature. Case summary A 48-yr-old Chinese man with CHB-related cirrhosis developed severe progressive generalized bone pain and muscle weakness after receiving ADV 10 mg daily for 54 months. The laboratory results showed severe hypophosphatemia and features of proximal renal tubule dysfunction. Imaging studies were consistent with osteomalacia. After discontinuation of ADV, his symptoms resolved, laboratory abnormalities normalized and imaging studies showed improvement. In addition to our case, 12 other patients have been reported to have developed hypophosphatemic osteomalacia induced by low-dose ADV. Most of the reported cases were of subjects of East-Asian ethnicity. After discontinuation or reduction of ADV, serum phosphate level increased and clinical symptoms significantly improved in all cases. What is new and Conclusion Hypophosphatemic osteomalacia and renal Fanconi syndrome can be associated with low-dose ADV. Clinicians treating CHB patients with ADV 10 mg daily over long periods of time should be aware of this infrequent but serious complication [ABSTRACT FROM AUTHOR]

Tenofovir (TFV) is a widely used and effective treatment for HIV infection. Numerous studies have shown that TFV exposure is associated with small but significant declines in estimated glomerular filtration rate (eGFR). However, TFV toxicity is targeted mainly at the proximal tubule (PT), and in severe cases can cause the renal Fanconi syndrome or acute kidney injury. Severe toxicity occurs in a minority of patients, but milder PT dysfunction is more common; the long-term significance of this on kidney and bone health is uncertain. Recent work suggests that changes in eGFR on TFV therapy might be explained by inhibition of PT creatinine secretion rather than actual alterations in glomerular function. Risk factors for nephrotoxicity include pre-existing kidney disease, increased age, and low body mass. Mitochondria in the PT are the targets of TFV toxicity, but the exact mechanisms remain unclear. Substantial improvement of renal function occurs in many patients with TFV toxicity upon stopping therapy, but function does not always return to baseline. In recent years, TFV usage has been extended to new clinical spheres, including pediatrics, resource-poor settings and treatment of Hepatitis B infection; theoretical reasons exist as to why some of these patients might be at higher or lower risk of TFV toxicity. Finally, strategies have been proposed to prevent TFV toxicity or enhance recovery. [ABSTRACT FROM AUTHOR]