The Fanconi Syndrome

This chapter focuses on a group of congenital and acquired disorders that are not due to a single solute transporter defect but rather appear to result from more generalized and heterogeneous dysfunctions of the entire proximal tubule. The complete or classical Fanconi syndrome may be defined as an impairment of proximal tubule reabsorption of sodium, bicarbonate, potassium, phosphate, glucose, amino acids, uric acid and low-molecular-weight proteins, and peptides, as well as other organic solutes. Cystinosis is the most common cause of the Fanconi syndrome in childhood and serves as a prototype of this syndrome. Cystinosis is a lysosomal storage disorder where the defective cystine transporter, cystinosin, fails to extrude cystine from the lysosomes. Fumarylacetoacetate hydrolase (FAH) in liver and kidney is the last enzyme in the tyrosine catabolic pathway. Patients with mutations of FAH have tyrosinemia and develop the Fanconi syndrome after ingestion of tyrosine or phenylalanine. One of the accumulated metabolites, succinylacetone, is nephrotoxic but may not be the sole agent. Over the short term, the Fanconi syndrome is reversible upon removal of tyrosine or phenylalanine from the diet. Heavy metals (cadmium, mercury, lead, chromium, and platinum) are transported by the proximal tubule and are also toxic to the same epithelia. The kidney is a prime target of heavy metal toxicity because of its ability to accumulate metals. In humans, chronic exposure can result in a range of disorders from the Fanconi syndrome to renal failure. Cadmium-induced Fanconi syndrome usually only occurs after years of industrial exposure, while lead-induced Fanconi syndrome can occur after a large acute exposure or after prolonged chronic low levels of exposure.