Urinary Megalin Deficiency Implicates Abnormal Tubular Endocytic Function in Fanconi Syndrome

ABSTRACT. Normal reabsorption of glomerular filtrate proteins probably requires recycling of the endocytic receptors megalin (gp330) and cubilin. Both receptors are located on the luminal surface of the renal proximal tubule epithelium. Whether abnormal amounts of receptor are present in the urine of patients with Dent’s disease, Lowe’s syndrome, or autosomal dominant idiopathic Fanconi syndrome was explored. They are all forms of the renal Fanconi syndrome and are associated with tubular proteinuria. Urine samples of equal creatinine contents were dialyzed, lyophilized, and subjected to electrophoresis on nonreducing sodium dodecyl sulfate-5% polyacrylamide gels. Proteins were blotted and probed with anti-megalin IgG, anti-cubilin IgG, or receptor-associated protein. Megalin and cubilin levels detected by immunochemiluminescence were measured as integrated pixels and expressed as percentages of the normal mean values. A striking deficiency of urinary megalin, compared with normal individuals (n = 42), was observed for eight of nine families with Dent’s disease (n = 10) and for the two families with Lowe’s syndrome (n = 3). The family with autosomal dominant idiopathic Fanconi syndrome (n = 2) exhibited megalin levels within the normal range. The measured levels of cubilin were normal for all patients. These results are consistent with defective recycling of megalin to the apical cell surface of the proximal tubules and thus decreased loss into urine in Dent’s disease and Lowe’s syndrome. This defect would interfere with the normal endocytic function of megalin, result in losses of potential ligands into the urine, and produce tubular proteinuria.

Idiopathic Fanconi syndrome in a family. Part I. Clinical aspects.

Fanconi syndrome is a rare cause of rickets in children. Only six families with Fanconi syndrome following an autosomal dominant pattern of inheritance have been reported. In this report, the results of clinical studies performed in three generations of a family of 39 members with autosomal dominant Fanconi syndrome are presented. Twenty-one members of this family provided blood and urine for biochemical evaluation. Many family members have one or more tubular reabsorptive abnormalities; however, the complete Fanconi syndrome was not present in most members. Three children with the complete syndrome all occur in the last generation. When the characteristic features of this family were compared with those of previously reported families with autosomal dominant Fanconi syndrome, several differences became apparent. Two serious manifestations, diabetes mellitus and renal failure, which occur in previous reports did not occur in this family. This report provides information on apparently the largest number of affected individuals in a single family with Fanconi syndrome. In addition, variable expressivity of tubular reabsorptive defects in a family with Fanconi syndrome has never been reported.