Abnormal renal phenotype in L1 knockout mice: a novel cause of CAKUT

L1, a member of the immunoglobulin superfamily, is a cell adhesion and signal transducing molecule. In the kidney, L1 is expressed in the mesonephric duct and the metanephros throughout collecting duct development. We show that mice with a targeted deletion of the L1 gene display diverse renal malformations including (i) a duplex kidney with two ureters partially or totally separated, accompanied by hydronephrosis; and (ii) an enlarged elongated kidney with a malformed or incorrectly positioned inner medulla. The type, penetrance and severity of these phenotypes are influenced by the genetic background. The development of a duplex kidney is initiated by double ureteral budding from the Wolffian duct or by an accessory budding from the main ureter, whereas medullary malformation is due to an improper growth and branching pattern of ureteral branches. Multiple developmental defects in formation of the collecting system promote subsequent renal damage and progression to renal insufficiency. Various features of mouse ureteral duplication resemble the human congenital anomalies of the kidney and urinary tract (CAKUT) although disturbances of medulla development have not yet been reported in men.