ABSTRACT:Renal hypoplasia, defined as abnormally small kidneys with normal morphology and reduced nephron number, is a common cause of pediatric renal failure and adult-onset disease. Genetic studies performed in humans and mutant mice have implicated a number of critical genes, in utero environmental factors and molecular mechanisms that regulate nephron endowment and kidney size. Here, we review current knowledge regarding the genetic contributions to renal hypoplasia with particular emphasis on the mechanisms that control nephron endowment in humans and mice. R enal hypoplasia is a common, yet poorly understood and misused term describing congenital renal anomalies. Renal hypoplasia is defined as abnormally small kidneys (Ͻ2 SD below the expected mean when correlated with age or parameters of somatic growth) with normal morphology and reduced nephron number. This definition predicts that ϳ2.2% of the population exhibit renal hypoplasia, whereas epidemiologic studies suggest an estimated incidence of 1 in 400 births (1). Much confusion and the misapplication of this definition have arisen because the majority of congenitally small kidneys also exhibit evidence of tissue maldifferentiation, defined as renal dysplasia. The exact incidence of pure renal hypoplasia (without dysplasia) is difficult to define as renal dysplasia has often been incorrectly described as hypoplasia. This has predominantly been due to the lack of noninvasive diagnostic tools (i.e. Ultrasound) with resolution power adequate enough to discriminate dysplasia from hypoplasia in such settings. Severe reductions in nephron number that are characteristic of renal hypoplasia/dysplasia are the leading cause of childhood end stage renal disease. Indeed, if severe enough, these conditions can lead to significant impairment of intrauterine renal function which can in turn lead to the oligohydramnios sequence, a condition not compatible with extrauterine life. This includes severe and modest bilateral renal hypoplasia. More subtle defects in nephron number, such as those at the lower end of the normal range caused by mild bilateral renal hypoplasia, have been associated with the development of adult-onset hypertension and chronic renal failure (2-6). Here, we focus on knowledge derived from the study of human syndromic forms of renal hypoplasia and mouse mutants that provide insights into the molecular mechanisms that underlie renal hypoplasia and control nephron endowment. Congenital renal abnormalities characterized by nephron number and other renal pathologies including renal dysplasia, hydroureter, cystic dysplasia, and agenesis are reviewed in detail elsewhere (7-9).
OVERVIEW OF KIDNEY DEVELOPMENTDevelopment of the mammalian metanephric kidney is dependent on reciprocal inductive interactions between two distinct cell lineages, the ureteric cell lineage and the metanephric mesenchyme (MM) cell lineage (Fig. 1). At the onset of metanephric development, signals emanating from the mass of uninduced MM initiate the formation of an epithelial bud (th...