Yet more ways to skin a cat: nephrin mutations outside the neonatal period.

The earliest descriptions of a genetic basis for the nephrotic syndrome concerned clinical phenotypes of relatively limited variability apparently as a result of classical Mendelian disorders with monogenic inheritance. Paradigmatic was the description by Tryggvason and his colleagues1 of the genetic basis of the Finnish type of congenital nephrotic syndrome (CNS) caused by autosomal recessive mutations in NPHS1 (encoding the protein nephrin). Subsequently, Antignac and colleagues2 described the gene NPHS2 (encoding the protein podocin) responsible for an autosomal recessive form of steroid-resistant nephrotic syndrome (SRNS) presenting later in childhood. To date, eight genes expressed in glomerular podocytes have been described, mutations of which are associated with development of the nephrotic syndrome with onset varying from the neonatal period to adulthood. The discovery of mutations in these same genes outside the clinical phenotypes of SRNS in which they were first described has shed new light on the complexity of the genetic basis of SRNS. Mutations in NPHS2 have been found in a large number of infants with CNS (more commonly than NPHS1 mutations in one study3). Genetic heterogeneity of CNS, including triallelic involvement of NPHS1 and NPHS2, has also been described.4 Even within the classical age range of SRNS caused by NPHS2 mutations, nonsense or homozygous p.R138Q mutations have been shown to present earlier than (other) missense mutations.5 Now Philippe et al.6 report in this issue of JASN that mutations in NPHS1 are found in some patients with SRNS presenting outside the neonatal period. As with neonatal presentation of triallelic NPHS1/ NPHS2 mutations,4 these individuals manifest histologic patterns quite different from classical CNS. Why are these mutations now being found outside their original phenotypes? In part, this may be because genotyping of such cases has so far been difficult and expensive, so mutations in a particular gene have generally not been sought in patients “outside” the classical phenotype. Could these extended phenotypes also be due to novel mutations? At least one mutation in NPHS2 (leading to p.R138Q in podocin) has been described both in CNS3,4 and in infantile and childhood-onset SRNS.2,4 This is also the case for at least two of the 14 mutations in NPHS1 reported by Philippe et al.,6 which have been reported before in the context of classical CNS, in one case (p.R460Q) associated as a heterozygous mutation with neonatal onset of nephrotic syndrome leading to kidney failure at 10 mo of age.7 Alternatively, does SRNS appear later in some patients because they are compound heterozygotes with one “mild” mutation in NPHS1? Philippe et al. present in vitro functional evidence for normal trafficking to the plasma membrane and protein–protein interactions of nephrin in most of their missense mutations. At the same time, two of the mutations with some preserved in vitro functions were severe enough to lead to kidney failure by 13 yr of age. Because Philippe et al. excluded in advance patients having detectable NPHS2 or WT1 mutations, the interesting question of oligogenic involvement of mutations in NPHS1 and in one or both of these other genes cannot be addressed from their study. A recent report8 suggested that bigenic (digenic) mutations in five genes (WT1, NPHS1, NPHS2, CD2AP, and PLCE1) may account for a substantial number (four of 19) of cases of sporadic childhoodonset SRNS, so undetected digenic mutations could conceivably explain nonclassical phenotypes associated with NPHS1 mutations. The occurrence of heterozygous NPHS1 mutations3,9 suggests that digenic mutations also may occur in classical presentations of CNS. Clearly, we do not as yet understand the genetic basis for phenotypic variation in SRNS; simple monogenic models are certainly inadequate. A complex form of inheritance has been well described in other genetic diseases affecting the kidney. Bardet-Biedl syndrome (BBS) is associated with mutations in 12 different genes and shows broad phenotypic variability.10 Katsanis11 proposed that this variability is in part due to an oligogenic (triallelic) mode of inheritance, based on epistatic interactions of heterozygous (missense) mutations in a BBS-associated gene with homozygous or compound heterozygous mutations in another BBS-associated gene. Mutations in these genes presumably interact because the genes underlie a system (or module12) of interacting proteins (in this case, proteins of the sensory cilia and central body in tubule cells). Similarly, because a large number of Published online ahead of print. Publication date available at www.jasn.org.