Recessive NPHS2 (Podocin) Mutations Are Rare in Adult-Onset Idiopathic Focal Segmental Glomerulosclerosis

Abstract

Recessive NPHS2 (podocin) mutations account for up to approximately 30% of steroid-resistant idiopathic FSGS in children and are associated with a reduced risk for disease recurrence after renal transplantation. R229Q, a missense variant that is present in 3.6% of the white population, has been implicated as a common disease-causing mutation. Given these clinical implications, we examined the role of NPHS2 mutations in a cohort of patients with adult-onset FSGS. We used denaturing HPLC to screen for heterozygous and homozygous gene variants in PCR-amplified DNA fragments that contained all exons and splice junctions of NPHS2. Bidirectional sequencing was performed to define all of the gene variants detected. With the use of the denaturing HPLC in a single-blind pilot study, 40 of 43 known NPHS2 mutations were detected from 22 pediatric patients with FSGS to establish a test sensitivity of 93%. This screen then was applied to 87 adult patients with idiopathic FSGS (15 steroid-sensitive, 63 steroidresistant, and nine familial cases). In this latter cohort, compound heterozygous mutations were detected only in one patient with steroid-sensitive FSGS (R229Q and Q285fsX302) and no homozygous mutations. Overall, R229Q accounted for eight (80%) of ten of the putative mutant alleles that were detected in the study cohort. Contrary to the pediatric experience, recessive NPHS2 mutations are rare in this study population, suggesting that the pathogenesis of FSGS in adults may differ from that in children. These data do not support R229Q as a disease-causing mutation for steroid-resistant FSGS.

Calcium-sensing receptor mutations and denaturing high performance liquid chromatography

“…A newer technique that detects heteroduplexes in PCR amplicons by ion-pair reverse-phase high performance liquid chromatography has been developed (Oefner & Underhill 1998, Xiao & Oefner 2001. Denaturing highperformance liquid chromatography (DHPLC) has proved useful for mutational analysis of genes such as MEN1 (Crépin et al 2006), RB1 (Houdayer et al 2004), ENG and ALK-1 (Lenato et al 2006), NPHS2/podocin (He et al 2007), and BRCA1/2 (Gerhardus et al 2007). We (Hendy et al 2003) and others (Waller et al 2004) have successfully utilized the DHPLC method for CASR mutational analysis in a few cases.…”

Treatment of Primary FSGS in Adults

“…[12][13][14] These mutations are most often associated with familial forms of FSGS, but sporadic mutations in podocin have been reported and the presentation can be indistinguishable from that of primary FSGS. Sporadic podocin mutations have been observed in up to 30% of children with steroid-resistant FSGS 15 but are extremely rare in adults; 16,17 therefore, routine screening is not advocated in adults. 16 Recently, patients with highrisk variants of two other proteins important in podocyte function-nonmuscle myosin heavy chain-9 and apolipoprotein L1-have been found to be at markedly increased risk of developing FSGS.…”

“…Sporadic podocin mutations have been observed in up to 30% of children with steroid-resistant FSGS 15 but are extremely rare in adults; 16,17 therefore, routine screening is not advocated in adults. 16 Recently, patients with highrisk variants of two other proteins important in podocyte function-nonmuscle myosin heavy chain-9 and apolipoprotein L1-have been found to be at markedly increased risk of developing FSGS. 18,19 These high-risk variants may be seen in up to 60% of African American patients but in less than 5% of European American patients, and it is felt that this difference may in part explain the excessive risk for FSGS and ESRD observed in African American patients.…”

Dosage-dependent role of Rac1 in podocyte injury

“…The activity of the podocin promoter has been proposed to be an indicator of podocyte health (12), so it is unclear whether the decline in podocin and nephrin expression in the CA-RAC1-expressing glomerulus is a direct or indirect regulatory effect of Rac1 or even a comprehensive consequence of podocyte injury.…”