Mutational and Biological Analysis of α-Actinin-4 in Focal Segmental Glomerulosclerosis

Abstract

Mutations in the ␣-actinin-4 gene (ACTN4) cause an autosomal dominant form of focal segmental glomerulosclerosis (FSGS).A mutational analysis was performed of ACTN4 in DNA from probands with a family history of FSGS as well as in individuals with nonfamilial FSGS. The possible contribution of noncoding variation in ACTN4 to the development of FSGS also was assessed. Multiple nucleotide variants were identified in coding and noncoding sequence. The segregation of nonsynonymous coding sequence variants was examined in the relevant families. Only a small number of nucleotide changes that seemed likely to be causing (or contributing to) disease were identified. Sequence changes that predicted I149del, W59R, V801M, R348Q, R837Q, and R310Q changes were identified. For studying their biologic relevance and their potential roles in the pathogenesis of FSGS, these variants were expressed as GFP-fusion proteins in cultured podocytes. F-actin binding assays also were performed. Three of these variants (W59R, I149del, and V801M) showed clear cellular mislocalization in the form of aggregates adjacent to the nucleus. Two of these mislocalized variants (W59R and I149del) also showed an increased actin-binding activity. The I149del mutation segregated with disease; W59R was found to be a de novo mutation in the proband. A total of five ACTN4 mutations that are believed to be disease causing (three reported previously and two novel) as well as a number of variants with unclear contribution to disease now have been identified. The possibility that some of these other variants increase the susceptibility to FSGS cannot be excluded. ACTN4 mutations seem to account for approximately 4% of familial FSGS.

Essential Role of Integrin-Linked Kinase in Podocyte Biology

“…In ILK-deficient podocyte, ␣-actinin-4 forms aggregated dots ( Figure 7F). Such mislocalization of ␣-actinin-4 certainly would change the actin cytoskeleton dynamics (40) and may cause the collapse of the actin meshwork underneath or near the SD of the foot processes. Because actin cytoskeleton defines cell shape and morphology, altered cytoskeletal structure of the foot processes will lead to disappearance of the SD structures and development of an "effaced" phenotype, as demonstrated by electron microscopy studies (Figure 6).…”

Genomic and clinical profiling of a national nephrotic syndrome cohort advocates a precision medicine approach to disease management

“…It has been previously noted that mutations in these genes can sometimes cause disease with early childhood onset. 5,[13][14][15] Patients 30 and 38 were diagnosed at 7 and 3 years respectively, and found to have TRPC6 mutations, and patient 29 was diagnosed at 12 years and had a novel ACTN4 mutation. All 3 patients were sporadic.…”

Genetic testing in steroid-resistant nephrotic syndrome: why, who, when and how?

“…Since then, TRPC6 mutations have been implicated in childhood-onset FSGS, and even SRNS presenting within the first year of life, with variable disease severity [8,10,77,79,80]. Similarly, mutations in ACTN4, which encodes alphaactinin 4, typically cause late-onset FSGS with slow progression to ESRD [29,81], but mutations in this gene have been reported in children presenting with SRNS and rapid progression to ESRD [8,82]. Mutations in INF2, encoding inverted formin 2, were originally identified in patients with autosomal dominant SRNS, with age of onset ranging from adolescence and throughout adulthood [31,83].…”