MAGI2 Mutations Cause Congenital Nephrotic Syndrome

Abstract

Steroid-resistant nephrotic syndrome (SRNS), a heterogeneous disorder of the renal glomerular filtration barrier, results in impairment of glomerular permselectivity. Inheritance of genetic SRNS may be autosomal dominant or recessive, with a subset of autosomal recessive SRNS presenting as congenital nephrotic syndrome (CNS). Mutations in 53 genes are associated with human SRNS, but these mutations explain ≤30% of patients with hereditary cases and only 20% of patients with sporadic cases. The proteins encoded by these genes are expressed in podocytes, and malfunction of these proteins leads to a universal end point of podocyte injury, glomerular filtration barrier disruption, and SRNS. Here, we identified novel disease-causing mutations in membrane-associated guanylate kinase, WW, and PDZ domain-containing 2 (MAGI2) through whole-exome sequencing of a deeply phenotyped cohort of patients with congenital, childhood-onset SRNS. Although MAGI2 has been shown to interact with nephrin and regulate podocyte cytoskeleton and slit diaphragm dynamics, MAGI2 mutations have not been described in human SRNS. We detected two unique frameshift mutations and one duplication in three patients (two families); two siblings shared the same homozygous frameshift mutation, whereas one individual with sporadic SRNS exhibited compound heterozygosity. Two mutations were predicted to introduce premature stop codons, and one was predicted to result in read through of the normal translational termination codon. Immunohistochemistry in kidney sections from these patients revealed that mutations resulted in lack of or diminished podocyte MAGI2 expression. Our data support the finding that mutations in the MAGI2 gene are causal for congenital SRNS.

Keywords: familial nephropathy; genetic renal disease; nephrin; nephrotic syndrome; podocyte; proteinuria.

Figure 1.

MAGI2 mutations causing CNS. (A) The exon structure of MAGI2 cDNA (NM_012301.3); 22 coding exons with start and stop codons are indicated. (B) Domain structure of the MAGI2 protein. Six PDZ domains (PDZ0–PDZ5) are shown in blue, one guanylate kinase (GK) domain is shown in yellow, and two WW domains are shown in green. (C) Frameshift variants found in three patients with CNS. Individual 180 is a compound heterozygote: the variant in exon 1 was inherited from the father (180F), and the variant in exon 20 was inherited from the mother (180M). Individual 175 and her sister, 175S, are homozygous for a single-nucleotide deletion in exon 22. The mother of the siblings is heterozygous for the variant.

Figure 2.

MAGI2 expression is altered in patients with mutations. Immunohistochemical staining with human anti–MAGI2 antibody (Sigma-Aldrich) is shown. (A) A kidney that was not suitable for transplant was used as a control. (B) Biopsy specimen obtained from an individual with minimal change disease; MAGI2 staining is seen in the glomeruli at the periphery of capillary loops, consistent with podocyte localization. Staining in tubules is also seen. (C) Nephrectomy specimen obtained from patient 175 (homozygous mutation in MAGI2) shows weak but positive glomerular MAGI2 staining. (D) Biopsy specimen obtained from patient 180 (compound heterozygous mutation in MAGI2) shows complete lack of MAGI2. Original magnification, ×20. Scale bar, 100 μm.

Figure 3.

Nephrin immunostaining is altered in patients with MAGI2 mutations. Nephrin immunostaining on paraffin–embedded renal tissue sections. Confocal microscopy images show nephrin staining in green and DAPI (4′,6-diamidino-2-phenylindole) nuclear staining in blue. (A) Positive control; normal human kidney. (B) Negative control (no nephrin antibody); normal human kidney. (C) Nephrectomy specimen obtained from patient 175 (homozygous mutation in MAGI2). (D) Biopsy specimen obtained from patient 180 (compound heterozygous mutation in MAGI2). Upper panels show the whole glomerulus (×40±3), and lower panels in A, C, and D show higher original magnification (×100±7). Lower panels show decreased/disrupted nephrin expression between normal and patient podocytes.