LMX1B mutations cause hereditary FSGS without extrarenal involvement

Abstract

LMX1B encodes a homeodomain-containing transcription factor that is essential during development. Mutations in LMX1B cause nail-patella syndrome, characterized by dysplasia of the patellae, nails, and elbows and FSGS with specific ultrastructural lesions of the glomerular basement membrane (GBM). By linkage analysis and exome sequencing, we unexpectedly identified an LMX1B mutation segregating with disease in a pedigree of five patients with autosomal dominant FSGS but without either extrarenal features or ultrastructural abnormalities of the GBM suggestive of nail-patella-like renal disease. Subsequently, we screened 73 additional unrelated families with FSGS and found mutations involving the same amino acid (R246) in 2 families. An LMX1B in silico homology model suggested that the mutated residue plays an important role in strengthening the interaction between the LMX1B homeodomain and DNA; both identified mutations would be expected to diminish such interactions. In summary, these results suggest that isolated FSGS could result from mutations in genes that are also involved in syndromic forms of FSGS. This highlights the need to include these genes in all diagnostic approaches to FSGS that involve next-generation sequencing.

Figure 1.

Patients with p.R246Q or p.R246P LMX1B mutations have FSGS without extrarenal involvement. (A) Trichrome staining of kidney sections from patient V.II:1. Segmental sclerotic lesions are observed in the two glomeruli along with tubulointerstitial fibrosis. Original magnification, x80. (B) Electron microscopy of kidney sections from patient F.III:6. The glomerular basement membrane has a regular thickness and normal structure, and no specific ultrastructural change usually observed in NPS. Original magnification, x6500. (C) Photograph of the right hand of patient F.III:6. Nails display no features of dysplasia, such as triangular lunulae, longitudinal nail ridging, or splitting. (D–F) Radiographs of the elbows (D), pelvis (E), and knees (F) of patient V.II:1. No elbow dysplasia, radial head hypoplasia, iliac horn, patella dysplasia, or hypoplasia of the lateral femoral condyle can be noticed.

Figure 2.

All identified mutations involve Arginine 246 of LMX1B. (A) Pedigrees of the three families that carry LMX1B mutations. The allele status is given below each tested individual. A heterozygous (het) mutation was detected in all affected persons. Family studies confirmed the segregation of the mutant allele as an AD trait. Normal individuals are represented by an open square or circle, depending on the sex, and affected individuals by a solid square or circle. Strikethrough represents deceased individual. (B) Linkage analyses in family F. Seventeen regions of interest spanning 118 Mb were identified on chromosomes 1, 2, 3, 4, 6, 9, 11, 15, 17, and 19 (with a maximum Lod score of 1.2). One 8.9-Mb region on chromosome 9, indicated here by an arrow, contains the LMX1B locus. (C) Three-dimensional representation of the LMX1B homeodomain. Left panel: Full view of the homology model of the two monomers of LMX1B (shown as yellow schematic) bound to a DNA fragment (orange schematic). The homeobox model only covers residues 218–275 of LMX1B because it is based on the x-ray template of the transcription factor NKX2.5 homeodomain dimer bound to the proximal promoter -242 site of the atrial natriuretic factor gene (ANF-242 DNA). The arginine 246 residue is depicted with blue spheres, in the predicted bound conformation to a DNA fragment, shown in stick representation (cyan and green carbon, blue nitrogen, red oxygen, and orange phosphate atoms). Right panel: Zoomed-in insert showing the predicted polar interactions (dotted yellow lines) between the R246 nitrogen atoms (shown in blue stick representation) and the ribose and phosphate oxygen atoms (red sticks) of DNA.