Identification of LMX1B gene point mutations in italian patients affected with Nail-Patella syndrome

Nail-Patella Syndrome: clinical and molecular data in 55 families raising the hypothesis of a genetic heterogeneity

Nail-Patella Syndrome (NPS) is a rare autosomal dominant condition comprising nail and skeletal anomalies. Skeletal features include dysplastic patellae and iliac horns, as well as scapula and elbow dysplasia. Nephropathy and glaucoma or intra-ocular hypertension can sometimes be present. NPS is due to variants affecting function in LMX1B, which encodes a LIM-homeodomain protein critical for limb, kidney and eye development. We describe the phenotype and the molecular data of 55 index patients and their 39 relatives presenting with typical NPS. We identified 38 different LMX1B anomalies, 19 of which were not reported before. In our series, 9% of families are not carriers of a LMX1B genomic alteration after extensive study of the coding and non-coding regions of the gene. One of the families showed no linkage to the LMX1B locus, raising the hypothesis of a genetic heterogeneity.

A microdeletion of chromosome 9q33.3 encompasses the entire LMX1B gene in a Chinese family with nail patella syndrome

Nail patella syndrome (NPS) is an autosomal dominant disorder characterized by nail malformations, patellar apoplasia, or patellar hypoplasia. Mutations within the LMX1B gene are found in 85% of families with NPS; thus, this gene has been characterized as the causative gene of NPS. In this study, we identified a heterozygous microdeletion of the entire LMX1B gene using multiplex ligation-dependent probe amplification (MLPA) in a Chinese family with NPS. The determination of the deletion breakpoints by Illumina genome-wide DNA analysis beadchip showed that the deletion was located in chromosome 9q33.3 and spanned about 0.66 Mb in size. This heterozygous deletion provides strong evidence for haploinsufficiency as the pathogenic mechanism of NPS.

LMX1B mutations cause hereditary FSGS without extrarenal involvement

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.

In vivo expression of putative LMX1B targets in nail-patella syndrome kidneys

The nail-patella syndrome (NPS) is characterized by nail and bone abnormalities, associated with glomerular involvement in approximately 40% of patients. Typical glomerular changes consist of fibrillar material in the irregularly thickened glomerular basement membrane. NPS is inherited as an autosomal dominant trait and caused by heterozygous loss of function mutations in LMX1B, a member of the LIM homeodomain protein family. Mice with homozygous inactivation of the gene exhibit nail and skeletal defects, similar to those observed in patients, associated with glomerular abnormalities. Strong reduction in the glomerular expression of the alpha3 and alpha4 chains of type IV collagen, and of podocin and CD2AP, two podocyte proteins critical for glomerular function, has been observed in Lmx1b null mice. The expression of these proteins appeared to be regulated by Lmx1b. To determine whether these changes in podocyte gene expression are involved in the development of NPS nephropathy, using immunohistological techniques, we analyzed the podocyte phenotype and the renal distribution of type IV collagen chains in the kidneys of seven NPS patients with severe glomerular disease. We also examined the nature of the fibrillar material present within the glomerular extracellular matrix. The glomerular basement membrane fibrillar material was specifically labeled with anti-type III collagen antibodies, suggesting a possible regulation of type III collagen expression by LMX1B. The expression of the alpha3 and alpha4 chains of type IV collagen, and of podocin and CD2AP, was found to be normal in the seven patients. These findings indicate that heterozygous mutations of LMX1B do not appear to dramatically affect the expression of type IV collagen chains, podocin, or CD2AP in NPS patients.

Nail patella syndrome: a review of the phenotype aided by developmental biology

Nail patella syndrome (NPS) is an autosomal dominant condition affecting the nails, skeletal system, kidneys, and eyes. Skeletal features include absent or hypoplastic patellae, patella dislocations, elbow abnormalities, talipes, and iliac horns on x ray. Kidney involvement may lead to renal failure and there is also a risk of glaucoma. There is marked inter- and intrafamilial variability. The results of a British study involving 123 NPS patients are compared with previously published studies and it is suggested that neurological and vasomotor symptoms are also part of the NPS phenotype. In addition, the first data on the incidence of glaucoma and gastrointestinal (GI) symptoms in NPS are presented. NPS is caused by loss of function mutations in the transcription factor LMX1B at 9q34. The expansion of the clinical phenotype is supported by the role of LMX1B during development.