A dominant-negative mutation of mouse Lmx1b causes glaucoma and is semi-lethal via LDB1-mediated dimerization [corrected

Sally H Cross,Alison L Kearney,Ian J Jackson,Caroline Thaung,Lisa Mckie,Fiona Young,Margaret Keighren,Danilo G Macalinao,Gareth R Howell,Rob Van T' Hof,Joe Rainger,Shalini Jadeja,Katrine West,Richard S Smith,Stephen C Kneeland,Simon W M John,Morag Robertson,Lorraine Rose,Gregory S Barsh

doi:10.1371/journal.pgen.1004359

Abstract

Mutations in the LIM-homeodomain transcription factor LMX1B cause nail-patella syndrome, an autosomal dominant pleiotrophic human disorder in which nail, patella and elbow dysplasia is associated with other skeletal abnormalities and variably nephropathy and glaucoma. It is thought to be a haploinsufficient disorder. Studies in the mouse have shown that during development Lmx1b controls limb dorsal-ventral patterning and is also required for kidney and eye development, midbrain-hindbrain boundary establishment and the specification of specific neuronal subtypes. Mice completely deficient for Lmx1b die at birth. In contrast to the situation in humans, heterozygous null mice do not have a mutant phenotype. Here we report a novel mouse mutant Icst, an N-ethyl-N-nitrosourea-induced missense substitution, V265D, in the homeodomain of LMX1B that abolishes DNA binding and thereby the ability to transactivate other genes. Although the homozygous phenotypic consequences of Icst and the null allele of Lmx1b are the same, heterozygous Icst elicits a phenotype whilst the null allele does not. Heterozygous Icst causes glaucomatous eye defects and is semi-lethal, probably due to kidney failure. We show that the null phenotype is rescued more effectively by an Lmx1b transgene than is Icst. Co-immunoprecipitation experiments show that both wild-type and Icst LMX1B are found in complexes with LIM domain binding protein 1 (LDB1), resulting in lower levels of functional LMX1B in Icst heterozygotes than null heterozygotes. We conclude that Icst is a dominant-negative allele of Lmx1b. These findings indicate a reassessment of whether nail-patella syndrome is always haploinsufficient. Furthermore, Icst is a rare example of a model of human glaucoma caused by mutation of the same gene in humans and mice.

Highlights

Nail-patella syndrome (NPS) (OMIM 161200) is an autosomal dominant human disease, cardinal features of which are nail dysplasia, absent or hypoplastic patellae and abnormal elbows along with iliac horns
Nail-patella syndrome is a human genetic disease caused by an inactivating mutation in one copy of a gene called LMX1B, with the amount of protein produced from the remaining copy of the gene not being enough for normal function
Icst is a missense mutation of the Lmx1b gene We identified the N-ethyl-N-nitrosourea-induced mouse mutation, iris-corneal strands (Icst), in a screen for dominant eye mutations

Summary

Introduction

Nail-patella syndrome (NPS) (OMIM 161200) is an autosomal dominant human disease, cardinal features of which are nail dysplasia, absent or hypoplastic patellae and abnormal elbows along with iliac horns. Mutations of the transcription factor LMX1B have been found to be the underlying cause of NPS [3,4,5,6]. Disease-causing mutations range from complete gene deletion to various frameshift, nonsense, splice and missense mutations. The majority of missense mutations are found in the homeodomain and the LIM domains. Several missense NPS homeodomain mutations tested in vitro have shown no dominant-negative effect on the transcriptional activity of wild-type protein [7,8] and NPS is thought to be a haploinsufficient disorder. In a comprehensive study of 106 NPS patients from 32 families, patients with mutations in the homeodomain had more severe proteinurea than those with mutations of the LIM domains, other aspects of NPS showed no phenotype-genotype correlation [9].

Methods

Results

Discussion

Conclusion