Genetic studies towards determining disease mechanisms in Lmx1b mutant mice

Abstract

Purpose : Mutations in LMX1B cause glaucoma. There is a wide range of phenotypic variability between individuals with LMX1B mutations suggesting that genetic modifiers impact individual susceptibility to glaucoma. Mice with dominant Lmx1b mutations (Lmx1b^V265D and Lmx1b^Q82X) display a similar range of phenotypes to their human counterparts. We characterized ocular phenotypes across multiple inbred mouse strain backgrounds with Lmx1b mutations and identified a modifier locus.

Methods : The Lmx1b^V265D allele (also known as Lmx1b^Icst) was backcrossed onto the C57BL/6J (B6), 129S6/SvEvTac (129), C3H/HeJ (C3H), and DBA/2J.Gpnmb(wt) (D2) backgrounds for at least 6 generations. The Lmx1b^Q82X allele was backcrossed onto B6 and D2 backgrounds. Mice were examined using a slit-lamp. IOP measurements, histology, and immunofluorescence on whole-mounted tissue were also performed. To identify genetic modifiers of the Lmx1b^V265D mutant phenotype, we performed a mapping cross between B6 and 129 backgrounds.

Results : Differences in the presentation of ocular phenotypes between the different strain backgrounds were evident for both mutant alleles. D2.Lmx1b^Q82X mice had age-related IOP elevation and glaucomatous nerve damage. In contrast, B6.Lmx1b^Q82X mice did not develop elevated IOP or nerve damage at any examined age. B6.Lmx1b^V265D mice had elevated IOP and severe anterior segment developmental anomalies, including malformed eccentric pupils, irido-corneal strands, corneal abnormalities, and abnormal Schlemm’s canal at multiple ages. They also developed glaucomatous nerve damage. In contrast, 129.Lmx1b^V265D mice had very mild anterior segment abnormalities, modestly elevated IOP and did not develop glaucomatous nerve damage. On the C3H and D2 backgrounds, Lmx1b^V265D mutants had intermediate phenotypes compared to the 129 (resistant) and B6 (susceptible) backgrounds. A mapping cross between B6 and 129 mice with the Lmx1b^V265D allele identified a modifier locus on chromosome 18, with the 129 allele substantially lessening phenotypic severity.

Conclusions : Genetic background profoundly influences susceptibility to glaucoma-relevant phenotypes in Lmx1b mutant mice. There is an important modifier locus on chromosome 18 and its future characterization is expected to provide new molecular information about pathways modulating glaucoma-relevant phenotypes.

This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.