Dissection of a QTL Locus on Mouse Proximal Chromosome 5 that Modulates Intraocular Pressure (IOP) leads to Identification of a Potential New Drug Target for Glaucoma

Abstract

Purpose: Current IOP lowering therapies are used on a trial and error basis without a pharmacogenomic rationale, leading to variability in drug effect. Within our large high-throughput gene screening, we identify genetic modifiers of IOP using the enlarged BXD glaucoma murine reference panel in combination with human GWAS glaucoma cohorts. Our objective in the study was to identify new gene loci modulating IOP using systems genetics and design new drug targets for personalized glaucoma therapy.

Methods: We acquired IOP measurements for parents and 80 progeny BXD lines at 1-2, 3-5, 6-9, 10-13 and >13 months of age, using a TONOLAB. Conventional arrays and RNA-seq data were used to estimate gene expression from eyes of parents and progeny. IOP datasets were mapped using GeneNetwork (www.genenetwork.org). Candidate genes for high IOP/ primary open angle glaucoma (POAG) were nominated using the following criteria: (1) genes are located within confidence intervals of QTLs; (2) genes have coding differences segregating among progeny; (3) genes are expressed in the eye, and are associated with cis-expression QTL (eQTL); (4) expression of transcripts covary with IOP; (5) genes have a plausible biological link to IOP and glaucoma; (6) genes are close to linkage peaks in the GLAUGEN and NEIGHBOR human POAG GWAS studies; and (7) genes are druggable. Subsequently, a druggable gene drug target and a small molecule drug were selected for further studies. The IOP lowering effect on high and low expresser BXD strains of the selected gene was studied (n=6) and the pharmacodynamic profile computed.

Results: IOP varied from 9.3±0.8 to 21.8±1.7 mmHg across BXDs. We identify a robust eQTL on proximal Chr 5 in BXD mice strains aged 10-13 mo. Within this QTL, candidates were nominated and the best SNP in each candidate were identified using human data from the GLAUGEN/NEIGHBOR GWAS data. Within the candidates, we identified a druggable gene target (a cation channel; p-value= 0.007526) that regulated ion transport in the eye. A single topical dose of the cation channel blocker reduced IOP by >4mmHg (>20%) in high BXD expressers from baseline within 3 h and returned to baseline after 6 h.

Conclusions: We have identified potential candidate genes that modulate IOP and further evaluated the suitability of a druggable gene target for personalized POAG therapy.

Keywords: 568 intraocular pressure • 536 gene modifiers • 535 gene microarray