Central Corneal Thickness Demonstrates No Correlation with TonoLab-measured IOP in Strains of Mice with Single Transgenic Deletions of Matricellular Proteins

Abstract

Purpose: : Intraocular pressure (IOP) is a major risk factor for glaucoma. The mouse model has become useful due to recent advances in IOP measurement instrumentation. A recent study of five strains of mice found no correlation between mean central corneal thickness (CCT) and mean IOP across the strains. Matricellular proteins affect extracellular matrix which could affect the cornea. We hypothesized that CCT and IOP, both within and across several strains of mice with single matricellular gene deletions, would not correlate despite the theoretical possibility of altered CCT.

Methods: : Over 1800 mice representing wild-type (WT) and transgenic mouse strains with single gene deletions (KO) of thrombospondin-1 (TSP1), thrombospondin-2 (TSP2), osteopontin (OPN), hevin, and secreted protein acidic rich in cysteine (SPARC) were imaged using optical coherence tomography (Stratus, Zeiss) under anesthesia to determine CCT, measuring peak-to-peak amplitude. IOP was measured between 11am and 3pm using TonoLab, one week later, at 7 weeks of age. Mice were anesthetized by intraperitoneal injection of a ketamine:xylazine mixture (100mg/kg:9mg/kg). Only measurements with "no significant variability" were accepted, the average taken from three sets of six alternating measurements in each eye.

Results: : Two-tailed student t-tests were used to compare both CCT and IOP between KO strains and their WT counterparts. Mean CCT was reduced by 5-6% compared to WT for TSP1, OPN, and SPARC KO mice (p=1.6x10^-7 [n=113], p=1.6x10^-11 [n=160], and p=1.9x10^-7 [n=114], respectively). As previously demonstrated, mean IOP was reduced by 15% in SPARC KO mice (p=3.8x10^-9 [n=114]), and more modestly in TSP1 and TSP2KO strains (p=2.1x10^-5 [n=113] and p=0.0029 [n=92], respectively). There was no correlation between CCT and IOP both across (y=0.0528x+11.3; r²=0.0512) and within (TSP1 r²=0.080, TSP2 r²=0.0003, OPN r²=0.0002, hevin r²=0.0174, SPARC r²=0.0628) transgenic mouse strains with single gene deletions of matricellular proteins.

Conclusions: : Our findings suggest that CCT does not affect TonoLab IOP readings and may not be needed to interpret IOP in mice. We also confirmed the previously reported IOP differences between SPARC, TSP1 and TSP2 KO and WT mice.

Keywords: intraocular pressure • cornea: basic science