Quantification of proteoglycan 4 (PRG4) / lubricin in normal and Sjögren Syndrome human tears

Abstract

Purpose : Sjögren’s syndrome (SS) is an autoimmune disease with hallmark clinical symptoms of dry eye and dry mouth. Proteoglycan 4 (PRG4), or lubricin, is a boundary lubricant that is naturally present on the ocular surface and in tears. Recently PRG4 in human tears was shown to be susceptible to proteolytic digestion by cathepsin S, an enzyme with increased activity in SS tears, which destroyed the in vitro ocular surface boundary lubricating ability. However, whether levels of PRG4 are diminished in SS tears remains to be determined. The objective of this study was to quantify PRG4 levels in normal and SS human tears.

Methods : Tears were collected from 17 SS (15 F, 2 M, 56.2±16.7 years old) and 20 asymptomatic (n=20, 7 M, 13 F, 31.2±11.4 years old) participants, with approval from the Office of Research Ethics (UWaterloo). SS participants were diagnosed using the American European Consensus Criterion. Tears were collected without anaesthetic, from the inferior temporal tear meniscus of each eye, using a disposable microcapillary tube and frozen at -80C until use. The concentration of PRG4 was determined via a sensitive, competitive amplified luminescent proximity homogeneous assay using recombinant human PRG4 as the control. Total mass of PRG4 was calculated by normalizing concentration by tear volume, using 5.0 ul for normal tears and measured SS tear volume (0.1 to 2.3 ul). Data is reported as mean±SD, nonparametric statistics were employed (Mann-Whitney U & Levine tests).

Results : The concentration of PRG4 in SS (28.6±44.3 ug/ml) was not significantly different than that of normal tears (2.6±2.0 ug/ml, p=0.15), but did demonstrate significantly greater variation (p<0.001). The mass of PRG4 in SS tears (10.6±4.8 ng) was significantly diminished compared to normal tears (12.8±1.4 ng, p<0.05).

Conclusions : PRG4 concentration is significantly more variable in SS tears, and when normalized by volume, the PRG4 mass in SS tears is diminished compared to normal tears. These data suggest either a reduction in PRG4 production or an increase in PRG4 catabolism in SS tears relative to normal tears, which could be the cause of the variability of PRG4 concentration in SS tears. Given the role PRG4 plays in ocular surface health and its susceptibility to degradation by cathepsin S in SS tears, diminished PRG4 could contribute to signs and symptoms of dry eye in SS.

This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.