Tear levels of SFRP1 are significantly reduced in keratoconus patients

Abstract

Purpose: To measure secreted frizzled-related protein 1 (SFRP1) levels in human tears and to investigate tear SFRP1 as a potential biomarker for keratoconus (KC).

Methods: Tears were collected from control (n = 33) and KC patients (n = 33) using micropipette tubes. Total tear protein was measured using a FluoroProfile Protein Quantification kit. An in-house enzyme-linked immunosorbent assay (ELISA) was developed to measure SFRP1 in control and KC tears. Statistical analyses of age, gender, the association of SFRP1, and total tear protein with KC were conducted.

Results: Tear SFRP1 was significantly decreased in KC, compared to age-matched controls (3.41 ng/μl ± 3.12 versus 5.55 ng/μl ± 5.62, respectively; p = 0.039). Conversely, total tear protein was significantly increased in KC, compared to age-matched controls (12.38 μg/μl ± 4.76 versus 9.40 μg/μl ± 3.88, respectively; p = 0.038). The ratio of SFRP1/total tear protein was also found to be significantly decreased in the KC group (p = 0.007). No significant association between tear SFRP1 and total tear protein was detected.

Conclusions: Tear SFRP1 was significantly decreased in age-matched KC versus control patients, and may be further reduced in moderate KC. Tear-SFRP1 levels alone do not provide an obvious biomarker for KC; however, our results provide further evidence that tear-protein profiles are altered in KC, and suggest the involvement of SFRPs in the pathogenesis of KC.

Figure 1

A representative standard curve for the tear secreted frizzled-related protein 1 (SFRP1) ELISA, with a correlation coefficient - R²-close to 1 (R²=0.9995). A high R² shows that the standard curve is reliable and can be used for sequential calculations.

Figure 2

Tear secreted frizzled-related protein 1 (SFRP1) and ratio of SFRP1/total tear protein were significantly lower in age matched KC, compared to control patients. A: Tear SFRP1 concentration was significantly lower in age-matched KC than controls. B: Total tear-protein concentration in age-matched KC was significantly higher than controls. C: Ratio of SFRP1/ total tear protein was significantly lower in age-matched KC compared to controls. Error bar: standard deviation.

Figure 3

Tear secreted frizzled-related protein 1 (SFRP1) concentration was highest in controls (Grade 0), compared to individual KC grades whereas the total tear-protein concentration was highest in KC Grade 3 and 4. A: Tear-SFRP1 concentration for control and KC patients showing that controls had the highest concentration. B: Total tear-protein concentration for control and KC showing highest concentration in advanced KC (Grade 3 and 4). Error bar: standard deviation.