Distribution of DNA Sequence Variants in COL8A1 and COL8A2 in Glaucoma Patients With Thin CCT

Abstract

Purpose: : Previous studies of COL8A2 knock out mice suggested that loss of COL8A2 function is associated with thinning of the central cornea. The purpose of this study is to examine the COL8A2 gene in human glaucoma patients to determine if variants in the COL8A2 gene are associated with thin central corneal thickness.

Methods: : A total of 463 glaucoma patients were enrolled in this study. Glaucoma was defined as an IOP greater than 22 without medications and optic nerve changes consistent with glaucomatous disease. CCT was measured three times in each eye and averaged. Initially genomic DNA from 8 individuals with average CCT less than 550 was sequenced for the entire COL8A2 coding sequence and flanking intron sequence after PCR amplification. Sequencing reactions were performed with BIGDYE chemistries and an automated ABI 3100 sequencer and were analyzed using Vector NTI software. The entire study population was subsequently evaluated for two missense changes discovered in exon 2 in the initial 8 patients, also using direct genomic sequencing.

Results: : Two missense mutations were identified in the COL8A2 gene in this patient population: R155Q, which has been previously associated with Fuchs dystrophy, and a novel change, P678L. Eight of the 230 glaucoma patients with a CCT less than 530 had one of these two missense mutations, while none of the 233 glaucoma patients with CCT greater than 530 had any mutation in COL8A2 (p <.007, Fisher’s exact test). Two common intronic SNPs did not demonstrate an association with CCT. The average CCT of the 8 patients carrying a COL8A2 missense mutations was 516 +12.

Conclusions: : The results of this study suggest that DNA sequence variants in COL8A2 may contribute to thin central corneal thickness in some glaucoma patients. Both the missense changes identified in this study affect evolutionarily conserved amino acids and represent nonconservative changes. These results would support a hypothesis that loss of function of COL8A2 can predispose to thin central corneas, consistent with the phenotype of the COL8A2 knock-out mouse. Further evaluation of patients with these mutations may help define the relationship between central corneal thickness and the development of glaucoma.

Keywords: genetics • cornea: basic science • clinical (human) or epidemiologic studies: risk factor assessment