An association study of PITX2 polymorphism in a cohort of patients with primary open angle glaucoma and considerations on the genetics of glaucoma

Abstract

Background: Glaucoma is a major cause of blindness world-wide. There is a need for methods to identify individuals at risk of developing glaucoma, so that early treatment can prevent visual loss. Genetic screening tests offer the prospect of pre-symptomatic diagnosis of at risk individuals. There is now strong evidence that a number of different genes are associated with glaucoma susceptibility. Mutations in the PITX2 homeobox transcription factor gene disrupt normal development of the anterior segment and cause overt structural abnormalities. It is possible that, as yet undetected mutations/polymorphisms in PITX2 may produce subtle and undetected abnormalities in anterior segment structure and function that could predispose to glaucoma. Purpose: The aim of this thesis is two fold: 1. Screening for the presence of single nucleotide polymorphisms in PITX2 gene in a cohort of 100 unrelated primary open angle glaucoma/ ocular hypertension patients, 10 Posterior embryotoxon subjects and 100 age and ethnically matched controls to establish the mutation spectrum. 2. Identification, phenotyping and recruitment for genetic studies of primary open angle glaucoma patients with strong family history of glaucoma. Materials and methods: 1. 100 primary open angle glaucoma patients and 60 age and ethnically matched controls were enrolled in the study. Patients and controls were phenotyped and a blood sample for DNA extraction collected. PITX2 exon-specific primers were used to PCR amplify patient and control DNA. Direct sequencing was used to screen for sequence alterations in the entire coding sequence of PITX2 gene. Concurrently, polymorphic sites reported in the PITX2 gene were identified from the NCBI and Ensemble databases and the frequency of polymorphic sites was investigated. The SHEsis and UNPHASED software packages were used for statistical analysis. 2. Patients diagnosed with glaucoma and strong family history were identified from Glaucoma Unit at Sunderland Eye Infirmary, phenotyped and enrolled in the study. The pedigrees were constructed and interested relatives enrolled in the study and phenotyped. A sample of blood for DNA extraction was collected from all people enrolled in the study. Results: 1. Direct sequencing did not identify any sequence variation in the coding region. 26 PITX2 polymorphic sites were identified from the internet databases, including five in the coding sequence. Sixteen non coding SNPs were confirmed within our study group and SNP frequencies were examined. None of the coding sequence SNPs was identified in our cohort, demonstrating a high degree of sequence conservation. Also, none of the SNPs confirmed in this study group showed an increased frequency in the primary open angle glaucoma group compared with the control group. 2. Thirty-three pedigrees were identified with strong family of glaucoma during the time allowed for patient recruitment. Of these, twenty-two agreed to take part in the study. Thirteen pedigrees are presented in this study, mostly demonstrating autosomal dominant inheritance. Conclusion: There is ample evidence to suggest that genetics play an important role in unravelling the pathogenesis of glaucoma. Identification and recruitment of patients for genetic studies is an essential step and the role of the clinician in this process is paramount. Also, developmental glaucoma genes are an important group of genes to be screened in primary open angle glaucoma/ocular hypertension patients, as they may play a role in the pathogenesis of this preventable blinding disease.