Digenic Inheritance of Early-Onset Glaucoma: CYP1B1, a Potential Modifier Gene

"Early-onset glaucoma" refers to genetically heterogeneous conditions for which glaucoma manifests at age 5-40 years and for which only a small subset is molecularly characterized. We studied the role of MYOC, CYP1B1, and PITX2 in a population (n=60) affected with juvenile or early-onset glaucoma from the greater Toronto area. By a combination of single-strand conformation polymorphism and direct cycle sequencing, MYOC mutations were detected in 8 (13.3%) of the 60 individuals, CYP1B1 mutations were detected in 3 (5%) of the 60 individuals, and no PITX2 mutations were detected. The range of phenotypic expression associated with MYOC and CYP1B1 mutations was greater than expected. MYOC mutations included cases of juvenile glaucoma with or without pigmentary glaucoma and mixed-mechanism glaucoma. CYP1B1 mutations involved cases of juvenile open-angle glaucoma, as well as cases of congenital glaucoma. The study of a family with autosomal dominant glaucoma showed the segregation of both MYOC and CYP1B1 mutations with disease; however, in this family, the mean age at onset of carriers of the MYOC mutation alone was 51 years (range 48-64 years), whereas carriers of both the MYOC and CYP1B1 mutations had an average age at onset of 27 years (range 23-38 years) (P=.001). This work emphasizes the genetic heterogeneity of juvenile glaucoma and suggests, for the first time, that (1) congenital glaucoma and juvenile glaucoma are allelic variants and (2) the spectrum of expression of MYOC and CYP1B1 mutations is greater than expected. We also propose that CYP1B1 may act as a modifier of MYOC expression and that these two genes may interact through a common pathway.     

α-Lipoic Acid Antioxidant Treatment Limits Glaucoma-Related Retinal Ganglion Cell Death and Dysfunction

Oxidative stress has been implicated in neurodegenerative diseases, including glaucoma. However, due to the lack of clinically relevant models and expense of long-term testing, few studies have modeled antioxidant therapy for prevention of neurodegeneration. We investigated the contribution of oxidative stress to the pathogenesis of glaucoma in the DBA/2J mouse model of glaucoma. Similar to other neurodegenerative diseases, we observed lipid peroxidation and upregulation of oxidative stress-related mRNA and protein in DBA/2J retina. To test the role of oxidative stress in disease progression, we chose to deliver the naturally occurring, antioxidant α-lipoic acid (ALA) to DBA/2J mice in their diet. We used two paradigms for ALA delivery: an intervention paradigm in which DBA/2J mice at 6 months of age received ALA in order to intervene in glaucoma development, and a prevention paradigm in which DBA/2J mice were raised on a diet supplemented with ALA, with the goal of preventing glaucoma development. At 10 and 12 months of age (after 4 and 11 months of dietary ALA respectively), we measured changes in genes and proteins related to oxidative stress, retinal ganglion cell (RGC) number, axon transport, and axon number and integrity. Both ALA treatment paradigms showed increased antioxidant gene and protein expression, increased protection of RGCs and improved retrograde transport compared to control. Measures of lipid peroxidation, protein nitrosylation, and DNA oxidation in retina verified decreased oxidative stress in the prevention and intervention paradigms. These data demonstrate the utility of dietary therapy for reducing oxidative stress and improving RGC survival in glaucoma.     

Endoplasmic Reticulum Stress-Unfolding Protein Response-Apoptosis Cascade Causes Chondrodysplasia in a col2a1 p.Gly1170Ser Mutated Mouse Model

The collagen type II alpha 1 (COL2A1) mutation causes severe skeletal malformations, but the pathogenic mechanisms of how this occurs are unclear. To understand how this may happen, a col2a1 p.Gly1170Ser mutated mouse model was constructed and in homozygotes, the chondrodysplasia phenotype was observed. Misfolded procollagen was largely synthesized and retained in dilated endoplasmic reticulum and the endoplasmic reticulum stress (ERS)-unfolded protein response (UPR)-apoptosis cascade was activated. Apoptosis occurred prior to hypertrophy, prevented the formation of a hypertrophic zone, disrupted normal chondrogenic signaling pathways, and eventually caused chondrodysplasia. Heterozygotes had normal phenotypes and endoplasmic reticulum stress intensity was limited with no abnormal apoptosis detected. Our results suggest that earlier chondrocyte death was related to the ERS-UPR-apoptosis cascade and that this was the chief cause of chondrodysplaia. The col2a1 p.Gly1170Ser mutated mouse model offered a novel connection between misfolded collagen and skeletal malformation. Further investigation of this mouse mutant model can help us understand mechanisms of type II collagenopathies.