"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.
POAG lymphoblasts in the study cohort exhibited a defect in complex-I of the oxidative phosphorylation pathway, leading to decreased rates of respiration and ATP production. Studies in LHON and other diseases have established that lymphocyte oxidative phosphorylation measurement is a reliable indicator of systemic dysfunction of this pathway. While these defects did not impact lymphoblast growth when the cells were forced to rely on oxidative ATP supply, the authors suggest that in the presence of a multitude of cellular stressors as seen in the early stages of POAG, these defects may lead to a bioenergetic crisis in retinal ganglion cells and an increased susceptibility to cell death.
Keratoconus is a progressive and non-inflammatory thinning of the cornea, which may result in severe visual impairment due to irregular curvature and scarring. It can occur in isolation but is often seen in association with other systemic or ocular disorders. There is a well-recognised genetic component to keratoconus, as evidenced by family and twin studies; however, the aetiology of the disease is complex with both genetic and environmental factors playing a role. Over the last decade significant progress has been made in identifying genetic risk factors for keratoconus. Multiple approaches have been taken including candidate gene studies and genome-wide studies. VSX1 remains as the best characterised keratoconus gene but only accounts for rare cases. Other candidate genes with a role to play include SOD1, other corneal dystrophy genes such as ZEB1 and TGFBI and collagen genes. Family-based studies have recently led to the identification of the MIR184 gene for keratoconus with cataract and to the DOCK9 gene in a family with isolated keratoconus. Numerous other linkages have been reported and new sequencing technologies are set to rapidly expand the number of identified keratoconus genes in these regions. Similarly, recent genome-wide association studies in case-controlled cohorts have identified common variations in and around HGF, RAB3GAP1 and LOX as candidate risk factors for keratoconus. These gene identifications are beginning to reveal the molecular aetiology of keratoconus but despite this recent progress, there remain numerous genetic risk factors to be identified for this relatively common yet complex disease.
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