Glaucoma 1, Open Angle, A

A number sign (#) is used with this entry because of evidence that one form of primary open angle glaucoma (POAG), designated GLC1A, is caused by heterozygous mutation in the MYOC gene (601652) on chromosome 1q.

Heterozygous mutations in the CYP1B1 gene (601771) may also contribute to the phenotype by digenic inheritance.

For a general phenotypic description and a discussion of genetic heterogeneity of POAG, see 137760.

Clinical Features

Impressive 'dominant' pedigrees of juvenile glaucoma were reported by Stokes (1940), Allen and Ackerman (1942), and others. In a Scottish family that settled in Virginia, Courtney and Hill (1931) described 18 cases (10 males, 8 females) in 5 generations with 2 instances of failure of penetrance in the third generation. Onset was usually in the second or third decade and the course was rapid.

Crombie and Cullen (1964) described juvenile open angle glaucoma in 11 members of 5 generations (see 601652.0009 for a description of the gln337-to-arg mutation in the MYOC gene in that kindred). Harris (1965) observed 16 cases in 3 generations. The age of onset in 8 of these averaged 26 years. The angles of the anterior chambers were open in 1 patient on whom gonioscopy was performed early in the progress of the disease.

Juvenile glaucoma of the form present in the kindreds studied by Sheffield et al. (1993) and Richards et al. (1994) is an uncommon form of open angle glaucoma, usually recognized during childhood or early adulthood and often showing a strong family history. Johnson et al. (1993) found that the average age at diagnosis was 18 years. Affected family members tended to be myopic, but lacked other ocular or systemic abnormalities. The intraocular pressures of affected individuals were commonly more than 50 mm Hg when they were first examined. Gonioscopy showed open angles, with no abnormal pigmentation, iris processes, or embryonic tissue. Topical medications were initially effective in controlling intraocular pressure, but surgery was usually required for long-term pressure control.

Wiggs et al. (1995) reported the clinical features of 5 pedigrees in which primary juvenile glaucoma showed linkage to 1q21-q31. In the 23 affected patients, the average age of diagnosis was 18.5 years (range, 5 to 30 years), and the average initial intraocular pressure was 38.5 mm Hg (range, 30 to 53 mm Hg). Myopia was present in 87% of affected individuals, and 83% required surgical treatment for glaucoma. A difference in phenotypic expression was observed in a pair of monozygotic twins who carried the affected haplotype. In one twin, the diagnosis of severe glaucoma was made at the age of 19 years. Because of this diagnosis, his twin brother was examined and found to have somewhat elevated intraocular pressures but minimal optic nerve deterioration and full visual fields. Linkage to 1q21-q31 was excluded in 2 other families with juvenile glaucoma. One of these families was African American. In the other family, early onset of the disease was associated with severe optic nerve deterioration before 10 years of age, despite only moderate elevations of intraocular pressures.

Lotufo et al. (1989) found that juvenile-onset open angle glaucoma is more frequent among Americans of African ancestry. Johnson et al. (1996) described the clinical phenotype of juvenile-onset primary open angle glaucoma in a previously unpublished family showing linkage to 1q. The family included 22 affected individuals over 5 generations, of whom 12 were still living. The average age of diagnosis for living affected individuals was 26 years. An association between myopia and glaucoma was observed in this family, but the glaucoma was not associated with iris processes or other structural anomalies. The clinical course and response to treatment were similar to those in other families with this disease. The phenotype can range from mild ocular hypertension to blindness, with age of diagnosis ranging from 6 to 62 years. However, most affected individuals display a characteristic phenotype that includes onset in the first 3 decades of life, unusually high intraocular pressures, and the need for surgical therapy to prevent loss of vision.

Mapping

In a family in which 22 persons were affected with an autosomal dominant form of juvenile-onset open angle glaucoma (Johnson et al., 1993), Sheffield et al. (1993) used short tandem repeat markers in linkage analysis to map the gene to 1q21-q31. The highest lod score was provided by D1S212; lod = 6.5 at theta = 0.0. They noted that one of the atrial natriuretic peptide receptor genes (108960) maps to 1q21-q22. Richards et al. (1994) extended the linkage studies in a different family, confirming the assignment to 1q21-q31. Wiggs et al. (1994) confirmed the assignment to 1q by the study of 3 affected pedigrees. Meyer et al. (1994) confirmed linkage to 1q21-q23 in 2 large French kindreds; maximum lod = 7.60 with D1S212 at theta = 0.44.

In a Danish 5-generation dominant juvenile-onset glaucoma family, Graff et al. (1995) confirmed linkage to 1q; a maximum 2-point lod score of 6.67 was obtained for linkage to D1S210. However, multipoint linkage analysis in a 9-generation Swedish family with dominant juvenile-onset glaucoma and iris hypoplasia excluded linkage to the region of approximately 18 cM between loci D1S104 and D1S218, shown to contain the previously mapped glaucoma gene. The diagnosis in the Swedish family that failed to show linkage to 1q may be 'iris hypoplasia with early onset glaucoma, autosomal dominant' (see 137600), which in some instances has been mapped to chromosome 4q.

Morissette et al. (1995) studied 142 members of a huge multigenerational French-Canadian family affected with autosomal dominant primary open angle glaucoma. In 40 patients, either juvenile open angle glaucoma (JOAG) or chronic open angle glaucoma (COAG) with diagnosis after age 40 years was identified. In 6 subjects, ocular hypertension, a possible precursor of primary open angle glaucoma, was found. JOAG/COAG was tightly linked to 7 microsatellite markers on 1q23-q25. The same characteristic haplotype, composed of 14 markers spanning 12 cM between loci D1S196 and D1S212, was recognized in all persons affected by JOAG, COAG, or ocular hypertension, but did not occur in unaffected spouses or in normal family members more than 35 years of age, except for 3 obligatory carriers. From these observations, Morissette et al. (1995) concluded that the GLC1A gene is responsible for both adult-onset and juvenile glaucoma.

Wiggs et al. (1996) suggested that although the findings of Morissette et al. (1995) may indicate that variable expressivity of the GLC1A gene may lead to a broader range of onset for the form of juvenile glaucoma that maps to 1q, it does not follow that the GLC1A gene is an important cause of adult-onset primary open angle glaucoma (137760). Wiggs et al. (1996) noted that the adult-onset primary open angle glaucoma usually has its onset after the age of 50 and is probably inherited as a complex trait, without an obvious segregation pattern. The rare juvenile form, inherited as an autosomal dominant with high penetrance, almost always develops before the age of 40. Morissette et al. (1995) studied 40 members of the family who showed a common haplotype derived from microsatellite markers located in the 1q21-q31 region and found that 36 developed the disease before the age of 40. The remaining 4 were first diagnosed at ages 44, 47, 53, and 62. However, because of the insidious character of glaucoma, pinpointing the actual onset of the disease is difficult.

Richards et al. (1996) appeared to have excluded mutation at the GLC1A locus as the cause of primary open angle glaucoma with onset in middle age (between 42 and 57 years) in a large affected family.

Brezin et al. (1997) tested linkage to the GLC1A locus in 8 French families with open angle glaucoma and ocular hypertension. The median age at diagnosis was 28.5 years. When analysis was based on a phenotype consisting of both open angle glaucoma and ocular hypertension, linkage to the GLC1A locus was found in 4 families and excluded in 3 families. When the phenotype was limited to only those with open angle glaucoma, linkage to the GLC1A locus was excluded in 2 families. Brezin et al. (1997) also demonstrated that while there was no difference in peak intraocular pressure between the linked and unlinked families, those linked to GLC1A had an increased risk of developing open angle glaucoma and of having severe glaucomatous optic neuropathy. Belmouden et al. (1997) used recombinant haplotypes to reduce the GLC1A interval to a maximum of 3 cM in the region 1q23-q25. Findings in a contig from this region were compatible with its cytogenetic location on 1q24 G-band.

Molecular Genetics

Stone et al. (1997) used analyses of sequence tagged site (STS) content and haplotype sharing between families affected with the 1q-linked form of glaucoma to prioritize candidate genes for mutation screening. A gene encoding the trabecular meshwork protein myocilin (MYOC; 601652) mapped to the narrowest disease interval by both STS content and radiation hybrid mapping. In 13 glaucoma probands, including the proband from the family with 22 affected members originally described by Johnson et al. (1993) and in which Sheffield et al. (1993) demonstrated linkage to 1q, Stone et al. (1997) identified heterozygosity for 1 of 3 different mutations in the MYOC gene (601652.0001-601652.0003). Initially, a search for mutations in the MYOC gene by Stone et al. (1997) was applied to the screening of affected members of 4 different 1q-linked glaucoma families and affected members of 4 smaller families implicated by haplotype data. Amino acid-altering mutations were detected in 5 of the 8 families. A tyrosine-to-histidine mutation in codon 430 (601652.0001) was detected in all 22 affected members of the original family in which 1q linkage was demonstrated (Sheffield et al., 1993). A lysine-to-valine mutation in codon 357 (601652.0002) was detected in 2 families, including 1 previously unreported adult-onset open angle glaucoma family with 15 affected members. A nonsense mutation (gln361 to ter; 601652.0003), which would be expected to result in a 136-amino acid truncation of the gene product, was detected in 2 families. The prevalence of these mutations was then estimated by screening 4 different populations: glaucoma patients with a family history of the disease; unselected primary open angle glaucoma probands seen in a single clinic; the general population (approximated by patients with heritable retinal disease and spouses from families who participated in prior linkage studies); and unrelated volunteers over the age of 40 with normal intraocular pressures and no personal or family history of glaucoma. These experiments revealed 8 additional individuals harboring the gln361-to-ter mutation and 1 additional individual harboring the tyr430-to-his mutation. Overall, missense or nonsense mutations were found in 13 of 330 unrelated glaucoma patients (3.9%) and 1 of 471 controls (0.2%). Stone et al. (1997) speculated that the MYOC gene product may cause increased intraocular pressure by obstruction of aqueous outflow. Its expression in trabecular meshwork and ciliary body (structures of the eye involved in the regulation of intraocular pressure) is consistent with this hypothesis. The prevalence of the sequence changes observed in this study, coupled with the prevalence of glaucoma in the general population, suggested to the authors that mutations in GLC1A cause glaucoma in nearly 100,000 individuals in the U.S. This would make GLC1A the most common molecularly recognizable form of blindness. For comparison, only 2,000 people in the U.S. would be expected to harbor mutations in the rhodopsin gene (180380), which is the most common form of molecularly recognizable retinitis pigmentosa.

Alward et al. (1998) screened 716 patients with primary open angle glaucoma and 596 control subjects for sequence changes in the GLC1A gene. They identified 16 sequence variations that met the criteria for probable disease-causing mutations because they altered the predicted amino acid sequence and were found in one or more patients with glaucoma and in less than 1% of the control subjects. These 16 mutations were found in 33 patients (4.6%). Six of the mutations were found in more than 1 subject (total, 99). Clinical features associated with these 6 mutations included an age at diagnosis ranging from 8 to 77 years and maximal recorded intraocular pressures ranging from 12 to 77 mm Hg. The spectrum of disease, as indicated by this survey, can range from juvenile glaucoma to typical late-onset primary open angle glaucoma. Quigley (1996) commented on the fact that glaucoma is the second leading cause of blindness in the world after cataract, affecting approximately 70 million people, about half of whom are estimated to have open angle glaucoma. Nearly as many people have angle-closure glaucoma, a disorder that is particularly prevalent among Asians. Among persons of European and African ancestry in the United States, open angle glaucoma affects an estimated 2.5 million, half of whom are unaware of their disease. The disorder is present in 2% of those over the age of 40 years, and the prevalence increases with age. The risk among blacks is 4 times that among whites. Quigley (1998) stated that it is premature to suggest that screening of genetic loci for open angle glaucoma will be widely useful, however.

Yoon et al. (1999) described a Korean case of apparent autosomal recessive inheritance of juvenile-onset POAG (601652.0011).

Wiggs and Vollrath (2001) examined a patient with a complex deletion of the maternal copy of chromosome 1 that included the entire TIGR/MYOC gene. Neither the patient nor her family showed evidence of glaucoma. The authors concluded that haploinsufficiency of the TIGR/MYOC protein is not the cause of early-onset glaucoma associated with mutations in TIGR/MYOC even though missense and nonsense mutations in the gene have been associated with juvenile- and adult-onset primary open angle glaucoma.

Craig et al. (2001) studied the phenotype and age-related penetrance of primary open angle glaucoma in families with the most common myocilin mutation in Australia (gln368 to ter; Q368X; 601652.0003). They found that the Q368X mutation was associated with primary open angle glaucoma with younger onset and higher peak intraocular pressure than nonmutation glaucoma cases. In addition, Q368X mutation glaucoma cases were more likely to have undergone glaucoma drainage surgery. They did not observe simple autosomal dominant inheritance patterns for POAG in the 8 pedigrees studied. They concluded that other factors were involved in expression of the POAG phenotype in Q368X pedigrees.

Approximately 10 to 20% of all cases of JOAG are caused by mutations in the MYOC gene. Wiggs et al. (2004) identified 25 pedigrees with typical JOAG, demonstrating autosomal dominant inheritance. They sequenced the myocilin gene in probands from each family and found mutations in 8%.

In 2 of 100 unrelated Indian patients with glaucoma, Sripriya et al. (2004) identified a heterozygous mutation in the MYOC gene (601652.0014). One patient with juvenile-onset glaucoma was from northern India; 4 affected family members with JOAG had the same mutation.

Baird et al. (2005) studied a large 6-generation Tasmanian family of British ancestry with POAG originally described by Craig et al. (2001), in which 9 of 24 affected individuals had a Q368X mutation in the MYOC gene (601652.0003), but the remaining 15 patients did not. Using Markov chain Monte Carlo multipoint estimations of identity-by-descent sharing and allele-sharing methods, they identified a second disease region in this family on the short arm of chromosome 3. The disease locus was initially mapped to D3S1298 and subsequently narrowed to a minimum 9-cM region between markers D3S1298 and D3S1289. A multiplicative relative risk model revealed a positive association between this region and the Q368X mutation on chromosome 1 in affected individuals. Baird et al. (2005) concluded that there is an autosomal dominant glaucoma locus (GLC1L) on chromosome 3p.

Bhattacharjee et al. (2007) screened 315 Indian patients with POAG for mutation in the MYOC gene and identified 7 mutations in 11 patients, indicating that MYOC mutations account for 2.2% of POAG cases in this population.

Hewitt et al. (2007) compared findings in 66 patients heterozygous for a range of MYOC mutations with those of 105 patients with open angle glaucoma known not to have an MYOC mutation. Patients with an MYOC mutation had glaucoma diagnosed earlier (P less than 0.001) and had higher maximum recorded intraocular pressures (P less than 0.001) than those without an MYOC mutation. There were no significant structural or morphologic differences between the 2 groups. Disc hemorrhages were identified more frequently in those without MYOC mutations (14/209 vs 1/129) but this was not significant after correction for multiple hypothesis testing.

In affected members of a US family segregating POAG, Wirtz et al. (2007) identified an asp380-to-his (D380H; 601652.0017) mutation in the MYOC gene. The disease presented in this family with extremely high IOPs requiring trabeculectomies to control the pressure. The age at diagnosis ranged from 30 to 45 years. Wirtz et al. (2007) cited reports of 3 other substitutions at asp380 to ala, asn, and gly, all of which resulted in a similar presentation of POAG that was intermediate between the more severe clinical presentations observed in individuals with the pro370-to-leu (601652.0004) or lys423-to-glu (601652.0010) variant and the milder findings observed in patients with the gln368-to-ter (601652.0003) mutation.

Digenic Inheritance

Vincent et al. (2002) described a Canadian family segregating both primary adult-onset and juvenile forms of open angle glaucoma, which were associated with digenic mutations in the MYOC (601652.0013) and the CYP1B1 (601771.0012) genes. All affected family members carried the MYOC mutation; those who also carried the CYP1B1 mutation had juvenile glaucoma, whereas those with only the MYOC mutation had the adult-onset form. The mean age at onset of disease among carriers of the MYOC mutation alone was 51 years, whereas carriers of both MYOC and CYP1B1 mutations had an average age at onset of only 27 years. Individuals carrying only the CYP1B1 mutation were not clinically affected. Thus, in this family, CYP1B1 appeared to be acting as a modifier of MYOC.

Reviews

Kwon et al. (2009) reviewed the clinical features of primary open-angle glaucoma and the mechanisms of elevated intraocular pressure and optic nerve damage, focusing on mutations in the MYOC gene.

Animal Model

Gould et al. (2004) found that mice overexpressing Myoc to a level similar to that induced by corticosteroids did not develop elevated intraocular pressure or glaucoma. They hypothesized that disease pathogenesis in primary open-angle glaucoma patients may depend upon expression of abnormal mutant MYOC protein.

Zillig et al. (2005) used the chicken beta-B1-crystallin promoter to overexpress human wildtype and Y437H (601652.0001)-mutated myocilin in the lenses of transgenic mice. They found that increasing amounts of myocilin were not secreted in vivo but remained in the rough endoplasmic reticulum, causing severe alterations of cellular structure and function. Lenses expressing mutated Y437H myocilin developed nuclear cataracts, completely lost transparency, and eventually ruptured.

Shepard et al. (2007) demonstrated that mutations in human MYOC induce exposure of a cryptic peroxisomal targeting sequence, which must interact with PTS1R (PEX5; 600414) to elevate intraocular pressure (IOP). They noted that the lack of a PTS1 signal on mouse myocilin explains why IOP was unchanged in mice overexpressing mouse wildtype myocilin (Gould et al., 2004) and in knockin mice expressing the mouse ortholog of human Y437H myocilin (Gould et al., 2006). In contrast, expression of human MYOC glaucomatous mutations in mouse eyes did cause elevation of IOP. Shepard et al. (2007) stated that this was the first disease-gene-based animal model of human primary open-angle glaucoma.