Michael J. Pro MD
Mauricio Pons MD
It is not hyperbole to say that discoveries in genetics will lead to a revolution in medical care. Gene therapy now enables physicians to cure diseases where once no treatment existed. In the future drugs could be individually tailored to a patient’s genetic make-up, and nanotechnology promises to allow targeted gene delivery. The field of glaucoma has been fertile ground for advances in genetic research. Several genes have been identified which are responsible for specific forms of glaucoma. However, to date, there have been no new therapies from these genetic discoveries, illustrating the difficulties in translating genetic research into clinical practice.
Genes associated with glaucoma that demonstrate a mendelian inheritance pattern, either autosomal-dominant or autosomal-recessive, have been found by using large affected pedigrees and standard linkage analysis.1 Linkage analysis is the practice of examining large families with many affected individuals. Markers or DNA sequence variants can trace the inheritance patterns between members of the same family. The assumption is that affected individuals would preferentially inherit the markers from their parents.2 Currently the most popular markers are single-nucleotide polymorphisms (SNPs). SNPs are being assembled into large SNP databases that promise to elucidate the genetic basis of many diseases.3 However, the most common adult-onset forms of glaucoma, such as primary open angle glaucoma (POAG) or primary angle closure glaucoma (PACG), do not demonstrate mendelian inheritance patterns. Linkage analysis has not been as successful for identifying genes for these multifactorial diseases. This is due to genetic and phenotypic heterogeneity, modest sample sizes, and epistasis (interactions between different genes).
Open Angle Glaucoma
According to age of onset, POAG is divided into juvenile-onset POAG (JOAG) and adult-onset POAG. JOAG has an age of onset between 3 and 30 years; usually presents with high IOP, visual field loss, and optic disc damage; and requires early surgical treatment.4,5 JOAG is typically inherited as an autosomal dominant trait, whereas adult-onset POAG is inherited as a complex trait.6
It has long been known that a family history of glaucoma is an important risk factor.
Cross-sectional studies have suggested that more than 50% of all glaucoma is familial and a positive family history of glaucoma conveys up to a three-fold increase in risk of developing POAG.7,8 Population studies show an increased prevalence of POAG among first-degree relatives of patients with the disease.9,10 Interestingly, results from the Ocular Hypertension Treatment Study (OHTS) suggested that family history was not a significant risk factor influencing progression from ocular hypertension to glaucoma.11 Although perhaps subject to the same recall biases as OHTS, a clinic-based investigation concluded that a positive family history of POAG did not influence disease severity at the time of diagnosis.12 However, OHTS did describe elevated intraocular pressure (IOP) and enlarged cup to disc ratio to be independent risk factors for conversion from OHTN to POAG.13 In an elderly population, cup to disc ratio (CDR) was found to be highly heritable and IOP to be moderately heritable. On average, siblings of glaucoma patients had higher IOPs and larger CDRs than siblings of nonglaucomatous probands.14
Recent studies have been even more definitive regarding the importance of family history. Approximately 60% of POAG in Tasmania is familial.15 The recent Los Angeles Latino Eye Study and Barbados Eye Study both found increased risk for individuals who have first-degree relatives with POAG.16,17
POAG is the most common type of glaucoma and is defined by an absence of any identifiable secondary etiology. Inheritance of most cases of POAG is non-mendelian and multifactorial in nature. Genetic contributions to this disease may influence intraocular pressure, optic nerve degeneration, or both. Mendelian autosomal dominant and recessive forms of glaucoma are caused by single gene defects that are associated with extreme phenotypes: either highly elevated intraocular pressure or severe optic nerve degeneration. Genes that contribute to POAG may not cause clinical evidence of the disease unless they are coupled with other genes or environmental factors. Because disease features are dependent on the combined effects of multiple factors, the identification and characterization of any one disease-predisposing factor can be difficult when using traditional linkage approaches.18 It is becoming increasingly apparent that human complex disorders arise because of multiple genetic interactions (epistasis) and gene environment interactions.19 However, linkage analysis has led to the mapping of numerous genetic loci, from GLC1A to GLC1N.20,21 For instance, Suriyapperuma et al used a group of families with adult-onset glaucoma that previously were unlinked to any locus and identified a new POAG locus (GLC1H) at the 2p15-p16 region. This region contains 61 known genes22 and thus illustrates the difficulty in identifying causative glaucoma genes within the multiple loci. Specific other loci have been identified as being associated either with late-onset glaucoma on 2cen-q13 (GLC1B);23 3q21-24 (GLC1C);24 and 8q2325 or with normal-pressure glaucoma on chromosome 10p15-14.26,27 Fan and Ko recently identified a novel glaucoma locus on 5q22.1-q32 (GLC1M) in a family from the Philippines with autosomal dominant juvenile-onset primary open angle glaucoma (JOAG).28 To date, only three genes have been identified: myocilin, optineurin, and WD-repeat protein 36.
The GLC1A gene myocilin (MYOC) is responsible for about 36% of juvenile-onset29 and 2% to 4% of adult-onset POAG cases.30 The GLC1E gene optineurin (OPTN) is responsible for familial normal-tension glaucoma.31 The GLC1G gene WD-repeat protein 36 (WDR36) originally showed mutations in 5% to 7% of POAG cases32 but other studies reported mutations in 10% to 17% of cases.33
One of the loci for autosomal dominant JOAG, named GLC1A, has been mapped by linkage analysis to chromosome 1q21-q31.34 The JOAG cases linked to the GLC1A locus are characterized by an age at onset of younger than 30 years and increased intraocular pressure.35 In 1997, the gene associated with GLC1A was identified36 and found to code for a 57-kd protein called trabecular meshwork-induced glucocorticoid response protein, originally described by Polansky et al,37 and now known as the myocilin gene (MYOC). The clinical spectrum of the disease can range from juvenile glaucoma to typical late-onset POAG.
The MYOC gene is the only glaucoma gene that has been widely evaluated and is accepted to be a causal gene for glaucoma, whereas the evidence of OPTN and WDR36 causality is not as strong. MYOC gene codes for a glycoprotein called myocilin. The ocular expression of this gene is primarily found in the ciliary body, sclera, and trabecular meshwork. Myocilin is a membrane-associated protein that causes protein-protein interactions. The C-terminus, coded for the third exon, shares significant homology with several olfactomedinlike glycoproteins. To date, most of the POAG and JOAG mutations identified in myocilin are missense mutations located in the third exon.38 This domain may be important for protein uptake and metabolism by the trabecular meshwork cells. Mutant MYOC proteins are retained in the endoplasmic reticulum of the trabecular meshwork, but wild types are excreted39 and it is thought that myocilin mutations cause a reduction in aqueous outflow facility. Application of timolol to human trabecular meshwork cell cultures causes a reduction in myocilin mRNA levels40 while glucocorticoid treatment causes a significant induction of myocilin mRNA.41
In the Spanish POAG population, disease-causing mutations in this domain are present in 2.7% of sporadic cases.42 The prevalence of MYOC mutations in Taiwanese JOAG patients was 12.5%.43 In India, MYOC mutations were found in 2.2% of POAG cases, which is similar to the reported incidence of 2% to 4% in POAG patients.44
Different mutations in myocilin may cause disease of varying severity, with severe clinical presentations observed in individuals with the Pro370Leu or Lys423Glu variant and milder findings in patients with the Gln368Stop mutation. All known Caucasian subjects with myocilin Gly252Arg mutation have a common founder and may lead to a disease of intermediate severity.45 In the Greek village of Taxiarchis, the incidence of the Thr377Met MYOC variant in the glaucoma patients was 59% (13/22). The Arg76Lys polymorphism was most common, but not strongly associated with glaucoma. Incomplete penetrance of the Thr377Met MYOC mutation was found, suggesting that this particular variant is more likely to be a susceptibility factor than a major glaucoma gene.46 Worldwide, the Thr377Met MYOC mutation is one of the most commonly identified POAG-causing mutations and the mutation has arisen independently at least three separate times.47 This specific mutation has been identified in four Australian-based families; two families residing in the United States; and one each from Greece, the former Yugoslavian Republic of Macedonia (FYROM), India, Finland, and Morocco.48,49 In general, the Thr377Met mutation confers disease of intermediate severity, with patients typically being diagnosed in their fourth decade. This is younger than the age at which patients with the Gln368Stop mutation are usually diagnosed, yet somewhat older than those carrying other MYOC mutations such as Pro370Leu, which is generally diagnosed around 15 years of age. Patients with the Thr377Met mutation tend to have a maximum recorded intraocular pressure around 30 mm Hg.50
The optineurin (OPTN) gene, locus (GLC1E), consists of 16 exons and the first three are noncoding. It maps to a 5-centimorgan (cM) region on chromosome 10p14-p15.51 The OPTN gene protein product optineurin, for “optic neuropathy inducing,” is expressed in ocular tissues such as retina, trabecular meshwork, and nonpigmented ciliary epithelium.52 In a study of 54 families with autosomal dominantly inherited adult-onset POAG, mutations in the optineurin gene (OPTN) were initially reported in 16.7% of families with hereditary POAG, with most of them having NTG.53 However, other reports have indicated that OPTN sequence variants are only a rare cause of POAG or NTG.54,55 OPTN DNA sequence variations are not involved in high-pressure POAG in the Spanish population.56
Mutations in the WDR36 gene have been demonstrated in about 5% of POAG patients. Although the function of the WDR36 protein and its role in glaucoma is not known, there is some evidence to suggest that WDR36 may participate in the activation of T cells in response to IL-2.57 Previous studies have suggested that some patients with glaucoma may have an alteration of cellular immunity that is IL-2 dependent.58 Recently, other studies have suggested that T-cell responses may influence optic nerve degeneration in glaucoma in humans59 and in a mouse glaucoma model. WDR36 may contribute to increased susceptibility of optic nerve degeneration in the setting of elevated intraocular pressure. But other research has suggested that the WDR36 gene may not be the defective gene in the GLC1G locus.60 Like other genes associated with POAG, it may be the case that abnormalities in WDR36 alone are not sufficient to cause POAG.61
CYP1B1 is the major gene that causes primary congenital glaucoma (PCG), but heterozygous mutations in CYP1B1 have been identified in 4% to 9% of affected POAG subjects from France,62,63 India,64 and Spain,65 and 17% of JOAG patients from Iran where an autosomal recessive inheritance pattern was observed.66
Researchers have noted that the myocilin and optineurin mutations account for <5% of known causes of POAG and speculate that mitochondrial abnormalities may play a role.67 Abu-Amero et al found a spectrum of mitochondrial abnormalities in Saudi Arabian patients with POAG, implicating oxidative stress and implying that mitochondria dysfunction may be a risk factor for POAG.68
Pigment Dispersion Syndrome and Pigmentary Glaucoma
Pigment dispersion syndrome is characterized by liberation of pigment from the iris pigment epithelium and deposition of iris pigment granules on anterior segment structures including corneal endothelium and trabecular meshwork.
The classic triad consists of pigment deposition on the trabecular meshwork, iris midperipheral transillumination defects, and an endothelial Krukenberg spindle.
Gonioscopy demonstrates a densely pigmented trabecular meshwork and often a concave peripheral iris contour. A Sampaolesi line, typical of pseudoexfoliation syndrome, can be observed. Phenotypic appearance is characteristically bilateral and symmetric.69
Pigmentary glaucoma typically develops in young myopic patients with pigment dispersion syndrome. Pigment dispersion syndrome is seen in 2.45% of white individuals screened.70 Pigmentary glaucoma represents 1% to 1.5% of all glaucomas. It is more common in males (78%), and affected woman are somewhat older.71 Patients are usually myopic, with a mean refractive error of -4 diopters. A gene affecting some aspect of the development of the middle third of the eye early in the third trimester has been thought to be responsible for the syndrome.72 In patients of European descent, pigment dispersion syndrome is inherited as an autosomal dominant trait as it has been described in four affected pedigrees. The gene responsible for the syndrome in these four families maps to the telomeric end of the long arm of chromosome 7 (i.e., 7q35-q36).73 Stankovic described in 1961 a four-generation family with pigmentary glaucoma.74 Mandelkorn et al observed direct linear transmission from parent to sibling in three families.75 Dorairaj et al. recently described two families with pigment dispersion that displayed autosomal dominant inheritance.76 Phenotypic variability is common in pigment dispersion syndrome and has been correlated with the severity of myopia.77 Inheritance in persons of African descent is not as well understood.