Optineurin gene may be a cause of POAG

A mutation in the optineurin gene can be a causative factor for both open-angle and normal-tension glaucoma, a recent study suggests.

FARMINGTON, Conn. — A gene that causes open-angle glaucoma has been identified by researchers at the University of Connecticut here. The optineurin gene, located on chromosome 10p14, has been dubbed OPTN. The investigators’ work is described in a recent article in Science.

A study of 54 families found sequence alterations in OPTN in 16.7% of families with hereditary primary open-angle glaucoma, including individuals with normal intraocular pressure.

The OPTN gene codes for a conserved 66-kilodalton protein of unknown function that has been implicated in the tumor necrosis factor-signaling pathway and that interacts with diverse proteins.

The researchers, led by Mansoor Sarfarazi, PhD, professor of human genetics here, have determined that the optineurin gene is expressed in trabecular meshwork, nonpigmented ciliary epithelium, retina and brain. They speculate that it plays a neuroprotective role.

Genetically affected glaucoma

Investigators claim that glaucoma is genetically heterogeneous and at least eight loci have been linked to the disorder. Two genes have been previously identified: CYP1B1, encoding cytochrome P4501B1 enzyme, is mutated in primary congenital glaucoma and MYOC, encoding myocilin, is mutated in juvenile-onset POAG.

With approval from the Human Genome Organization (HUGO) no-menclature committee, the investigators added a third glaucoma gene to the list by designating the gene OPTN and its protein product optineurin. Optineurin is short for “optic neuropathy-inducing protein.”

The investigators had previously mapped an adult-onset POAG locus (GLC1E) to a 21-centimorgan (cM) region on chromosome 10p14-p15 and subsequently reduced the critical region to 5 cM. After excluding four genes, they selected OPTN as a candidate gene on the basis of its physical location in this region and its expression in retina. The OPTN gene has previously been identified as FIP-2 and NRP and its product as an interacting protein for Huntingtin, transcription factor IIIA and RAB8.

After identifying a missense mutation [Glu50 Lys (E50K)] in their original kindred, the investigators studied 54 families with autosomal dominant adult-onset glaucoma with at least one member having normal-tension glaucoma. The majority of these families presented with normal IOP, less than 22 mm Hg, where others had mixed clinical pictures of both normal and moderately raised IOP, 23 to 26 mm Hg, in the same family.

Optineurin mutation

The investigators’ analysis revealed four sequence alterations in OPTN. A recurrent E50K mutation was identified in seven families; it segregated in 124 members, including 38 affected, 16 asymptomatic gene carriers, 50 unaffected and 20 spouses. Of the 38 affected subjects, seven had elevated IOP, 23 to 26 mm Hg, and the remaining individuals had normal IOP (11 to 21 mm Hg). Two additional mutations were identified in two other families with normal IOP.

According to investigators, the recurrent E50K is located within a putative bZIP motif, conserved in the mouse, bovine and macaque genomes, and may have a dominant-negative effect. The bZIP motif is a transcription factor domain normally involved in DNA binding and protein dimerization.

The 2-bp “AG” insertion truncates the protein by 76% and presumably leads to loss of function or haploin-sufficiency. Although the R545Q mutation is not part of a known protein domain, it is situated near the only zinc finger motif within optineurin. This motif is normally seen in transcription factors.

A fourth sequence change, Met⁹⁸ Lys (M98K), was initially detected in both glaucoma families and in normal control subjects. To determine the significance of this change, the investigators screened 124 sporadic cases of glaucoma with predominantly normal IOP. M98K was identified in 23 of 169 (13.6%) glaucoma subjects and 9 of 422 (2.1%) normal control chromosomes. Only three of these 23 (13.0%) subjects had IOP values above normal, 23, 26, and 40 mm Hg.

“The average age of the normal control subjects with the M98K sequence change was 54.3 years, so it remains possible that a subset will eventually develop glaucoma. Nevertheless, because the difference in frequency between the affected (13.6%) and normal (2.1%) chromosomes is highly significant and because M98K is lo-cated within a putative bZIP domain and also conserved in macaque, it may represent a risk-associated factor or a dominant susceptibility allele,” Dr. Sarfarazi said.

According to the investigators, the study data suggest that mutations in OPTN may be responsible for 16.7% of hereditary forms of normal-tension glaucoma with an additional attributable risk factor of 13.6% in both familial and sporadic cases.

“However, because we sequenced the entire OPTN gene in only one family and thereafter used single-strand conformational polymorphism analysis to screen the remaining glaucoma cases, it is possible that additional mutations have been missed. We also identified additional non-disease-causing sequence alterations in the OPTN gene,” Dr. Sarfarazi said.

“We next investigated the intra-cellular localization of optineurin by immunocytochemistry using both primary and transformed cell lines,” he said. “Endogenous optineurin showed granular staining that was associated with vesicular structures near the nucleus. The staining colocalized with a marker specific for the Golgi apparatus. In a dermal fibroblast culture from a patient with an E50K mutation, optineurin was present at much lower levels than in a similar culture from a normal subject.”

The investigators believe that optineurin has no significant homology to any protein, but said it is known to interact with adenovirus E3-14.7K, Huntingtin, transcription factor IIIA, RAB8 and two unknown kinases.

“Optineurin’s ability to block the protective effect of E3-14.7K on tumor necrosis factor- (TNF-)-mediated cell killing suggests this protein may be a component of the TNF- signaling pathway that can shift the equilibrium toward induction of apoptosis. TNF- can markedly increase the severity of damage in optic nerve heads of both POAG and LTG subjects. We speculate that wild-type optineurin, operating through the TNF- pathway, plays a neuroprotective role in the eye and optic nerve, but when defective, it produces visual loss and optic neuropathy as typically seen in normal and high-pressure glaucoma,” Dr. Sarfarazi said.

Because OPTN mutations are a contributing factor in patients with normal tension glaucoma, the gene may be a useful tool for presymptomatic screening of the general population.

For Your Information:

Mansoor Sarfarazi, PhD, and Tayebeh Rezaie, MS, can be reached at the Molecular Ophthalmic Genetics Laboratory, Surgical Research Center, Department of Surgery, University of Connecticut Health Center, 263 Farmington Ave., Farmington, CT 06030; (860) 679-3629; fax: (860) 679-7524; e-mail: mansoor@neuron.uchc.edu; Web site: www.uchc.edu.

Reference:

Rezaie T, Child A, et al. Adult-onset primary open-angle glaucoma caused by mutations in optineurin. Science. 2002;295:1077-1079.