|
|
||||||||
1From the Center for Human Genetics and 2Department of Ophthalmology, Duke University School of Medicine, Durham, North Carolina; the 3Department of Ophthalmology, Harvard Medical School, Boston, Massachusetts; and the 4Center for Human Genetics, Vanderbilt University School of Medicine, Nashville, Tennessee.
| Abstract |
|---|
|
|
|---|
METHODS. All of the 23 coding exons and flanking introns of the WDR36 gene were sequenced in 118 probands from families with at least two members affected by POAG, 6 probands from juvenile-onset POAG families, and 108 control individuals.
RESULTS. Thirty-two WDR36 sequence variants were found in this population of patients with POAG. Nonsynonymous single-nucleotide polymorphisms (SNPs), including those previously described as "disease-causing" and "disease susceptibility," were found in 17% of POAG patients and 4% of control subjects. Although the distribution of WDR36 variants in the pedigrees did not show consistent segregation with the disease, the WDR36 sequence variants were found more frequently in patients with more severe disease.
CONCLUSIONS. The results of this study suggest that abnormalities in WDR36 alone are not sufficient to cause POAG. The association of WDR36 sequence variants with more severe disease in affected individuals suggests that defects in the WDR36 gene can contribute to POAG and that WDR36 may be a glaucoma modifier gene.
Two genes have been identified as factors that contribute to POAG, defects in the myocilin gene (GLC1A) primarily causes elevated pressure1 2 and the optineurin gene (GLC1E), appears to contribute to disease in familial low-tension glaucoma.3 4 In addition to these genes, five other glaucoma gene loci (GLC1B, GLC1C, GLC1D, GLC1F, GLC1G) have been identified using large affected pedigrees and Mendelian linkage approaches (Samples JR, et al. IOVS 2004;44:ARVO E-Abstract 4622).5 6 7 8 9 10 Sib pair-based whole genome analyses of typical late-onset families have also identified chromosomal regions likely to contain POAG susceptibility genes, including 14q,11 15q,12 2q,13 and 10p.13
GLC1G was initially defined as a 2-Mb region by linkage studies using POAG pedigrees (Samples JR, et al. IOVS 2004;44:ARVO E-Abstract 4622).9 10 The WDR36 gene is located within the critical genetic interval defined by these linkage studies, and recently Monemi et al.,9 characterized WDR36 as the third POAG gene by identifying WDR36 DNA sequence variants in patients with high- and low-tension glaucoma. A total of 24 sequence variants were identified: Four were defined as disease-causing (5% of 130 POAG families, and 0% of 200 control subjects), 3 as disease-predisposing (11% of POAG families and 2% of control subjects), and 17 as polymorphisms with equal distribution between POAG cases and control subjects.
WDR36 is a member of the WD40 repeat protein family and may be involved in T-cell activation.14 Recently, T-cell-mediated responses have been hypothesized to participate in glaucoma associated optic nerve degeneration.15 The purpose of this study is to determine the distribution of WDR36 DNA sequence variants in a well-studied cohort of patients with POAG in the United States.
| Methods |
|---|
|
|
|---|
POAG family probands were defined as age of diagnosis greater than 35 years, intraocular pressure greater than 22 mm Hg in both eyes without medications or 19 mm Hg on two or more medications, glaucomatous optic nerve damage in both eyes, and visual field loss in at least one eye. Glaucomatous optic nerve damage was defined as cup-to-disc ratio higher than 0.7 or focal loss of the nerve fiber layer (notch) associated with a specific visual field defect. Visual fields were performed using automated perimetry (mainly Humphrey) and were scored with a modified six-stage system adapted from that published by Quigley.12 16 Level-I patients are the most severely affected and met all criteria including elevated IOP, bilateral optic nerve damage, and visual field loss in at least one eye. Level-II patients were less severely affected and satisfied any two of these three criteria. Control subjects were age matched and were collected from the same geographic regions as the probands. Control subjects had no evidence of glaucoma and no family history of glaucoma.
For this study 118 probands from families with at least two members affected by POAG (a level-I proband with all other affected family members either level I or level II) were screened for WDR36 sequence variants. We also included six probands from juvenile-onset POAG (JOAG) families (defined as POAG except with age of onset before 35 years).
PCR Amplification and DNA Sequencing
Genomic DNA from peripheral blood was prepared from all individuals by using standard techniques (Gentra, Minneapolis, MN). All the 23 coding exons and flanking introns of the WDR36 gene (GenBank accession no. NM_139281/ http://www.ncbi.nlm.nih.gov/Genbank; provided in the public domain by the National Center for Biotechnology Information, Bethesda, MD) were sequenced using nested PCR strategies for amplification. Amplification conditions for PCR were (per 25 µL PCR reaction): 20 mM Tris-HCl (pH 8.4); 50 mM KCl; 1.5 mM MgCl2; 200 µM each of dATP, dCTP, dGTP, and dTTP; 100 ng forward PCR primer; 100 ng reverse PCR primer; 30 ng genomic DNA; and 0.5 U Taq DNA polymerase (Platinum Taq; Invitrogen-Life Technologies, Rockville, MD). PCR products were amplified using a "touchdown" strategy whereby the annealing temperature is lowered incrementally over the course of the reaction. Initial thermocycler conditions were as follows: 94°C for 30 seconds, 65°C for 30 seconds, and 72°C for 30 seconds. After two cycles at an annealing temperature of 65°C, the temperature was lowered to 63°C for two cycles, then to 61°C for two cycles, 59°C for two cycles, 57°C for two cycles, and finally 55°C for 30 additional cycles (40 cycles total).
Oligonucleotides for amplification and sequencing were selected using Primer3 software (http://frodo.wi.mit.edu/cgi-bin/primer3/primer3_www.cgi/ provided in the public domain by the Massachusetts Institute of Technology, Cambridge, MA) and were located at least 40 bases from each splice site. Primer sequences used to amplify each exon are available on request.
Amplified genomic DNA from an affected proband or control was directly sequenced using sequencing chemistries (BigDye ver. 3.1; Applied Biosystems, Inc. [ABI], Foster City, CA) and an automated sequencer (model 3100 or 3730; ABI). In some cases, pools of amplified genomic DNA from two individuals were initially sequenced and pools displaying DNA sequence variants were resequenced individually. Sequences from pooled samples were analyzed (Sequencher Software; Gene Codes Corp., Ann Arbor, MI).
| Results |
|---|
|
|
|---|
|
|
The distribution of WDR36 variants in the pedigrees did not show consistent segregation with the disease. In the 16 pedigrees with nonsynonymous SNPs (excluding the I264V common variant), a WDR36 sequence variant was found in only 25 of 44 affected individuals, and in 4 of 16 unaffected family members. However, among the affected individuals with WDR36 variants, we found that individuals with glaucoma and a WDR36 variant had more severe disease (level I) than those affected individuals without a WDR36 variant (Table 3 ; P < 0.001,
2, P = 0.0019, the Fisher exact test). Of interest, two of the pedigrees have known mutations in myocilin (Thr377Met, pedigree 27; Gln368Stop, pedigree 125) that are consistently present in every affected individual (both level-I and -II affected individuals). However, the level-I individuals in these two pedigrees had a WDR36 variant in addition to the myocilin mutation (Fig. 1) .
|
|
|
| Discussion |
|---|
|
|
|---|
However, in the affected pedigrees that carried a WDR36 sequence variant, we found that the sequence variants did not consistently segregate with the occurrence of the disease, arguing that abnormalities in WDR36 alone are not sufficient to cause POAG. We did find, however, that affected individuals who carried a WDR36 sequence variant were more likely to have more severe disease than were affected patients who did not carry a variant. These results suggest that while defects in the WDR36 gene may contribute to the glaucomatous disease process, WDR36 most likely acts as a glaucoma modifier gene.
Genes that modify the action or expression of a dominant gene are emerging as important determinants of the phenotypic variation observed in genetic disorders. In mouse studies, the segregating background genes can modify the age of onset, rate of progression, or severity of disease expression.17 It is becoming increasingly apparent that human complex disorders arise because of multiple genetic interactions (epistasis) and gene environment interactions.18 Epistatic interactions caused by modifier genes may make the disease more severe or less severe. The identification of modifier genes will help define the molecular pathways responsible for the disease as well as provide new information that may lead to the development of biomarkers for the disease as well as novel therapeutics.
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-2.14 Previous studies have suggested that some patients with glaucoma may have an alteration of cellular immunity that is IL-2 dependent.19 Recently, other studies have suggested that T-cell responses may influence optic nerve degeneration in glaucoma in humans15 and in a mouse glaucoma model.20 In our study most of the level-II patients who did not have WDR36 gene defects met the affected criterion for intraocular pressure, but were scored level II because of incomplete evidence of optic nerve disease or visual field defects, which could support the hypothesis that WDR36 may contribute to increased susceptibility of optic nerve degeneration in the setting of elevated intraocular pressure. It is interesting to speculate that WDR36 may contribute to glaucoma by modifying optic nerve degeneration; however, further work defining the role of the protein in POAG is necessary before this conclusion can be reached.
POAG is a common disorder with a complex inheritance that is likely to result from contributions of multiple genes and possibly environmental conditions. 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. Most patients with POAG do not have extreme phenotypes, and the underlying genetic etiologies are not thought to result from single gene defects, but from contributions of multiple genetic factors that independently cause moderate alterations in intraocular pressure and optic nerve disease but collectively cause more severe disease. Genes that contribute to POAG may not cause clinical evidence of the disease unless they are coupled with other genes or environmental factors. If disease features are dependent on the combined effects of multiple factors then the identification and characterization of any one disease-predisposing factor can be difficult when using traditional linkage approaches. In this study, we provide evidence to suggest that WDR36 may influence disease severity and may contribute to the disease process. The identification of glaucoma susceptibility and glaucoma modifying genes are important steps toward the complete molecular definition of POAG.
| Footnotes |
|---|
Submitted for publication November 17, 2005; revised January 24, 2006; accepted April 24, 2006.
Disclosure: M.A. Hauser, None; R.R. Allingham, None; K. Linkroum, None; J. Wang, None; K. LaRocque-Abramson, None; D. Figueiredo, None; C. Santiago-Turla, None; E.A. del Bono, None; J.L. Haines, None; M.A. Pericak-Vance, None; J.L. Wiggs, None
The publication costs of this article were defrayed in part by page charge payment. This article must therefore be marked "advertisement" in accordance with 18 U.S.C.
1734 solely to indicate this fact.
Corresponding author: Janey L. Wiggs, Associate Professor of Ophthalmology, Harvard Medical School, Massachusetts Eye and Ear Infirmary, 243 Charles Street, Boston, MA 02114; janey_wiggs{at}meei.harvard.edu.
| References |
|---|
|
|
|---|
This article has been cited by other articles:
![]() |
J. M. Skarie and B. A. Link The Primary open-angle glaucoma gene WDR36 functions in ribosomal RNA processing and interacts with the p53 stress-response pathway Hum. Mol. Genet., August 15, 2008; 17(16): 2474 - 2485. [Abstract] [Full Text] [PDF] |
||||
![]() |
Y. Liu, S. Schmidt, X. Qin, J. Gibson, K. Hutchins, C. Santiago-Turla, J. L. Wiggs, D. L. Budenz, S. Akafo, P. Challa, et al. Lack of Association between LOXL1 Variants and Primary Open-Angle Glaucoma in Three Different Populations Invest. Ophthalmol. Vis. Sci., August 1, 2008; 49(8): 3465 - 3468. [Abstract] [Full Text] [PDF] |
||||
![]() |
F Carbonaro, T Andrew, D A Mackey, T D Spector, and C J Hammond Heritability of intraocular pressure: a classical twin study Br. J. Ophthalmol., August 1, 2008; 92(8): 1125 - 1128. [Abstract] [Full Text] [PDF] |
||||
![]() |
F. Pasutto, C. Y. Mardin, K. Michels-Rautenstrauss, B. H. F. Weber, H. Sticht, G. Chavarria-Soley, B. Rautenstrauss, F. Kruse, and A. Reis Profiling of WDR36 Missense Variants in German Patients with Glaucoma Invest. Ophthalmol. Vis. Sci., January 1, 2008; 49(1): 270 - 274. [Abstract] [Full Text] [PDF] |
||||
![]() |
K. K. Abu-Amero, J. Morales, M. N. Osman, and T. M. Bosley Nuclear and Mitochondrial Analysis of Patients with Primary Angle-Closure Glaucoma Invest. Ophthalmol. Vis. Sci., December 1, 2007; 48(12): 5591 - 5596. [Abstract] [Full Text] [PDF] |
||||
![]() |
L. M. E. van Koolwijk, D. D. G. Despriet, C. M. van Duijn, L. M. Pardo Cortes, J. R. Vingerling, Y. S. Aulchenko, B. A. Oostra, C. C. W. Klaver, and H. G. Lemij Genetic Contributions to Glaucoma: Heritability of Intraocular Pressure, Retinal Nerve Fiber Layer Thickness, and Optic Disc Morphology Invest. Ophthalmol. Vis. Sci., August 1, 2007; 48(8): 3669 - 3676. [Abstract] [Full Text] [PDF] |
||||
![]() |
J. L. Wiggs Genetic Etiologies of Glaucoma Arch Ophthalmol, January 1, 2007; 125(1): 30 - 37. [Abstract] [Full Text] [PDF] |
||||
![]() |
S. P. Suriyapperuma, A. Child, T. Desai, G. Brice, A. Kerr, R. P. Crick, and M. Sarfarazi A New Locus (GLC1H) for Adult-Onset Primary Open-angle Glaucoma Maps to the 2p15-p16 Region Arch Ophthalmol, January 1, 2007; 125(1): 86 - 92. [Abstract] [Full Text] [PDF] |
||||
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |