- Email: [email protected]
Aniridia with Preserved Visual Function: A Report of Four Cases with No Mutations in PAX6
Aniridia with Preserved Visual Function: A Report of Four Cases with No Mutations in PAX6 ELIAS I. TRABOULSI, JAY ELLISON, JONATHAN SEARS, IRENE H. MAUMENEE, JOHN AVALLONE, AND BRIAN G. MOHNEY ● PURPOSE: To report four patients with aniridia, preserved visual function, and no detectable mutations in PAX6. ● DESIGN: Retrospective case series. ● METHODS: The clinical records and molecular genetic findings of four patients from three clinical practices were reviewed retrospectively. ● RESULTS: All four patients had anterior segment findings characteristic of aniridia with good vision, no nystagmus in three of four patients, and no mutations on PAX6. An optical coherence tomography study from one of the patients showed a very shallow foveal pit. At the latest examination, none of the patients demonstrated a Wilms tumor. ● CONCLUSIONS: These four cases provide evidence for genetic heterogeneity in aniridia. In aniridic patients without a PAX6 mutation, vision seems to be relatively well preserved. (Am J Ophthalmol 2008;145: 760 –764. © 2008 by Elsevier Inc. All rights reserved.)
A
NIRIDIA IS A MISNOMER THAT REFERS TO THE CON-
genital absence of the iris. Most often there is circumferential loss of tissue that includes all of the pupillary sphincter and midperipheral iris, with preservation of the most peripheral part of the iris into the anterior chamber angle. In some patients, there are instead sectoral or other iris defects that are not in the inferior and nasal iris, as one would expect in patients with typical iris colobomas.1 In addition to the iris defects, most patients with aniridia have reduced vision from foveal hypoplasia; with time, keratopathy that has been referred to as pannus or keratitis develops in some. This keratopathy seems to be the result of defective or absent corneoscleral limbal stem cells, the development and maintenance of which require a normal activity of PAX6.2 The only known chromosomal locus for aniridia has been mapped to 11p13,3 which was found to harbor PAX6, the only gene that has been shown to cause aniridia.4 PAX6 is an essential transcription factor for eye formation. Accepted for publication Dec 5, 2007. From the Cole Eye Institute, Cleveland Clinic, Cleveland, Ohio (E.I.T., J.S.); Departments of Medical Genetics (J.E.) and Ophthalmology (B.G.M.), Mayo Clinic, Rochester, Minnesota; The Wilmer Eye Institute, Baltimore, Maryland (I.H.M.); and Ophthalmology Associates, Annapolis, Maryland (J.A.). Inquiries to Elias I. Traboulsi, i32, 9500 Euclid Avenue, Cleveland, OH 44195; e-mail: [email protected]
760
The great majority of PAX6 mutations cause aniridia, but some result in congenital cataracts, Peters anomaly, anterior segment dysgenesis, ectopia pupillae, and dominant keratitis.5– 8 Mutation analysis of cohorts of patients with typical aniridia (iris defects, nystagmus, reduced vision from foveal hypoplasia) has led to the identification of PAX6 sequence variations in more than 80% of patients.9 One study that extensively sequenced PAX6 in 12 patients with aniridia detected mutations in 90% of cases.10 The remaining 10% to 20% are postulated to have mutations in other genes or in parts of the PAX6 gene that are not sequenced routinely. Rare families with aniridia and preserved visual function have been reported.11 No mutation in PAX6 has been identified to date in the family reported by Elsas and associates.11 We have identified four patients with an anterior segment malformation clinically indistinguishable from aniridia, but with preserved visual function, a normalappearing fovea in some, and no detectable mutations in PAX6.
©
2008 BY
METHODS THIS WAS A CHART REVIEW OF FOUR PATIENTS FROM
three sources and a tabulation of their clinical ocular findings. Institutional review board approval was obtained from the Cleveland Clinic Foundation. All patients had clinical molecular testing of the PAX6 gene at GeneDx (Gaithersburg, Maryland, USA), a commercial certified clinical laboratory. Exons 1 through 13 and the intron– exon boundaries of the PAX6 gene were amplified and were sequenced bidirectionally from the deoxyribonucleic acid samples of all patients. The alternatively spliced exon 5a also was sequenced. All patients also were genotyped for the presence of at least three polymorphisms: IVS4-195 a/g, IVS9-12 c/t, and IVS12⫹43 g/t. These polymorphisms were ascertained to ensure that both copies of the PAX6 gene were sequenced and that the patient did not have a complete deletion of the gene.
RESULTS ● CLINICAL FINDINGS:
All patients had almost total absence of iris tissue, good visual acuity, and foveal reflexes
ELSEVIER INC. ALL
RIGHTS RESERVED.
0002-9394/08/$34.00 doi:10.1016/j.ajo.2007.12.012
TABLE. Clinical Findings in Four Patients with Aniridia and No Detectables Mutation in PAX6 Case No.
Age at Last Visit
Vision
1
35 mos
VA, 20/40 in both eyes; no nystagmus
2
Two mos Good fix and follow; no nystagmus
3
10 yrs
4
33 mos
Iris/Anterior Segment Findings
Typical aniridia (Figure 1): persistent pupillary membranes Typical aniridia with inferior colobomas
Fundus/Fovea
Other Eye or Systemic Findings
Normal/⫹2.00 in both eyes
Normal renal ultrasound results
Good foveal reflexes
normal MRI results; normal renal ultrasound results None
No nystagmus; latest VA, 20/25 in both eyes
Typical aniridia with remnants Poor foveal reflexes (Figure 2)/ of pupillary membrane; ⫹6.00 in both eyes inferior colobomas (Figure 2) VA, 20/30 with Allen pictures Typical aniridia (Figure 3) Poor foveal reflex (Figure 3)/plano Has an identical twin on near card; history of ⫹2.75 x 90 in the right eye/ with normal eye nystagmus plano ⫹1.75 x 90 in the left examination eye/OCT showed blunted results; normal foveal reflexes (Figure 3) karyotype; normal renal ultrasound results
MRI ⫽ magnetic resonance imaging; OCT ⫽ optical coherence tomography; VA ⫽ visual acuity.
FIGURE 1. Photographs of the right and left eyes of Patient 1 showing aniridia and no detectable mutation in PAX6. Note the circumferential absence of iris tissue and pupil sphincter.
described as normal to variably blunted on clinical examination. The Table summarizes the clinical findings in the four patients. ● MOLECULAR ANALYSIS: No PAX6 mutations were detected in any of the four patients. Heterozygous polymorphisms at IVS4-195 a/g, IVS9-12 c/t, and IVS12⫹43 g/t were detected in all patients. If two copies of any or all of these polymorphisms are detected in an individual samples, this indicates the presence of both copies of the gene and rules out total gene deletions. ● CASE:
A 7 day-old girl was referred to Mayo Clinic for bilaterally dilated and unresponsive pupils since birth. The child was the product of an uncomplicated pregnancy, born at 41 weeks’ gestational age with a birth weight of 8 pounds 1 ounce. Her birth was complicated by arrest of descent and she required a vacuum-assisted delivery, with Apgar scores of 7 at one minute and 8 at five minutes.
VOL. 145, NO. 4
ANIRIDIA
WITHOUT
There was no family history of ocular disorders including aniridia or other forms of anterior segment dysgenesis. The ocular examination demonstrated normal external appearance, ocular alignment, and motility. Her corneas were of normal size and free of posterior embryotoxon and iris strands. The anterior segments were deep. Her irides were remarkable for being truncated and lacking normal architecture for 360 degrees (Figure 1). There were a few iris strands from the iris to the anterior capsule of the lens bilaterally, but no cataracts were noted in either eye. Her pupils did not respond to light. The foveal reflex appeared blunted bilaterally, and this was interpreted as evidence of mild foveal hypoplasia. Her intraocular pressure was 13 mm Hg in both eyes under mask anesthesia at nine weeks of age and again at 32 months of age. Results from testing for PAX6 mutations were negative. The child was followed up in the Departments of Genetics and Ophthalmology. At last follow-up, at 35 months of age, no medical or developmental disorders had PAX6 MUTATIONS
761
FIGURE 2. (Top) Slit-lamp photographs and (Bottom) fundus photographs showing the undilated appearance of iris in both eyes of Patient 3 with aniridia and no PAX6 mutations. The posterior poles in both eyes appear to be normal.
developed in the child, and her ocular examination results, except for her attenuated irides, were completely normal, including a visual acuity of 20/40 in each eye with Allen pictures and a complete resolution of what was interpreted as foveal hypoplasia in the first week of life.
One patient had a twin sister who was not affected. One possible explanation is that the mutation that led to aniridia developed very early during postzygotic fetal development in the patient, but not in her twin. The most interesting aspect of these patients lies in the absence of detectable mutations in PAX6. The gene was totally sequenced, including exon 5a. Heterozygosity for polymorphic markers in the gene for all patients confirmed the presence of both copies and eliminated the possibility of a total gene deletion. The presence of a mutation upstream or downstream of the gene cannot be eliminated entirely, but one would have expected a much more classical clinical picture of the aniridia phenotype that would have included nystagmus and reduced visual acuity. Mutations in PAX6 are detectable in approximately 80% of patients with aniridia, and most result in loss of function of one copy of the gene or haploinsufficiency.9 The phenotypes of patients with aniridia and no detectable mutations in PAX6 have not been described in detail, so we do not know if these patients in general have better vision than those with mutations in this gene. A mild degree of visual impairment and an absence of glaucoma have been described in occasional patients and families with aniridia.8,12 Missense mutations and those at the N-terminus of the gene have been associated with a mild phenotype, usually an iris that appears almost normal and isolated foveal hypoplasia.13,14 For an extensive review of reported PAX6 mutations and their phenotypic correlations, the reader is referred to the article by Tzoulaki and associates.15
DISCUSSION THE FOUR CASES IN THIS REPORT HAD ANTERIOR SEGMENT
features typical for aniridia. All had absence of large portions of iris tissue in a circumferential pattern, with a few iris strands to the anterior lens capsule in some. None of the patients exhibited features of anterior segment dysgenesis characteristic of the Axenfeld-Rieger spectrum of malformations, such as a prominent and anteriorly displaced Schwalbe line, iridocorneal adhesions, or pseudopolycoria. None of the patients had evidence of poor vision, either, on clinical testing. Case 4 did have nystagmus that disappeared on follow-up; an exact measurement of visual acuity in this patient was not possible at her last examination. Although the fovea appeared slightly hypoplastic and the foveal reflex was blunted in some patients, this did not seem to affect vision. Optical coherence tomography was performed in one patient, and the results were suggestive of mild foveal hypoplasia. None of the patients had elevated intraocular pressure, nor did a Wilms tumor develop in any by their final examination, although this tumor may occur rarely in children 7 years or older. Karyotypes were obtained in all patients and were normal. 762
AMERICAN JOURNAL
OF
OPHTHALMOLOGY
APRIL 2008
FIGURE 3. (Top) Fundus photographs showing bilateral aniridia, a blunted foveal reflex on (Bottom left) ophthalmoscopy and (Bottom right) optical coherence tomography in Patient 4, who had no detectable PAX6 mutations.
One likely explanation for the clinical findings in our patients is that the aniridia phenotype is the result of mutations in a gene other than PAX6. It may be a fresh dominant mutation in each of the patients or possibly a recessive trait inherited from both parents. It is also possible that there is more than one gene involved in our four patients; however, the consistent phenotype is more consistent with mutations in the same gene. It is also possible that the mutation is on a chromosomal area adjacent to the PAX6 gene and somehow affects its expression. The cases described in this report support the presence of genetic heterogeneity in aniridia. Unfortunately, in the absence of other affected family members and large informative pedigrees, it is not possible to identify the responsible gene(s) at the present time. The normal vision and absence of glaucoma in these patients suggests that the genetic defect does not interfere with retinal development and anterior chamber angle
development to a significant extent. However, we are uncertain about the risk of elevation of intraocular pressure as patients get older, and we continue to monitor intraocular pressure at regular intervals in all patients. The absence of a detectable deletion on 11p in these patients and the probable location of the affected gene away from the PAX6 locus make the risk of developing a Wilms tumor to be most similar to that of the general population. We have documented the presence of an aniridia phenotype with good vision that is not the result of mutations in PAX6. We estimate that this form accounts for some of the 10% to 20% of cases of aniridia in which a mutation of PAX6 does not occur. Now that clinical genetic testing is available, this variant of aniridia should be considered in babies without detectable mutations in PAX6. From our limited experience with these four patients, the visual outcome seems to be significantly better than with PAX6-related aniridia.
THIS STUDY WAS SUPPORTED BY AN UNRESTRICTED GRANT FROM RESEARCH TO PREVENT BLINDNESS, INC, NEW YORK, NEW York (Drs Traboulsi and Mohney). The authors indicate no financial conflict of interest. Involved in design and conduct of study (E.I.T., B.G.M.); collection (E.I.T., J.A., B.G.M.); management (E.I.T., J.A., J.S., J.E., B.G.M.); analysis (E.I.T., J.A., J.S., J.E., B.G.M.); and interpretation of the data (E.I.T., J.A., J.S., J.E., B.G.M.); and preparation, review, or approval of the manuscript (E.I.T., J.A., J.S., J.E., B.G.M.). The Cleveland Clinic Institutional Review Board approved this research.
VOL. 145, NO. 4
ANIRIDIA
WITHOUT
PAX6 MUTATIONS
763
9. Gronskov K, Olsen JH, Sand A, et al. Population-based risk estimates of Wilms tumor in sporadic aniridia. A comprehensive mutation screening procedure of PAX6 identifies 80% of mutations in aniridia. Hum Genet 2001;109:11–18. 10. Axton R, Hanson I, Danes S, Sellar G, van Heyningen V, Prosser J. The incidence of PAX6 mutation in patients with simple aniridia: an evaluation of mutation detection in 12 cases. J Med Genet 1997;34:279 –286. 11. Elsas F, Maumenee I, Kenyon K, Yoder F. Familial aniridia with preserved ocular function. Am J Ophthalmol 1977;83: 718 –724. 12. Ferrell R, Chakravarti A, Hittner H, Riccardi V. Autosomal dominant aniridia: probable linkage to the acid phosphatase-1 locus on chromosome 2. Proc Natl Acad Sci U S A 1980;77:1580 –1582. 13. Azuma N, Nishina S, Yanagisawa H, Okuyama T, Yamada M. PAX6 missense mutation in isolated foveal hypoplasia. Nat Genet 1996;13:141–142. 14. Gronskov K, Rosenberg T, Sand A, Brondum-Nielsen K. Mutational analysis of PAX6: 16 novel mutations including 5 missense mutations with a mild aniridia phenotype. Eur J Hum Genet 1999;7:274 –286. 15. Tzoulaki I, White IMS, Hanson IM. PAX6 mutations: genotype-phenotype correlations. BMC Genet 2005; 6:27.
REFERENCES 1. Pearce W. Variability of iris defects in autosomal dominant aniridia. Can J Ophthalmol 1994;29:25–29. 2. Koroma BM, Yang JM, Sundin OH. The PAX6 homeobox gene is expressed throughout the corneal and conjunctival epithelia. Invest Ophthalmol Vis Sci 1997;38:108 –120. 3. Mannens M, Bleeker-Wagemakers E, Bliek J, et al. Autosomal dominant aniridia linked to the chromosome 11p13 markers catalase and D11S151 in a large Dutch family. Cytogenet Cell Genet 1989;52:32–36. 4. Glaser T, Walton D, Maas R. Genomic structure evolutionary conservation and aniridia mutations in the human PAX6 gene. Nature Genet 1992;2:232–239. 5. Mirzayans F, Pearce W, MacDonald I, Walter M. Mutation of the PAX6 gene in patients with autosomal dominant keratitis. Am J Hum Genet 1995;57:539 –548. 6. Hanson I, Fletcher J, Jordan T, et al. Mutations at the PAX6 locus are found in heterogeneous anterior segment malformations including Peters’ anomaly. Nature Genet 1994;6:168 –173. 7. Hanson I, Churchill A, Love J, et al. Missense mutations in the most ancient residues of the PAX6 paired domain underlie a spectrum of human congenital eye malformations. Hum Mol Genet 1999;8:165–172. 8. Mitchell TN, Free SL, Williamson KA, et al. Polymicrogyria and absence of pineal gland due to PAX6 mutation. Ann Neurol 2003;53:658 – 663.
764
AMERICAN JOURNAL
OF
OPHTHALMOLOGY
APRIL 2008
A
NIRIDIA IS A MISNOMER THAT REFERS TO THE CON-
genital absence of the iris. Most often there is circumferential loss of tissue that includes all of the pupillary sphincter and midperipheral iris, with preservation of the most peripheral part of the iris into the anterior chamber angle. In some patients, there are instead sectoral or other iris defects that are not in the inferior and nasal iris, as one would expect in patients with typical iris colobomas.1 In addition to the iris defects, most patients with aniridia have reduced vision from foveal hypoplasia; with time, keratopathy that has been referred to as pannus or keratitis develops in some. This keratopathy seems to be the result of defective or absent corneoscleral limbal stem cells, the development and maintenance of which require a normal activity of PAX6.2 The only known chromosomal locus for aniridia has been mapped to 11p13,3 which was found to harbor PAX6, the only gene that has been shown to cause aniridia.4 PAX6 is an essential transcription factor for eye formation. Accepted for publication Dec 5, 2007. From the Cole Eye Institute, Cleveland Clinic, Cleveland, Ohio (E.I.T., J.S.); Departments of Medical Genetics (J.E.) and Ophthalmology (B.G.M.), Mayo Clinic, Rochester, Minnesota; The Wilmer Eye Institute, Baltimore, Maryland (I.H.M.); and Ophthalmology Associates, Annapolis, Maryland (J.A.). Inquiries to Elias I. Traboulsi, i32, 9500 Euclid Avenue, Cleveland, OH 44195; e-mail: [email protected]
760
The great majority of PAX6 mutations cause aniridia, but some result in congenital cataracts, Peters anomaly, anterior segment dysgenesis, ectopia pupillae, and dominant keratitis.5– 8 Mutation analysis of cohorts of patients with typical aniridia (iris defects, nystagmus, reduced vision from foveal hypoplasia) has led to the identification of PAX6 sequence variations in more than 80% of patients.9 One study that extensively sequenced PAX6 in 12 patients with aniridia detected mutations in 90% of cases.10 The remaining 10% to 20% are postulated to have mutations in other genes or in parts of the PAX6 gene that are not sequenced routinely. Rare families with aniridia and preserved visual function have been reported.11 No mutation in PAX6 has been identified to date in the family reported by Elsas and associates.11 We have identified four patients with an anterior segment malformation clinically indistinguishable from aniridia, but with preserved visual function, a normalappearing fovea in some, and no detectable mutations in PAX6.
©
2008 BY
METHODS THIS WAS A CHART REVIEW OF FOUR PATIENTS FROM
three sources and a tabulation of their clinical ocular findings. Institutional review board approval was obtained from the Cleveland Clinic Foundation. All patients had clinical molecular testing of the PAX6 gene at GeneDx (Gaithersburg, Maryland, USA), a commercial certified clinical laboratory. Exons 1 through 13 and the intron– exon boundaries of the PAX6 gene were amplified and were sequenced bidirectionally from the deoxyribonucleic acid samples of all patients. The alternatively spliced exon 5a also was sequenced. All patients also were genotyped for the presence of at least three polymorphisms: IVS4-195 a/g, IVS9-12 c/t, and IVS12⫹43 g/t. These polymorphisms were ascertained to ensure that both copies of the PAX6 gene were sequenced and that the patient did not have a complete deletion of the gene.
RESULTS ● CLINICAL FINDINGS:
All patients had almost total absence of iris tissue, good visual acuity, and foveal reflexes
ELSEVIER INC. ALL
RIGHTS RESERVED.
0002-9394/08/$34.00 doi:10.1016/j.ajo.2007.12.012
TABLE. Clinical Findings in Four Patients with Aniridia and No Detectables Mutation in PAX6 Case No.
Age at Last Visit
Vision
1
35 mos
VA, 20/40 in both eyes; no nystagmus
2
Two mos Good fix and follow; no nystagmus
3
10 yrs
4
33 mos
Iris/Anterior Segment Findings
Typical aniridia (Figure 1): persistent pupillary membranes Typical aniridia with inferior colobomas
Fundus/Fovea
Other Eye or Systemic Findings
Normal/⫹2.00 in both eyes
Normal renal ultrasound results
Good foveal reflexes
normal MRI results; normal renal ultrasound results None
No nystagmus; latest VA, 20/25 in both eyes
Typical aniridia with remnants Poor foveal reflexes (Figure 2)/ of pupillary membrane; ⫹6.00 in both eyes inferior colobomas (Figure 2) VA, 20/30 with Allen pictures Typical aniridia (Figure 3) Poor foveal reflex (Figure 3)/plano Has an identical twin on near card; history of ⫹2.75 x 90 in the right eye/ with normal eye nystagmus plano ⫹1.75 x 90 in the left examination eye/OCT showed blunted results; normal foveal reflexes (Figure 3) karyotype; normal renal ultrasound results
MRI ⫽ magnetic resonance imaging; OCT ⫽ optical coherence tomography; VA ⫽ visual acuity.
FIGURE 1. Photographs of the right and left eyes of Patient 1 showing aniridia and no detectable mutation in PAX6. Note the circumferential absence of iris tissue and pupil sphincter.
described as normal to variably blunted on clinical examination. The Table summarizes the clinical findings in the four patients. ● MOLECULAR ANALYSIS: No PAX6 mutations were detected in any of the four patients. Heterozygous polymorphisms at IVS4-195 a/g, IVS9-12 c/t, and IVS12⫹43 g/t were detected in all patients. If two copies of any or all of these polymorphisms are detected in an individual samples, this indicates the presence of both copies of the gene and rules out total gene deletions. ● CASE:
A 7 day-old girl was referred to Mayo Clinic for bilaterally dilated and unresponsive pupils since birth. The child was the product of an uncomplicated pregnancy, born at 41 weeks’ gestational age with a birth weight of 8 pounds 1 ounce. Her birth was complicated by arrest of descent and she required a vacuum-assisted delivery, with Apgar scores of 7 at one minute and 8 at five minutes.
VOL. 145, NO. 4
ANIRIDIA
WITHOUT
There was no family history of ocular disorders including aniridia or other forms of anterior segment dysgenesis. The ocular examination demonstrated normal external appearance, ocular alignment, and motility. Her corneas were of normal size and free of posterior embryotoxon and iris strands. The anterior segments were deep. Her irides were remarkable for being truncated and lacking normal architecture for 360 degrees (Figure 1). There were a few iris strands from the iris to the anterior capsule of the lens bilaterally, but no cataracts were noted in either eye. Her pupils did not respond to light. The foveal reflex appeared blunted bilaterally, and this was interpreted as evidence of mild foveal hypoplasia. Her intraocular pressure was 13 mm Hg in both eyes under mask anesthesia at nine weeks of age and again at 32 months of age. Results from testing for PAX6 mutations were negative. The child was followed up in the Departments of Genetics and Ophthalmology. At last follow-up, at 35 months of age, no medical or developmental disorders had PAX6 MUTATIONS
761
FIGURE 2. (Top) Slit-lamp photographs and (Bottom) fundus photographs showing the undilated appearance of iris in both eyes of Patient 3 with aniridia and no PAX6 mutations. The posterior poles in both eyes appear to be normal.
developed in the child, and her ocular examination results, except for her attenuated irides, were completely normal, including a visual acuity of 20/40 in each eye with Allen pictures and a complete resolution of what was interpreted as foveal hypoplasia in the first week of life.
One patient had a twin sister who was not affected. One possible explanation is that the mutation that led to aniridia developed very early during postzygotic fetal development in the patient, but not in her twin. The most interesting aspect of these patients lies in the absence of detectable mutations in PAX6. The gene was totally sequenced, including exon 5a. Heterozygosity for polymorphic markers in the gene for all patients confirmed the presence of both copies and eliminated the possibility of a total gene deletion. The presence of a mutation upstream or downstream of the gene cannot be eliminated entirely, but one would have expected a much more classical clinical picture of the aniridia phenotype that would have included nystagmus and reduced visual acuity. Mutations in PAX6 are detectable in approximately 80% of patients with aniridia, and most result in loss of function of one copy of the gene or haploinsufficiency.9 The phenotypes of patients with aniridia and no detectable mutations in PAX6 have not been described in detail, so we do not know if these patients in general have better vision than those with mutations in this gene. A mild degree of visual impairment and an absence of glaucoma have been described in occasional patients and families with aniridia.8,12 Missense mutations and those at the N-terminus of the gene have been associated with a mild phenotype, usually an iris that appears almost normal and isolated foveal hypoplasia.13,14 For an extensive review of reported PAX6 mutations and their phenotypic correlations, the reader is referred to the article by Tzoulaki and associates.15
DISCUSSION THE FOUR CASES IN THIS REPORT HAD ANTERIOR SEGMENT
features typical for aniridia. All had absence of large portions of iris tissue in a circumferential pattern, with a few iris strands to the anterior lens capsule in some. None of the patients exhibited features of anterior segment dysgenesis characteristic of the Axenfeld-Rieger spectrum of malformations, such as a prominent and anteriorly displaced Schwalbe line, iridocorneal adhesions, or pseudopolycoria. None of the patients had evidence of poor vision, either, on clinical testing. Case 4 did have nystagmus that disappeared on follow-up; an exact measurement of visual acuity in this patient was not possible at her last examination. Although the fovea appeared slightly hypoplastic and the foveal reflex was blunted in some patients, this did not seem to affect vision. Optical coherence tomography was performed in one patient, and the results were suggestive of mild foveal hypoplasia. None of the patients had elevated intraocular pressure, nor did a Wilms tumor develop in any by their final examination, although this tumor may occur rarely in children 7 years or older. Karyotypes were obtained in all patients and were normal. 762
AMERICAN JOURNAL
OF
OPHTHALMOLOGY
APRIL 2008
FIGURE 3. (Top) Fundus photographs showing bilateral aniridia, a blunted foveal reflex on (Bottom left) ophthalmoscopy and (Bottom right) optical coherence tomography in Patient 4, who had no detectable PAX6 mutations.
One likely explanation for the clinical findings in our patients is that the aniridia phenotype is the result of mutations in a gene other than PAX6. It may be a fresh dominant mutation in each of the patients or possibly a recessive trait inherited from both parents. It is also possible that there is more than one gene involved in our four patients; however, the consistent phenotype is more consistent with mutations in the same gene. It is also possible that the mutation is on a chromosomal area adjacent to the PAX6 gene and somehow affects its expression. The cases described in this report support the presence of genetic heterogeneity in aniridia. Unfortunately, in the absence of other affected family members and large informative pedigrees, it is not possible to identify the responsible gene(s) at the present time. The normal vision and absence of glaucoma in these patients suggests that the genetic defect does not interfere with retinal development and anterior chamber angle
development to a significant extent. However, we are uncertain about the risk of elevation of intraocular pressure as patients get older, and we continue to monitor intraocular pressure at regular intervals in all patients. The absence of a detectable deletion on 11p in these patients and the probable location of the affected gene away from the PAX6 locus make the risk of developing a Wilms tumor to be most similar to that of the general population. We have documented the presence of an aniridia phenotype with good vision that is not the result of mutations in PAX6. We estimate that this form accounts for some of the 10% to 20% of cases of aniridia in which a mutation of PAX6 does not occur. Now that clinical genetic testing is available, this variant of aniridia should be considered in babies without detectable mutations in PAX6. From our limited experience with these four patients, the visual outcome seems to be significantly better than with PAX6-related aniridia.
THIS STUDY WAS SUPPORTED BY AN UNRESTRICTED GRANT FROM RESEARCH TO PREVENT BLINDNESS, INC, NEW YORK, NEW York (Drs Traboulsi and Mohney). The authors indicate no financial conflict of interest. Involved in design and conduct of study (E.I.T., B.G.M.); collection (E.I.T., J.A., B.G.M.); management (E.I.T., J.A., J.S., J.E., B.G.M.); analysis (E.I.T., J.A., J.S., J.E., B.G.M.); and interpretation of the data (E.I.T., J.A., J.S., J.E., B.G.M.); and preparation, review, or approval of the manuscript (E.I.T., J.A., J.S., J.E., B.G.M.). The Cleveland Clinic Institutional Review Board approved this research.
VOL. 145, NO. 4
ANIRIDIA
WITHOUT
PAX6 MUTATIONS
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9. Gronskov K, Olsen JH, Sand A, et al. Population-based risk estimates of Wilms tumor in sporadic aniridia. A comprehensive mutation screening procedure of PAX6 identifies 80% of mutations in aniridia. Hum Genet 2001;109:11–18. 10. Axton R, Hanson I, Danes S, Sellar G, van Heyningen V, Prosser J. The incidence of PAX6 mutation in patients with simple aniridia: an evaluation of mutation detection in 12 cases. J Med Genet 1997;34:279 –286. 11. Elsas F, Maumenee I, Kenyon K, Yoder F. Familial aniridia with preserved ocular function. Am J Ophthalmol 1977;83: 718 –724. 12. Ferrell R, Chakravarti A, Hittner H, Riccardi V. Autosomal dominant aniridia: probable linkage to the acid phosphatase-1 locus on chromosome 2. Proc Natl Acad Sci U S A 1980;77:1580 –1582. 13. Azuma N, Nishina S, Yanagisawa H, Okuyama T, Yamada M. PAX6 missense mutation in isolated foveal hypoplasia. Nat Genet 1996;13:141–142. 14. Gronskov K, Rosenberg T, Sand A, Brondum-Nielsen K. Mutational analysis of PAX6: 16 novel mutations including 5 missense mutations with a mild aniridia phenotype. Eur J Hum Genet 1999;7:274 –286. 15. Tzoulaki I, White IMS, Hanson IM. PAX6 mutations: genotype-phenotype correlations. BMC Genet 2005; 6:27.
REFERENCES 1. Pearce W. Variability of iris defects in autosomal dominant aniridia. Can J Ophthalmol 1994;29:25–29. 2. Koroma BM, Yang JM, Sundin OH. The PAX6 homeobox gene is expressed throughout the corneal and conjunctival epithelia. Invest Ophthalmol Vis Sci 1997;38:108 –120. 3. Mannens M, Bleeker-Wagemakers E, Bliek J, et al. Autosomal dominant aniridia linked to the chromosome 11p13 markers catalase and D11S151 in a large Dutch family. Cytogenet Cell Genet 1989;52:32–36. 4. Glaser T, Walton D, Maas R. Genomic structure evolutionary conservation and aniridia mutations in the human PAX6 gene. Nature Genet 1992;2:232–239. 5. Mirzayans F, Pearce W, MacDonald I, Walter M. Mutation of the PAX6 gene in patients with autosomal dominant keratitis. Am J Hum Genet 1995;57:539 –548. 6. Hanson I, Fletcher J, Jordan T, et al. Mutations at the PAX6 locus are found in heterogeneous anterior segment malformations including Peters’ anomaly. Nature Genet 1994;6:168 –173. 7. Hanson I, Churchill A, Love J, et al. Missense mutations in the most ancient residues of the PAX6 paired domain underlie a spectrum of human congenital eye malformations. Hum Mol Genet 1999;8:165–172. 8. Mitchell TN, Free SL, Williamson KA, et al. Polymicrogyria and absence of pineal gland due to PAX6 mutation. Ann Neurol 2003;53:658 – 663.
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