Pax6-Related Aniridia

Clinical characteristics.

PAX6-related aniridia occurs either as an isolated ocular abnormality or as part of the Wilms tumor-aniridia-genital anomalies-retardation (WAGR) syndrome. Aniridia is a pan ocular disorder affecting the cornea, iris, intraocular pressure (resulting in glaucoma), lens (cataract and lens subluxation), fovea (foveal hypoplasia), and optic nerve (optic nerve coloboma and hypoplasia). Individuals with aniridia characteristically show nystagmus and impaired visual acuity (usually 20/100 - 20/200); however, milder forms of aniridia with subtle iris architecture changes, good vision, and normal foveal structure do occur. Other ocular involvement may include strabismus and occasionally microphthalmia. Although the severity of aniridia can vary between and within families, little variability is usually observed in the two eyes of an affected individual.

WAGR syndrome. The risk for Wilms tumor is 42.5%-77%; of those who develop Wilms tumor, 90% do so by age four years and 98% by age seven years. Genital anomalies in males can include cryptorchidism and hypospadias (sometimes resulting in ambiguous genitalia), urethral strictures, ureteric abnormalities, and gonadoblastoma. While females typically have normal external genitalia, they may have uterine abnormalities and streak ovaries. Intellectual disability (defined as IQ <74) is observed in 70%; behavioral abnormalities include attention deficit hyperactivity disorder (ADHD), autism spectrum disorder, anxiety, depression, and obsessive-compulsive disorder. Other individuals with WAGR syndrome can have normal intellect without behavioral problems.

Diagnosis/testing.

The diagnosis of PAX6-related aniridia is established in a proband with one of the two following clinical and molecular genetic findings:

• Isolated aniridia (i.e., without systemic involvement) and a heterozygous PAX6 pathogenic variant, ranging in size from a single nucleotide (e.g., those resulting in a nonsense, missense, or splice site variant or single-nucleotide deletion or duplication) to a partial- or whole-gene deletion (or in rare instances deletions telomeric to PAX6 that do not include PAX6); or
• Aniridia and one or more additional findings of WAGR syndrome and a deletion of PAX6 and the upstream adjacent gene, WT1

Management.

Treatment of manifestations:

• Aniridia. Correction of refractive errors, use of tinted or photochromic lenses to reduce light sensitivity, occlusion therapy in childhood for amblyopia, use of low-vision aids. Treatment of severe cataracts requires attention to potential complications caused by poor zonular stability. Glaucoma: Initial treatment is usually topical anti-glaucoma medication; surgery is reserved for eyes that do not respond to medical therapy. Ocular surface disease: medical treatment (lubricants, mucolytics, and punctal occlusion) may help slow the progression of corneal opacification. When corneal opacification causes significant visual reduction, penetrating keratoplasty with limbal stem cell transplantation may be considered; however, this has a high risk of failure and possible lifelong systemic immunosuppression to prevent rejection.
• WAGR syndrome. Wilms tumor, genital anomalies, and developmental delay / intellectual disability are managed as per standard practice.

Surveillance:

• Aniridia. Monitor children younger than age eight years every four to six months for refractive errors and detection and treatment of incipient or actual amblyopia; annual ophthalmology follow up of all individuals to detect problems such as corneal changes, raised intraocular pressure, and cataracts.
• WAGR syndrome. Children with aniridia and a WT1 deletion require renal ultrasound examinations every three months and follow up by a pediatric oncologist until age eight years. Because of the increased risk for renal impairment in WAGR syndrome (especially in those with bilateral Wilms tumor), lifelong evaluation of renal function is recommended. Developmental progress and educational needs require regular monitoring. Behavioral assessment for anxiety, ADHD, and aggressive or self-injurious behavior as needed.

Agents/circumstances to avoid: Intraocular surgery may increase the likelihood of (or exacerbate existing) keratopathy; repeated intraocular surgery predisposes to severe aniridic fibrosis syndrome.

Evaluation of relatives at risk: Early clarification of the genetic status of infants who are offspring or sibs of an individual with PAX6-related isolated aniridia (by either an eye examination or molecular genetic testing for the PAX6 variant in the family) is recommended in order to identify those who would benefit from prompt treatment and surveillance of complications of aniridia.

Genetic counseling.

Isolated aniridia and WAGR syndrome are inherited in an autosomal dominant manner.

• Isolated aniridia. ~70% of individuals have an affected parent; ~30% have a de novo PAX6 pathogenic variant or deletion of a regulatory region controlling PAX6 expression. Each child of an individual with isolated aniridia has a 50% chance of inheriting the causative genetic alteration and developing aniridia. In rare instances of mosaicism for the PAX6 pathogenic variant in the proband, the risk to offspring may be lower.
• WAGR syndrome is associated with contiguous-gene deletions including PAX6 and WT1. If the proband has a de novo contiguous-gene deletion and neither parent has evidence of mosaicism for the deletion, the risk to sibs is no greater than that in the general population.

When the PAX6 genetic alteration in a family is known, prenatal testing for a pregnancy at increased risk and preimplantation genetic testing are possible.

Suggestive Findings

PAX6-related isolated aniridia should be suspected in individuals who have the following clinical and imaging findings of aniridia with no other associated systemic abnormalities.

Clinical findings

• Aniridia. Complete or partial iris hypoplasia best seen on slit lamp examination. Iris translucency or abnormal architecture and pupillary abnormalities may also be seen.
• Reduced visual acuity secondary to:
  • Absence of or reduction in the normal foveal architecture (usually [not always] observed)
  • Optic nerve abnormalities (e.g., optic nerve hypoplasia or coloboma)
• Early-onset nystagmus (usually apparent by age 6 weeks)
• Microphthalmia and ocular coloboma (iris, chorioretinal, and/or optic disc)

Imaging findings

• Optical coherence tomography (OCT) may be used to document foveal hypoplasia. Although OCT is difficult to perform in the presence of nystagmus, useful images can be obtained with persistence. This should be routinely performed to support a clinical diagnosis, especially where iris defects may be subtle. Anterior segment OCT can also be used to delineate the detailed anatomy of the anterior segment structures, even in those with corneal opacity [Majander et al 2012].
• Ultrasound B-scan should be performed routinely to assess axial length due to the association with microphthalmia.
• High-frequency ultrasound biomicroscopy. In infants with corneal opacity or severe corneal edema resulting from associated congenital glaucoma, high-frequency anterior segment ultrasound examination, usually performed under anesthesia, can demonstrate iris hypoplasia and/or absence [Nischal 2007].
  Note: Iris fluorescein angiography may identify subtle iris hypoplasia but is rarely used clinically.

Wilms tumor-aniridia-genital anomalies-retardation (WAGR) syndrome should be suspected in individuals with aniridia and no family history of aniridia who also have at least one of the following findings:

• Wilms tumor (also known as nephroblastoma), a childhood kidney malignancy. Of children with WAGR who develop Wilms tumor, 90% do so by age four years and 98% by age seven years (see Wilms Tumor Predisposition).
• Genitourinary abnormalities. In males: cryptorchidism, hypospadias, ambiguous genitalia; in females: normal external female genitalia, but uterine abnormalities (heart-shaped bicornate uterus) and streak ovaries. In males and females: end-stage renal disease, ureteric abnormalities, and gonadoblastoma.
• Intellectual disability and/or behavior abnormalities including depression, anxiety, ADHD, obsessive-compulsive disorder, and autism.
• Childhood-onset obesity and pancreatitis

Establishing the Diagnosis

The diagnosis of PAX6-related aniridia is established in a proband with one of the two following clinical and molecular genetic findings (Table 1):

• Isolated aniridia (i.e., without systemic involvement) and a heterozygous PAX6 pathogenic variant, ranging in size from a single nucleotide (e.g., those resulting in a nonsense, missense, or splice site variant or single-nucleotide deletion or duplication) to a partial or whole-gene deletion [Richardson et al 2016]
  Note: Deletions telomeric to PAX6 that do not include PAX6 have been reported.
  • A heterozygous variant in the ultraconserved PAX6 cis-regulatory element (SIMO) that resides 150 kb downstream from PAX6 in intron 9 of ELP4 (NM_001288726.1) causes isolated aniridia [Bhatia et al 2013].
  • MLPA detected a 0.6-Mb deletion downstream of PAX6 on chromosome 11 that encompasses DCDC1, DPH4, IMMP1L, and ELP4 [Wawrocka et al 2012].
  Coverage of these regions on chromosomal or gene-targeted arrays will vary [Blanco-Kelly et al 2017, Franzoni et al 2017].
• Wilms tumor-aniridia-genital anomalies-retardation (WAGR) syndrome and EITHER of the following:
  • A deletion of PAX6 and the upstream adjacent gene, WT1
    Note: Reported deletions include the recurrent 11p13 deletion (see Table 1).
  • One or more additional findings of WAGR syndrome found on physical examination in individuals with aniridia.
    Note: If the child has not had genetic testing, the clinical diagnosis of WAGR syndrome usually cannot be established or ruled out until a child has passed through the age of risk for Wilms tumor, intellectual disability, and behavior abnormalities.

Molecular genetic testing can establish the molecular basis of aniridia, and thus distinguish between isolated aniridia (no increased risk for Wilms tumor) and WAGR (markedly increased risk for Wilms tumor). In the following scenarios molecular genetic testing approaches are based on the individual's age, clinical findings, family history, and testing methods available.

Clinical Description

PAX6-related aniridia occurs either as an isolated ocular abnormality or as part of the Wilms tumor-aniridia-genital anomalies-retardation (WAGR) syndrome. Aniridia, a congenital eye anomaly, is usually detected at birth if fully penetrant. It is often the presenting feature of WAGR; children with WAGR are at significant risk of developing Wilms tumor during early childhood.

Genotype-Phenotype Correlations

Isolated aniridia. PAX6 haploinsufficiency produces classic and severe aniridia with a high incidence of sight-reducing pathology including optic nerve malformations, glaucoma, cataract, and corneal changes [Kleinjan & van Heyningen 1998, Prosser & van Heyningen 1998, Grønskov et al 1999, Hanson et al 1999, Lauderdale et al 2000, van Heyningen & Williamson 2002, Chao et al 2003, Tzoulaki et al 2005, Dansault et al 2007, Hingorani et al 2009].

PAX6 pathogenic missense variants, particularly those that are in the paired domain and therefore likely to significantly reduce the DNA binding ability, tend to produce atypical/milder or variable-phenotype aniridia with better vision, more residual iris tissue, and a lower frequency of sight-reducing malformations and complications [Hingorani et al 2009].

C-terminal extension (CTE) pathogenic variants, which generate a longer protein product, are associated with a moderately severe aniridic phenotype with poor vision, keratopathy, and cataracts; however, individuals with CTE pathogenic variants are less likely to have glaucoma and are more likely to have preservation of iris tissue than individuals who have pathogenic null variants [Hingorani et al 2009, Aggarwal et al 2011]. For reasons that are not clear, the rare reports of significant non-foveal retinal abnormalities (exudative retinopathy, chorioretinal degeneration) are all associated with CTE pathogenic variants [Hingorani et al 2009, Aggarwal et al 2011].

Penetrance

Isolated aniridia has almost complete penetrance.

Aniridia in WAGR also has almost complete penetrance. The risk of Wilms tumor is up to 77%.

Prevalence

The prevalence of aniridia is 1:40,000 to 1:100,000. No racial or sexual differences are recognized.

The prevalence of WAGR syndrome is approximately 1:500,000.

Individuals with Aniridia and No Identifiable PAX6 Pathogenic Variant

Heterozygous pathogenic variants in the following genes are included in the differential diagnosis of PAX6-related aniridia:

• FOXC1. Phenocopies exist and include dominant alleles of FOXC1, which can cause diagnostic difficulties [Khan et al 2008, Ito et al 2009].
• PITX2. Perveen et al [2000]
• PITX3. Semina et al [1998]
• Unknown. Ansari et al [2016] could not identify the cause of aniridia in 20 patients despite PAX6, FOXC1, and PITX2 sequence analysis, FISH, and aCGH, suggesting there may be further genetic heterogeneity with potentially new disease loci and/or novel mutational mechanisms.

Rieger anomaly, a form of anterior segment mesenchymal dysgenesis, is characterized by severe iris atrophy, corectopia (displaced pupils), iris holes, and, frequently, childhood-onset glaucoma. Rieger anomaly may be distinguished from aniridia by the presence of posterior embryotoxon (visible Schwalbe's line seen as a white line just inside the corneal limbus) with attached iris strands, relatively good visual acuity, and the absence of nystagmus or foveal abnormality.

Iris coloboma is a developmental defect resulting in a focal absence of the iris and a keyhole-shaped pupil; the rest of the iris is normal. Chorioretinal coloboma may be associated. Most iris colobomas are not associated with reduced visual acuity or nystagmus unless accompanied by a large posterior coloboma that involves the optic nerve and fovea; such large chorioretinal colobomas are apparent on fundoscopic examination.

Gillespie syndrome (OMIM 206700), characterized by partial iris hypoplasia, cerebellar ataxia, and intellectual disability, can be distinguished from aniridia by a characteristic iris configuration in Gillespie syndrome showing a scalloped pupillary edge with iris strands extending onto the anterior lens surface [Nelson et al 1997]. In five simplex cases of Gillespie syndrome (i.e., a single occurrence in a family), three were found to have biallelic ITPR1 pathogenic variants and two were found to have a de novo heterozygous ITPR1 pathogenic variant [Gerber et al 2016].

Oculocutaneous albinism (OCA) and ocular albinism typically present in early infancy with nystagmus but a structurally complete iris, typical diffuse iris transillumination (resulting from reduced pigment in the iris pigment epithelium), hypopigmented fundus, and, in the case of OCA, skin and hair hypopigmentation, which distinguish these disorders from aniridia (see Oculocutaneous Albinism Type 4 and X-Linked Ocular Albinism).

The other causes of nystagmus and poor vision in infancy (e.g., retinal dysplasia, retinal dystrophy, congenital cataracts, optic nerve hypoplasia, congenital infections) lack the iris changes seen in aniridia.

Causes of partial or complete absence of iris tissue in adults include trauma, prior ocular surgery, and the iridocorneal endothelial syndromes. The age at onset, medical history, and absence of other ocular features in aniridia should prevent diagnostic confusion with aniridia.

Evaluation Following Initial Diagnosis

To establish the extent of disease and needs in an individual diagnosed with aniridia (whether isolated or part of WAGR syndrome), the following are recommended:

• Evaluation of visual acuity (not easily performed in infants) and documentation of the degree of iris tissue deficiency, and the presence of foveal and optic nerve hypoplasia in order to predict future visual function.
• Evaluation for the degree of involvement of the cornea and lens and measurement of intraocular pressure, as they are potentially treatable causes of further visual reduction; however, treatable changes may not appear until later in life.
• Consultation with a clinical geneticist and/or genetic counselor

To establish the extent of disease and needs in an individual diagnosed with Wilms tumor-aniridia-genital anomalies-retardation (WAGR) syndrome, the following are recommended:

• Evaluation by a pediatrician to assess growth and feeding
• Evaluation for Wilms tumor
• Evaluation by a urologist for urogenital abnormalities
• Developmental assessment

Treatment of Manifestations

Aniridia. Simple measures are often the most important:

• Regular eye examinations and correction of refractive errors. Refractive errors range from high myopia through emmetropia to high hypermetropia. Spectacle correction of refractive errors is usually recommended as use of contact lenses can be difficult in the presence of keratopathy and reduced tear production.
• Tinted or photochromic lenses to reduce light sensitivity associated with the large pupillary aperture. Colored, tinted, or artificial pupil contact lenses may reduce light sensitivity or restore a more normal appearance to the eye but, as above, may be difficult to wear because of a poor ocular surface and tear film.
• Occlusion therapy in childhood for anisometropic amblyopia or strabismic amblyopia
• Optical low-vision aids and other devices such as closed-circuit television systems to help adults and children of school age
• Advice and help with schooling
• Social support

Note: Corrective surgery for strabismus can be undertaken to improve alignment and appearance but will not result in improved visual function.

Lens. Cataract extraction can significantly improve visual acuity in those with severe lens opacities. It should be remembered that in aniridia visual improvement after surgery is limited by foveal hypoplasia; thus, mild to moderate lens opacities may not require surgery:

• Children rarely require surgery (lensectomy).
• In adults, phacoemulsification and intraocular lens implantation can improve visual function if the cataract is severe.

Note: (1) A significant number of individuals with aniridia have poor zonular stability, which increases the risk for intraoperative complications and influences the choice of surgical technique and options for intraocular lens (IOL) implantation [Schneider et al 2003]. (2) The use of various types of black diaphragm aniridic IOLs may reduce glare or light sensitivity but are associated with a higher rate of surgical complications [Reinhard et al 2000, Menezo et al 2005, Pozdeyeva et al 2005].

Intraocular pressure

• Glaucoma is usually initially treated with topical anti-glaucoma medication.
• Surgery is reserved for eyes that do not respond to medical therapy:
  • Trabeculectomy with or without antimetabolites (e.g., 5-fluorouracil, mitomycin C) is often used but is associated with a higher risk of treatment failure than that seen in patients with primary glaucoma who undergo the same treatment.
  • Drainage tube surgery (with or without antimetabolites) or cyclodiode laser treatment may be necessary in refractory cases; however, this treatment is increasingly being undertaken as a primary procedure [Khaw 2002, Kirwan et al 2002, Arroyave et al 2003, Lee et al 2010].

Note: (1) Glaucoma presenting in infancy is more difficult to treat. Medical treatment is generally ineffective and surgery is required. Goniotomy and trabeculotomy have a low success rate, but trabeculectomy with or without antimetabolites is often successful [Nelson et al 1984, Okada et al 2000, Khaw 2002]. (2) While goniosurgery has been suggested as a preventive measure, glaucoma never develops in a significant proportion of those with aniridia [Swanner et al 2004].

Cornea

• Ocular surface disease can be treated medically using lubricants, mucolytics, and punctal occlusion, which may help slow the progression of sight-threatening corneal changes. Note: Drops without preservatives are often required to avoid preservative-related ocular surface toxicity.
• When corneal opacification causes significant visual reduction, penetrating keratoplasty (PK) may be considered; however, in the presence of the significant limbal stem cell deficiency observed in aniridia, PK alone has a poor prognosis [Tiller et al 2003].
• Limbal stem cell transplantation alone, preceding or concurrent with keratoplasty, may be undertaken but requires an allograft as both eyes are usually affected. This may take the form of a cultured stem cell sheet or a limbal tissue transplant [Lee et al 2008, Pauklin et al 2010]. However, this therapy is associated with a high risk of failure, and lifelong systemic immunosuppression may be required to prevent rejection. Whether the use of cultured oral mucous membrane cells may have a beneficial role is as yet uncertain.

Aniridic fibrosis syndrome. Surgical intervention is recommended at the first sign of aniridic fibrosis syndrome [Tsai et al 2005].

Wilms tumor. See Wilms Tumor Predisposition overview.

Genital abnormalities. This may need specialist care for functional and cosmetic management including endocrine therapy and surgical correction (e.g., hypospadias repair). There may be fertility issues that require support or active management.

Developmental delay / intellectual disability. There are a range of developmental, intellectual, psychiatric, and behavioral issues, as well as the challenges of visual impairment. Children may require:

• Special educational support including extra or different teaching resources and a specialized educational setting, specialist teachers of the visually impaired, educational psychologists, and formal statements of educational needs;
• Involvement of a pediatrician and sometimes a pediatric psychiatrist.

Surveillance