PITX2

Iridogoniodysgenesis, Type 2

Clinical Characteristics
Ocular Features: 

The iris stroma is hypoplastic resulting in a usually dark chocolate color which can suggest the diagnosis at birth.  It may, however, appear slate gray in lightly pigmented individuals.  The pupil is usually normal in morphology and location.  Glaucoma may detectable in the newborn period but it may also not be diagnosed until the 4th decade or later.  It is widely accepted that the anterior chamber angle is anomalous but the architectural and cellular details are lacking.

Systemic Features: 

No systemic abnormalities have been described.

Genetics

This is an autosomal dominant disorder resulting from heterozygous mutations in the PITX2 gene (4q25).

The same gene may be mutated in ring dermoid of the cornea (180550), Axenfeld-Rieger syndrome 1 (180500), Peters anomaly (604229), and in Axenfeld-Rieger anomaly plus (109120).

Type 1 iridogoniodysgenesis (IRID1) (601631) has many clinical similarities but is caused by DNA alterations in the FOXC1 gene.

Pedigree: 
Autosomal dominant
Treatment
Treatment Options: 

Glaucoma is the most frequent result of the anterior chamber dysgenesis in IRID2.  It is often difficult to control.  Early detection is of the utmost importance and all members of at-risk families require lifelong surveillance.

References
Article Title: 

Cornea, Ring Dermoid

Clinical Characteristics
Ocular Features: 

Dermoids in this condition are found at the limbus extending onto the cornea anteriorly and into the conjunctiva posteriorly.  They may be unilateral or bilateral and some contain functional hair follicles.  They are present at birth and appear as raised yellow-white tissue which can be segmental or extend for the full limbal circumference.  Some present as a dark ring around the cornea.  The apophyses can be elevated 2-3mm and extend for 3-5mm laterally.  Corneal changes, primary or secondary, lead to progressive vision loss in older individuals. Corneal distortion can result in significant astigmatism.  Some patients have glaucoma and congenital cataracts.

Histological studies have not been reported.

Systemic Features: 

No systemic disease is part of this condition.

Genetics

Two families, one Peruvian and one Chinese, have been reported with autosomal dominant patterns of transmission.  A G185A substitution in the PITX2 gene (4q25) cosegregated with the ocular disease in the Chinese family.

PITX2 encodes a transcription factor important to the development of multiple organs including the eye.  Mutations in this gene have also been found in patients with Peters anomaly (604229), a form of iris hypoplasia with goniodysgenesis (IRID2) (137600), and in Type 1 Axenfeld-Rieger syndrome (180500).

Pedigree: 
Autosomal dominant
Treatment
Treatment Options: 

Surgical excision may be necessary in patients with extensive disease.  Patients need to be monitored for cataracts, amblyopia, and glaucoma.

References
Article Title: 

Peters Anomaly

Clinical Characteristics
Ocular Features: 

Peters anomaly occurs as an isolated malformation but also as a feature of other syndromes.  It is often unilateral.  A wide variety of other ocular findings may occur with Peters anomaly as well. Here we limit our description to 'simple' Peters anomaly in which the findings are limited to the eye having the classic findings of adhesions of the iris to the posterior cornea and a central or paracentral corneal leukoma.  The lens may also be adherent to the cornea and is often opacified to some degree.  Descemet's membrane and portions of the posterior stroma are usually missing as well.  Glaucoma is frequently present.  Importantly, there is a wide range in the presentation of clinical features.

Systemic Features: 

Peters anomaly is a frequent feature of numerous syndromes, both ocular and systemic, among them the Peters-plus (261540) syndrome (sometimes called the Kivlin-Krause (261540) syndrome) and has been reported in a case with aniridia (106210).

Genetics

Isolated Peters anomaly usually occurs in an autosomal recessive pattern but autosomal dominant patterns have been reported as well.  The recessive disorder may be caused by a mutation in several genes, notably PAX6, PITX2CYP1B1, FOXC1, and FOXE3.  The latter gene is also mutated in anterior segment mesenchymal dysgenesis (107250) and congenital primary aphakia (610256).  The variety of clinical features are likely the result of a disruption in some common pathway or pathways.  Mutations in B3GALTL associated with the Peters-Plus syndrome have not been identified in isolated Peters anomaly.

This is a genetically and clinically heterogeneity condition as whole genome sequencing reveals numerous additional gene mutations in patients with both syndromic and isolated Peters anomaly.

PITX2 is also mutated in ring dermoid of the cornea (180550) and in Axenfeld-Rieger syndrome type 1 (180500).  PAX6 mutations also cause diseases of the cornea, fovea, optic nerve and iris.

Pedigree: 
Autosomal dominant
Autosomal recessive
Treatment
Treatment Options: 

Glaucoma is the most serious threat to vision on Peters anomaly but also the most difficult to treat.  Less than a third of patients achieve control of intraocular pressure even with the most vigorous combinations of therapy.  Corneal opacities can be treated with transplantation but the prognosis is often guarded when glaucoma is present.

From eye bank and other data, it has been estimated that 65% of penetrating keratoplasties in infants for visually significant congenital corneal opacities are performed in patients with Peters anomaly. 

References
Article Title: 

Whole exome sequence analysis of Peters anomaly

Weh E, Reis LM, Happ HC, Levin AV, Wheeler PG, David KL, Carney E, Angle B, Hauser N, Semina EV. Whole exome sequence analysis of Peters anomaly. Hum Genet. 2014 Sep 3. [Epub ahead of print].

PubMed ID: 
25182519

Axenfeld-Rieger Syndrome, Type 1

Clinical Characteristics
Ocular Features: 

Axenfeld-Rieger syndrome consists of a heterogeneous group of disorders with overlapping features.  Common to all types are the presence of ocular, dental, facial, skeletal abnormalities and autosomal dominant inheritance.  Anterior chamber dysgenesis of some form is universally present and severe glaucoma occurs in 50% of patients.  This may have its onset in childhood with typical symptoms of congenital glaucoma such as photophobia, excessive tearing and corneal clouding.  Hypoplasia of the iris is common and when progressive may result in an ectopic pupil and/or pseudopolycoria.  Iris insertion and Schwalbe's line are often anteriorly displaced with iridocorneal adhesions, a pattern that leads to the inclusion of this disorder among those with iridogoniodysgenesis or anterior chamber dysgenesis.  Pupillary ectropion of the posterior pigmented layer of the iris may be seen.

There is considerable clinical overlap among conditions with iris dysgenesis.  Some patients with typical systemic features of Axenfeld-Rieger syndrome may even have typical anterior chamber features of Axenfeld-Rieger anomaly in one eye and severe iris hypoplasia resembling aniridia in the other.

Systemic Features: 

Dental anomalies and mid-facial hypoplasia secondary to underdeveloped maxillary sinuses are among the most common systemic features in type 1.  The nasal root often appears abnormally broad and the lower lip appears to protrude. The teeth are frequently small and conical in shape with wide spaces between them (diastema).  Some teeth may be missing.  The umbilicus may fail to involute normally and retains excessive, redundant skin that sometimes leads to the erroneous diagnosis of an umbilical hernia for which unnecessary surgery may be performed.  Hypospadius is frequently present while cardiac defects, sensorineural deafness, and anal stenosis are less common.

Genetics

There is clinical and genetic heterogeneity in this syndrome and precise classification of many families remains elusive without knowing the genotype.  Mutations in at least four genes are responsible and all are are responsible for phenotypes transmitted in autosomal dominant patterns.  Type 1 discussed here is caused by a mutation in the homeobox transcription factor gene, PITX2, located at 4q25-q26.  A type of iris hypoplasia (IH)/iridogoniodysgenesis (IGDS) (IRID2; 137600) disorder has been classified separately but is caused by a mutation in PITX2 as well and many cases have the same systemic features.  Mutations in the same gene have also been found in ring dermoid of the cornea (180550) and in some cases of Peters anomaly (604229).

RIEG2 (601499) is rare but a deletion of 13q14 has been reported in several cases.  Mapping in a large family with 11 affected individuals yielded a locus in the same region.  Clinical signs overlap types 1 and 3 with dental, craniofacial, and ocular features, but with hearing impairment and rare umbilical anomalies.

Mutations in the FOXC1 gene (6p25) may be responsible for RIEG3 (602482).  However, a family has been reported with a severe 'Axenfeld-Rieger phenotype' in which a digenic etiology may have been responsible: patients had mutations in both FOXC1 and PITX2

Heterozygous mutations in the PRDM5 gene (4q25-q26) have been identified in 4 members of a Pakistani family with typical features of the Axenfeld-Rieger syndrome. It is labeled type 4 Axenfeld-Rieger syndrome in this database. 

Pedigree: 
Autosomal dominant
Treatment
Treatment Options: 

The presence of glaucoma requires prompt and vigorous treatment but control is difficult with blindness too often the result.  Oral surgery may be beneficial for dental problems.  Low vision aids can be useful.

References
Article Title: 

The Rieger syndrome

Jorgenson RJ, Levin LS, Cross HE, Yoder F, Kelly TE. The Rieger syndrome. Am J Med Genet. 1978;2(3):307-18.

PubMed ID: 
263445
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