Axenfeld-Rieger Syndrome Panel

Axenfeld-Rieger syndrome (ARS) is a rare, highly penetrant disorder characterized by varying degrees of eye anterior segment anomalies with systemic malformations such as dental hypoplasia and a protuberant umbilicus (Hjalt and Semina. 2005. PubMed ID: 16274491; Berry et al. 2006. PubMed ID: 16449236; Waldron et al. 2010. PubMed ID: 20831741; Tümer and Bach-Holm. 2009. PubMed ID: 19513095; Chang et al. 2012. PubMed ID: 22199394). Dental abnormalities in this syndrome help in diagnosis and to distinguish ARS from other eye anterior segment abnormalities. Early diagnosis of ARS from its dento-facial and systemic features is essential in treating or preventing the most serious consequence of ARS (O'Dwyer and Jones. 2005. PubMed ID: 16238657). The major clinical concern is high risk of developing open-angle glaucoma, which represents the main challenge in terms of treatment. Approximately 50% of ARS affected patients develop glaucoma (Shields et al. 1985. PubMed ID: 389274; Alward. 2000. PubMed ID: 11004268; Hjalt and Semina. 2005. PubMed ID: 16274491; Chang et al. 2012. PubMed ID: 22199394). ARS affected patients also need surveillance and management of sensorineural hearing loss, and cardiac, endocrinological, craniofacial and orthopaedic defects (Chang et al. 2012. PubMed ID: 22199394). Rieger syndrome (RIEG) and Peters anomaly both are in the anterior chamber cleavage group of anomalies and are considered to be variations of a single developmental disorder (the ARS group) (Reese and Ellsworth. 1966. PubMed ID: 5948260). RIEG is characterized by malformations of the eyes, teeth, and umbilicus; whereas Peters anomaly displays only ocular features (Amendt et al. 2000. PubMed ID: 11092457; Doward et al. 1999. PubMed ID: 10051017).

Pathogenic variants in the FOXC1 or PITX2 genes cause autosomal dominant (AD) Axenfeld-Rieger syndrome (ARS). Linkage studies identified four chromosomal loci (4q25, 6p25, 13q14 and 16q24) that are associated with ARS and related or overlapping phenotypes. The genes that have been identified on chromosomes 4q25 and 6p25 are PITX2 (the pituitary homeobox 2 gene) and FOXC1 (the forkhead box C1 gene, also known as FKHL7) respectively (Alward. 2000. PubMed ID: 11004268; Lines et al. 2002. PubMed ID: 12015277; Tümer and Bach-Holm. 2009. PubMed ID: 19513095). PITX2 and FOXC1 are transcription factor genes which are expressed throughout eye ontogeny (Lines et al. 2002. PubMed ID: 12015277). The genes at 13q14 and 16q24 have not yet been discovered.

PITX2 encodes a Paired-like homeodomain transcription factor (PITX2), which plays a critical role in cell proliferation, differentiation, hematopoiesis and organogenesis (Huang et al. 2009). Also, PITX2 integrates retinoic acid and canonical Wnt signaling during eye anterior segment development (Gage et al. 2008. PubMed ID: 18367164; Gage and Zacharias. 2009. PubMed ID: 19623614 ). The product of the FOXC1 gene (FOXC1) is reported to maintain homeostasis in trabecular meshwork (TM) cells by regulating genes that play an important role in stress response (Ito et al. 2014. PubMed ID: 24556684; Paylakhi et al. 2013. PubMed ID: 23541832). TM helps in regulating intraocular pressure by acting as a drainage structure for aqueous humor (Tamm. 2009. PubMed ID: 19239914). Pathogenic mutations in FOXC1 significantly decrease the TM cell viability and subsequently contribute to the development of glaucoma.

A Genotype-Phenotype correlation study indicated that patients with PITX2 pathogenic variants have a more severe prognosis for glaucoma as compared to FOXC1 pathogenic variants. Within FOXC1 pathogenic variants, patients with FOXC1 duplications are at higher risk for the development of glaucoma, which emphasizes the importance of genetic testing. It’s been reported that PITX2 and FOXC1 interact with each other, which is essential for the regulation of common downstream target genes within specific cell lineages (Berry et al. 2006. PubMed ID: 16449236). Also, co-inheritance of PITX2 and FOXC1 pathogenic variants has been reported in a family which segregated with the disease and showed variable phenotypic expression. The most severely affected individual had pathogenic variants in both genes, whereas single heterozygous variants caused milder ARS phenotypes (Kelberman et al. 2011). So far, about 80 and 100 ARS causative variants have been identified in the PITX2 and FOXC1 genes, respectively (Human Gene Mutation Database).

Pathogenic variants in COL4A1 also cause AD ARS (Sibon et al. 2007. PubMed ID: 17696175). Pathogenic variants in SH3PXD2B cause autosomal recessive (AR) Frank-ter Haar syndrome. SH3PXD2B analysis in Axenfeld-Rieger syndrome patients identified several rare variants. However, the pathogenicity of these have not been well documented (Mao et al. 2012. PubMed ID: 22509100). Pathogenic variants in PAX6 and CYP1B1 have been shown to be associated with AD Peters anomaly (Hanson et al. 1994. PubMed ID: 8162071; Vincent et al. 2001. PubMed ID: 11403040). Pathogenic variants in ASPH cause AR Traboulsi syndrome or facial dysmorphism, lens dislocation, anterior-segment abnormalities, and spontaneous filtering blebs disorder (Patel et al. 2014. PubMed ID: 24768550). Pathogenic variants in FOXE3 cause AR anterior segment dysgenesis 2, multiple subtypes including Peters anomaly (Doucette et al. 2011. PubMed ID: 21150893). Pathogenic variants in B3GLCT cause AR Peters-plus syndrome (Lesnik Oberstein et al. 2006. PubMed ID: 16909395).

A wide variety of causative variants (missense, nonsense, splicing, small deletions and insertions) have been reported in Axenfeld-Rieger syndrome and overlapping phenotypes -associated genes including large deletions/duplications and complex genomic rearrangements in a few genes (FOXC1, PAX6, CYP1B1, B3GLCT and PITX2) (Human Gene Mutation Database).

See individual gene test descriptions for more information on molecular biology of gene products.

Approximately 25%-60% of Axenfeld-Rieger syndrome cases are due to FOXC1 or PITX2 pathogenic variants (Tümer and Bach-Holm. 2009. PubMed ID: 19513095; Reis et al. 2012. PubMed ID: 22569110; Alward. 2000. PubMed ID: 11004268). In one study, two out of 30 Axenfeld-Rieger syndrome patients (6.7%) carried SH3PXD2B sequence variants (Mao et al. 2012. PubMed ID: 2250910). Due to the phenotypic and genotypic heterogeneity, clinical sensitivity of the other genes in this panel is difficult to predict.

Large deletions/duplications or complex genomic rearrangements have been reported in FOXC1, PAX6, CYP1B1, B3GLCT and PITX2 (Human Gene Mutation Database).

This test is performed using Next-Gen sequencing with additional Sanger sequencing as necessary.

This panel typically provides 99.0% coverage of all coding exons of the genes plus 10 bases of flanking noncoding DNA in all available transcripts along with other non-coding regions in which pathogenic variants have been identified at PreventionGenetics or reported elsewhere. We define coverage as ≥20X NGS reads or Sanger sequencing. A small region in exon 1 of FOXE3 is not covered for technical reasons.

Dependent on the sequencing backbone selected for this testing, discounted reflex testing to any other similar backbone-based test is available (i.e., PGxome panel to whole PGxome; PGnome panel to whole PGnome).