Identification of four new PITX2 gene mutations in patients with Axenfeld-Rieger syndrome

Axenfeld Rieger syndrome (ARS) is an autosomal dominant inherited disorder affecting development of the ocular anterior chamber, abdomen, teeth and facial structures. The PITX2 gene is a major gene encoding a major transcription factor associated with ARS.

ARS patients were collected from six unrelated families. Patients and their families were ophthalmologically phenotyped and their blood was collected for DNA extraction. We screened the coding region of human PITX2 gene by direct sequencing. The consequences of the mutations described were investigated by generating crystallographic representations of the amino acid changes. In order to better understand the occurrence of glaucoma in ARS patients, we studied the PITX2 gene expression in human embryonic and fetal ocular tissue sections.

We identified four novel PITX2 genetic alterations in four unrelated families with ARS. These mutations included two nonsense mutations (E55X and Y121X), an eight nucleotides insertion (1251 ins CGACTCCT) and a substitution (F58L), in familial and sporadic cases of ARS. We also showed for the first time that PITX2 is expressed at early stages of the human embryonic and fetal periocular mesenchyme, as well as at later stages of human development in the fetal ciliary body, ciliary processes, irido corneal angle and corneal endothelium. The human fetal eye PITX2 gene expression pattern reported here for the first time provides a strong basis for explaining the frequent occurrence of glaucoma in patients affected by PITX2 gene mutations.

Two mutations identified affect the homeodomain (E55X and F58L). The E55X nonsense mutation is likely to alter dramatically the DNA-binding capabilities of the PITX2 homeodomain. Furthermore, there is a complete loss of the carboxy-terminal part of the PITX2 protein beyond the site of the mutation. The phenylalanine F58 is known to contribute to the hydrophobic network of the homeodomain. The crystallographic representations of the mutation F58L show that this mutation may change the conformation of the helical core. The F58L mutation is very likely to modify the homeodomain conformation and probably alters the DNA binding properties of PITX2. The other mutations (Y121X and the eight-nucleotide insertion (1251 ins CGA CTC CT) CGA CTC CT, at position 224 in PITX2A) result in partial loss of the C-terminal domain of PITX2. Pitx2 synergistically transactivates the prolactin promoter in the presence of the POU homeodomain protein Pit-1. Pitx2 activity is regulated by its own C-terminal tail. This region contains a highly conserved 14-amino-acid element involved in protein-protein interactions. The C-terminal 39-amino-acid tail represses DNA binding activity and is required for Pitx2 interactions with other transcription factors, for Pitx2-Pit-1 interaction and Pit-1synergism. Pit-1 interaction with the Pitx2 C terminus masks the inhibitory effect and promotes increased DNA binding activity. Thus, the partial or complete loss of the C terminus tail can lead to decreased or absent DNA binding activity and trigger severe ARS phenotypes. Our in situ hybridization results obtained on human embryonic and fetal ocular tissue sections constitute the first molecular histological data providing an explanation for the occurrence of precocious glaucoma in human patients affected by ARS caused by PITX2 mutations. Further structural and biochemical studies are needed for understanding the wide spectrum of clinical phenotypes caused by the increasing number of new PITX2 mutations found in ARS affected patients.