Variation in residual PITX2 activity underlies the phenotypic spectrum of anterior segment developmental disorders

The autosomal dominant disorders iris hypolasia (IH), iridogoniodysgenesis syndrome (IGDS) and Axenfeld-Rieger syndrome (ARS) are characterized by maldevelopment of the anterior segment of the eye associated with an increased risk of early-onset glaucoma. IH, IGDS and ARS are allelic disorders, as all three can result from mutations of the transcription factor PITX2. IH is the mildest of the three, whereas ARS exhibits the most severe ocular malformations. We hypothesize that varying amounts of residual PITX2 activity could underlie the severity of these phenotypes. Missense mutations of the PITX2 homeodomain identified in IH (Arg46Trp), IGDS (Arg31His) and ARS patients (Leu16Gln; Thr30Pro; Arg53Pro) were introduced into recombinant PITX2 cDNA by site-directed mutagenesis. PITX2 mutant proteins expressed in COS-7 cells were determined to be stable and localized to the nucleus; however, the Arg53Pro ARS mutant also displayed cytoplasmic staining. Our findings are consistent with the possibility of a novel nuclear localization signal (NLS) within helix 3 of the PITX2 homeodomain, homologous to the NLS of the related transcription factor PDX-1. Analysis of the five mutant PITX2 proteins by DNA-binding shifts and transactivation studies demonstrated reduced activity of the IH and IGDS mutant PITX2 proteins, with the IH mutant retaining the most activity in both studies, whereas the ARS mutant PITX2 proteins proved to be non-functional. In addition to providing insight into the etiological mechanism of IH, IGDS and ARS, these results are consistent with the hypothesis that mutant PITX2 proteins that retain partial function result in milder anterior segment aberrations.