Enamel defects of Axenfeld-Rieger syndrome and the role of PITX2 in its pathogenesis

OBJECTIVES

To investigate the detailed ultrastructural patterns of dental abnormalities affected by Axenfeld-Rieger syndrome (ARS) with a heterozygous microdeletion involving paired-like homeodomain 2 (PITX2) and explored the underlying molecular mechanisms driving enamel defects.

SUBJECTS AND METHODS

Sanger sequencing, genomic quantitative PCR analysis, and chromosomal microarray analysis (CMA) were used to screen the disease-causing mutation in one ARS proband. An exfoliated tooth from an ARS patient was analyzed with scanning electron microscopy and micro-computerized tomography. A stable Pitx2 knockdown cell line was generated to simulate PITX2 haploinsufficiency. Cell proliferation and ameloblast differentiation were analyzed, and the role of the Wnt/β-catenin pathway in proliferation of ameloblast precursor cells was investigated.

RESULTS

An approximately 0.216 Mb novel deletion encompassing PITX2 was identified. The affected tooth displayed a thinner and broken layer of enamel and abnormal enamel biomineralization. PITX2 downregulation inhibited the proliferation and differentiation of inner enamel epithelial cells, and LiCl stifmulation partially reversed the proliferation ability after Pitx2 knockdown.

CONCLUSIONS

Enamel formation is disturbed in some patients with ARS. Pitx2 knockdown can influence the proliferation and ameloblast differentiation of inner enamel epithelial cells, and PITX2 may regulate cell proliferation via Wnt/β-catenin signaling pathway.

Case report of the rare Peters' anomaly complicated with Axenfeld-Rieger syndrome: A case report and brief review of the literature

INTRODUCTION

Peters' anomaly (PA) and Axenfeld-Rieger syndrome (ARS) are typical classifications of anterior segment dysgenesis (ASD) and ascribed to congenital eye diseases that encompass developmental defects in anterior segment structures. The aim of this study is to discuss the unusual association between PA and ARS and to determine the results of penetrating keratoplasty combined with extracapsular cataract extraction and anterior vitrectomy for this unusual ophthalmic phenotype.

PATIENT CONCERNS

A 72-year-old female was referred to Changzhou No. 2 People's Hospital for a progressive decrease in visual acuity in both eyes in the past few decades.

DIAGNOSES

The patient was diagnosed with PA with cone-shaped polar cataracts in the left eye based on a series of ophthalmic examinations. ARS with retinal detachment was diagnosed in the right eye 2 years prior.

INTERVENTIONS

Penetrating keratoplasty combined with extracapsular cataract extraction and anterior vitrectomy were performed to manage PA with cataracts in the left eye.

OUTCOMES

Her best corrected visual acuity did not improve significantly after the operation. Patients with ARS and PA should be treated cautiously because of fundus lesions.

CONCLUSION

This study revealed that cases with PA accompanied by iridocorneal adhesions, or other ocular anomalies, need to be treated cautiously for a very low success rate. It is of reference value for the evaluation of treatment prognosis for this joint occurrence of ophthalmic phenotypes.

Identification of a novel frameshift mutation in PITX2 gene in a Chinese family with Axenfeld-Rieger syndrome

OBJECTIVE

Axenfeld-Rieger syndrome (ARS) is phenotypically and genetically heterogeneous. In this study, we identified the underlying genetic defect in a Chinese family with ARS.

METHODS

A detailed family history and clinical data were recorded. The ocular phenotype was documented using slit-lamp photography and systemic anomalies were also documented where available. The genomic DNA was extracted from peripheral blood leukocytes. All coding exons and intron-exon junctions of paired-like homeodomain transcription factor 2 (PITX2) gene and the forkhead box C1 (FOXC1) gene were amplified by polymerase chain reaction (PCR) and screened for mutation by direct DNA sequencing. Variations detected in exon 5 of PITX2 were further evaluated with cloning sequencing. The exon 5 of PITX2 was also sequenced in 100 healthy controls, unrelated to the family, for comparison. Structural models of the wild type and mutant homeodomain of PITX2 were investigated by SWISS-MODEL.

RESULTS

Affected individuals exhibited variable ocular phenotypes, whereas the systemic anomalies were similar. After direct sequencing and cloning sequencing, a heterozygous deletion/insertion mutation c.198_201delinsTTTCT (p.M66Ifs*133) was revealed in exon 5 of PITX2. This mutation co-segregated with all affected individuals in the family and was not found either in unaffected family members or in 100 unrelated controls.

CONCLUSIONS

We detected a novel frameshift mutation p.M66Ifs*133 in PITX2 in a Chinese family with ARS. Although PITX2 mutations and polymorphisms have been reported from various ethnic groups, we report for the first time the identification of a novel deletion/insertion mutation that causes frameshift mutation in the homeodomain of PITX2 protein.

PITX2 and FOXC1 spectrum of mutations in ocular syndromes

Anterior segment dysgenesis (ASD) encompasses a broad spectrum of developmental conditions affecting anterior ocular structures and associated with an increased risk for glaucoma. Various systemic anomalies are often observed in ASD conditions such as Axenfeld-Rieger syndrome (ARS) and De Hauwere syndrome. We report DNA sequencing and copy number analysis of PITX2 and FOXC1 in 76 patients with syndromic or isolated ASD and related conditions. PITX2 mutations and deletions were found in 24 patients with dental and/or umbilical anomalies seen in all. Seven PITX2-mutant alleles were novel including c.708_730del, the most C-terminal mutation reported to date. A second case of deletion of the distant upstream but not coding region of PITX2 was identified, highlighting the importance of this recently discovered mechanism for ARS. FOXC1 deletions were observed in four cases, three of which demonstrated hearing and/or heart defects, including a patient with De Hauwere syndrome; no nucleotide mutations in FOXC1 were identified. Review of the literature identified several other patients with 6p25 deletions and features of De Hauwere syndrome. The 1.3-Mb deletion of 6p25 presented here defines the critical region for this phenotype and includes the FOXC1, FOXF2, and FOXQ1 genes. In summary, PITX2 or FOXC1 disruptions explained 63% of ARS and 6% of other ASD in our cohort; all affected patients demonstrated additional systemic defects with PITX2 mutations showing a strong association with dental and/or umbilical anomalies and FOXC1 with heart and hearing defects. FOXC1 deletion was also found to be associated with De Hauwere syndrome.

An unusual class of PITX2 mutations in Axenfeld-Rieger syndrome

BACKGROUND

Mutations in the PITX2 homeobox gene are known to contribute to Axenfeld-Rieger syndrome (ARS), an autosomal-dominant developmental disorder. Although most mutations are in the homeodomain and result in a loss of function, there is a growing subset in the C-terminal domain that has not yet been characterized. These mutations are of particular interest because the C-terminus has both inhibitory and stimulatory activities.

METHODS

In this study we used a combination of in vitro DNA binding and transfection reporter assays to investigate the fundamental issue of whether C-terminal mutations result in gain or loss of function at a cellular level.

RESULTS

We report a new frameshift mutation in the PITX2 allele that predicts a truncated protein lacking most of the C-terminal domain (D122FS). This newly reported mutant and another ARS C-terminal mutant (W133Stop) both have greater binding than wild-type to the bicoid element. Of interest, the mutants yielded approximately 5-fold greater activation of the prolactin promoter in CHO cells, even though the truncated proteins were expressed at lower levels than the wild-type protein. The truncated proteins also had greater than wild-type activity in 2 other cell lines, including the LS8 oral epithelial line that expresses the endogenous Pitx2 gene.

CONCLUSIONS

The results indicate that the PITX2 C-terminal domain has inhibitory activity and support the notion that ARS may also be caused by gain-of-function mutations.