Aniridia refers specifically to a panocular disorder, noticeable iris hypoplasia was one of its characteristics, which was usually accompanied by foveal hypoplasia. The visual acuity of patients were impaired seversly and they often displayed nystagmus. Some cases could also present other manifestations including cataract, keratopathy, glaucoma and optic nerve malformations[15, 16]. The severity of iris hypoplasia was different, which could range from complete absence of the iris, through enlargement and irregularity of the pupil mimicking a coloboma, to small slit-like defects in the anterior layer seen only on transillumination with a slit-lamp. In this study, we screened the pathogenic gene mutation for this Chinese aniridia family using an iris diseases panel including 37 targeted genes. Then a novel PAX6 nonsense mutation c.619A > T (p.K207*) was identified in this family and it was separated from disease phenotype. This mutation induced premature termination codons (PTCs) into the PAX6 open reading frame, and the mRNAs containing PTCs were degraded by the nonsense-mediated decay process, which resulted in a single-dose deficiency. The phenotype of this novel PAX6 nonsense mutation was line with classical Aniridia related to PAX6 haploinsufficiency.
A high proportion of cases of aniridia is associated with mutations in PAX6 frameshift mutations, splicing site mutations or nonsense mutations and thess kinds of variations have been considered to produce premature truncation of the protein or nonsense transcripts, leading to haploinsufficiency while few of them were missense mutations. Aniridia phenotype associated with PAX6 haploinsufficiency almost present various anterior segment and fundus abnormalities including iris hypoplasia, cataract, glaucoma, corneal opacity, microcornea, lens subluxation, foveal hypoplasia, optic nerve malformations and so on, while that variant aniridia phenotype involving dysplasia of skeleton and central nervous system is mostly caused by missense mutations[16, 17]. The most common clinical manifestations associated with aniridia haploinsufficiency were iris anomalies, nystagmus and foveal hypoplasia, followed by cataracts, glaucoma and corneal opacity. In this Chinese family, the inheritance pattern coincided with autosomal dominant inheritance, and the phenotype of proband is similar to his mother's. The proband and his mother felt photophobia from childhood and ocular examination demonstrated nystagmus, aniridia, cataract and foveal hypoplasia. It was reported that
the severity of iris hypoplasia varied in different PAX6 cases and lens abnormalities include various degrees and types of cataracts and lens ectopic. The patients of this Chinese family associated with PAX6 nonsense mutation c.619A > T presented complete absence of the iris and spotted opacification of lens cortex, and other eye abnormalities including glaucoma, corneal opacity could not be observed. However, it was interesting that the phenotypes between two nonsense mutation located in close position and same domain were different. It was reported that a 24-year-old patient with PAX6 c.607C > T, p.Arg203* presented glaucoma and keratopathy besides nystagmus, aniridia and foveal hypoplasia, who has worse visual function. While the patients in this Chinese family with PAX6 c.619A > T (p.K207*) have milder phenotype in comparison.
The human PAX6 gene was cloned in 1991 and has been isolated from both vertebrates and invertebrates. It consists of 14 exons and encodes a transcriptional regulator which has a paired-type DNA-binding domain. There are 2 distinct DNA-binding subdomains, the N-terminal subdomain (NTS) and the C-terminal subdomain (CTS), in the paired domain, which bind respective consensus DNA sequences to recognize target genes. The human PAX6 gene produces 2 alternatively spliced isoforms that have the distinct structure of the paired domain.[20, 21].There have been more than 100 mutations reported in the human PAX6 database since firstly identified as the genetic cause of aniridia in the small eye mouse. While it is remarkable that the alterations in a conserved non-coding element within the ‘critical region’ and other cis-regulatory elements also can cause aniridia. PAX6 whole-gene deletions and telomeric cis-regulatory elements deletions were also identified in some Aniridia patients of negative for intragenic PAX6 mutations. Consequently, it was suggested that PAX6 whole-gene direct sequencing combined with and molecular methods of detecting copy number alterations (CNV) such as high-resolution comparative hybridization (HR-CGH) arrays, fluorescence in situ hybridization (FISH), and multiplex ligation-dependent probe amplification (MLPA) is important to improve detection rate for Aniridia associated with PAX6 variations, which could be more suitable for using in the Aniridia cases of iris diseases panel screening negative. It was suggested greater locus heterogeneity might exist in both isolated and syndromic aniridia than was previously appreciated, therefore the improvement of detection method is helpful to improve the detection rate of pathogenic genes for aniridia. In this study, we just have screened protein-coding region of 37 genes associated iris diseases but ignored conserved non-coding element and CNV of PAX6, which should be improved in the future.