Novel Intragenic PAX6 Deletion in a Pedigree with Aniridia, Morbid Obesity, and Diabetes

Abstract

Purpose: Aniridia is a rare congenital eye disease, characterized by a constellation of symptoms including hypoplastic irides, foveal hypoplasia, early cataract, corneal stem cell deficiency, and glaucoma. Large chromosomal deletions spanning the PAX6 gene cause WAGR syndrome (Wilms tumor, aniridia, genitourinary anomalies, and intellectual disability [formerly called mental retardation]). We describe clinical and genetic studies of a three-generation pedigree with aniridia along with additional systemic conditions (morbid obesity, diabetes) suggesting the possibility of a contiguous-gene syndrome like WAGR.Methods: Clinical records were obtained and DNA was prepared from blood samples from three of the four patients and tested for mutations in the coding sequences of the PAX6 gene. The index patient also had cardiomyopathy and was tested for known cardiomyopathy genetic mutations using a next-generation DNA sequencing assay.Results: We discovered a novel intragenic PAX6 mutation, a 16 bp heterozygous deletion c.203delCCAGGGCAATCGGTGG, with Sanger sequencing that is the likely cause of autosomal dominant aniridia in this pedigree. This PAX6 deletion causes a frameshift in predicted protein translation and a subsequent premature termination, p.Pro68Leufs*6. The PAX6 deletion was detected in all three available family members with aniridia, the index patient, his mother, and his maternal aunt but was not observed in the Exome Aggregation Consortium (ExAC) database. Targeted sequencing of known cardiomyopathy genes in the index patient identified a second mutation, a 1.7 Mp deletion that spans the MYBPC3 gene.Conclusions: We report a pedigree with aniridia and other systemic abnormalities that were initially suspicious for a contiguous-gene syndrome like WAGR. However, genetic analysis of the pedigree revealed two independent genetic abnormalities on chromosome 11p: 1) a novel PAX6 mutation, and 2) a large chromosome deletion spanning MYBPC3, a known cardiomyopathy gene. It is unclear if morbid obesity and type II diabetes mellitus have a related genetic cause.

Keywords: Aniridia; PAX6; diabetes; glaucoma; obesity.

Figure 1.

A three-generation aniridia pedigree. Patients that were either diagnosed with aniridia by one of the authors or that were affected by a report from other family members are indicated by black symbols. Square symbols indicate males, round symbols indicate females, and diamond symbols indicate a family member with the gender that was not reported. The family members that were available for clinical examination are indicated with an asterisk. A slash marking indicates a family member is deceased.

Figure 2.

Clinical features of aniridia in the pedigree. Clinical photos of three members with aniridia, II-4 (A. OD, B. OS), II-6 (B. OD, C. OS), and III-1 (E. OD, F. OS). All have varying degrees of corneal stem cell deficiency and pannus, including one who has undergone a Boston KPro keratoprosthesis (II-4, Frame A). Patients II-4 and II-6 have no clinically visible iris (Frames A, B, C, D). Patient III-1 has symmetric supratemporal crescents of iris remnants (Frames E, F). Tube-shunts are visible in both eyes of patient III-1 (Frames E, F).

Figure 3.

Model of PAX6 protein structure. A. Wild-type PAX6 protein structure includes a paired box domain sequence (amino adds 4–130), a Gln-Gly-rich domain (amino acids 131–209), a homeobox domain (amino acids 210–269), and a Pro-Ser-Thr-rich domain (amino acids 279–422). B. The Pro68Leufs*6 mutation generates a truncated PAX6 protein with disrupted and missing domains. The Pro68Leufs*6 mutation causes a premature termination within the paired domain produces a truncated protein with an abnormal terminal five amino acids (LeuValAspArgGlu shown with a stick representation) that disrupts the paired domain and eliminates the homeobox domain. These structures were generated using a rotamer optimization algorithm and a potential energy function in the program Force Field X using the known crystal structure of PAX6.