The genetic architecture of aniridia and Gillespie syndrome

Abstract

Absence of part or all of the iris, aniridia, is a feature of several genetically distinct conditions. This review focuses on iris development and then the clinical features and molecular genetics of these iris malformations. Classical aniridia, a panocular eye malformation including foveal hypoplasia, is the archetypal phenotype associated with heterozygous PAX6 loss-of-function mutations. Since this was identified in 1991, many genetic mechanisms of PAX6 inactivation have been elucidated, the commonest alleles being intragenic mutations causing premature stop codons, followed by those causing C-terminal extensions. Rarely, aniridia cases are associated with FOXC1, PITX2 and/or their regulatory regions. Aniridia can also occur as a component of many severe global eye malformations. Gillespie syndrome-a triad of partial aniridia, non-progressive cerebellar ataxia and intellectual disability-is phenotypically and genotypically distinct from classical aniridia. The causative gene has recently been identified as ITPR1. The same characteristic Gillespie syndrome-like iris, with aplasia of the pupillary sphincter and a scalloped margin, is seen in ACTA2-related multisystemic smooth muscle dysfunction syndrome. WAGR syndrome (Wilms tumour, aniridia, genitourinary anomalies and mental retardation/intellectual disability), is caused by contiguous deletion of PAX6 and WT1 on chromosome 11p. Deletions encompassing BDNF have been causally implicated in the obesity and intellectual disability associated with the condition. Lastly, we outline a genetic investigation strategy for aniridia in light of recent developments, suggesting an approach based principally on chromosomal array and gene panel testing. This strategy aims to test all known aniridia loci-including the rarer, life-limiting causes-whilst remaining simple and practical.

Fig. 1

The structure and development of the human iris. a Cartoon of the iris musculature, showing the position of the sphincter (shaded yellow) and dilator muscles. b Macroscopic photo of normal adult iris (courtesy of Chris Moody), showing the inner pupillary portion and outer ciliary portion. c Cartoon of the developing eye in a 7–8-week-old human embryo, showing the migration of periocular mesenchymal cells (shaded in teal) into the developing anterior chamber, which will go on to form the mesenchymal iris, pupillary membrane and corneal endothelium and stroma. The tips of the optic cup are shaded in purple showing the specification of iris epithelial progenitor cells. d Cartoon of developing human eye at 4–5 months’ gestation, showing the iris growing out from the optic cup margins. The iris musculature (shaded orange) is just starting to develop. e Cartoon of adult eye, showing a well-formed anterior chamber and iridocorneal angle; the iris stroma, musculature (orange) and epithelium (black); lens attached by zonules to the ciliary muscle. The cartoons in a, b are derived from multiple sources (primarily Mann 1925)

Fig. 2

Iris phenotypes in classical aniridia, Gillespie syndrome and multisystemic smooth muscle dysfunction syndrome. a Right eye of an individual with classical aniridia, showing near complete absence of the iris and mild ptosis (image courtesy of David Hall); b iris of a Gillespie syndrome patient with an ITPR1 mutation. Note the scalloped edge of the iris remnant (arrow), with aplasia of the iris central to the collarette (image courtesy of Abhijit Dixit); c iris of a patient with an ACTA2 mutation and multisystemic smooth muscle dysfunction syndrome, showing the aplasia of the iris central to the collarette, with a scalloped pupillary margin and iridolenticular strand (arrow) (image courtesy of Françoise Meire)

Fig. 3

a Protein bar showing the canonical 422-amino acid human PAX6 protein, annotated with the ten commonest aniridia alleles. The domains and main secondary structural elements are shown (UnitProt P26367). The coding exons are 4–13, with exon boundaries indicted by a white vertical line. The location of exon 5a, part of the alternatively spliced isoform PAX6(5a), is indicated. b PAX6 paired domain bound to dsDNA. Note the two subdomains, each containing three alpha helices [PDB ID: 6PAX (Xu et al. 1999), from http://www.rcsb.org. Image of “6PAX” created using Protein Workshop (Moreland et al. 2005)]

Fig. 4

Molecular pathology of WAGR syndrome. Cartoon of the 11p genomic region which encompasses PAX6, WT1 and BDNF. The numbers and lines at the top represent the hg19 human genomic coordinates, below which are the gene models. If the gene is an OMIM morbid gene the cognate OMIM number is given below the gene symbol. The boxed genes are those that have been confidently associated with components of the WAGR syndrome. The bar at the bottom is an empirically-derived critical region for deletions associated with WAGR. However, it is likely there are many possible causes of the intellectual disability associated with WAGR, so it is reasonable to consider that any deletion which encompasses PAX6 and WT1 may be WAGR-associated

Fig. 5

Spectrum of ITPR1 pathogenic variants. A linear protein schematic of ITPR1 is shown in the bottom panel, with domains and features demarcated and labelled in colour. The position of the 15-amino acid insertion in the longer isoform, UniProt Q14643-1, is shown in yellow. Amino acid numbering and domain positions are based on the 2743-amino acid isoform 2: Q14643-2, encoded by the canonical transcript GenBank NM_001168272.1; ENST00000302640. Shown above the protein schematic are all of the published variants associated with Gillespie syndrome and other neurological conditions, and all of the published substitution variants associated with spinocerebellar ataxia (SCA). All variants shown have had their amino acid numbering unified to isoform 2 (to facilitate an accurate collation and comparison of the ITPR1 dataset) and may therefore differ from the numbering used in the original publication. Variants in brackets indicate recessive alleles, of which *compound heterozygous alleles identified in a single proband; **cases reported as having a SCA29 phenotype and with substitutions at residues associated with Gillespie syndrome (identical variant) or pontocerebellar hypoplasia (different variant); ***variant associated with adult onset SCA in a mother and early onset SCA in her daughter

Fig. 6

a Flowchart showing an example approach to the genetic investigation of new congenital aniridia/congenital mydriasis cases. This will usually consist of an aniridia NGS gene panel—in the UK this includes PAX6, FOXC1, PITX2 and ITPR1—and copy number analysis (e.g., chromosomal array or array-based comparative genomic hybridisation). The latter is important to look for contiguous deletion of PAX6 and WT1. We have suggested the combination of iris and cardiac features which should prompt consideration of ACTA2 sequencing—this could be tested with other genes such as ITPR1 depending on clinical judgement. *Gillespie syndrome-like iris (see Fig. 2b, c); **a suggested surveillance regimen for the serious complications of ACTA2 multisystemic smooth muscle dysfunction has recently been outlined (Regalado et al. 2018). b Brief outline of the management of aniridia and WAGR syndrome. c Pie chart showing the genetic causes of isolated and syndromic aniridia. Note that these frequencies are calculated as a percentage of all aniridia cases (using the term broadly and including Gillespie syndrome). For example, whilst ITPR1 accounts for 2–3% aniridia cases, it is the only known cause of Gillespie syndrome. They are estimated and approximate frequencies derived from published sources (Grønskov et al. ; Crolla and van Heyningen ; Robinson et al. ; Bobilev et al. ; Ansari et al. 2016) or, where no published data is available, estimated from our cohort of > 400 aniridia patients