Mutations in TCF8 Cause Posterior Polymorphous Corneal Dystrophy and Ectopic Expression of COL4A3 by Corneal Endothelial Cells

Abstract

Posterior polymorphous corneal dystrophy (PPCD, also known as PPMD) is a rare disease involving metaplasia and overgrowth of corneal endothelial cells. In patients with PPCD, these cells manifest in an epithelial morphology and gene expression pattern, produce an aberrant basement membrane, and, sometimes, spread over the iris and nearby structures in a way that increases the risk for glaucoma. We previously mapped PPCD to a region (PPCD3) on chromosome 10 containing the gene that encodes the two-handed zinc-finger homeodomain transcription factor TCF8. Here, we report a heterozygous frameshift mutation in TCF8 that segregates with PPCD in the family used to map PPCD3 and four different heterozygous nonsense and frameshift mutations in TCF8 in four other PPCD probands. Family reports of inguinal hernia, hydrocele, and possible bone anomalies in affected individuals suggest that individuals with TCF8 mutations should be examined for nonocular anomalies. We detect transcripts of all three identified PPCD genes (VSX1, COL8A2, and TCF8) in the cornea. We show presence of a complex (core plus secondary) binding site for TCF8 in the promoter of Alport syndrome gene COL4A3, which encodes collagen type IV alpha 3, and we present immunohistochemical evidence of ectopic expression of COL4A3 in corneal endothelium of the proband of the original PPCD3 family. Identification of TCF8 as the PPCD3 gene provides a valuable tool for the study of critical gene regulation events in PPCD pathology and suggests a possible role for TCF8 mutations in altered structure and function of cells lining body cavities other than the anterior chamber of the eye. Thus, this study has identified TCF8 as the gene responsible for approximately half of the cases of PPCD, has implicated TCF8 mutations in developmental abnormalities outside the eye, and has presented the TCF8 regulatory target, COL4A3, as a key, shared molecular component of two different diseases, PPCD and Alport syndrome.

Heterozygous deletions at the ZEB1 locus verify haploinsufficiency as the mechanism of disease for posterior polymorphous corneal dystrophy type 3

“…20,24 PPCD often presents in small nuclear families or as an isolated case such that linkage to a specific genomic region is not conclusive, or is confounded by incomplete penetrance in some cases, so a substantial proportion of patients currently lack a molecular diagnosis. [9][10][11][12][14][15][16][17] In this study we demonstrate that, in some of these cases, PPCD can be attributed to heterozygous deletions at the ZEB1 locus, providing insights into the molecular diagnosis of previously unexplained PPCD cases and verifying haploinsufficiency as the mechanism of disease for PPCD3.…”

“…[2][3][4] PPCD is genetically heterogeneous with three identified loci; PPCD1 (OMIM #122000) on chromosome 20p with a currently undefined causative gene, [5][6][7] PPCD2 (OMIM #609140) associated with mutations in COL8A2, 8 and PPCD3 (OMIM #609141) caused by mutations in ZEB1. 9 To date, over 30 heterozygous nonsense and frameshifting mutations in ZEB1 have been identified. [9][10][11][12][13][14][15][16][17] There are regional variations in the contribution of these PPCD subtypes, and the proportion attributed to ZEB1 mutations (PPCD3) has been reported to be 9% in New Zealand, 20% in the Czech Republic, 30% in the United Kingdom and Canada, and 34% in the United States.…”

“…[9][10][11][12][13][14][15][16][17] There are regional variations in the contribution of these PPCD subtypes, and the proportion attributed to ZEB1 mutations (PPCD3) has been reported to be 9% in New Zealand, 20% in the Czech Republic, 30% in the United Kingdom and Canada, and 34% in the United States. [9][10][11][12][13][14][15][16][17] In contrast to other PPCD subtypes, PPCD3 is often associated with corneal steepening. 16,18 Systemic features such as inguinal hernias have also been described.…”

“…16,18 Systemic features such as inguinal hernias have also been described. 9,10,16 There is a substantial variability in disease onset and presentation even among affected members of the same family. 19 PPCD3 can arise as a result of de novo mutations, and some individuals with a pathogenic mutation in ZEB1 do not exhibit clinical signs of the disease indicating either incomplete, or age-related, penetrance.…”

“…19 PPCD3 can arise as a result of de novo mutations, and some individuals with a pathogenic mutation in ZEB1 do not exhibit clinical signs of the disease indicating either incomplete, or age-related, penetrance. 9,14,16,19 ZEB1 encodes a zinc finger transcription factor (zinc finger E-box binding homeobox 1) that has a role in many cellular processes, including epithelial-mesenchymal transition (EMT) that involves the transformation of adhesive, non-mobile, epithelial cells into cells with a mesenchymal phenotype that have the ability to migrate. 20,21 ZEB1 induces EMT by suppressing the expression of epithelial-specific factors such as E-cadherin.…”