Genetic characterization of posterior polymorphous corneal dystrophy.

The purpose of this dissertation is to examine the genetic origins of posterior polymorphous corneal dystrophy (PPCD). The proband of a large PPCD family, UM:139, was examined clinically and histopathologically to confirm the diagnosis of PPCD. In addition to the proband, 12 of the 28 individuals sampled in this family were found to have PPCD. We used linkage analysis with an autosomal dominant model to exclude (lod < -2.0) the PPCD1 locus on 20q11, the congenital hereditary endothelial dystrophy (CHED1) locus on 20q11, the loci containing the type IV collagen genes associated with Alport Syndrome (which includes PPCD as a sequela), and the locus containing the collagen type VIII alpha-2 gene. We also used linkage analysis with an autosomal recessive model to exclude the CHED2 locus on 20p13. Linkage analysis of whole-genome scan data identified a previously unknown PPCD locus (PPCD3) on chromosome 10p11 in the 8.55 cM region between D10S213 and D10S578 with a maximum lod score at D10S1780 of 4.35. Informatic examination of the more than 25 genes known to lie within this region revealed a homeodomain transcription factor, TCF8. Sequencing of TCF8 in UM:139 revealed a cosegregating frame-shift mutation. Sequencing of DNA from 10 additional PPCD probands revealed two additional frame-shift and two nonsense mutations. Studies of total RNA isolated from human and mouse tissue confirm the expression of TCF8 in cornea and other bodily tissues. These studies, along with the mutation screening, support the conclusion that variants in TCF8 cause PPCD. We propose two models for the role of TCF8 variants in the PPCD phenotype including ectopic expression of some epithelial characteristics not typical of corneal endothelial cells. The first model supposes a reduced ability to repress the epithelial phenotype and the second model supposes a reduced ability to regulate the expression of corneal collagens. The identification of TCF8 as the PPCD3 gene has implications for the further investigation of PPCD, for the study of similar endothelial cells elsewhere in the body, and for the understanding of the regulation of corneal endothelial cellular morphology and gene expression.Ph.D.Biological SciencesGeneticsHealth and Environmental SciencesPublic healthUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/124850/2/3163853.pd

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

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 α3, 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