Genetic characterization of posterior polymorphous corneal dystrophy.

Abstract

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.