ARTICLE Missense Mutations in TCF8 Cause Late-Onset Fuchs Corneal Dystrophy and Interact with FCD4 on Chromosome 9p S. Amer Riazuddin, 1 Norann A. Zaghloul, 1 Amr Al-Saif, 1 Lisa Davey, 1 Bill H. Diplas, 1 Danielle N. Meadows, 2 Allen O. Eghrari, 2 Mollie A. Minear, 3 Yi-Ju Li, 3 Gordon K. Klintworth, 4 Natalie Afshari, 4 Simon G. Gregory, 3 John D. Gottsch, 2 and Nicholas Katsanis 1,5, * Fuchs corneal dystrophy (FCD) is a degenerative genetic disorder of the corneal endothelium that represents one of the most common causes of corneal transplantation in the United States. Despite its high prevalence (4% over the age of 40), the underlying genetic basis of FCD is largely unknown. Here we report missense mutations in TCF8, a transcription factor whose haploinsufficiency causes posterior polymorphous corneal dystrophy (PPCD), in a cohort of late-onset FCD patients. In contrast to PPCD-causing mutations, all of which are null, FCD-associated mutations encode rare missense changes suggested to cause loss of function by an in vivo complementation assay. Importantly, segregation of a recurring p.Q840P mutation in a large, multigenerational FCD pedigree showed this allele to be sufficient but not necessary for pathogenesis. Execution of a genome-wide scan conditioned for the presence of the 840P allele identified an addi- tional late-onset FCD locus on chromosome 9p, whereas haplotype analysis indicated that the presence of the TCF8 allele and the disease haplotype on 9p leads to a severe FCD manifestation with poor prognosis. Our data suggest that PPCD and FCD are allelic vari- ants of the same disease continuum and that genetic interaction between genes that cause corneal dystrophies can modulate the expres- sivity of the phenotype. Introduction FCD represents the most common form of genetic disor- ders of the corneal endothelium. 1,2 The disorder affects as much as 4% of the population over the age of 40 and accounts for a significant fraction of the corneal transplan- tation performed in the United States every year. 2–4 Clini- cally, FCD is marked by the development of guttae, excres- cences of Descemet membrane that appear in the fourth or fifth decade and increase in number over time. 5,6 As the disease progresses, visual acuity decreases secondary to corneal edema and endothelial cell loss, with end-stage disease evidenced by the formation of painful epithelial bullae. 7 FCD is genetically heterogeneous, exhibiting an auto- somal-dominant mode of inheritance with variable pene- trance and expressivity. The rare form of early-onset FCD is causally associated with mutations in COL8A2 (MIM 120252), whereas the more common late-onset FCD has been localized to three loci: FCD1, FCD2, and FCD3 on chromosomes 13, 18, and 5, respectively. 8–10 Although clinically distinct, corneal endothelial dystrophies share clinical features suggesting that genes implicated in one corneal dystrophy may also harbor mutations liable for other dystrophies. This premise was strengthened when pathogenic mutations in SLC4A11 (MIM 610206), a borate transporter in which loss-of-function mutations cause autosomal-recessive congenital hereditary endothelial dystrophy (CHED2 [MIM 217700]), were identified in sporadic late-onset FCD patients. 11 Therefore, we hypoth- esized that TCF8 (MIM 189909), a transcription factor shown to be causally associated with another corneal dystrophy, PPCD (MIM 609141), 12,13 may contribute to the genetic load of late-onset FCD. Here, we report five missense mutations in TCF8 associ- ated with late-onset FCD, which appear to be causal because of a loss of protein function based on an in vivo complementation assay. One of these mutations, encoding a p.Q840P change, was present in both a sporadic FCD patient and in a large family and was transmitted in a manner consistent with an autosomal-dominant trait. However, the 840P allele could not explain FCD in all patients, suggesting the presence of a second pathogenic allele in this pedigree. A genome-wide scan conditioned to the presence/absence of the TCF8 mutant allele identi- fied a new locus for late-onset FCD, FCD4 on chromosome 9p subsequent genetic and clinical analyses suggested that although mutations in each of TCF8 and FCD4 are suffi- cient for pathogenesis, genetic interaction between these two loci can lead to a more severe form of the disease. Our data represent the first familial evidence for mutations that cause late-onset FCD, supporting the hypothesis that several corneal dystrophies, although clinically distinct, share the same molecular etiology. Additionally, our data suggest that the quality and quantity of mutations in FCD-associated loci can have a profound impact on the 1 McKusick-Nathans Institute of Genetic Medicine, Johns Hopkins University School of Medicine, Baltimore, MD 21205, USA; 2 Center for Corneal Genetics, Cornea and External Disease Service, Wilmer Eye Institute, Johns Hopkins Hospital, Baltimore, MD 21205, USA; 3 Center for Human Genetics, 4 Department of Ophthalmology, Duke University Medical Center, Durham, NC 27710, USA; 5 Center for Human Disease Modeling, Department of Cell Biology, Duke University, Durham, NC 27710, USA *Correspondence: katsanis@cellbio.duke.edu DOI 10.1016/j.ajhg.2009.12.001. ª2010 by The American Society of Human Genetics. All rights reserved. The American Journal of Human Genetics 86, 45–53, January 8, 2010 45