Comparisons of demographic features between cases and controls were performed by a 2-sample
t-test for age and by Fisher's exact test for sex. For the trinucleotide repeat CTG18.1 polymorphism, we dichotomized alleles, such that CTG
≥40 was considered an expanded allele, denoted as X, and CTG
<40 was considered a normal allele, denoted as S. Hardy-Weinberg equilibrium (HWE) was examined in cases and controls, separately, by an exact test.
28 The degree of linkage disequilibrium (LD) between rs613872 and the CTG18.1 polymorphism was estimated by
r 2 and
D'.
29,30 Logistic regression models were fit to test association between genotype and FECD affection status by the likelihood ratio test.
31 Risk factors age and sex were included as covariates.
32–34 The genotypic value was coded in an additive manner; that is, 0, 1, and 2 denoted TT, TG, and GG genotypes, respectively, for SNP rs613872, and SS, SX, XX denote genotypes, respectively, for the CTG18.1 polymorphism. Association between FECD and haplotypes between the two loci also was evaluated using haplo.stats.
35
Segregation of these two
TCF4 polymorphisms in 29 families with FECD was examined. Considering the late-onset nature of FECD, we took a conservative approach, such that individuals with FECD were classified as “affected,” those unaffected after age 40 were classified as “unaffected,” and all others were assigned an unknown affection status. In particular, model-based linkage analysis using MERLIN
36 was performed in a multiplex family with four affected individuals (family F10,
Fig. 3), which were genotyped by the Illumina HumanLinkage-24 array (Illumina, Inc., San Diego, CA). A dominant genetic model with penetrance of 0.95, sporadic rate of 0.001, and disease-predisposing allele frequency of 0.001 was assumed.