A spectrum of FOXC1 mutations suggests gene dosage as a mechanism for developmental defects of the anterior chamber of the eye

Abstract

Mutations in the forkhead transcription-factor gene (FOXC1), have been shown to cause defects of the anterior chamber of the eye that are associated with developmental forms of glaucoma. Discovery of these mutations was greatly facilitated by the cloning and characterization of the 6p25 breakpoint in a patient with both congenital glaucoma and a balanced-translocation event involving chromosomes 6 and 13. Here we describe the identification of novel mutations in the FOXC1 gene in patients with anterior-chamber defects of the eye. We have detected nine new mutations (eight of which are novel) in the FOXC1 gene in patients with anterior-chamber eye defects. Of these mutations, five frameshift mutations predict loss of the forkhead domain, as a result of premature termination of translation. Of particular interest is the fact that two families have a duplication of 6p25, involving the FOXC1 gene. These data suggest that both FOXC1 haploinsufficiency and increased gene dosage can cause anterior-chamber defects of the eye.

Figure 1

Summary of the mutations detected in this study and of those that have been published elsewhere (Mears et al. ; Nishimura et al. ; Swiderski et al. ; Mirzayans et al. 2000). The hatched boxes indicate the location of the forkhead domain within the FOXC1 coding sequence. The two white boxes represent the locations of two polymorphic poly-glycine tracts (Mears et al. 1998) that are labeled “Fkh8” and “Fkh10” in figure 4. The distribution of missense, nonsense (indicated by *), and frameshift mutations are illustrated. The predicted protein translations are shown below the gene diagram for the frameshift mutations. The blackened boxes represent those areas of the protein that, because of the frameshift mutations, are translated differently than the normal FOXC1 protein.

Figure 2

Physical map of the 6p25 region that surrounds the FOXC1 gene. The approximate location of the proposed IRID1B locus (Mears et al. 1998) is shown. The approximate location of PACs sequenced by The Sanger Centre is shown at the bottom of the figure. Genetic markers derived from these sequences are listed in table 2.

Figure 3

Genotyping results showing evidence for partial 6p25 duplication in families RG-5 and FG-253. A, Results for the marker 668J24-GATA. Sample identification is given at the top of the gel image. Families FG-253 (sample numbers 1–6) and RG-5 (sample numbers 7–9) have a partial duplication of 6p25. The “2/6” label denotes a sample with an unbalanced translocation involving chromosomes 2 and 6 (partial deletion of 6p25). The “6/13” label denotes a sample with a balanced translocation of chromosomes 6 and 13. The 01 and 02 samples are CEPH control samples (1331-01 and 1331-02). Samples 1 and 6 have three alleles, whereas samples 3, 5, and 7 demonstrate allele-intensity differences. B, Example of three alleles (sample 7) for the marker 118B18-GT in family RG-5.

Figure 4

Genetic analysis of the two families with partial duplication (FG-253 and RG-5), as well as of the sample with partial deletion (2/6). The markers are listed in order, according to their placement on the physical map. Markers derived from the same PAC are listed in their correct order as derived from the sequence data, but the orientation of the cluster could not be determined accurately. The boxes indicate the proposed extent of each duplication or deletion.