A candidate gene for congenital bilateral isolated ptosis identified by molecular analysis of a de novo balanced translocation

Abstract

Ptosis is defined as drooping of the upper eyelid and can impair full visual acuity. It occurs in a number of forms including congenital bilateral isolated ptosis, which may be familial and for which two linkage groups are known on chromosomes 1p32–34.1 and Xq24–27.1. We describe the analysis of the chromosome breakpoints in a patient with congenital bilateral isolated ptosis and a de novo balanced translocation 46,XY,t(1;8)(p34.3;q21.12). Both breakpoints were localized by fluorescence in situ hybridisation with yeast artificial chromosomes, bacterial artificial chromosomes and P1 artificial chromosomes. The derived chromosomes were isolated by flow-sorting, amplified by degenerate oligonucleotide-primed polymerase chain reaction and analyzed by sequence tagged sites amplification to map the breakpoints at a resolution that enabled molecular characterization by DNA sequencing. The 1p breakpoint lies ~13 Mb distal to the previously reported linkage locus at 1p32–1p34.1 and does not disrupt a coding sequence, whereas the chromosome 8 breakpoint disrupts a gene homologous to the mouse zfh-4 gene. Murine zfh-4 codes for a zinc finger homeodomain protein and is a transcription factor expressed in both muscle and nerve tissue. Human ZFH-4 is therefore a candidate gene for congenital bilateral isolated ptosis.