Ocular Colobomata Associated With a Chromosomal Deletion - Identification of a Candidate Gene

Purpose: To determine the genetic cause of anterior and posterior segment colobomata in a patient with a segmental chromosomal deletion. Methods: The chromosomal anomaly was originally identified as a result of a broader than usual clinical phenotype (developmental delay). Microsatellite marker genotyping, and subsequent array comparative genome hybridisation defined the extent of the deletion, whose breakpoint was identified with long-range PCR. RT-PCR, in situ hybridisation and in silico analyses were used to characterise the 27 genes within or adjacent to the deleted interval. Morpholino anti-sense oligonucleotides were used to inhibit expression of a strong candidate gene (OC1) in a zebrafish model. Results: The breakpoints lie adjacent to blocks of repetitive sequence and the 10.3 Mb segmental deletion is syntenic with a murine ocular anomaly locus. OC1 is expressed at the appropriate timepoint and tissue [embryonic murine and zebrafish retina] to induce colobomata. Morpholino inhibition of OC1 gene function results in anomalies that recapitulate the patient phenotype, including dorsal and ventral colobomata as well as decreased ocular and lenticular size. Higher morpholino doses cause regression to vestigial ocular remnants resembling human anophthalmia. Conclusions: Study of chromosomal anomalies represents a fruitful starting point from which to investigate the genetic basis of disease. The segmental deletion is caused by non-allelic homologous recombination, and decreases OC1 copy number. Altered gene dosage and recapitulation of the patient’s phenotype using OC1 morpholinos (plus the syntenic murine locus), provide evidence for OC1’s role in retinal development. These data illustrate the value of zebrafish models and the benefits of integrated clinical and basic science approaches to human disease.