Novel Ciliary Function for TOPORS Associated With Autosomal Dominant Retinitis Pigmentosa

Abstract

Purpose: : We have recently identified mutations in TOPORS responsible for autosomal dominant retinitis pigmentosa. TOPORS is a ubiquitously expressed gene; and its protein shows multi-functional character. The purpose of our work is to characterise the role TOPORS plays in the photoreceptors, which may explain the retinal degeneration seen with mutations in this gene.

Methods: : A commercially available antibody was used to localise WT endogenous TOPORS in human and mouse retinal tissue sections. Tissue sections were stained for TOPORS with several proteins of known localisation in the retina (RP1, MAP2 and γ-tubulin), to ascertain a more detailed localisation of TOPORS and indicate potential interactants. Cellular localisation experiments were carried in cell lines and tissue sections on a single cell level to determine the exact sub-cellular localisation. To identify functional partners of TOPORS, immuno-precipitation experiments were carried out to substantiate data obtained from localisation studies in retinal sections.

Results: : Expression studies show a novel localisation for TOPORS in the retina and in confluent, non-diving cells. TOPORS localises to the connecting cilium of the photoreceptors in retinal sections, and to the primary cilia of cells grown at confluence, but did not co-localise with any of the marker proteins. Immuno-precipitation experiments showed that TOPORS does not interact with many known connecting cilium proteins or with proteins involved in anterograde transport. However, TOPORS does show interaction with proteins involved in retrograde transport.

Conclusions: : This is the first report of a ubiquitous and multi-functional gene causing only adRP. TOPORS is expressed in many cell and tissue types, and is primarily a nuclear protein. However, it shows a unique localisation in the retina and association with proteins involved in retrograde transport, which together suggest it plays a novel role in the retina that, when compromised, results in only photoreceptor degeneration.

Keywords: retinal degenerations: cell biology • photoreceptors • genetics