Disease mechanisms in late-onset retinal macular degeneration associated with mutation in C1QTNF5

Late-onset retinal macular degeneration (L-ORMD) is an autosomal dominant condition resemblingage-related macular degeneration (AMD) in which a keypathological feature is a thick extracellular sub-retinal pigment epithelial (RPE) deposit. L-ORMD is caused by mutation in the C1QTNF5 (CTRP5) short-chaincollagen gene, but the disease mechanism is unknown. Here, we first show that wild-type C1QTNF5 issecreted, whereas mutant C1QTNF5 is misfolded and retained within the endoplasmic reticulum (ER).Secondly, the ER retained mutant protein has a shorter half-life than wild-type C1QTNF5 and is preferentiallydegraded by proteasomes. Thirdly, C1QTNF5 is shown to interact with the membrane-type frizzled relatedprotein (MFRP), on the basis of yeast two-hybrid, protein pull-down and co-immunoprecipitation assaysand RPE co-localization. These data suggest that L-ORMD is due to insufficient levels of secretedC1QTNF5, compromised RPE cell function resulting from ER retention of the mutant protein or bothmechanisms.INTRODUCTIONAge-related macular degeneration (AMD) is the commonestcause of blind registration in the developed world (1). It ischaracterized by a late-onset degeneration of the retinalmacula and represents the advanced stage of a morecommon disorder, age-related maculopathy. An importantpathological feature of AMD is the accumulation of bothfocal (drusen) and diffuse extracellular (basal) deposits inthe macula, between the retinal pigment epithelium (RPE)and the adjacent Bruch’s membrane (2). These deposits leadto dysfunction and later death of RPE and associated photo-receptors. The nature of the proteins within the diffuse extra-cellular (basal) deposits have not been elucidated, but the focaldeposits (drusen) include .100 proteins, together with lipidsand glycosaminoglycans (3–5). Genetic factors are implicatedin AMD on the basis of twin, family and association studiesbut it appears to be a genetically complex disorder (6).An important means of elucidating disease mechanisms in agenetically complex disorder such as AMD, is to take advan-tage of information from a Mendelian disorder in which AMDis also present. Late-onset retinal macular degeneration(L-ORMD), originally named late-onset retinal degeneration(7–10), and also referred to as autosomal dominanthaemorrhagic macular dystrophy (11) or late-onset maculardegeneration (12), is a rare disorder with onset in the fifth tosixth decade with drusen-like deposits, followed by bothmacular degeneration and, in late stages, choroidal neovascu-larization and disciform scarring and severe peripheral chorior-etinal atrophy (7–12). L-ORMD is caused by a single founderSer163Arg mutation in the Complement 1q Tumour NecrosisFactor 5 gene (C1QTNF5, previously called CTRP5) (10,12).A primary feature of the disease is a thick ( 50 m