Humans carrying the CORD7 (cone-rod dystrophy 7) mutation possess increased verbal IQ and working memory. This autosomal dominant syndrome is caused by the single-amino acid R844H exchange (human numbering) located in the 310 helix of the C2A domain of RIMS1/RIM1 (Rab3-interacting molecule 1). RIM is an evolutionarily conserved multi-domain protein and essential component of presynaptic active zones, which is centrally involved in fast, Ca2+-triggered neurotransmitter release. How the CORD7 mutation affects synaptic function has remained unclear thus far. Here, we established Drosophila melanogaster as a disease model for clarifying the effects of the CORD7 mutation on RIM function and synaptic vesicle release.

To this end, using protein expression and X-ray crystallography, we solved the molecular structure of the Drosophila C2A domain at 1.92 Å resolution and by comparison to its mammalian homolog ascertained that the location of the CORD7 mutation is structurally conserved in fly RIM. Further, CRISPR/Cas9-assisted genomic engineering was employed for the generation of rim alleles encoding the R915H CORD7 exchange or R915E,R916E substitutions (fly numbering) to effect local charge reversal at the 310 helix. Through electrophysiological characterization by two-electrode voltage clamp and focal recordings we determined that the CORD7 mutation exerts a semi-dominant rather than a dominant effect on synaptic transmission resulting in faster, more efficient synaptic release and increased size of the readily releasable pool but decreased sensitivity for the fast calcium chelator BAPTA. In addition, the rim CORD7 allele increased the number of presynaptic active zones but left their nanoscopic organization unperturbed as revealed by super-resolution microscopy of the presynaptic scaffold protein Bruchpilot/ELKS/CAST.

We conclude that the CORD7 mutation leads to tighter release coupling, an increased readily releasable pool size and more release sites thereby promoting more efficient synaptic transmitter release. These results strongly suggest that similar mechanisms may underlie the CORD7 disease phenotype in patients and that enhanced synaptic transmission may contribute to their increased cognitive abilities.

Introduction
Cone-rod dystrophy is characterized by the early loss of visual acuity and color vision, followed by night blindness and peripheral visual field loss [1]. Autosomal-dominant, X-linked, and recessive modes of inheritance have been described, and recent genetic studies have implicated a variety of different genetic loci in the etiology of this set of heterogeneous disorders, although the disease loci that underlie most of the cone and cone-rod dystrophies have yet to be identified.

An autosomal-dominant cone-rod dystrophy, CORD7, was originally mapped in a four-generation British family to a region of chromosome 6q14 that is flanked by markers D6S430 and D6S1625 [2]. This localization for CORD7 overlaps or is adjacent to the map locations of a number of other retinal disorders. These include, in the overlapping category, a recessive form of retinitis pigmentosa (RP25) [3], Leber congenital amaurosis type 5 (LCA5) [4], and a dominant drusen and macular degeneration [5] and in the nonoverlapping category, North Carolina macular dystrophy (MCDR1) [6], a dominant Stargardt-like disease (STGD3) [7], [8], and a dominant macular atrophy [9]. STGD3 has recently been shown to arise from mutations in ELOVL4, a gene encoding a protein with a possible activity in the biosynthesis of very long-chain fatty acids [10].

The onset of reduced color vision and visual acuity in affected members of the CORD7 family varies between the ages of 20 and 40 years [2]. As the disorder progresses, difficulties of seeing in bright light become apparent, and one individual also reported visual problems in dim light. At the onset of symptoms, retinal pigmentary changes are already present around the fovea, which develops into macular atrophy. Electrophysiological examination shows that scotopic rod responses in patients with advanced disease are barely detectable, and all cone responses are severely attenuated but with no change in implicit time. Pattern electroretinogram is extinguished in keeping with the severe macular dysfunction [2].

Our strategy for identifying the disease gene has been to prioritize the screening of candidate genes on the basis of function and pattern of gene expression. Three loci were considered excellent candidates, the interphotoreceptor matrix proteoglycan gene, IMPG1 [11], atypical myosin VI, MYO6 [12], [13], and Rab3-interacting molecule, RIM1 [14]...