Primary cilium dysfunctions cause complex oligogenic disorders called the ciliopathies. Among the ciliopathies are Nephronophthisis (NPHP), and Meckel-Gruber syndrome (MKS). MKS/NPHP patients exhibit multiple phenotypes including midgestation lethality, left-right body asymmetry defects, skeletal abnormalities, cystic kidney disease, retinal degeneration and CNS malformation. In NPHP4 patients phenotypes can be limited to renal dysfunction or may include retinal degeneration, with no apparent genotype-phenotype correlation. Many of the genes identified in NPHP and MKS patients encode proteins that localize to the base of the cilium. Most of these proteins are conserved in Caenorhabditis elegans. Compound mutations in C.elegans nphp and mks genes result in the appearance of more severe cilia phenotypes that are not observed in any of the single mutants, for example inability to absorb lipophilic dye DiI into the sensory neurons (Dyf phenotype). The goal of this study was to identify mutations in new genes producing synergistic Dyf phenotypes with
nphp-4(
tm925) null mutation using C. elegans. These genes would be potential targets for analysis in human patients. For this we performed a large scale EMS mutagenesis screen on
nphp-4 (
tm925) mutants and isolated 40 synthetic Dyf mutant lines. SNP mapping was done to place new mutations on chromosomes. Interval mapping and gene rescue experiments performed for one of the lines (YHW66) identified a new missense mutation in
osm-3 gene encoding kinesin motor protein required for building of ciliary distal segments in subset of sensory neurons. Similarly, Zebrafish homolog of
osm-3, Kif17 is essential for vertebrate photoreceptor sensory outer segment development. Multispecies alignment revealed that the position of the new mutation is highly conserved in humans. Dye-filling and osmotic avoidance experiments showed that new
osm-3 allele does not produce noticeable cilia defects compared to wild type worms. In contrast, when combined with
nphp-4(
tm925) mutation, but not mutations in other transition zone proteins, it caused severe dye-filling and osmotic avoidance defects comparable to
osm-3 null allele. Further, OSM-3::GFP containing the new missense mutation properly localizes to the cilium in wild type background. Interestingly, it abnormally accumulates at the base of the cilium in YHW66 mutant background. Together our data suggest that NPHP-4 may be involved in controlling OSM-3 entry to the cilium. This can potentially explain retinal degeneration in a subgroup of NPHP4 human patients. Collectively, our studies identify a new genetic modifier of
nphp-4 and help to understand complex genetic interactions contributing to the diversity of phenotypes associated with cilia disorders.