Generation and characterization of Ccdc28b mutant mice links the Bardet-Biedl associated gene with social behavioral phenotypes

CCDC28B (coiled-coil domain-containing protein 28B) was identified as a modifier in the ciliopathy Bardet-Biedl syndrome (BBS). Our previous work in cells and zebrafish showed that CCDC28B plays a role regulating cilia length in a mechanism that is not completely understood. Here we report the generation of a Ccdc28b mutant mouse using CRISPR/Cas9 (Ccdc28b mut). After confirming the depletion of Ccdc28b we performed a phenotypic characterization showing that Ccdc28b mut animals present a mild phenotype: i) do not present clear structural cilia affectation, although we did observe mild defects in cilia density and cilia length in some tissues, ii) reproduce normally, and iii) do not develop retinal degeneration or obesity, two hallmark features of reported BBS murine models. In contrast, Ccdc28b mut mice did show clear social interaction defects as well as stereotypical behaviors suggestive of autism spectrum disorder (ASD). This finding is indeed relevant regarding CCDC28B as a modifier of BBS since behavioral phenotypes have been documented in BBS. Importantly however, our data suggests a possible causal link between CCDC28B and ASD-like phenotypes that exceeds the context of BBS: filtering for rare deleterious variants, we found CCDC28B mutations in eight probands from the Simmons Simplex Collection cohort. Furthermore, a phenotypic analysis showed that CCDC28B mutation carriers present lower BMI and mild communication defects compared to a randomly selected sample of SSC probands. Thus, our results suggest that mutations in CCDC28B lead to mild autism-like features in mice and humans. Overall, this work reports a novel mouse model that will be key to continue evaluating genetic interactions in BBS, deciphering the contribution of CCDC28B to modulate the presentation of BBS phenotypes. In addition, our data underscores a novel link between CCDC28B and ASD-like phenotypes, providing a novel opportunity to further our understanding of the genetic, cellular, and molecular basis of ASD. AUTHOR SUMMARYBardet-Biedl syndrome (BBS) is caused by mutations in any of 21 genes known to date. In some families, BBS can be inherited as an oligogenic trait whereby mutations in more than one BBS gene collaborate in the presentation of the syndrome. In addition, CCDC28B was identified as a modifier of BBS, associated with a more severe presentation of the syndrome. Different mechanisms, all relying on functional redundancy, have been proposed to explain this genetic interaction and the characterization of different BBS proteins supported this possibility as they were shown to play roles in the same cellular organelle, the primary cilium. Similarly, CCDC28B also participates in cilia biology regulating the length of the organelle: knockdown of CCDC28B in cells results in cilia shortening and depletion in zebrafish also results in early embryonic phenotypes characteristic of other cilia mutants. Here, we sought to generate a mouse Ccdc28b mutant to determine whether it would be sufficient to cause a ciliopathy phenotype, and to generate a reagent critical to further dissect its modifying role in the context of BBS. Overall, Ccdc28b mutant mice presented a mild phenotype, a finding fully compatible with a modifier rather than a causal BBS gene. In addition, we found that Ccdc28b mutants showed a clear autism-like behavior, and autism is indeed a feature of several BBS patients. Importantly, we identified multiple individuals with autism from the Simmos Simplex Collection to carry disruptive mutations in CCDC28B suggesting that this gene is causally associated with autism independently of BBS.