Update of genetic variants in CEP120 and CC2D2A-With an emphasis on genotype-phenotype correlations, tissue specific transcripts and exploring mutation specific exon skipping therapies

BACKGROUND

Mutations in ciliary genes cause a spectrum of both overlapping and distinct clinical syndromes (ciliopathies). CEP120 and CC2D2A are paradigmatic examples for this genetic heterogeneity and pleiotropy as mutations in both cause Joubert syndrome but are also associated with skeletal ciliopathies and Meckel syndrome, respectively. The molecular basis for this phenotypical variability is not understood but basal exon skipping likely contributes to tolerance for deleterious mutations via tissue-specific preservation of the amount of expressed functional protein.

METHODS

We systematically reviewed and annotated genetic variants and clinical presentations reported in CEP120- and CC2D2A-associated disease and we combined in silico and ex vivo approaches to study tissue-specific transcripts and identify molecular targets for exon skipping.

RESULTS

We confirmed more severe clinical presentations associated with truncating CC2D2A mutations. We identified and confirmed basal exon skipping in the kidney, with possible relevance for organ-specific disease manifestations. Finally, we proposed a multimodal approach to classify exons amenable to exon skipping. By mapping reported variants, 14 truncating mutations in 7 CC2D2A exons were identified as potentially rescuable by targeted exon skipping, an approach that is already in clinical use for other inherited human diseases.

CONCLUSION

Genotype-phenotype correlations for CC2D2A support the deleteriousness of null alleles and CC2D2A, but not CEP120, offers potential for therapeutic exon skipping approaches.

Expression patterns of ciliopathy genes ARL3 and CEP120 reveal roles in multisystem development

BACKGROUND

Joubert syndrome and related disorders (JSRD) and Jeune syndrome are multisystem ciliopathy disorders with overlapping phenotypes. There are a growing number of genetic causes for these rare syndromes, including the recently described genes ARL3 and CEP120.

METHODS

We sought to explore the developmental expression patterns of ARL3 and CEP120 in humans to gain additional understanding of these genetic conditions. We used an RNA in situ detection technique called RNAscope to characterise ARL3 and CEP120 expression patterns in human embryos and foetuses in collaboration with the MRC-Wellcome Trust Human Developmental Biology Resource.

RESULTS

Both ARL3 and CEP120 are expressed in early human brain development, including the cerebellum and in the developing retina and kidney, consistent with the clinical phenotypes seen with pathogenic variants in these genes.

CONCLUSIONS

This study provides insights into the potential pathogenesis of JSRD by uncovering the spatial expression of two JSRD-causative genes during normal human development.

Disease-Associated Mutations in CEP120 Destabilize the Protein and Impair Ciliogenesis

Ciliopathies are a group of genetic disorders caused by a failure to form functional cilia. Due to a lack of structural information, it is currently poorly understood how ciliopathic mutations affect protein functionality to give rise to the underlying disease. Using X-ray crystallography, we show that the ciliopathy-associated centriolar protein CEP120 contains three C2 domains. The point mutations V194A and A199P, which cause Joubert syndrome (JS) and Jeune asphyxiating thoracic dystrophy (JATD), respectively, both reduce the thermostability of the second C2 domain by targeting residues that point toward its hydrophobic core. Genome-engineered cells homozygous for these mutations have largely normal centriole numbers but show reduced CEP120 levels, compromised recruitment of distal centriole markers, and deficient cilia formation. Our results provide insight into the disease mechanism of two ciliopathic mutations in CEP120, identify putative binding partners of CEP120 C2B, and suggest a complex genotype-phenotype relation of the CEP120 ciliopathy alleles.