Cell-based analysis of CAD variants identifies individuals likely to benefit from uridine therapy

8 pages, 4 figures, 1 table. The online version of this article (https://doi.org/10.1038/s41436-020-0833-2) contains supplementary material, which is available to authorized users.
Purpose: Pathogenic autosomal recessive variants in CAD, encoding the multienzymatic protein initiating pyrimidine de novo biosynthesis, cause a severe inborn metabolic disorder treatable with a dietary supplement of uridine. This condition is difficult to diagnose given the large size of CAD with over 1000 missense variants and the nonspecific clinical presentation. We aimed to develop a reliable and discerning assay to assess the pathogenicity of CAD variants and to select affected individuals that might benefit from uridine therapy. Methods: Using CRISPR/Cas9, we generated a human CAD-knockout cell line that requires uridine supplements for survival. Transient transfection of the knockout cells with recombinant CAD restores growth in absence of uridine. This system determines missense variants that inactivate CAD and do not rescue the growth phenotype. Results: We identified 25 individuals with biallelic variants in CAD and a phenotype consistent with a CAD deficit. We used the CAD-knockout complementation assay to test a total of 34 variants, identifying 16 as deleterious for CAD activity. Combination of these pathogenic variants confirmed 11 subjects with a CAD deficit, for whom we describe the clinical phenotype. Conclusions: We designed a cell-based assay to test the pathogenicity of CAD variants, identifying 11 CAD-deficient individuals who could benefit from uridine therapy.
This work was supported by The Rocket Fund, by R01DK99551 to H.H.F., and by grants BFU2016–80570-R and RTI2018–098084- B-100 from the Spanish Ministry of Science and Innovation (AEI/ FEDER, UE) and with partial support from U54 NS115198. The University of Washington Center for Mendelian Genomics for exome sequencing and analysis of CDG-0117