ZNF423 patient variants, truncations, and in-frame deletions in mice define an allele-dependent range of midline brain abnormalities

Interpreting rare variants remains a challenge in personal genomics, especially for disorders with several causal genes and for genes that cause multiple disorders. ZNF423 encodes a transcriptional regulatory protein that intersects several developmental pathways. ZNF423 has been implicated in rare neurodevelopmental disorders, consistent with midline brain defects in Zfp423-mutant mice, but pathogenic potential of most patient variants remains uncertain. We engineered ~50 patient-derived and small deletion variants into the highly-conserved mouse ortholog and examined neuroanatomical measures for 791 littermate pairs. Three substitutions previously asserted pathogenic appeared benign, while a fourth was effectively null. Heterozygous premature termination codon (PTC) variants showed mild haploabnormality, consistent with loss-of-function intolerance inferred from human population data. In-frame deletions of specific zinc fingers showed mild to moderate abnormalities, as did low-expression variants. These results affirm the need for functional validation of rare variants in biological context and demonstrate cost-effective modeling of neuroanatomical abnormalities in mice. AUTHOR SUMMARYGene identification in rare disorders is typically supported by finding different mutations of the same gene in multiple families with the same disorder. However, causal evidence for any specific mutation found in one or a few related individuals is weaker, especially if the disorder can be caused by any of several genes and the functional effect of the mutation is not certain. Experimental models can be helpful in testing causal effects, but only to the extent that the model is validated to recapitulate one or more aspects of the disorder. We used CRISPR/Cas9-based genome engineering to create a wide range of mutations in mouse Zfp423, whose human cognate is implicated in neurodevelopmental disorders, especially cerebellar vermis hypoplasia and Joubert syndrome. This large collection of animal models shows that both reduced Zfp423 expression, including heterozygosity for loss-of-function mutations, and normally-expressed domain deletions, including specific zinc finger domains, produce measureable abnormalities in midline development. Despite this high level of validation, most patient-derived amino acid substitution variants tested do not produce measureable effects. The single exception is a substitution, H1277Y, that destroys a structural element in the last zinc finger domain and results in dramatic loss of steady-state Zfp423 protein level.