Summary A recent convergence of technological innovations has re‐energized the ability to apply genetics to research in human craniofacial development. Next‐generation exome and whole genome sequencing have significantly dropped in price, making it relatively trivial to sequence and analyze patients and families with congenital craniofacial anomalies. A concurrent revolution in genome editing with the use of the CRISPR‐Cas9 system enables the rapid generation of animal models, including mouse, which can precisely recapitulate human variants. Here, we summarize the choices currently available to the research community. We illustrate this approach with the study of a family with a novel craniofacial syndrome with dominant inheritance pattern. The genomic analysis suggested a causal variant in AMOTL1 which we modeled in mice. We also made a novel deletion allele of Amotl1. Our results indicate that Amotl1 is not required in the mouse for survival to weaning. Mice carrying the variant identified in the human sequencing studies, however, do not survive to weaning in normal ratios. The cause of death is not understood for these mice complicating our conclusions about the pathogenicity in the index patient. Thus, we highlight some of the powerful opportunities and confounding factors confronting current craniofacial genetic research.

Frizzled 2 (FZD2) is a transmembrane Wnt receptor. We previously identified a pathogenic human FZD2 variant in individuals with FZD2-associated autosomal dominant Robinow syndrome. The variant encoded a protein with a premature stop and loss of 17 amino acids, including a region of the consensus dishevelled-binding sequence. To model this variant, we used zygote microinjection and i-GONAD-based CRISPR/Cas9-mediated genome editing to generate a mouse allelic series. Embryos mosaic for humanized Fzd2W553* knock-in exhibited cleft palate and shortened limbs, consistent with patient phenotypes. We also generated two germline mouse alleles with small deletions: Fzd2D3 and Fzd2D4. Homozygotes for each allele exhibit a highly penetrant cleft palate phenotype, shortened limbs compared with wild type and perinatal lethality. Fzd2D4 craniofacial tissues indicated decreased canonical Wnt signaling. In utero treatment with IIIC3a (a DKK inhibitor) normalized the limb lengths in Fzd2D4 homozygotes. The in vivo replication represents an approach for further investigating the mechanism of FZD2 phenotypes and demonstrates the utility of CRISPR knock-in mice as a tool for investigating the pathogenicity of human genetic variants. We also present evidence for a potential therapeutic intervention.

A recent convergence of technological innovations has re-energized the ability to apply genetics to research in human craniofacial development. Next-generation exome and whole genome sequencing have dropped in price significantly making it relatively trivial to sequence and analyze patients and families with congenital craniofacial anomalies. A concurrent revolution in genome editing with the use of the CRISPR-Cas9 system enables the rapid generation of animal models, including mouse, which can precisely recapitulate human variants. Here we summarize the choices currently available to the research community. We illustrate this approach with the study of a family with a novel craniofacial syndrome with dominant inheritance pattern. The genomic analysis suggest a causal variant in AMOTL1 which we modeled in mice. We also made a novel deletion allele of Amotl1. Our results indicate that Amotl1 is not required in the mouse for survival to weaning. Mice carrying the variant identified in the human sequencing studies, however, do not survive to weaning in normal ratios. The cause of death is not understood for these mice complicating our conclusions about the pathogenicity in the index patient. Thus, we highlight some of the powerful opportunities and confounding factors confronting current craniofacial genetic research.

FRIZZLED-2 (FZD2) is a transmembrane Wnt ligand receptor. We previously identified a pathogenic human FZD2 variant, encoding for a protein with a premature stop and loss of 17 amino acids. This includes a portion of the consensus DISHEVELLED binding sequence required for Wnt signal transduction. Patients with this variant exhibited FZD2-associated autosomal dominant Robinow Syndrome. To model this variant, we utilized zygote microinjection and i-GONAD-based CRISPR/Cas9-mediated genome editing to generate an allelic series in the mouse. Embryos mosaic for humanized Fzd2W553* knock-in exhibited cleft palate and shortened limbs, consistent with FZD2W548* patient phenotypes. We also generated two germline mouse alleles with small deletions, Fzd2D3 and Fzd2D4. Homozygotes for each allele survive embryonic development at normal ratios but exhibit a highly penetrant cleft palate phenotype, shortened limbs compared to wild-type and perinatal lethality. Fzd2D4 craniofacial tissues indicated decreased canonical WNT signaling. In utero treatment with IIIC3a (DKK inhibitor) normalized the limb lengths in Fzd2D4 homozygotes. The in vivo replication represents an approach to further investigate the mechanism of FZD2 phenotypes and validates the utility of CRISPR knock-in mice as a tool for demonstrating pathogenicity of human genetic variants. We also present evidence for a potential therapeutic intervention.