Published November 2019 | PLoS Genetics

Scientists have known that disruption of primary cilia function during embryonic development can lead to microcephaly, based on studies of mice with a mutated form of the gene Ttc21b. However, the severity of the brain size loss has varied when this mutation is maintained in different inbred strains of mice, possibly because of influence from unknown “background” genes.

Now, a team of genetic experts at Cincinnati Children’s led by first author John Snedecker, PhD, and senior author Rolf Stottmann, PhD, has isolated a gene mutation that appears to be a key contributor to the severity of microcephaly.

The hunt began with a Quantitative Trait Locus (QTL) analysis to determine what other gene mutations beyond Ttc21b may be contributing to differential forebrain size. The team chose to use B6 and FVB mice for this project because these strains have been used in medical studies for decades and their genomics have been deeply documented.

The QTL produced 253 potential gene candidates, which meant the team faced a lengthy mapping process to identify which ones actually played a causative role. However, using CRISPR-Cas9 genome editing, the team whittled the field down to one gene of interest: an orphan G-protein coupled receptor, Gpr63.

The team ultimately found a SNP in Gpr63 (rs13477613) that differed between B6 and FVB mice. Further testing demonstrated the SNP alters localization to the cilium, where it interacts with Ttc21b, resulting in more severe brain size reduction in the FVB mice.

“We found that the Gpr63 mutation is the likely causal variant affecting brain size,” Stottmann says. “This is significant because the approach taken here may be quite useful in understanding modifier effects in any number of structural birth defects.”

GPR63 Expression Across Mouse Models