Background

Horizontal gaze palsy with progressive scoliosis (HGPPS; OMIM #607313) is a rare autosomal recessive disorder caused by mutations to the roundabout guidance receptor 3 gene (ROBO3; OMIM 608630), which encodes a protein responsible for axonal guidance. The diagnosis of HGPPS is established in the presence of clinical features, specific neuroimaging and electrophysiology findings and genetic testing for ROBO3 gene mutations. Brain MRI shows a ‘butterfly’ configuration of the medulla, dorsal pontine clefting and absence of the pontine crossing tract on diffusion tensor imaging.

Objective

We conduct a population-level analysis to test whether adult carriers of ROBO3 variants have detectable differences in brain structure and ultrastructure as measured by quantitative MRI, using data from the UK Biobank.

Methods

Using the AstraZeneca PheWAS portal, we performed variant-level analysis of ROBO3 across 2077 brain MRI-derived phenotypes using the dominant model. ROBO3 variants identified by the PheWAS as being associated with MRI-derived phenotypes were compared to a list of 27 known pathogenic or likely pathogenic ROBO3 variants identified from the ClinVar database.

Results

Three variants were identified as having significant associations with MRI derived phenotypes: 11-124872489-G-A, 11-124878289-TCTC-T and 11-124876284-C-T. All three variants are relatively common (heterozygous variant genotype in 9.1%, 9.5% and 10.2%, homozygous variant genotype in 0.2%, 0.2% and 0.3% respectively), and are not known to be pathogenic / likely pathogenic variants causative of HGPPS. All three identified variants associate with altered mean diffusion metrics in the pontine crossing tract, including the mode of anisotropy (effect sizes -0.25 to -0.28, p = 3.90e-41 to 2.42e-45), orientation dispersion (effect sizes 0.17 to 0.18, p = 3.09e-19 to 6.05e-20) and second eigenvalue (effect size of 0.12, p = 3.53e-10 to 7.71e-10).

Conclusions

Our population-level analysis shows that subtle ultrastructural differences are detectable in the pontine crossing tract using diffusion imaging in people with common non-pathogenic variants in the ROBO3 gene. The finding provides in vivo evidence supporting the role of ROBO3 in guiding pontine axonal architecture during normal human development.