Impact of Pathogenic Variants of the Ras-MAPK Pathway on Major White Matter Tracts in the Human Brain

Noonan syndrome (NS) and Neurofibromatosis type-1 (NF1) are genetic conditions linked to pathogenic variants in genes of the Ras-MAPK signaling pathway. Both conditions hyperactivate signaling of the Ras pathway and exhibit a high prevalence of neuropsychiatric disorders. Further, animal models of NS and NF1 and human imaging studies show white-matter abnormalities in both conditions. While these findings suggest Ras pathway hyper-activation effects on white-matter, it is unknown whether these effects are syndrome-specific or pathway-specific. To characterize the effect of NS and NF1 on human white-matter microstructural integrity and discern potential syndrome-specific influences on microstructural integrity of individual tracts, we collected diffusion-weighted imaging data from children with NS (n=24), NF1 (n=28), and age and sex-matched controls (n=31). We contrasted the clinical groups (NS or NF1) and controls using voxel-wise, tract-based, and along-tract analyses. Outcomes included voxel-wise, tract-based and along-tract fractional anisotropy (FA), axial diffusivity (AD), radial diffusivity (RD) and mean diffusivity (MD). NS and NF1 showed similar patterns of reduced FA and increased AD, RD, and MD on white-matter relative to controls and different spatial patterns. NS presented a more extensive spatial effect than NF1 on white-matter integrity as measured by FA. Tract-based analysis also demonstrated differences in effect magnitude with overall lower FA in NS compared to NF1 (d=0.4). At the tract-level, NS specific-effects on FA were detected in association tracts (superior longitudinal, uncinate and arcuate fasciculi; ps <0.012) and NF1 specific-effects were detected in the corpus callosum (ps<0.037) compared to controls. Results from along-tract analyses aligned with results from tract-based analyses and indicated that effects are pervasive along the affected tracts. In conclusion, we find that pathogenic variants in the Ras-MAPK pathway are associated with white-matter abnormalities as measured by diffusion in the developing brain. Overall NS and NF1 show common effects on FA and diffusion scalars, as well as specific unique effects, namely on temporoparietal-frontal tracts (intra-hemispheric) in NS and on the corpus callosum (inter-hemispheric) in NF1. The observed specific effects not only confirm prior observations from independent cohorts of NS and NF1 but also inform on syndrome-specific susceptibility of individual tracts. Thus, these findings suggest potential targets for precise, brain-focused outcome measures for existing medications, such as MEK inhibitors, that act on the Ras pathway.