T1w/T2w ratio suggests reduced intracortical myelin content in youth with RASopathies

Myelin may represent a modifiable treatment target in neurodevelopmental disorders, however, a reliable tool for in vivo assessment of myelin alterations in clinical settings is needed. Prior work shows commonly acquired T1-weighted (T1w) and T2-weighted (T2w) scans can be transformed into ratio maps and subsequently provide a reasonable estimate of cortical myelin content. We investigated T1w/T2w ratios for measuring cortical myelin in children with neurodevelopmental disorders of the RAS-MAPK signaling pathway.

In this prospective study, 112 children (86 RASopathies, 26 typical developing (TD), aged 6-17 years) completed T1w and T2w MRI scans and NIH Toolbox cognitive assessments. Parent-rated mobility and strength impact scores were also acquired. T1w/T2w ratio maps were calculated at three levels of the cortex in FreeSurfer, and average values were extracted from 68 regions-of-interest (ROIs) at each level across the brain and from eight subcortical ROIs. Group differences were assessed using analyses of covariance, including a false discovery rate adjustment for multiple comparisons. As an exploratory analysis, differences in cognitive scores and parent-rated health were assessed across quartiles of whole-brain T1w/T2w ratios.

Widespread decreases in T1w/T2w ratios were found in the RASopathies group, suggesting decreased cortical myelin content. Of the T1w/T2w ratios sampled along the midline, 63 of 68 cortical ROIs were significantly reduced in RASopathies (p _FDR <.050). Of the subcortical ROIs, only the T1w/T2w ratio in the accumbens was significantly reduced in RASopathies compared to TD (Cohen's d=0.520, p _FDR =.024). Exploratory quartile analyses across the whole sample indicated significant effects of T1w/T2w ratio quartile on mobility (p=.002) and strength impact ratings (p<.001), such that subjects in higher T1w/T2w ratio quartiles had better physical health ratings.

These results support the utility of T1w/T2w ratio mapping as a sensitive tool for detecting cortical myelin alterations in genetically defined neurodevelopmental disorders. Exploratory analyses suggest a relationship between cortical myelin content and physical mobility and stamina. Future work should explore the clinical relevance of these findings for cognitive and functional outcomes and assess their potential as biomarkers for targeted therapeutic interventions.