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Background/Objectives: Neurofibromatosis type 1 (NF1) and Noonan syndrome spectrum disorders (NSSD) are the most common RASopathies, resulting from germline mutations that affect the RAS-MAPK signaling pathway. Both are associated with increased risk for neurodevelopmental and psychiatric conditions, yet few studies have used structured diagnostic interviews to compare their psychiatric comorbidities. Methods: We conducted clinician-administered DSM-5 diagnostic assessments (KSADS) in 123 children with RASopathies (NF1 = 29, NSSD = 94; ages 5–15). Diagnosis prevalence was compared within each group and to population-based estimates. Results: Psychiatric diagnoses were highly prevalent, at 79.3% in NF1 and 76.6% in NSSD, with ADHD (NF1 = 72.4%, NSSD = 51.1%) and anxiety disorders (NF1 = 37.9% and NSSD = 43.6%) being the most common, rates substantially higher than those reported in general population estimates. Behavioral and sleep disorders were identified in approximately 25% of both groups. Notably, social anxiety disorder was identified in 14.9% of NSSD but not in NF1. Full-scale IQ did not significantly differ by diagnosis status. Specific anxiety disorders, elimination disorders, obsessive–compulsive disorder, and post-traumatic stress disorder were characterized, expanding the known psychiatric phenotype of RASopathies. Conclusions: Children with NF1 and NSSD demonstrate similarly high rates of ADHD, anxiety, and behavioral disorders compared to the general population; in addition, we report sleep disorders in NSSD and characterize psychiatric disorders not previously described in RASopathies. The shared psychiatric profiles may reflect the common effect of RAS-MAPK pathway dysregulation on psychiatric outcomes. These findings highlight the need for early, syndrome-informed mental health screening and intervention in the clinical care of individuals with RASopathies.

Background Noonan syndrome (NS) and Noonan syndrome with multiple lentigines (NSML) are neurodevelopmental conditions caused by genetic variants leading to upregulated signaling in the RAS-MAPK pathway. While previous research has focused on genetic variability in cognitive and cardiac phenotypes, behavioral phenotypes, and their correlations across genetic variants and within the PTPN11 gene remain poorly characterized. Methods This study included 121 individuals with NS ( PTPN11 : 88, SOS1 : 18, RAF1 : 6, KRAS : 2, RIT1 : 3, NRAS : 2, LZTR1 : 2, SOS2 : 1) and seven individuals with NSML ( PTPN11 ), compared to age- and sex-matched typically developing (TD) (N = 71). Behavioral questionnaires assessed social responsiveness and ASD-related traits (using SRS-2), and emotional problems (using CBCL) to identify genetic variant-specific behavioral profiles. Biochemical profiling of SHP2 activity in PTPN11 -associated NS variants examined genotype–phenotype relationships. Results Compared to TD individuals, those with PTPN11 -associated NS, NSML, and SOS1 -associated NS exhibited clinically elevated scores, indicating increased ASD-related behaviors, poorer social functioning, and heightened emotional problems. Genetic variant comparisons revealed that individuals with PTPN11 -associated NS and NSML exhibited greater ASD-related challenges than those with RAF1 . Individuals with NSML exhibit elevated attention problems compared to all other genetic groups. Logistic regression results suggested each one-unit increase in SHP2 fold activation for PTPN11 -associated NS corresponded to a 64% higher likelihood of markedly elevated restricted and repetitive behaviors, suggesting genotype–phenotype links. Limitations Small sample sizes for rarer variants, leading to unequal group sizes across subgroups, with PTPN11 variants comprising most of the NS group. Future research should address these sampling constraints and conduct functional studies to clarify variant impacts. Longitudinal assessments could elucidate behavioral phenotype trajectories. Conclusions This study underscores the importance of genetic variant-specific research to understand unique behavioral phenotypes in NS and NSML. Our findings indicate a higher risk for ASD-related symptoms in PTPN11 -associated NS and NSML compared to other variants. Additionally, individuals with PTPN11 -associated NS and higher SHP2 fold activation exhibited greater impairments in restricted and repetitive behaviors, suggesting SHP2 activation variations may contribute to phenotypic variability. By linking ASD-related symptoms to biochemical predictors in PTPN11 -associated NS, this study may inform future targeted treatment approaches.