Complex Genetic Architecture in RASopathies: Constitutional PTPN11 and Mosaic RIT1 Pathogenic Variants Underlying Severe Noonan Syndrome With Adult-Onset Acute Myeloid Leukemia.

Noonan syndrome (NS) is a genetically heterogeneous disorder characterized by a broad spectrum of clinical features resulting from dysregulation of the RAS/MAPK pathway. Although complex genotypes are increasingly recognized in NS, cases harboring two distinct pathogenic variants in different NS genes remain extremely rare. We describe the case of a 53-year-old female presenting a severe NS phenotype-including short stature, facial dysmorphism, congenital heart defect, and developmental delay-and concurrent acute myeloid leukemia (AML). Targeted NGS analysis of a RASopathy-specific gene panel identified a constitutional heterozygous PTPN11 variant (c.1472C>T, p.(Pro491Leu)) and a mosaic RIT1 variant (c.229G>C, p.(Ala77Pro), 16.45% VAF), both meeting criteria for pathogenicity. The RIT1 variant was validated via PCR-RFLP across multiple tissues, excluding leukemia-driven clonal expansion, and further quantified by high-depth amplicon-based sequencing. To our knowledge, this case represents a unique example of NS associated with pathogenic variants in two distinct RASopathy genes. Our findings underscore that comprehensive molecular characterization and multi-tissue validation are essential for accurate diagnosis, genetic counseling, and personalized management, while also revealing that combinatorial RAS/MAPK alterations may influence disease severity and clinical outcomes.