Clinical features and molecular genetics of patients with RASopathies: expanding the phenotype with rare genes and novel variants

The RASopathies are a group of disorders resulting from a germline variant in the genes encoding the Ras/mitogen-activated protein kinase pathway. These disorders include Noonan syndrome (NS), cardiofaciocutaneous syndrome (CFC), Costello syndrome (CS), Legius syndrome (LS), and neurofibromatosis type 1 (NF1), and have overlapping clinical features due to RAS/MAPK dysfunction. In this study, we aimed to describe the clinical and molecular features of patients exhibiting phenotypic manifestations consistent with RASopathies. The study included 149 patients from 146 unrelated families who were admitted between 2019 and 2023 with a clinical suspicion of RASopathy spectrum disorder. Clinical and laboratory characteristics of the patients at the time of the diagnosis were obtained from hospital records. Variant analysis of twenty-four RASopathy genes was performed using a targeted next-generation sequencing (NGS) panel, and the variants were classified according to American College of Medical Genetics and Genomics Standards and Guidelines recommendations. Pathogenic/likely pathogenic variants were detected in 39 out of 149 patients (26.1%). Thirty-two patients were diagnosed as NS (32/39; 82%). The variants detected in NS patients were PTPN11 (21/32; 65.6%), LZTR1 (3/32; 9.3%), SOS1 (2/32; 6.2%), RAF1 (2/32; 6.2%), RIT1 (2/32; 6.2%), KRAS (1/32; 3.1%), and RRAS (1/32; 3.1%) genes, respectively. The remaining patients were diagnosed with CS (2/39; 5.1%), NF1 (2/39; 5.1%), NF-NS (2/39; 5.1%), and CFC (1/39; 2.5%). We observed rare clinical findings including lymphangioma circumscriptum, Meckel’s diverticulum, and omphalocele in three patients with PTPN11 gene variations. Additionally, we detected corpus callosum thickness in a patient with the SOS1 gene variant, which has not been previously described in NS. We also identified three novel variants in RIT1 , BRAF , and NF1 genes. Conclusion : In this study, we described rare clinical manifestations and detected three novel variants in NF1 , BRAF , and RIT1 genes. We propose that NGS technology enables the detection of variants in rare genes responsible for the etiology of RASopathies. The study, therefore, not only contributes to the existing literature but also expands the spectrum of genotype and phenotype of RASopathies. What is Known: • RASopathies are a group of disorders caused by germline variants in genes involved in the Ras/mitogen-activated protein kinase (RAS/MAPK) pathway. • These disorders, includin