Plastic Bronchitis in Noonan Syndrome: Further Evidence Suggesting a Higher Risk of Lymphatic Abnormalities in Individuals Harboring Variants in PTPN11 Residue p.Phe285.

Introduction: Noonan syndrome (NS) is a Mendelian phenotype, member of the RASopathies, a group of clinically overlapping multisystem disorders caused by germline variants in the RAS-MAPK signaling pathway genes. Among the clinical findings in NS, lymphatic abnormalities (LAs) are diagnosed in approximately 30%, mostly in individuals harboring variants in and . This genotype-phenotype correlation is not precise, and recent evidence suggests a higher prevalence of LAs in individuals harboring variants on p.Phe285 residue in , the main gene responsible for NS.

Case Presentation: Here, we report a novel case of NS harboring the :p.Phe285Ser variant that evolved with chylothorax and presented the rare finding of plastic bronchitis, an uncommon and underdiagnosed pulmonary disease, characterized by production of cohesive and branching casts filling the airways. We also provide a review of other individuals with NS and LA harboring variants on Phe285 residue in from our service and from the literature and compared its prevalence with the most commonly affected residue in -related NS (p.Asn308), which indicated that variants in the p.Phe285 residue might predispose to LA.

Conclusion: We suggest that, when this variant is identified in an individual, clinicians should be warned of a possible higher prevalence of LA and a prompt evaluation should be performed if any clinical signs are noticed.