Mutations in RIT1 cause Noonan syndrome - additional functional evidence and expanding the clinical phenotype

Area of Science:

• Genetics
• Molecular Biology
• Developmental Biology

Background:

• RASopathies are a diverse group of genetic disorders stemming from mutations in the RAS-MAPK signaling pathway.
• Recent findings implicated mutations in RIT1 as a novel genetic cause of Noonan syndrome.

Purpose of the Study:

• To provide further functional validation for RIT1 mutations in Noonan syndrome.
• To expand the known clinical and phenotypic spectrum associated with RIT1 pathogenic variants.
• To investigate the functional consequences of specific RIT1 variants on cellular signaling and embryonic development.

Main Methods:

• Identification and characterization of two de novo missense variants (p.Met90Ile and p.Ala57Gly) in RIT1.
• In vitro functional assays to assess the impact of RIT1 variants on MEK-ERK signaling.
• Zebrafish (Danio rerio) embryo studies to model human phenotypes and investigate developmental effects.
• Clinical evaluation of patients with identified RIT1 variants.

Main Results:

• Both identified RIT1 variants (p.Met90Ile and p.Ala57Gly) demonstrated increased MEK-ERK signaling compared to wild-type RIT1, indicating a gain-of-function mechanism.
• Zebrafish embryos injected with mutant RIT1 variants exhibited phenotypes mirroring human conditions and revealed novel eye development abnormalities.
• One patient presented with severe lymphedema of the lower extremity and genitalia, expanding the RIT1-associated phenotype.

Conclusions:

• Pathogenic RIT1 mutations are causally linked to Noonan syndrome and related RASopathies through enhanced RAS-MAPK/MEK-ERK signaling.
• RIT1 plays a critical role in embryonic development, including eye formation and organogenesis.
• The phenotypic spectrum associated with RIT1 mutations is broader than previously recognized and may include significant lymphedema.