Germline gain-of-function mutations in SOS1 cause Noonan syndrome

Area of Science:

• Genetics
• Molecular Biology
• Developmental Biology

Background:

• Noonan syndrome is a common genetic disorder linked to congenital heart disease, with PTPN11 mutations causing about 50% of cases.
• SHP2 phosphatase, encoded by PTPN11, is crucial for RAS-ERK MAP kinase (MAPK) pathway activation.
• The genetic causes for the remaining Noonan syndrome cases, beyond PTPN11 and KRAS mutations, were largely unknown.

Purpose of the Study:

• To identify the genetic basis for Noonan syndrome in patients without PTPN11 mutations.
• To investigate the role of SOS1 mutations in the pathogenesis of Noonan syndrome.
• To understand the molecular mechanisms by which SOS1 mutations contribute to Noonan syndrome.

Main Methods:

• Genetic sequencing to identify mutations in patients with Noonan syndrome.
• Functional studies to assess the impact of identified mutations on protein activity.
• Analysis of patient cohorts to correlate genotype with clinical phenotype, including cardiac defects.

Main Results:

• Missense mutations in SOS1 were identified in approximately 20% of Noonan syndrome cases lacking PTPN11 mutations.
• SOS1 encodes a RAS guanine nucleotide-exchange factor (RAS-GEF) essential for RAS-ERK MAPK pathway activation.
• Mutations in SOS1 lead to enhanced RAS and ERK activation, classifying them as hypermorphs.
• Specific cardiac defect patterns were observed in Noonan syndrome associated with SOS1 mutations.

Conclusions:

• SOS1 mutations represent a major genetic cause of Noonan syndrome, second only to PTPN11 mutations.
• This study provides the first evidence of disease-associated activating mutations in a RAS-GEF.
• The findings offer new insights into the regulation of RAS-GEF proteins and their role in human developmental disorders.