Noonan syndrome (NS; MIM 163950) is an autosomal dominant developmental disorder affecting approximately 1 in every 1,000 to 2,500 live births. NS is characterized by wide phenotypic variability and approximately 80% of patients with clinically diagnosed NS also have congenital cardiovascular abnormalities, including pulmonary valve stenosis, cardiac hypertrophy, or septal defects. Although NS is a genetically heterogeneous disorder, nearly all genes associated with NS encode proteins that are components or regulators of the RAS mitogen-activated protein kinase (MAPK) signaling pathway. And while understanding of the genetic underpinnings of NS has continued to grow over the last decade, approximately 20% of all NS cases remain genetically elusive. Identifying the remaining NS genes is critical for proper patient diagnosis and management, elucidating genotype-phenotype correlations, family screening and development of potential treatments. Recently, whole exome sequencing (WES) and trio-based genomic triangulation performed on a 15-year-old female with a clinical diagnosis of NS and concomitant cardiac hypertrophy (NS+CH) and her unaffected parents elucidated a de novo variant in MRAS- encoded RAS-related protein 3 (MRAS) as the cause of her disease. Biochemical assays demonstrated a significant increase in MRAS activation for the patient-identified mutant MRAS, p.Gly23Val-MRAS, as well as enhanced activation of both RAS/MAPK pathway signaling and downstream gene expression. Subsequent direct sequencing of MRAS in a cohort of 109 unrelated, genetically elusive NS+CH patients revealed an additional, potentially pathogenic mutation, p.Thr68Ile-MRAS, in one patient supporting the hypothesis that MRAS is a novel NS+CH susceptibility gene. In order to further bolster the evidence that mutations in MRAS cause NS+CH, it is necessary to establish pathogenicity of these mutations in multiple in vitro models. To do this, we will take a multimodal approach utilizing traditional in vitro models as well as patient-derived induced pluripotent stem cells (iPSCs). Gene editing of these iPSCs will further demonstrate that mutations in MRAS not only cause disease, but are in fact sufficient to do so.
Despite growing knowledge of the genetic underpinnings of Noonan syndrome (NS), nearly 20% of all NS cases remain genetically elusive, while genotype/phenotype correlations for known NS-susceptibility genes have remained limited. MRAS was recently identified as a novel NS- susceptibility gene; however, it remains unclear how mutations in MRAS lead to the development of NS and its particular phenotypes, especially cardiac hypertrophy. Therefore, the goal of our research is to provide new details and insight into the pathobiology of MRAS- mediated NS, leading to a better overall understanding of NS and how mutations in disease- susceptibility genes lead to disease.
