Novel de novo CDC42 variants cause variable functional alterations and heterogeneous spectrum of neurodevelopmental and immune-hematologic rare diseases

CDC42 (Cell Division Cycle 42) is a small GTPase of the RAS super family regulating key developmental processes. We have recently characterized different CDC42 missense mutations associated with a broad spectrum of immune-haematological and neurodevelopmental disorders, which include RASopathies-related phenotypes. Here, we report the identification and molecular characterization of six additional CDC42 variants (p.K16R, p.Y23H, p.P34L, p.D118G, p.G164R, and p.D170N) associated with neurodevelopmental and immune-haematological conditions. In particular, patients present with various degree of cardiac defects, neurodevelopmental delay, facial dysmorphisms, and recurrent infections. CDC42 mutants variably affect protein expression and localization (Fig. 1) , GTPase activity (Fig. 2A), effector binding (i.e. RHOGDI (Fig. 2B), IQGAP1 (Fig. 3A), N-WASP (Fig. 3B), and, and RAS-mitogen-activated protein kinase (MAPK) pathway (Fig. 4). The comparative functional analysis of the CDC42 variants so far described indicates that only the IQGAP1-binding defective CDC42 mutants are associated with the immune-hematologic phenotype, suggesting a common pathogenic mechanism for these variants. Moreover, the majority of CDC42 variants significantly increases MAPK activation, indicating a role of the RAS-MAPK pathway in the pathogenesis of CDC42-associated disorders. Finally, our study expands the spectrum of CDC42 pathogenic variants and confirms the relevance of functional validation of unclassified variants to assess their possible role in disease pathogenicity.