A De Novo Truncating Mutation of the Gene Encoding Nucleolin in an ASD Individual Disrupts Its Nucleolar Localization

Background: Nucleolin (NCL / C23; OMIM: 164035) is a major nucleolar protein that plays a critical role in multiple processes including ribosome assembly and maturation, chromatin decondensation, and pre-rRNA transcription. Due to its diverse functions, nucleolin role has frequently been implicated in pathological processes, including cancer and viral infection.

Objectives: To study the functional effect of a de novo frameshifting indel mutation (Gly664Glufs*70), identified through whole exome sequencing in an autism spectrum disorder trio.

Methods: Nucleolar localization and real-time protein dynamics studies using constructs encoding either wild type human nucleolin or a mutant nucleolin with the same C-terminal sequence predicted for the autism proband into mammalian cells.

Results: Using co-localization with nucleophosmin (NPM; B23) protein, we have shown that the mutant nucleolin leads to mislocalization of the NCL protein from the nucleolus to the nucleoplasm. Moreover, a construct with a nonsense mutation at the same residue, p.Gly664*, shows a very similar effect on the location of the NCL protein, thus confirming the presence of the nucleolar location signal of NCL protein in this region. Real time fluorescent recovery experiments show significant changes in the kinetics and mobility of mutant NCL protein in the nucleoplasm of HEK293Tcells.

Conclusions: Several other studies report de novo mutations in NCL in ASD or neurodevelopmental disorders. The altered mislocalization and dynamics of mutant NCL (p.G664Glufs*70/ p.G664*) may have relevance to the etiopathlogy of NCL-related neurodevelopmental phenotypes.