Heterozygous De Novo Truncating Mutation of Nucleolin in an ASD Individual Disrupts Its Nucleolar Localization.

Taimoor I Sheikh,Majid Ghadami,Muhammad Ayub,Nasim Vasli,Roksana Sasanfar,Ricardo Harripaul,Susan L Santangelo,John B Vincent

doi:10.3390/genes13010051

Abstract

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 has frequently been implicated in pathological processes, including cancer and viral infection. We recently identified a de novo frameshifting indel mutation of NCL, p.Gly664Glufs*70, through whole-exome sequencing of autism spectrum disorder trios. Through the transfection of constructs encoding either a wild-type human nucleolin or a mutant nucleolin with the same C-terminal sequence predicted for the autism proband, and by using co-localization with the nucleophosmin (NPM; B23) protein, we have shown that the nucleolin mutation 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 a predicted nucleolar location signal in this region of the NCL protein. Real-time fluorescence recovery experiments show significant changes in the kinetics and mobility of mutant NCL protein in the nucleoplasm of HEK293Tcells. Several other studies also report de novo NCL mutations 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 ASD and other neurodevelopmental phenotypes.

Highlights

Nucleolin (C23; OMIM: 164035) is one of the most abundant proteins in the nucleolus, accounting for almost 10% of the total protein content [1,2]
The mutation lies within the penultimate exon of the gene NCL, encoding the nucleolin protein; despite causing a frameshift, this does not result in a premature stop codon, and the nonsense-mediated mRNA decay is not predicted
Gene-disrupting de novo mutations have become a common theme in the genetics of autism spectrum disorder [20]

Summary

Introduction

Nucleolin (C23; OMIM: 164035) is one of the most abundant proteins in the nucleolus, accounting for almost 10% of the total protein content [1,2]. The nucleolus is formed by three components: the granular component (GC), the dense fibrillar component (DFC), and the fibrillar center (FC) [3]. During mitotic cell division (interphase), the nucleolin protein is localized within the nucleolar DFC and GC [5] and close to the kinetochore of chromosomes; from the prometaphase to anaphase stages, it has been observed to be associated with the spindle poles [6]. A depletion of nucleolin protein leads to a disruption in the congression of chromosomes, improper kinetochore attachments, and an effect on the spindle assembly in the HeLa cells [6]. Nucleolin has been reported to have been found in the cytoplasm, such as under heat shock, γ-irradiation, and CPT (camptothecin) treatment [7,8]

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