Elisa Bettella

Fragile X syndrome (FXS) is a genetic neurodevelopmental disorder that causes a range of developmental problems including cognitive and behavioral impairment and learning disabilities. FXS is caused by full mutations (FM) of the FMR1 gene expansions to over 200 repeats, with hypermethylation of the cytosine–guanine–guanine (CGG) tandem repeated region in its promoter, resulting in transcriptional silencing and loss of gene function. Female carriers of FM are typically less impaired than males. The Activation Ratio (AR), the fraction of the normal allele carried on the active X chromosome, is thought to play a crucial modifying role in defining phenotype severity. Here, we compare the cognitive, neuropsychological, adaptive, and behavioral profile of two FXS girls (10 and 11 years old) with seemingly identical FMR1 genotypic profile of FM but distinctive AR levels (70% vs. 30%). A multi-method protocol, combining molecular pathophysiology and phenotypical measures, parent reports, lab-based tasks, gait analyses, and eye-tracking was employed. Results showed that lower AR corresponds to worse performances in most (cognitive, neuropsychological, adaptive, behavioral, social, mathematical skills), but not all the considered areas (i.e., time perception and gait analysis). These observations underscore the importance of AR as a phenotypic modifying parameter in females affected with FXS.

Fragile X syndrome (FXS) is a genetic condition caused by the inheritance of alleles with >200 CGG repeats in the 5′ UTR of the fragile X messenger ribonucleoprotein 1 (FMR1) gene. These full mutation (FM) alleles are associated with DNA methylation and gene silencing, which result in intellectual disabilities, developmental delays, and social and behavioral issues. Mosaicism for both the size of the CGG repeat tract and the extent of its methylation is commonly observed in individuals with the FM. Mosaicism has also been reported in carriers of premutation (PM) alleles, which have 55–200 CGG repeats. PM alleles confer risk for the fragile X premutation-associated conditions (FXPAC), including FXTAS, FXPOI, and FXAND, conditions thought to be due to the toxic consequences of transcripts containing large CGG-tracts. Unmethylated FM (UFM) alleles are transcriptionally and translationally active. Thus, they produce transcripts with toxic effects. These transcripts do produce some FMRP, the encoded product of the FMR1 gene, albeit with reduced translational efficiency. As a result, mosaicism can result in a complex clinical presentation. Here, we review the concept of mosaicism in both FXS and in PM carriers, including its potential clinical significance.

WAC (WW Domain Containing Adaptor With Coiled-Coil) mutations have been reported in only 20 individuals presenting a neurodevelopmental disorder characterized by intellectual disability, neonatal hypotonia, behavioral problems, and mildly dysmorphic features. Using targeted deep sequencing, we screened a cohort of 630 individuals with variable degrees of intellectual disability and identified five WAC rare variants: two variants were inherited from healthy parents; two previously reported de novo mutations, c.1661_1664del (p.Ser554*) and c.374C>A (p.Ser125*); and a novel c.381+2T>C variant causing the skipping of exon 4 of the gene, inherited from a reportedly asymptomatic father with somatic mosaicism. A phenotypic evaluation of this individual evidenced areas of cognitive and behavioral deficits. The patient carrying the novel splicing mutation had a clinical history of encephalopathy related to status epilepticus during slow sleep (ESES), recently reported in another WAC individual. This first report of a WAC somatic mosaic remarks the contribution of mosaicism in the etiology of neurodevelopmental and neuropsychiatric disorders. We summarized the clinical data of reported individuals with WAC pathogenic mutations, which together with our findings, allowed for the expansion of the phenotypic spectrum of WAC-related disorders.

The PCDH19 gene encodes protocadherin-19, a transmembrane protein with six cadherin (EC) domains, containing adhesive interfaces likely to be involved in neuronal connection. Over a hundred mostly private mutations have been identified in girls with epilepsy, with or without intellectual disability (ID). Furthermore, transmitting hemizygous males are devoid of seizures or ID, making it difficult to establish the pathogenic nature of newly identified variants. Here, we describe an integrated approach to evaluate the pathogenicity of four novel PCDH19 mutations. Segregation analysis has been complemented with an in silico analysis of mutation effects at the protein level. Using sequence information, we compared different computational prediction methods. We used homology modeling to build structural models of two PCDH19 EC-domains, and compared wild-type and mutant models to identify differences in residue interactions or biochemical properties of the model surfaces. Our analysis suggests different molecular effects of the novel mutations in exerting their pathogenic role. Two of them interfere with or alter functional residues predicted to mediate ligand or protein binding, one alters the EC-domain folding stability; the frame-shift mutation produces a truncated protein lacking the intracellular domain. Interestingly, the girl carrying the putative loss of function mutation presents the most severe phenotype.