Recent advances in assays for the fragile X-related disorders

Abstract

The Fragile X-related disorders are a group of 3 clinical conditions resulting from the instability of a CGG-repeat tract at the 5’ end of the FMR1 transcript. Fragile X-associated tremor/ataxia syndrome (FXTAS) and Fragile X-associated primary ovarian insufficiency (FXPOI) are disorders seen in carriers of FMR1 alleles with 55–200 repeats. Female carriers of these premutation (PM) alleles are also at risk of having a child who has an FMR1 allele with >200 repeats. Most of these full mutation (FM) alleles are epigenetically silenced resulting in a deficit of the FMR1 gene product, FMRP. This results in Fragile X Syndrome (FXS), the most common heritable cause of intellectual disability and autism. The diagnosis and study of these disorders is challenging in part because the detection of alleles with large repeat numbers has, until recently, been either time-consuming or unreliable. This problem is compounded by the mosaicism for repeat length and/or DNA methylation that is frequently seen in PM and FM carriers. Furthermore, since AGG interruptions in the repeat tract affect the risk that a FM allele will be maternally transmitted, the ability to accurately detect these interruptions in female PM carriers is an additional challenge that must be met. This review will discuss some of the pros and cons of some recently described assays for these disorders, including those that detect FMRP levels directly, as well as emerging technologies that promise to improve the diagnosis of these conditions and to be useful in both basic and translational research settings.

Modeling Fragile X Syndrome in Drosophila

“…The Fragile X Syndrome (FXS), previously known as Martin-Bell syndrome or marker X syndrome or FRAXA, is the first X-linked intellectual disability (ID) syndrome described involving a DNA alteration and the most frequent heritable monogenic form of ID (reviewed in Penagarikano et al, 2007 ; Santoro et al, 2012 ; Hayward et al, 2017 ). Human FXS patients present severe ID often accompanied by an increase in Autism Spectrum Disorder (ASD) traits and other phenotypes like delayed development, hyperactivity, attention deficit, hypersensitivity to sensorial stimuli, anxiety, aggression, sleep, cardiac disorders, and epileptic seizures (reviewed in Hagerman, 2002 ; Garber et al, 2008 ; Utari et al, 2010 ; Santoro et al, 2012 ; Hagerman et al, 2014 ; Kidd et al, 2014 ; Maurin et al, 2014 ; Schaefer et al, 2015 ; Dahlhaus, 2018 ).…”

“…Through alternative splicing, at least 12 different isoforms of 67–80 kD are produced. The CGG repeats are polymorphic in the population ranging from 5 to 54 repeats in normal individuals to more than 200 (full mutation) in severely affected patients (reviewed in Hayward et al, 2017 ). The repeat expansion results in hypermethylation of the CGG repeat, of a 5′ CpG island, and of flanking promoter sequences causing the reduction or absence of FMR1 expression through an epigenetic mechanism involving FMR1 mRNA (Colak et al, 2014 ).…”

“…Although FMRP mainly acts as a repressor, an activator function has been observed (reviewed in Maurin et al, 2014 ). Many methods have been used to identify FMRP targets (reviewed in Maurin et al, 2014 ; Davis and Broadie, 2017 ; Hayward et al, 2017 ): binding to biotinylated RNAs (Ashley et al, 1993 ), Cross-Link Immuno Precipitation (CLIP) (Darnell et al, 2011 ; Ascano et al, 2012 ; Anderson et al, 2016 ), Systematic Evolution of Ligands by Exponential Enrichment (SELEX) (Chen et al, 2003 ), yeast two-hybrid system (Ma et al, 2016 ), yeast three-hybrid system (Dolzhanskaya et al, 2003 ), and Antibody-Positioned RNA Amplification (APRA) (Miyashiro et al, 2003 ). Many of the identified targets have been involved in autism, other neuronal pathologies or gonadal development and many of them encode synaptic proteins (reviewed in Maurin et al, 2014 ).…”

Repeat Detector: versatile sizing of expanded tandem repeats and identification of interrupted alleles from targeted DNA sequencing

“…Moreover, some of the genetic modifiers of HD implicated in repeat expansion may also modify disease onset in other repeat disorders [18]. Interruptions in SCA1, SCA2, Fragile X syndrome and myotonic dystrophy type 1 are associated with lower repeat instability, delayed symptom onset, and/or modified clinical manifestations [16,17,[27][28][29][19][20][21][22][23][24][25][26].…”

“…Interruptions in SCA1, SCA2, Fragile X syndrome, and myotonic dystrophy type 1 are associated with lower repeat instability, delayed symptom onset, and/or modified clinical manifestations (15,16,(26)(27)(28)(18)(19)(20)(21)(22)(23)(24)(25).…”