CGG allele size somatic mosaicism and methylation inFMR1premutation alleles

Abstract

Background Greater than 200 CGG repeats in the 5′UTR of the FMR1 gene leads to epigenetic silencing and lack of the FMR1 protein, causing Fragile X Syndrome. Individuals carriers of a premutation (PM) allele with 55–200 CGG repeats are typically unmethylated and can present with clinical features defined as FMR1 associated conditions. Methods Blood samples from 17 male PM carriers were assessed clinically and molecularly by Southern Blot, Western Blot, PCR and QRT-PCR. Blood and brain tissue from additional 18 PM males were also similarly examined. Continuous outcomes were modeled using linear regression and binary outcomes were modeled using logistic regression. Results Methylated alleles were detected in different fractions of blood cells in all PM cases (n= 17). CGG repeat numbers correlated with percent of methylation and mRNA levels and, especially in the upper PM range, with greater number of clinical involvements. Inter/intra- tissue somatic instability and differences in percent methylation were observed between blood and fibroblasts (n=4) and also observed between blood and different brain regions in three of the 18 premutation cases examined. CGG repeat lengths in lymphocytes remained unchanged over a period of time ranging from 2–6 years, three cases for whom multiple samples were available. Conclusion In addition to CGG size instability, individuals with a PM expanded alleles can exhibit methylation and display more clinical features likely due to RNA toxicity and/or FMR1 silencing. The observed association between CGG repeat length and percent of methylation with the severity of the clinical phenotypes underscores the potential value of methylation in affected PM to further understand penetrance, inform diagnosis and to expand treatment options.

Psychosis and catatonia in fragile X: Case report and literature review

“…Moreover, Aziz et al (32) reported that FMR1 expanded CGG repeat of premutation and gray zone alleles (45)(46)(47)(48)(49)(50)(51)(52)(53)(54) repeats), may demonstrate some clinical features of fragile X syndrome (FXS) in those who presented clinically. It is uncertain if the gray zone allele has FMRP deficits but the premutation demonstrates FMRP deficits that increase according to the CGG repeat number increases (33).…”

Clinical and molecular implications of mosaicism in FMR1 full mutations

“…Overexpression of expanded CGG-repeat alleles is currently believed to lead to “toxicity” of the FMR1 mRNA (Hagerman, 2013; Pretto et al, 2014). However, because expanded CGG-repeat alleles are translated with reduced efficiency (Kenneson et al, 2001; Primerano et al, 2002; Brouwer et al, 2007; Ludwig et al, 2011) FMRP expression is negatively correlated with the CGG repeat number particularly in the upper premutation and full size range (Ludwig et al, 2014; Pretto et al, 2014). Thus, FMR1 and FMRP expression in a mosaic background could add complexity to the clinical presentation and widen the spectrum of involvement in FXS mosaics.…”

Recent advances in the study of somatic mosaicism and diseases other than cancer

“…One can now say that many, if not most, of these children reflect the consequences of new mutations, both germ-line and somatic. Again, NGS is increasing the discovery rate for mutations and pre-mutations, for example, fragile X pre-mutation alleles [35].…”