Analysis of FMR1 gene expression in female premutation carriers using robust segmented linear regression models

Abstract

Fragile X syndrome is caused by the absence or reduction of the fragile X mental retardation protein (FMRP) because FMR1 gene expression is reduced. Alleles with repeat sizes of 55-200 are classified as premutations, and it has been demonstrated that FMR1 expression is elevated in the premutation range. However, the majority of the studies reported were performed in males. We studied FMR1 expression in 100 female fragile X family members from the northern region of Spain using quantitative (fluorescence) real-time polymerase chain reaction. Of these 100 women, 19 had normal alleles, 19 were full mutation carriers, and 62 were premutation carriers. After confirming differences between the three groups of females, and increased levels of the FMR1 transcript among premutation carriers, we found that the relationship between mRNA levels and repeat size is nonlinear. These results were obtained using a novel methodology that, based on the size of the CGG repeats, allows us to find out the most probable threshold from which the relationship between CGG repeat number and mRNA levels changes. Using this approach, a significant positive correlation between CGG repeats and total mRNA levels has been found in the premutation range <100 CGG, but this correlation diminishes from 100 onward. However, when correcting by the X inactivation ratio, mRNA levels increase as the number of CGG repeats increases, and this increase is highly significant over 100 CGG. We suggest that due to skewed X inactivation, mRNA levels tend to normalize in females when the number of CGG repeats increases.

Microdeletions Including FMR1 in Three Female Patients with Intellectual Disability - Further Delineation of the Phenotype and Expression Studies

“…However, our collective findings are consistent with the presence of an intrafamilial effect, with mRNA levels being more similar among female members of the same family than among unrelated females [García-Alegría et al, 2007]. Since in all 3 patients the significant reduction in FMR1 transcript levels compared to their respective mothers appears to be specific for their phenotype, our data may be interpreted as suggesting a family-specific 'threshold' expression level, below which an FXS phenotype may present.…”

“…In females, however, the relationship between FMR1 transcript levels and CGG repeat length is more complex, as it mirrors that observed for males only after accounting for the XCI status [Allen et al, 2004;García-Alegría et al, 2007]. In general, FMR1 transcript levels vary considerably between females [Allen et al, 2004;Schuettler et al, 2011].…”

“…Full-length CGG repeat expansions cause hypermethylation of the FMR1 promoter and the loss of both FMR1 mRNA and protein expression [Feng et al, 1995;Allen et al, 2004]. For patients with alleles within the premutation range (55-200 repeats), FMR1 transcript levels are elevated and increase with increasing repeat length [Tassone et al, 2000;Allen et al, 2004;García-Alegría et al, 2007].…”

“…In general, FMR1 transcript levels vary considerably between females [Allen et al, 2004;Schuettler et al, 2011]. However, there is preliminary evidence suggesting a possible intrafamilial effect, with mRNA levels being more similar among members of the same family than among unrelated females [García-Alegría et al, 2007]. This might, in part, explain the differences in FMR1 mRNA levels observed between unrelated healthy females.…”

Segmented regression, a versatile tool to analyze mRNA levels in relation to DNA copy number aberrations

“…Segmented regression has previously been applied to link CGG repeats to mRNA levels in trinucleotide repeat disorders patients (Garcia-Alegria et al, 2007;Minguez et al, 2009) and gene compactness driven nonmonotonic expression levels in eukaryote cells (Carmel and Koonin, 2009).…”