The expansion of DNA repeats in the FMR1 gene, which leads to fragile X syndrome, is independent of changes that stop production of the resulting FMRP protein, according to an analysis of a family with fragile X.
The study, “Methylated premutation of the FMR1 gene in three sisters: correlating CGG expansion and epigenetic inactivation,” was published in the European Journal of Human Genetics.
People with fragile X have an expansion of the CGG repeat within the FMR1 gene in the X chromosome. (Of note, C refers to cytosine and G to guanine, which comprise two of the four building blocks of DNA).
A full mutation, as seen in people with fragile X, typically has more than 200 CGG repeats. Along with the extra repeats, the addition of methyl groups (called methylation) to these DNA sections stops production of the FMRP protein, which is important for nerve cell protein synthesis. This loss-of-function of the FMR1 gene, leading to loss of FMRP protein, causes fragile X.
The full mutation originates from a change in FMR1 known as a premutation, which consists of 56 to 200 CGG repeats in the gene copy that’s inherited from the mother. Unlike the full mutation, these repeats are not methylated and actively produce the FMRP protein, although they are associated with related disorders such as fragile X‑associated tremor/ataxia syndrome.
These observations support the hypothesis that CGG expansions of more than 200 are coupled to methylation, which then silences FMR1 gene activity.
However, studies have described healthy males from families with fragile X whose CGG expansions were above 200 repeats but not methylated, and thereby their FMR1 gene was active. These observations suggest that CGG expansion and DNA methylation are independent processes in fragile X.
In support, researchers in Italy identified a family with three sisters carrying the premutation (with around 140 CGG repeats), but, in contrast to healthy carriers, their FRM1 gene was completely methylated. All three had mild cognitive impairments.
The family consisted of six siblings: four females and two males. Among the three females carrying the premutation, two of them each had a daughter who carried the full, methylated FRM1 mutation, with one daughter having two children herself, one of whom (a male) also carried the full mutation.
Cognitive evaluations of all three sisters carrying the premutation found lower-than-normal scores measuring verbal, performance, and global intelligence quotients (IQ), as assessed by the Wechsler Adult Intelligence Scale-Revised.
“The presence of a methylated PM allele [premutation gene copy] is the likely cause of these findings,” the researchers wrote, adding that the three sisters had a low level of education.
Additional genetic analysis revealed that the CGG expansion was most likely transmitted by the father of the three sisters and remained relatively stable around 140 repeats.
“The study of these atypical individuals demonstrates that the size of the CGG expansion is not as tightly coupled to methylation as previously thought,” the researchers said.