Contraction of a Maternal Fragile X Mental Retardation 1 Premutation Allele

Abstract

Fragile X syndrome (FXS) is mainly caused by an expansion of a cytosine-guanine-guanine (CGG) trinucleotide repeat (greater than 200) in the 5 untranslated region of the fragile X mental retardation 1 (FMR1) gene resulting in gene silencing, methylation, and absence of FMRP. FXS involves a wide spectrum of phenotypes including intellectual disabilities, autism spectrum disorder (ASD), maladaptive behaviors or symptoms, social anxiety, mood disorders and physical features. Carriers of premutation alleles (55 - 200 CGG repeats) can manifest with neurodevelopmental phenotypes and are at risk for the neurodegenerative fragile X-associated tremor/ataxia syndrome (FXTAS) and about 20% of women with a premutation develop fragile X-associated primary ovarian insufficiency (FXPOI). These phenotypes are believed to be caused by a distinct molecular mechanism involving increased FMR1 mRNA production and toxicity. Premutation alleles are unstable and depending on the CGG length and the presence of AGG interruptions have the potential to expand to larger premutation or full mutation alleles during transmission from mothers to their children. The contraction of a CGG repeat allele, although rare, has been observed. Here, we report the case of a three-generation family, who was identified as part of a pilot study of newborn screening for FXS. Specifically, the maternal grandmother carried a premutation allele (109 CGG repeats), which contracted to an intermediate size allele (52 CGG repeats) in her son, the father of a newborn girl (proband). This allele expanded to a premutation allele of 59 CGG repeats in the proband newborn. A premutation allele of 61 CGG repeats was detected in the probands sister who demonstrated social phobia but normal cognition. The proband newborn showed some fragile X associated physical stigmata but normal cognition. Intriguingly, all FMR1 CGG expanded allele showed no AGG interruptions. This case shows a remarkable instability of premutation alleles throughout the three generations likely arising from different mechanisms.




J Med Cases. 2015;6(12):547-553
doi: https://doi.org/10.14740/jmc2301w