ASFMR1 splice variant: A predictor of fragile X-associated tremor/ataxia syndrome

摘要

ObjectiveTo explore the association of a splice variant of the antisense fragile X mental retardation 1 (ASFMR1) gene, loss of fragile X mental retardation 1 (FMR1) AGG interspersions and FMR1 CGG repeat size with manifestation, and severity of clinical symptoms of fragile X-associated tremor/ataxia syndrome (FXTAS).MethodsPremutation carriers (PMCs) with FXTAS, without FXTAS, and normal controls (NCs) had a neurologic evaluation and collection of skin and blood samples. Expression of ASFMR1 transcript/splice variant 2 (ASFMR1-TV2), nonspliced ASFMR1, total ASFMR1, and FMR1 messenger RNA were quantified and compared using analysis of variance. Least absolute shrinkage and selection operator (LASSO) logistic regression and receiver operating characteristic analyses were performed.ResultsPremutation men and women both with and without FXTAS had higher ASFMR1-TV2 levels compared with NC men and women (n = 135,135, p ASFMR1-TV2 had good discriminating power for FXTAS compared with NCs but not for FXTAS from PMC. After adjusting for age, loss of AGG, larger CGG repeat size (in men), and elevated ASFMR1-TV2 level (in women) were strongly associated with FXTAS compared with NC and PMC (combined).ConclusionsThis study found elevated levels of ASFMR1-TV2 and loss of AGG interruptions in both men and women with FXTAS. Future studies will be needed to determine whether these variables can provide useful diagnostic or predictive information.Fragile X-associated tremor/ataxia syndrome (FXTAS) is an inherited neurodegenerative disorder in adults older than 50 years because of 55–200 (permutation range) CGG repeat expansion in the 5′ untranslated (UTR) region fragile X mental retardation 1 (FMR1) gene causing adult-onset ataxia.1 Primary features include progressive action tremor and cerebellar ataxia, whereas associated findings include parkinsonism, neuropathy, cognitive decline, and dysautonomia.2 The CGG repeat in the 5′ UTR is unstable and its repeat length in the normal population ranges from 6 to 55 repeats.3,4 More than 200 repeats (full mutation) is associated with methylation and transcriptional silencing of FMR1 and consequent absence or deficiency of fragile X mental retardation protein, resulting in fragile X syndrome5 (FXS). The estimated prevalence of the FMR1 premutation is approximately 1/430 males and 1/209 females.6 The molecular mechanism of FXTAS includes elevated levels of the expanded CGG repeat–containing messenger RNA (mRNA) transcript and abnormal repeat-associated non–AUG-initiated (RAN) translation products that lead to cellular toxicity and neuronal intranuclear inclusions (including DNA damage response proteins).7,–9Although there is a relationship in a large group analysis between the age at onset/motor severity of FXTAS and CGG repeat size10 with increasing risk and severity at higher repeat lengths, the CGG repeat size alone does not fully explain risk. Women have a protective normal X allele, and hence, the propensity for developing FXTAS is different between men and women. In carrier women, a higher activation ratio (AR) (percentage of cells expressing the normal FMR1 gene) may lower the presence and severity of motor signs of FXTAS.10,–12 It is probable, however, that there are additional secondary genes, medical conditions, and environmental factors that affect the effects of the premutation expansion and contribute to the risk of developing symptoms.A second gene, antisense FMR1 (ASFMR1) overlaps a portion of the FMR1 gene including the CGG repeat sequence, is transcribed in the reverse direction from FMR1 with 5 splice variants of unclear exact clinical impact.13 One of the variants previously described13 ASFMR1 splice variant 2 (ASFMR1 transcript variant 2 [ASFMR1-TV2]) was further explored in this study. Elevated levels of the expanded GCC repeat–containing ASFMR1 transcript are observed in premutation carriers (PMCs),13 and small unpublished studies have suggested that certain splice variants of the ASFMR1 transcript are associated with the presence of neurologic symptoms in PMC men with FXTAS. Within the CGG repeat element of the FMR1 gene, there are AGG trinucleotide interruptions (typically separated by 9–11 CGG repeats), which are known to disrupt the otherwise pure CGG repeat motif. Normal FMR1 alleles typically possess 2 or 3 AGG interruptions; premutation alleles generally possess 2 or less interruptions, whereas larger premutation alleles tend to have fewer AGG interruptions. The loss of AGG interruptions is believed to increase the probability that large normal and small premutation size alleles will be unstable on transmission and expand to a full mutation allele on transmission from small or moderate size alleles.14 The rationale for investigating AGG interspersions is that pilot data from our group show that a lack of AGG interspersions i