The Fragile X Mental Retardation 1 (FMR1) gene, located on the long (q) arm of the X chromosome at position 27.3, contains 5–44 cytosine–guanine–guanine (CGG) triplets. The FMR1 gene is responsible for the synthesis of the FMR1 protein (FMRP), which is present in many tissues, including brain, testicular and ovarian tissues. FMRP is a selective, mRNA–binding protein that regulates the translation of hundreds of mRNAs into appropriate proteins that are important for normal cell function. FMRP plays a role in embryonic brain development, which involves differentiation and migration of neurons and glial cells. In addition, this protein regulates synapse function by controlling the relationship between excitatory (glutamate) and inhibitory (GABA) neurotransmitters, as well as the presynaptic release of other neurotransmitters. In the absence of FMRP, there is a deficit of GABA–A receptor activity and enhanced activity of metabotropic glutamate receptors 5 (mGluR5), which in turn leads to impaired function of the dopaminergic and cholinergic pathways in the central nervous system. At the neuron level, the most significant role of FMRP is expressed at the dendrite level. In addition, FMRP is important for the regulation of the endocannabinoid system and the proper functioning of ion channels such as Na–dependent K channels, Ca–dependent K channels (specifically BK channels) and N–type Ca channels at the nerve cell level.
Mutation of the FMR1 gene
Mutations of the FMR1 gene are expressed as an increase in the number of CGG triplets repeats within the gene (normal is 5–44 CGG triplets repeats).
The number of repeats from 45 to 54 is referred to as the "gray zone" and is not clinically defined.
Premutation (PM) of the FMR1 gene: number of CGG triplets’ repeats from 55 to 200.
Full mutation (FM) of the FMR1 gene: the number of CGG triplets’ repeats is greater than 200.
It is important to note that the pathophysiology mechanisms of pre– and full mutations are different, leading to the development of a different clinical presentations in individuals with these two types of mutations. While FM manifests as fragile X syndrome, PM of the FMR1 gene remain unrecognized most commonly until adulthood or older.
Fragile X syndrome (FXS)
Fragile X Syndrome (FXS) is the most common cause of inherited intellectual disability with prevalence rates estimated at 1:5.000 males and 1:8.000 females. An expansion of >200 CGG repeats in the FMR1 gene gives rise to the full mutation (FM) and FXS. The full mutation results in transcriptional silencing of the FMR1 gene with subsequent reduced or absent fragile X mental retardation protein (FMRP), an RNA binding protein involved in the maturation and elimination of synapses. FMRP is important to dendritic maturity and synaptic plasticity, and reduced levels therefore lead to intellectual impairment and FXS.
Physical features have been described but are often nonspecific, making diagnostic testing based on alterations found in the FMR1 gene essential for the diagnosis of FXS. Common physical and medical features in FXS include increased risk for chronic otitis media, esotropia, hyperextensible finger joints, long face, prominent ears, high arched palate, low muscle tone, seizures (occurring in 16% of patients with FXS) and macroorchidism with puberty. As FXS is an X–linked disorder, the symptoms manifest markedly in males, who commonly present with moderate to severe cognitive impairment. Females have two X chromosomes with variable activation ratios and are thus generally less affected, presenting with a spectrum of impairments from mild learning difficulties to intellectual disability. The behavioral phenotype involves poor eye contact, excessive shyness, anxiety, hand flapping, hand biting, aggression, tactile defensiveness, attention deficits, hyperactivity, impulsivity, hyperarousal to sensory stimuli, and autism.
Autism and Fragile X syndrome
Fragile X syndrome is the most commonly known single gene cause of Autism Spectrum Disorders (approximately 60% individuals with FXS are diagnosed with ASD). Whereas autism is a behavioural diagnosis, Fragile X Syndrome is a medical, or more accurately, a genetic diagnosis. When associated with Fragile X Syndrome, autism is caused by the genetic change or mutation in the Fragile X gene. The genetic diagnosis of Fragile X syndrome is possible using FMR1 gene testing.
Disorders associated with the premutation (PM) of the FMR1 gene
In the general population, the rate of PM is found in approximately 1:130–1:250 females and in 1:250–1:810 males, which is 10 times higher than the rate of the FM. It is noteworthy that the prevalence of premutation is highest in Columbia and Israel (1:100 females) and lowest in Japan (1:1,674 females). Clinical manifestations of the premutation include:
Fragile–X–associated tremor/ataxia syndrome (FXTAS)
FXTAS as a late–onset neurodegenerative disorder that affects some (the majority of ageing male premutation carriers but in fewer than 20% of ageing women with the premutation) but not all carriers of small, noncoding CGG–repeat expansions (55–200 repeats; premutation) within the fragile X gene (FMR1). Principal features of FXTAS include intention tremor, cerebellar ataxia, Parkinsonism, memory and executive function deficits, autonomic dysfunction, brain atrophy with white matter disease, and cognitive decline.
Fragile–X–associated primary ovarian insuﬃciency (FXPOI) in adult females:
Fragile X associated primary ovarian insufficiency (FXPOI) is a chronic disorder characterized by oligo/amenorrhea and hypergonadotropic hypogonadism before age of 40 years. Approximately 20% of women with PM will develop FXPOI.
Fragile X–associated Neuropsychiatric Disorders (FXAND)
Neuropsychiatric disorders are the most common problems associated with the premutation, and they affect approximately 50% of individuals with 55 to 200 CGG repeats in the FMR1 gene. Neuropsychiatric disorders in children with the premutation include anxiety, ADHD, social deﬁcits, or autism spectrum disorders (ASD). In adults with the premutation, anxiety and depression are the most common problems, although obsessive compulsive disorder, ADHD, and substance abuse are also common. These problems are often exacerbated by chronic fatigue, chronic pain, ﬁbromyalgia, autoimmune disorders and sleep problems, which are also associated with the premutation.
Diagnosis of the FMR1 gene mutation
The AmplideX® PCR/CE FMR1 Kit is an in vitro diagnostic device for professional use in clinical laboratories to amplify and detect the cytosine–guanine–guanine (CGG) repeat region in the 5’–untranslated region of the fragile X mental retardation–1 (FMR1) gene. The device is intended as an aid in diagnosis of fragile X syndrome and fragile X associated disorders, e.g. tremor and ataxia syndrome (FX–TAS) and primary ovarian insufficiency (FX–POI), through determination of CGG repeat length up to 200 CGG and detection of alleles greater than 200 CGG. The test consists of a polymerase chain reaction (PCR) of genomic DNA purified from whole blood or buccal cells, followed by fragment sizing on a general laboratory–validated genetic analyzer or capillary electrophoresis platform and conversion of product size to the number of CGG repeats.