What triplet repeat is expanded in fragile X syndrome?
Abstract. Fragile X syndrome, a common form of inherited mental retardation, is mainly caused by massive expansion of CGG triplet repeats located in the 5′-untranslated region of the fragile X mental retardation-1 (FMR1) gene.
Is fragile X syndrome a triplet repeat syndrome or disease?
The child has the “full mutation.” An example of a trinucleotide repeat disorder is fragile X syndrome.
How many CGG repeats does fragile X have?
Individuals with Fragile X Syndrome have over 200 CGG repeats. Males with over 200 repeats are almost always affected.
How do CGG repeats lead to fragile X syndrome?
People with different numbers of CGG repeats have different risks of developing fragile X-associated disorders and of having children with FXS. A female has two copies of the FMR1 gene, one on each of her two X chromosomes. The number of CGG repeats on each copy of the FMR1 gene is usually different.
What causes triplet repeat expansion?
Mechanism. Triplet expansion is caused by slippage during DNA replication or during DNA repair synthesis. Because the tandem repeats have identical sequence to one another, base pairing between two DNA strands can take place at multiple points along the sequence.
What is triplet repeat mutation?
The mutation, referred to as “trinucleotide repeat (TNR) expansion,” occurs when the number of triplets present in a mutated gene is greater than the number found in a normal gene [1–3]. Additionally, the number of triplets in the disease gene continues to increase as the disease gene is inherited (Fig.
What causes FXS?
FXS is caused by changes in a gene that scientists called FMR1 gene when it was first discovered. The FMR1 gene usually makes a protein called FMRP. FMRP is needed for brain development. People who have FXS do not make this protein.
Is FXS dominant or recessive?
Fragile X syndrome is considered to be an X-linked dominant condition with variable expressivity and reduced penetrance. [5] However, due to X-inactivation in females and genetic anticipation, the inheritance of FXS does not follow standard X-linked dominant inheritance.
What is CGG triplet?
CGG triplet repeat-binding protein 1 (CGGBP1) is a repetitive DNA-binding transcription regulator with target sites at CpG-rich sequences such as CGG repeats and Alu-SINEs and L1-LINEs. . CGGBP1 mitigates cytosine methylation at repetitive DNA sequences. . It also functions in DNA damage/repair and telomere metabolism.
What causes trinucleotide repeat disorders?
Trinucleotide repeat disorders are caused due to an abnormal number of triplet repeat sequences either in the coding or the non-coding regions and are a result of either maternal or paternal transmission.
How is Huntington’s disease diagnosed?
To make a diagnosis of Huntington’s disease (HD), a genetic test, using a blood sample, will be performed. This test is usually combined with a complete medical history and other neurological and laboratory tests. For individuals who are at risk of carrying the HD gene, testing can be performed before symptoms occur.
Why do triplet repeats expand?
The mutation, referred to as “trinucleotide repeat (TNR) expansion,” occurs when the number of triplets present in a mutated gene is greater than the number found in a normal gene [1–3]. Additionally, the number of triplets in the disease gene continues to increase as the disease gene is inherited (Fig. 1a).