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Encyclopedia > Trinucleotide repeat disorders

Trinucleotide repeat disorders (also known as trinucleotide repeat expansion disorders, expansion disorders or codon reiteration disorders) are due to stretches of DNA in a gene that contain the same trinucleotide sequence repeated many times. These repeats are a subset of unstable microsatellite repeats that occur throughout all genomic sequences. If the repeat is present in a gene, an expansion of the repeat results in a defective gene product and often disease. Anita Harding was the first to identify the correlation between trinucleotide repeat expansion and diseases causing neurological dysfunction. At present there are 14 documented trinucleotide repeat disorders that affect humans. A nucleotide is a chemical compound that consists of a heterocyclic base, a sugar, and one or more phosphate groups. ... Microsatellites, or Simple Sequence Repeats (SSRs), are polymorphic loci present in nuclear DNA that consist of repeating units of 1-4 base pairs in length [1]. They are typically neutral, co-dominant and are used as molecular markers which have wide-ranging applications in the field of genetics, including kinship... In biology the genome of an organism is the whole hereditary information of an organism that is encoded in the DNA (or, for some viruses, RNA). ... For a non-technical introduction to the topic, see Introduction to Genetics. ... Anita Harding (September 17, 1952 - September 11, 1995) was a British neurologist. ...


Eight of these disorders have the same repeated codon, CAG, that codes for glutamine (Q). These diseases are commonly referred to as polyglutamine ( or PolyQ) diseases. The other six disorders do not have similar repeats and are classified as non-polyglutamine diseases. Glutamine is one of the 20 amino acids encoded by the standard genetic code. ...


A common symptom of Polyq diseases is characterized by a progressive degeneration of nerve cells usually affecting people later in life. Although these diseases share the same repeated codon (CAG) and some symptoms, the repeats for the different polyglutamine diseases occur on different chromosomes. The non-Polyq diseases do not share any specific symptoms and are unlike the Polyq diseases. Neurons (also called nerve cells) are the primary cells of the nervous system. ... This article is about the biological chromosome. ...


Trinucleotide repeat disorders generally show genetic anticipation, where their severity increases with each successive generation that inherits them. In medicine, anticipation is the severity of a genetic disorder increasing with each generation. ...


Trinucleotide repeat disorders are the result of extensive duplication of a single codon. In fact, the cause is trinucleotide expansion up to a repeat number above a certain threshold level.

Contents

Why three nucleotides?

An interesting question is why three nucleotides are expanded, rather than two or four or some other number. Dinucleotide repeats are a common feature of the genome in general, as are larger repeats (e.g. VNTRs - Variable Number Tandem Repeats). One possibility is that repeats that are not a multiple of three would not be viable. Trinucleotide repeat expansions tend to be near coding regions of the genome, and therefore repeats that are not multiples of three could cause frameshift mutations that would be deadly. VNTRs, or Variable Number of Tandem Repeats are short sequences of chromosomal DNA that are repeated many times. ... A frameshift mutation (also called a frameshift or a framing error) is a genetic mutation that inserts or deletes a number of nucleotides that is not evenly divisible by three from a DNA sequence. ...


Polyglutamine (PolyQ) Diseases

  • DRPLA (Dentatorubropallidoluysian atrophy)
Caused by the DRPLA gene on chromosome 12. The normal DRPLA allele has between 6 and 35 copies of CAG, however, in people with the disorder the allele has between 49 and 88 copies.
Caused by the huntington gene. The normal huntington allele has between 10 and 35 copies of CAG, however, in people with the disorder the allele has more than 35 copies.
Caused by the Androgen receptor (AR) gene on the X chromosome. The normal AR allele has between 9 and 36 copies of CAG, however, in people with the disorder the allele has between 38 and 62 copies.

Kennedys disease (KD) or X-linked spinal-bulbar muscular atrophy (SBMA) is a neuromuscular disease associated with mutations of the androgen receptor (AR). ... Kennedys disease (KD) or X-linked spinal-bulbar muscular atrophy (SBMA) is a neuromuscular disease associated with mutations of the androgen receptor (AR). ... The androgen receptor is an intracellular steroid receptor that specifically binds testosterone and dihydrotestosterone. ... ‹ The template below (Expand) is being considered for deletion. ... Spinocerebellar ataxia (SCA) is a genetic disease with multiple types, each of which could be considered a disease in its own right. ... Spinocerebellar ataxia (SCA) is a genetic disease with multiple types, each of which could be considered a disease in its own right. ... Spinocerebellar ataxia (SCA) is a genetic disease with multiple types, each of which could be considered a disease in its own right. ... Spinocerebellar ataxia (SCA) is a genetic disease with multiple types, each of which could be considered a disease in its own right. ... Spinocerebellar ataxia (SCA) is a genetic disease with multiple types, each of which could be considered a disease in its own right. ... Spinocerebellar ataxia (SCA) is a genetic disease with multiple types, each of which could be considered a disease in its own right. ...

Non-Polyglutamine Diseases

Caused by the FMR1 gene on the X-chromosome. The normal FMR1 allele has between 6 and 53 copies of CGG, however, in people with the disorder the allele has over 230 copies.
  • FRAXE (Fragile XE mental retardation)
Caused by the FMR2 gene on the X-chromosome. The normal FMR2 allele has between 6 and 35 copies of GCC, however, in people with the disorder the allele has over 200 copies.
Caused by the X25 (frataxin) gene. The normal X25 allele has between 7 and 34 copies of GAA, however, in people with the disorder the allele has over 100 copies.
Caused by the DMPK gene. The normal DMPK allele has between 5 and 37 copies of CTG, however, in people with the disorder the allele has over 50 copies.
Caused by the SCA8 gene. The normal SCA8 allele has between 16 and 37 copies of CTG, however, in people with the disorder the allele has between 110 and 250 copies.
Caused by the SCA12 gene. The normal SCA8 allele has between 7 and 28 copies of CAG (in the 3' UTR and therefore does not translate to glutamine), however, in people with the disorder the allele has between 66 and 78 copies.

Fragile X syndrome, also known as the MikyD syndrome, is a syndrome of X-linked mental retardation. ... Trinucleotide repeat disorders (also known as trinucleotide repeat expansion disorders, expansion disorders or codon reiteration disorders) are due to stretches of DNA in a gene that contain the same trinucleotide sequence repeated many times. ... Friedreichs ataxia is a rare autosomal recessive disorder caused by a mutation in Gene X25 that codes for frataxin, located on chromosome 9. ... Frataxin is a small protein, localized to the mitochondrion. ... Myotonic dystrophy (DM) is a chronic, slowly progressing, highly variable inherited multisystemic disease that can manifest at any age from birth to old age. ... Spinocerebellar ataxia (SCA) is a genetic disease with multiple types, each of which could be considered a disease in its own right. ... Spinocerebellar ataxia (SCA) is a genetic disease with multiple types, each of which could be considered a disease in its own right. ... In genetics, the 3 UTR (read as 3 prime untranslated region) is a particular section of messenger RNA (mRNA). ...

Trinucleotide repeat expansion

Trinucleotide repeat expansion, also known as triplet repeat expansion, is the DNA mutation responsible for causing any type of disorder categorized as a trinucleotide repeat disorder. Robert I. Richards and Grant R. Sutherland called these phenomena, in the framework of dynamical genetics, dynamic mutations. Triplet expansion is caused by slippage during DNA replication. Due to the repetitive nature of the DNA sequence in these regions 'loop out' structures may form during DNA replication while maintaining completmentary base paring between the parent strand and daughter strand being snythesised. If the loop out structure is formed from sequence on the daughter strand this will result in an increase in the number of repeats. However if the loop out structure is formed on the parent strand a decrease in the number of repeats occurs. It appears that expansion of these repeats is more common than reduction. Generally the larger the expansion the more likely they are to cause disease or increase the severity of disease. This property results in the characteristic of anticipation seen in trinucleotide repeat disorders. Anticipation describes the tendency of age of onset to decrease and severity of sympotoms to increase through successive generations of an affected family due to the expansion of these repeats. The structure of part of a DNA double helix Deoxyribonucleic acid, or DNA, is a nucleic acid molecule that contains the genetic instructions used in the development and functioning of all known living organisms. ... It has been suggested that mutant be merged into this article or section. ... The term disease refers to an abnormal condition of an organism that impairs function. ... Dynamical genetics concerns the study and the interpretation of those phenomena in which physiological enzymatic protein complexes alter the DNA, in a more or less sophisticated way. ...


See also

  • HOPES: Huntington's Outreach Project for Education, at Stanford

These repeats make a secondary structure which harms the functionality of protein.


 
 

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