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Repeated sequences are short or long patterns of nucleic acids that occur in multiple copies throughout the genome. In many organisms, a significant fraction of the genomic DNA is repetitive, with over two-thirds of the sequence consisting of repetitive elements in humans. Some of these repeated sequences are necessary for maintaining important genome structures such as telomeres or centromeres.
Spinocerebellar ataxia (SCA) is a progressive, degenerative, genetic disease with multiple types, each of which could be considered a neurological condition in its own right. An estimated 150,000 people in the United States have a diagnosis of spinocerebellar ataxia at any given time. SCA is hereditary, progressive, degenerative, and often fatal. There is no known effective treatment or cure. SCA can affect anyone of any age. The disease is caused by either a recessive or dominant gene. In many cases people are not aware that they carry a relevant gene until they have children who begin to show signs of having the disorder.
Machado–Joseph disease (MJD), also known as Machado–Joseph Azorean disease, Machado's disease, Joseph's disease or spinocerebellar ataxia type 3 (SCA3), is a rare autosomal dominantly inherited neurodegenerative disease that causes progressive cerebellar ataxia, which results in a lack of muscle control and coordination of the upper and lower extremities. The symptoms are caused by a genetic mutation that results in an expansion of abnormal "CAG" trinucleotide repeats in the ATXN3 gene that results in an abnormal form of the protein ataxin which causes degeneration of cells in the hindbrain. Some symptoms, such as clumsiness and rigidity, make MJD commonly mistaken for drunkenness or Parkinson's disease.
Trinucleotide repeat disorders, also known as microsatellite expansion diseases, are a set of over 50 genetic disorders caused by trinucleotide repeat expansion, a kind of mutation in which repeats of three nucleotides increase in copy numbers until they cross a threshold above which they become unstable. Depending on its location, the unstable trinucleotide repeat may cause defects in a protein encoded by a gene; change the regulation of gene expression; produce a toxic RNA, or lead to chromosome instability. In general, the larger the expansion the faster the onset of disease, and the more severe the disease becomes.
Slipped strand mispairing is a mutation process which occurs during DNA replication. It involves denaturation and displacement of the DNA strands, resulting in mispairing of the complementary bases. Slipped strand mispairing is one explanation for the origin and evolution of repetitive DNA sequences.
A trinucleotide repeat expansion, also known as a triplet repeat expansion, is the DNA mutation responsible for causing any type of disorder categorized as a trinucleotide repeat disorder. These are labelled in dynamical genetics as dynamic mutations. Triplet expansion is caused by slippage during DNA replication, also known as "copy choice" DNA replication. Due to the repetitive nature of the DNA sequence in these regions, 'loop out' structures may form during DNA replication while maintaining complementary base pairing between the parent strand and daughter strand being synthesized. If the loop out structure is formed from the 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. Other proposed mechanisms for expansion and reduction involve the interaction of RNA and DNA molecules.
Ataxin-1 is a DNA-binding protein which in humans is encoded by the ATXN1 gene.
Ataxin 7 (ATXN7) is a protein of the SCA7 gene, which contains 892 amino acids with an expandable poly(Q) region close to the N-terminus. The expandable poly(Q) motif region in the protein contributes crucially to spinocerebellar ataxia (SCA) pathogenesis by the induction of intranuclear inclusion bodies. ATXN7 is associated with both olivopontocerebellar atrophy type 3 (OPCA3) and spinocerebellar ataxia type 7 (SCA7).
Ataxin is a type of nuclear protein. The class is called ataxin because mutated forms of these proteins and their corresponding genes were found to cause progressive ataxia.
Spinocerebellar ataxia type 6 (SCA6) is a rare, late-onset, autosomal dominant disorder, which, like other types of SCA, is characterized by dysarthria, oculomotor disorders, peripheral neuropathy, and ataxia of the gait, stance, and limbs due to cerebellar dysfunction. Unlike other types, SCA 6 is not fatal. This cerebellar function is permanent and progressive, differentiating it from episodic ataxia type 2 (EA2) where said dysfunction is episodic. In some SCA6 families, some members show these classic signs of SCA6 while others show signs more similar to EA2, suggesting that there is some phenotypic overlap between the two disorders. SCA6 is caused by mutations in CACNA1A, a gene encoding a calcium channel α subunit. These mutations tend to be trinucleotide repeats of CAG, leading to the production of mutant proteins containing stretches of 20 or more consecutive glutamine residues; these proteins have an increased tendency to form intracellular agglomerations. Unlike many other polyglutamine expansion disorders expansion length is not a determining factor for the age that symptoms present.
Ataxin-2 is a protein that in humans is encoded by the ATXN2 gene. Mutations in ATXN2 cause spinocerebellar ataxia type 2 (SCA2).
Cav2.1, also called the P/Q voltage-dependent calcium channel, is a calcium channel found mainly in the brain. Specifically, it is found on the presynaptic terminals of neurons in the brain and cerebellum. Cav2.1 plays an important role in controlling the release of neurotransmitters between neurons. It is composed of multiple subunits, including alpha-1, beta, alpha-2/delta, and gamma subunits. The alpha-1 subunit is the pore-forming subunit, meaning that the calcium ions flow through it. Different kinds of calcium channels have different isoforms (versions) of the alpha-1 subunit. Cav2.1 has the alpha-1A subunit, which is encoded by the CACNA1A gene. Mutations in CACNA1A have been associated with various neurologic disorders, including familial hemiplegic migraine, episodic ataxia type 2, and spinocerebellar ataxia type 6.
Ataxin-3 is a protein that in humans is encoded by the ATXN3 gene.
Ataxin-10 is a protein that in humans is encoded by the ATXN10 gene.
Ataxin-2-like protein was initially identified in 1996 and designated Ataxin-2 Related protein (A2RP) as the search for the gene causing SCA2 lead to the identification of 2 cDNA clones with high similarity to ATXN2. It was later renamed as ATXN2L. It is a protein that in humans is encoded by the ATXN2L gene.
Junctophilin-3 is a protein in humans that is encoded by the JPH3 gene. The gene is approximately 97 kilobases long and is located at position 16q24.2. Junctophilin proteins are associated with the formation of junctional membrane complexes, linking the plasma membrane with the endoplasmic reticulum in excitable cells. Junctophilin-3 is specific to the brain and has an active role in neurons involved in motor coordination and memory.
Kelch-like protein 20 is a protein that in humans is encoded by the KLHL20 gene.
Kelch-like protein 1 is a protein that in humans is encoded by the KLHL1 gene.
Autosomal dominant cerebellar ataxia (ADCA) is a form of spinocerebellar ataxia inherited in an autosomal dominant manner. ADCA is a genetically inherited condition that causes deterioration of the nervous system leading to disorder and a decrease or loss of function to regions of the body.
Spinocerebellar ataxia type 1 (SCA1) is a rare autosomal dominant disorder, which, like other spinocerebellar ataxias, is characterized by neurological symptoms including dysarthria, hypermetric saccades, and ataxia of gait and stance. This cerebellar dysfunction is progressive and permanent. First onset of symptoms is normally between 30 and 40 years of age, though juvenile onset can occur. Death typically occurs within 10 to 30 years from onset.
References
- ↑ "Human PubMed Reference:". National Center for Biotechnology Information, U.S. National Library of Medicine.
- 1 2 "Entrez Gene: ATXN8OS ataxin 8 opposite strand".
- ↑ Ikeda, Yoshio; Daughters, Randy S.; Ranum, Laura P. W. (2008). "Bidirectional expression of the SCA8 expansion mutation: one mutation, two genes". Cerebellum (London, England). 7 (2): 150–158. doi:10.1007/s12311-008-0010-7. ISSN 1473-4230. PMID 18418692. S2CID 31192490.
