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Microsatellite instability

 

 
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Microsatellite instability
 
 
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Microsatellites are repeated sequences of DNA. Although the length of these microsatellites is highly variable from person to person, each individual has microsatellites of a set length. These repeated sequences are common, and normal. The most common microsatellite in the humans is a dinucleotide repeat of CA which occurs tens of thousands of times across the genome.

In cells with mutations in DNA repair genes, however, some of these sequences accumulate errors and become longer or shorter.






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Microsatellites are repeated sequences of DNA. Although the length of these microsatellites is highly variable from person to person, each individual has microsatellites of a set length. These repeated sequences are common, and normal. The most common microsatellite in the humans is a dinucleotide repeat of CA which occurs tens of thousands of times across the genome.

In cells with mutations in DNA repair genes, however, some of these sequences accumulate errors and become longer or shorter. The appearance of abnormally long or short microsatellites in an individual's DNA is referred to as microsatellite instability. Microsatellite instability (MSI) is a condition manifested by damaged DNA
DNA

Deoxyribonucleic acid is a nucleic acid that contains the genetics instructions used in the development and functioning of all known living organisms and some viruses....
 due to defects in the normal DNA repair
DNA repair

DNA repair refers to a collection of processes by which a cell identifies and corrects damage to the DNA molecules that encode its genome. In human cells, both normal metabolism activities and environmental factors such as UV light and Radiation can cause DNA damage, resulting in as many as 1 million individual molecular lesions per cell pe...
 process. Sections of DNA called microsatellite
Microsatellite

Microsatellites, or Simple Sequence Repeats , are Polymorphism loci present in nuclear DNA and organellar DNA DNA that consist of repeating units of 1-6 base pairs in length....
s, which consist of a sequence
DNA sequence

A DNA sequence or genetic sequence is a succession of letters representing the primary structure of a real or hypothetical DNA molecule or strand, with the capacity to carry information as described by the central dogma of molecular biology....
 of repeating units of 1-6 base pair
Base pair

In molecular biology, two nucleotides on opposite complementarity DNA or RNA strands that are connected via hydrogen bonds are called a base pair ....
s in length, become unstable and can shorten or lengthen. Microsatellites are also known as simple sequence repeats (SSRs).

MSI is a key factor in several cancers including colorectal, endometrial, ovarian and gastric cancers. Colorectal cancer studies have demonstrated two mechanisms for MSI occurrence. The first is in hereditary nonpolyposis colorectal cancer
Hereditary nonpolyposis colorectal cancer

Hereditary nonpolyposis colorectal cancer , also known as Lynch syndrome, is characterised by a risk of colorectal cancer and other cancers of the Endometrial cancer, Ovarian cancer, Stomach cancer, Gastrointestinal cancer, Gallbladder cancer, upper urinary tract, Brain tumor, and Skin cancer....
 (HNPCC) or Lynch Syndrome, where an inherited mutation in a mismatch-repair gene causes a microsatellite repeat replication error to go unfixed. The replication error results in a frameshift mutation that inactivates or alters major tumor suppressor genes - key genes in the regulation of the cell cycle and, ultimately, the prevention of cancer. The second mechanism whereby MSI causes colorectal cancer is an epigenetic change which silences an essential mismatch-repair gene. In both cases, microsatellite insertions and deletions within tumor suppressor gene coding regions result in uncontrolled cell division and tumor growth.

Five markers have been recommended by the National Cancer Institute to screen for MSI in HNPCC tumors (often called Bethesda markers). Generally, MSI detection in two of the markers is considered a positive result or high probability of MSI (MSI-H). Recently, Promega Corporation released a microsatellite instability assay called . The kit includes five mononucleotide markers which were chosen for their sensitivity and specificity.

See also

  • Adenomatous polyposis coli, or APC
  • Chromosomal instability


External links

Software for analyzing AFLP data
  • GeneMarker fragment analysis software