|Year : 2010 | Volume
| Issue : 2 | Page : 141-143
Microsatellites in varied arenas of research
KS Remya, Sigimol Joseph, PK Lakshmi, S Akhila
Nazareth College of Pharmacy, Othera P O, Thiruvalla, Pathanamthitta (Dist), Kerala, India
|Date of Submission||18-Feb-2010|
|Date of Decision||23-Mar-2010|
|Date of Acceptance||15-May-2010|
|Date of Web Publication||2-Aug-2010|
K S Remya
Nazareth College of Pharmacy, Othera P O, Thiruvalla, Pathanamthitta (Dist), Kerala
Source of Support: UGC, Conflict of Interest: None
| Abstract|| |
Microsatellites known as simple-sequence repeats (SSRs) or short-tandem repeats (STRs), represent specific sequences of DNA consisting of tandemly repeated units of one to six nucleotides. The repetitive nature of microsatellites makes them particularly prone to grow or shrink in length and these changes can have both good and bad consequences for the organisms that possess them. They are responsible for various neurological diseases and hence the same cause is now utilized for the early detection of various diseases, such as, Schizophrenia and Bipolar Disorder, Congenital generalized Hypertrichosis, Asthma, and Bronchial Hyperresponsiveness. These agents are widely used for forensic identification and relatedness testing, and are predominant genetic markers in this area of application. The application of microsatellites is an extending web and covers the varied scenarios of science, such as, conservation biology, plant genetics, and population studies. At present, researches are progressing round the globe to extend the use of these genetic repeaters to unmask the hidden genetic secrets behind the creation of the world.
Keywords: Microsatellites, neurological diseases, genetic markers
|How to cite this article:|
Remya K S, Joseph S, Lakshmi P K, Akhila S. Microsatellites in varied arenas of research. J Pharm Bioall Sci 2010;2:141-3
A human's genetic code consists of roughly three billion bases of DNA, the familiar 'letters' of the DNA alphabet. However, a mere 10 - 15% of those bases make up the genes, the blueprint cells used to build proteins. Some of the remaining base sequences in humans - and in many other organisms - perform crucial functions, such as, helping to turn the genes 'on' and 'off' and holding the chromosomes together. Much of the DNA, however, seems to have no obvious purpose at all, leading some to refer to it as 'junk'.
Part of this 'junk DNA' includes strange regions known as DNA satellites or microsatellites.  Microsatellites known as simple-sequence repeats (SSRs) or short-tandem repeats (STRs), represent specific sequences of DNA consisting of tandemly repeated units of one to six nucleotides. For example, (A) 11 , (GT) 9 , (CTC) 12 , (GATA) 8 , (ATTGCC) 5 , and (ATCGC) 4 represent mono-, di-, tri-, tetra-, penta-, and hexa-nucleotide repeats, respectively. The sequences are abundant in prokaryotic and eukaryotic genomes, occurring in both the coding and non-coding regions. These are classified as:
The simplest microsatellites are mononucleotide SSRs of A (eg., AAA = (A) 3 ). The most commonly used (and most abundant in humans and probably most other mammals) microsatellite is AC (wherein CA = GT = TG).
- Perfect (uninterrupted run of repeats), for example, (GTG) 15
- Imperfect (interrupted with base substitution), for example, (GTG) 7 CTCTG(GTG) 8
- Compound (two or more runs of different repeat units), for example, (GTG) 8 (AT) 16 . 
Satellite DNA was first identified in the 1960s. Scientists are discovering that the repetitive nature of microsatellites makes them particularly prone to grow or shrink in length, and these changes can have both good and bad consequences for the organisms that possess them.
| Diagnosis and Identification of Human Diseases|| |
Microsatellites - the repeating DNA sequences can be the cause of many diseases. For example, in one part of chromosome No.4, the CAG nucleotide is repeated excessively. If the trinucleotides are repeated excessively, it will cause the person to get Huntington's disease in adult life. About 14 neurological disorders have been seen to result from the expansion of trinucleotide repeats. For example, myotonic dystrophy, Spinal bulbar muscular atrophy, Friedrich's ataxia, and so on.
Normal CTG repeats in the non-coding region of the DMPK gene is between 5 and 37. The disease may involve CTG repeats from the 50s to 1000s.
Spinal bulbar muscular atrophy
A normal CAG repeat in the first coding exon of the androgen receptor gene is between 9 and 36, and people with 38 - 62 repeats develop the disease.
A normal GAA repeat in non-coding region of gene X25 is between 7 and 34, whereas, the disease gene has 34 - 80 repeats.
Not all diseases are caused by a mistake in one gene. Sometimes many genes may be involved in a disease, for example, in Schizophrenia. For these diseases, the microsatellite sequences have been used as markers for locating the diseased region of the chromosome. This method is called positional cloning. Microsatellite markers close to the disease gene correlate with the heredity of the disease and by analysis of these markers within the families, scientists can predict how the disease will be inherited.  Few examples of diseases found by positional cloning are Scizophrenia and Bipolar Disorder, Congenital generalized Hypertrichosis, and Asthma and Bronchial Hyperresponsiveness.
Microsatellites provide another method for early cancer detection, as the overall rate of expansion or contraction in cells turns out to be markedly increased in some types of cancer. Such bursts of change, often involving many different microsatellites, can be detected fairly easily. The approach can currently detect one cancerous cell out of about 500 normal ones. ,,
Successful clinical trials of early detection symptoms employing microsatellites have been carried out for colorectal and bladder cancer, and are now being extended to many other types of cancer. ,
Microsatellites are widely used for forensic identification, as Short Tandem Repeat (STR) loci are widely used for forensic identification and relatedness testing, and are predominant genetic markers in this area of application. In forensic identification cases, the goal is to typically link a suspect with a sample of blood, semen or hair taken from the crime scene. Alternatively, the goal may be to link a sample found on the suspect's clothing with the victim. Relatedness testing in criminal work may involve investigating paternity in order to establish rape or incest. Another application involves linking the DNA samples with relatives of a missing person. As the lengths of the microsatellites may vary from one person to the next, scientists have begun to use them to identify criminals and to determine paternity, a procedure known as DNA profiling or 'fingerprinting'. The features that have made the use of microsatellites attractive are, they are easy to use, accuracy of typing, and high levels of polymorphism. The ability to employ polymerase chain reaction ((PCR), a method used to amplify fragments of DNA) is particularly valuable in this setting, as in criminal casework only minute samples of DNA may be available. 
Applications in population studies
By looking at the variation of microsatellites in the population, inferences can be made about population structures and differences, genetic drift, genetic bottlenecks, and even the date of a common ancestor. ,,
Applications in conservation biology
Microsatellites can be used to detect sudden changes in population, effects of population fragmentation and interaction of different populations. Microsatellites are useful in the identification of new and incipient populations. ,,,
Application in plant genetics
Microsatellite markers are useful for a variety of applications in plant genetics and breeding, on account of their reproducibility, multiallelic nature, co-dominant inheritance, relative abundance, and good genome coverage. It is also useful for integrating genetic, physical, and sequence-based physical maps in the plant species and has simultaneously provided breeders and geneticists with an efficient tool to link phenotypic and genotypic variation. 
Studies conducted in microsatellites
Sandra H et al. developed the X-linked tetrameric microsatellite marker HumDXS6789 for forensic purposes. This was found to be a useful tool for solving complicated cases of kinship testing.
Xiao L et al. conducted molecular studies of loss of heterozygosity in Chinese sporadic retinoblastoma patients. They concluded that there was no relationship between loss of heterozygosity and any of the clinicopathological features but age at diagnosis.
Michitaka O et al. studied the involvement of microsatellite instability (MSI) in the lymph node metastasis of endometrial carcinoma. It was concluded that MSI was involved in lymph node metastasis, in the development and progression of endometrial carcinoma in some patients.
Temduang L et al. suggested that the genetic alterations of DNA mismatch repair genes and tumor suppressor gene p53 might be involved in cholangiocarcinogenesis, and these alterations may be of value as prognostic indicators of liver fluke related cholangiocarcinoma.
| Conclusion|| |
In recent years microsatellite marker applications in various disciplines has increased rapidly. Microsatellites, characterized as independent and highly variable markers, are most noted for their usefulness and popularity among researchers. Microsatellites can be constructed in-house from a vendor or borrowed from a species that is closely related to the target species. In addition, microsatellite application in natural population can reveal the present as well as historical impacts of stressors of the particular population.
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