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Role of DNA in Disease Diagnosis and Medical Forensics
It's a universal and a scientifically proven fact that DNA carries the genetic material of an organism. DNA analysis is diagnostically proven as very useful and a sensitive tool. In this modern diagnostic DNA analyses can diagnose inherit genetic defect and also disease causing pathogen can be detected by identifying genes of that organism. Furthermore, each person’s genome is structurally unique which helps in the field of medical forensics. The DNA analysis in disease diagnosis and medical forensics involves procedures like- hybridization technique, DNA profiling, etc.

In the laboratory, to begin with, it is very essential to identify a specific DNA sequence. Nucleic acid hybridization is one such tool to be reliable on for DNA analysis. The DNA probes, are used in this technique are synthetic single-stranded DNA molecule which recognizes and binds to the specific target DNA to be analyzed. The technique basically involves the single stranded target DNA bound to membrane support and addition of DNA probes, which in presence of specific circumstances binds to the complementary target DNA. Now the sequence of the target DNA can be analyzed on the basis of the sequence of known DNA probes complementary to the target DNA. The hybridization technique involves two kinds: radioactive and non-radioactive i.e., DNA probe tagged with radioactive isotope and DNA probed untagged. Non-radioactive labeled system has an advantage over radioactive labeled system that they are quite stable (biotin labeled DNA) at room temperature even for an year. The use of probes for disease diagnosis has several advantages over conventional methods like, they are simple, rapid, highly specific, powerful when combined with PCR, and viral infections can also be detected.

Another technique is DNA chip-microarray which contains thousands of DNA probes on a glass slide on which thousands of target DNA molecules can be scanned at a time. In this method, by the use of restriction enzymes DNA molecules are cut into fragments and labeled by fluorescent markers. These reacts with probes on the DNA chip i.e. bind according to their complementary sequences. The target DNA is identified by fluorescence emission which is recorded by the computer and DNA is identified.

In the diagnosis of infectious diseases the DNA analysis is a novel approach to identify the very specific pathogen. This is incorporated by genetically engineered techniques or DNA probes or direct DNA analysis. A specific DNA diagnostic test for malaria for identification of P. falciparum is developed, where in even as little as 1ng of P. falciparum is detected; in this method the DNA probe is bound and hybridized with DNA of P. falciparum genome and not with any other species of Plasmodium. Now even radioisotope labeled DNA probes are available for diagnosis of HIV DNA. Diagnostic tool for tuberculosis is developed by genetic engineering in bacteriophage using luciferase enzyme, this involves flash of light which confirms tuberculosis. Also diseases like lyme, periodontals and chagas’ have been diagnosed using PCR amplification, DNA probes and genetically engineered DNA probes.

Genetic diseases have ailments that are manageable but there is no cure except for gene therapy. But if, the gene identification is done for that genetic disease there are chances of development of management therapy, development of precautions to reduce its risk and helpful in terminating the foetuses affected. Foetal cells are obtained by amniocentesis or biopsies of trophoblastic villi and are detected by methods like- karyotype analysis, enzyme assays, hybridization technique or RFLP analysis. Then comes identification of gene causing genetic diseases in which first step is to do pedigree analysis where in the inheritance of the disease showing high incidences in the family is examined, followed by analysis of the identified region using STRs (Short Tandem Repeats) which would result in genome mapping and identification of the most likely gene causing genetic disorder is confirmed using blotting techniques, RT-PCR or northern hybridization.

Few examples of DNA analysis in genetic disease diagnosis are: (i) Sickle cell anemia can be detected by digesting mutant and normal β-globin gene and performing hybridization with a cloned β-globin DNA probe. (ii) Huntington’s disease can be diagnosed by the analysis of RFLPs in blood related individuals. (iii) Alzheimer’s disease, researchers have found a gene on chromosome number 21 which is believed to be responsible for inheriting this disease and they developed a DNA probe that located the genetic marker for this disease. There are also diagnostic systems developed and are is underdevelopment to detect the disease by DNA analysis for diseases like- fragile X syndrome, Friedrich’s ataxia, cystic fibrosis, muscle dystrophy, diabetes, cancers, obesity, etc.

There are other human diseases which are detected by DNA analysis. These includes: deafness- where mutation of the gene on chromosome 5 causes defective protein synthesis and disassembly of actin molecules, which in turn results is deafness, baldness- alopecia universalis known as inherited form of baldness is associated with a gene located on chromosome 12. Other diseases include Glaucoma, Parkinson’s disease, etc.

In the medical forensics, DNA analysis has proven to be genetic detective in settling paternity disputes, identification of criminals, thieves, rapists, etc. This novel term and technique was developed by Alec Jaffrey’s in 1985 called as DNA Profiling also widely better known as DNA fingerprinting. This procedure does not require a large quantity of DNA but minute quantities of DNA from skin fragments, hair, semen, blood stains are enough which are amplified using PCR. DNA fingerprinting is analysis of the nitrogenous base sequence in DNA of an individual which is unique in each person. The markers used in DNA fingerprinting as well as in disease diagnosis are microsatellites or simple tandem repeats, minisatellites or variable number tandem repeats, single nucleotide polymorphisms and Restriction Fragment Length Polymorphism. Also, it is now possible to carry out DNA profiling by automated DNA detection system.
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Medical Forensics was first discussed in lecture during the mid 18th century in Europe. Several scientists like Carl Liman, Johann Casper, and Auguste Tardieu helped build forensic pathology into an important field of science. It was in 1959 that forensic pathology was acknowledged by the American Board of Pathology in the United States. Canada formally welcomed it in 2003.

There are several duties a forensic pathologist does in the application of medical jurisprudence. He must:
a. Be a medical doctor who has complete anatomical pathology and sub-specialized forensic pathology, though varying from country to country;
b. Does postmortem examination to identify cause of death. Such report may consist of the disease, injury, or pathologic process and the manner of death;
c. Occasionally examine injuries in clinical settings;
d. Determine the presence or absence of disease;
e. Interpret body tissue toxicological analyses to identify chemical cause of injury;
f. Collaborate with medico-legal authority to investigate sudden deaths; and
g. Appear as professional witness in criminal or civil law cases.

Aside from medical forensics, DNA or genetic testing is also used in the following fields:
• Newborn screening – identify genetic disorders for early treatment
• Diagnostic testing – diagnose or rule out a genetic condition
• Carrier testing – determine couple’s risk of having an offspring with a genetic condition
• Prenatal testing – identify fetus’s genes before birth
• Parental testing – match DNA sequences
• Research testing – advance understanding of genetic conditions
• Pharmacogenomics – test for genetic variation on drug reaction

DNA probes, as discussed in the article, are tiny segments of DNA which are used to detect a gene’s presence from a long DNA sequence. They are prepared for commercial purposes and are considered to be the most sensitive and sophisticated means to identify genes or particular DNA sequences. They are used for diagnosis of infectious disease, identification of contaminants in food, and in other micro tests. It is commended for being a much simpler, cheaper, faster, and cleaner way than other traditional microbiological tests.
Lyka Candelario, RN
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