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Genetic testing is the analysis of human DNA,RNA, and chromosomes in order to detect heritable disease mutations, genotypes, and phenotypes for clinical use. The purpose of genetic testing is to diagnose genetic diseases in newborns,children, and adults and to identify the future health risks.
There are many different types of genetic testing for example:
Prental diagnosis is the testing of the fetus. Sickle cell anemia is one of the diseases screened using this method. The following amniocentesis, chorionic villus samplying , and fetal blood sampling are methods used in detecting mutations in single gene disorders by enriching fetal cells from maternal blood by magnetic cell sorting followed by the isolation of fetal cells by microdissection. Sickle cell disease is a disorder affecting the molecule haemoglobin.In sickle cell anaemia a mutation in gene encoding β- globlin causes a substituation of 1 of 146 amino acids in the protein. Individuals inherited with sickle cell anaemia have mutant copies of the β-globlin gene. The β-globlin of the inherited individual causes haemoglobin in red blood cells to polymerize when the oxygen's concentration is low causing a distort in the shape of red blood cells. It was demonstrated in 1949 by James Neel and E.A.Beet that sickle cell anaemia is inheriated as a mendelian trait. Linus pauling discovered that haemoglobins isolated from diseased and normal individuals differ in their rates of elctrophertic migration. The experiement involved seeing the difference in normal and sickle cell haemoglobin. The results showed all molecules moved towards the anode, indicated a net negative charge. However HbA migrated further than HbS suggesting net charge was greater. The result from electrophoresis showed carriers had both HbA and HbS. People with sickle cell have atypical haemoglobin molecules called haemoglobin S, which can distort red blood cells into sickle shape. Sickle cell disease affects millions of people worldwide common among people from Africa. It is the most common inherited blood disorder in the USA affecting around 80,000 americans. Sickle cell disease is caused by mutations in the HBB gene.HBB gene makes a protein called beta-globin. Beta-globin is a subunit of haemoglobin which is located in the red blood cell. This gene causes sickle cell anemia. Mutaions in the HBB gene results in the production haemoglobin S. In sickle cell anemia the hemoglobin S replaces both beta-globin subunits in hemoglobin.The mutation changes the amino acid glutamic acid to valine at postion 6 in beta-globin. This changes causes the haemoglobin S to stick together to form rigid molecules. The rigid molecules bend red blood cells into cresent shapes. Many other types of sickle cell diseases can be found from mutation in the HBB gene such as Hemoglobin C and E. A method of genetic testing is restriction fragment length polymorphism(RFLP) analysis. Prenatl diagnosis is done on sickle cell anaemia.The single nucleotide substituation eliminates a cutting site in the β-globin gene for the restriction enzymes MstII and CvnI. This results in the mutation altering the pattern of restriction fragments which can be seen on southern blots. These differences in the restriction cutting sites are used to prentally diagnose sickle cell anaemia. Testing adults using this analysis requires blood samples and DNA from white blood cells is used . RFLP is carried out on check cells and collected by swabbing the inside of the mouth.(1)(2)
Figure1. the location of HBB gene of sickle cell anemia
The HBB gene is located on the short (p) arm of chromosome 11 at position 15.5.
The HBB gene can be found on the short arm of chromosome 11 at position 15.5 from base pair ,246,695 to base pair 5,248,300.
Medium-chain acyl-CoA dehydrogenase (MCAD) deficiency is a inherited defect in the beta-oxidation of fatty acids with autosomal recessive inheritance. In humans it is the most frequently diagnosed defect of mitochondrial β-oxidation. This disease can occur at any time in life from neonatal phase to adulthood. The majority of patients with this defiency are present with metabolic crisis during the first years of life when metabolically challenged by fasting/ or viral illness. The phenotypes include coma, hypoketotic hypoglycemmia, and death. Around 20% of patients die during their first metabolic crisis. In contrast patients using the treatment regimen that avoids fasting and low diets, it is been seen to reduce and elimate recurrent disease episodes. It has been found that the majority of patients around 80% with MCAD deficiency are homozygous for a common mutation,985A→G and another 18% have this mutation in one disease allele. At present no further mutations have been identified, but a great amount of mutations have been detected and characterized with MCAD defiency.
MCAD is caused my mutations in the ACADM gene. The gene provides the key to making the medium chain acyl coenzyme. The dehydrogenase breaks down the group of fats called medium-chain fatty acids. 80 mutations in this gene have been found to cause this deficiency. Many of the mutations change single amino acids in the MCAD enzyme. The common change replaces the amino acid lysine with glutamic acid to position 304 in the enzyme. This mutation alters the enzyme's structure and reduces its activity. A defiency in the MCAD enzyme leads to medium-chain fatty acids not functionally properly. This results in fats not being converted to energy this leads to lack of energy and low blood pressure. Medium chain fatty acids may build up in tissues and damage the liver and brain. This build up causes symptoms of MCAD deficiency.
Figure2.The location of the ACADM gene.
The ACADM gene is located on the short (p) arm of chromosome 1 at position 31.
The gene is located on the short arm of chromosome 1 at position 31 from base pair 76,190,042 to base pair 76,229,354 on chromosome 1.
Medium-chain acyl-CoA dehydrogenase (MCAD) deficiency is an inherited condition with an autosomal recessive pattern; this means both copies of the gene have mutations. The parents of an individual with an autosomal recessive condition each carry a copy of the mutated gene, but don't show the symptoms of the condition.(3)(6)(7)
Late-onset Disorders Huntingtons disease
Huntington disease is a brain disorder that causes uncontrolled movements and loss of thinking ability.Huntington disease is inherited as an autosomal dominant disorder that affects 1 in 10,000 people. It occurs in adult onset and appears in a person around thirties or forties. Symptoms of the disease include depression, irritability, and poor coordination and appear in a person in the fifth decade of life. As the disease progresses on in adult onset the individual may have trouble walking, speaking, and swallowing. Patients with this adult onset form of Huntington disease live to around 15-20 years after symptoms begin. Another form of huntingtons disease is an early onset form this begins in childhood.This forms involves movement problems and slurred speech. The early-onset of Huntington disease progresses quickly than adult onsent form. The Huntington disease gene is located on the short arm of chromosome 4 and was one of the first genes to be mapped using restriction fragment lengh polymorphism. Mutations in the HTT gene cause huntingtons disease.The gene encodes a protein of 35 KDa called huntingtin(HTT). This protein plays an important role in nerve cells in the brain and is important in the development before birth. The protein is found in the body tissues with the highest levels of activity in the brain. One region of the gene contains a DNA segement known as CAG trinucleotide repeat. This region is made up of cytosine,adenine, and guanine that appear multiple times in a row. Huntington disease is caused by mutations in the HTT gene. The inherited mutation causing Huntington disease is known as a CAG trinuclotide repeat. The mutation causes an increase in size of the CAG segment in the HTT gene. Normal individuals have 7 to 34 repeats. People with the disease have 36 to 12 CAG repeats. People with 36 to 40 CAG repeats may or may not develop the symptoms of huntingtons disease, but people with more than 40 repeats develop the disorder. People with the adult onset form of Huntington disease have around 40-50 CAG repats in the HTT gene, in contrast people with early onset form of the disorder have more than 60 CAG repeats. As the HTT gene is passed from generation from parent to child the size of the CAG trinucleotide repeat increases in the range associated with Huntington disease. Huntingtons disease is a class of inherited neurodegenerative disorders that are characterized by the expansion of CAG repeats within exons, resulting in polyglutamine tracts in the encoded proteins. The polyglutamine diseases are inherited as dominant traits with sysptoms appearing in adult onset.(4)(5)
Figure 3. the location of HTT gene
The HTT gene is located on the short (p) arm of chromosome 4 at position 16.3.
The HTT gene is located on the short arm of chromosome 4 at position 16.3 from base pair 3,076,407 to base pair 3,245,686.