Practise medical genetic technology by comparing recombinant-protein treatments, interpreting screening choices and weighing gene-therapy benefits and risks.
One cause of the genetic disease severe combined immunodeficiency (SCID) is a mutation in the ADA gene. This mutation results in a deficiency of the enzyme adenosine deaminase (ADA).
Although ADA is found throughout the body, it is especially active in lymphocytes. The absence of functional ADA causes the build-up of toxic metabolites that kill lymphocytes and damage organs.
Babies are often diagnosed with SCID by six months old. Treatment can greatly improve the life expectancy of children with SCID.
Some treatment options are available.
- Enzyme replacement therapy with recombinant human ADA made by genetically modified (GM) Escherichia coli. Weekly intra-muscular injections are given.
- Bone marrow transplant if a well-matched donor, such as a close relative, can be found.
- Gene therapy.
Suggest and explain why it may be more appropriate to use enzyme replacement therapy to treat SCID instead of a bone marrow transplant.
Outline the procedure used for gene therapy treatment of a person with SCID.
Suggest the social and ethical implications of gene therapy for SCID that need to be considered before treatment is carried out.
The polymerase chain reaction (PCR) is used to produce large quantities of DNA from a very small original sample. The main steps of one PCR method are shown in Fig. 4.1.
Fig. 4.1
The presence of a faulty allele of the gene BRCA2 can lead to an increased chance of developing breast cancer. There are many different faulty alleles of the gene BRCA2.
People who are considered to be at risk of breast cancer may choose to be tested for the presence of these alleles in their genomes.
A microarray can be used to test blood samples for the presence of these alleles. The microarray contains DNA probes for different faulty alleles of the BRCA2 gene.
Outline the advantages of screening for faulty alleles of the BRCA2 gene.
Lung epithelial cells have a thin layer of watery mucus on their surface.
The normal allele of the CFTR gene codes for a transport protein that transports chloride ions out of epithelial cells.
Fig. 4.1 is a diagram of part of the cell surface membrane and the mucus layer of an epithelial cell with normal CFTR proteins.
Fig. 4.1
Cystic fibrosis (CF) is a genetic disorder caused by having two recessive alleles of CFTR. In severe cases of CF, the transport proteins are not added to the cell surface membrane. This causes the mucus layer to be thick and sticky.
The probability of a baby having CF when both parents are heterozygous carriers for CF is 25 %.
It is possible to carry out prenatal screening to check for CF by using one of these tests:
- amniocentesis, using cells from the amniotic fluid
- chorionic villus sampling, using cells from the placenta.
Both tests slightly increase the probability of the pregnancy failing (miscarriage).
Outline the advantages of carrying out prenatal screening for CF.
Embryos produced by IVF may be screened for genetic abnormalities:
- to test for a specific genetic disease, such as cystic fibrosis
- to check whether there is an abnormal number of chromosomes present.
To improve the success of implantation and pregnancy, only embryos without any form of genetic abnormality are transferred to the woman's uterus.
A new double screening method was trialled where a single embryo biopsy was taken and used to test for a specific genetic disease and to check the number of chromosomes. In the trial, 1122 embryos were tested using this double screening method.
In the trial, of the 1122 embryos tested:
- 50.6 % did not have a genetic disease
- 27.5 % did not have a genetic disease and did not have an abnormal number of chromosomes (normal embryos).
Only normal embryos were transferred into the women. The percentage of embryo transfers that resulted in pregnancy was calculated.
The results of the trial using double screening of a single biopsy were compared to the results of IVF procedures that used standard screening methods, as shown in Table 4.1.
Table 4.1
Using the data in Table 4.1, discuss the social and ethical considerations of double screening for cystic fibrosis and chromosomal abnormalities in a single biopsy.
One cause of the genetic disease severe combined immunodeficiency (SCID) is a mutation in the ADA gene. This mutation results in a deficiency of the enzyme adenosine deaminase (ADA).
Although ADA is found throughout the body, it is especially active in lymphocytes. The absence of functional ADA causes the build-up of toxic metabolites that kill lymphocytes and damage organs.
Babies are often diagnosed with SCID by six months old. Treatment can greatly improve the life expectancy of children with SCID.
Some treatment options are available.
- Enzyme replacement therapy with recombinant human ADA made by genetically modified (GM) Escherichia coli. Weekly intra-muscular injections are given.
- Bone marrow transplant if a well-matched donor, such as a close relative, can be found.
- Gene therapy.
Suggest and explain why it may be more appropriate to use enzyme replacement therapy to treat SCID instead of a bone marrow transplant.
One cause of the genetic disease severe combined immunodeficiency (SCID) is a mutation in the ADA gene. This mutation results in a deficiency of the enzyme adenosine deaminase (ADA).
Although ADA is found throughout the body, it is especially active in lymphocytes. The absence of functional ADA causes the build-up of toxic metabolites that kill lymphocytes and damage organs.
Babies are often diagnosed with SCID by six months old. Treatment can greatly improve the life expectancy of children with SCID.
Some treatment options are available.
- Enzyme replacement therapy with recombinant human ADA made by genetically modified (GM) Escherichia coli. Weekly intra-muscular injections are given.
- Bone marrow transplant if a well-matched donor, such as a close relative, can be found.
- Gene therapy.
Outline the procedure used for gene therapy treatment of a person with SCID.
Lung epithelial cells have a thin layer of watery mucus on their surface.
The normal allele of the CFTR gene codes for a transport protein that transports chloride ions out of epithelial cells.
Fig. 4.1 is a diagram of part of the cell surface membrane and the mucus layer of an epithelial cell with normal CFTR proteins.
Fig. 4.1
Cystic fibrosis (CF) is a genetic disorder caused by having two recessive alleles of CFTR. In severe cases of CF, the transport proteins are not added to the cell surface membrane. This causes the mucus layer to be thick and sticky.
Embryos produced by IVF may be screened for genetic abnormalities:
- to test for a specific genetic disease, such as cystic fibrosis
- to check whether there is an abnormal number of chromosomes present.
To improve the success of implantation and pregnancy, only embryos without any form of genetic abnormality are transferred to the woman's uterus.
A new double screening method was trialled where a single embryo biopsy was taken and used to test for a specific genetic disease and to check the number of chromosomes. In the trial, 1122 embryos were tested using this double screening method.
In the trial, of the 1122 embryos tested:
- 50.6 % did not have a genetic disease
- 27.5 % did not have a genetic disease and did not have an abnormal number of chromosomes (normal embryos).
Only normal embryos were transferred into the women. The percentage of embryo transfers that resulted in pregnancy was calculated.
The results of the trial using double screening of a single biopsy were compared to the results of IVF procedures that used standard screening methods, as shown in Table 4.1.
Table 4.1
Using the data in Table 4.1, discuss the social and ethical considerations of double screening for cystic fibrosis and chromosomal abnormalities in a single biopsy.