19.2. Genetic technology applied to medicine
A subsection of Biology, 9700, through 19. Genetic technology
Listing 10 of 37 questions
Leber’s congenital amaurosis (LCA) is an autosomal recessive eye disease. LCA results in eye disorders, including severe loss of vision, at birth. LCA has been successfully treated by gene therapy, using a virus instead of a plasmid as the vector. Adeno‑associated virus (AAvectors containing the therapeutic allele were injected directly into the retina, the layer at the back of the eye containing the photoreceptor cells. People who had been blind from a young age were able to see again. There is a risk associated with the injection method used to deliver the vectors, as it might cause the retina to detach, damaging vision. This method of delivery was first used for LCA before being trialled on other retinal diseases that gradually reduce the vision of people as they get older. Suggest the main steps involved in creating recombinant DNA for this example of gene therapy. Explain why the fact that LCA is an autosomal recessive genetic disease makes it suitable for treatment with gene therapy. Suggest why the retinal injection method of gene therapy was used for LCA before it was trialled on other retinal diseases that gradually reduce the vision of people as they get older. Scientists tried to create an improved virus vector for gene therapy. step 1 – The scientists used a special form of the polymerase chain reaction (PCR). This form of PCR causes mutations in the DNA sequence of AAV by base substitution. step 2 – The viruses containing different base substitutions were tested. This was done by using the different viruses to deliver a new gene, the gene for green fluorescent protein (GFP), into the photoreceptor cells of mice, using the retinal injection method. step 3 – The best virus, known as 7m8, caused the photoreceptor cells in the retina of the mouse to fluoresce brightly, even when the recombinant virus was injected into the fluid inside the eye instead of into the retina itself. step 4 – The 7m8 virus was used to cure a mouse with LCA by injecting this virus containing the therapeutic allele into the fluid inside the eye of the mouse. Suggest how errors occurring during PCR can cause base substitution mutations in the DNA sequence of AAV. Explain why the photoreceptor cells of the mouse fluoresced in step 3. Predict the impact of the 7m8 AAV on treatment for age‑related retinal diseases.
9700_w18_qp_43
THEORY
2018
Paper 4, Variant 3
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Adenosine deaminase (ADA) deficiency is an immune system disorder caused by a recessive autosomal mutation. Severe combined immunodeficiency caused by a lack of ADA is called ADA-SCID. Genetic engineering is used to make a recombinant human protein to treat people with ADA-SCID. Outline the principles of genetic engineering. In 2016 gene therapy to cure ADA-SCID was approved in Europe. The gene therapy involves three main steps. • Blood stem cells are taken from the bone marrow of the person with ADA-SCID. • The functional gene and its promoter are inserted into the blood stem cells. • A single infusion of the gene-corrected cells is given to the patient. Explain why a single infusion of gene-corrected stem cells is enough to cure the disease. Explain why a promoter has to be transferred as well as the desired gene. A modified retrovirus is used to insert the new gene into the DNA of the blood stem cells. State two ethical considerations of using a retrovirus for gene therapy. The gene therapy technique used to cure ADA-SCID is not suitable for treating the genetic disease called Huntington’s disease. A newer technique called gene editing could potentially be used instead to cure Huntington’s disease. Explain why gene editing is more suitable as a potential cure for Huntington’s disease.
9700_w22_qp_41
THEORY
2022
Paper 4, Variant 1
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Haemophilia is a blood clotting disorder in humans caused by a mutant allele on the X chromosome. Table 4.1 compares two forms of haemophilia: haemophilia A and haemophilia B. Table 4.1 haemophilia A haemophilia B gene F8 F9 clotting factor protein factor VIII factor IX proportion of males born with haemophilia 1 in 5000 1 in 30 000 length of functional gene (exons only) / kilobase pairs 7.0 1.6 Genetic engineering is used to make recombinant human proteins to treat people with haemophilia A and haemophilia B. Outline the principles of genetic engineering. Scientists are working towards a goal of treating haemophilia by gene therapy. They plan to use a common, harmless virus to introduce the functional gene. The virus has a genome that is 4.7 kilobase pairs long. With reference to Table 4.1 and the introduction to , assess: • which form of haemophilia, A or B, scientists should try to treat first • whether they should attempt to treat haemophilia with gene therapy at all. Explain your reasoning. In gene therapy trials to treat haemophilia, the gene coding for the clotting factor needs to be introduced together with a promoter. Explain why a promoter has to be introduced as well as the desired gene. Some individuals taking part in gene therapy trials have been naturally exposed to the virus carrying the functional gene, so that their blood already contains antibodies to the virus. Predict how this will affect the success of the gene therapy treatment. Gene editing is a newer technique for modifying DNA. Some scientists are researching the use of gene editing, instead of introducing a functional gene, to treat haemophilia. State two possible advantages of using gene editing as a method of treating haemophilia.
9700_w22_qp_42
THEORY
2022
Paper 4, Variant 2
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Adenosine deaminase (ADA) deficiency is an immune system disorder caused by a recessive autosomal mutation. Severe combined immunodeficiency caused by a lack of ADA is called ADA-SCID. Genetic engineering is used to make a recombinant human protein to treat people with ADA-SCID. Outline the principles of genetic engineering. In 2016 gene therapy to cure ADA-SCID was approved in Europe. The gene therapy involves three main steps. • Blood stem cells are taken from the bone marrow of the person with ADA-SCID. • The functional gene and its promoter are inserted into the blood stem cells. • A single infusion of the gene-corrected cells is given to the patient. Explain why a single infusion of gene-corrected stem cells is enough to cure the disease. Explain why a promoter has to be transferred as well as the desired gene. A modified retrovirus is used to insert the new gene into the DNA of the blood stem cells. State two ethical considerations of using a retrovirus for gene therapy. The gene therapy technique used to cure ADA-SCID is not suitable for treating the genetic disease called Huntington’s disease. A newer technique called gene editing could potentially be used instead to cure Huntington’s disease. Explain why gene editing is more suitable as a potential cure for Huntington’s disease.
9700_w22_qp_43
THEORY
2022
Paper 4, Variant 3
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The treatment of many diseases has been improved by the use of genetic technology in medicine. Explain what is meant by genetic engineering. Leber Congenital Amaurosis (LCA) is an inherited eye disease. In LCA, the photoreceptor cells in the retina die at an early age. This causes impaired vision (reduced eyesight) in children, which can progress to blindness. Mutations in different genes cause different forms of LCA. One form of this disease, LCA2, is caused by a mutation in the RPE65 gene. Gene therapy has been used to treat LCA2. Outline how an inherited eye disease, such as LCA2, is treated with gene therapy. Suggest why the eye is a suitable organ for gene therapy. LCA10 is a different form of LCA caused by a recessive mutation in the CEP290 gene. This gene codes for the protein CEP290, which is involved in the correct functioning of photoreceptor cells in the retina. The mutation in CEP290 causes an error to be made when the primary transcript is spliced to form messenger RNA (mRNA). The abnormal mRNA that is formed has an extra sequence of RNA nucleotides, known as exon X, between exon 26 and exon 27. Exon X contains a STOP codon. compares the effect of the mutation in CEP290 with the normal gene expression. normal LCA10 exon intron exon section of DNA of CEP290 exon mutation exon exon intron exon primary transcript exon mutation exon mRNA exon exon X exon CEP290 polypeptide product shortened CEP290 polypeptide In 2022, research was carried out into possible treatment of LCA10 using genetic technology. A human clinical trial investigated a treatment of LCA10 using a short RNA nucleotide sequence known as Sepofarsen. Sepofarsen binds to the section of the primary transcript containing the CEP290 mutation so that normal splicing occurs and functional CEP290 protein is synthesised. • People in the clinical trial received regular treatment with Sepofarsen to the eye with the greatest loss of vision (treated eye) for a period of 12 months. • Changes in the light perception (visual acuity) of both eyes were measured using a vision chart. • A negative change in the visual acuity score shows an improvement in visual acuity. shows the results of the clinical trial over 12 months. exon exon Another method being investigated to treat LCA10 is to use a gene editing tool known as the CRISPR/Cas9 system. The CRISPR/Cas9 system uses a short length of RNA called guide RNA. Guide RNA is complementary to the target DNA and is linked to a nuclease enzyme called Cas9. Cas9 breaks phosphodiester bonds in DNA. The cell repair mechanisms repair the cut in DNA after the modification has taken place. • A vector delivers Cas9 and two specific guide RNAs to the photoreceptor cells. • They act on the section of DNA which contains the mutation. • Exon X is no longer added to the mRNA. Explain how this method used to treat LCA10 is an example of gene editing.
9700_w24_qp_42
THEORY
2024
Paper 4, Variant 2
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Cancer is a disease in which normal controls over cell division are lost and malignant tumours form. An early diagnosis of many types of cancer can result in successful treatment. The BRCA2 protein is involved in suppressing the development of tumours. The gene that codes for this protein is on chromosome 13. Several different dominant alleles of this gene, BRCA2, code for faulty versions of the protein. The presence of any one of these faulty alleles leads to an increased chance of developing several types of cancer, including breast cancer. Not everyone with one of these alleles develops cancer. This is because environmental factors, including lifestyle, are also involved. is a pedigree (family tree) showing the occurrence of cancers in four generations of a family. The presence of a faulty BRCA2 allele was confirmed in person 15. The other individuals with cancer were not tested for the presence of the allele. Individuals 24–30 are all under twelve years old. female cancer allele present but no cancer no information male cancer allele present but no cancer no information Discuss the extent to which provides evidence that a faulty BRCA2 allele increases the risk of a person developing cancer. People whose families are suspected of having a faulty BRCA2 allele may choose to be tested for its presence in their own genome. A company based in the USA sells a microarray containing DNA probes for 20 different alleles that are associated with an increased risk of cancer, including the faulty BRCA2 alleles. This microarray can be used in a medical facility or research laboratory to test blood samples for the presence of these alleles. Explain the meaning of the term genome. Suggest a type of cell from a blood sample that is suitable for testing for the presence of this allele and explain your choice. Outline how a microarray enables the detection of particular alleles. Suggest one advantage and one disadvantage of screening for faulty alleles of BRCA2 before any symptoms occur. advantage disadvantage
9700_s17_qp_42
THEORY
2017
Paper 4, Variant 2
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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. is a diagram of part of the cell surface membrane and the mucus layer of an epithelial cell with normal CFTR proteins. outside cell thin mucus layer cell surface membrane inside cell Cl – water 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. Explain why the absence of CFTR proteins will cause 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 . 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 IVF method percentage of embryo transfers that resulted in pregnancy IVF with standard screening IVF with double screening 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.
9700_s20_qp_43
THEORY
2020
Paper 4, Variant 3
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Questions Discovered
37