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CBL1 Down Syndrome Notes.docx

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University of Ottawa
Dr Kingswell

FOUNDATIONS - CBL 1 DOWN SYNDROME properly of Melissa Pasqua Learning objectives 1.Explain how nondisjunction can occur during oogenesis and spermatogenesis. 2.Describe normal chromosome structure, function, and nomenclature and the types of karyotypes resulting in Down syndrome. 3.Describe the techniques involved in obtaining a karyotype and what information it provides. 4.Describe alternative techniques for chromosome analysis besides karyotype. 5.Discuss what factors influence the risk of having a child with Down syndrome, the prenatal options available, and ethical problems posed by prenatal diagnosis of Down syndrome. 6.Describe the differences between a diagnostic and screening test. 7.Characterize the features and health issues for individuals with Down syndrome and what services are available to them. 8. Discuss the impact of disability on the family, identify challenges the family may face and the services are available to them. Resources Objective #1 - Nondisjunction describes the error of separation of chromosomes, when an unequal amount of chromosomes are separated into the two resulting daughter cells after division. This often occurs during anaphase (in particular, meiosis I). (Concepts of Genetics) - During oogenesis and spermatogenesis, this is the same case during either anaphase I or II, an uneven separation may occur, resulting in an N that is not 23. When the zygote is formed, though one gamete might have the correct N, there will not be 46 chromosomes Objective #2 - Each chromosome contains a centromere, which has a different location depending on which chromosome. From the centromere extends two arms the shorter arm (p arm) or the longer arm (q arm). Chromosomes can either be metacentric (centromere in the middle), submetacentric (between middle and end), acrocentric (close to end), or telocentric (at end). [Concepts of Genetics] th st - Nomenclature e.g. 5 p21 = 5 chromosome, short arm, 21 band from the centromere (going from the centromere outward) - Chromosome function carry DNA in a way that facilitates distribution during mitosis - Chromosome nomenclature # from 1 22, where 23 is X or Y - For Down Syndrome, 95% of cases is due to nondisjunction, due to a third copy of chromosome 21. The extra chromosome is usually from the mother, usually an error in Meiosis I of the oocytes. 5% of cases occur as an error in mitotic cell division after fertilization. Partial trisomy 21 comes from having 3 copies of the long q arm of chromosome 21. The genes in this area are responsible for the features of DS (Public Health Agency of Canada) Objective #3 - To do a karyotype, DNA needs to be collected by obtaining patients cells. If this is to be retrieve from a foetus, foetal cells are obtained by either amniocentesis or chorionic villus sampling - Karyotyping involves using actively dividing cells, as mitotic chromosomes are needed for the karyotype. Cells are treated with a chemical so that the chromosomes all go into metaphase. They are then treated with a lysis buffer on a microscope slide. Because chromosomes are usually colourless, Giemsa dye is added. Chromosomes are then sorted - in amniocentesis, amniotic fluid is removed via a needle. This fluid is then centrifuged, and the cells used for karyotyping. [Concepts of Genetics]. This can be done at 14 20 weeks, though 1 in 200 babies may be miscarried due to this. - for chorionic villus sampling, cells from the chorionic villi are sampled through a vacuum tube, and analyses can be carried out with that tissue [Concepts of Genetics]. This can be done at 10 13 weeks, though this can result in miscarriage 1 in 100 times Objective #4 Other analyses to test for Down Syndrome are: - Combined First Trimester Screen for Fetal Aneuploidy - can be done at 10 14 weeks - detects fetal Down Syndrome 80 90% of the time, though can detect other birth defects like heart problems - measures free beta-HCG and PAPPA from maternal blood + looking at ultrasound and maternal age to check overall risk for DS - Maternal biochemistry - can be done at 15 20 weeks - measures maternal blood levels of the protein, AFP (alpha-fetaprotein), where low levels can help identify if foetus has DS. However, 3 other proteins are used in combination with AFP to identify DS. With AFP only, its 65% likely, with all four its 70 75% likely - ultrasounds in the 2 trimester can also be used to look for Down Syndrome - other proteins/molecules to look for: - estriol is found to be decreased in DS pregnancy - inhibin A inhibits production of FSH by pituitary - is increased during DS pregnancy Objective #5 - Down Syndrome is the most common chromosome anomaly about 1/700 to 1/1000, which does not change much between countries (Public Health Agency of Canada) - the only well-established risk is maternal age (Public Health Agency of Canada) risk increases after 35 years of age. - Parents of those who believe want to know if their fetus may have DS are able to do genetic testing using any of the procedures listed above. - Ethical issues: abortion debate, whether or not having a foetus with Down Syndrome is bad Objective #6 - A diagnostic test is a procedure that can tell whether a foetus has a certain condition with a high level of certainty (though the information is very narrow). Examples include genetic amniocentesis and chorionic villus sampling. However, these tests often have their own issues associated with them, such as risk of miscarriage - A screening test is less sensitive but less risky, and tests for which patients are likely to be at risk for a certain condition. Examples for Down Syndrome are: maternal age, combined screen for fetal aneuploidy, biochemical testing first or second trimesters, integrated screen, sequential or step wise screen, or second trimester genetic sonogram
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