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Down Syndrome: The Power of an Extra Chromosome

Down syndrome, or trisomy 21, is a genetic condition in which people are born with an extra chromosome 21. This leads to a varying level of physical, developmental and intellectual disabilities. According to the Centres for Disease Control and Development, each year around 1 in 700 babies are born with Down syndrome (CDC, 2022). Better understanding of the syndrome as well as advances in medicine now offer an improved quality of life for individuals with Down syndrome, allowing them to become active members of the society. This article goes through the genetic background of the condition, the main symptoms and some details about the lifestyle of individuals with Down syndrome.


Genetic Background of Down Syndrome

Before going into the details of the genetic background of Down syndrome, it is important to understand the basic genetics behind it. Genomic information is saved in the DNA of the nucleus of every cell. In order for the big DNA molecules to fit into the tiny cell nucleus, DNA is tightly packed with proteins, forming threadlike structures, called chromosomes. Humans have 23 pairs of chromosomes, 2 of which are the sex chromosomes (XX and XY). Except for the sex chromosomes, the other 21 pairs are numbered from 1 to 21. Chromosome 1 is the largest one and chromosome 21 is the smallest one.


The abnormal number of chromosomes is called aneuploidy. Down syndrome is the result of an aneuploidy, called trisomy 21. Instead of a pair of chromosomes 21, individuals with Down syndrome have three chromosomes 21 (MacLennan, Crichton, Playfoot, & Adams, 2015). Except from some rare cases, Down syndrome is not inheritable. It is the result of problematic cell division during the early development of the fetus.


Other common trisomies are trisomy 18, also known as Edward syndrome and trisomy 13, named Patau syndrome. Due to the bigger size of the chromosomes involved in these conditions, individuals do not survive after the neonatal stage. There is also a number of chromosomal abnormalities that just concern the sex chromosomes (e.g. Turner syndrome and Klinefelter syndrome) or the morphology of chromosomes (Lannoo et al., 2022).


Figure 1: Complete set of chromosomes (Karyotype) of a healthy individual. (National Human Research Institute)

Symptoms, Clinical Implications and Diagnosis

Down syndrome is characterized by phenotypic variability. Individuals with Down syndrome however have some distinctive physical and intellectual characteristics. Some of the most common facial features include a flattened face, short neck, almond-shaped slanted eyes, small ears, poor muscle tone and short height (Sinet et al., 1994). Down syndrome also causes some serious clinical implications. Firstly, there is an increased risk of up to 70% for dementia as well as early onset Alzheimer’s Disease (Holland, Hon, Huppert, & Stevens, 2001). Other implications include cardiovascular defects such as congenital heart disease and ventricular septal defect (Maslen et al., 2006) and haematological diseases such as leukaemia (Hasle, Clemmensen, & Mikkelsen, 2000). Finally, Down syndrome individuals often have to face hypertension and gastrointestinal problems. However, it is important to mention that recent advances in medicine and pharmaceuticals have increased the life span of Down syndrome individuals to almost 60 years (Glasson et al., 2002).


Diagnostic methods are usually applied during pregnancy. The first step is a prenatal screening test. The Non-invasive prenatal test (NIPT) is the standard screening test that detects congenital abnormalities in the DNA of the fetus. This test does not diagnose conditions, but offers an estimation of how likely it is for the fetus to have a genetic condition. If this test suggests that there is an increased likelihood of aneuploidy, the mother proceeds to prenatal diagnostic testing (Hartwig, Ambye, Sørensen, & Jørgensen, 2017). The most popular techniques are amniocentesis, chorionic villus sampling (CVS) and percutaneous umbilical blood sampling (PUBS). Amniocentesis examines the amniotic fluid whereas CVS examines a small sample of cells from the placenta. Finally, PUBS examines a sample of blood from the fetal umbilical cord which is reached through the uterus (Caughey, Hopkins, & Norton, 2006). All three techniques search for changes in the number chromosomes that would indicate trisomy in chromosome 21. However, they are all invasive techniques which involve an elevated risk of miscarriage (NIH, 2017).


Figure 2: Common physical characteristics of Down syndrome (CDC, 2021)

Management of Down Syndrome

As mentioned above, Down Syndrome presents phenotypic variability, meaning that the condition affects each individual differently. Therefore, there is a number of implications that need to be addressed from each person. A small percentage of people are born with congenital cataract. This should be extracted soon after birth, so that is does not ruin the child’s vision. Moreover, people with cardiac implications should undergo surgical treatments as early as possible to minimise health risks and improve the quality of life. Finally, a general suggestion is that individuals follow a balanced diet and exercise regularly in order to maintain their weight, which tends to increase due to their condition (Asim, Kumar, Muthuswamy, Jain, & Agarwal, 2015).


Ethical Considerations and Societal Issues

Ethical issues usually surround the topic of Down syndrome. The main ethical issue is screening for Down’s syndrome. Interestingly, before Down syndrome was declared a serious condition, screening for the disorder was considered to be unethical. However, families wish to know before the birth of the child, in order to provide the best possible quality of life to them. Currently, NIPT, amniocentesis and CVS are standard procedures during pregnancy, available for every prospective mother who is interested in being tested for chromosomal abnormalities (Reynolds, 2003).


Nowadays, individuals with Down syndrome are actively included in society. Numerous individuals with Down syndrome are thriving in various professional fields. A characteristic example is the case of Ellie Goldstein, the famous model featured in world-known magazines such as Vogue and Elle. Ellie was the first person with Down syndrome starring in a luxury fashion campaign. Special Olympics also give the opportunity to individuals with Down syndrome to express their passion for sports, set goals and be active members of the society.


Figure 3: Ellie Goldstein (BBC News, 2023)

Conclusion

Down syndrome is a condition that is interwoven with lifelong developmental and clinical implications for the individual. However, in western societies, advances in medicine as well as social structures allow these individuals to have a good quality of life and thrive professionally. The next goal is for individuals from developing countries to also have the same chances to succeed professionally and have the best possible quality of life.

Bibliographical References

Asim, A., Kumar, A., Muthuswamy, S., Jain, S., & Agarwal, S. (2015). “Down syndrome: An insight of the disease.” Journal of Biomedical Science, 22(1). doi:10.1186/s12929-015-0138-y

https://doi.org/10.1186/s12929-015-0138-y


Caughey, A. B., Hopkins, L. M., & Norton, M. E. (2006). Chorionic villus sampling compared with amniocentesis and the difference in the rate of pregnancy loss. Obstetrics & Gynecology, 108(3, Part 1), 612–616. doi:10.1097/01.aog.0000232512.46869.fc

https://journals.lww.com/greenjournal/Abstract/2006/09000/Chorionic_Villus_Sampling_Compared_With.21.aspx


Centers for Disease Control and Development (CDC) (2022). Data and Statistics from Down Syndrome. Retrieved from

https://www.cdc.gov/ncbddd/birthdefects/downsyndrome/data.html


Glasson, E., Sullivan, S., Hussain, R., Petterson, B., Montgomery, P., & Bittles, A. (2002). The changing survival profile of people with Down’s syndrome: Implications for genetic counselling. Clinical Genetics, 62(5), 390–393. doi:10.1034/j.1399-0004.2002.620506.x

https://doi.org/10.1034/j.1399-0004.2002.620506.x



Hartwig, T. S., Ambye, L., Sørensen, S., & Jørgensen, F. S. (2017). Discordant non-invasive prenatal testing (NIPT) - A systematic review. Prenatal Diagnosis, 37(6), 527–539. doi:10.1002/pd.5049

https://doi.org/10.1002/pd.5049


Hasle, H., Clemmensen, I. H., & Mikkelsen, M. (2000). Risks of leukaemia and solid tumours in individuals with Down’s syndrome. The Lancet, 355(9199), 165–169. doi:10.1016/s0140-6736(99)05264-2

https://doi.org/10.1016/S0140-6736(99)05264-2



Holland, A. J., Hon, J., Huppert, F. A., & Stevens, F. (2001). Incidence and course of dementia in people with Down’s syndrome: Findings from a population-based study. Journal of Intellectual Disability Research, 44(2), 138–146. doi:10.1046/j.1365-2788.2000.00263.x

https://doi.org/10.1046/j.1365-2788.2000.00263.x


Lannoo, L., van Straaten, K., Breckpot, J., Brison, N., De Catte, L., Dimitriadou, E., … Devriendt, K. (2022). Rare autosomal trisomies detected by non-invasive prenatal testing: An overview of current knowledge. European Journal of Human Genetics, 30(12), 1323–1330. doi:10.1038/s41431-022-01147-1

https://doi.org/10.1038/s41431-022-01147-1


MacLennan, M., Crichton, J. H., Playfoot, C. J., & Adams, I. R. (2015). Oocyte development, meiosis and aneuploidy. Seminars in Cell & Developmental Biology, 45, 68–76. doi:10.1016/j.semcdb.2015.10.005

https://doi.org/10.1016/j.semcdb.2015.10.005


Maslen, C. L., Babcock, D., Robinson, S. W., Bean, L. J., Dooley, K. J., Willour, V. L., & Sherman, S. L. (2006). CRELD1 mutations contribute to the occurrence of cardiac atrioventricular septal defects in down syndrome. American Journal of Medical Genetics Part A, 140A(22), 2501–2505. doi:10.1002/ajmg.a.31494

https://doi.org/10.1002/ajmg.a.31494


Reynolds, T. M. (2003). Down’s syndrome screening is unethical: Views of today’s Research Ethics Committees. Journal of Clinical Pathology, 56(4), 268–270. doi:10.1136/jcp.56.4.268

http://dx.doi.org/10.1136/jcp.56.4.268


Sinet, P. M., Théophile, D., Rahmani, Z., Chettouh, Z., Blouin, J. L., Prieur, M., … Delabar, J. M. (1994). Mapping of the down syndrome phenotype on chromosome 21 at the molecular level. Biomedicine & Pharmacotherapy, 48(5–6), 247–252. doi:10.1016/0753-3322(94)90140-6

https://doi.org/10.1016/0753-3322(94)90140-6


Eunice Kennedy Shriver National Institute of Child Health and Human Development (NIH). (2017). How do Healthcare Providers diagnose Down Syndrome? Retrieved from https://www.nichd.nih.gov/health/topics/down/conditioninfo/diagnosis


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