Difficulty Index ★★★☆☆
Iyla Kwon '27
Imagine you are reading a comic series with a missing book. You will struggle to understand the rest of the stories. It is the same in human DNA, a missing or extra chromosome referred to in a missing comic book. Chromosomes contain genetic information, the DNA, which contributes to the formation and management of the human body. They are set in the exact number of 46 in total, each functioning respectively. So, if the chromosomes appear in aneuploidy in the human body, this leads to genetic disorders such as Down Syndrome, Turner Syndrome, and Cri du Chats Syndrome, resulting in developmental and health issues. This literature review is to delve into each chromosomal abnormality alongside the influence of the absence or surplus of a piece of chromosome in humans.
The most common chromosomal disorder is Down syndrome, which occurs at a rate of approximately 1 in every 700 births. Down Syndrome is an abnormal genetic condition caused by an extra copy of chromosome 21. This syndrome is also known as Trisomy 21, with trisomy representing the presence of an extra chromosome. If the baby is born with Down Syndrome, the extra copy will deform the development of its brain and body. The representative physical features of people with Down Syndrome are a flattened face, often with the bridge of the nose, slanted almond-shaped eyes, a short neck, and small ears, hands, and feet. These traits most likely accompany intellectual disability and genetic risk factors for congenital heart attack. However, there is no cure for Down Syndrome, so the current treatment focuses on speech therapies and intervention programs for additional health issues. Mouse modeling has been contributing to the development of treating Down Syndrome. It started in the early 1990s, and these model mice mimic the trisomy of chromosome 21, found in humans with Down Syndrome, having similar brain changes. For example, they discovered an imbalance in hippocampal signals in mice, having trisomy, which indicates to target in improv cognitive function. Additionally, researchers found out that the treatments for the SHH(sonic hedgehog) pathway can normalize the growth of the cerebellum(Katheleen 1). These models also contributed to the study of brain plaques and tangles, which are perilous symptoms of Down Syndrome. Overall, people are looking forward to ongoing research on the treatments for Down Syndrome, the trisomic genetic disorder.
Unlike Down Syndrome which occurs regardless of gender, Turner Syndrome is a distinctive abnormality, which is only discovered in females. It is caused by the absence of an X chromosome, while others usually have a pair of chromosomes, a total of 2 chromosomes. The failure in the separation of X chromosomes in the process of cell division or merely a mutation in X chromosomes itself leads to abnormality in X chromosomes, resulting in Turner Syndrome. The absence of a completed X chromosome causes various developmental issues in females, like shorter height as the usual growth is constrained. The other possible physical characteristics include a webbed neck, low-set ears, a broad chest, and swollen hands and feet. Heart defects and kidney problems are common to those who have Turner Syndrome, susceptible to autoimmune diseases. Most girls with Turner Syndrome have less-developed ovaries, which causes infertility and a lack of puberty without any medical help. They might also have difficulties with spatial reasoning despite their normal intelligence. The treatments for Turner Syndrome focus on two major treatments broadly; growth hormone therapy and estrogen replacement. Hormone therapy can help increase height during childhood and estrogen replacement therapy will be used to start puberty and maintain bone health. Majority of the girls with Turner Syndrome need hormone replacement therapy to start and maintain puberty, develop secondary sexual characteristics, and ensure healthy bones and uterine growth. The best method involves starting low-dose estrogen (transdermal patches) around age 11 to 12, gradually increasing the dose, and eventually combining it with progestin (Klein 1). Although there's no cure for Turner Syndrome as Down Syndrome did, these treatments will gradually develop as the cure for those suffering from this syndrome.
Cri du chat syndrome would be the rarest genetic disorder, among other syndromes in this review, caused by a deletion of a portion of the short arm of chromosome 5. This mutation occurs randomly during the formation of reproductive cells or in the process of early fetal development. However, the exact cause of the deletion remains unknown usually. Cri du chat syndrome is named after a high-pitched cry from infants that sounds like a mewing of a cat, which is due to problems with the nervous system. Other symptoms are intellectual disability, particularly in speech, and delayed development with distinctive facial features, small head size, and low birth weight. There is also no cure for Cri du chat syndrome, but early intervention programs, including speech, physical, and occupational therapies, can help to improve the disabilities derived from this syndrome. Continuous medical care, educational support, and counseling for infants are also crucial to help those with Cri du chat syndrome.
In conclusion, Down Syndrome, Turner Syndrome, and Cri du Chat Syndrome are some examples of chromosomal abnormalities, each presenting unique aneuploidy in humans. Down Syndrome is caused by an extra copy of chromosome 21, and Turner Syndrome results from a missing X chromosome in females, while Cri du Chat Syndrome occurs due to a deletion on chromosome 5. As none of the syndromes has a cure, advancements in genetic research promise more effective treatments and potential cures for those who are suffering from these syndromes.
Understanding the genetic bases of these syndromes will facilitate the discovery of successful therapies and remedies and be able to prevent and avoid potential chromosomal abnormalities in the future.
References
1. Gardiner, Katheleen, Yann Herault, Ira T. Lott, Stylianos E. Antonarakis, Roger H. Reeves, and Mara Dierssen. “Down Syndrome: From Understanding the Neurobiology to Therapy.” Journal of Neuroscience, November 10, 2010. https://www.jneurosci.org/content/30/45/14943.full
2. Gardiner, Katheleen, Yann Herault, Ira T. Lott, Stylianos E. Antonarakis, Roger H. Reeves, and Mara Dierssen. “Down Syndrome: From Understanding the Neurobiology to Therapy.” Journal of Neuroscience, November 10, 2010. https://www.jneurosci.org/content/30/45/14943.full
3. Cleveland Clinic medical. “Cri-Du-Chat (Cat’s Cry) Syndrome: Symptoms & Causes.” Cleveland Clinic. Accessed May 17, 2024. https://my.clevelandclinic.org/health/diseases/24084-cri-du-chat-syndrome
4. “Cri Du Chat Syndrome - Symptoms, Causes, Treatment: Nord.” National Organization for Rare Disorders, November 20, 2023. https://rarediseases.org/rare-diseases/cri-du-chat-syndrome/
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