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Medical Genetics : Down Syndrome

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Question:

Discuss about the Medical Genetics for Down Syndrome.
 
 

Answer:

Introduction

DS is a persistent state caused by the extra copy of chromosome 21(Hsa21). The parents of DS affected children are normal atypically on context to genetics. The chance of having an extra copy of chromosome 21 is 95%. This extra copy is presented randomly when there is some form of abnormality during the process of formation of egg or sperm. In some cases the parents may pass on the genes, increasing the chances of the grandchildren to have Down syndrome. For the rest of the 5% cases, the syndrome is inherited from a healthy parent. The detection for the presence of the silent risk for Down syndrome in the DNA can b done with the help of some very useful tests that give accurate results.

DS results when a complete or partial copy of the chromosome 21 is present. The chromosomes are the collection of genetic information present in every cell within the human body. Generally, 46 chromosomes present in human body are organized in 23 pairs of chromosomes. Among them, 22 pairs are known as autosomes and one pair referred to as sex chromosomes. However, in individuals with DS, an extra copy of the chromosome 21 is found and this condition is referred to as trisomy 21. Hence, such individuals have 47 chromosomes instead of 46 chromosomes. As a result, present of this three copies of chromosome 21 with genetic material leads to the occurrence of DS (Malt et al., 2013).The risk of being affected by the disorder increases from 0.1% among the 20 years old mothers to 3% within the mothers in the age group of 45 years (nads.org, 2016).  Until date, there has not been found any activity related to behavioral patterns or environmental determinants that alter the risk of the occurrence of disorder.The individuals with this genetic disorder have some distinguishing facial as well as physical characteristics, medical complications and cognitive disabilities. The facial traits include upturned eyes, flat nose, small nose, small mouth with proportionally larger tongue and round face with flattened profile on a average. The physical features include small feet and short finger (Langlois et al., 2013). They have higher chances of developing medical complications.  They have vision and hearing difficulties too.  They are found with heart defects that might be mild or severe.  In addition to the above-mentioned issues, they might suffer from gastrointestinal problems, thyroid functioning, chances of developing cancer and mental illness. A pedigree chart for DS is a rough outlining of the likelihood of a child for developing with this condition because of the previous generations. Since, the DS is not an inherited disorder, so accurate diagram for predicting the chances of developing it is not possible. It occurs in case where a child possesses an extra copy of chromosome (reference).

Down syndrome phenotype is the result of a dosage imbalance occurring in the genes found on human chromosome 21 (Hsa 21).  The genetic characteristics of Down syndrome have been widely investigated by scientists in the past one decade. The length of 21 p is 5–15 Mb [12] while the length of 21q is 33.5 Mb [11]. At the time of initial sequencing of 21q, it was estimated that a total number of 225 genes are presented in the sequence. Around 40.06% repetitions in the sequences are found in Hsa 21 [11]. Out of the sequence repetitions, 10.84% are SINE’s, 15.15% are LINE’s and 9.21% are LTR.

Genetics of Down syndrome

Down syndrome babies are generally formed due to the presence of an extra copy of chromosome 21 that results in trisomy.  Down syndromes can be caused in babies also due ring chromosome or isochromosomal condition and Robertsonian translocation. In  Isochrome condition two long arms of the chromosome are found to be separating together, in contrast to the normal condition in which the short arm and the long arm separate together at the time of development of egg or sperm. Trisomy 21 is characterised by karyotype 47, XY, + 21 in a case of males and karyotype 47, XX, + 21 in a case of females. This condition is a result of the erroneous separation of the chromosome 21 at the time of sperm or egg development.  Robertsonian translocation occurs only in 2-4% cases when the long arm of the chromosome 21 erroneously gets attached to another chromosome instead of the chromosome 14 for normal cases.  Mosaicism is referred to the condition in which two dissimilar genotypes present in humans after developing from a single egg due to misdivision. This condition gives rise to two different cell lineages in individuals contributing to tissues and organs having mosaicism. One of the cell lineages has the normal number of chromosome while the second lineage has an extra number of chromosome 21 [15].

 

Correlation between genotype and phenotype

As per the gene dosage imbalance, patients suffering from Down syndrome carry an increased copy number of the gene on Hsa 21 resulting in increased gene expression [13-15]. Within the hypothesis, the chances of particular genes and gene subsets controlling particular Down syndrome phenotypes have been encompassed [16]. As per the Amplified developmental instability hypothesis, genetic imbalance results from a non-specific dosage of a number of trisomic genes. The regulation and expression of a number of genes are impacted due to this along the whole genome [13-14]. Another related hypothesis on this subject is the critical region hypothesis. Phenotypic analysis gave rise to the conclusion that individuals having partial trisomy for Hsa 21 possess one or few small “Down syndrome critical regions” (DSCR) that are 3.8-6.5 Mb in length, located at the 21q region. Around 30 genes are responsible for the majority of phenotypes leading to Down syndrome [15-16]. A region of 1.6-2.5 Mb was previously recognised as the sole reason for a phenotype of the syndrome [17-18]. The research on Down syndrome witnessed a milestone with the advent of the sequencing of Hsa 21 [19]. With the possibility of Hsa 21 sequencing, more information about the relation between phenotype and genotype was able to be gathered, and accurate characterisation of DSCR regions became possible [13]. A number of Down syndrome phenotypes, like, clinodactyly of the fifth finger, congenital heart defects of the endocardial cushions, craniofacial abnormalities, and mental retardation were believed to be resulting from a critical region within 21q22 [20]. Scientists have suggested that Down syndrome cell adhesion molecule (DSCAM), acting as a regulator of calcineurin 1 (RCAN1), and phosphorylation-regulated kinase (DYRK1A) have a pivotal role in the brain development. It also has a role in augmented risks of congenital heart diseases (CHD) in people suffering from Down syndrome [21-22]. Neural differentiation, development of neural networks and axon guidance involves DSCAM to be a significant factor in their functioning. Any form of disruption occurring in these processes leads to the Down syndrome neuro-cognitive phenotype [22]. Different studies on mouse models and humans suffering from Down syndrome have indicated that no single region of genes is responsible for all of the phenotypes of this syndrome. On the other hand, there are chances that multiple critical regions or genes are responsible for a particular phenotype or a group of phenotypes that are associated with Down syndrome [23].

Risk factors

There are a number of risk factors which have been suggested to contribute to the birth of a child with DS, like maternal grandmother’s diet, lifestyles, genotype, occupational and environmental exposures. All of these factors cause recombination errors, giving rise to nondisjunction of chromosome 21 at maternal meiosis I. This happens at the time of fetal development in the body of the maternal grandmother or first events in sequential meiosis I and meiosis II errors (Coppedè 2015).  Age of mother at conception, maternal diet, lifestyles, genotype and occupational and environmental exposures increase the chances of meiotic errors at maternal meiosis II or the second event in sequential meiosis I and II errors giving rise to chromosome 21 nondisjunction (Rowsey et al. 2013). Dietary habits of the father, along with the genotype, lifestyles and occupational and environmental exposure is a contributing factor towards chromosome 21 disomy in spermatozoa. The trisomy 21 of paternal origin is the result of this (Oliver et al. 2009). Complex association of the genome of developing trisomic embryo with maternal environmental exposures and genotype, diet increases the chances of miscarriage of trisomy 21 pregnancies or survival up to the birth. Genetic variants that are inherited from environmentally induced epigenetic modifications and parents at the time of early development can lead to survival to the birth.

 

Screening and Diagnostic methods

There are two types of tests available for DS, which can be done prior to the birth of a baby; they are screening tests and diagnostic tests. Though the screening tests do not provide information that the disorder would take place in the baby surely, however, it merely states the chances that a baby may develop such a disorder. Whereas, the diagnosis tests facilitate 100% accurate results and provide absolute diagnosis. A new screening technique is available for prenatal screening among the pregnant mothers. The blood tests help in measuring the amount of substances present in the mother’s blood. These tests help in estimating the chances of the mother to give birth to a baby with DS.  Mostly, the screening tests include blood or serum tests and sonogram.

These advanced techniques are now available for detection of fetal chromosomes that circulates in mother’s blood. Unlike the diagnostic tests, these screening tests are non-invasive but give accurate results. The diagnostic tests that are available for prenatal detection of DS include Chronic Villus Sampling (CVS) as well as amniocentesis. These procedures although carry a risk factor of 1% chance of miscarriage, nevertheless these methodologies provide with 100% accurate diagnosis of DS. Amniocentesis is generally done during the second trimester of gestation period between 15 and 20 weeks, CVS is performed mostly between 9 and 14 weeks of pregnancy. The recognition of DS is generally performed at birth. It is detected by the presence of certain physical characteristics such as low tonality of muscles, a single crease in the palm are of the hand, flat profile, rounded face and upturned eyes. However, such traits are sometimes present in normal babies, so a chromosomal analysis is done referred to as karyotype in order to reaffirm the diagnosis. Generally, blood is drawn from cells of the baby for performing the karyotype. The chromosomes are photographed. They are grouped according to their size, shape and number. In this way, the doctors diagnose DS by examination of the karyotype.  There is another test for examining genetic disorders known as uorescent in situ hybridization(FISH). This is applied using the same principle as that of the karyotype for confirming the diagnosis for shorter time-period (Palomaki et al., 2012).

After it was discovered in the year 1969 that fetal lymphocytes are present in maternal blood, cell-free fetal nucleic acids (cffNA) has proved to be a suitable method for early non-invasive prenatal diagnosis [64]. The genetic status that a fetus has can be easily detected by this appropriate approach. A small fraction of the total circulating cell-free DNA, around 3-6% is cell-free fetal DNA. Research has indicated that cffDNA is initially observable in the time frame of 7 weeks of gestation. With the progress of pregnancy, there is an increase in the amount of cffDNA. cffDNA is found to be reducing after the baby is born and cannot be detected in the blood of the mother [65]. The possible applications of the advanced technology can be categorised into two groups; (i) high genetic risks families who have inheritable monogenic diseases, sex determination and detection of paternally inherited single gene disorders [66,67]; and (ii) regular antenatal care given to all women who are pregnant, encompassing prenatal diagnosis, screening for aneuploidy, like Down syndrome, and Rhesus factor diagnosis in women who are RhD negative [65].

 

Management of Down syndrome

DS is not curable. However, early interventions and treatment procedures may help the individuals to lead productive life. The treatments include exercises, speech and occupational therapy for improving the motor abilities. Special educational patterns and proper care with attention can help them further. The medical complications like hearing impairments, thyroid and related disorders can be corrected surgically. The use of amino acid supplements as well as drugs like Piracetam is believed to provide improved ability to apprehend and teach (Bartesaghi et al., 2015).

In attempts to cure DS, many researchers have made great efforts in tackling DS through gene therapy. Gene therapy has already been tested on animals and success has been achieved.

Some researchers in July 2013have attempted to bring developments in the ‘chromosome therapy’ for Down syndrome. Inactivation of one copy of chromosome 21 was possible by the insertion of a copy of the X-inactive specific transcript (XIST) gene into the chromosome. The gene is normally responsible for the inactivation of one X female chromosome out of the two present. XIST is responsible for encoding an RNA that is non-coding in nature. This RNA silences expression of genes through the process that covers the X-chromosome with a blanket of RNA. This RNA blanket is then responsible for triggering a sequence of chromosomal changes for silencing the expression of genes. Such changes include chemical modifications resulting in the histones to be more tightly wrapped by the DNA. A condensed X chromosome, termed as the Barr body, is therefore formed, with a silent gene expression.

A research publication in August 2015 discussed a novice therapy to be utilised for Down syndrome. The researchers were able to generate cells having the typical chromosome number through the introduction of the ZSCAN gene 4 into the cell line of individuals suffering from this trisomic condition. They were successful in converting a full trisomic condition into a mosaic syndrome, of which some had an extra chromosome while some did not. Mosaicism takes place in a natural process in some people suffering from Down syndrome; however, this therapy marked the possibility of inducing a mosaic condition with the help of genetic therapy.

Researchers at Boston University and Yale University in the year 2016 did a comparison of Down syndrome across development and adulthood with gene expressions in different parts of the human brain. The findings were such that white matter is established in the brain in a changing process from infancy to adulthood. White matter is responsible for insulating the nerve fibers of the brain that is axons. The finding that the study led to was unanticipated since the present theory states that many of the changes that lead to intellectual disability take place prenatally in Down syndrome. The results also indicated that changes in white matter are a result of particular defects in a certain group of brain cells, termed as ‘oligodendrocytes,' occurring during the developmental stages. Oligodendrocytes are behind the formation of white matter and any defects in it lead to a stalled nerve transmission.

Ethical issues

The factors that contribute to the availability of screening services to patients include financial status, subject knowledge, and patient education on the information relevant to the syndrome. This concern of patient’s access to services raises an equity issue. For instance, the cost of services may reach up to $1000 in the private sector. The prevailing inequality in relation to access to services for the couple to have a child forms a vital argument supporting the universal availability of the test (Bassett, Lee et al. 2004). Some argue the need of setting guidelines for ensuring standardisation in the process of prenatal screening (Pratte, 2003, p. 197).

Another challenge that draws attention is that there is an absence of skilled healthcare professionals capable of providing information to the patients regarding the process of screening. The general population would be benefitted by the implementation of a well-developed screening program that can provide them with guidance and information on the standardisation of the information. The result of such approach would be the proper assessment of practices for ensuring optimal quality. Parents need to be provided with balanced and accurate information regarding the definite effects of child birth with Down syndrome. They are also required to be provided with this information in a manner that is not affected by the education level and social class they have. The underpinning principle behind these needs is that reproductive decisions are taken up by the parents. This need for imparting medical information is applicable for testing options as this would make it possible for parents to evaluate the risk of Down syndrome and confirm it. At present, the accessibility of tests is dependent on the financial condition of the parents as well as local resources.

Since pregnant women over the age of 35 have a risk to carry a fetus with this syndrome, these women are offered amniocentesis. However, there is a major risk associated with the diagnostic test, and that is the fetal loss (Wilson, Langlois et al. 2007). The loss of fetus also includes cases of foetuses that are healthy, and chances were there to have them born alive in case the test was not performed. There is a possibility of identifying the risks prior to the diagnostic test is done, and this approach would eliminate the need of performing the amniocentesis test (Bassett, Lee et al., 2004, p. 109). The risk has been a reason why researchers have suggested other diagnostic methods. For instance, the group of researchers had been making efforts to develop methods for establishing a diagnostic method that would involve analysis of fetal cells and DNA from the blood of the mother (Audibert, 2006).

Pregnancy termination is the only possible action for preventing giving birth to Down syndrome babies as there are no suitable medical solutions (Kohut, Rusen et al. 2002, p. 5). Abortion has been made illegal in Canada since the year 1969 (Pratte, 2203, p. 72) and the instance of its practice is not confined (Doucet, Létourneau et al., 2007, p. 31). Pregnancy termination is not an option for many individuals. Some consider screening to raise a question of a life worth living as the condition cannot be cured at present.

 

Conclusion

DS is one of the most commonly occurring genetic disorders that occur due to formation of an extra copy of chromosome 21. Many theories have provided insight into the genotype and phenotype correlation and increased our knowledge in this matter. A “critical region” within 21q22 was thought to be the main reason behind different Down syndrome phenotyping, like congenital heart defects of the endocardial cushions, clinodactyly of the fifth finger craniofacial abnormalities, and mental retardation. The modern diagnostic procedures are however capable of detecting the disorder with accuracy. In this way, early detection can be helpful for treating such patients by speech therapy, exercises so that they can lead a prospective life. However, further advancement in the field of genetics will help in adding on to more technologies for treating DS.

 

References:

Bartesaghi, R., Haydar, T. F., Delabar, J. M., Dierssen, M., Martínez-Cué, C., & Bianchi, D. W. (2015). New Perspectives for the Rescue of Cognitive Disability in Down Syndrome. The Journal of Neuroscience, 35(41).

Epstein, C. J. (2013). of Down Syndrome. Molecular Genetic Medicine, 2, 105.

Langlois, S., Brock, J. A., Wilson, R. D., Audibert, F., Carroll, J., Cartier, L., ... & Okun, N. (2013). Current status in non-invasive prenatal detection of Down syndrome, trisomy 18, and trisomy 13 using cell-free DNA in maternal plasma. Journal of Obstetrics and Gynaecology Canada, 35(2).

Malt, E. A., Dahl, R. C., Haugsand, T. M., Ulvestad, I. H., Emilsen, N. M., Hansen, B., ... & Davidsen, E. M. (2013). Health and disease in adults with Down syndrome. Tidsskrift for den Norske laegeforening: tidsskrift for praktisk medicin, ny raekke, 133(3).

Palomaki, G. E., Deciu, C., Kloza, E. M., Lambert-Messerlian, G. M., Haddow, J. E., Neveux, L. M., ... & Nelson, S. F. (2012). DNA sequencing of maternal plasma reliably identifies trisomy 18 and trisomy 13 as well as Down syndrome: an international collaborative study. Genetics in medicine, 14(3).

Vos, T., Barber, R. M., Bell, B., Bertozzi-Villa, A., Biryukov, S., Bolliger, I., ... & Duan, L. (2015). Global, regional, and national incidence, prevalence, and years lived with disability for 301 acute and chronic diseases and injuries in 188 countries, 1990–2013: a systematic analysis for the Global Burden of Disease Study 2013. The Lancet, 386(9995).

www.nads.org,. (2016). Facts About Down Syndrome- National Association for Down Syndrome. Retrieved 31 October 2016, from https://www.nads.org/resources/facts-about-down-syndrome/

www.scicarlyrae.files.wordpress.com,. (2016). Retrieved 31 October 2016, from https://scicarlyrae.files.wordpress.com/2011/02/pedigree1.png

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