Down Syndrome

Introduction:-
Down syndrome (DS) is a condition in which extra genetic material causes delays in the way a child develops, and often leads to mental retardation. It affects 1 in every 800 babies born.

The symptoms of Down syndrome can vary widely from child to child. While some kids with DS need a lot of medical attention, others lead very healthy and independent lives.

Individuals with Down syndrome tend to have a lower than average cognitive ability, often ranging from mild to moderate learning disabilities. A small number have severe to profound mental disability. The incidence of Down syndrome is estimated at 1 per 800 to 1,000 births, although these statistics are heavily influenced by, in particular, the age of the mother. Other factors may also play a role.

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Many of the common physical features of Down syndrome also appear in people with a standard set of chromosomes. They may include a single transverse palmar crease (a single instead of a double crease across one or both palms, also called the Simian crease), an almond shape to the eyes caused by an epicanthic fold of the eyelid, upslanting palpebral fissures, shorter limbs, poor muscle tone, a larger than normal space between the big and second toes, and protruding tongue. Health concerns for individuals with Down syndrome include a higher risk for congenital heart defects, gastroesophageal reflux disease, recurrent ear infections, obstructive sleep apnea, and thyroid dysfunctions.

Early childhood intervention, screening for common problems, medical treatment where indicated, a conducive family environment, and vocational training can improve the overall development of children with Down syndrome. Although some of the physical genetic limitations of Down syndrome cannot be overcome, education and proper care will improve quality of life

Though Down syndrome can’t be prevented, it can be detected before a child is born. The health problems that can go along with DS can be treated, and there are many resources within communities to help kids and their families who are living with the condition.


Causes:-

Normally, at the time of conception a baby inherits genetic information from its parents in the form of 46 chromosomes: 23 from the mother and 23 from the father. In most cases of Down syndrome, however, a child gets an extra chromosome – for a total of 47 chromosomes instead of 46. It’s this extra genetic material that causes the physical and cognitive delays associated with DS.

Although no one knows for sure why DS occurs and there’s no way to prevent the chromosomal error that causes it, scientists do know that women age 35 and older have a significantly higher risk of having a child with the condition. At age 30, for example, a woman has less than a 1 in 1,000 chance of conceiving a child with DS. Those odds increase to 1 in 400 by age 35. By 42, it jumps to about 1 in 60.

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Characteristics:-
Individuals with Down syndrome may have some or all of the following physical characteristics: oblique eye fissures with epicanthic skin folds on the inner corner of the eyes, muscle hypotonia (poor muscle tone), a flat nasal bridge, a single palmar fold, a protruding tongue (due to small oral cavity, and an enlarged tongue near the tonsils), a short neck, white spots on the iris known as Brushfield spots, excessive joint laxity including atlanto-axial instability, congenital heart defects, excessive space between large toe and second toe, a single flexion furrow of the fifth finger, and a higher number of ulnar loop dermatoglyphs. Most individuals with Down syndrome have mental retardation in the mild (IQ 50–70) to moderate (IQ 35–50) range, with individuals having Mosaic Down syndrome (explained below) typically 10–30 points higher. In addition, individuals with Down syndrome can have serious abnormalities affecting any body system. They also may have a broad head and a very round face.

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Kids with Down syndrome tend to share certain physical features such as a flat facial profile, an upward slant to the eyes, small ears, a single crease across the center of the palms, and an enlarged tongue. A doctor can usually tell if a newborn has the condition through a physical exam.

Low muscle tone and loose joints are also characteristic of children with DS, and babies in particular may seem especially “floppy.” Though this can and often does improve over time, most children with DS typically reach developmental milestones – like sitting up, crawling, and walking – later than other kids. At birth, kids with DS are usually of average size, but they tend to grow at a slower rate and remain smaller than their peers. For infants, low muscle tone may contribute to sucking and feeding problems, as well as constipation and other digestive issues. In toddlers and older children, there may be delays in speech and self-care skills like feeding, dressing, and toilet teaching.

Down syndrome affects kids’ cognitive abilities in different ways, but most have mild to moderate mental retardation. Kids with DS can and do learn, and are capable of developing skills throughout their lives. They simply reach goals at a different pace – which is why it’s important not to compare a child with DS with typically developing siblings or even other children with the condition. Kids with DS have a wide range of abilities, and there’s no way to tell at birth what they will be capable of as they grow up.

Medical Problems Associated with Down Syndrome:-
While some kids with DS have no other health problems, others may experience a host of medical issues that require extra care. For example, half of all children born with DS also have congenital heart defects and are prone to developing pulmonary hypertension (high blood pressure in the lungs). A pediatric cardiologist can monitor these types of problems, many of which can be treated with medication or surgery.

Approximately half of all kids with DS also have problems with hearing and vision. Hearing loss can be related to fluid buildup in the inner ear or to structural problems of the ear itself. Vision problems commonly include amblyopia (lazy eye), near- or farsightedness, and an increased risk of cataracts. Regular evaluations by an audiologist and an ophthalmologist are necessary to detect and correct any problems before they affect a child’s language and learning skills.

Other medical conditions that may occur more frequently in children with DS include thyroid problems, intestinal abnormalities, seizure disorders, respiratory problems, obesity, an increased susceptibility to infection, and a higher risk of childhood leukemia. Fortunately, many of these conditions are treatable.

Prenatal Screening and Diagnosis:-
There are two types of prenatal tests available to detect Down syndrome in a fetus: screening tests and diagnostic tests. Screening tests estimate the risk that a fetus has DS; diagnostic tests can tell whether the fetus actually has the condition.

Screening tests are noninvasive and generally painless. But because they can’t give a definitive answer as to whether a baby has DS, mostly they’re used to help parents decide whether to have more diagnostic tests.

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Diagnostic tests are about 99% accurate in detecting Down syndrome and other chromosomal abnormalities. However, because they are performed inside the uterus, they are associated with a risk of miscarriage and other complications. For this reason, they are generally recommended only for women age 35 or older, those with a family history of genetic defects, or those who’ve had an abnormal result on a screening test. If you’re unsure about which test, if any, is right for you, your doctor or a genetic counselor can help you sort through the pros and cons of each.

Screening tests include:

Nuchal translucency testing. This test, performed between 11 and 14 weeks of pregnancy, uses ultrasound to measure the clear space in the folds of tissue behind a developing baby’s neck. (Babies with DS and other chromosomal abnormalities tend to accumulate fluid there, making the space appear larger.) This measurement, taken together with the mother’s age and the baby’s gestational age, can be used to calculate the odds that the baby has DS. Nuchal translucency testing correctly detects DS about 80% of the time; when performed with a maternal blood test, it may offer greater accuracy.
The triple screen (also called the multiple marker test) and the alpha fetoprotein plus. These tests measure the quantities of various substances in the mother’s blood, and together with the woman’s age, estimate the likelihood that her baby has Down syndrome. They are typically offered between 15 and 20 weeks of pregnancy.
A detailed ultrasound. This is often performed in conjunction with the blood tests, and it checks the fetus for some of the physical traits associated with Down syndrome. However, these screening tests are only about 60% accurate and often lead to false-positive or false-negative readings.
Diagnostic tests include:
Amniocentesis. This test, performed between 16 and 20 weeks of pregnancy, involves the removal of a small amount of amniotic fluid through a needle inserted in the abdomen. The cells can then be analyzed for the presence of chromosomal abnormalities. Amniocentesis carries a small risk of complications, such as preterm labor and miscarriage.
Chorionic villus sampling (CVS). CVS involves taking a tiny sample of the placenta, also through a needle inserted in the abdomen. The advantage of this test is that it can be performed earlier than amniocentesis, between 8 and 12 weeks. The disadvantage is that it carries a slightly greater risk of miscarriage and other complications.
Percutaneous umbilical blood sampling (PUBS). Usually performed after 20 weeks, this test uses a needle to retrieve a small sample of blood from the umbilical cord. It carries risks similar to those associated with amniocentesis.
After a baby is born, a diagnosis of Down syndrome can usually be made just by looking at the baby. If the doctor suspects DS, a karyotype – a blood or tissue sample stained to show chromosomes grouped by size, number, and shape – can be performed to verify the diagnosis.

Health:-
The medical consequences of the extra genetic material in Down syndrome are highly variable and may affect the function of any organ system or bodily process. The health aspects of Down syndrome encompass anticipating and preventing effects of the condition, recognizing complications of the disorder, managing individual symptoms, and assisting the individual and his/her family in coping and thriving with any related disability or illnesses.

Down syndrome can result from several different genetic mechanisms. This results in a wide variability in individual symptoms due to complex gene and environment interactions. Prior to birth, it is not possible to predict the symptoms that an individual with Down syndrome will develop. Some problems are present at birth, such as certain heart malformations. Others become apparent over time, such as epilepsy.

The most common manifestations of Down syndrome are the characteristic facial features, cognitive impairment, congenital heart disease (typically a ventricular septal defect), hearing deficits (maybe due to sensory-neural factors, or chronic serous otitis media, also known as Glue-ear), short stature, thyroid disorders, and Alzheimer’s disease. Other less common serious illnesses include leukemia, immune deficiencies, and epilepsy.

However, health benefits of Down syndrome include greatly reduced incidence of many common malignancies except leukemia and testicular cancer — although it is, as yet, unclear whether the reduced incidence of various fatal cancers among people with Down syndrome is as a direct result of tumor-suppressor genes on chromosome 21 (such as Ets2), because of reduced exposure to environmental factors that contribute to cancer risk, or some other as-yet unspecified factor. In addition to a reduced risk of most kinds of cancer, people with Down syndrome also have a much lower risk of hardening of the arteries and diabetic retinopathy.

Life expectancy :-
These factors can contribute to a shorter life expectancy for people with Down syndrome. One study, carried out in the United States in 2002, showed an average lifespan of 49 years, with considerable variations between different ethnic and socio-economic groups. However, in recent decades, the life expectancy among persons with Down Syndrome has increased significantly up from 25 years in 1980. The causes of death have also changed, with chronic neurodegenerative diseases becoming more common as the population ages.


Fertility:-

Fertility amongst both males and females is reduced, with only three recorded instances of males with Down syndrome fathering children.

Genetic research:-
Main article: Research of Down syndrome-related genes
Down syndrome is “a developmental abnormality characterized by trisomy of human chromosome 21″ (Nelson 619). The extra copy of chromosome-21 leads to an over expression of certain genes located on chromosome-21.

Research by Arron et al shows that some of the phenotypes (displayed genetic characteristics), associated with Down Syndrome can be related to the dysregulation of gene-regulating proteins (596). The gene-regulating proteins bind to DNA and initiate certain segments of DNA to be replicated for the production of a certain protein (Arron et al. 596). The gene-regulator in interest is called NFATc. Its activities are controlled by two proteins, DSCR1 and DYRK1A; these genes are located on chromosome-21 (Epstein 582). In people with Down Syndrome, these proteins have 1.5 times greater concentration than normal (Arron et al. 597). The elevated levels of DSCR1 and DYRK1A mean that most of the NFATc is located in the cytoplasm rather than in the nucleus promoting DNA replication which will produce vital proteins (Epstein 583).

This dysregulation was discovered by testing in transgenic mice. The mice had segments of their chromosomes duplicated to simulate a human chromosome-21 trisomy (Arron et al. 597). A common characteristic of Down Syndrome is poor muscle tone, so a test involving the grip strength of the mice showed that the genetically modified mice had a significantly weaker grip (Arron et al. 596). The mice squeezed a probe with a paw; the modified mice displayed a .2 Newton (measurement of force) weaker grip (Arron et al. 596). Down syndrome is also characterized by increased socialization. Both modified and unmodified mice were observed for social interaction. The modified mice showed as many as 25% more interactions per time period as the unmodified mice (Arron et al. 596).

The genes that may be responsible for the phenotypes associated may be located proximal to 21q22.3. Testing by Olson et al, in transgenic mice show the duplicated genes presumed to cause the phenotypes are not enough to cause the exact features. While the mice had sections of multiple genes duplicated to approximate a human chromosome-21 triplication, they only showed slight craniofacial abnormalities (688-690). The transgenic mice were compared to mice that had no gene duplication by measuring distances on various points on their skeletal structure and comparing them to the normal mice (Olson et al. 687). The exact characteristics of Down Syndrome were not observed, so more genes involved for Down Syndrome phenotypes have to be located elsewhere.

Reeves et al, using 250 clones of chromosome-21 and specific gene markers, were able to map the gene in mutated bacteria. The testing had 99.7% coverage of the gene with 99.9995% accuracy due to multiple redundancies in the mapping techniques. In the study 225 genes were identified (311-313).

The search for major genes that may be involved in Down syndrome symptoms is normally in the region 21q21–21q22.3. However, studies by Reeves et al. show that 41% of the genes on chromosome-21 have no functional purpose, and only 54% of functional genes have a known protein sequence. Functionality of genes was determined by a computer using exon prediction analysis (312). Exon sequence was obtained by the same procedures of the chromosome-21 mapping.

Research has led to an understanding that two genes located on chromosome-21, that code for proteins that control gene regulators, DSCR1 and DYRK1A can be responsible for some of the phenotypes associated with Down Syndrome. DSCR1 and DYRK1A cannot be blamed outright for the symptoms; there are a lot of genes that have no known purpose. Much more research would be needed to produce any appropriate or ethically acceptable treatment options.

Recent use of transgenic mice to study specific genes in the Down syndrome critical region has yielded some results. APP[44] is an Amyloid beta A4 precursor protein. It is suspected to have a major role in cognitive difficulties.[45] Another gene, ETS2[46] is Avian Erythroblastosis Virus E26 Oncogene Homolog 2. Researchers have “demonstrated that over-expression of ETS2 results in apoptosis. Transgenic mice over-expressing ETS2 developed a smaller thymus and lymphocyte abnormalities, similar to features observed in Down syndrome.

Getting Help:-
If you’re the parent of a child diagnosed with Down syndrome, you may at first feel overwhelmed by feelings of loss, guilt, and fear. Talking with other parents of kids with DS may help you deal with the initial shock and grief and find ways to look toward the future. Many parents find that learning as much as they can about DS helps alleviate some of their fears.

Experts recommend enrolling kids with Down syndrome in early intervention services as soon as possible after your child is born. Physical, occupational, and speech therapists and early-childhood educators can work with your child to develop motor skills and language, and show you how to encourage these skills at home. Many states provide free early-intervention services to kids with disabilities from birth to age 3, so check with your child’s doctor or a social worker to determine what resources are available in your area.

Once your child is 3 years old, he or she is guaranteed educational services under the Individuals with Disabilities Education Act (IDEA). Under IDEA, local school districts must provide “a free appropriate education in the least restrictive environment” and an individualized education plan (IEP) for each child.

Where to send your child to school can be a difficult decision. Some kids with Down syndrome have needs that are best met in a specialized program, while many others do well attending neighborhood schools alongside peers who don’t have DS. Studies have shown that this type of situation, known as inclusion, is beneficial for both the child with DS as well as the other children. Your school district’s child study team can work with you to determine what’s best for your child, but remember, any decisions can and should involve your input, as you are your child’s best advocate.

Today, many children with Down syndrome grow up going to school and enjoying many of the same activities as other kids their age. A few go on to college. Many transition to semi-independent living. Still others continue to live at home but are able to hold jobs, thus finding their own success in the community.
CLICK TO READ MODERN RESEARCH ON DOWN SYNDROME

Resources:
http://www.kidshealth.org/parent/medical/genetic/down_syndrome.html
http://en.wikipedia.org/wiki/Down_syndrome

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