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Clinical Genetics

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What is Down syndrome?
Down syndrome is a common chromosomal disorder that occurs in about 1 in 700 live births. The scientific name for Down syndrome is Trisomy 21.

How is it caused?
Down syndrome or Trisomy 21 is caused when a baby receives an extra copy of the 21st chromosome from one of the parents during conception.  Typically during conception, a baby receives 23 chromosomes from each parent, making a total of 46. The chromosomes are arranged in pairs in every cell of the body. Sometimes, for reasons unknown, an extra chromosome may be present so the baby gets three chromosomes instead of the usual two.  The presence of this extra genetic material is what causes the problems associated with Down syndrome.

What are the symptoms of Down syndrome?
The usual features of Down syndrome are mental retardation, birth defects and characteristic facial features. While the degree of mental retardation can differ greatly from one child to another, most of them have mild-moderate mental retardation. Some of the most common birth defects seen in babies with Down syndrome are heart defects, hearing loss, abnormal eye features as well as gastrointestinal and thyroid problems. Babies with Down syndrome are often born with low muscle tone and can appear ‘floppy’.

How is Down syndrome diagnosed?
Down syndrome can often be identified by the physical features. Trained physicians can recognize the unique signs when examining at a patient.  To confirm a diagnosis of Down syndrome, a blood test can be done to look at the patient’s chromosomes to see if there is an extra 21st chromosome.
During pregnancy, women get offered a number of tests to detect their baby’s risk of having Down syndrome.  Fetal ultrasound can pick up certain signs that are associated with Down syndrome although it is not 100% accurate. Two tests called chorionic villus sampling (CVS) and amniocentesis can detect Down syndrome in a pregnancy.

Is it hereditary?
Although the underlying cause of Down syndrome is due to genetic changes, the condition itself is not usually inherited; this means that it usually does not run in families and happens by chance in 97% of cases.  In about 2-3% of cases, it can run in families due to a rearrangement of chromosomes.  A genetic counselor can help give the risk for a future pregnancy to be affected by considering different factors like family history and age.

What is Prader-Willi syndrome?
Prader-Willi syndrome is a childhood-onset genetic disorder that affects about 1 in 15,000 live births. It is the most common genetic cause of childhood obesity.

How is it caused?
Prader-Willi can occur due to different types of genetic changes involving chromosome 15.  About 75% of cases are due to missing genetic material (called a ‘deletion’) from chromosome 15, 25% of cases are caused when individuals inherit two copies of chromosome 15 from the mother and none from the father and 5% of cases are due to mutations in the chromosome 15 genes which allow only one parent’s genes to be expressed or ‘switched on.’

What are the symptoms of Prader-Willi syndrome?
Prader-Willi syndrome is associated with low muscle tone, short stature, developmental delay, compulsive eating and behavioral problems. Right after birth, babies appear to be ‘floppy’ with low muscle tone and initially may have feeding difficulties. During infancy or later childhood however, children with Prader-Willi develop a constant, insatiable hunger and compulsive eating habits which often lead to overeating and obesity. However, early intervention and controlled diet can really benefit these children and help to significantly improve the outcome.

How is it diagnosed?
Prader-Willi syndrome is typically diagnosed by identifying the main features seen with the condition like low muscle tone and hormone imbalance.  A test called ‘Methylation study’ is done to confirm a clinical diagnosis. Pre and postnatal genetic testing is also available.

Is it hereditary?
The risk of Prader-Willi syndrome occurring again in future pregnancies depends on the underlying cause of the condition in an affected child. If it is due to missing chromosome material called a  ‘deletion’ or due to inheritance of two chromosome 15 copies from the mother, then the risk of it occurring again is very low (close to 1%).  However, if it is found to be due to a mutation, then that means that the child inherited it from one of the parents and there is a 50% risk for each future pregnancy to be affected as well.


What is Turner syndrome?
Turner syndrome is a chromosomal condition that occurs in about 1 in 2000 female live births. It only affects girls.

How is it caused?
Typically during conception, a baby receives 23 chromosomes from each parent (a total of 46) which then get arranged into pairs. The last pair of chromosomes is called the ‘sex chromosomes’ and they determine the gender of a child. The sex chromosomes are named X and Y.  Boys should receive an X chromosome from the mother and a Y chromosome from the father, while girls should receive two X chromosomes- one from each parent.
Sometimes, for reasons unknown, girls may only receive one X chromosome instead of the usual two, so they are missing one sex chromosome.  The absence of the second X chromosome in girls is what causes Turner syndrome.

What are the symptoms of Turner syndrome?
The usual features of Turner syndrome are short stature, early ovarian failure, birth defects, certain physical features such as ‘webbed neck’ (excessive fluid in the lymph nodes cause the neck to appear swollen and ‘webbed’.) About 33% of children with Turner syndrome have heart defects. Kidney problems, thyroid issues and ear infections are also commonly seen.

How is Turner syndrome diagnosed?
Sometimes, the physical features of Turner syndrome can be very subtle so it may be detected clinically. A test called chromosome karyotype is used to diagnose Turner syndrome.  This test looks at the chromosomes in a cell to see if any are missing. Postnatally, the test is done through a blood draw and prenatally it is done by an amniocentesis or chorionic villus sampling.

Is it hereditary?
Most cases of Turner syndrome are sporadic and do not run in families. Rarely, an affected child may have a chromosomal rearrangement called a translocation which is inherited from a parent. In these cases, there is an increased risk for each future pregnancy to be affected as well.  A chromosome karyotype can help determine if Turner syndrome in an affected child is sporadic or due to a translocation.


What are neural tube defects?
Neural tube defects (NTDs) are a group of birth defects that prevent the brain and spinal cord from forming properly. They are one of the most common birth defects seen and occur in 1 in every 1000 live births.

How are they caused?
NTDs occur when the sides of a fetus’ spine fail to join together and close during development.

The underlying cause of NTDs is not really known.  Studies have shown that a combination of environmental factors and genes may play an important role in causing them. Exposures to certain drugs or lack of folic acid during pregnancy can increase a woman’s risk to have a child with an NTD.

How are they diagnosed?
During pregnancy, a woman can find out her risk to have a baby with NTD by doing a blood test called ‘maternal serum AFP’ test. A more accurate test to detect NTD during pregnancy is the ‘fetal AFP’ test. It involves a procedure called an amniocentesis which uses the amniotic fluid surrounding the baby to detect a pregnancy’s risk of NTD.  This test detects about 96% of babies with NTDs. After birth, NTDs are usually picked up by ultrasound.

What are the features of NTD?
There are many different types of NTDs which can range in level of severity. Depending on the type of NTD, the effects can differ. Anencephaly is considered to be the most severe, where the top most part of the spinal cord does not close and so the brain and skull do not form completely. The least severe type is spina bifida occulta where the spine does not close but the hole is still covered by a membrane so individuals usually show no signs or symptoms.

Are they hereditary?
NTDs are usually sporadic occurrences although sometimes a familial pattern is seen where multiple individuals in a family are affected. This could be due to shared environmental factors or genetic factors; since no gene mutation has currently been known to cause NTD, it is not considered to be a genetic defect.

Dr. Lysette Iglesias M.D.

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