1. Sickle Cell Anemia

Sickle Cell Anemia is a hereditary genetic disorder in which the body makes an abnormal form of hemoglobin, a protein in red blood cells that carries oxygen. HBB gene is responsible for production of beta-globin protein of the hemoglobin molecule. It is located on chromosome 11p15.5. Different kind of mutations in HBB gene are responsible for formation of faulty hemoglobin. One particular HBB gene mutation produces an abnormal version of beta-globin known as hemoglobin S (HbS). HbS has a substitution of valine for glutamic acid at the sixth position of the beta-globin chain from normal. People with this disorder have a deformed hemoglobin in blood called hemoglobin S, which can distort red blood cells into a sickle or crescent shape. The sickle-shaped red blood cells are incapable of carrying oxygen and die prematurely, which often lead to severe anemia. Sickle Cell Anemia can cause shortness of breath, fatigue, and delayed growth and development in children

2. Huntingtons DiseaseHD is an inherited neurological condition affecting the brain and nervous system. It is a genetic condition that causes involuntary muscle movements, personality changes, and reduced memory and reasoning abilities. Symptoms commonly associated with HD, previously known as Huntingtons chorea, were recorded in European populations in the 1600s. The condition is named after Dr George Huntington who described the inheritance of HD as early as 1872.

Prenatal genetic testing optionsChorionic villus sampling (CVS)CVS is usually carried out between 11 and 12 weeks of pregnancy. Therefore, it is advisable to see a doctor as soon as a pregnancy begins. CVS involves taking a small sample of cells from the chorion, which is the lining of the uterus that later develops into the placenta. Under the guidance of ultrasound, a small sample of chorion is removed for testing by passing a fine needle through the cervix or through the abdominal wall. A local anaesthetic is usually used prior to the procedure. The sample is tested for an expanded huntingtin gene copy. Laboratories aim to provide a result within a short time frame. When CVS is carried out by an obstetrician who is experienced in the technique the risk of miscarriage related to the test is less than 1% (less than 1 in 100 pregnancies).

AmniocentesisAmniocentesis is usually carried out between 15 and 19 weeks of pregnancy. Under the guidance of ultrasound, a small sample of amniotic fluid is removed for testing by passing a fine needle through the abdominal wall and into the amniotic fluid surrounding the baby. The amniotic fluid contains cells of the developing baby. A local anaesthetic is usually used prior to the procedure. The sample is tested for an expanded huntington gene copy. Laboratories aim to provide a result within a short time frame. When amniocentesis is carried out by an obstetrician who is experienced in this technique, the risk of miscarriage related to the test is less than 1% (less than 1 in 100 pregnancies).

3. Duchenne Muscular Dystrophy (DMD):

Overview: Duchenne muscular dystrophy is one of the most common muscular dystrophies and is characterised by the progressive break-down and deterioration of muscle, due to the lack of a protein known as dystrophin. This leads to progressive difficulty walking and loss of general mobility. Other problems that can be associated with the lack of the dystrophin protein include cardiac and cognitive impairment in a small percentage of people. Unlike many other genetic disorders, carriers of the DMD may experience some muscle weakness or have a cardiac disorder, known as cardiomyopathy. This means it is important to determine if female members of the family carry the gene change.

Inheritance: DMD is inherited in an X-linked recessive pattern (see X-linked recessive inheritance in the types of inheritance section for more detail). Only two thirds of DMD cases are inherited, with the other one third of cases resulting spontaneously without a genetic link. Genetic testing may be required to determine if a case of DMD is sporadic or genetic if there is no family history to provide information that may affect other members of the family. DMD results from an alteration in the dystrophin gene that leads to the gene producing a malformed dystrophin protein. This means an individual that is affected with DMD produces dystrophin that is changed or mutated. The types of gene alterations include a large deletion or duplication that affects 65% of people with DMD or a small deletion or other mutation that affects between 30 35% of people with DMD. Because DMD is an X-linked disorder, only boys will be affected (with extremely rare exceptions).

Testing: 1) CK testing: Your genetic counselor or doctor will organize testing for the boy suspected to have DMD after you have had an appointment with them. To confirm the diagnosis of DMD, they will do 3 simple blood tests, over a period of time, which tests for an enzyme known as Creatine Phosphokinase (also referred to as CK). This is because individuals with DMD lack the protein dystrophin, the muscle cells in their bodies become damaged. This allows for the CK to leak out of the cells and is found in large amounts in the blood. Individuals with DMD have 50 100 times the normal CK levels in their blood. This same blood test can be done to determine if the boys mother is a carrier, with 70% of carriers having a slightly raised CK level. However, this is not a conclusive test for carriers, as it only tells us that there is muscle weakness, not what is causing it. Therefore, further genetic testing may be needed. The 3 CK blood tests get an accurate average CK level measurement over a period of time. The reason that this is needed is because normal day-to-day activities and exercise can also cause muscle cells to become damaged temporarily, resulting in an increase in that individuals CK levels. This means if you work your body too hard in the days before the test, an inaccurate CK level will be recorded. The average ensures that the results seen are what truly represent your normal CK level. If one of the 3 results is extremely different to the other two, a fourth test may be required just to confirm that it is an incorrect result. Your doctor or genetic counselor will advise that the week before the test you do as little physical work as possible and 48 hrs before the test you stop all physical activity. This is an important factor to consider when deciding on genetic testing, as it can have an impact on your professional and personal life.

2) Genetic Testing: If the CK test shows an increase in the CK levels that are expected in a person with DMD or a carrier of DMD, a genetic test can be undertaken to specifically find the gene alteration in the dystrophin gene. Your genetic counselor or doctor will organize the testing after an appointment with them. This is done most commonly by taking a simple blood test (occasionally by tissue sample) that is tested by the Laboratory. Testing can confirm the type of alteration that has occurred in the gene, therefore confirming a diagnosis of DMD, making it easier for carriers in the family to be identified as they are able to search for the same alteration. Occasionally, after DNA testing, a person who is suspected to have DMD, may get an uninformative result. This means that they were unable to find an alteration in the gene. Although the technology is improving all the time, at the moment, some very small alterations can be missed. Very rarely, the mother of an affected boy can have a DNA test that comes back to show that she is negative for carrying the gene alteration, however is a carrier. This is because the mother has what is called germline mosacism. This means, the mother has an unaltered copy of the dystrophin gene throughout her body except in her germline, or her eggs. Although it is very rare, it is something that needs to be considered especially if the mother is planning to have another child.

3) Muscle Biopsy: There is a final test that can be performed if the DNA test does not show any gene changes in the person suspected with DMD. This is a muscle biopsy. This involves taking a small piece of muscle tissue, usually from the leg, by using a needle. The sample is then taken to the laboratory where it is stained to look for the presence of dystrophin microscopically. If there is no dystrophin present, then the boy is diagnosed with DMD. If there is a small amount of Dystrophin present, then it is likely that the boy has a related disorder Becker Muscular Dystrophy, which affects the same gene as DMD.

4. CharcotMarieTooth Disease (CMT)

Charcot-Marie-Tooth disease is the most commonly inherited disease that affects the peripheral nervous system. The peripheral nervous system controls our abilities to move and feel parts of our bodies, such as the hands and feet. Initial symptoms of CMT include weakness of the hands and feet, muscle atrophy (decrease in size), sensory loss and foot irregularities. When we wish to move a part of our body, such as our leg, signal through the spinal cord, to the motor neurons (that connect the spinal cord to the muscle), which pass the message to move onto our muscles in the leg. If this movement is the result of a stimulus, for example, stepping on something hot, the sensory receptors send a message to the brain via the sensory neurons, causing the brain to signal to be sent back to the leg to move. Although there are many different forms of CMT, all of which have different severity and symptoms, generally all individuals affected with CMT have muscle weakness and wasting as a result of the loss of stimulation from the affected nerves. Many people also experience some loss of sensory nerve function, which means the sensation of touch is reduced.

There are many different types of CMT, all of which initially present similar symptoms. They are;

Type 1: Our nerves are covered by what is known as a Myelin Sheath, it helps to speed up the delivery of messages from our brain to parts of the body. If the myelin sheath is damaged or absent, the messages sent from the brain travel much slower to the part of the body we wish to move. This is known as demyelination and results in CMT type 1. This type is further broken down into subtype including; 1A, 1B, 1C, 1D, 1E, 1F, 1X (also known as X-linked CMT) and Hereditary Neuropathy with Liability to Pressure Palsies (HNPP).

Type 2: If the nerve itself is damaged or if there are a reduced number of nerves to the muscle, the brain has trouble trying to send a message to the muscles. This reduction of nerves is said to be an axonal form of CMT. This means the muscle is not able to get as strong a signal as it should. This type is further broken down also into subtypes, which include; 2A, 2B, 2C, 2D and 2E.

Type 3: The most severe form of CMT is type 3 with most individuals being diagnosed very early in life. These individuals are found to have severe demyelination or severe reduction in the number of nerves. There are two subtypes of CMT type 3, they being; Dejerine-Sottas Syndrome (or DSS) and Congenital hypomyelination (CH).

Type 4: CMT type 4 is considered to be very rare. Depending on their subtype, they may be either demyelinating or axonal. Once again, this type is further broken down into sub types, they being; 4A, 4B, 4C 4D and 4F.

InheritanceThere are 3 different ways that CMT can be inherited. All cases of CMT type 1 (except the subgroup CMT type 1X), CMT type 2 and HNPP are inherited in an autosomal dominant pattern (see autosomal dominant inheritance in the types of inheritance section for more detail). Depending on the type and subtype of CMT, the gene and the chromosomes involved in causing the disorder vary. Because they are inherited in an autosomal dominant pattern, it is likely that the parents of an affected child have CMT, but have very mild symptoms that go unnoticed. Spontaneous cases of CMT can also occur in families who have no family history of the disorder. CMT type 1X has been linked to the X chromosome and therefore is inherited via X-linked dominant (see X-linked dominant inheritance in the types of inheritance section for more detail).

CMT type 3 arises by autosomal recessive inheritance (see autosomal recessive inheritance in the types of inheritance section for more detail). This means both parents are carriers of the disorder but are not affected. As CMT type 3 results in very severe symptoms in infants, it is very unlikely that either parent would be affected with CMT type 3.

CMT type 4 also arises as a result of autosomal recessive inheritance. Occasionally, a person can also develop CMT sporadically with out any family history. If this is confirmed to be the case (by genetic testing), then it is very likely that no one else in the family is affected. This sporadic gene change can however, be passed onto the affected individuals children.

Testing: There is a process by which a diagnosis of CMT is determined. Genetic testing is just one step in this process.

1) Physical examination: If the signs of CMT are noticed by a doctor, they will be referred to a neurologist. Examples of these signs include the legs don't have stretch reflexes (especially ankle jerks), and the person may have trouble lifting their feet (dorsiflexion) and bringing the thumb upwards (thumb abduction). A physical examination by a neurologist will be needed prior to testing to look for signs of distal weakness and sensory loss.

2) Nerve Conduction Velocity test (NCV): This test is used to measure the strength and speed of electrical signals moving down the peripheral nerves. It is done by stimulating a nerve using an electrode with a mild electrical current, whilst a second electrode records the electrical signal down stream (or further away from the spine). Surface electrodes are used on the skins surface meaning it is not invasive; however, mild discomfort may be experienced as a result of the electric currents. The speed of the signal is used to determine if there is a problem with the nerve. The results of the NCV can be used to separate type 1 and type 2 CMT. Axonal CMT (i.e. type 1) is confirmed if the speed of nerve transmission is slightly slowed, whilst CMT resulting from demyelination (type 2) shows dramatically slowed signals.

3) Electromyography testing: After a physical examination has been completed and other disorders have been ruled out, Electromyography testing (also known as an EMG) may be performed and involves 2 parts. The first part involves a small needle that is gently inserted into the arm or thigh that shows electrical patterns of those muscles, which are recorded by a specialist. The second part involves a small electrical pulse that stimulates the nerves of either the arm or the leg to determine how fast the messages are being sent from the brain to the nerves. This procedure usually uses the same needle and equipment used in the first test. The test can be uncomfortable, however, your doctor may be able to suggest ways to minimize the discomfort and an experienced electromyographer can minimize the pain and length of the procedure. The age of onset and characteristic symptoms are combined with this result to determine a diagnosis.

4) Genetic tests: Genetic testing is also an option to confirm a diagnosis of CMT and to confirm the gene change that has caused the disorder to develop. This may be important in distinguishing the disorders sub type (i.e. CMT type 1A, etc). Because there are different genes responsible for each subtype, it is important to narrow the possible type of CMT as much as possible using the previously mentioned tests. Currently the subtypes that have a known gene alteration that can be tested for include; CMT types 1A, 1B, 1C, 1D, 1X (X-linked form), 2A, 2E, 2F, 4F and HNPP). If the gene change can been found and confirmed, this information can then be used to help in testing other family members to determine if the are carriers of the disease. It is important to remember that siblings under the age of 18 will not be tested unless they show symptoms of the disorder. Partners of an individual that carries the gene change will also be able to get genetic testing if the couple is planning to have children. Genetic testing usually involves as simple blood test (occasionally can require a tissue sample) to determine if a person is affected or a carrier of CMT. A positive result can confirm a diagnosis of CMT; however it is important to note that a negative result cannot rule out a diagnosis of CMT.

5) Nerve and/or Biopsy: Small samples of tissue are removed and examined in a laboratory. Either nerve or muscle tissue (or both) may be examined. This is not commonly done, and is unnecessary if a genetic abnormality is found, however, it can confirm a diagnosis of CMT if other tests fail to do so.

5. Beta Thalassemia Thalassemia is a hereditary genetic disorder in which the body makes an abnormal form of hemoglobin, a protein in red blood cells that carries oxygen. Thalassemia occurs when our body is not able to produce sufficient amount of hemoglobin. It is caused by mutations in gene encoding Beta globin protein of the hemoglobin . In such case excessive destruction of red blood cells is observed which results in severe anemia. Children affected with Beta Thalassemia exhibits the symptoms of anemia, poor growth and skeletal abnormalities during infancy. Patients with Thalassemia major can develop life threatening anemia and require regular blood transfusions for survival and may develop significant implications including iron overload, bone deformities, cardiovascular failure, spleen enlargement and even premature death.

HBB gene is responsible for production of Beta globin protein of the hemoglobin molecule. It is located on chromosome 11p15.5. Mutations in HBB gene leads to Beta Thalassemia. This disorder is of two different types- Thalassemia Major and Thalassemia Minor, depending upon the type of mutations present in patients. In Thalassemia Minor patients there is reduced ability to produce Beta globin protein and reduced functional capacity of hemoglobin. In Thalassemia Major patients Beta globin is not produced and has dysfunctional hemoglobin.

Medina, Ma. Beatrix DL.3Bio1