Role of Genetics in Ataxia

Role of Genetics in Ataxia

Ataxia is a neurological disorder that is characterized by unsteady walking, loss of muscle coordination and slurred speech. It is no longer a rare condition in that more is known about it. It is dominant hereditary, recessive or sporadic. It is dominant hereditary if both parents pass the gene on to their offspring and DNA tests detect this condition. It is recessive when only one parent transmits the gene. And it is sporadic if there is no evidence to show this. Friedrich ataxia is the most common recessive type of ataxia, with first symptoms developing in childhood. The ataxia gene may also be mutated and develop into ATM. No cures are known for ataxia and the prospects are dim.

Ataxia is a neurological condition, wherein spinal and cerebellum nerve cells slowly but progressively deteriorate (Unicorn Self-Help Committee 2000). It is characterized by unsteady gait, clumsiness and lack of muscle coordination. Stumbling or drunken walk, un-coordinated hand movement and slurred speech are initial symptoms, which may quickly progress in 10-15 years or slowly in 20 or more years. Ataxia is hereditary or sporadic or idiopathic. It is hereditary when a defective gene is responsible for it, either as a dominant or recessive trait, and crosses generations. And it is sporadic or idiopathic when no family history of the disease can explain it. Although no longer a rare condition, it is still not as recognized as multiple sclerosis and muscular dystrophy, which have similar symptoms as ataxia.

Recent studies showed that there are approximately 2,500 with recessive ataxia, 800 with dominant ataxia and about 2,000 with the sporadic type (Unicorn Self-Help Committee 2000). There is as yet no known cure or treatment to ataxia, which inevitably confines the victim to the wheelchair. Besides the general practitioner, a speech therapist, a physiotherapist and even alternative practitioners like a reflexologist or aromatherapist may help control the symptoms of ataxia. The most common inherited and dominant type is Friedreich's ataxia, which usually begins in childhood. The recessive inherited and early onset types include spastic ataxia, cerebellar ataxia and ataxia telangiectasia, while late onset ataxias include cerebellar atrophy, cerebellar ataxia, spastic ataxia, cerebellar degeneration and olivopontocerebellar atrophy or degeneration. It is believed that 1 in every 100 in the U.S. is a carrier of the altered type of Friedreich's ataxia gene. Many carriers are not even aware of it until a child develops the symptoms. The frataxin gene is behind Friedreich's ataxia.

Every person ha approximately 100,000 genes in every cell of his or her body and not all of them are active (Unicorn Self-Help Committee 2000). He or she has copies of each gene of each parent, the mother and the father. This genetic pattern, known as autosomal recessive inheritance, means that the disease is hereditary, that a double dose of the altered or nonfunctioning gene is needed to produce symptoms, that the disease can affect male and female equally and that a person can carry the gene without the symptoms of the disease. A person who has only one copy of the altered Friedreich's gene is only a carrier of this type of ataxia and can pass it on to his or her children without showing symptoms. The second copy is needed to develop the symptoms.

Freidreich's ataxia or FA affects 3,000 to 5,000 persons in the U.S. (Robinson 1999). It is an autosomal recessive disease. Couples with one child carrying the gene have a 25% chance of replicating the condition with each succeeding pregnancy. The gene for FA is a protein called frataxin. Normal frataxin is located in the mitochrondria, cellular structures that contribute to the transport of iron in the body. The frataxin in FA creates a condition called triple repeat, wherein there is a production of superfluous information. The extra DNA disrupts the normal production of frataxin and transport of iron. Some believe that FA develops partly because of defects in the transport of iron, which interfere with the efficient use of cellular energy supplies. The spinal cord nerve cells that transmit information between muscles and the brain are the most affected in FA. Normal functioning requires the tight control of movement between the muscles that enable movement, those that restrain the movement and the brain. Without this tight control, the movements turn loose and uncoordinated, jerky and useless.

The first symptoms of FA show up between ages 8 and 15, although they may begin as early as 18 months and as late as 25 (Robinson 1999), usually in the form of gain un-coordination. A child with FA will be unsteady when standing still and usually suffer from a deterioration of position sense. There can be foot deformities, uneven muscle weakness in the legs and muscle spasms and cramps, especially at night. Within many years, ataxia in the arms usually follows and leads to reduced hand-eye coordination. Arm weakness occurs much later. Speech and swallowing then become hard and diabetes mellitus may develop. Other commonly observed symptoms include eye tremor or nystagmus, loss of visual acuity, hearing loss, severe scoliosis and heartbeat abnormalities, such as shortness of breath after exertion, swollen lower limbs and cold feet.

FA can be diagnosed through a careful review of one's medical history and a thorough neurological examination (Robinson 1999), which can include electromyography, an electrical test of muscles, a nerve conduction velocity test and a direct DNA test for FA and other types of ataxia. The siblings of the patient may also be diagnosed for the genetic defect. No cure or a treatment to slow the progress of FA has been found or developed. Administering amantadine has been used but not by patients with cardiac disorders. Physical and occupational therapy can address the problem of motion in weakened muscles as well as establish adaptive techniques and tools to make up for the loss of coordination and strength. One technique is the use of weights on the arms.

The progress of the condition varies highly (Robinson 1999). Most sufferers lose their ability to walk within 15 years from the first appearance of symptoms and 95% of them need a wheelchair to move about by age 45. The rate of deaths among victims of FA also widely varies from the mid-30s to the mid-60s. There has been no known way of preventing FA from developing in a person with two defective copies of the gene from both parents

Hereditary dominant ataxia differs from recessive ataxia. A child whose parent has hereditary dominant ataxia has a 50-50% chance of having it and passing it on to his own offspring by gene mutation. Symptoms of dominant ataxias show up in the 20s and 30s. Recessive ataxia, on the other hand, appears only when both parents carry the gene and pass it on to the child. In this case, the parents are only carriers and show no symptoms. Each of their children has a 25% chance of acquiring the disorder and 50% of becoming a carrier like them. Symptoms of recessive ataxia usually come out in the teenage years. Dominant ataxias are now relatively easy to detect through modern DNA tests. Hereditary ataxia, like Friedreich ataxia, is often mistaken as multiple sclerosis (Robinson).

Besides the triple repeat genetic condition, point mutation can lead to Freidreich's ataxia, although this type is less common (Unicorn Self-Help Committee 2000). It occurs when the person inherits two triple repeats or a point mutation and a triplet repeat. Two point mutations can also theoretically cause FA, but it has not been observed in patients. Research, instead, has revealed that those with fewer repeats develop the disease at a later time, with slower progression and less severe heart conditions.

The most affected parts of the body in FA are the cells of the nervous system that pass on sensory and movement signals through long fibers (Unicorn Self-Help Committee 2000). These fibers cross between the central nervous system and the brain. They degenerate over time. FA affects the cells in the heart and the pancreas too. Some of these cells die probably as a direct consequence of the loss of frataxin or of secondary effects.

A person with FA should, first of all, have himself or herself thoroughly examined by a physician, preferably a neurologist who is knowledgeable and sensitive to all the complications of FA (Unicorn Self-Help Committee 2000). A neurologic examination may evaluate the cerebellum and the spine through the computerized tomography or CT or the magnetic resonance imaging or MRI, and conduct other tests that will rule out other conditions, an eye examination, a hearing test, a bone test, an electrocardiogram, and an endocrine test. A speech pathologist, a physical or occupational therapist and urologist will also be helpful if the condition is ascertained and progresses. The patient and his or her family should also go through genetic counseling - they should be informed about the chances of passing on the gene to their offspring or become carriers. It will also help them to ask questions about sophisticated DNA techniques.

The most frequent symptom is difficulty in walking or gait ataxia (Unicorn Self-Help Committee 2000), which spreads slowly to the arms and the trunk. Foot deformities, such as clubfoot, flexion of the toes or foot inversion are other early signs. In time, muscles weaken and waste, especially the muscles in the feet, lower legs and hands and, at this time, deformities s begin to show. Other symptoms are the loss of tendon reflexes especially in the knees and ankles, the gradual disappearance of sensation in the extremities, dysarthria or slowness of speech or slurring, easy fatigue, rapid and involuntary movements of the eyes, scoliosis, chest pain, shortness of breath, palpitations, heart enlargement, myocardial fibrosis, tachycardia, heart block and heart failure. Studies showed that about 20% of FA patients also develop carbohydrate intolerance and 10%, of diabetes mellitus, while others lose their hearing or eyesight.

In most cases, the patient gets strapped to a wheelchair within 15 to 20 years from the appearance of the first symptoms (Unicorn Self-Help Committee 2000). In the later stage of the disease, the patient becomes completely incapacitated and his or her life expectancy is reduced. Most FA sufferers die in early adulthood, often because of a serious and resulting heart condition, which also becomes the immediate cause of death. Patients with less than severe symptoms fortunately live longer (Unicorn).

Another serious type of ataxia is ataxia-telangiectasia or at, also called Louis-Bar syndrome, although a rare type (Barrett 1999). At patients mostly die in their early 20s. The abnormal effects of at on the nervous system are first observed or detected at age 2, during which muscle control progressively decreases. It is also at this age that immune system defects and blood cancers are not uncommon and the patient becomes most sensitive to radiation.

It was first discovered and written about in the mid-1920s but got recognized as a specific disorder and given a name only in 1957 (Barrett 1999). At is a combination of ataxia and telangiectasia, which is characterized by tiny red spots, but is more than a combination of these disorders. At is liked with immune system deficiencies, extreme sensitivity to radiation, and blood cancers. Medical experts first suspected that it was caused by multiple genes. In 1995, the discovery of a single gene behind at denied their suspicion and they named the gene ATM or at mutated. Further tests and discoveries revealed that ATM plays a role in cell division. Under normal conditions, a proofreading and repair mechanism is able to correct a damage on the DNA before cell replication continues. At cells seem to ignore this mechanism and continue to grow and divide.

AT is a rare condition. It affects only about 500 persons in the U.S. (Barrett 1999), but approximately.5 to 14% of the population or roughly 2.5 million are carriers of one copy of the mutated gene (Barrett 1999). Many or most of these carriers do not experience signs or symptoms, such as increased sensitivity to radiation or a higher rate of cancer.

The ATM gene is autosomal recessive or occurs only if inherited from both parents. A child of such parents first appears healthy, but develops the symptoms of at when he or she turns 2. The root cause is cell death in the brain, specifically the large ones in the middle layer of the cerebellum. The toddler suddenly turns clumsy and loses balance, his speech becomes slurred and more difficult and symptoms progressively get worse. Up to age 8, there appear tiny, red spider veins on the cheeks, ears and the eyes. Between 10 and 12, the child with at can no longer control their muscles. Their immune system is affected. They have lower-than-normal level of antibodies and white blood cells. Their thymus gland is either missing or abnormally developed. Although intelligence remains normal, their growth may be retarded on account of their disturbed immune system functioning. Some at patients are stricken with diabetes, premature graying of the hair, and have difficulty swallowing. As children with at grow older, their immune system turns weaker and so they become less capable of fighting disease. In the later stages of at, recurrent respiratory infections and blood cancers, like leukemia and lymphoma, are common.