Neurological Disorders, Inherited
The enormous complexity of the brain's structure and function allows many possibilities for genetic errors to occur. The same is true for the eye, an extension of the brain.
Some of these errors are known as structural defects. One such defect, holoprosencephaly, results in a single brain ventricle where a pair usually occurs, and malformation of facial features, including cyclopia and the lack of a nose. Although such cases are often sporadic, they sometimes follow a Mendelian pattern. Hydrocephalus, which results from too much cerebrospinal fluid for the size of the brain, enlarges the ventricles, causing intracranial pressure. It can be X-linked or multifactorial in origin. In some 2-3/10,000 live births, the neural tube of the embryo fails to close completely. This can cause varying sensory and motor defects, depending on which part of the spinal cord and nerves are affected. The condition may be autosomal dominant, and can be detected in pregnancy by measuring the level of maternal serum alpha-fetoprotein.
Up to three percent of the general population are affectd by some type of mental (psychomotor) retardation. Down Syndrome, one of the more common types, is associated with characteristic facial features, congenital heart disease, and problems with vision and hearing. It is caused by a trisomy of the 21st chromosome, specifically, the genetic material in the region from q22.1-q.22.2. Fragile-X Syndrome is an X-linked disorder caused by high numbers of CCG trinucleotide repeats. In addition to retardation, it causes large testes, unusual facial features, connective tissue abnormalities, and often attention-deficit disorder, poor coordination, and seizures. Rhett Syndrome is X-linked, but lethal in males, so only females inherit it. These girls seem normal at birth, but at about one year of age, head growth stops, and developmental milestones are lost.
Seizure disorders tend to be more common in some families, but most have polygenic or multifactorial causes. A few, such as La Fora Body Disease, which is autosomal recessive, are associated with a specific gene.
Neurocutaneous syndromes result in developmental anomalies of the skin and nervous system. One of the better known examples is Neurofibromatosis, of which there are two types. Both are autosomal dominant, and cause cafe-au-lait spots on the skin and neurofibromas, which are soft tumors that develop from peripheral nerve sheaths. Also associated with the disease are demineralization of the bone and resulting fractures, scoliosis, and an increased risk of tumors in the nervous system. Neurofibromatosis 1 is often associated with learning disability or attention-deficit disorder. The neurofibromin gene has significant homology to a known tumor-suppressor gene.
Diseases of the motor neurons are genetically heterogeneous with different ages of onset. While all cause decreases in movement, some cause death in infancy while others allow a normal life span. Degeneration of motor neurons causes Amyotropic Lateral Sclerosis, which results in progressive weakness and eventually death from weakness of the respiratory muscles.
There is a host of diseases of the sensory neurons. Hereditary Radicular Neuropathy (autosomal dominant) causes progressive loss of pain and temperature perception. Congenital Sensory Neuropathy (autosomal recessive) results in total absence of pain, and absence of fungiform taste buds, gag reflexes, and deep tendon reflexes.
Huntington Disease is caused by decreased neurotransmitters in the basal ganglia. It causes lack of coordination, dementia, personality changes, and eventual death. An autosomal dominant disorder, it is the result of an increased number of CAG trinucleotide repeats in the Huntington gene. The higher the number of repeats, the earlier the onset of the disease. Another disease of the basal ganglia is Parkinson syndrome. It causes a progressive resting tremor, muscle rigidity, and slowness of movement. About ten percent of cases are familial, inherited in autosomal dominant fashion with variable penetrance.
Among inherited ophthalmological disorders, Retinitis Pigmentosa is one of the most common. It is associated with abnormal retinal vessels and changes of pigment within the retina. It causes night blindness, followed by loss of central vision. Mutations in any one of three different genes can cause this condition. Another hereditary vision disorder is Stationary Night Blindness. Caused by a dysfunction of the rods, it has an early onset but does not progress. Color Blindness is an X-linked recessive disorder that occurs in eight percent of Caucasian males. It is caused by mutations in the genes for red, green, and blue photopigments. Other disorders include congenital cataracts and aniridia, the absence of an iris.
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