Pol III-related leukodystrophy is a disorder that affects the nervous system and other parts of the body. Leukodystrophies are conditions that involve abnormalities of the nervous system's white matter, which consists of nerve cells (neurons) covered by a fatty substance called myelin. Myelin insulates nerve fibers and promotes the rapid transmission of nerve impulses.

Pol III-related leukodystrophy is a hypomyelinating disease, which means that the nervous system of affected individuals has a reduced ability to form myelin. Hypomyelination underlies most of the neurological problems associated with Pol III-related leukodystrophy. A small number of people with this disorder also have a loss of nerve cells in a part of the brain involved in coordinating movements (cerebellar atrophy) and underdevelopment (hypoplasia) of tissue that connects the left and right halves of the brain (the corpus callosum). These brain abnormalities likely contribute to the neurological problems in affected individuals.

People with Pol III-related leukodystrophy usually have intellectual disability ranging from mild to severe, which gradually worsens over time. Some affected individuals have normal intelligence in early childhood but develop mild intellectual disability during the course of the disease.

Difficulty coordinating movements (ataxia), which begins in childhood and slowly worsens over time, is a characteristic feature of Pol III-related leukodystrophy. Affected children typically have delayed development of motor skills such as walking. Their gait is unstable, and they usually walk with their feet wide apart for balance. Affected individuals may eventually need to use a walker or wheelchair. Involuntary rhythmic shaking (tremor) of the arms and hands may occur in this disorder. In some cases the tremor occurs mainly during movement (intention tremor); other affected individuals experience the tremor both during movement and at rest.

Development of the teeth (dentition) is often abnormal in Pol III-related leukodystrophy, resulting in the absence of some teeth (known as hypodontia or oligodontia). Some affected infants are born with a few teeth (natal teeth), which fall out during the first weeks of life. The primary (deciduous) teeth appear later than usual, beginning at about age 2. In Pol III-related leukodystrophy, the teeth may not appear in the usual sequence, in which front teeth (incisors) appear before back teeth (molars). Instead, molars often appear first, with incisors appearing later or not at all. Permanent teeth are also delayed, and may not appear until adolescence. The teeth may also be unusually shaped.

Some individuals with Pol III-related leukodystrophy have excessive salivation and difficulty chewing or swallowing (dysphagia), which can lead to choking. They may also have speech impairment (dysarthria). People with Pol III-related leukodystrophy often have abnormalities in eye movement, such as progressive vertical gaze palsy, which is restricted up-and-down eye movement that worsens over time. Nearsightedness is common in affected individuals, and clouding of the lens of the eyes (cataracts) has also been reported. Deterioration (atrophy) of the nerves that carry information from the eyes to the brain (the optic nerves) and seizures may also occur in this disorder.

Hypogonadotropic hypogonadism, which is a condition caused by reduced production of hormones that direct sexual development, may occur in Pol III-related leukodystrophy. Affected individuals have delayed development of the typical signs of puberty, such as the growth of body hair.

People with Pol III-related leukodystrophy may have different combinations of its signs and symptoms. These varied combinations of clinical features were originally described as separate disorders. Affected individuals may be diagnosed with ataxia, delayed dentition, and hypomyelination (ADDH); hypomyelination, hypodontia, hypogonadotropic hypogonadism (4H syndrome); tremor-ataxia with central hypomyelination (TACH); leukodystrophy with oligodontia (LO); or hypomyelination with cerebellar atrophy and hypoplasia of the corpus callosum (HCAHC). Because these disorders were later found to have the same genetic cause, researchers now group them as variations of the single condition Pol III-related leukodystrophy.

Pol III-related leukodystrophy is caused by mutations in the POLR3A or POLR3B gene. These genes provide instructions for making the two largest parts (subunits) of an enzyme called RNA polymerase III. This enzyme is involved in the production (synthesis) of ribonucleic acid (RNA), a chemical cousin of DNA. The RNA polymerase III enzyme attaches (binds) to DNA and synthesizes RNA in accordance with the instructions carried by the DNA, a process called transcription. RNA polymerase III helps synthesize several forms of RNA, including ribosomal RNA (rRNA) and transfer RNA (tRNA). Molecules of rRNA and tRNA assemble protein building blocks (amino acids) into working proteins; this process is essential for the normal functioning and survival of cells.

Researchers suggest that mutations in the POLR3A or POLR3B gene may impair the ability of subunits of the RNA polymerase III enzyme to assemble properly or result in an RNA polymerase III with impaired ability to bind to DNA. Reduced function of the RNA polymerase III molecule likely affects development and function of many parts of the body, including the nervous system and the teeth, but the relationship between POLR3A and POLR3B gene mutations and the specific signs and symptoms of Pol III-related leukodystrophy is unknown.

Normal Function

The POLR3A gene provides instructions for making the largest piece (subunit) of an enzyme called RNA polymerase III. This enzyme is involved in the production (synthesis) of ribonucleic acid (RNA), a chemical cousin of DNA. The RNA polymerase III enzyme attaches (binds) to DNA and synthesizes RNA molecules in accordance with the instructions carried by the DNA, a process called transcription. RNA polymerase III helps synthesize several forms of RNA, including ribosomal RNA (rRNA) and transfer RNA (tRNA). Molecules of rRNA and tRNA assemble protein building blocks (amino acids) into working proteins; this process is essential for the normal functioning and survival of cells.

Health Conditions Related to Genetic Changes

Pol III-related leukodystrophy

At least 70 POLR3A gene mutations have been associated with Pol III-related leukodystrophy. Leukodystrophies are conditions that involve abnormalities of the nervous system's white matter. White matter consists of nerve fibers covered by a fatty substance called myelin, which insulates nerve fibers and promotes the rapid transmission of nerve impulses. A reduced ability to form myelin (hypomyelination) leads to the signs and symptoms of Pol III-related leukodystrophy, which include intellectual disability and difficulty with coordinating movements (ataxia). Development of the teeth (dentition) is also abnormal in this disorder.

In Pol III-related leukodystrophy, POLR3A gene mutations may impair the ability of the subunits of the RNA polymerase III enzyme to assemble properly or result in an RNA polymerase III with impaired ability to bind to DNA. Reduced function of the RNA polymerase III molecule likely affects development and function of many parts of the body, but the relationship between POLR3A gene mutations and the specific signs and symptoms of this disorder is unknown.

People with Pol III-related leukodystrophy may have different combinations of its signs and symptoms. These varied combinations of clinical features were originally described as separate disorders. Affected individuals may be diagnosed with ataxia, delayed dentition, and hypomyelination (ADDH); hypomyelination, hypodontia, hypogonadotropic hypogonadism (4H syndrome); tremor-ataxia with central hypomyelination (TACH); leukodystrophy with oligodontia (LO); or hypomyelination with cerebellar atrophy and hypoplasia of the corpus callosum (HCAHC). Because these disorders were later found to have the same genetic cause, researchers now group them as variations of the single condition Pol III-related leukodystrophy.

Wiedemann-Rautenstrauch syndrome

Mutations in the POLR3A gene cause Wiedemann-Rautenstrauch syndrome, which is a condition characterized by the dramatic, rapid appearance of aging beginning early in life. Affected individuals grow slowly before and after birth. They have a characteristic facial appearance, a lack of fatty tissue under the skin (lipodystrophy), sparse hair on their head, and abnormal tooth development. Some of the mutations that cause this condition alter the blueprint for making proteins. Other mutations lead to an abnormally short blueprint. Both types of mutations result in production of abnormal subunit proteins that are thought to impair the function of RNA polymerase III. The resulting shortage of RNA synthesis likely impairs protein production in cells, affecting development and function of many parts of the body. However, it is not known exactly how changes in the POLR3A gene lead to the features of Wiedemann-Rautenstrauch syndrome.

Researchers suspect that mutations that cause Wiedemann-Rautenstrauch syndrome reduce RNA polymerase III function more drastically than mutations that cause Pol III-related leukodystrophy (described above), which may explain why development is affected even before birth in individuals with Wiedemann-Rautenstrauch syndrome. Because of its distinct signs and symptoms, Wiedemann-Rautenstrauch syndrome is not thought to be part of the Pol III-related leukodystrophy spectrum.

Shingles

MedlinePlus Genetics provides information about Shingles

Other disorders

Mutations in the POLR3A gene have been found in individuals with ataxia and dental abnormalities. Unlike in Pol III-related leukodystrophy (described above), these individuals do not have reduced myelination in the nervous system. When associated with POLR3A gene mutations, ataxia typically begins in adolescence. Affected individuals may also develop rhythmic shaking (tremor) in the arms or legs and have a reduced ability to sense vibrations, particularly in the feet. It is unclear why mutations in the same gene lead to several disorders of varying severity. Researchers are unsure if this condition is part of the spectrum of Pol III-related leukodystrophies.

Other Names for This Gene

• ADDH
• DNA-directed RNA polymerase III largest subunit
• DNA-directed RNA polymerase III subunit A
• DNA-directed RNA polymerase III subunit RPC1
• HLD7
• HRPC155
• polymerase (RNA) III (DNA directed) polypeptide A, 155kDa
• polymerase (RNA) III subunit A
• RNA polymerase III 155 kDa subunit
• RNA polymerase III subunit C1
• RNA polymerase III subunit C160
• RNA polymerase III subunit RPC155-D
• RPC1
• RPC155
• RPC1_HUMAN

Genomic Location

Normal Function

The POLR3B gene provides instructions for making one part (subunit) of an enzyme called RNA polymerase III. This enzyme is involved in the production (synthesis) of ribonucleic acid (RNA), a chemical cousin of DNA. The RNA polymerase III enzyme attaches (binds) to DNA and synthesizes RNA in accordance with the instructions carried by the DNA, a process called transcription. RNA polymerase III helps synthesize several forms of RNA, including ribosomal RNA (rRNA) and transfer RNA (tRNA). Molecules of rRNA and tRNA assemble protein building blocks (amino acids) into working proteins; this process is essential for the normal functioning and survival of cells.

Health Conditions Related to Genetic Changes

Pol III-related leukodystrophy

At least 54 POLR3B gene mutations have been associated with Pol III-related leukodystrophy. Leukodystrophies are conditions that involve abnormalities of the nervous system's white matter. White matter consists of nerve fibers covered by a fatty substance called myelin, which insulates nerve fibers and promotes the rapid transmission of nerve impulses. A reduced ability to form myelin (hypomyelination) leads to the signs and symptoms of Pol III-related leukodystrophy, which include intellectual disability and difficulty with coordinating movements (ataxia). Development of the teeth (dentition) is also abnormal in this disorder.

In the Pol III-related leukodystrophies, POLR3B gene mutations may impair the ability of the subunits of the RNA polymerase III enzyme to assemble properly or result in an RNA polymerase III with impaired ability to bind to DNA. Reduced function of the RNA polymerase III molecule likely affects development and function of many parts of the body, but the relationship between POLR3B gene mutations and the specific signs and symptoms of these disorders is unknown.

People with Pol III-related leukodystrophy may have different combinations of its signs and symptoms. These varied combinations of clinical features were originally described as separate disorders. Affected individuals may be diagnosed with ataxia, delayed dentition, and hypomyelination (ADDH); hypomyelination, hypodontia, hypogonadotropic hypogonadism (4H syndrome); tremor-ataxia with central hypomyelination (TACH); leukodystrophy with oligodontia (LO); or hypomyelination with cerebellar atrophy and hypoplasia of the corpus callosum (HCAHC). Because these disorders were later found to have the same genetic cause, researchers now group them as variations of the single condition Pol III-related leukodystrophy.

Other Names for This Gene

• C128
• DNA-directed RNA polymerase III 127.6 kDa polypeptide
• DNA-directed RNA polymerase III subunit B
• DNA-directed RNA polymerase III subunit RPC2
• DNA-directed RNA polymerase III subunit RPC2 isoform 1
• DNA-directed RNA polymerase III subunit RPC2 isoform 2
• FLJ10388
• HLD8
• polymerase (RNA) III (DNA directed) polypeptide B
• polymerase (RNA) III subunit B
• RNA polymerase III subunit C2
• RPC2
• RPC2_HUMAN

Genomic Location

This condition is inherited in an autosomal recessive pattern, which means both copies of the gene in each cell have mutations. The parents of an individual with an autosomal recessive condition each carry one copy of the mutated gene, but they typically do not show signs and symptoms of the condition.

Pol III-related leukodystrophy is a rare disorder; its prevalence is unknown. More than 100 affected individuals have been described in the medical literature.