Facioscapulohumeral muscular dystrophy is a disorder characterized by muscle weakness and wasting (atrophy). This condition gets its name from the muscles that are affected most often: those of the face (facio-), around the shoulder blades (scapulo-), and in the upper arms (humeral). The signs and symptoms of facioscapulohumeral muscular dystrophy usually appear in adolescence. However, the onset and severity of the condition varies widely. Milder cases may not become noticeable until later in life, whereas rare severe cases become apparent in infancy or early childhood.

Weakness involving the facial muscles or shoulders is usually the first symptom of this condition. Facial muscle weakness often makes it difficult to drink from a straw, whistle, or turn up the corners of the mouth when smiling. Weakness in muscles around the eyes can prevent the eyes from closing fully while a person is asleep, which can lead to dry eyes and other eye problems. For reasons that are unclear, weakness may be more severe in one side of the face than the other. Weak shoulder muscles tend to make the shoulder blades (scapulae) protrude from the back, a common sign known as scapular winging. Weakness in muscles of the shoulders and upper arms can make it difficult to raise the arms over the head or throw a ball.

The muscle weakness associated with facioscapulohumeral muscular dystrophy worsens slowly over decades and may spread to other parts of the body. Weakness in muscles of the lower legs can lead to a condition called foot drop, which affects walking and increases the risk of falls. Muscular weakness in the hips and pelvis can make it difficult to climb stairs or walk long distances. Additionally, affected individuals may have an exaggerated curvature of the lower back (lordosis) due to weak abdominal muscles. About 20 percent of affected individuals eventually require the use of a wheelchair.

Additional signs and symptoms of facioscapulohumeral muscular dystrophy can include mild high-tone hearing loss and abnormalities involving the light-sensitive tissue at the back of the eye (the retina). These signs are often not noticeable and may be discovered only during medical testing. Rarely, facioscapulohumeral muscular dystrophy affects the heart (cardiac) muscle or muscles needed for breathing.

Researchers have described two types of facioscapulohumeral muscular dystrophy: type 1 (FSHD1) and type 2 (FSHD2). The two types have the same signs and symptoms and are distinguished by their genetic cause.

Facioscapulohumeral muscular dystrophy has an estimated prevalence of 1 in 20,000 people. About 95 percent of all cases are FSHD1; the remaining 5 percent are FSHD2.

Facioscapulohumeral muscular dystrophy is caused by genetic changes involving the long (q) arm of chromosome 4. Both types of the disease result from changes in a region of DNA near the end of the chromosome known as D4Z4. This region consists of 11 to more than 100 repeated segments, each of which is about 3,300 DNA base pairs (3.3 kb) long. The entire D4Z4 region is normally hypermethylated, which means that it has a large number of methyl groups (consisting of one carbon atom and three hydrogen atoms) attached to the DNA. The addition of methyl groups turns off (silences) genes, so hypermethylated regions of DNA tend to have fewer genes that are turned on (active). Facioscapulohumeral muscular dystrophy results when the D4Z4 region is hypomethylated, with a shortage of attached methyl groups. In FSHD1, hypomethylation occurs because the D4Z4 region is abnormally shortened (contracted), containing between 1 and 10 repeats instead of the usual 11 to 100 repeats. In FSHD2, hypomethylation most often results from mutations in a gene called SMCHD1, which provides instructions for making a protein that normally hypermethylates the D4Z4 region. However, about 20 percent of people with FSHD2 do not have an identified mutation in the SMCHD1 gene, and the cause of the hypomethylation is unknown.

Hypermethylation of the D4Z4 region normally keeps a gene called DUX4 silenced in most adult cells and tissues. The DUX4 gene is located in the segment of the D4Z4 region closest to the end of chromosome 4. In people with facioscapulohumeral muscular dystrophy, hypomethylation of the D4Z4 region prevents the DUX4 gene from being silenced in cells and tissues where it is usually turned off. Although little is known about the function of the DUX4 gene when it is active, researchers believe that it influences the activity of other genes, particularly in muscle cells. It is unknown how abnormal activity of the DUX4 gene damages or destroys these cells, leading to progressive muscle weakness and atrophy.

The DUX4 gene is located next to a regulatory region of DNA on chromosome 4 known as a pLAM sequence, which is necessary for the production of the DUX4 protein. Some copies of chromosome 4 have a functional pLAM sequence, while others do not. Copies of chromosome 4 with a functional pLAM sequence are described as 4qA or "permissive." Those without a functional pLAM sequence are described as 4qB or "non-permissive." Without a functional pLAM sequence, no DUX4 protein is made. Because there are two copies of chromosome 4 in each cell, individuals may have two "permissive" copies of chromosome 4, two "non-permissive" copies, or one of each. Facioscapulohumeral muscular dystrophy can only occur in people who have at least one "permissive" copy of chromosome 4. Whether an affected individual has a contracted D4Z4 region or a SMCHD1 gene mutation, the disease results only if a functional pLAM sequence is also present to allow DUX4 protein to be produced.

Studies suggest that mutations in the SMCHD1 gene, which cause FSHD2, can also increase the severity of the disease in people with FSHD1. Researchers suspect that the combination of a contracted D4Z4 region and a SMCHD1 gene mutation causes the D4Z4 region to have even fewer methyl groups attached, which allows the DUX4 gene to be highly active. In people with both genetic changes, the overactive gene leads to severe muscle weakness and atrophy.

Normal Function

The DUX4 gene is located near the end of chromosome 4 in a region known as D4Z4. This region consists of 11 to more than 100 repeated segments, each of which is about 3,300 DNA base pairs (3.3 kb) long. The entire D4Z4 region is normally hypermethylated, which means that it has a large number of methyl groups (consisting of one carbon atom and three hydrogen atoms) attached to the DNA. The addition of methyl groups turns off (silences) genes, so hypermethylated regions of DNA tend to have fewer genes that are turned on (active).

Each of the repeated segments in the D4Z4 region contains a copy of the DUX4 gene; the copy closest to the end of chromosome 4 is called DUX4, while the other copies are described as "DUX4-like" or DUX4L. Hypermethylation of the D4Z4 region keeps the DUX4-like genes silenced all the time. No protein is produced from these genes. The DUX4 gene is also silenced in most adult cells and tissues, although it is active during early development and in the testes of adult males. Little is known about the function of the protein produced from the active DUX4 gene; it appears to help control the activity of other genes.

The DUX4 gene (the copy closest to the end of chromosome 4) is located next to a regulatory region of DNA known as a pLAM sequence, which is necessary for the production of the DUX4 protein. Some copies of chromosome 4 have a functional pLAM sequence, while others do not. Copies of chromosome 4 with a functional pLAM sequence are described as 4qA or "permissive." Those without a functional pLAM sequence are described as 4qB or "non-permissive." Without a functional pLAM sequence, no DUX4 protein is made. Because there are two copies of chromosome 4 in each cell, individuals may have two "permissive" copies of chromosome 4, two "non-permissive" copies, or one of each.

Health Conditions Related to Genetic Changes

Facioscapulohumeral muscular dystrophy

Changes in the D4Z4 region of chromosome 4, which contains the DUX4 gene, cause facioscapulohumeral muscular dystrophy. This disorder is characterized by muscle weakness and wasting (atrophy) that worsens slowly over time. Two types of the disorder have been described: type 1 (FSHD1) and type 2 (FSHD2). Both types result from hypomethylation of the D4Z4 region, in which the DNA has fewer methyl groups attached than normal. In FSHD1, hypomethylation occurs because the D4Z4 region is abnormally shortened (contracted), containing between 1 and 10 repeats instead of the usual 11 to 100 repeats. In FSHD2, hypomethylation most often results from mutations in a gene called SMCHD1, which normally hypermethylates the D4Z4 region.

Hypomethylation of the D4Z4 region prevents the DUX4 gene from being silenced in cells and tissues where it is usually turned off, such as adult muscle cells. However, hypomethylation of the D4Z4 region results in facioscapulohumeral muscular dystrophy only when it occurs with a "permissive" chromosome 4. The "permissive" chromosome contains a working pLAM sequence, which allows protein to be produced from the abnormally active DUX4 gene. Researchers believe that the protein influences the activity of other genes, particularly in muscle cells. However, it is unknown how presence of the DUX4 protein damages or destroys these cells, leading to progressive muscle weakness and atrophy.

Other Names for This Gene

• double homeobox 4-like
• double homeobox protein 10
• double homeobox protein 4
• double homeobox protein 4/10

Genomic Location

Normal Function

The SMCHD1 gene provides instructions for making a protein that is involved in regulating gene activity by altering the structure of DNA. Specifically, the SMCHD1 protein is associated with DNA methylation, which is the addition of methyl groups (consisting of one carbon atom and three hydrogen atoms) to DNA molecules. The addition of methyl groups is associated with the turning off (silencing) of genes, so regions of DNA with many methyl groups (hypermethylated regions) tend to have fewer genes that are turned on (active).

The SMCHD1 protein is involved in a process called X-inactivation. Early in embryonic development in females, one of the two X chromosomes is randomly and permanently inactivated in cells other than egg cells. X-inactivation ensures that females, like males, have only one active copy of the X chromosome in each body cell. The SMCHD1 protein appears to be involved in the hypermethylation of certain areas of DNA called CpG islands, although the mechanism is unclear. This hypermethylation is critical for inactivation of the X chromosome. The SMCHD1 protein then remains attached (bound) to the inactive X chromosome to help keep it inactivated.

The SMCHD1 protein also plays a role in hypermethylation of a region near the end of chromosome 4 called D4Z4. This region consists of 11 to more than 100 repeated segments, each of which is about 3,300 DNA building blocks (3.3 kb) long. The segment closest to the end of chromosome 4 contains a gene called DUX4. Because the D4Z4 region is hypermethylated, the DUX4 gene is silenced in most adult cells and tissues. Little is known about the function of the protein produced from the DUX4 gene; it appears to help control the activity of other genes.

The SMCHD1 protein appears to play a role in normal development of the nose, eyes, and other structures of the head and face and seems to be involved in repairing damaged DNA. However, little is known about its roles in these processes.

Health Conditions Related to Genetic Changes

Bosma arhinia microphthalmia syndrome

At least nine SMCHD1 gene mutations have been found to cause Bosma arhinia microphthalmia syndrome (BAMS). Individuals with this rare condition have an abnormally small or absent nose (arhinia), unusually small eyes (microphthalmia), and a shortage of certain hormones that direct sex development (hypogonadotropic hypogonadism).

The gene mutations involved in BAMS change single protein building blocks (amino acids) in the SMCHD1 protein. Researchers are unsure how these changes affect the protein's function and lead to the developmental problems characteristic of BAMS. SMCHD1 gene mutations may lead to abnormal silencing of genes involved in development of the head and face. Problems with nasal development may affect gonadotropin-releasing hormone (GnRH) neurons, which are nerve cells that control the release of reproductive hormones. GnRH neurons originate in the developing nose and then move to the brain. Impaired development of these neurons could explain hypogonadotropic hypogonadism in affected individuals.

Some family members of individuals with BAMS have milder symptoms, such as a reduced sense of smell (anosmia), arhinia without other features of BAMS, or less severe abnormalities of the nose. Researchers suspect that additional genetic factors contribute to the severity of the symptoms, although these factors are not yet known.

Facioscapulohumeral muscular dystrophy

Dozens of mutations in the SMCHD1 gene have been found to cause facioscapulohumeral muscular dystrophy, a disorder characterized by muscle weakness and wasting (atrophy) that worsens slowly over time. Two forms of the disorder have been described: type 1 (FSHD1) and type 2 (FSHD2). Changes in the SMCHD1 gene appear to play a role in both types.

SMCHD1 gene mutations cause most cases of FSHD2. These mutations reduce the amount of SMCHD1 protein available to silence the D4Z4 region. This region has fewer methyl groups than usual (hypomethylation), which is associated with impaired silencing of the DUX4 gene in cells and tissues where it is usually turned off, such as adult muscle cells. However, hypomethylation of the D4Z4 region results in facioscapulohumeral muscular dystrophy only when it occurs in people who also have at least one copy of chromosome 4 that is described as "permissive." A "permissive" chromosome 4 has a functional region of DNA known as a pLAM sequence located next to the DUX4 gene. The pLAM sequence is necessary for the production of the DUX4 protein. (Conversely, a "non-permissive" chromosome 4 does not contain a functional pLAM sequence, preventing the production of any DUX4 protein.) Researchers believe that the DUX4 protein influences the activity of other genes, particularly in muscle cells. However, it is unknown how the presence of this protein damages or destroys these cells, leading to progressive muscle weakness and atrophy.

Studies suggest that mutations in the SMCHD1 gene can increase the severity of disease in people with the other type of facioscapulohumeral muscular dystrophy, FSHD1. FSHD1 results when the D4Z4 region is abnormally shortened (contracted), containing between 1 and 10 repeats instead of the usual 11 to 100 repeats. Researchers suspect that the combination of a contracted D4Z4 region and a SMCHD1 gene mutation causes the D4Z4 region to have even fewer methyl groups attached, which allows the DUX4 gene to be highly active. In people with both genetic changes, the overactive gene leads to severe muscle weakness and atrophy.

Other disorders

SMCHD1 gene mutations have also been found in individuals with arhinia without other features of BAMS (described above), which is referred to as isolated arhinia. Researchers are working to understand why some people with mutations in this gene develop only arhinia and others have additional abnormalities.

Other Names for This Gene

• KIAA0650
• SMC hinge domain-containing protein 1
• structural maintenance of chromosomes flexible hinge domain-containing protein 1

Genomic Location

Chromosome 4 spans about 191 million DNA building blocks (base pairs) and represents more than 6 percent of the total DNA in cells. Chromosome 4 likely contains 1,000 to 1,100 genes that provide instructions for making proteins. These proteins perform a variety of different roles in the body.

The following chromosomal conditions are associated with changes in the structure or number of copies of chromosome 4.

Cancers

Changes in chromosome 4 have been identified in several types of human cancer. These genetic changes are somatic, which means they are acquired during a person's lifetime and are present only in certain cells. For example, rearrangements (translocations) of genetic material between chromosome 4 and several other chromosomes have been associated with leukemias, which are cancers of blood-forming cells.

A specific translocation involving chromosome 4 and chromosome 14 is commonly found in multiple myeloma, which is a cancer that starts in cells of the bone marrow. The translocation, which is written as t(4;14)(p16;q32), abnormally fuses the WHSC1 gene on chromosome 4 with part of another gene on chromosome 14. The fusion of these genes overactivates WHSC1, which appears to promote the uncontrolled growth and division of cancer cells.

Facioscapulohumeral muscular dystrophy

Facioscapulohumeral muscular dystrophy is caused by genetic changes involving the long (q) arm of chromosome 4. This condition is characterized by muscle weakness and wasting (atrophy) that worsens slowly over time. It results from changes in a region of DNA known as D4Z4, located near the end of the chromosome at a position described as 4q35. The D4Z4 region consists of 11 to more than 100 repeated segments, each of which is about 3,300 DNA base pairs (3.3 kb) long. The entire D4Z4 region is normally hypermethylated, which means that it has a large number of methyl groups (consisting of one carbon atom and three hydrogen atoms) attached to the DNA. Facioscapulohumeral muscular dystrophy results when the region is hypomethylated, with too few methyl groups attached. In facioscapulohumeral muscular dystrophy type 1 (FSHD1), hypomethylation occurs because the D4Z4 region is abnormally shortened (contracted), containing between 1 and 10 repeats instead of the usual 11 to 100 repeats. In facioscapulohumeral muscular dystrophy type 2 (FSHD2), hypomethylation most often results from mutations in a gene called SMCHD1, which normally hypermethylates the D4Z4 region.

The segment of the D4Z4 region closest to the end of chromosome 4 contains a gene called DUX4. Hypermethylation of the D4Z4 region normally keeps the DUX4 gene turned off (silenced) in most adult cells and tissues. In people with facioscapulohumeral muscular dystrophy, hypomethylation of the D4Z4 region prevents the DUX4 gene from being silenced in cells and tissues where it is usually turned off. Although little is known about the function of the DUX4 gene when it is turned on (active), researchers believe that it influences the activity of other genes, particularly in muscle cells. It is unknown how abnormal activity of the DUX4 gene damages or destroys these cells, leading to progressive muscle weakness and atrophy.

The DUX4 gene is located next to a regulatory region of DNA known as a pLAM sequence, which is necessary for the production of the DUX4 protein. Some copies of chromosome 4 have a functional pLAM sequence, while others do not. Copies of chromosome 4 with a functional pLAM sequence are described as 4qA or "permissive." Those without a functional pLAM sequence are described as 4qB or "non-permissive." Without a functional pLAM sequence, no DUX4 protein is made. Because there are two copies of chromosome 4 in each cell, individuals may have two "permissive" copies of chromosome 4, two "non-permissive" copies, or one of each. Facioscapulohumeral muscular dystrophy can only occur in people who have at least one "permissive" copy of chromosome 4. Whether an affected individual has a contracted D4Z4 region or a SMCHD1 gene mutation, the disease results only if a functional pLAM sequence is also present to allow DUX4 protein to be produced.

PDGFRA-associated chronic eosinophilic leukemia

PDGFRA-associated chronic eosinophilic leukemia is caused by genetic abnormalities that involve the PDGFRA gene, a gene found on chromosome 4. This condition is a type of blood cell cancer characterized by an increased number of eosinophils, a type of white blood cell involved in allergic reactions.

The PDGFRA gene abnormalities are somatic mutations, which are mutations acquired during a person's lifetime that are present only in certain cells. The most common of these abnormalities is a deletion of genetic material from chromosome 4 that removes approximately 800 DNA building blocks (nucleotides) and brings together parts of two genes, FIP1L1 and PDGFRA, creating the FIP1L1-PDGFRA fusion gene. Occasionally, through mechanisms other than deletion, genes other than FIP1L1 are fused with the PDGFRA gene. Rarely, mutations that change single DNA building blocks in the PDGFRA gene (point mutations) cause this condition.

The protein produced from the FIP1L1-PDGFRA fusion gene (as well as other PDGFRA fusion genes) has the function of the PDGFRA protein, which stimulates signaling pathways inside the cell that control many important cellular processes, such as cell growth and division (proliferation) and cell survival. Unlike the normal PDGFRA protein, however, the fusion protein is constantly turned on (constitutively activated), which means the cells are always receiving signals to proliferate. Similarly, point mutations in the PDGFRA gene can result in a constitutively activated PDGFRA protein. When the FIP1L1-PDGFRA fusion gene or point mutations in the PDGFRA gene occur in blood cell precursors, the growth of eosinophils (and occasionally other blood cells) is poorly controlled, leading to PDGFRA-associated chronic eosinophilic leukemia. It is unclear why eosinophils are preferentially affected by this genetic change.

Wolf-Hirschhorn syndrome

Wolf-Hirschhorn syndrome is caused by a deletion of genetic material near the end of the short (p) arm of chromosome 4 at a position described as 4p16.3. The signs and symptoms of this condition are related to the loss of multiple genes from this part of the chromosome. The size of the deletion varies among affected individuals; studies suggest that larger deletions tend to result in more severe intellectual disability and physical abnormalities than smaller deletions.

The region of chromosome 4 that is deleted most often in people with Wolf-Hirschhorn syndrome is known as Wolf-Hirschhorn syndrome critical region 2 (WHSCR-2). This region contains several genes, some of which are known to play important roles in early development. A loss of these genes leads to developmental delay, a distinctive facial appearance, and other characteristic features of the condition. Scientists are working to identify additional genes at the end of the short arm of chromosome 4 that contribute to the characteristic features of Wolf-Hirschhorn syndrome.

Other chromosomal conditions

Some deletions of genetic material from the short (p) arm of chromosome 4 do not involve the critical region WHSCR-2. These deletions cause signs and symptoms that are distinct from those of Wolf-Hirschhorn syndrome, including mild intellectual disability and, in some cases, rapid (accelerated) growth. People with this type of deletion usually do not have seizures.

Trisomy 4 occurs when cells have three copies of chromosome 4 instead of the usual two copies. Full trisomy 4, which occurs when all of the body's cells contain an extra copy of chromosome 4, is not compatible with life. A similar but somewhat less severe condition called mosaic trisomy 4 occurs when only some of the body's cells have an extra copy of chromosome 4. The signs and symptoms of mosaic trisomy 4 vary widely and can include heart defects, abnormalities of the fingers and toes, and other birth defects. Mosaic trisomy 4 is very rare; only a few cases have been reported.

Other changes in the number or structure of chromosome 4 can have a variety of effects including delayed growth and development, intellectual disability, distinctive facial features, heart defects, and other medical problems. Changes involving chromosome 4 include an extra piece of the chromosome in each cell (partial trisomy 4), a missing segment of the chromosome in each cell (partial monosomy 4), and a circular structure called a ring chromosome 4. Ring chromosomes occur when a chromosome breaks in two places and the ends of the chromosome arms fuse together to form a circular structure.

Cancers

Changes in chromosome 4 have been identified in several types of human cancer. These genetic changes are somatic, which means they are acquired during a person's lifetime and are present only in certain cells. For example, rearrangements (translocations) of genetic material between chromosome 4 and several other chromosomes have been associated with leukemias, which are cancers of blood-forming cells.

A specific translocation involving chromosome 4 and chromosome 14 is commonly found in multiple myeloma, which is a cancer that starts in cells of the bone marrow. The translocation, which is written as t(4;14)(p16;q32), abnormally fuses the WHSC1 gene on chromosome 4 with part of another gene on chromosome 14. The fusion of these genes overactivates WHSC1, which appears to promote the uncontrolled growth and division of cancer cells.

FSHD1 is inherited in an autosomal dominant pattern, which means one copy of the shortened D4Z4 region on a "permissive" chromosome 4 is sufficient to cause the disorder. In most cases, an affected person inherits the altered chromosome from one affected parent. Other people with FSHD1 have no history of the disorder in their family. These cases are described as sporadic and are caused by a new (de novo) D4Z4 contraction on one copy of a "permissive" chromosome 4.

FSHD2 is inherited in a digenic pattern, which means that two independent genetic changes are necessary to cause the disorder. To have FSHD2, an individual must inherit a mutation in the SMCHD1 gene (which is located on chromosome 18) and, separately, they must inherit one copy of a "permissive" chromosome 4. Affected individuals typically inherit the SMCHD1 gene mutation from one parent and the "permissive" chromosome 4 from the other parent. (Because neither parent has both genetic changes in most cases, they are typically unaffected.)