Facioscapulohumeral muscular dystrophy - FSHD-) - Haplotype 4qA161 and PAS polymorphism (Polyadenylation signal) in pLAM region (FSHD1) and SMCHD1 gene (FSHD2).
Muscular dystrophy facio-scapulohumeral (FSHD: Facio Scapulo Humeral Dystrophy) is a process characterized by weakness and muscle atrophy. His name is determined by the areas of the body that are affected most often: muscles of the face (facio-), muscles around the shoulders (escapulo-), and upper extremities (humeral-). Signs and symptoms of this dystrophy usually occur in adolescence, but both the time of onset, and severity vary widely; Thus, mild cases can start in elderly while, although rare, severe cases can occur in infancy or early childhood.
The weakness of the facial muscles and shoulders are usually the first symptoms, and often difficult to lift the mouth when smiling corners; the weakness of the eye muscles can prevent completely close my eyes to sleep, resulting in dry eyes and problems arising from it. The demonstration may affect more than half of the face relative to each other. The weakness of shoulder muscles causes the shoulder blades to separate from the back (winged scapula), which prevents lift his arms above his head to throw an object. Muscle weakness worsens over the years and can spread to other parts of the body. In the lower extremities, when affected, leads to foot drop, affecting gait and increasing the risk of falls. Muscle weakness in the hips and pelvis can make it difficult to climb stairs or walk long distances. In addition, affected individuals may exhibit lordosis due to the weakness of the abdominal muscles. Other additional signs and symptoms may include mild hearing loss and abnormalities affecting the retina. Rarely, muscular dystrophy facio-scapular-humeral affects the heart muscle or respiratory muscles.
They described two types of muscular dystrophy facio-scapulohumeral: type 1 (FSHD1) and type 2 (FSHD2). Both types have the same signs and symptoms and are distinguished by their genetic cause. Both types are due to changes in a region of the long arm of chromosome 4 called D4Z4 (macrosatélite), located near the end of this chromosome. This region normally has between 11 and 150 repeated segments of DNA, each with 3,300 bp (3.3 kb) in length.
Normally, the entire region is hypermethylated D4Z4, ie has a large number of methyl groups attached to the DNA. Adding methyl stops gene expression, so hypermethylated DNA regions tend to express less genes. Muscular dystrophy facio-scapulohumeral develops when D4Z4 region has fewer methyl groups attached. In FSHD1, hypomethylation is because the region D4Z4 in one copy of chromosome 4 is abnormally short, with only 1 to 10 repetitions instead of the usual 11 to 150 repetitions. In FSHD2, hypomethylation is due most often to mutations in the gene SMCHD1, located on the short arm of chromosome 18 (18p11.32), encoding a protein involved in the regulation of gene activity by altering the structure of DNA. Specifically, this protein plays a role in the region hypermethylation D4Z4 near the end of chromosome 4. However, approximately 20% of people with FSHD2 not have an identified mutation in the gene SMCHD1, and the cause is unknown hypomethylation .
Hypermethylation of the region D4Z4 normally maintains an OPen Reading Frame (ORF) DUX4, located on chromosome 4, silenced in most cells and adult tissues. The ORF DUX4 gene is located in the segment region D4Z4 closer to the end of chromosome 4. In people with muscular dystrophy facio-scapulohumeral, hypomethylation of the region D4Z4 prevents the ORF DUX4 gene is silenced in cells and tissues in which usually is not expressed. Although little is known about the function of the ORF DUX4 when active, is believed to influence the activity of other genes, particularly muscle cells.
They have identified more than a dozen mutations in the gene SMCHD1 as responsible for muscular dystrophy facio-scapular-humeral type 2. SMCHD1 changes in the gene appear to play a role in both types: FSHD1 and FSHD2, however, genetic mutations in the gene SMCHD1 cause most cases of FSHD2. These mutations reduce the amount of available SMCHD1 to add methyl groups to the region D4Z4 protein. Hypomethylation resulting from this region prevents the ORF DUX4 is silenced in cells and tissues in which usually is not expressed, as in adult muscle cells.
The ORF-DUX4 is located next to a regulatory region of DNA on chromosome 4 known as a sequence of pLAM, which is necessary for encoding the protein DUX4. Some copies of chromosome 4 have a functional pLAM sequence, whereas others do not. Copies of chromosome 4 with a pLAM functional sequence described as 4qA or "permissive". Which do not have a functional sequence pLAM described as 4qB or "nonpermissive." Without a functional pLAM sequence, no protein DUX4 not encoded. Because there are two copies of chromosome 4 in each cell, individuals can have two "permissive" copies of chromosome 4, two "non - permissive" or one of each copy. Muscular dystrophy facio-scapular-humeral can only occur in people who have at least one "permissive" copy of chromosome 4. If an affected individual has a shortened D4Z4 region or a genetic mutation SMCHD1, the disease is expressed only if a sequence pLAM functional is also present to allow protein coding DUX4. In addition, it is believed that mutations in the gene SMCHD1, causing FSHD2, may also increase the severity of illness in people with FSHD1. It is likely that the combination of a region D4Z4 shortened and a genetic mutation SMCHD1 makes D4Z4 region has fewer methyl groups attached, allowing the ORF-DUX4 is highly active. Although it is believed that the DUX4 protein influences the activity of other genes, in particular in muscle cells, it is not known how the presence of this protein damages or destroys these cells, leading to progressive muscle weakness and atrophy.
They have also been linked FSHD cases with certain haplotypes of chromosome 4, located in 4q subtelomeric level. Here, we have studied the existence of long sequence variations (SSLP: Single Sequence Length Polymorphism) about 3 kb in D4Z4, and differences in a rich region single nucleotide polymorphisms (SNPs: Single Nucleotide Polymorphisms) adjacent the proximal end of D4Z4.
The analysis of the relationship between the findings with respect to these different possibilities have allowed to find that in many patients with FSHD exists regarding the haplotype 4qA161, although this haplotype was also found in the control population unaffected. This haplotype 4qA161, differs from haplotype 10qA166 corresponding to 10q (chromosome 10), representing 96% of the alleles of this chromosome, which could quite possibly exclude this possible source of confusion. Haplotype identification 4qA161 has a G nucleotide (guanosine) instead of A (adenosine) in the SNP3, and nucleotide A (adenosine) instead of a G nucleotide at SNP6. It has been found that all cases of FSHD have at least one of these two alleles, while in the controls, approximately half of them have only one nucleotide of A (adenosine) in SNP3, and only one G nucleotide (guanosine) in SNP6, so there would be no 4qA161 allele. The 4qA161 allele was found in 70% of patients with FSHD and 33% of control, not affected with FSHD cases, so their presence indicates a significantly higher probability for the diagnosis of FSHD.
Muscular dystrophy facio-scapulohumeral type 1 (FSHD1) is inherited as an autosomal dominant, which means that a copy of the region on a chromosome D4Z4 shortened 4 "permissive" is sufficient to express the disease. In most cases, an affected person inherits the altered chromosome of an affected parent. FSHD1 other people with no history of disease in your family. These cases are described as sporadic and are due to a further contraction D4Z4 on a copy of a chromosome 4 "permissive". FSHD2 inherited a digenic pattern, which means that two independent genetic changes are necessary to express the disease. To have FSHD2, a person must inherit a mutation in the gene SMCHD1 and, separately, must inherit a copy of chromosome 4 "permissive". Affected individuals usually inherit the mutated gene from one parent SMCHD1 and chromosome 4 "permissive" the other parent.
Tests in IVAMI: In IVAMI for the diagnosis of FSHD type 1, we performed the detection of the 4qA161 haplotype, with its corresponding single nucleotide polymorphisms (SNPs), by PCR amplification and direct sequencing. In addition, PAS (Polyadenylation signal) polymorphism in the pLAM region will be studied by PCR and sequencing. The results of these two tests, when they are in accordance with what is found in the cases of FSHD type 1. In addition, for the diagnosis of FSHD type 2, we perform the detection of mutations in the SMCHD1 gene, by fully amplifying all its exons and subsequent sequencing.
Samples recommended: EDTA blood collected for separation of blood leukocytes, or impregnated sample card with dried blood (IVAMI may mail the card to deposit the blood sample).