Question: how common is fragile x syndrome

Reference:
 Fragile X Syndrome Fragile X syndrome or Fragile X Martin-Bell syndrome Overview The genetic disorder Fragile X syndrome, which results from mutations in a gene on the X chromosome, is the most commonly inherited form of developmental and intellectual disability. The NICHD supports and conducts research on the diagnosis, treatment, management, prevention, and inheritance of Fragile X and its associated conditions, Fragile X-Associated Primary Ovarian Insufficiency (FXPOI) and Fragile X-Associated Tremor/Ataxia Syndrome (FXTAS). All three conditions result from changes in the same gene on the X chromosome. Condition Information What is Fragile X syndrome? Fragile X syndrome is a genetic disorder that affects a person’s development, especially that person’s behavior and ability to learn. In addition, Fragile X can affect: - Communication skills - Physical appearance - Sensitivity to noise, light, or other sensory information Fragile X syndrome is the most common form of inherited intellectual and developmental disability (IDD) People with Fragile X syndrome may not have noticeable symptoms, or they can have more serious symptoms that range from learning disabilities to cognitive and behavior problems. How is FMR1 mutation related? » How is the FMR1 mutation related to Fragile X-associated disorders? Fragile X syndrome and its associated conditions are caused by changes (mutations) in the FMR1 gene found on the X chromosome. This mutation affects how the body makes the Fragile X Mental Retardation Protein, or FMRP. The mutation causes the body to make only a little bit or none of the protein, which can cause the symptoms of Fragile X. In a gene, the information for making a protein has two parts: the introduction, and the instructions for making the protein itself. Researchers call the introduction the promoter because of how it helps to start the process of building the protein. The promoter part of the FMR1 gene includes many repeats—repeated instances of a specific DNA sequence called the CGG sequence. A normal FMR1 gene has between 6 and 40 repeats in the promoter; the average is 30 repeats. People with between 55 and 200 repeats have a premutation of the gene. The premutation may cause the gene to not work properly, but it does not cause intellectual and developmental disability (IDD). The premutation is linked to the disorders FXPOI and FXTAS. However, not all people with the premutation show symptoms of FXPOI or FXTAS. People with 200 or more repeats in the promoter part of the gene have a full mutation, meaning the gene might not work at all. People with a full mutation often have Fragile X syndrome. The number of repeats, also called the “size of the mutation,” affects the type of symptoms and how serious the symptoms of Fragile X syndrome will be. Inheriting Fragile X Syndrome Fragile X syndrome is inherited, which means it is passed down from parents to children. Anyone with the FMR1 gene mutation can pass it to their children. However, a person who inherits the gene mutation may not develop Fragile X syndrome. Males will pass it down to all of their daughters and not their sons. Females have a 50/50 chance to pass it along to both their sons and daughters. In some cases, an FMR1 premutation can change to a full mutation when it is passed from parent to child. Read more about how FMR1 changes as it is passed from parent to child. « Condition Information What causes it? » What causes it? Fragile X results from a change or mutation in the Fragile X Mental Retardation 1 (FMR1) gene, which is found on the X chromosome. The gene normally makes a protein called Fragile X Mental Retardation Protein, or FMRP. This protein is important for creating and maintaining connections between cells in the brain and nervous system. The mutation causes the body to make only a little bit or none of the protein, which often causes the symptoms of Fragile X. Not everyone with the mutated FMR1 gene has symptoms of Fragile X syndrome, because the body may still be able to make FMRP. A few things affect how much FMRP the body can make: - The size of the mutation. Some people have a smaller mutation (a lower number of repeats) in their FMR1 gene, while others have big mutations (a large number of repeats) in the gene. If the mutation is small, the body may be able to make some of the protein. Having the protein available makes the symptoms milder. - The number of cells that have the mutation. Because not every cell in the body is exactly the same, some cells might have the FMR1 mutation while others do not. This situation is called mosaicism (pronounced moh-ZAY-uh-siz-uhm). If the mutation is in most of the body’s cells, the person will probably have symptoms of Fragile X syndrome. If the mutation is in only some of the cells, the person might not have any symptoms at all or only mild symptoms. - Being female. Females have two X chromosomes (XX), while males have only one. In females, if the FMR1 gene on one X chromosome has the mutation, the FMR1 gene on the other X chromosome might not have the mutation. Even if one of the female’s genes has a very large mutation, the body can usually make at least some FMRP, leading to milder symptoms. « How is FMR1 mutation related? How many people are affected? » How many people are affected? About 1 in 4,000 males and 1 in 8,000 females have Fragile X syndrome. How many people have the Fragile X premutation? Although Fragile X syndrome is relatively rare, premutations in the FMR1 gene are relatively common: A recent study of 6,747 people found that 1 in 151 women and 1 in 468 men had the premutation. People with the premutation might not have any symptoms of Fragile X. However, the premutation can sometimes expand in the next generation, which can cause Fragile X syndrome. « What causes it? What are common symptoms » What are common symptoms? People with Fragile X do not all have the same signs and symptoms, but they do have some things in common. Symptoms are often milder in females than in males. - Intelligence and learning. Many people with Fragile X have problems with intellectual functioning. - These problems can range from the mild, such as learning disorders or problems with mathematics, to the severe, such as an intellectual or developmental disability. - The syndrome may affect the ability to think, reason, and learn. - Because many people with Fragile X also have attention disorders, hyperactivity, anxiety, and language-processing problems, a person with Fragile X may have more capabilities than his or her IQ (intelligence quotient) score suggests. - Physical. Most infants and younger children with Fragile X don’t have any specific physical features of this syndrome. When these children start to go through puberty, however, many will begin to develop certain features that are typical of those with Fragile X. - These features include a narrow face, large head, large ears, flexible joints, flat feet, and a prominent forehead. - These physical signs become more obvious with age. - Behavioral, social, and emotional. Most children with Fragile X have some behavioral challenges. - They may be afraid or anxious in new situations. - They may have trouble making eye contact with other people. - Boys, especially, may have trouble paying attention or be aggressive. - Girls may be shy around new people. They may also have attention disorders and problems with hyperactivity. - Speech and language. Most boys with Fragile X have some problems with speech and language. - They may have trouble speaking clearly, may stutter, or may leave out parts of words. They may also have problems understanding other people’s social cues, such as tone of voice or specific types of body language. - Girls usually do not have severe problems with speech or language. - Some children with Fragile X begin talking later than typically developing children. Most will talk eventually, but a few might stay nonverbal throughout their lives. - Sensory. Many children with Fragile X are bothered by certain sensations, such as bright light, loud noises, or the way certain clothing feels on their bodies. - These sensory issues might cause them to act out or display behavior problems. « How many people are affected? How is it diagnosed? » How is it diagnosed? Health care providers often use a blood sample to diagnose Fragile X. The health care provider will take a sample of blood and will send it to a laboratory, which will determine what form of the FMR1 gene is present. Prenatal Testing (During Pregnancy) Pregnant women who have an FMR1 premutation or full mutation may pass that mutated gene on to their children. A prenatal test allows health care providers to detect the mutated gene in the developing fetus. This important information helps families and providers to prepare for Fragile X syndrome and to intervene as early as possible. Possible types of prenatal tests include: - Amniocentesis (pronounced am-nee-oh-sen-TEE-sis). A health care provider takes a sample of amniotic (pronounced am-nee-OT-ik) fluid, which is then tested for the FMR1 mutation. - Chorionic villus (pronounced KOHR-ee-on-ik VILL-uhs) sampling. A health care provider takes a sample of cells from the placenta, which is then tested for the FMR1 mutation. Because prenatal testing involves some risk to the mother and fetus, if you or a family member is considering prenatal testing for Fragile X, discuss all the risks and benefits with your health care provider. Prenatal testing is not very common, and many parents do not know they carry the mutation. Therefore, parents usually start to notice symptoms in their children when they are infants or toddlers. The average age at diagnosis is 36 months for boys and 42 months for girls. Diagnosis of Children Many parents first notice symptoms of delayed development in their infants or toddlers. These symptoms may include delays in speech and language skills, social and emotional difficulties, and being sensitive to certain sensations. Children may also be delayed in or have problems with motor skills such as learning to walk. A health care provider can perform developmental screening to determine the nature of delays in a child. If a health care provider suspects the child has Fragile X syndrome, he/she can refer parents to a clinical geneticist, who can perform a genetic test for Fragile X syndrome. « What are common symptoms? What are the treatments? » ​​ What are the treatments? There is no single treatment for Fragile X syndrome, but there are treatments that help minimize the symptoms of the condition. Individuals with Fragile X who receive appropriate education, therapy services, and medications have the best chance of using all of their individual capabilities and skills. Even those with an intellectual or developmental disability can learn to master many self-help skills. Early intervention is important. Because a young child’s brain is still forming, early intervention gives children the best start possible and the greatest chance of developing a full range of skills. The sooner a child with Fragile X syndrome gets treatment, the more opportunity there is for learning. « How is it diagnosed? Educational treatments » Other FAQs Are there specific disorders or conditions associated with Fragile X syndrome? How is Fragile X syndrome inherited? How does the FMR1 gene change as it is passed from parent to child? Are there specific disorders or conditions associated with Fragile X syndrome? Among the other conditions associated with Fragile X syndrome are the following: - Autism spectrum disorder. From 30% to 50% of people with Fragile X also meet the criteria for autism spectrum disorder. - Mitral valve prolapse. In mitral valve prolapse (pronounced MY-truhl valv PROH-laps), a heart condition, the valve that separates the upper and lower left chambers of the heart does not work properly. This condition is usually not life-threatening, but in severe cases, surgery might be required to correct the problem. - Seizures. Between 6% and 20% of children with Fragile X syndrome have seizures. Seizures associated with the syndrome are more common in boys than in girls. [top] How is Fragile X syndrome inherited? The gene for Fragile X is carried on the X chromosome. Because both males (XY) and females (XX) have at least one X chromosome, both can pass on the mutated gene to their children. - A father with the altered gene for Fragile X on his X chromosome will pass that gene on only to his daughters. To his sons he will pass on a Y chromosome, which doesn’t transmit Fragile X syndrome. Therefore, a father with the altered gene on his X chromosome and a mother with normal X chromosomes would have daughters with the altered gene for Fragile X, while none of their sons would have the mutated gene. - A father can pass on the premutation form of the FMR1 gene to his daughters but not the full mutation. Even if the father himself has a full mutation of this gene, it appears that sperm can carry only the premutation. Scientists don’t understand how or why fathers can pass on only the milder form of Fragile X to their daughters. This remains an area of focused research. - Mothers pass on only X chromosomes to their children, so if a mother has the altered gene for Fragile X, she can pass that gene to either her sons or her daughters. If a mother has the mutated gene on one X chromosome and has one normal X chromosome, and the father has no mutations, all the children have a 50-50 chance of inheriting the mutated gene. - These 50-50 odds apply for each child the parents have. Having one child with the FMR1 mutation does not increase or decrease the chances of having another child with the mutated FMR1 gene. This is also true for the severity of the symptoms. Having one child with mild symptoms does not mean that the other children will have severe symptoms, and having a child with severe symptoms does not mean that the other children will have mild symptoms. [top] How does the FMR1 gene change as it is passed from parent to child? The repeats in the promoter part of the FMR1 gene are unstable, and sometimes the number of repeats increases from one generation to the next. A premutation gene is less stable than a full mutation gene. So as it passes from parent to child, a premutation gene might expand to become a full mutation gene. The chances of expansion depend on the number of repeats in the promoter of the premutation gene: Normal FMR1 genes that have 5 to 44 CGG repeats in the promoter are considered normal. When these genes are passed from parent to child, the number of repeats does not increase or decrease. Intermediate FMR1 genes with 45 to 54 CGG repeats in the promoter are considered intermediate, or borderline. An intermediate gene may expand from one generation to the next, depending on which parent has the gene. Mother to Child About 14% of the time, when a mother passes an intermediate gene to her child, the CGG repeats increase to a number seen with premutations. Research shows that an intermediate gene will not become a full mutation gene in one generation, and so a mother with an intermediate gene will not have a child with a full mutation. Father to Child When intermediate genes are transmitted from father to child, they are generally stable and do not increase to premutations. Premutations Premutation (55 to 199 CGG repeats) FMR1 genes can expand to a full mutation from one generation to the next. The risk of expansion depends on which parent has the gene and the number of repeats in that gene. Mother to Child An FMR1 gene from the mother with 100 CGG repeats is very likely to expand to a full mutation when passed to the child. An FMR1 gene from the mother with 70 to 79 CGG repeats has about a 30% chance of expanding to a full mutation in one generation. Father to Child Premutations passed from father to child have almost no chance of expanding to full mutations. [top] « What are the Treatments? 

Answer: Fragile X syndrome occurs in about 1 in 4,000 males and 1 in 8,000 females. Although Fragile X syndrome is relatively rare, people may have a premutation in the gene responsible for the syndrome without showing symptoms of the syndrome. The premutations of the FMR1 gene are relatively common: A recent study of 6,747 people found that 1 in 151 women and 1 in 468 men had the premutation.