Aug. 05, 2020
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What is fragile X syndrome?
Fragile X syndrome (FXS) is a genetic condition that causes intellectual disability, behavioral and learning challenges and various physical characteristics. Though FXS occurs in both genders, males are more frequently affected than females, and generally with greater severity.
Features of fragile X syndrome in males
• The majority of males with fragile X syndrome demonstrate significant intellectual disability (formerly referred to as “mental retardation”). Disabilities in FXS include a range from moderate learning disabilities to more severe intellectual disabilities.
• Physical features may include large ears, long face, soft skin and large testicles (called “macroorchidism”) in post-pubertal males. Connective tissue problems may include ear infections, flat feet, high arched palate, double-jointed fingers and hyper-flexible joints.
• Behavioral characteristics can include ADD, ADHD, autism and autistic behaviors, social anxiety, hand-biting and/or flapping, poor eye contact, sensory disorders and increased risk for aggression.
• No one individual will have all the features of FXS, and some features, such as a long face and macroorchidism, are more common after puberty.
Features of fragile X syndrome in females
• The characteristics seen in males can also be seen in females, though females often have milder intellectual disability and a milder presentation of the syndrome’s behavioral and physical features.
• About one-third of females with FXS have a significant intellectual disability.
• Others may have moderate or mild learning disabilities, emotional/mental health issues, general anxiety and/or social anxiety.
• A small percentage of females who have the full mutation of the FMR1 Gene that causes FXS will have no apparent signs of the condition—intellectual, behavioral or physical. These females are often identified only after another family member has been diagnosed.
While individuals with FXS will experience a number of challenges in their lives, given effective interventions and support they can be engaging and productive members of their families, schools, workplaces and communities.
How common is fragile X syndrome?
There have been a number of studies aimed to determine the prevalence of FXS in males and females. Studies have been undertaken both in the “special needs” population and the general population. The agreed upon prevalence of FXS in males is approximately 1 in 3600 to 4000 and in females is approximately 1 in 4000 to 6000. The reason it is lower in females is that, while all males with an FMR1 full mutation will have fragile X syndrome, some females with an FMR1 full mutation will not have behavioral, cognitive or physical features of FXS.
How many individuals are carriers of the FMR1 premutation?
General population studies in both the newborn and adult populations have found that approximately 1/260 females and 1/800 males are carriers of the FMR1 premutation worldwide. There may be some variation in some populations. For example a large Israeli study found approximately 1/130 women were FMR1 carriers. However, at this time, the accepted carrier rates internationally stand at approximately 1/260 females and 1/800 males.
The prevalence of FXTAS and FXPOI
• Studies report that approximately 1/3 (33%) of all men over 50 years of age with an FMR1 premutation will develop symptoms of FXTAS. Not all of these men will have symptoms that fulfill the diagnostic criteria for FXTAS, as some develop mild features that do not progress.
• Studies report approximately 5-8% of women over 50 years of age with an FMR1 premutation, will develop features of FXTAS, though females tend to have fewer and milder symptoms than men.
• Approximately 20-25% of women with an FMR1 premutation will develop FXPOI. FXPOI covers a range of ovarian difficulties including early menopause, irregular menstrual cycles, infertility, sub fertility and premature ovarian failure (cessation of menstrual periods prior to age 40).
How common are intermediate (grey area) alleles?
Approximately 1 in 50 (2%) of individuals have an intermediate allele. There appear to be no clinical associations with intermediate alleles. Most intermediate alleles are stable and do not change over generations. In a small number of families intermediate alleles show some slight instability and can lead to a premutation in future generations. Individuals with an intermediate allele are not at risk for any for the FXD’s or to have children with fragile X syndrome.
Based on the best available evidence:
• Approximately one-million Americans carry the Fragile X mutation, including approximately 100,000 with fragile X syndrome, and are at risk for developing a Fragile X-associated Disorder.
• Approximately 1 in 3600 to 4000 males in the world are born with the full mutation for Fragile X.
Note: The vast majority of males with the full mutation will have fragile X syndrome.
• Approximately 1 in 4000 to 6000 females in the world are born with the full mutation for Fragile X.
Note: Approximately 50% of females with the full mutation will have some features of fragile X syndrome.
• Approximately 1 in 800 men in the world are carriers of the Fragile X premutation.
• Approximately 1 in 260 women in the world are carriers of the Fragile X premutation.
What it means to be a carrier
When you or someone in your family are told you are a “carrier” for a Fragile X mutation, many questions arise. Do you have a premutation or full mutation? What does it mean for your health, your family and your future? Does the size of your FMR1 gene matter?
A carrier has an altered form of a gene that can lead to having a child or grandchild with a genetic disorder. We are all carriers of gene mutations, many of which are “silent.” This means we can pass the gene on but suffer no ill effects from it ourselves. It is only through genetic testing that we know which ones we carry. Some genes are on the non-sex chromosomes (numbered 1-22), which are the same in males and females, and some are on the sex chromosomes, which are the X and Y chromosomes.
Traditionally, a carrier of a genetic mutation is defined as a person who inherits an altered form of a gene but shows no effects of that mutation. However, in Fragile X this definition does not exactly fit, as carriers of a premutation are at risk to develop Fragile X-associated Disorders including fragile X-associated tremor/ataxia syndrome (FXTAS) and fragile X-associated primary ovarian insufficiency (FXPOI).
Why can both females and males be fragile X carriers?
The gene for Fragile X (the FMR1 gene) is on the X chromosome, which is why fragile X syndrome is called an X-linked disorder. Often in these disorders, only females are carriers and only males are affected. However, in Fragile X, both males and females can be carriers, and both can be affected by the condition. This occurs because the changes in the FMR1 gene go through stages as it is passed down in a family. These stages start with the normal gene and then proceed to the premutation and then the full mutation. The differences in the stages are determined by the number of “CGG repeats” (repeats of a DNA pattern). In most other X-linked conditions, there is no middle “premutation” state, so males with the mutation are either affected or non-carriers.
CGG repeat ranges
A premutation carrier is an individual, male or female, who has between 55-200 CGG repeats in the Fragile X (FMR1) gene. The full mutation is defined as over 200 CGG repeats. Occasionally a female with a full mutation shows little or no effect of the full mutation and is sometimes referred to as “full mutation carrier.” However, most of the time, the term “carrier” is used for those with a premutation.
Intermediate or “grey area” alleles
An allele is a term to describe one’s gene, like green apples are a specific form of an apple. Some individuals have what is called an “intermediate” or “grey area” sized allele. These are alleles with 45-54 CGG repeats. They are not considered to be mutations and do not appear to be associated with any clinical or medical issues, developmental disabilities or social/emotional difficulties. These alleles are identified as such because there is a small chance that they are mildly unstable and may expand to a premutation in future generations. There is no reported risk for an individual with an intermediate sized allele to have a child with a full mutation. Generally, we don’t use the term “carrier” for those with an intermediate allele.
Issues for females with a premutation
Females with a premutation are at risk to have a child, male or female, with fragile X syndrome. The magnitude of this risk is related to the number of CGG repeats identified in her FMR1 gene. The larger the number, the higher the risk for expansion from a permutation to a full mutation if it is passed on.
-- Physical effects: Among the first effects reported in premutation carriers were an increased rate of twin births and of premature ovarian failure (POI)... We now know that approximately 20-25% of women with a premutation experience FXPOI, which is characterized by infertility, decreased ovarian function, early menopause or irregular cycles. Women with the full mutation do not experience FXPOI or increased twinning rates. FXTAS is another medical effect of the premutation. It is more common in male carriers, but has been reported in about 5-8 percent of female carriers over the age of 50.
-- Social-emotional effects: Given the stresses and emotional aspects of parenting a child with disabilities, along with stresses that can accompany the various reproductive issues associated with having a premutation (risks for an affected child, fertility and prenatal issues), researchers find it difficult to establish emotional effects which are a direct biological result of the premutation. Though most women with a premutation show no significant mental health issues, some have reported increased general anxiety, shyness and social anxiety. In addition, there is evidence that women with the premutation are at increased risk for depression. It is therefore recommended that any premutation carrier who is concerned about depression seek the services of a mental health professional.
Male can also carry the Fragile X premutation. A male carrier will pass his premutation (as a premutation, not a full mutation) on to all of his daughters and none of his sons. There is no reported risk for a male premutation carrier to have a daughter with fragile X syndrome. However, there is a risk for fragile X syndrome in his grandchildren through his daughters.
The most significant issue for males with a premutation is the risk for FXTAS.
Other than the FXTAS risk in older males, the vast majority of males with the premutation are clinically unaffected. There have been reports of a small subset of boys with a premutation who have an additional diagnosis of autism, ADHD, or other learning/behavioral disorders. It is possible that in a small subset of boys, these developmental disorders may be caused by an interaction of various genetic and non-genetic factors, one of which may be the FMR1 premutation. Research regarding this possible association is ongoing.
Genetics and inheritance
What are chromosomes?
Our bodies are made up of about 60 trillion cells. Each one of those cells manufactures proteins. The kinds of proteins any given cell makes determine its particular characteristics, which in turn create the characteristics of the entire body.
The instructions for making these proteins are stored in chemicals or molecules called DNA, which is organized into chromosomes. Chromosomes are found in the center, or nucleus, of all of our cells, including the eggs and sperm.
Chromosomes are passed down from generation to generation through the egg and sperm. Typically, we all have 46 chromosomes in our cells, two of which are sex chromosomes. In females, these are two Xs; in males they are an X and a Y.
The role of genes?
Genes are sections of DNA that are passed from generation to generation and perform one function. If we think of DNA as letters in the alphabet, the genes are words and the chromosome is a full sentence. All 46 chromosomes then make up the whole book.
There are many genes on each chromosome; we all have tens of thousands of genes that instruct our bodies on how to develop.
Genes are given names to identify them and the gene responsible for fragile X syndrome is called FMR1. The FMR1 Gene is on the X chromosome.
How do changes in the FMR1 gene lead to FXS?
The FMR1 gene appears in four forms that are defined by the number of repeats of a pattern of DNA called CGG repeats.
Individuals with less than 45 CGG repeats have a normal FMR1 gene. Those with 45-54 CGG repeats have what is called an “intermediate” or “grey zone allele,” which does not cause any of the known fragile X associated disorders.
Individuals with 55-200 CGG repeats have a “premutation,” which means they carry an unstable mutation of the gene that can expand in future generations and thus cause fragile X syndrome in their children or grandchildren. Individuals with a premutation can also develop FXTAS or FXPOI themselves.
Individuals with over 200 CGG repeats have a full mutation of the FMR1 gene, which causes fragile X syndrome.
The full mutation causes the FMR1 gene to shut down or “methylate” in one region. Normally, the FMR1 gene produces an important protein called FMRP. When the gene is turned off, the individual does not make this protein. The lack of this specific protein is what causes fragile X syndrome.
The fragile X premutation
Fragile X-associated Disorders are a group of conditions called trinucleotide repeat disorders. A common feature of these conditions is that the gene can change sizes over generations, becoming more unstable, and thus the conditions may occur more frequently or severely in subsequent generations. These conditions are often caused by a gene change that begins with a premutation and then expands to a full mutation in subsequent generations.
Approximately 1 in 250 females and 1 in 800 males carry the FMR1 premutation. They are thus “carriers” of the premutation.
Premutations are defined as having 55-200 CGG repeats and can occur in both males and females. When a father passes the premutation on to his daughters, it usually does not expand to a full mutation. A man never passes the fragile X gene to his sons, since he passes only his Y chromosome to them, which does not contain a fragile X gene.
A female with the FMR1 premutation will often pass on a larger version of the mutation to her children (more on this point below). She also has a 50 percent chance of passing on her normal X chromosome in each pregnancy, since usually only one of her X chromosomes has the FMR1 mutation.
The chance of the premutation expanding to a full mutation is related to the size of the mother’s premutation. The larger the mother’s CGG repeat number, the higher the chance that it will expand to a full mutation if it is passed on.
Typically, the premutation has no immediate and observable impact on a person’s appearance or health. However, some females with a premutation will experience fragile X-associated primary ovarian insufficiency (FXPOI), which causes infertility, irregular or missed menstrual cycles, and/or early menopause.
Additionally, some older adults with a premutation may develop a neurological condition called FXTAS, (fragile X-associated tremor/ataxia syndrome), an “adult onset” neurodegenerative disorder.
FXTAS and FXPOI are part of the family of conditions called Fragile X-associated Disorders.
The FMR1 full mutation
A full mutation is defined as having over 200 CGG repeats and causes that indicate the presence of fragile X syndrome in males and some females. Most full mutation expansions have some degree of Methylation (the process which “turns off” the gene). Males with a full mutation will have Fragile X Syndrome, though with varying degrees of severity
About 65-70 percent of females with a full mutation exhibit some difficulties with cognitive, learning, behavioral, or social functioning, and may also have some of the physical features of FXS (such as large ears or a long face). The remaining 30-35 percent are at risk to develop mental health issues such as anxiety or depression, or they may have no observable effects of the full mutation.
Fragile X in an “X-linked” condition, which means that the gene is on the X chromosome.
Since a woman has two X chromosomes a woman with a premutation or full mutation has a 50% chance of passing on the X with the mutation in each pregnancy, and a 50% chance of passing on her normal X.
If she has a premutation, and it is passed on (to either males or females), it can remain a premutation or it can expand to a full mutation. If she has a full mutation and it is passed on (to either males or females), it will remain a full mutation.
Because males have only one X chromosome, fathers who carry the premutation will pass it on to all their daughters and none of their sons (they pass their Y chromosome on to their sons). There have been no reports of premutations that are passed from a father to his daughter expanding to a full mutation. This appears to only occur when passed from a mother to her children.
Unique features of fragile X inheritance
In many X-linked conditions only males who inherit the abnormal gene are affected. Fragile X syndrome is one of the X-linked conditions that can also affect females.
Additionally, in other X-linked conditions all males who carry the abnormal form of the gene are affected. In fragile X syndrome, unaffected males can carry the gene in the premutation form while themselves having no symptoms of the condition.
During the 1970s and 1980s the only available tool for diagnosing fragile X syndrome (FXS) was the chromosome (“cytogenetic”) test. While it was helpful, it was not always accurate. In the 1990s, scientists identified the FMR1 gene that causes FXS, and accurate DNA testing became available. The “FMR1 DNA Test” (sometimes called the “Fragile X DNA Test”) thus replaced chromosome testing, and became the “standard of care” for determining the presence of Fragile X. DNA testing detects more than 99 percent of individuals (both males and females) with FXS, as well as Fragile X carriers.
Who should have fragile X testing?
There are three general circumstances in which Fragile X testing should be considered:
1. Clinical symptoms that suggest Fragile X Syndrome, FXTAS or infertility/FXPOI.
2. A family history of fragile X syndrome, FXTAS, intellectual or learning disabilities or autism of unknown cause, or infertility.
3. Family or personal history of a Fragile X Genetics and Inheritance (carrier).
Specific indications for testing include:
• Any male or female with intellectual disabilities, developmental delay, speech and language delay, autism or learning disabilities of unknown cause.
• Any female with infertility, elevated FSH levels, premature ovarian failure, primary ovarian insufficiency or irregular menses.
• Any adult over 50 with features of FXTAS, including intention tremors, ataxia, memory loss, cognitive decline, personality change, especially in combination with a positive family history of Fragile X.
• Any preconception or pregnant woman who expresses interest in or requests Fragile X carrier testing.
Lab tests for fragile X
The FMR1 DNA Test can be administered with two different lab procedures.
1. The Southern blot analysis test determines if the gene has a full mutation, its approximate size, whether the gene has been “methylated” and if there is mosaicism of the gene (a mixture of different cell types).
2. The polymerase chain reaction (PCR) analysis can determine the actual number of “CGG repeats” (a pattern of DNA) that are present in the Fragile X gene. For various technical reasons, PCR has been not the test of choice to diagnose a full mutation, but is quite accurate in determining premutation and normal gene repeat numbers. However, PCR is less expensive and quicker than Southern blot, and recent advances in technology have increased its ability to identify Fragile X full mutations. PCR may thus be the only test used in the near future.
Testing cost and reporting time
The blood test usually ranges from $300-$600, and results are usually available in 2-4 weeks.
The test must be ordered by a genetic counselor or physician. A genetic counselor often will facilitate the testing, and make certain that the correct drawing, shipping and processing of the sample occurs. The genetic counselor is trained and experienced in the interpretation and explaining of the test results. She/he can coordinate any follow-up appointments and work with your physician to make referrals to resources in your area. You can locate a genetic counselor in your area by logging on to the “Find a counselor” feature on the National Society of Genetic Counselors website or by contacting the genetic specialist (firstname.lastname@example.org) at the NFXF for a referral to a counselor in your area.
In most cases you will be directed to go to a blood drawing station for a blood drawing procedure. Although this facility is often referred to as a “lab,” it is not the actual lab doing the DNA test. After the blood is drawn it is sent to a specific genetics or “reference” lab for the genetic test.
If your insurance company (or state/public insurance such as Medicaid) is paying for the testing, it is likely that the sample will go to the laboratory with which they are contracted. If you are paying for the test yourself, your physician or genetic counselor can locate a genetics lab in your area or one with which they have a relationship. The National Fragile X Foundation can assist your provider in locating a laboratory as well. For providers only, there is also a listing of laboratories that provide genetic testing; log onto GeneTests for information.
CPT and ICD-9 codes for fragile X testing
When ordering the FMR1 DNA test, if the CPT codes are needed, check with the lab where the test will be performed. However, here is a list of the CPT codes most commonly associated with FMR1 DNA testing:
Pertinent ICD-9 codes:
• Fragile X Syndrome: 759.83
• Testing of male for genetic disease carrier status: V26.34
• Testing for genetic disease carrier status of female: V26.31
What do test results report?
The test will determine the number of CGG repeats that are present in the area of the Fragile X gene that is studied. If Southern blot is ordered, the test will also tell you the methylation pattern of the Fragile X gene. If the testing is performed on a female, two numbers will be provided—one for each X chromosome. In males, only one result will be provided, though that number might be a range, such as 200-400, because the gene often expands in an unpredictable manner.
Who receives test results?
Test results go to the provider who ordered the test, such as the genetic counselor or physician. Other recipients may include a pediatrician, obstetrician, or other health care provider involved in the patient’s or family’s care.
Other tests for children with developmental delay
Physicians and other providers ordering Fragile X testing often arrange for other genetic testing to provide more information on a child’s condition. Depending on the child’s presenting features or symptoms, this might include chromosome analysis, chromosome microarray (CGH), FISH testing (for other single gene conditions such as Velo-cardio-facial syndrome) and metabolic/ biochemical tests. Other types of medical evaluations might be recommended as well, including magnetic resonance imaging (MRI), electroencephalography (EEG), computer tomography (CT scan) or X-rays.
This information was developed by the National Fragile X Foundation and is herewith used with permission.
National Fragile X Foundation. What Is Fragile X? Available at: https://www.fragilex.org/fragile-x-associated-disorders/. Last accessed January 30, 2014.
The information in this document is for general educational purposes only. It is not intended to substitute for personalized professional advice. Although the information was obtained from sources believed to be reliable, MedLink Corporation, its representatives, and the providers of the information do not guarantee its accuracy and disclaim responsibility for adverse consequences resulting from its use. For further information, consult a physician and the organization referred to herein.