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.
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.
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.
For More Information
Advances in Understanding Premutation Involvement in Women & Advice to Stay Healthy. Be sure to look through Footnote #6 – by Polussa – and open the Figures and the Table that are embedded in the article.Women’s Health and the Fragile X Premutation
Many couples who find out that the woman or the man is an FMR1 mutation carrier ask “what are my options for the future?” or “How can I increase my chance of having a child without Fragile X syndrome or to test a pregnancy in the future?” There are a number of ways to build a family for carriers including conceiving naturally and testing the pregnancy (prenatal diagnosis), testing a fertilized egg outside the woman’s body and implanting only unaffected embryos PGD), using an egg donor, adoption and conceiving naturally and not doing any testing.
Prenatal diagnosis refers to testing the fetus early in pregnancy. There are two different methods to evaluate the genetic status of the fetus; amniocentesis and chorionic villus sampling (CVS). They both use fluid and/or tissue that is made up of fetal cells and the fetal DNA is analyzed in the cells obtained from the procedure. These tests are commonly offered to women for other indications, like “advanced maternal age” (AMA) or a positive maternal screening test. If a woman is having a procedure for AMA or any other indication, which does not include Fragile X, the fluid will not be analyzed for FXS unless it is known that the mother is an FMR1 carrier.
Amniocentesis uses a small (two tablespoons) sample of the amniotic fluid that surrounds the fetus. This fluid has fetal cells that can be grown and studied for various genetic conditions. The procedure is usually performed between 15 and 20 weeks of gestation, which is determined by ultrasound (sonogram) and /or by the first day of the last menstrual period. With the ultrasound affording a view of the womb, the physician inserts a very thin needle through the abdomen to withdraw the fluid from the uterus. The fluid is then taken to the laboratory to begin the testing process. For many women the procedure feels similar to a blood draw, though some feel mild cramping of the uterus. After the procedure the woman is given care instructions regarding rest, travel, etc. The risks of the procedure include miscarriage, bleeding, cramping, and amniotic fluid leakage. The risk for miscarriage is approximately 1 in 300.
Chorionic villus sampling (CVS)
Chorionic villus sampling involved obtaining a small sample from the developing placenta. The placenta contains genetic material that is of fetal origin. Chorionic villi are small fingerlike projections on the edge of the placenta. The cells can be studied for chromosome abnormalities such as Down syndrome and FXS, if the mother is a known carrier.
Unlike amniocentesis, CVS does not detect neural tube defects such as spina bifida. A blood test is therefore recommended at 15-20 weeks gestation to detect neural tube defects.
CVS is performed earlier in pregnancy than amniocentesis, usually between 10-13 weeks of gestation. The procedure is performed under ultrasound guidance.
There are two methods of withdrawing the sample, depending on the position of the placenta. If the placenta is in back of the fetus, the physician inserts a thin tube through the vagina/cervix. In the other method, when the placenta is in front of the fetus, a small needle is inserted through the abdomen, similar to amniocentesis. Using gentle suction, the doctor removes a very small amount of chorionic villi for laboratory analysis.
Some women feel cramping or pressure, while others do not find it uncomfortable. Like amniocentesis, there is a small risk of miscarriage (less than 1 percent). If performed earlier than 10 weeks’ gestation, there is an associated risk of limb defects.
Prenatal Fragile X DNA testing is reliable and accurate. A maternal blood sample may be requested to ensure the prenatal sample does not contain any maternal cells.
The laboratory techniques used include both PCR and Southern analyses. PCR is able to determine the size of premutation, intermediate and normal alleles. Southern analysis better defines full mutation size alleles and whether the gene is working or not Methylation. This process occurs early in the pregnancy in a fetus with a full mutation.
The CGG repeat size can be accurately determined in amniotic cells. However, in CVS it may be difficult to determine the methylation status in large premutation and small full mutation alleles. Though this is rare, in these cases an amniocentesis is recommended to clarify the fetal status. On rare occasions, the CVS may yield no result (less than 1% of the time) and an amniocentesis is then recommended.
One of the challenges emerging from prenatal testing is the difficulty in predicting intellectual, behavioral or psychological involvement in females with the full mutation. Females demonstrate a broad range of effects, from very mild to more severe intellectual disability and autism. Prenatal testing cannot determine where in this range a female may fall.
In most cases, results of prenatal testing can take from 3-5 weeks; the cells must be cultured in the laboratory for 1-2 weeks and then analysis requires 7-12 days. However, recently some labs have offered “direct” testing of CVS which involves testing the tissue directly before culturing the cells and getting a “preliminary” result in about one week. (Southern analysis on DNA from cultured cells is also performed to confirm the “preliminary” result).
Preimplantation Genetic Diagnosis (PGD)
PGD is a process whereby a woman achieves a pregnancy by in-vitro fertilization (IVF) with an unaffected embryo. IVF is an assisted reproductive technology in which after stimulating the ovaries to produce multiple eggs for fertilization, the eggs are removed from the ovary, and fertilized in the laboratory. Then a single cell from each fertilized egg is sent to a PGD laboratory. The cells are then tested for the FMR1 mutation and then only those eggs without a mutation are implanted into the woman’s womb. This procedure has been used for other genetic conditions.
One of the difficulties when attempting PGD for Fragile X carriers is that some carriers have ovarian insufficiency (ranging from premature menopause to reduced ovarian functioning). This makes it difficult to stimulate the ovaries to make the multiple eggs needed for PGD. Another complexity in PGD is that the laboratory technology used, called PCR, does not accurately distinguish between a full and premutation. It will only tell you if the fetus has the normal FMR1 allele or the one with the mutation, not how large the mutation is. The PGD analysis for Fragile X is done by a process called linkage—the lab uses genetic markers very close to the gene to distinguish the mother’s two different X chromosomes. This linkage technique will require blood samples from other family members (such as a sibling, other affected individuals, grandparents or other relatives).
The accuracy of PGD is 95-98 percent; therefore prenatal testing by CVS or amniocentesis is recommended later in the pregnancy.
There are two parts to this process. The IVF, which is performed at many infertility centers, is the process of withdrawing and then implanting the fertilized egg. The PGD lab work, i.e. the genetic testing of the egg, is done separately and only at a small number of PGD laboratories in the world. When undergoing PGD, the woman may have her IVF at a center close to home or at the center performing the PGD analysis. There are only a few centers around the world that offer PGD for Fragile X. Therefore, if the woman has the IVF close to home the IVF center will ship the cells to the PGD center, which will then alert the IVF center as to which embryos that are unaffected and available for implantation. It is usually a five-day process from when the eggs are removed to the day they are transferred back.
Prior to any PGD process a woman must have a fertility evaluation to determine her ovarian function and quantity of eggs. Then she will begin medications to stimulate her ovaries prior to the egg removal.
Choosing a center
Parents exploring a PGD center might consider asking the following questions:
- How much experience does the center have with Fragile X testing?
- How many unaffected babies have been born using their services?
- What is the percentage of women who don’t get pregnant using their services?
- How many “cycles” are typical for most patients?
- Have they had any misdiagnoses?
- What technology do they use to determine the FMR1 status?
- Have they worked with your IVF group before?
- What is their rate of inconclusive results?
- Do they have a genetic counselor on staff?
Costs and advantages
PGD is an expensive endeavor. The cost of the IVF alone can be up to $20,000, with up to $10,000 more for the PGD. These fees will depend on the number of cycles, distance from the PGD center, other testing (such as chromosome analysis) medications, etc. Health insurance will sometimes pay for a portion of it—coverage will vary from plan to plan and may depend on whether or not the couple is infertile.
The advantages of PGD include:
- It is often an acceptable option for those who would not consider a pregnancy termination.
- It allows for the possibility of having a child who is biologically related to both parents.
- Other testing such as chromosome analysis can take place at the same time, thus reducing the risk for other disorders.
Achieving Pregnancy Using Donated Eggs
Some couples choose to attempt using eggs donated from a non-FMR1 carrier. The benefits of this option are that it does not require the carrier (recipient) to have adequate eggs of her own, and it eliminates the risk for Fragile X syndrome. Though it is expensive because it involves in vitro fertilization, it is not as expensive as PGD. However, if a couple wishes to have a child who is biologically related to both the mother and father this option would not meet those needs.
This process also utilizes in-vitro fertilization. The carrier (recipient) begins the process with medical tests for any infectious diseases, RH type, thyroid and general health screening, etc. The egg donors go through their own screening, which includes hormone levels, genetic screening usually including FMR1 screening, and other carrier testing, family history assessment, psychological consultation, and more. The recipient couple meets with a “donor coordinator” to discuss their criteria for a good match, and then, with the coordinator, selects an egg donor whom they feel would meet these criteria.
Following the donor selection, the menstrual cycles of both the donor and recipient are synchronized by medications that stop and then restart their cycles. Then the recipient is given hormones to prepare her body to receive the fertilized egg, and the donor is given medications to stimulate her ovaries to produce multiple eggs. The eggs are retrieved from the donor at the optimal time (this is called “harvesting”), and then the eggs are fertilized in the laboratory by sperm from the recipient’s husband or a donor. Healthy-appearing embryos are implanted into the recipient’s womb two- to three days later. Sometimes unused healthy embryos are frozen for future use or embryo donation. Hormone levels and ultrasound are used to determine if pregnancy is achieved, and whether there is more than one fetus. The recipient stays on hormones for three months to support the early stages of the pregnancy.
There are obvious benefits to using donor eggs. These include eliminating the risk of Fragile X syndrome as well as the ovarian dysfunction which many carriers experience. The drawbacks include potential uncertainty about the donor’s family history (outside of those genetic conditions which are tested for in the assessment) or feelings about having a child with a different biological background than the recipient mother.