CGG Repeat Ranges
An individual with the Fragile X permutation is a 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. (A typical FMR1 gene has 6-54 CGG repeats.) Occasionally a female with a full mutation shows little or no effect of the full mutation and is sometimes referred to as a “full mutation carrier.” However, most of the time, the term “carrier” is used and associated with those with a premutation.
Intermediate or “Gray 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 “gray 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.
Both Females and Males Can Have the Fragile X Premutation
The gene for Fragile X (the FMR1 gene) is on the X chromosome, which is why Fragile X syndrome is called an X-linked condition or disorder. Often in these conditions or disorders, only females are carriers and only males are affected. However, in Fragile X, both males and females can be carriers or have the Fragile X premutation, 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 typical 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.
Issues for Females
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 premutation to a full mutation if it is passed on.
Premutation: About 20% of women with the Fragile X premutation develop primary ovarian insufficiency over their reproductive life span, compared with only 1% in the general population.
Evidence shows women the Fragile X premutation, on average, experience natural menopause at an earlier age compared to those without the Fragile X premutation — the mean age of natural menopause being reduced by about five years from the typical age of about 51 years.
Approximately 3% of women with the Fragile X premutation will have menstrual cycle irregularities in their teens or 20s due to Fragile X-associated primary ovarian insufficiency (FXPOI). 1% of women with the Fragile X premutation will stop having periods prior to age 18, and about one-third — equivalent to 7% of women with the Fragile X premutation — stop having periods at or before age 29.
Not all women with the Fragile X premutation experience FXPOI. One well-documented risk factor is the premutation repeat size: the highest risk for ovarian dysfunction is for women carrying premutation alleles in the 80–100 CGG repeat range, not the highest alleles of >100 repeats (although these group ranges are not exact).
One study estimated the risk of FXPOI to be 38% in this high-risk group, although more studies are needed to better define high-risk alleles and the reason for this association.
Full mutation: Women with the full mutation do not experience FXPOI or increased twinning rates.
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 the Fragile X premutation (risks for an affected child, fertility and prenatal issues), researchers find it difficult to establish emotional effects that are a direct biological result of the Fragile X 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 Fragile X premutation are at increased risk for depression. It is therefore recommended that any individual with the Fragile X premutation who is concerned about depression seek the services of a mental health professional. Research regarding these possible associations and incidences is ongoing.
Issues for Males
Males can also have the Fragile X premutation. A male with the Fragile X premutation 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 with the Fragile X premutation 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 the Fragile X premutation is the risk for FXTAS.
Other than the FXTAS risk in older males, the vast majority of males with the Fragile X 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.
How many individuals are have the Fragile X premutation? According to a 2012 study by the CDC, the frequency of the Fragile X premutation in the U.S. is as follows:
- 1 in 151 females, or about 1 million women.
- 1 in 468 males, or about 320,000 men.
These statistics are important because both men and women are at risk for having symptoms linked to Fragile X-associated conditions and disorders.
Women with a Fragile X premutation reported their last menstrual cycle at an earlier age than women without a Fragile X premutation (48 vs. 51 years).
Men and women with a Fragile X premutation were more than four times as likely to report dizziness or fainting as people without a Fragile X premutation (18% vs. 4%). Men and women with a Fragile X premutation were more than twice as likely to report numbness as people without a Fragile X premutation (29% vs. 13%).