Fragile X syndrome occurs in individuals who carry the gene, FMR1 on an X-chromosome. This condition is inherited as a dominant X-linked disorder. With a dominant disorder, the condition results when there is only one copy of the altered gene in each cell.
Fragile syndrome occurs twice as frequently in males (1:1,200) as compared to females (1:2,500) A striking characteristic of X-linked inheritance is that fathers cannot pass X-linked traits to their sons. The Fragile X gene, FMR1, can be passed on in a family by individuals who have no apparent signs of this genetic condition. In some families, a number of members appear to be affected, whereas in other families a newly diagnosed individual may be the first family member to exhibit symptoms.
By and large, Fragile X syndrome results from a mutation in the FMR1 gene where a segment, known (CGG triplet repeat), is expanded. Under normal circumstances, the CGG triplet is repeated from 5 to approximately 55 times. In contrast, those who have Fragile X syndrome will have more than 200 repeats. CGG segments prevent the FMR from propagating the formation of a specific protein needed to protect against the development of Fragile X syndrome. Thus over-expression of CGG triplet (>200 times) on an X chromosome represents a degree of loss of this “protective protein” as to lead to the development of fragile X syndrome. Since boys have only one X chromosome, Fragile X syndrome tends to manifest much more severely in males than in females, (who have two X chromosomes).
In a normal population, the number of repeated FMR1 genes varies from 5 to about 55. Those with 55 to 200 repeats of the CGG segment are said to have an FMR1 premutation (carriers”). In women, this is liable to increase to >200 repeats in the developing eggs. Accordingly, such women are at increased risk of having a child with fragile X syndrome. Conversely, when passed by men to the next generation, CGG repeats either remain the same in size or shorten. This is why men with a permutation do not transmit the disease. However they do transmit the permutation which if carried to a subsequent female offspring can result in them transmitting Fragile X syndrome in subsequent generations.
Both males and females with fragile X pre-mutation are by and large intellectually and physically normal in outward appearance. Some may manifest with mild but often socially harmful intellectual or behavioral symptoms,. They are however usually not infertile.
Some men with a premutation are at risk of developing a manifestation of fragile X-associated tremor/ataxia syndrome (FXTAS) a condition characterized by loss of balance, tremors and memory loss. It occurs in some older male carriers of the gene. Heart bone and skin problems are also often present. Age distribution is a s follows: Seventeen percent (17%) of males aged 50-59 years, in 38 percent of males aged 60-69 years, in 47 percent of males aged 70-79 years, and in 75 percent or males aged 80 years or older. Some female premutation carriers may have diminished ovarian reserve (DOR), premature ovarian failure and FXTAS.
It is important to bear in mind that women who have approximately 55 to 200 repeats. There is no clear cut-off between the upper limit of normal and the lower limit of the premutation range. Accordingly, cases with 45-55 repeat copies fall into the so called “gray zone.” In some cases, premutations expand from generation to generation such that over time they ultimately express as full Fragile X syndrome. The larger the premutation in cases that fall in the “gray zone”, the greater is the risk of subsequent expansion to a full mutation in the offspring.
Boys with full FMR1 mutation (Fragile X syndrome) will almost routinely have moderately severe mental retardation. They will tend to have a characteristic facial appearance with a long face, enlarged cranium, protruding ears and an elongated face with a protuberant chin and forehead. Affected boys after puberty tend also to experience enlargement of the scrotum and laxicity of joints. There will also usually be characteristic behavioral problems such as lack of impulse control, temper tantrums, delay in speech and language development and perseverative speech. Hand biting, hand flapping and attention deficit /hyperactivity are other common manifestations. Fragile X syndrome is also the most common known cause of autism or “autistic-like” behaviors.
Girls with Fragile X on the other hand, tend to only have mild mental retardation. Women who have fewer repeats of the FMR-1 gene usually do not have mental retardation but often will have prematurely diminishing of ovarian reserve (DOR) with early menopause and infertility. Both men and women may develop FXTAS.
While most males with full blown clinical fragile X syndrome are mentally retarded and exhibit some or all the physical and behavioral characteristics, only about one third of females are mentally retarded. Another one third are partially mentally impaired, and the remaining third are unaffected.
Fragile X syndrome is diagnosed through DNA testing of cells using one of two methods:
- Polymerase Chain Reaction (PCR) or
- Southern blot analysis
Both methods exhibit a high degree of interpersonal variability and thus when it comes to interpreting results, there are significant limitations. This is especially the case when diagnosing a “carrier state.” Interpretation is further complicated by the presence of other fragile sites in the same region of the X chromosome.
It is recommended that in the following circumstances, patients undergoing assisted reproduction be tested for Fragile-X:
- All mentally challenged individuals, those who are autistic, and in cases of developmental delay
- Women with unexplained premature reduction in ovarian reserve or premature ovarian failure (menopause)
- Individuals who have physical or behavioral characteristics of fragile X syndrome
- Those with a family history of fragile X syndrome
- Those with a family history of mentally challenged male or female relatives where no definitive cause has been ascertained.
- Offspring of known carrier mothers
Prenatal diagnosis can be made by 2nd trimester amniocentesis, which yields definitive results. In contrast, results obtained from 1st trimester chorionic villus sampling (CVS) should be interpreted with caution, because the status of the FMR1 gene often will not fully manifest in chorionic villi until the second trimester.