Fragile X

Question 1

True or False: Fragile X syndrome is the most common cause of inherited mental retardation

Answer 1

True

Question 2

Mutation that causes Fragile X:

Answer 2

FMR1 gene on the X chromosome

Question 3

Symptoms of Fragile X Syndrome

Answer 3

• Intellectual Disability (usually in the moderate range)
• Characteristic physical features and behavior
• Males are more severely affected
• Affected females tend to have mild intellectual disability and variable associated physical features

Question 4

98% of the cases of fragile X syndrome are caused by what mutation

Answer 4

An expansion of an unstable CGG repeat sequence located in the 5’ untranslated region (UTR) of the FMR1 gene
The full mutation form of FMR1 gene consists of over 200 repeats and is abnormally hypermethylated

Question 5

What is responsible for the intellectual disability?

Answer 5

The lack of the gene product, FMRP (an RNA-binding protein)

Question 6

Prevalence of Fragile X

Answer 6

1/4000 males have fragile X syndrome

Question 7

A pre-mutation of Fragile X is associated with:

Answer 7

Pre-mutation (most often clinically reported as 55+ repeats) is most strongly associated with premature ovarian failure (POF) which is clinically defined as the cessation of menses before the age of 40.

Among women who carry the premutation, approximately 21% have POF compared to only 1% in the general population.

Question 8

What late-onset neurodegenerative disorder has recently been associated with the premutation?

Answer 8

Late onset neurodegenerative disorder with tremor/ataxia syndrome (FXTAS) has been identified in men who carry the premutation and a small proportion of women with the premutation.

Clinical symptoms: cerebellar ataxia and intention tremor

Penetrance: in men 50+: ~20-40% but more research is needed to accurately define risks of age-related penetrance

Question 9

Testing Techniques for Fragile X

Answer 9

• PCR: The size of PCR products are indicative of the approximate number of repeats present. Large mutations are more difficult to amplify and may fail to yield a detectable product in the PCR assay. PCR can not determine methylation status of FMR1. Allows accurate sizing of alleles in the normal zone, the “gray zone,” and the premutation sizes in a relatively short turnaround time. The assay is not affected by skewed X-inactivation
• Southern Blotting: Allows a crude measure of the size of the repeat segments and an accurate assessment of the methylation status to be assayed simultaneously. Is more labor intensive and requires larger quantities of genomic DNA than PCR does. Precise sizing is not possible, though it accurately detects alleles in all size ranges. Highly skewed X-inactivation can lead to the lack of resolution of the premutation allele.
• Labs should therefore perform both methods in clinical circumstances

Question 10

When should Fragile X testing be considered?

Answer 10

• Individuals of either sex with intellectual disability, developmental delay, or autism especially combined with one of the following: family hx of fragile X, any physical or behavioral characteristic of fragile X, relatives with undiagnosed intellectual disability
• Individuals in prenatal settings who have a family history of fragile X or undiagnosed intellectual disability.
• Fetuses of known carrier mothers
• Affected individuals or their relatives in the context of a positive cytogenetic fragile X test result who are seeking further counseling related to the risk of carrier status. The cytogenetic test was used prior to the ID of the FMR1 gene and is significantly less accurate than the current DNA test. DNA testing on such individuals is warranted to accurately identify premutation carriers from full mutation carriers
• Women who are experiencing reproductive or fertility problems associated with elevated follical stimulating hormone (FSH) levels, especially those with fam hx of intellectual disability

Question 11

Approaches to Fragile X Testing

Answer 11

• DNA analysis if one is testing specifically for fragile X
• For isolated cognitive impairment, DNA analysis for fragile X should be performed as part of a comprehensive genetic evaluation (Cytogenetic studies are critical since chromosome abnormalities are as common or more common than fragile X)
• Prenatal testing of a fetus should be offered when the mother of a known carrier ideally through amniocentesis after 15 weeks gestation. CVS can be performed but results should be interpreted with caution because the methylation status of the FMR1 gene is often not yet established in chorionic villi at the time of sampling.
• If a woman has ovarian failure before the age of 40, DNA testing for premutation size alleles should be considered as part of the infertility evaluation and prior to in vitro
• If a patient has cerebellar ataxia and intentional tremor, DNA testing for premutation size alleles, especially among men, should be considered as part of the diagnostic evaluation