Fragile X DNA Analysis (Fragile X FMR1 Repeat Analysis)

Fragile X syndrome is the most common form of inherited intellectual disability. FMR1, the gene associated with fragile X syndrome, full mutations are frequently associated with autism spectrum disorder, behavioral changes and cognitive impairment. 

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Fragile X syndrome is the most common form of inherited intellectual disability. FMR1, the gene associated with fragile X syndrome, full mutations are frequently associated with autism spectrum disorder, behavioral changes and cognitive impairment. 

FMR1 is actively involved during embryogenesis, contributing to fetal brain and CNS development. Mutations within the FMR1 gene are associated with Fragile X syndrome, Fragile X tremor/ataxia syndrome (FXTAS) and premature ovarian failure (POF). Fragile X syndrome diagnosis involves the expansion of an unstable CGG trinucleotide repeat sequence to a full mutation. Females who are known to be premutation carriers are at an increased risk of having premature ovarian failure (POF). FMR1 premutation can expand to a full mutation in subsequent generations when transmitted from a female carrier. Additionally, males with a premutation are at increased risk of developing Fragile X-associated tremor/ataxia syndrome (FXTAS).

The Ambry Test: Fragile X DNA Analysis includes FMR1 CGG repeat detection by PCR. If a full mutation allele is detected, reflex to Southern blot for expansion confirmation and promoter methylation analysis. Greater than 99% of the mutations in the FMR1 gene responsible for Fragile X syndrome are due to CGG repeat expansions.

Disease Name 
Fragile X Syndrome
X-Linked Intellectual Disabilities
Disease Information 

Fragile X syndrome is the most common form of inherited intellectual disability. FMRP (the FMR1 protein) is localized in neurons and is active during embryogenesis, primarily during formation of neuronal synapses.1,2 As a result, loss-of-function mutations commonly result in autism spectrum disorder, intellectual disability, behavioral changes, and cognitive impairment.3

Mutations within the FMR1 gene are associated with fragile X syndrome, fragile X tremor/ataxia syndrome (FXTAS) and premature ovarian failure (POF). The result of testing the size of the triplet repeat region of the promoter of the FMR1 gene  may be looked as divided into three categories: normal, premutation, and full mutation, based on the number of CGG trinucleotide repeats.

The general population carries between 6-50 CGG repeats. Premutation carriers can have anywhere between 55 to 200 CGG trinucleotide repeats, while a full mutation result indicates > 200 repeats in the 5’UTR region with hypermethylation of the promotor region.3 Individuals affected with Fragile X syndrome have >200 methylated repeats which leads to a silencing effect of the gene.4,5

Premutation carriers can exhibit social, emotional and cognitive differences along the Fragile X spectrum in addition to behavioral differences suggestive of autism spectrum disorder.6 Premutation female carriers are at an increased risk of premature ovarian failure (POF)7 and for repeat expansion to a full mutation in the next generation.8  
 

Testing Benefits & Indication 

Fragile X syndrome testing should be considered for any individual with intellectual disability, developmental delay, and learning disabilities of unknown cause, autism or autism-like characteristics. Testing should also be considered for families with a history of Fragile X syndrome or intellectual disability due to unknown cause.

Prenatal testing is recommended for fetuses of mothers known to be Fragile X carriers, as well as for those with a family history of Fragile X syndrome or intellectual disability of unknown cause.

Test Description 

If FMR1 promoter CGG repeat detection is requested, a triple primed polymerase chain reaction (PCR) using a standardized kit is used to selectively amplify the regions of gDNA corresponding to the FMR1 promoter gene followed by capillary electrophoresis for fragment size analysis. Determination of FMR1 expansions and methylation status is performed as a reflex test if PCR indicates a full mutation or a premutation (>53 repeats) allele. mPCR CGG analysis is done using Asuragen AmplideX mPCR kit.  

Mutation Detection Rate 

CGG repeat expansion in the FMR1 gene have been associated with >99% of clinically affected Fragile X syndrome patients. A small portion (<1%) of Fragile X cases are due to rare mutations in areas of the FMR1 gene not evaluated by this test.

Specimen Requirements 

Blood: Collect 5 ml minimum, 8-10 ml preferred, in an EDTA purple/lavender-top tube. Store at room temperature or refrigerate. Ship at room temperature. Specimen must be less than 5 days old upon receipt.
DNA: Minimum sample amount required at 2.1 mL at 100 nanogram concentration. Extraction protocol required.
Prenatal: Ambry only accepts amniotic fluid cultures for prenatal testing of Fragile X syndrome.  Please call for more information.

Billing Codes 
Test Code Technique
4544 Fragile X DNA Analysis (FMR1)

 

Turnaround Time 
Technique Days
Fragile X DNA Analysis (FMR1) 7-14

 

Specialty 
Genes 
References 

1. Bear M. et al. The mGluR theory of fragile X mental retardation. Trends Neurosci. 2004; 27:370-7. [PMID: 15219735]

2. Abitbol M. et al. Nucleus basalis magnocellularis and hippocampus are the major sites of FMR-1 expression in the human fetal brain. Nature Genet. 1993; 4:147-153. [PMID: 8348153]

3. Verkerk A. et al. Identification of a gene (FMR-1) containing a CGG repeat coincident with a breakpoint cluster region exhibiting length variation in fragile X syndrome. Cell. 1991;65:905-914 [PMID: 1710175]

4. Rife M et al. Analysis of CGG variation through 642 meioses in Fragile X families. Mol Hum Repro. 2004;10(10):773-776. [PMID: 15322225]

5. Fu YH et al. Variation of the CGG repeat at the fragile X site results in genetic instability: resolution of the Sherman paradox. Cell. 1991;67:1047-1058. [PMID: 1760838]

6. Hagerman P, Hagerman R. The fragile-X premutation: a maturing perspective. Am J Hum Genet. 2004;74:805-816 [PMID: 15052536]

7. Hessl D et al. Amygdala dysfunction in men with the fragile X premutation. Brain. 2007;130:404-416. [PMID: 17166860]

8. Pieretti M et al. Absence of expression of the FMR-1 gene in fragile X syndrome. Cell. 1991;66:817-822. [PMID: 1878973]

9. Grigsby, J et al. Impairment in the cognitive functioning of men with fragile X-associated tremor/ataxia syndrome (FXTAS). J of Neurol Sci. 2006;248: 227-233. [PMID: 16780889]