Fragile X Syndrome Testing | FMR1 Gene Analysis | Cytogenomix® Malaysia
Trinucleotide Repeat Disorder

Fragile X Syndrome Testing

Comprehensive FMR1 gene analysis including CGG repeat sizing, methylation studies, and AGG interruption analysis. Essential for intellectual disability evaluation and carrier screening.

1:4,000 MALES AFFECTED
1:8,000 FEMALES AFFECTED
1:150-300 FEMALE PREMUTATION
7-10 DAYS TAT

Test Specifications

Technical details for Fragile X genetic testing

Test Code FXS001 (CGG Repeat Analysis), FXS002 (Methylation Study), FXS003 (Comprehensive)
Methodology TP-PCR (Triplet-Primed PCR) for repeat sizing, Southern blot for large expansions and methylation status
Gene Analyzed FMR1 (Xq27.3) - CGG repeat region in 5' UTR
Mutations Detected CGG repeat number, methylation status, AGG interruption pattern
Sample Type 3-5 mL whole blood (EDTA purple top) | Prenatal: Amniotic fluid/CVS
Turnaround Time 7-10 working days (PCR) | 14-21 days (Southern blot)
Reporting Repeat category, methylation status, risk assessment for expansion
Genetic Counseling Pre- and post-test counseling included

Understanding Fragile X Syndrome

Fragile X syndrome is the most common inherited cause of intellectual disability and autism spectrum disorder. It is caused by an expanded CGG trinucleotide repeat in the FMR1 gene, leading to gene silencing and loss of FMRP protein.

Key Facts:

  • Inheritance: X-linked dominant with anticipation
  • Gene: FMR1 (Fragile X Mental Retardation 1)
  • Mechanism: CGG repeat expansion → hypermethylation → gene silencing
  • Anticipation: Increasing severity in successive generations

Fragile X syndrome demonstrates genetic anticipation - the repeat expands when transmitted through females, leading to earlier onset and more severe symptoms in subsequent generations.

Fragile X Inheritance

CGG Repeat Categories

Classification based on repeat number

Normal

5-44
CGG Repeats

Stable, no risk of expansion

Normal FMRP production

Intermediate (Gray Zone)

45-54
CGG Repeats

Unstable, may expand in future generations

Minimal expansion risk

Premutation

55-200
CGG Repeats

Unstable, at risk for expansion to full mutation

Elevated FMR1 mRNA

Full Mutation

>200
CGG Repeats

Methylated, gene silenced

Absent FMRP → Fragile X syndrome

FMR1-associated Conditions

Spectrum of disorders related to FMR1 mutations

Fragile X Syndrome (FXS)

Full mutation (>200 CGG, methylated)

  • Intellectual disability (moderate to severe)
  • Autistic-like behaviors
  • Anxiety, ADHD, sensory issues
  • Distinctive facial features (long face, prominent ears)
  • Macroorchidism in males
Affected: Males (100%), Females (50-70%)

FXPOI

Premutation carriers (55-200 CGG)

  • Primary ovarian insufficiency
  • Menopause before age 40
  • Infertility, irregular cycles
  • Affects ~20% of female premutation carriers
Females only

FXTAS

Fragile X-associated Tremor/Ataxia Syndrome

  • Late-onset (after age 50)
  • Intention tremor, cerebellar ataxia
  • Parkinsonism, cognitive decline
  • Affects ~40% of male premutation carriers
Males > Females

Methylation Status

Determining gene silencing and clinical significance

Unmethylated
Active Gene

Normal FMRP production

Normal, Intermediate, Premutation

Methylated
Silenced Gene

No FMRP production

Full Mutation >200 CGG

Southern blot analysis determines both repeat size and methylation status, essential for diagnosing full mutation and predicting phenotype.

Risk of Repeat Expansion

Probability of premutation expanding to full mutation during transmission

Maternal Repeat Size Risk of Expansion to Full Mutation AGG Interruptions
55-59 CGG ~3-5% Risk lower with AGG interruptions
60-69 CGG ~5-30% Risk decreases with each AGG
70-79 CGG ~30-80% Presence of AGG interruptions reduces risk
80-89 CGG ~80-95% Fewer interruptions = higher risk
90-99 CGG 95-99% Almost certain expansion
>100 CGG >99% Virtually all expand to full mutation

AGG Interruption Analysis: AGG trinucleotides interspersed within the CGG repeat increase stability. The absence of AGG interruptions increases the risk of expansion during transmission.

Clinical Features by Age

Developmental trajectory in Fragile X syndrome

Infants & Toddlers

  • Hypotonia (low muscle tone)
  • Developmental delay
  • Autistic-like behaviors
  • Poor eye contact

Children

  • Intellectual disability
  • ADHD, anxiety
  • Speech/language delay
  • Sensory processing issues

Adolescents

  • Social anxiety
  • Macroorchidism (males)
  • Distinctive facial features
  • Joint laxity

Adults

  • Ongoing cognitive deficits
  • Behavioral challenges
  • Mitral valve prolapse
  • Seizures (15-20%)

Who Should Be Tested?

Indications for Fragile X genetic testing

Developmental Delay/ID

Individuals with unexplained intellectual disability, autism, or developmental delay

Family History

Family history of Fragile X syndrome, intellectual disability, or autism

POI/FXTAS Symptoms

Women with primary ovarian insufficiency, adults with tremor/ataxia of unknown cause

Prenatal/Carrier Screening

Women with family history or seeking carrier screening, especially those with POI

X-Linked Dominant Inheritance

Understanding transmission and anticipation

Premutation Mother × Normal Father

Xᴾ X
X Y
Xᴾ X
X X
Xᴹ Y*
X Y

*If premutation expands to full mutation

50% daughters premutation carriers
50% sons at risk for FXS

Normal Mother × Premutation Father

X X
Xᴾ Y
Xᴾ X
X X
X Y
X Y

100% daughters premutation carriers
0% sons affected (no expansion through male)

Important: Expansion to full mutation ONLY occurs during female transmission. Male premutation carriers pass stable premutations to all daughters.

Frequently Asked Questions

Common questions about Fragile X testing

What is the difference between premutation and full mutation?

Premutation (55-200 CGG) carriers have elevated FMR1 mRNA but produce some FMRP. They are at risk for FXPOI and FXTAS but not intellectual disability. Full mutation (>200 CGG, methylated) silences the gene, causing Fragile X syndrome.

Why are some females with full mutation unaffected?

Due to X-inactivation, females with full mutation may have a skewed pattern where the normal X is active in most cells, producing enough FMRP. Approximately 30-50% of females with full mutation have intellectual disability.

What is AGG interruption analysis?

AGG trinucleotides interspersed within the CGG repeat increase stability. The number and position of AGG interruptions help predict the risk of a premutation expanding to full mutation during transmission.

Can Fragile X be detected prenatally?

Yes, prenatal diagnosis is available through CVS (10-12 weeks) or amniocentesis (15-18 weeks). Both repeat size and methylation status can be determined. PGT for IVF cycles is also available.

What is the recurrence risk for siblings?

If mother is a premutation carrier: 50% of siblings at risk for inheriting the premutation, with expansion risk depending on repeat size. If mother has full mutation: all children inherit an expanded allele.

Is there treatment for Fragile X syndrome?

There is no cure, but multidisciplinary management includes educational interventions, behavioral therapy, medications for ADHD/anxiety, and targeted treatments in development (mGluR5 antagonists, GABA agonists).

Ready to Order Fragile X Testing?

Our genetic counselors and neurodevelopmental specialists are ready to assist with test selection, result interpretation, and family counseling.

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