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Condition report · Neurological

Fragile X Syndrome

The most common inherited cause of intellectual disability, caused by an expanded CGG repeat in the FMR1 gene. It can also cause autism features and anxiety. Carriers of smaller expansions (premutations) are at risk of FXTAS and early menopause.

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About this condition

Fragile X syndrome (FXS) is the most common inherited cause of intellectual disability and the most common single-gene cause of autism spectrum disorder. Prevalence is approximately 1 in 4,000 males and 1 in 8,000 females. FXS is caused by a CGG trinucleotide repeat expansion in the 5' untranslated region of the FMR1 gene. Normal alleles have 5-44 repeats; premutation alleles have 55-200 repeats; full mutation alleles have more than 200 repeats, which triggers hypermethylation and gene silencing. Inheritance is X-linked, meaning affected males are typically more severely symptomatic (intellectual disability, behavioral issues, characteristic facial features, macroorchidism) while affected females have more variable and milder phenotypes due to random X-inactivation.

Premutation carriers are at risk for distinct conditions: fragile X-associated tremor/ataxia syndrome (FXTAS) (progressive neurodegeneration primarily affecting older males) and fragile X-associated primary ovarian insufficiency (FXPOI), occurring in approximately 20% of female premutation carriers and affecting fertility. The CGG repeat exhibits unstable inheritance: premutation alleles (55-200 repeats) undergo expansion during maternal meiosis and can expand to full mutation in a single generation. Maternal repeat length correlates with expansion risk: mothers carrying higher repeat counts have higher probability of transmitting a full mutation to offspring.

A confirmed FMR1 full mutation diagnosis enables early intervention services (speech, occupational, behavioral therapy from infancy), targeted pharmacotherapy for associated anxiety and ADHD, and educational planning informed by intellectual disability severity. It enables cascade testing of female relatives, identifying premutation carriers at risk for FXTAS (progressive neurodegeneration) and FXPOI (premature menopause affecting fertility planning). Preconception carrier screening for FMR1 premutation is now recommended by ACOG for all women considering pregnancy. Genetic counseling guides prenatal diagnosis options and family planning decisions.

Fragile X has two distinct clinical presentations: full mutation (FXS with intellectual disability) and premutation carriers (who may develop FXTAS or FXPOI without FXS).

The 2 conditions we analyse

Fragile X Syndrome

X-linked dominant

An FMR1-related neurodevelopmental disorder caused by CGG-repeat expansion, with intellectual disability and behavioural features.

What a diagnosis changes

Early intervention services, speech and occupational therapy, behavioral support, pharmacotherapy for anxiety and ADHD, preconception carrier screening, prenatal diagnosis, gene therapy in development

Genes analysed

Also known as

fra(X) syndrome; fragile 10 mental retardation syndrome; fragile X intellectual disability syndrome; Fragile X syndrome, X-linked dominant; FraX syndrome; FRAXA syndrome; FXS; marker X syndrome; Martin-Bell syndrome; mental retardation, X-linked, associated with Marxq28; X-linked mental retardation and macroorchidism

Includes 4 related subtypes ↓
  • fragile X syndrome type 1
  • fragile X syndrome type 2
  • fragile X syndrome type 3
  • symptomatic form of fragile X syndrome in female carrier
MONDO:0010383OMIM 300624Orphanet 908MedGen 8912

Fragile X-Associated Tremor/Ataxia Syndrome (FXTAS)

A late-onset neurodegenerative condition in some carriers of an FMR1 premutation, with tremor, ataxia and cognitive decline, more often affecting men.

Genes analysed

Also known as

fragile X tremor/ataxia syndrome; Fragile X tremor/ataxia syndrome, X-linked dominant; FXTAS; FXTAS syndrome

MONDO:0010382OMIM 300623Orphanet 93256MedGen 333403

Why whole genome sequencing

Standard short-read sequencing cannot detect CGG repeat expansions. Fragile X is systematically missed unless specifically tested.

Standard WGS cannot detect CGG trinucleotide repeats: Fragile X is invisible on most genetic tests

Standard short-read exome and genome sequencing universally FAIL at detecting CGG repeat expansions in FMR1. The repetitive nature of the region causes sequencing reads to fail or collapse, making the repeat invisible to standard bioinformatic pipelines. Fragile X testing has historically required specialized molecular methods: PCR-based repeat sizing followed by Southern blot analysis for methylation status. This means Fragile X is systematically missed by standard next-generation sequencing panels and even many 'comprehensive' genetic tests unless Fragile X-specific testing is explicitly ordered.

Early testing identifies full mutation cases and premutation carriers, enabling prevention of transmission

A confirmed FMR1 full mutation diagnosis guides early intervention services (speech, occupational, behavioral therapy from infancy), enabling optimal neurodevelopmental outcomes with early support. Carrier testing of female relatives identifies premutation carriers who are at risk for FXTAS (progressive neurodegeneration), FXPOI (premature menopause and fertility planning), and transmission of full mutation to offspring. Preconception carrier screening is now ACOG-recommended for all women considering pregnancy. Prenatal diagnosis via CVS or amniocentesis is available for at-risk pregnancies. Genetic diagnosis connects families to the robust Fragile X research community and emerging therapies including gene therapy approaches in development.

Limitations

  • Fragile X Syndrome cannot be called with short read sequencing, our standard whole genome sequencing offering. This report is provided for any clinical correlations that might be relevant. Learn more

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