Fragile X Syndrome

Slide 1 · Fragile X Syndrome

Molecular pathogenesis, clinical phenotype, inheritance and diagnosis

Dr Abu Suraih Sakhri

MBBS, MD (Pathology)

www.abusuraihsakhri.com

Slide 2 · Learning objectives

Explain how a CGG-repeat expansion disrupts FMR1 expression.

Connect FMRP deficiency to altered synaptic translation and plasticity.

Recognize the characteristic clinical spectrum and its variability.

Interpret the unusual transmission pattern and rationale for molecular testing.

Slide 3 · Fragile X syndrome: A causal overview

CGG expansion in FMR1

5′ region methylation

FMR1 transcription reduced

FMRP deficiency

Neuronal dysfunction

The disease is a loss-of-function consequence of an unstable noncoding repeat.

Slide 4 · Clinical and genetic significance

FXS as the leading genetic cause of intellectual disability in males.

It is a single-gene disorder with nonclassic transmission patterns.

Molecular repeat analysis has superseded the historical chromosome-fragility assay.

Cognitive

Intellectual disability

Behavioral

Autism; Anxiety; Hyperactivity

Somatic

Face; Macroorchidism; Joints

Slide 5 · Why “fragile X”?

The name describes a former cytogenetic finding on the X chromosome.

Folate-deficient cell culture can reveal a gap or constriction by Giemsa staining.

The apparent break maps near the FMR1 locus; DNA assays now establish the molecular lesion.

Xq27.3 / FMR1

Simplified chromosome schematic

Slide 6 · FMR1 maps to Xq27.3

FMR1 lies on the long arm of the X chromosome at Xq27.3.

This localization linked the historical fragile site to a single disease gene.

The abnormal repeat sequence is located within the FMR1 5′ untranslated region.

Xq27.3 / FMR1

Simplified chromosome schematic

Slide 7 · The causal sequence is CGG

The unstable repeat consists of tandem CGG trinucleotides.

Expansions arise in the 5′ untranslated region, not the protein-coding segment.

The pathogenic effect depends on expansion size and its effect on gene regulation.

Promoter

5′ UTR

Coding region

CGG · CGG · CGG · …

Repeat expansion occurs upstream of the coding sequence

Slide 8 · Repeat length is biologically unstable

The number of CGG units can change during transmission.

Expansion beyond a critical range alters the biological behavior of the locus.

This repeat instability explains inheritance patterns that do not fit a simple static allele model.

Usual allele

Premutation

Full mutation

Slide 9 · Normal, premutation and full mutation

Repeat category

CGG length

Usual implication

Normal

Approximately 6–55

FMR1 remains functionally expressed

Premutation

Approximately 55–200

May be transmitted and further expand

Full mutation

Approximately 200–4000

Often associated with gene silencing and FXS

Textbook intervals are approximate; The extracted source overlaps at the category boundaries.

Slide 10 · The FMR1 repeat lies in the 5′ UTR

A 5′ untranslated region is transcribed but is not translated into protein.

An expanded CGG tract can therefore act through transcriptional control.

The disease need not arise from an altered FMRP amino-acid sequence.

Promoter

5′ UTR

Coding region

CGG · CGG · CGG · …

Repeat is in untranslated, noncoding sequence

Slide 11 · Noncoding repeats and coding repeats differ

Fragile X: Noncoding CGG

Expanded repeat lies in the 5′ UTR.

Promoter methylation reduces FMR1 transcription.

FMRP becomes deficient.

Huntington disease: Coding CAG

Expanded repeat lies in a protein-coding region.

An expanded polyglutamine protein is produced.

Protein toxicity is mechanistically distinct.

Slide 12 · Large expansions trigger abnormal methylation

Abnormal methylation when CGG expansions exceed roughly 230 repeats.

Methylation involves the 5′ region and extends into the promoter.

The promoter modification suppresses FMR1 transcription.

Promoter

5′ UTR

Coding region

CGG · CGG · CGG · …

DNA methylation at 5′ region / promoter

Slide 13 · Promoter methylation silences FMR1

Expressed locus

Promoter remains transcriptionally permissive.

FMR1 mRNA is produced.

FMRP is available.

Silenced full-mutation locus

Extensive DNA methylation involves the promoter.

FMR1 transcription decreases markedly.

FMRP production is reduced or absent.

Slide 14 · The full-mutation phenotype reflects loss of function

Expanded CGG tract

Promoter methylation

Transcriptional suppression

Low or absent FMRP

FXS manifestations

The repeat does not need to encode a novel toxic protein to produce FXS.

Slide 15 · FMRP is the central affected protein

FMRP is the protein product of FMR1.

It is a cytoplasmic RNA-binding protein expressed in multiple tissues.

The brain and testes have particularly abundant FMRP and are prominently affected in FXS.

FMR1

Gene expression

FMRP

RNA-binding regulator

MRNA cargo

Synaptic protein transcripts

MRNA localization and translational control

Slide 16 · FMRP binds selected neuronal mRNAs

FMRP associates with messenger RNAs linked to polyribosomes.

Its cargo includes transcripts for proteins used at pre- and postsynaptic sites.

The FMRP–mRNA complex helps position transcripts where neuronal activity requires them.

FMR1

Gene expression

FMRP

RNA-binding regulator

MRNA cargo

Synaptic protein transcripts

MRNA localization and translational control

Slide 17 · Neurons synthesize proteins locally

Neuronal protein synthesis occurs in the soma and at dendritic spines.

Local translation permits protein production near synapses.

Control of the timing and amount of this translation is fundamental to synaptic regulation.

Soma

Dendrite and spines

FMRP-mediated RNA control

Slide 18 · FMRP transports RNA to dendrites

FMRP associates with mRNA

Complex exits nucleus

MRNA is transported

Complex reaches dendrites

Local translation is controlled

FMRP as both an mRNA trafficking and translation-regulatory factor.

Slide 19 · FMRP regulates translation at synapses

At synaptic junctions, FMRP restrains translation of selected bound mRNAs.

Its activity is described in relation to group I metabotropic glutamate receptor signaling.

Loss of this restraint changes the balance of proteins produced at synapses.

Soma

Dendrite and spines

FMRP-mediated RNA control

Slide 20 · The local translation balance is disturbed

With FMRP

RNA cargo reaches dendrites.

Translation remains under FMRP-mediated restraint.

Synaptic protein production is regulated.

Without FMRP

Translation of regulated transcripts increases.

Synaptic protein abundance becomes imbalanced.

Normal synaptic adaptation is impaired.

Slide 21 · Synaptic plasticity provides the clinical bridge

Low FMRP

Excess translation of cargo mRNAs

Synaptic protein imbalance

Reduced synaptic plasticity

Learning and memory impairment

Plasticity is the ability of synapses to change in response to signals.

Slide 22 · Why intellectual disability develops

FXS disrupts neuronal regulation rather than simply removing a structural brain component.

FMRP deficiency alters the translation-dependent responses of synapses.

Impaired synaptic plasticity provides a mechanism linking the mutation to cognitive dysfunction.

Soma

Dendrite and spines

FMRP-mediated RNA control

Slide 23 · The clinical phenotype is multisystem

Domain

Textbook manifestations

Cognition

Intellectual disability; Learning and memory impairment

Behavior

Autism spectrum features, anxiety, hyperactivity, aggression

Neurology

Epilepsy in a subset of affected individuals

Somatic

Facial, gonadal and connective tissue-like features

Slide 24 · Intellectual disability is a core feature

Affected males often have substantial intellectual disability.

Clinical severity is variable across individuals and between sexes.

Neurodevelopmental findings should be integrated with molecular confirmation.

Cognitive

Intellectual disability

Behavioral

Autism; Anxiety; Hyperactivity

Somatic

Face; Macroorchidism; Joints

Slide 25 · Characteristic craniofacial findings

A long face and prominent mandible are commonly described.

Large, everted ears contribute to the recognizable phenotype.

A high-arched palate is another reported somatic feature.

These findings are supportive, not individually diagnostic.

Craniofacial

Long face · prominent jaw

Ears

Large and everted

Palate

High arched

Slide 26 · Macroorchidism is age-dependent

Testicular enlargement is a particularly distinctive physical feature.

Macroorchidism in at least 90% of affected postpubertal males.

Its absence in young boys does not exclude the disease.

Childhood

After puberty

Absence of macroorchidism does not rule out FXS

Slide 27 · Connective tissue-like manifestations

Hyperextensible joints may be present.

A high-arched palate and mitral valve prolapse are reported.

These findings resemble a connective-tissue phenotype but occur in the FXS clinical spectrum.

Joints

Hyperextensible

Cardiac

Mitral valve prolapse

Palate

High-arched

Slide 28 · Physical signs can be subtle

The characteristic facial and somatic features do not appear uniformly.

Some affected individuals have only mild or inconspicuous dysmorphology.

Molecular testing cannot be replaced by recognition of a “classic face.”

Cognitive

Intellectual disability

Behavioral

Autism; Anxiety; Hyperactivity

Somatic

Face; Macroorchidism; Joints

Slide 29 · Behavioral manifestations broaden the phenotype

Aggression, anxiety and hyperactivity are described in affected individuals.

Autism spectrum disorder is an important associated phenotype.

Behavioral presentation may dominate the referral history.

Cognitive

Intellectual disability

Behavioral

Autism; Anxiety; Hyperactivity

Somatic

Face; Macroorchidism; Joints

Slide 30 · Autism and fragile X are not synonymous

Fragile X syndrome

Defined by a pathogenic FMR1 repeat expansion.

May include autistic features.

Also has specific inheritance and somatic findings.

Autism spectrum disorder

A clinical neurodevelopmental diagnosis.

Occurs with many different genetic backgrounds.

Only a subset have FMR1 mutations.

Slide 31 · Epilepsy is a recognized association

Epileptic seizures occur in a subset of patients with FXS.

Epilepsy in approximately 30% of affected cases.

The seizure phenotype is an associated feature, not a molecular diagnostic criterion.

Cognitive

Intellectual disability

Behavioral

Autism; Anxiety; Hyperactivity

Somatic

Face; Macroorchidism; Joints

Slide 32 · Recognize the breadth of expression

Feature

Clinical interpretation

Intellectual disability

Central neurodevelopmental manifestation

Facial configuration

Helpful pattern; May be subtle

Macroorchidism

More conspicuous after puberty

Autism/anxiety/hyperactivity

Frequent but variable behavioral associations

Epilepsy

Associated finding in a subset

Slide 33 · X linkage explains male predominance

FMR1 is located on the X chromosome.

Males usually have a single X chromosome and thus lack a second allele on another X.

Females can show a spectrum of impairment rather than uniform expression.

Male

X · Y

Female

X · X

The number of X chromosomes shapes genetic expression

Slide 34 · Female phenotype varies with X inactivation

Some females carrying a full mutation have intellectual disability and related findings.

That the active-versus-inactive X pattern influences clinical expression.

Skewed activation of the mutation-bearing X is a proposed explanation, not a deterministic rule.

Male

X · Y

Female

X · X

Different X-inactivation patterns can modify expression

Slide 35 · Transmission from fathers follows X inheritance

Father: X (premutation) + Y

Daughter inherits paternal X

Son inherits paternal Y

May carry paternal FMR1 premutation

Does not inherit paternal FMR1 allele

Slide 36 · The “normal transmitting male” phenomenon

Some men carrying an FMR1 premutation lack the classic childhood FXS phenotype.

They can transmit their repeat-bearing X chromosome to daughters.

A daughter’s maternal transmission can expand that repeat in the next generation.

Carrier fatherpremutation

Daughterpremutation

Childfull mutation

Expansion during maternal transmission

Slide 37 · A carrier daughter can have affected offspring

Paternal premutation

Carrier daughter

Expansion during oogenesis

Full-mutation X in child

FXS phenotype may appear

An apparently unaffected family branch can precede a generation with affected children.

Slide 38 · Large repeat expansion is linked to oogenesis

That dramatic premutation-to-full-mutation expansion occurs during maternal transmission.

Paternal transmission usually introduces only smaller repeat-length changes.

This sex-of-transmitting-parent difference explains the unusual pedigree pattern.

Carrier fatherpremutation

Daughterpremutation

Childfull mutation

Expansion during maternal transmission

Slide 39 · Anticipation follows unstable repeat transmission

Clinical effects can become more conspicuous in successive generations.

A premutation can expand to a full mutation when passed through a female.

This molecular mechanism explains the textbook’s anticipation phenomenon.

Carrier fatherpremutation

Daughterpremutation

Childfull mutation

Expansion during maternal transmission

Slide 40 · Why ordinary X-linked models are insufficient

Typical X-linked model

Allele state is treated as stable across meiosis.

Risk is modeled largely by the transmitted X.

Phenotype reflects inheritance of that allele.

Fragile X transmission

Repeat number itself may change.

Maternal repeat expansion changes allele class.

Phenotype may differ markedly across generations.

Slide 41 · Pedigree position changes observed risk

Different observed risks among relatives at different pedigree positions.

This reflects the possibility of a premutation expanding during intervening maternal transmissions.

A pedigree percentage from a particular family model is not a universal individual risk estimate.

Carrier fatherpremutation

Daughterpremutation

Childfull mutation

Expansion during maternal transmission

Slide 42 · Inheritance reasoning: Father to daughter

An unaffected man carries an FMR1 premutation. Which children inherit his repeat-bearing X chromosome?

Mechanistic interpretation

All of his daughters inherit his X; None of his sons inherit his paternal X.

Slide 43 · Inheritance reasoning: Daughter to grandson

A daughter inherits a premutation from her father. Why might her son be affected even though the grandfather was not?

Mechanistic interpretation

During the daughter’s oogenesis, the premutation may expand to a full mutation.

Slide 44 · Repeat number, methylation and expression

Level

Key event

Consequence

DNA sequence

CGG repeat amplification

Unstable allele length

Epigenetic state

Methylation of 5′ region/promoter

FMR1 silencing

RNA

Reduced FMR1 transcription

Less FMR1 mRNA

Protein

Reduced FMRP

Dysregulated synaptic translation

Cell phenotype

Impaired synaptic plasticity

Cognitive/behavioral effects

Slide 45 · Historical cytogenetics is not the modern test

Former cytogenetic approach

Cultured cells under folate-deficient conditions.

Looked for a Giemsa-negative gap/constriction.

Provided a visible “fragile site” phenotype.

Molecular approach

Analyzes CGG repeat length directly.

Connects test to the disease-causing allele.

Has replaced fragile-site microscopy for diagnosis.

Slide 46 · PCR-based repeat analysis establishes the lesion

PCR-based repeat detection as the preferred diagnostic approach.

The test interrogates the FMR1 CGG tract rather than an unrelated phenotype marker.

Interpretation requires correlation with repeat class, clinical findings and inheritance.

Clinical suspicion

FMR1 repeat test

Molecular result

DNA-based confirmation replaces fragile-site culture

Slide 47 · Molecular diagnosis and clinical recognition

Intellectual disability or syndromic features

Consider FXS in differential

Molecular FMR1 repeat analysis

Classify expansion

Integrate genetic and clinical findings

The defining lesion is molecular; Physical features direct suspicion.

Slide 48 · Premutation is not the same as FXS full mutation

Full mutation: Classic FXS

Large CGG expansion.

5′ region/promoter becomes methylated.

Reduced FMRP: Loss-of-function pathogenesis.

Premutation carrier

Smaller expanded CGG tract.

FMR1 transcription can persist.

Possible risk of distinct RNA-mediated disorders.

Slide 49 · Premutation RNA toxicity contributes to FXTAS

FMR1 premutation

Transcript remains expressed

Abnormal CGG-bearing mRNA

RNA-binding protein sequestration

FXTAS manifestations

Ovarian disease also occurs in carriers, but its exact molecular pathogenesis is less certain.

Slide 50 · FXTAS differs clinically from classic FXS

Fragile X syndrome

Primarily neurodevelopmental presentation.

FMRP deficiency.

Intellectual disability and behavioral manifestations.

FXTAS in premutation carriers

Typically later-life neurologic syndrome.

Intention tremor and cerebellar ataxia.

Toxic RNA-mediated mechanism.

Slide 51 · Ovarian dysfunction is a premutation association

A subset of female premutation carriers develop primary ovarian insufficiency.

Reduced fertility, menstrual irregularity and early menopause are described.

FSH may rise and anti-Müllerian hormone may fall as ovarian function declines.

The precise role of RNA aggregates in follicular loss remains uncertain.

FMR1 premutation

Expanded CGG-bearing RNA

Ovarian dysfunction: Mechanism uncertain

Slide 52 · Case: Intellectual disability and somatic signs

A postpubertal male has intellectual disability, large everted ears, a long face and macroorchidism. What molecular lesion should be investigated?

Mechanistic interpretation

A large CGG-repeat expansion in the 5′ untranslated region of FMR1.

Slide 53 · Case: Explain the gene-expression result

A boy carries a full FMR1 CGG expansion. Why may the encoded FMRP be absent despite no amino-acid–coding variant?

Mechanistic interpretation

The noncoding expansion promotes promoter methylation and transcriptional silencing.

Slide 54 · Case: A girl with a full mutation

Two girls have full FMR1 expansions, but their cognitive findings differ. Which source-discussed mechanism may contribute to the variable phenotype?

Mechanistic interpretation

Differences in X-inactivation patterns may change the proportion of cells expressing the altered X.

Slide 55 · Case: Expanded repeats without childhood FXS

A male relative with an FMR1 premutation is neurologically well in youth. Why should the premutation not be considered equivalent to a full mutation?

Mechanistic interpretation

Premutations can remain transcribed and have distinct intergenerational and later-life implications.

Slide 56 · Mechanism integration: From DNA to behavior

CGG expansion

FMR1 gene silencing

FMRP deficiency

Excess synaptic translation

Reduced synaptic plasticity

Cognitive and behavioral phenotype

Each arrow represents the mechanistic sequence emphasized.

Slide 57 · Interpretation pitfalls to avoid

A classic appearance does not establish a molecular diagnosis.

An absent physical sign does not exclude an FMR1-associated condition.

A premutation is not interchangeable with a full mutation.

The inheritance pattern changes because repeat length may expand in maternal transmission.

Slide 58 · Synthesis: Five points to retain

CGG expansion in the 5′ UTR of FMR1 is the underlying genetic lesion.

Full mutations commonly silence FMR1 through abnormal methylation.

FMRP loss disturbs neuronal mRNA transport and synaptic translation.

Cognition, behavior and somatic findings vary across affected individuals.

Maternal repeat expansion explains anticipation and unusual pedigrees.

Slide 59 · Primary textbook source

Robbins, Cotran & Kumar: Pathologic Basis of Disease, 11th edition (2026).

Chapter 5, Genes and Human Diseases: Fragile X Syndrome and related trinucleotide-repeat mechanisms.

Lecture content is based on the supplied textbook Markdown extraction; original chapter PDF was unavailable for image/caption verification.