FRAGILE X

SYNDROMEDIVYA NARAYANAN

OCT 13TH

, 2014

HISTORY

1943 - Discovery by Martin & Bell

1969 - Development of Chromosome Tests

by Herbert Lubs

1999 – FMR1 gene and mutational basis

discovered

FMR1 5’ LEADER

FMR1 GENE

METHYLATION OF CYTOSINE LEADS TO

TRANSCRIPTIONAL SILENCING IN FXS

CARRIERS ARE MORE PREMUTATION AFFECTED

CHARACTERIZATION OF

PREMUTATION CGG EXPANSION

WHEN THE CGG EXPANSION HAPPENS?

TYPES OF FMR1 REPEAT EXPANSION MUTATIONS

Type No. of CGG

repeats

Methylation

status of FMR1

Phenotype

in males

Phenotype in

females

Normal 5-50 Unmethylated Unaffected Unaffected

Premutation 51-200 Unmethylated Unaffected Unaffected

Full mutation >200 Hypermethylated Affected 50% affected;

50% unaffected

Mosaicism Varies 51 to

>200

Partially

methylated

Affected Highly variable

Methylation

Mosaicism

>200 Partially

methylated

Affected Highly variable

Unmethylated

full mutation

>200 Unmethylated Nearly all

affected

Highly variable

FMRP PRESENCE

FMRP ABSENCE

FRAGILE X SYNDROME INDIVIDUALS EXHIBIT

ALTERED DENDRITIC SPINE MORPHOLOGY

Weiler et al. Am J Med Genet (1999)

DIAGNOSIS

Pedigree analysis

DIAGNOSIS

Karyotyping

DIAGNOSIS

Direct DNA analysis

DIAGNOSIS

Southern Blotting

DIAGNOSIS

Immunocytochemical tests

Sample : Maternal Blood;amniotic fluid

Tests : PCR, Southern Blot; Karyotype, FISH analysis

Enzymes : EcoRI; EagI

Results : a) Deletion in one FMR1 gene in Linda

b) one 33 CGG repeat allele (normal) and one 110 CGG repeat allele (premutation) in fetus

CASE STUDY

35 years old

10 years old (first child)

CASE STUDY

1. Molecular size marker

2. Control (female with premutation)

3. Control (female with full mutation)

4. Normal male

5. Linda’s result

6. Linda’s fetus result

7. Female with intermediate result

8. Female with normal result

9. Male with full mutation

Electropherogram

Largest peak : 33 CGG repeats

Smallest peak : 110 CGG repeats

CASE STUDY

• After PCR analysis, how many repeat length results are expected?

• What steps can be taken by the laboratory to investigate this apparent discrepancy?

• Is Linda a carrier of fragile X syndrome?

• Is amniotic fluid an appropriate specimen type for prenatal fragile X testing?

• Should these results alleviate Linda’s parental anxiety?

CASE STUDY

• Weiler et al, Synaptic Synthesis of the Fragile X Protein: Possible Involvement in Synapse Maturation

and Elimination; American Journal of Medical Genetics 83:248–252 (1999)

• Dobson et al, Identifying intrinsic and extrinsic determinants that regulate internal initiation of

translation mediated by the FMR1 5' leader; BMC Molecular Biology 2008, 9:89 doi:10.1186/1471-

2199-9-89

• Tassone et al, Elevated Levels of FMR1 mRNA in Carrier Males: A New Mechanism of Involvement in

the Fragile-X Syndrome; Am. J. Hum. Genet. 66:6–15, 2000

• Haas, History Tends to Repeat: FMR-1 Silencing in Fragile X Syndrome; Eukaryon, Vol. 3, February

2007

• Berman et al, Mouse Models of the Fragile X-Associated Tremor/Ataxia Syndrome (FXTAS) and the

Fragile X Premutation; Movement Disorders: Genetics and Models, second edition

• Jin et al, RNA-Mediated Neurodegeneration Caused by the Fragile X Premutation rCGG Repeats in

Drosophila; Neuron, Vol. 39, 739–747, August 28, 2003

• Cheng et al, Astrocytes and Developmental Plasticity in Fragile X; Neural Plasticity Volume 2012,

Article ID 197491,12 pages doi:10.1155/2012/197491

• Fragile X syndrome, Atlas of Genetic Diagnosis and Counselling (2006)

• Eliez et al, Genetics of Childhood Disorders: XI. Fragile X Syndrome; J . Am. Acad. Child Adolesc.

Psychiatry, 39:2. February 2000

• Jaquemont et al, Fragile X Premutation Tremor/Ataxia Syndrome : Molecular, clinical and

Neuroimaginf Correlates; Am J Hum Genet. Apr 2003; 72(4): 869-878

• Damell et al, FMRP Stalls Ribosomal Translocation on mRNAs Linked to Synaptic Function and

Autism; Cell 146, 247–261, July 22, 2011

• Heim, Fragil X syndrome; Diagnostic Molecular Pathology in Practice, 2011

REFERENCES

THANKYOU