fragile x laboratory testing: background and quality improvement opportunities (part 1 of 2) elaine...
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Fragile X Fragile X Laboratory Testing:Laboratory Testing:Background and Quality Background and Quality
Improvement OpportunitiesImprovement Opportunities(part 1 of 2)(part 1 of 2)
Elaine Lyon, Ph.D.University of Utah/ARUP Laboratories
Association for Molecular Pathology, Chair, Clinical Practice Committee
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OutlineOutline
Clinical Features of Fragile X Molecular Basis of Disease Molecular Testing/Interpretation Opportunities for Improvement
Quality control material Interlaboratory study (AMP, CDC, NIST)
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Fragile X SyndromeFragile X Syndrome Most common inherited form of mental retardation.
Incidence 1:4000 males and 1:8000 females.
Affected males have mental retardation, characteristic physical features and behavior.
Affected females exhibit a less severe phenotype.
Found in all populations.
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FeaturesFeatures Mental impairment Attention deficit/autistic-like Long, thin face - prominent forehead Large ears Flexible joints Low muscle tone Enlarged testicles
Jones KL. Smith’s Recognizable Patterns of Human Malformation, 4 th Ed.4
Folate-sensitive fragile site at Xq27.3 (FRAXA).Other sites: FRAXD, FRAXE, & FRAXF.
Chromosome LevelChromosome Level
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Molecular Level
Tri Nucleotide Repeat (CGG) at the 5' Untranslated Region (UTR). A small expansion (pre-mutation) associated with
increased mRNA A large expansion associated with methylation,
inactivating gene expression.
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Molecular SchematicMolecular Schematic
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Protein LevelProtein Level
Normal – protein widely expressed (nerve, brain, etc.) RNA binding protein
Pre-mutation – normal protein, increased mRNA
Full mutation – no protein produced Protein expression by immunohistochemistry
(IHC) suspected deletions/point mutations in males
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TransmissionTransmission
Female pre-mutation carriers 50/50 chance of transmitting unstable allele
May stay within pre-mutation range May expand to full mutation (higher pre-mutations more likely to
fully expand in one generation)
Male pre-mutation carriers Will transmit pre-mutation to all daughters Unlikely to expand
Intermediate May expand to pre-mutation, but not full mutation, in one
generation
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Risk of Expansion byRisk of Expansion byPre-mutation Size Pre-mutation Size
Number of Maternal Pre-Mutation CGG Repeats
Approximate % Risk that a Son Will be Affected with Fragile X Syndrome
56-59 7%
60-69 10%
70-79 29%
80-89 36%
90-99 47%
> 100 50%
Adapted originally from Warren & Nelson 1994; modified according to Nolin et al. 1996.
GENEReviews at www.genetests.org, FMR1-Related Disorders.
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Fragile X-associated Tremor/Ataxia Syndrome (FXTAS)
Symptoms Late-onset, progressive cerebellar ataxia/intention tremor Short-term memory loss, executive function deficits, cognitive decline Lower-limb proximal muscle weakness, and autonomic dysfunction
Genetics FMR1 pre-mutation mRNA accumulation
Jacquemont, JAMA 2004
Penetrance is age related
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Distribution of Subjects
020406080
100120140160180
18 - 29 30 - 39 40 - 49 50+
Age at Interview
Nu
mb
er
of
Su
bje
cts
Full Mutation
Premutation
Control
Proportion of Subjects Experiencing POF
0 0 0 0
0.05
0.11
0.22
0.26
0.020 0 0
0
0.05
0.1
0.15
0.2
0.25
0.3
18 - 29 30 - 39 40 - 49 50+
Age at Interview
Pro
po
rtio
n o
f S
ub
ject
s
Allingham-Hawkins, AJMG, 199912
Premature Ovarian Failure (POF)
Cessation of menses before age 40
21% of females with pre-mutations